Thesis 1 - Integration of Ferroelectric and Piezoelectric Thin Films

Integration of Ferroelectric Thin Filmsinto Silicon Based Microsystems

Von der Fakultät für Elektrotechnik und Informationstechnikder Rheinisch-Westfälischen Technischen Hochschule Aachen

zur Erlangung des akademischen Grades eines Doktors derIngenieurwissenschaften genehmigte Dissertation

vorgelegt von

Diplom-IngenieurCarsten Kügeler

aus Rheda-Wiedenbrück

Berichter: Univ.-Prof. Dr.-Ing. Rainer WaserUniv.-Prof. Dr. rer. nat. Wilfried Mokwa

Tag der mündlichen Prüfung: 13.07.2006

Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar.

III

PrefaceThis dissertation was written during my Ph.D.-studies at the Institut für Werkstoffe der Elek-trotechnik of the Rheinisch-Westfälische Technische Hochschule Aachen, Germany (RWTHAachen).

I would like to express my gratitude to Prof. R. Waser for giving me the opportunity to do re-search at the Institut für Werkstoffe der Elektrotechnik in the exciting field of ferroelectric basedMEMS and for providing an excellent working and learning environment. I highly appreciatehis advice and support.

I am also indebted to Prof. W. Mokwa who kindly agreed to be the co-examiner in the jury.

Many thanks also to: Dr. U. Böttger for his support and stimulating discussions; S. Tappe andR. Plonka for the fruitful discussions, careful proof-reading and being great office mates; D.Bräuhaus, C. Dehoff and K. Herbertz for their dedicated work during the diploma thesis; Dr. T.Schneller and R. Thelen for preparing the precursor solutions and the help regarding questionsof chemistry; D. Erdoglija for preparing Pt electrodes and for performing lithography so manytimes; G. Wasse for taking SEM pictures.

Last but not least I would like to thank all colleagues and all the students at the Institut fürWerkstoffe der Elektrotechnik and at the Forschungszentrum Jülich for their valuable supportand many great moments spent together.

IV

V

Contents

1 Introduction 1

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.1 Piezoelectric Cantilever Structures on Silicon Substrates . . . . . . . 2

1.2.2 Piezoelectric Cantilever Structures on Metal Substrates . . . . . . . . 2

1.2.3 Integrated Piezoelectric Driven Micro-Relays . . . . . . . . . . . . . 2

1.2.4 Piezoelectric Driven Microwave Switches . . . . . . . . . . . . . . . 3

1.2.5 Thin Film Bulk Acoustic Resonators for High Frequency Applications 3

1.2.6 Dielectric Bolometers . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Ferroelectric Materials 7

2.1 Spontaneous Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Piezoelectric Effect and Electrostriction . . . . . . . . . . . . . . . . . . . . 10

2.3 Lead Zirconate Titanate (PZT) . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4 Lanthanum doped Lead Hafnate Titanate (PLHT) . . . . . . . . . . . . . . . 13

2.5 Barium Strontium Titanate (BST) . . . . . . . . . . . . . . . . . . . . . . . 14

3 Thin Film Deposition 17

3.1 Chemical Solution Deposition . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.2 Microstructural Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2.1 Lead Zirconate Titanate (PZT) . . . . . . . . . . . . . . . . . . . . . 20

3.2.2 Lanthanum doped Lead Hafnate Titanate (PLHT) . . . . . . . . . . . 23

3.2.3 Barium Strontium Titanate (BST) . . . . . . . . . . . . . . . . . . . 27

4 Material Characterization with Respect to MEMS Devices 29

4.1 Electrical Characterization Methods . . . . . . . . . . . . . . . . . . . . . . 29

4.2 Electromechanical Characterization Methods . . . . . . . . . . . . . . . . . 30

4.3 Lead Zirconate Titanate (PZT) . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.3.1 Electromechanical Fatigue . . . . . . . . . . . . . . . . . . . . . . . 35

VI

4.4 Lanthanum doped Lead Hafnate Titanate (PLHT) . . . . . . . . . . . . . . . 38

4.5 Barium Strontium Titanate (BST) . . . . . . . . . . . . . . . . . . . . . . . 42

5 Technology 45

5.1 Photolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.2 Silicon Bulk Micromachining . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.3 Surface Micromachining . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.3.1 Plasma Etching of Pt and BST . . . . . . . . . . . . . . . . . . . . . 52

5.3.2 Plasma Etching of PZT . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.3.3 Plasma Etching of SiO2 and Si3N4 . . . . . . . . . . . . . . . . . . . 54

5.3.4 Anisotropic Silicon Etching . . . . . . . . . . . . . . . . . . . . . . 55

5.3.5 Isotropic Silicon Etching . . . . . . . . . . . . . . . . . . . . . . . . 56

5.4 HF-Vapor Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

5.5 Stress Compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

5.6 Electroplating of Freestanding Copper Contacts . . . . . . . . . . . . . . . . 66

6 Piezoelectric Cantilever Structures on Silicon Substrates 69

6.1 Cantilevers Fabricated by Silicon Bulk Micromachining . . . . . . . . . . . . 69

6.2 Cantilevers Fabricated by Silicon Surface Micromachining . . . . . . . . . . 76

6.3 Cantilevers Fabricated on Silicon On Insulator Substrates . . . . . . . . . . . 79

7 Devices 85

7.1 Integrated Piezoelectric Driven Micro-Relays . . . . . . . . . . . . . . . . . 85

7.2 Piezoelectric Driven Microwave Switches . . . . . . . . . . . . . . . . . . . 91

7.3 Thin Film Bulk Acoustic Wave Resonators . . . . . . . . . . . . . . . . . . . 97

7.4 Dielectric Bolometers for Infrared Detection . . . . . . . . . . . . . . . . . . 103

7.5 Piezoelectric Cantilevers on Metal Foil Substrates . . . . . . . . . . . . . . . 110

8 Conclusions 117

8.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

8.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

References 121

VII

Used Symbols and AbbreviationsSymbols

A AreaC CapacitanceCf Capacitance on membraneCref Capacitance on bulkCCurie Curie-Weiss constantdij Tensor of the piezoelectric coefficientsD Dielectric displacementDi Components of the dielectric displacement vectoreij ij-th component of transversal piezoelectric charge coefficientE Electric fieldEC Coercive fieldESi Young’s modulus of siliconf Frequencyfbias Large signal frequencyfosc Small signal frequencyh displacementI Electric currentkB Boltzmann’s constant kB = 1.38066·10−23 JK−1

kt Electromechanical coupling constantl LengthL Power densityP Polarization or PowerPr Remanent polarizationPS Saturation polarizationQij ij-th component of electrostriction tensorr radiusr0 initial radiusS Mechanical strain or microwave measurement parametersij ij-th component of the elastic compliance tensort Time or thicknessT TemperatureTint Intrinsic stressText Extrinsic stressTtherm Thermal stressT0 Curie-Weiss temperatureTC Transition temperatureTk k-th component of the mechanical stress tensorv sound velocityV VoltageVosc Small signal voltageVC Coercive voltagezi Normalized acoustic impedance

VIII

Z0 Acoustic impedance

Greek Symbols

γ phase delayε0 Permittivity of free space (8.854·1012 As/Vm)ε, εr, εrev Permittivity, relative and reversible permittivityεtotal Total permittivityεintrinsic Intrinsic permittivityεextrinsic Extrinsic permittivityεij Components of the permittivity tensorνSi Poisson ratio of siliconρ Mass densityω Angular frequencyσ electric conductance

Abbreviations

AFM Atomic Force MicroscopyAlN Aluminum NitrideBAW Bulk Acoustic ResonatorBS Beam SplitterBST Barium Strontium TitanateBTO Barium TitanateBOX Buried oxideCH3COOH Acetic acidCHF3 TrifluoromethaneCF4 TetrafluoromethaneCMOS Complementary Metal Oxide SemiconductorCSD Chemical Solution DepositionCuSO4 · 5H2O CoppersulphatepentahydrateCVD Chemical Vapor DepositionECR Electro cyclotron resonance plasma sourceFeRAM Ferroelectric Random Access MemoryFEM Finite Element MethodHF Hydrofluoric AcidH2SO4 Sulfuric acidIR Infra redKOH Potassium HydroxideLPCVD Low Pressure Chemical Vapor DepositionL LensLNO Lanthanum Nickel OxideM MirrorMBE Molecular Beam EpitaxyMEMS Microelectromechanical Systems

IX

MOCVD Metal-Organic Chemical Vapor DepositionMOD Metal-Organic DecompostionMPB Morphotropic Phase BoundaryMW MEMS Microwave Microelectromechanical SystemsHNO3 Nitric acidPBS Polarizing Beam SplitterPLD Pulsed Laser DepositionPLHT Lanthanum doped Lead Hafnate TitanatePLZT Lanthanum doped Lead Zirconate TitanatePHT Lead Hafnate TitanatePZT Lead Zirconate TitanatePVD Physical Vapor DepositionRF MEMS Radio Frequency Microelectromechanical SystemsRTA Rapid Thermal AnnealingSEM Scanning Electron MicroscopeSF6 Sulfur hexafluorideSiO2 Silicon dioxideSi3N4 Silicon nitrideSTO Strontium TitanateTFBAR Thin Film Bulk Acoustic ResonatorTMAH Tetramethylammonium HydroxideUMTS Universal Mobile Telecommunication SystemWLAN Wireless Local Area NetworkXRD X-Ray Diffraction

X

1

1 Introduction

1.1 Motivation

The continued interest on integrated ferroelectric thin films is based on their manifold

properties, which are applied in memory devices, automotive applications, RF and

microwave components as well as sensors and actuators. This class of materials promises cost

effective, future devices with a new functionality, a higher performance and a lower energy

consumption.

Ferroelectric capacitors are promising candidates for non-volatile Ferroelectric Random

Access Memories (FeRAMs), where the direction of the spontaneous polarization is used to

store informations digitally [1–5]. The high dielectric coefficients over a wide temperature and

frequency range are used in integrated or surface mounted device capacitors. Furthermore, the

electric field tunable permittivity is applied for the realization of thin film varactors in RF and

microwave components for wireless communication [6] and the high pyroelectric coefficients

are utilized in infrared sensing applications [7].

Microsystems and especially microelectromechanical systems (MEMS) have become

synonymous for many types of microfabricated devices, whereby the lateral dimensions are

in the range of 10 µm - 10 mm and the devices are often limited to the functionality of

traditional materials like Si, SiO2, Si3N4, Pt, Al etc. [8]. The ultimate goal of microsystems

is a self-contained system of interrelated sensing and actuating devices together with signal

processing and control electronics on silicon substrates.

The superior piezoelectric properties of ferroelectric ceramics make them ideal materials for

the use in MEMS realizing new functions. In addition ferroelectric actuators do not dissipate

power in static mode and the excellent dielectric properties result in extremely low noise

during operation. In the past, a number of devices have been demonstrated on research level

e.g. micro motors [9–11], cantilever beam actuators [12], meanderline micropositioners [13],

micropumps [14], piezoelectric accelerometers [15], piezoelectric microphones for photo

acoustic gas detectors [16] or microwave acoustic resonators [17–21].

Especially the market potential of MEMS for microwave applications is tremendous and

estimated to exceed US$ 1 billion in 2007 [22]. Piezoelectric based microwave switches and

microwave resonators are expected to gain a part of this market.

2 1 Introduction

1.2 State of the Art

This thesis deals with the integration of ferroelectric thin films into microsystems and therefore

the following section gives an overview about the current status of a number of devices, which

are of interest for this work.

1.2.1 Piezoelectric Cantilever Structures on Silicon Substrates

Cantilevers are of primary interest for the integration of piezoelectric thin films into MEMS,

since they promise large deflections and low driving voltages. Hoffmann [23] reported the

fabrication of a piezoelectric cantilever with a maximum tip deflection of 70 µm at 12 V. A

very popular approach is the application of piezoelectric cantilevers in atomic force microscopy

(AFM) to enhance the resolution [24, 25].

1.2.2 Piezoelectric Cantilever Structures on Metal Substrates

The integration of functional materials into MEMS is concentrated on the world of CMOS using

silicon substrates. This is a limitation in the broad field of MEMS applications, since mostly

the silicon determines the mechanical properties and the applied micromachining technologies

limit the structure size. To benefit in the macro world from the functionality of those materi-

als, the covering of large, low cost substrates with functional thin films and mass production

technologies are desired. Recently, several reports have described attempts of depositing ferro-

electric thin films on metal foil substrates, including copper, aluminum, nickel alloys, titanium,

Hastelloy or stainless steel [26–30]. Li et al. [31] deposited 22 µm PZT films on stainless steel

and suggested the use for ultrasonic devices. However, films in this thickness range require

driving voltages of more than 100 V. To achieve a voltage reduction thinner films are needed.

1.2.3 Integrated Piezoelectric Driven Micro-Relays

It turned out that the assumption that switches based on semiconductors can fully replace slow

electromechanical relays is not correct. The galvanic separation of excitation and signal circuit

exhibits advantages regarding electric losses and crosstalk. Using miniaturized micro-relays

will enhance the switching time and reduce the dimensions down to the range of semiconductor

switches and therefore combine the advantages of both technologies. The research focus is

1.2 State of the Art 3

still on integrated switches with electrostatic actuation for microwave applications (see next

paragraph). Up to date only few publications deal with piezoelectric actuated switches [32].

1.2.4 Piezoelectric Driven Microwave Switches

Integrated high frequency switches working in the microwave range are of huge interest in or-

der to replace currently used expensive and space consuming off-chip components. Microwave

switches based on electromagnetic and electrostatic actuation have been demonstrated. Since

electromagnetic MEMS consume much more power than electrostatic switches, the research

activities are focused in the latter area [33].

The demonstrated electrostatic switches suffer from a high activation voltage, low displace-

ments and device failure due to stiction. Typical operation voltages are around 50 V and contact

distances are in the range between 1.5 - 3.5 µm [34–38]. In a recent work the operation voltage

could be reduced to 15 V by exhibiting a deflection of 14 µm [39]. Reported contact forces are

around 11 µN, if the electrostatic switch is driven with 58 V [40].

1.2.5 Thin Film Bulk Acoustic Resonators for High Frequency Applications

Bulk acoustic wave (BAW) resonators base on a planar capacitor structure of a piezoelectric ma-

terial clamped between two electrodes, which are acoustically isolated from the substrate [41].

By means of an alternating electric field the structure is excited to the fundamental thickness

mode resonance. If piezoelectric thin films are incorporated with this design the structure is

referred as thin film bulk acoustic resonator (TFBAR).

Sound velocities of piezoelectric materials are in the range from 4000 to 11000 m/s, so typical

resonance frequencies of 2 - 5.5 GHz are achieved with 1 µm thick films. This is the range of

modern applications like the universal mobile telecommunications system (UMTS) or wireless

local area network (WLAN) and therefore of great industrial interest.

During recent years a couple of devices using lead zirconate titanate (PZT) for the genera-

tion of acoustic waves are demonstrated [42–45], but for TFBAR applications PZT is not the

most encouraging candidate, since the use of platinum electrodes increases the high frequency

losses enormously [46]. Therefore, aluminum nitride (AlN) films are mostly applied in TF-

BARs [47–51].

4 1 Introduction

1.2.6 Dielectric Bolometers

Uncooled infrared detectors base on the thermal detection of incoming IR flux instead of the

quantum mechanism used in superconducting, high performance detectors. In recent years they

have attracted much attention since they are highly suitable for detecting a thermal point such

as a human body and eliminates the large and expensive cooling systems. There are different

types of infrared detectors: Resistive bolometers, dielectric bolometers and pyroelectric sensors.

Resistive bolometers are fabricated from amorphous silicon and detect the change of resistance,

if the silicon is heated [52–54]. The advantage of full compatibility with CMOS processing is

overruled by the disadvantage that a voltage drop across the detector resistance leads to current

consumption which is not negligible. It causes a temperature increase in the bolometer and

requires a thermal stabilizer.

Pyroelectric bolometers base on the pyroelectric effect, whereby a polar dielectric generates

charge when it is heated. In such pyroelectric IR detectors, the infrared energy is absorbed in

a thin film of the pyroelectric material and the temperature change gives rise to a pyroelectric

current, which is detected in an external circuit. Mostly PZT and calcium substituted lead

titanate thin films are in use due to a high pyroelectric coefficient in the range of ≈ 1µC/cm2K

[55–59]. However, one drawback of pyroelectric bolometers is the requirement of a chopper

technique, because only temperature changes are detectable.

Dielectric bolometers simply uses the permittivity dependence of ferroelectric materials on the

temperature. They are not necessarily reliant on fast temperature changes and since a reference

capacitor belongs to the device, no additional cooling or thermal stabilization is required.

1.3 Objectives 5

1.3 Objectives

The final goal of this thesis is the demonstration of microsystems using integrated, ferroelectric

thin films as functional materials. Therefore this work addresses three major tasks:

(i) Deposition and Characterization of Ferroelectric Thin Films

Lead zirconate titanate (PZT) is a well known ferroelectric material [60–62] and thin films are

deposited on silicon and metal substrates. The characterization addresses the electrical and

electromechanical properties with regard to reliability aspects in piezoelectric MEMS.

The deposition and the characterization of lead hafnate titanate (PHT) and lanthanum doped

lead hafnate titanate (PLHT) thin films follows the demand of hysteresis free material proper-

ties.

Barium strontium titanate (BST) thin films are investigated with regard to infrared imaging

sensors [63–65] and therefore especially the dependence of the dielectric permittivity on the

temperature is of interest.

(ii) Development of Micromachining Technologies

The integration process of the piezoelectric thin films by maintaining CMOS compatibility

is one important issue [66–69] and therefore technologies for the integration of ferroelectric

materials into silicon based microsystems are developed, including e.g. release processes using

silicon bulk or surface micromachining, plasma etching of ferroelectric thin films and the

fabrication of free standing copper contacts.

(iii) Processing of Microsystems

The successful integration of ferroelectric thin films is demonstrated by a number of

microsystems, which are designed, simulated, fabricated and characterized:

1. Piezoelectric Cantilever Structures on Silicon Substrates

As basic elements, piezoelectric cantilevers on silicon substrates are requested, which

exhibit a large deflection at low operation voltages in order to close contacts in a distance

of 5 µm or more. For fast operation, high resonance frequencies are desired and modeling

of the devices by finite element method (FEM) is required to confirm the measurement

results and for the calculation of the contact forces.

6 1 Introduction

2. Piezoelectric Cantilever Structures on Metal Substrates

To benefit from the outstanding properties of ferroelectric thin films in the macro world

the deposition of PZT thin films on metal foils using chemical solution deposition (CSD)

is desired. Thereby, the covering of large areas by maintaining the piezoelectric properties

and the fabrication of cantilevers by applying low cost fine mechanical techniques is

requested to demonstrate the possibility of mass production.

3. Integrated Piezoelectric Driven Micro-Relays

Integrated piezoelectric micro-relays based on cantilever structures are one competitive

concept to electrostatic switches. For the device fabrication complex micromachining

technologies, which offers a high integration density need to be applied. The addition of

free standing metal contacts is one further task. The demands for the micro-relays are

the separation of the actuation and the signal circuit, low driving voltages, fast operation,

a completely integrated design, improved mechanical stability compared to single side

suspended cantilevers and fatigue free operation.

4. Piezoelectric Driven Microwave Switches

By the integration of piezoelectric micro-relays into a microwave circuit design one

achieves a microwave switch, which overcomes the drawbacks of conventional electro-

static switches. For good switching characteristics of more than 15 dB between "on"

and "off" state low loss microwave signal lines are required and the device needs to be

designed to work stable over a large frequency range, at least up to 20 GHz.

5. Thin Film Bulk Acoustic Resonators for High Frequency Applications

A new approach within this thesis is the exploitation of the electrostrictive behavior of

BST for the generation of acoustic waves within the thin film, whereby the device should

work in the microwave range. By the use of a membrane technology a good acoustically

insulation can be achieved so that the TFBARs show a pronounced resonance at certain

frequencies. The device design needs to be supported by theoretical considerations.

6. Dielectric Bolometers

To demonstrate infrared sensors, taking advantage of the dielectric working principle the

integration of high quality BST thin films into a bolometer is requested. The demands are

fast operation and a high sensibility.

7

2 Ferroelectric Materials

This chapter gives an overview over the relevant properties of ferroelectric materials. It

deals with crystallographic considerations and general aspects of spontaneous polarization

and piezoelectric properties. For more details the reader is referred to general textbooks or

review articles [70–73].

Ferroeletricity describes the presence of a spontaneous electric moment in a crystal, which

can be switched in its orientation between two or more distinct crystallographic directions

by applying an external electric field. The term was coined to indicate the close analogy to

ferromagnetism [74]. Together with the apparence of spontaneous polarization ferroelectric

materials also exhibit high dielectric coefficients and the piezoelectric effect.

2.1 Spontaneous Polarization

The ability of a crystal to exhibit spontaneous polarization is related to its symmetry. Among

the 32 different crystal classes, which are all electrostrictive, 11 groups belong to the cen-

trosymmetric classes and contain an inversion center. The remaining 21 groups belong to the

non-centrosymmetric classes and 20 of them exhibit the piezoelectric effect. Subjecting those

crystals to mechanical stress causes charge separation and therefore an electric field. The piezo-

electric effect is reversible. Applying an electric field to the crystal induces stress and deforma-

tion following Hook’s Law. The piezoelectric properties of ferroelectrics are essential for this

work and therefore the piezoelectric effect is explained more detailed in section 2.2.

Ten of those piezoelectric classes are polar with an unique polar axis. Such crystals may display

spontaneous polarization parallel to the polar axis.

Figure 2.1 (a) shows a perovskite crystal lattice. A central B-ion is positively charged and

surrounded by six negatively charged O2−-ions. The corners of the unit cell are occupied by

eight positively charged A-ions. The crystal is of cubic shape and therefore symmetric, ex-

hibiting no spontaneous polarization in this state. The material becomes ferroelectric, if the

crystal lattice loses the symmetry, e.g. by transforming from the cubic into the tetragonal phase

while cooling the material below the phase transition temperature TC (Figure 2.1 (b)). In this

non-centrosymmetric form the B-cation has two stable states. Hence, the material shows spon-

taneous polarization which can be switched by an external electric field.

The analogy of ferroelectrics to ferromagnetics is not only due to the exhibition of spontaneous

8 2 Ferroelectric Materials

(a) (b)

A+

B+O-

FIGURE 2.1:(a) Perovskite crystal in cubic phase. (b) Perovskite crystal in tetragonal phase.

polarization rather its reorientability by an external electric field is essential. Figure 2.2 (a)

displays the polarization characteristic of a single domain crystal whereby the polarization is a

superposition of dielectric ionic and electronic polarization (non-ferroelectric) and spontaneous

polarization (ferroelectric). The spontaneous polarization is reoriented, if an electric field E

opposite to the polarization, exceeds the coercive field EC leading to a jump in the P − E

curve. The non-ferroelectric part leads to the linear increasing polarization for E > +EC and

E < −EC respectively.

Polydomain crystals possess a statistical distribution of domains. If an electric field is applied,

the domains start to orientate in one direction (Figure 2.2 (b), line AB). The saturation pola-

P

E-E +Ec c

s

s

+P

-P

P

EA

BC

-E +Ec

s

c

P

(a) (b)

Pr

FIGURE 2.2:(a) Typical hysteresis loop of a single crystal ferroelectric material. (b) P−E characteristicfor a multidomain ferroelectric material [70].

2.1 Spontaneous Polarization 9

rization PS is defined by extrapolating the line BC to E=0. If the electric field is reduced to

zero, parts of the domains switch back in opposite direction due to energy constraints, but the

major part remains in the direction and results in a remanent polarization Pr. By increasing the

electric field into the opposite direction the domains switch continously until an equal distribu-

tion is obtained and the polarization is reduced to zero at the coercive field EC and by further

increasing of the electric field the negative saturation is reached. A complete cycle results in the

typical hysteresis loop. Actually, the y-Axis shows the dielectric displacement D:

D = ε0E + P (2.1)

where ε0 denotes the permittivity of free space. For ferroelectric materials the vacuum term only

contributes a small amount to the dielectric displacement and can be neglected, thus: D ≈ P .

Hysteresis curves reflect the large signal behavior of ferroelectrics where an electric field is

applied to the material and the current or electric charge respectively is monitored. If sufficiently

small electric fields are applied to dielectrics they follow a linear relation D = εE. This

dielectric small signal response is caused by reversible contributions of the electronic and ionic

polarization (intrinsic) [75, 76]. In polycrystalline ferroelectrics this effect is superimposed by

extrinsic mechanisms like movement of domain walls and the alignment of defects.

εtotal = εintrinsic + εextrinsic (2.2)

It has been shown that especially the electromechanical active non-180° domain walls, i. e. 90°

domain walls in tetragonal structures, 71° and 109° domain walls in rhombohedral structures

are responsible for a considerable contribution to the dielectric coefficient as well as to the

piezoelectric and the elastic coefficients.

E

permittivity

FIGURE 2.3:Typical small signal measurementof a polydomain ferroelectric mate-rial.


Figure 2.3 shows a typical small signal measurement of a polydomain ferroelectric material.

Thereby, a large signal (bias voltage) is supperimposed by the small signal. The curve shows

two maxima around EC and reduced permittivity at high electric fields.

2.2 Piezoelectric Effect and Electrostriction

All ferroelectric and therefore polar materials belong to the subgroup of piezoelectric crystals

which can be polarized by applying mechanical stress. This is called the direct piezoelectric

effect and can be expressed by

Di = dikTk + εXij Ej (2.3)

where Tk represents the stress and dik the piezoelectric coefficient [77]. Due to symmetric

constrains 6 of the 9 components of the stress and strain tensors are independent from each

other and it is customary to use a reduced notation [74]. The equation includes two parts. The

first one combines mechanical forces to electrical output and the second one represents the

dielectric behavior due to an applied electric field. Since rarely an electric field is applied, if

using the direct piezoelectric effect, equation 2.3 reduces to

Di = dikTk. (2.4)

Furthermore, the piezoelectric effect is a reversible effect. Applying an electric field to the

material results in an elastic deformation and is called the converse piezoelectric effect, which

can be expressed by

Sk = sEklTl + dikEi (2.5)

where sEkl is the tensor of the elastic compliance and Sk is the induced strain. Similar to the

reduction of the equation for the direct piezoelectric effect the mechanical part in equation 2.5

can be neglected and the equation reduces to

Sk = dikEi. (2.6)

The typical piezoelectric behavior of ferroelectrics is presented in Figure 2.4. One has to distin-

guish between large and small signal measurements. Figure 2.4 (a) shows the mechanical strain,

if an electric field is applied to a polydomain, ferroelectric material. An applied electric field

causes the orientation of the domains in field direction, thereby those extrinsic contributions

are superimposed by intrinsic contributions due to the shift of oppositely charged sublattices

in the crystal. At some point all domains are oriented in the direction of the electric field and

2.2 Piezoelectric Effect and Electrostriction 11

(a)

E

S

(b)d

33

E-EC

+EC

FIGURE 2.4:Typical piezoelectric behavior of a ploydomain ferroelectric thin film: (a) Strain (S) vs.electric field (E) and (b) piezoelectric coefficient (d33) vs. electric field (E).

therefore only the intrinsic part contributes to the strain and the slope of the curve is reduced

for E > EC . The elongation of the piezoelectric material reduces to zero, if the electric field is

removed. Switching of the direction of the applied field leads to contraction until all domains

are uniformly distributed before the domains start to switch in opposite direction causing an

increasing strain. This so called butterfly measurements (due to the shape of the measurement

curve) refer to the large signal measurements.

The small signal behavior is investigated by applying certain bias electric fields superimposed

by a small electric field. The small electric field activates the intrinsic contributions and there-

fore the linear response is measured. Figure 2.4 (b) presents the typical piezoelectric small

signal behavior of ferroelectrics indicating the piezoelectric coefficient d33 if the electric field

and the generated strain are in parallel direction. In the high electric field regime (E >> EC)

the piezoelectric coefficient shows a negative slope because all domains are aligned parallel to

the electric field and only the intrinsic effects contribute to the piezoelectric coefficient. This

effect is more and more reduced if the bias field increases and therefore the crystal is strained.

Around the coercive field also domain switching contributes to the piezoelectric coefficient,

hence the d33 decreases to zero if the domains are uniformly distributed at EC .

Electrostriction is a second electromechanical effect and occurs in all dielectric materials as a

result of the reorientation of the molecules, if an electric field is applied. Thereby, the reversal of

the electric field does not reverse the direction of the deformation. Consequently electrostriction

is a quadratic effect

Sk = QijP2j (2.7)

where Qij is the coefficient of electrostriction. Usually electrostriction is a small effect and in


E

S

FIGURE 2.5:Typical electrostrictive behavior of adielectric material.

piezoelectric materials covered by the piezoelectric effect. Figure 2.5 shows the typical strain

curve dependend on the applied electric field for purly electrostrictive materials.

2.3 Lead Zirconate Titanate (PZT)

PbZrxTi(1−x)O3 crystallizes in the perovskite structure shown in Figure 2.1. Thereby, the

Pb-ions occupy the A site of the primitive cell and the Ti- or Zr-ions occupy the B site. The

oxygen is placed in the center of the faces of the cell (see Table 2.1).

From the phase diagram in Figure 2.6 it can be seen that the crystal structure does not depend

only on temperature, but also on the ratio between Ti and Zr. For high Ti contents PZT crys-

tallizes in a tetragonal structure, while Zr-rich compositions have a rhombohedral shape. In

the high temperature regime the cell is always cubic. The transition from cubic to tetragonal

or rhombohedral structure appears at the phase transition temperature TC and depends on the

Zr/Ti ratio. Below TC , the morphotropic phase boundary (MBP) is observed, at a Zr content

of around x = 0.53. The shape of the lattice cell changes spontaneous from rhombohedral to

tetragonal and the other way around or in other words the tetragonal and the rhombohedral

phase coexists. Tetragonal PZT has six equivalent directions of the polar axis and in polydo-

main crystals it has 90° and 180° domain walls. In rhombohedral PZT the polarization distorts

along the body diagonal of the cubic prototype phase and has therefore eight possible domain

TABLE 2.1: Occupation of the perovskite unit cell of lead zirconate titanate (PZT).

A-ion B-ion O-ionPb2+ Ti4+ or Zr4+ O2−

2.4 Lanthanum doped Lead Hafnate Titanate (PLHT) 13

500

400

Cubic

cT

300

200

100

Tetragonal

Rhombohedral(high-tempertureform)

Rhombohedral(low-teperatureform)

Morphotrop cphaseboundary

i

Orthorhombic

Tem

per

atu

re/˚

C

1 0.8 0.6 0.4 0.2 0

x 3PbTiO3PbZrO

FIGURE 2.6:Phase diagram of PZT as a functionof Zr content [71].

states. Furthermore, not only 90° and 180° domain walls exists, but also 71° and 109° domain

walls have been observed. Since both phases coexist an external electric field can also switch

between the tetragonal and the rhombohedral domain states resulting in 14 possible directions.

Thus, the spontaneous polarization can be aligned more accurately in the field direction and

therefore PZT at the MPB exhibits high dielectric permittivity, high remanent polarization and

high piezoelectric coefficients [78–80].

2.4 Lanthanum doped Lead Hafnate Titanate (PLHT)

An alternative material to PZT also suggested for electromechanical actuation is lead hafnate

titanate PbHfxTi(1−x)O3 (PHT) [83, 84], which is composed by the substitution of the Zr4+

cations with Hf4+ cations on the B-site due to a similar ion radius. Although the material

exhibits strong ferroelectric properties it is less studied than PZT. Heremans et al. [85] reported

on the piezoelectric properties and the phase diagram for various PHT ceramics and Schorn

et al. [86] reported on polarization and fatigue properties compared to PZT.

By doping PHT with lanthanum the La3+ ions occupy the A-sites, since their ion radius (1.15

A) is similar to those of the Pb2+ (1.2 A) (Table 2.2) [87].

TABLE 2.2: Occupation of the perovskite unit cell of lanthanum doped lead hafnate titanate(PLHT).

A-ion B-ion O-ionPb2+ or La3+ Ti4+ or Hf4+ O2−


TABLE 2.3: Occupation of the perovskite unit cell of barium strontium titanate (BST).

A-ion B-ion O-ionBa2+ or Sr2+ Ti4+ O2−

2.5 Barium Strontium Titanate (BST)

BST is a combination of two perovskite materials barium titanate (BTO) and strontium titanate

(STO). The A-site of the ABO3 lattice is shared by Ba2+ and Sr2+ ions, whereas the B-site is

occupied by the Ti4+-ions (Table 2.3). Since BST crystallizes in the ABO3 lattice, it is also a

ferroelectric material, but unlike PZT the phase transition temperature can be much lower and

depends strongly on the ratio between the BTO and STO. Figure 2.7 shows the lattice constant

of pure BTO dependent on the temperature. The transition from the cubic (paraelectric) state

to the tetragonal (ferroelectric) phase occurs at 396 K. On the other hand, pure STO has a

cubic shape at room temperature, because the phase transition from cubic to tetragonal occurs

at 105 K [88, 89].

When cooling down from temperatures well above the critical temperature TC , the center ion

of the cubic unit cell is increasingly mobile to a shift along an applied electric field, i. e. the

polarization increases. The dielectric permittivity εr increases with decreasing temperature

according to:

εr =CCurie

T − T0(2.8)

Temperature [K]

BaTiO3-phase: rhombohedral orthorhombic tetragonal cubic

Latti

ce c

onst

ant [

Å]

FIGURE 2.7:Lattice constant of BaTiO3 as a function of the temperature [90].

2.5 Barium Strontium Titanate (BST) 15

FIGURE 2.8:Susceptibility ofBa1−xSrxTiO3 ceramic asfunction of composition andtemperature [91].

known as the Curie-Weiss-Law. CCurie is the Curie-Weiss constant and T0 is the Curie-Weiss

temperature, which is slightly lower than the critical or phase transition temperature TC .

In the mixed material system Ba1−xSrxTiO3 the Curie-Weiss constant as well as the Curie-

Weiss temperature are dependend on the mixing ratio between BTO and STO. Figure 2.8 shows

the susceptibility of BST for different compositions dependend on the temperature. At room

temperature the maximum susceptibility is observed for x=0.3, which makes this composition

specifically interesting for the use as high permittivity material.


17

3 Thin Film Deposition

The deposition of thin film functional layers on different substrates is an important step in

micro- and nanoelectronics. The next chapter gives an general overview over different

deposition methods with focus on the deposition from the liquid phase for ferroelectric thin

films and presents the microstructural analysis of the deposited thin films.

To fulfill the demands of a broad spectrum of applications currently a couple of different

deposition methods are under investigation. These are dominated by the deposition from the

vapor phase, but there are also techniques starting with a liquid phase. Special attention is

paid to the covering of three dimensional structures like surface steps or holes in the substrate.

Furthermore, the scalability to full wafer size is very important for the implementation of

ferroelectric thin films into the field of integrated circuits. Figure 3.1 summarizes the most

important deposition techniques.

With Physical Vapor Deposition (PVD) a target of the desired material composition is placed

into a vacuum chamber and becomes pulverized so that a gas is formed, containing molecules,

Physical Vapor Deposition Chemical

Vapor Deposition

Chemical Solution

Deposition Evaporation /

MBE Sputtering PLD CVD / MOCVD CSD

Mechanism of production of depositing

species

Thermal energy

Momentum transfer

Thermal energy

Chemical reaction

Chemical reaction

Deposition rate

High, up to 7500 nm/min Low Moderate

Moderate, up to 250

nm/min

Deposition species

Atoms and ions

Atoms and ions

Atoms, ions and clusters

Precursor molecules dissociate into atoms

Precursor molecules dissociate into atoms

Covering of complex shaped objects

Poor Nonuniform thickness Poor Good Moderate

Scalable to wafer size Up to large Up to large Limited Up to large Up to large

Low

FIGURE 3.1:Some characteristics of deposition methods for ferroelectric thin films (modified after [92])

18 3 Thin Film Deposition

ions and free electrons. The particles than diffuses to the substrate, where they are deposited.

Thereby the microstructure and the crystallinity of the film is controlled from amorphous to

epitaxial or from coarse- to fine grained films depending on numerous process conditions like

pressure, reactor atmosphere, substrate temperature etc. Evaporation is achieved by transferring

excessive heat to the target by conventional heating, by a focused electron beam or by a high

energy laser (pulsed laser deposition; PLD). Sputtering by highly energized ions (mostly argon

or oxygen) is another way to atomize the target surface. Sputtering and PLD are commonly

used methods for successful deposition of various perovskites like BST [93, 94], STO [95, 96]

or PZT [97–99].

The methods mentioned before need a target of the desired material composition, whereas

molecular beam epitaxy (MBE) uses multiple targets. Each of them contains one pure element.

By evaporating two or more materials at the same time and controlling with individual shutters,

the deposition is very flexible in composition and sequence of materials.

However, PVD techniques are limited in the structure size that can be successfully deposited

and due to the direct line between the target and the substrate, a conformal coverage of three

dimensional structured substrate surfaces can not be achieved. For smaller structure sizes

(i.e. high density applications) is the chemical vapor deposition (CVD) preferable. For the

deposition of materials with metallic elements such as PZT and BST a number of special

precursors in the form of organometallic compounds are developed and a special method the

so called metal-organic chemical vapor deposition (MOCVD) has therefore evolved. The

precursors are in separated bubblers at elevated temperatures (≈ 100°C) and pressure of a few

mbar. These conditions allow a vapor-phase mixing and the transport to the heated substrate

in the reaction chamber by a carrier gas where the precursor dissolves and the atoms form a

crystalline thin film.

3.1 Chemical Solution Deposition

The methods described above come along with enormous technical efforts and mostly very poor

flexibility in material composition. A low cost method, overcoming those disadvantages is the

chemical solution deposition (CSD). Here, the substrate is coated by a liquid precursor using

spin-coating or dip-coating. The commonly used CSD methods are sol/gel and metal organic

decomposition (MOD) with a wide spectrum of hybrid methods in between.

CSD based deposition methods are basically executed in the following manner:

3.2 Microstructural Analysis 19

1. Preparation of precursor solutions

2. Coating of the substrate

3. Heat treatment to pyrolyze the wet film

4. Final crystallization annealing

The preparation of the precursor solution is a chemical process whereby the raw material

such like lead-acetate, zirconium-n-propoxide or titanium-i-propoxide are mixed, dissolved by

addition of a solvent and then temperature treated in an appropriate way. The film is formed

by dropping a certain amount of precursor solution on the substrate, which is then rotated to

distribute the liquid homogeneous over the wafer using a conventional spin coater. Thereby

the film thickness depends on the viscosity of the precursor and the rotation speed. In the

third step the wet film is dried and pyrolyzed by heating at moderate temperatures, e.g. by

placing the substrate on a hot plate. To deposit thick films without changing the viscosity of the

precursor solution the spinning and pyrolysis step are repeated several times. Finally, the film

is crystallized at elevated temperatures (typically between 600 °C and 800 °C). Sol gel thin

films have been successfully demonstrated with PZT and other perovskite materials [100–102].

One of the main advantages of chemical solution deposition is the very flexible choice

of material compositions since the preparation of the precursor solution is fast and each desired

material composition is possible by mixing the starting components with the desired ratio.

Furthermore, no special and cost expensive technical effort is necessary. Standard CMOS

equipment like spin coaters and diffusion furnaces are sufficient to cover large amounts of

wafers.

3.2 Microstructural Analysis

To investigate the crystallographic texture and the phase purity of the deposited films, X-ray

diffraction (XRD) using the diffractometer X’PERT (Phillips) with a CuKα cathode in Bragg-

Brentano geometry (θ − 2θ) has been accomplished [103]. In this configuration the counter

records lattice planes parallel to the film surface. Comparing the results with those of powder

diffractometer databases 1 gives insight into the degree of preferential orientation of the film.

However, the lattice constants of ceramics differ slightly from those of thin films, hence the

1e.g. database of International Center of Diffraction Data (ICDD)


XRD peaks appear not exactly at the same angle [104, 105].

The film thickness and morphology are characterized by Scanning Electron Microscopy (SEM).

All pictures are made with a Gemini 982 (Zeiss) with field emission cathode.

For the investigation of the allocation of the included materials in a thin film a quadrupole mass

spectrometer (Hiden EQP 500) together with a reactive ion beam etching system (RIBE, Oxford

Ionfab 300 plus) is used. The thin film is etched physically with an inert argon beam and at the

same time the mass spectrometer analyzes the released particles of the film. Hence, in case of

PZT one can distinguish between titanium rich and zirconium rich phases in the film.

3.2.1 Lead Zirconate Titanate (PZT)

It is known that the properties of thin films diverge from those of bulk PZT. Therefore an in-

tensive investigation of the piezoelectric thin film properties depending on composition, film

thickness etc. has been conducted in recent years. It is reported that PZT thin films close to the

MPB with a ratio of 45 % zirconium and 55 % titanium and (111) orientation show the highest

piezoelectric coefficient d33 compared to other Zr/Ti ratios [23, 106, 107]. Therefore, further

investigations are mainly dealing with this composition.

PZT thin films with various compositions and different thicknesses are prepared by chemical

solution deposition on platinum coated silicon substrates. The films are prepared on a (100)

oriented silicon wafer (1 inch x 1 inch), which is wet oxidized to 450 nm thickness. On

top is a 100 nm thin platinum electrode ((111) orientation) with a 10-20 nm thin titanium

dioxide adhesion layer. The starting compounds Lead(II)-Acetate, Ti(IV)-isopropoxide and

Zr(IV)-propoxide are mixed with regard to the desired Zr/Ti-ratio dissolved in ethylenglycol-

monobutylether (EGMBE) and then treated in an appropriate way to get a solution, which is

spined onto the substrate. The exact route is shown in the flow chart (Figure 3.2). A pyrolysis

step is performed after each coating whereas the film is crystallized after three coatings. In

a last step 100 nm thick platinum electrodes are sputtered on top. Figure 3.3 (a) displays the

cross view of a PZT(45/55) layer, where the wafer is covered six times with the stock-solution

and two times crystallized at 700 °C. The film grows in columnar grains with an average grain

size of around 150 nm on the platinum electrode. The thickness is 223 nm. If the sample

is coated twelve times the resulting film is still dense and of columnar structure with around

twice the thickness (Figure 3.3 (b)). After up to 30 coatings the film is still columnar and

a thickness of 1325 nm is reached, but there are some pores included at the grain bound-

aries (Figure 3.3 (c)). Nevertheless, if the dilution of the stock-solution is reduced one can


Lead(II)-AcetateZirconium(IV)-n-propoxideTitanium(IV)-i-propoxide

Reflux for 2,5h at 130°Cin Ar-Atmosphere

Dissolving in EGMBE

Vacuum-Distillation[Removing of by-products)]

Hydrolysis I (EGMBE + HNO3)Hydrolysis II (EGMBE + H2O)

Dissolving the Gel in EGMBE

Stock-Solution (c = 1M)

Addition of EGMBE[to adjust the final concentration]

Dilution of the Stock-Solution &Filtration with 0,2µm PTFE-Filter

Spin-coating at 3000 rpmonto (Si/SiO2/TiO2/Pt)-Substrate

Drying and Pyrolysis on200°C & 400 °C Hotplates

for 2 min

Crystallization in O2at 700°C for 5 min

Multiple coating

Pb(Zrx,Ti1-x)O3

FIGURE 3.2:Flow chart for the deposi-tion of PZT thin films.

reach films with more than 2 µm. The PZT film in Figure 3.3 (d) is produced with three parts

stock-solution and one part EGMBE resulting in the same grain size and a thickness of 2400 nm.

The crystal structure of the films is characterized by XRD. Some exemplary results for

PZT(45/55) films with different thicknesses are given in Figure 3.4. The measurements are

slightly shifted vertically to give a clearer view. The dominant peak a 40° indicates the Pt

electrode and shows (111) orientation. For this reason the PZT grows also in (111) orientation

which is indicated by the peak nearby. A smaller signal is detected at around 22° signifying

an contingent of (100) oriented cells in the PZT. Other peaks belong to the silicon substrate.

Note, with increasing film thickness the height of the peaks also increases, especially visible in

the PZT (111) signal, because a thicker film gives a better signal. However, the ratio between

(111) and (100) orientation stays identical. Therefore the measurements show that PZT films

deposited with the CSD method maintain the orientation at least for films up to 700 nm and are

mainly (111) oriented.


(a)

PZT

PtTiO2

SiO2

(b)

(c) (d)

FIGURE 3.3:SEM pictures of different PZT(45/55) thin films with various thicknesses from (a) 223 nm,(b) 481 nm, (c) 1325 nm, (d) 2400 nm. (a) to (c) are deposited with a mix ratio of 1:1(stock solution : EGMBE), whereas (d) is deposited with three parts stock-solution : onepart EGMBE.

SEM and XRD experiments give an insight of the texture and reveal a steady growth of the film.

No interruptions are visible for subsequent coatings or annealing steps. Apparently, each PZT

layer nucleates nearly "epitaxially" in each grain on the previous layer. However, characteriza-

tion using a mass spectrometer during dry etching reveals a compositional gradient. Figure 3.5

shows the counts of the mass spectrometer during the dry etching from top to bottom electrode

of a 600 nm film with 15 coatings PZT(45/55) and annealing always after three coatings. The

Zr content decreases continuously from the surface and increases rapidly after three coatings,


20 25 30 35 40 45 50

PZT(101)

Pt(111)K

β

708 nm604 nm481 nm363 nm255 nm121 nm

PZT(200)

Pt(111)

PZT(111)

Si(200)

PZT(100)

Inte

nsity

[a.u

.]

θ−2θ [deg.]

PZT 45/55

FIGURE 3.4:XRD measurement of PZT45/55 thin film with differ-ent thicknesses

which is the point where the crystallization takes place (e.g. 4. RTA). On the contrary, the

Ti content exhibit an increasing concentration the surface to deeper layers and exhibits sharp

drops at the points, where the film is treated in the RTA for crystallization. This behavior repeats

for each annealing step. The composition gradient reflects the fact that for Ti rich compounds

the activation energy for nucleation is smaller and the free energy of formation is more neg-

ative [108, 109]. Considering the whole film, an overall decrease of the Ti content is found.

Hence, the composition of PZT films vary within the layer.

3.2.2 Lanthanum doped Lead Hafnate Titanate (PLHT)

The precursor preparation for PLHT is based on the preparation of the PZT precursor. The

raw material zirconium(IV)-n-propoxide is replaced by hafnium(IV)-n-butoxide, but an addi-

5. R

TA

4. R

TA

3. R

TA

2. R

TA

Cou

nts

[a.u

.]

Ti

Zr

growth direction

surfa

ce

1. R

TA

botto

m e

lect

rode

FIGURE 3.5:Zr/Ti distribution within a600 nm thin PZT(45/55) film.15 coatings are deposited andcrystallized with five annealingsteps.


tional stabilization with acetylacetone is needed. To dope the pure PHT an adequate amount

of lanthanum(III)-acetylacetonate is added in the beginning (Figure 3.6). Thin films with

PLHT(x/30/70) and PLHT(x/45/55) basic composition and a doping amount of La x = 2.5 %,

5 %, 7.5 % and 10 % are deposited and characterized by SEM and XRD.

Figure 3.7 displays PLHT(x/45/55) thin films with various La amount. All films consist of six

coatings with two crystallization steps resulting in a film thickness of 250 nm. The picture in

Figure 3.7 (a) on the left side shows the cross section of a pure PHT(45/55) film. Like PZT

the film is dense and of columnar grain growth. A look to the right side of the picture denotes

the top view and reveals that the grains are of irregular shape with a grain size from 200 nm to

500 nm. Figure 3.7 (b) and (c) show PLHT(2.5/45/55) and PLHT(7.5/45/55) films, respectively,

which are also dense and exhibit irregular columnar grains, but of smaller grain size. (in the

range between 30 nm and 300 nm). Films with 10 % La show slightly different growth prop-

erties (Figure 3.7 (d)). From the cross view it looks like the grains are not of columnar growth

Lead(II)-AcetateHafnium (IV)-n-butoxideTitanium(IV)-i-propoxide

(Lanthanum(III)-acetylacetonate)

Reflux for 3h at 130°Cin Ar-Atmosphere

Vacuum-Distillation[Removing of by-products)]

Stabilization withAcetylacetone

Dissolving the Gel in 1-BuOH

Stock-Solution (c = 1M)

Addition of 1-BuOH[to adjust the final concentration]


Dilution of the Stock-Solution &Filtration with 0,2µm PTFE-Filter


Pyrolysis on 350 °C Hotplatefor 2 min

Multiple coating

Pb(Hfx,Ti1-x)O3((Pb,La)(Hfx,Ti1-x)O3)

FIGURE 3.6:Flow chart for the deposi-tion of La doped PHT thinfilms.


(a) (b)

(c) (d)

FIGURE 3.7:SEM Pictures of PLHT(x/45/55) films with various La doping concentrations (a) pure PHT(b) 2.5 % La (c) 7.5 % La (d) 10 % La (left side: cross view; right side: top view).

anymore, instead the film exhibit polycrystalline character. From the top view it can be seen

that the grains are even more irregular, but the film is still dense.

XRD measurements of a pure PHT (45/55) film on platinum reveal a (111) and also a strong

(101) orientation, but with increasing La concentration the (101) peak decreases and the (100)

peak as well as the (111) orientation increases (Figure 3.8 (a)). Thereby the ratio between (111)

and (100) increases steady so that the (111) orientation becomes dominant with increasing La

doping. This trend is intercepted for PLHT(10/45/55) films. Here, the (100) orientation in-

creases dramatically and the (111) peak decreases to the same level so that both orientations

are coexistent in this film. For comparison also PLHT(30/70) films are characterized (Fig-

ure 3.8 (b)). They show a similar behaviour and are mainly (111) oriented apart from the film


(a)

20 25 30 35 40 45 50

PHT(101)

Si(200) Pt

(111)Kβ

Pt(111)

PHT(111)

PHT(200)

PHT(100)

0 % La

2.5 % La

5 % La

7.5 % La

10 % La

PLHT(x/45/55)

Inte

nsity

[a.u

.]

θ - 2θ [deg.]

(b)

20 25 30 35 40 45 50

PHT(200)

Pt(111)PHT

(111)

Si(200)

PHT(101)

PHT(100)

Pt(111)K

β

0 % La

2.5 % La

5 % La

PLHT(x/30/70)

Inte

nsity

[a.u

]

θ - 2θ [deg.]

FIGURE 3.8:XRD measurements of (a) PLHT(45/55) and (b) PLHT(30/70) with various La doping.

with 10 % La, where also the amount of (100) and (111) oriented cells is approximately the

same.


3.2.3 Barium Strontium Titanate (BST)

The preparation of the BST precursor solution starts with a reflux step of Ba- and Sr-carbonate

followed by the addition of stabilized titanium(IV)-butoxide (Figure 3.9). Spin-coating and

crystallization for 10 minutes at 700 °C in an oxygen atmosphere results in a thickness of

9 nm per coating. A minimum of three coatings is necessary to deposit dense films [110].

By repeating this procedure for 14 times a film thickness of 120 nm is reached. Figure 3.10

shows two pictures of BST(30/70) and BST(55/45) on platinum electrodes. Both exhibit dense,

columnar grains with an average grain size of 50 nm.

The XRD measurement reveals a dominant (111) orientation for the BST(70/30) film and only

a small amount of (100) cells (Figure 3.11), whereas the BST(55/45) film exhibits almost no

(111) orientation. The film is preferential (100) and (110) oriented.

Addition of 1-BuOH [to adjust the final concentration]

Stock-Solution (c = 0,1 M)



(Bax,Sr1-x)TiO3

Multiple Coating

Ba-CarbonateSr-Carbonate

Reflux for 10hat 140 °C

Titanium(IV)-butoxide

Stabilizing with Acetylacetone &1-BuOH

Addition of Propionic acid &Propionic anhydride

FIGURE 3.9:Flow chart forthe deposition ofBST thin films.


(a) (b)

FIGURE 3.10:SEM pictures of 120 nm thin BST films: (a) BST(70/30) (b) BST(55/45).

20 25 30 35 40 45 50

BST(70/30)

BST(55/45)

Inte

nsity

[a.u

.]

θ - 2θ

BST(110)BST

(100)BST(200)

Pt (111)

BST(111)

Pt(111)Kb

FIGURE 3.11:XRD measurementof BST(70/30) andBST(55/45).

29

4 Material Characterization with Respect to MEMS Devices

This chapter presents the electrical and electromechanical characterization of ferroelectric

thin films prepared on silicon substrates. (Thin films deposited on metal foils are charac-

terized in chapter 7.5.) A detailed overview is given over the measurement methods, followed

by the characterization of the materials regarding their application in piezoelectric MEMS or

dielectric working devices.

4.1 Electrical Characterization Methods

The samples are contacted in a probe station (Cascade Summit 9600), which is equipped with a

temperature control from -50 °C to 200 °C and BNC cables providing access to the used mea-

surement equipment e.g. LCR bridge (HP 4194A). The polarization properties of the films are

characterized with the aixACCT TF analyzer 2000 operating in virtual ground mode [111].

For the integration of ferroelectric materials one major concern is the reliability. Fatigue, which

means a loss of switchable polarization with increasing numbers of switching cycles, is still

a drawback for the successful incorporation of ferroelectric thin films (especially in FeRAM

technology). Since the polarization is directly related to the piezoelectric properties, also the

integration into piezoelectric devices suffers from these failure mechanism and is therefore of

special interest. To characterize the sample, it is exposed to a signal sequence of bipolar pulses

whereat complete switching is essential [112]. This fatigue treatment is interrupted at regular

points for hysteresis measurements.

Large signal (polarization) measurements and small signal (capacitance) measurements are per-

formed, whereby in the latter case the material behaves like a linear, time invariant, parallel

plate capacitor. So, the permittivity can be calculated by

C = ε0εrA

d(4.1)

where ε0 denotes the permittivity of free space, εr the permittivity of the ferroelectric material

and A and d the capacitor area and thickness. The permittivity of ferroelectrics depends strongly

on the polarization and therefore the small signal is superimposed with a larger bias signal. To

measure a full C−V curve the bias voltage is changed stepwise and a capacitance measurement

is performed at each step.

30 4 Material Characterization with Respect to MEMS Devices

4.2 Electromechanical Characterization Methods

For the characterization of the electromechanical properties several techniques are established.

One of them, often applied to bulk ceramics, but also to piezoelectric thin films, analyzes the

impedance spectrum that arises if the material is excited by an electric field with increasing

frequency [113]. Another popular method uses the direct piezoelectric effect. Thereby a stress

is applied to the sample and the generated charge is measured and integrated to calculate the

piezoelectric coefficient [114–119]. A third method uses the converse piezoelectric effect by

means of optical, interferometric principles [120–123]. Thereby, the piezoelectric film is excited

by applying an electric field and generating a displacement due to equation 2.6.

For the measurements a double beam laser interferometer with resolution of 0.2 pm is

used Figure 4.1. The laser beam of a He Ne-laser (wavelength 632.8 nm) is rotated with a

lambda/2-plate and then divided by the polarizing beam splitter (PBS1) into a measurement-

and a reference beam. The measurement beam leads through a lambda/4-plate and the lens

HeNe-LaserIsolator

M1

Lambda/2 plate

PBS1

PBS2

PBS3

BS.4

M2

M3

M4

Sample

Lambda/4 plateL1

L2Lambda/4 plate

Lambda/4 plate

Piezo actuator+reference mirror

L3Photodiode

FIGURE 4.1:Beam path of the double beam laser interferometer used to determine the piezoelectricproperties of thin films [124].

4.3 Lead Zirconate Titanate (PZT) 31

(L1) to the polished backside of the sample, which is placed perpendicular to the beam. The

beam is reflected and distracted by PBS1 to the mirrors M2 and M3 since the polarization of

the beam is two times turned 45° by the lambda/4-plate. Deflected at PBS2, passing the second

lambda/4-plate and L2 leads the beam to the top-side of the sample where it hits the top contact

and is then reflected toward M4. From here the beam is directed to the photodiode passing the

beam splitter 4 (BS4). The reference beam is deflected from PBS3 toward a reference mirror

passing a lambda/4-plate, where the beam is reflected passing PBS3 again and is then deflected

by BS4 to the photodiode, where the supperposition of the measurement and reference beam

causes an interference pattern.

If an electric field is applied to the sample the measurement beam becomes shorter due to the

dilation of the piezoelectric layer and therefore the intensity of the interference pattern changes,

which is measured by the photodetector. To keep the measurement conditions constant, the

reference mirror elongates or shortens the length of the reference beam, if the optical devices

moves due to mechanical vibrations or thermally induced instabilities (more details are given

in [124, 125]). One of the reasons to use a double beam laser interferometer is to eliminate

measurement errors because of sample bending since the piezoelectric stress of the clamped

films not only results in a displacement of the film but also induces strong bending in the

substrate [126, 127]. If the sample bends, the beam length on the backside elongates exactly

the same amount as the beam length is shortened on the front side resulting in an elimination

of the bending error.

Electromechanical large signal and small signal measurements are performed with this setup,

whereby typical measurement conditions are (i) large signal measurements: 0 to 10 V at

f=123 Hz and (ii) small signal measurements: bias signal 0 to 10 V at 1Hz, supperimposed

with a small signal of 100 mV at 8000 Hz.

4.3 Lead Zirconate Titanate (PZT)

Electrical Characterization

Figure 4.2 (a) displays an exemplary hysteresis measurement of a 150 nm thin PZT(45/55) film.

The sample shows a pronounced hysteresis with a remanent polarization Pr = 16 µC/cm2 and

a coercive voltage of VC = 0.7 V , which corresponds to a coercive field of EC = 50 kV/cm.

Measurements on films with various PZT compositions prove, that changing the Zr/Ti ratio


(a)

−5 −4 −3 −2 −1 0 1 2 3 4 5

−45

−30

−15

0

15

30

45 PZT(45/55)

Voltage [V]

Pol

ariz

atio

n [µ

C/c

m²]

(b)

25 30 35 40 45 50 55

10

15

20

25

30

35

40

P

r [µ C

/cm

2 ]

Zr content [%]

FIGURE 4.2:(a) P-V characteristic of a 150 nm thin PZT(45/55) film. (b) Remanent polarization of PZTfilms dependent on the Zr content.

to Zr rich compositions results in slanted hysteresis graphs with lower remanent polarization

and smaller coercive voltages. Figure 4.2 (b) summarizes the remanent polarization vs. Zr

content. PZT(30/70) exhibit a remanent polarization of around 35 µC/cm2. With increasing Zr

content a monotonous decrease of the remanent polarization is observed. Ti rich compositions

are therefore the main candidates for non-volatile memory applications since the remaining

polarization is used for read and write operations [128].

Also, small signal measurements are performed on various PZT thin films to determine the di-

electric permittivity (oscillation level = 50 mV, 10 kHz). Figure 4.3 (a) shows a typical C(V)

curve for PZT(45/55) thin films. The measurement shows that the permittivity strongly depends

on the applied bias voltage and exhibits two maxima of εr = 1300 around the coercive voltage.

Without an applied electric field decreases the permittivity down to εr = 1150 and by exceed-

ing the coercive voltage a minimum value of around 300 is reached. This tunable behavior

is observed on PZT films of all compositions whereby the minimum value in voltage satura-

tion is always around 300, but the maximum and the initial permittivity (without an applied

electric field) are strongly affected by the Zr/Ti ratio. Figure 4.3 (b) shows the permittivity at

E=0 kV/cm vs. the zirconium content and a strong increase is obseved, if the films changes to

zirconium rich compositions.


(a)

-5 -4 -3 -2 -1 0 1 2 3 4 5

200

400

600

800

1000

1200

1400

Voltage [V]

ε r

PZT 45/55

(b)

30 35 40 45 50 55 60

200

400

600

800

1000

1200

1400

1600

1800

Zr content [%]

ε r

FIGURE 4.3:(a) Measurement of the dielectric permittivity vs. applied bias voltage for PZT(45/55)(150 nm). (b) Dielectric permitivity at E = 0 kV/cm as function of the zirconium content.

Electromechanical Characterization

Figure 4.4 (a) displays two electromechanical small signal measurements of PZT(30/70)

and PZT(45/55) films (300 nm). PZT(45/55) exhibit a maximum d33 = 115 pm/V whereas

PZT(30/70) exhibit a d33 below 50 pm/V. In Figure 4.4 (b) a plot of the remanent d33 vs.

the zirconium content is shown. If the Zr/Ti ratio shifts to a higher zirconium content the

piezoelectric coefficient increases and the maximum is measured for PZT(45/55). Beyond this

compositions the piezoelectric coefficient decreases. This corresponds with the estimation of

the piezoelectric coefficient d31 using

d31 = 2ε0εRPrQ12. (4.2)

with the electrostrictive coefficient Q12 = −3.1 · 10−2 m4/C2 [129].

PZT(45/55) films are characterized with regard to the film thickness. The detailed electro-

mechanical investigation is mainly on films with thicknesses below 700 nm, because it is ex-

pected that this is the technical relevant range in order to keep the driving voltages low. An

electric field of 200 kV/cm corresponds to 14 V, if a film thickness of 700 nm is considered.

The d33 of a selection of different PZT(45/55) thicknesses dependent on the electric field is

presented in Figure 4.5. The measurements are almost identical, showing the distinctive behav-

ior of the electromechanical small signal measurements, which means symmetric graphs and

a negative slope of d33 for E >> EC . Therefore it can be concluded that the piezoelectric

coefficient d33 is independent of the film thickness.


(a)

-300 -200 -100 0 100 200 300

-120

-90

-60

-30

0

30

60

90

120

d 33 [p

m/V

]

Electric field [kV/cm]

PZT(30/70) PZT(45/55)

(b)

20 30 40 50

20

40

60

80

100

rem

. d33

[pm

/V]

Zr content [%]

FIGURE 4.4:(a) Piezoelectric coefficient d33 of PZT(30/70) compared with PZT(45/55) (film thickness300 nm). (b) d33 as function of the Zr content.

Theoretically, the strain can be calculated using equation 2.6 (contact forces applied by the

probe needles are neglected). However, the film is not a perfect crystal, and clamping on the

substrate hinders the PZT to evolve the full piezoelectric potential [77]. Therefore, the strain

of the films is investigated by performing electromechanical large signal measurements. Fig-

ure 4.6 (a) shows the results of various films. The measurements turn out that the strain is totally

independent on the film thickness and therefore the displacement is also independent on the film

thickness, if the applied voltage is constant.

S3 =∆t

t= d33E3 = d33

U3

t=⇒ ∆t = d33U3 (4.3)

with ∆t representing the total displacement, t the film thickness and U the applied voltage. On

the other hand, the total displacement is strongly dependent on the film thickness at the same

electric field. The thicker the film the higher the displacement. Figure 4.6 (b) displays the total

-400 -300 -200 -100 0 100 200 300 400

-100-80-60-40-20

020406080

100 121 nm 481 nm 604 nm

d 33 [p

m/V

]


FIGURE 4.5:d33 measurements of PZT(45/55)thin films with various thicknesses(4V bias signal).


(a)

-400 -200 0 200 400-1

0

1

2

3

4

121 nm 481 nm 604 nm

Stra

in [1

/100

0]


(b)

-400 -200 0 200 400

-400

0

400

800

1200

1600

Dis

plac

emen

t [pm

]

Electric Field [kV/cm]

121 nm 481 nm 604 nm

FIGURE 4.6:Electromechanical large signal measurements on PZT(45/55) thin films with various thick-ness: (a) Strain vs. electric field (b) Displacement vs. electric field.

displacement vs. the electric field and reveals a maximum displacement of around 1.4 nm at

200 kV/cm for a 604 nm thin film, whereas the 121 nm film reveal only 0.25 nm displacement

at E = 200 kV/cm.

4.3.1 Electromechanical Fatigue

As ferroelectric actuators are system-enabling components, long-term reliability is of para-

mount industrial concern. It needs to be ensured that the device will be stable over a long

time period and number of operation cycles depending on the application. Besides MEMS

packaging, the mechanical stability of the supporting substrate, the integrated smart material

needs to remain their functionality. To cover the reliability aspects of the integration, a closer

look to the fatigue behavior in terms of the electrical and electromechanical properties is neces-

sary [130–133].

It is known, that the polarization of PZT films decreases with an increasing number of switching

cycles, if platinum electrodes are used. In Figure 4.7 (a) a typical result on a 481 nm thin sample

is shown for several polarization measurements after a different number of cycles. In the ini-

tial state the material is clearly ferroelectric and shows a remanent polarization of 23 µC/cm2.

By increasing the number of switching cycles the hysteresis curve becomes shallow and the

remanent polarization decreases, whereas the coercive field remains constant. After 109 cycles


almost no ferroelectric properties are left. Figure 4.7 (b) summarizes the results for the piezo-

electric fatigue measurements. Like the polarization, the measured graph becomes shallow and

d33 decreases with an increasing number of cycles down to a d33 = 35 pm/V after 109 cycles.

It is obvious that there is a relation between the piezoelectric properties and the polarization.

If one look more into detail of the development of the remanent polarization and the remanent

d33 it is found that there is no continuous decrease. Both parameters remain constant until

106 switching cycles before a strong decrease occurs (Figure 4.8 (a)). After 108 cycles the

fatigue mechanism seems to be completed and at least Pr remain constant on a low level. This

fatigue effect is found for all samples with a tendency that in thinner films the fatigue of the

piezoelectric coefficient starts earlier than the fatigue of the polarization [134]. However, Gerber

et al. [135] performed similar fatigue characterization on 130 nm PZT(30/70) and PZT(40/60)

thin films. The fatigue of both parameters starts simultaneous at 105 switching cycles. Hence,

further investigation is needed to clarify this point.

Another interesting point is the comparison of the values in the initial state and in the fatigued

state. A 255 nm film shows a constant d33 of 87 pm/V at 0 V bias voltage before the fatigue

starts. After the fatigue is completed a d33 of 57 pm/V is left. This means that the d33 in the

fatigued state has still 65 % of its initial piezoelectric abilities. The 708 nm film shows a stronger

fatigue behavior. After 108 cycles just 22 % of the initial d33 value is left. The comparison of

the initial remanent polarization with the fatigued remanent polarization after 108 cycles gives

a similar behavior. In thinner films, the difference between the initial and fatigued state is less

(a)

-200 -100 0 100 200

-40

-20

0

20

40

Pol

ariz

atio

n [µ

C/c

m²]


initial 1E7 1E8 1E9

(b)

-200 -100 0 100 200

-100-80-60-40-20

020406080

100

d 33 [p

m/V

]


initial 1E6 1E7 5E7 1E8 1E9

FIGURE 4.7:(a) Polarization measurement and (b) piezoelectric coefficient d33of a 481 nm PZT(45/55)thin film after different numbers of fatigue cycles.


(a)

101 103 105 107 109

−80

−60

−40

−20

0

20

40

60

80

−40

−30

−20

−10

0

10

20

30

40

d 33 [p

m/V

]

Number of cycles

P r [µC

/cm

²]

(b)

100 200 300 400 500 600 70010

20

30

40

50

60

70

80

90

rem. d33

PrLo

ss @

108 c

ycle

s [%

]

Film thickness [nm]

FIGURE 4.8:(a) Comparison between Pr and d33 (@ 0 V) for a 708 nm PZT(45/55) thin film. (b)Piezoelectric and polarization losses vs. film thickness.

than in thicker films. Figure 4.8 (b) summarizes the loss of d33 and the remanent polarization

after 108 cycles vs. the film thickness. The losses are dependent on the film thickness whereas

the piezoelectric coefficient loss is stronger than the loss of polarization. Hence, the decrease

of the small signal parameter d33 and the decrease of the polarization, which is a large signal

parameter, originate from different effects.

Avoiding fatigue is an essential issue in memory devices as well as in piezoelectric devices.

Since ferroelectric materials exhibit a symmetric displacement it is not necessary to drive piezo-

(a)

-300 -200 -100 0 100 200 300

-100-80-60-40-20

020406080

100

d 33 [p

m/V

]


virgin 1E6 1E8

(b)

102 104 106 108 1010

-100-80-60-40-20

020406080

100

d 33 [p

m/V

]

Number of cycles

bipolar unipolar

FIGURE 4.9:(a) Piezoelectric coefficient d33 of a 255 nm PZT(45/55) thin film after different numbersof unipolar fatigue cycles. (b) Comparison of the effect of bipolar and unipolar voltagepulses to d33 (481 nm film thickness).


electric devices with bipolar voltages. Hence, the piezoelectric response is measured with

unipolar voltage pulses used for the fatigue procedure. The unipolar pulses always range be-

tween 0 V and a maximum bias voltage, dependent on the film thickness. Applying unipolar

pulses mean that the sample does not switch between negative and positive polarization. Fig-

ure 4.9 (a) shows the measurement result for the d33 coefficient of a 255 nm thin film. After

108 cycles the graph has still the same shape. Repeating the measurements with other films

gives the same result. Figure 4.9 (b) compares the remanent d33 in case of bipolar and unipolar

fatigue pulses for a 481 nm film. No decrease of the piezoelectric properties is observed after

109 unipolar cycles.

4.4 Lanthanum doped Lead Hafnate Titanate (PLHT)

In this section the electrical and electromechanical properties of PHT are investigated. Schorn

et al. [86] focused the research on pure PHT(30/70) and examined the electrical properties

with regard to FeRAM applications. He found a similar behavior to PZT(30/70). Therefore,

electromechanical measurements are performed on PHT(30/70) with and without La doping.

Since the characterization of PZT with a Zr/Ti ratio of 45/55 prooved to be the best material for

piezoelectric applications the emphasis is on the electrical and electromechanical characteriza-

tion of PHT(45/55) with additional doping.


A study on pure PHT(45/55) is performed in comparison to pure PZT(45/55). The polarization

measurements show a similar behavior (Figure 4.10 (a)). Both materials exhibit a hysteresis

loop with a coercive field of EC ≈ 50 kV/cm and a remanent polarization of Pr ≈ 25 µC/cm2.

Figure 4.10 (b) displays polarization measurements performed on films with various La

doping concentrations. All films exhibit hysteresis loops, but the shape of the curves varies

from a broad, square hysteresis to a small, slim hysteresis loop accompanied with a strong

decrease of Pr if the La concentration increases. The remanent polarization is reduced from

Pr = 25 µC/cm2 (no La doping) to Pr = 6 µC/cm2 (10 % La). At the same time the

decrease of EC is much less.

Small signal measurements are performed on the different films and are summarized in

Figure 4.11 (b). Pure PHT(45/55) shows the typical two maxima and a strong dependence on

the applied electric field. Thereby it exhibits the highest dielectric permittivity of εr = 1250


(at E = 0 kV/cm). Films with higher amounts of La exhibit the same strong dependence

on the electric field, but their maximum permittivity is decreased. Nevertheless, in the high

electric field regime (E EC) the measurements are identical. Furthermore, one notices

that the maxima come closer together and there is almost no difference any more for the film

doped by 10 % La. Another fact to be mention here, is the difference between appliying a

positive or negative electric field to bring the material into saturation. In Figure 4.11 (b) two

measurement cycles are plotted for each film. The curves for the pure PHT film are identical,

but the film with 10 % La doping shows a difference in the absolute values of the maxima. If

a positive electric field is applied and then reduced, the material passes the higher maximum.

If the material is in negative saturation and the electric field is reversed then the dielectric

permittivity reaches the smaller maximum. Nonuniform distribution of the high amount of

La within the film can be the reason for this effect. Further studies need to be done on this issue.

Electromechanical Characterization

Measurements of the piezoelectric coefficient are performed on PHT(45/55) and PHT(30/70)

thin films with various La doping. Figure 4.12 (a) shows the result of small signal measure-

ments. They exhibit a strong hysteresis shape. Pure PHT (square dots) exhibits a maximum

d33 ≈ 100pm/V, which is analog to pure PZT of the same composition, but also it shows a

very strong decrease of d33 with increasing applied electric fields. At 550 kV/cm the material

has lost 50 % of its initial d33 value.

(a)

-600 -400 -200 0 200 400 600

-50

-40

-30

-20

-10

0

10

20

30

40

50

Pol

aris

atio

n [µ

C/c

m2 ]


PZT(45/55) PHT(45/55)

(b)

-600 -400 -200 0 200 400 600

-50-40-30-20-10

01020304050

Pol

aris

atio

n [ µ

C/c

m2 ]


PLHT(45/55) 0.0 % La 5.0 % La10.0 % La

FIGURE 4.10:(a) Comparison of the polarization between PZT(45/55) and PHT(45/55) thin films(150 nm) (b) Polarization vs. electric field of PHT(45/55) films with various amount ofLa doping. (Measurements taken at 100 Hz.)


-600 -400 -200 0 200 400 6000

200

400

600

800

1000

1200

1400

ε r


PLHT(45/55) 0.0 % La 5.0 % La 10.0 % La

FIGURE 4.11:Dielectric permittivity vs. elec-tric field of PLHT(x/45/55) filmswith various amounts of La doping(50 mV oscillation level, 10 kHzmeasurement frequency.

Also the characteristics of the electromechanical small signal measurements are comparable

to the electrical large signal measurements, if films with various La doping concentrations are

considered. The maximum d33 decreases with increasing La amount and the shape of the curve

becomes smaller, while almost no difference exists between 2.5 % and 5 % La doping. The

characteristic of the 10 % La doped films looks different. The material exhibits no hysteresis

behavior, instead it shows a slim loop. A linear increase of d33 is observed until a maximum

value of d33 = 63 pm/V is reached at 200 kV/cm and then the piezoelectric coeficient remains

constant in the high voltage regime.

For comparison a number of La doped PHT(30/70) films are characterized by electromechan-

ically small signal measurements, whose results are plotted in Figure 4.12 (b). PHT(30/70)

exhibits a square hysteresis behavior with a maximum of d33 ≈ 50 pm/V and a negative slope

(a)

-600 -400 -200 0 200 400 600

-100-80-60-40-20

020406080

100

d 33 [p

m/V

]


PLHT(45/55) 0.0 % La 5.0 % La 10.0 % La

(b)

-400 -200 0 200 400

-60

-40

-20

0

20

40

60

d 33 [p

m/V

]


PLHT(30/70) 0.0 % La 2.5 % La 5.0 % La

FIGURE 4.12:(a) d33 vs. electric field of PHT(45/55) films with various amounts of La doping. (b) d33

vs. electric field of PHT(30/70) films with various amounts of La doping.


(a)

-600 -400 -200 0 200 400 600

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Dis

plac

emen

t [nm

]


PLHT (45/55)0.0 % La

(b)

-600 -400 -200 0 200 400 600

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Dis

plac

emen

t [nm

]


PLHT (45/55)10.0 % La

FIGURE 4.13:Displacement vs. electric field of PHT(45/55) films: (a) without La doping (b) with 10 %La doping (fbias = 123 Hz).

within higher electric fields. With increasing La concentration the d33 remains nearly constant,

but the curve becomes smaller and the coercive field decreases. These measurements prove that

PHT(45/55) generally is better qualified for piezoelectric applications than PHT(30/70).

How the different behavior of the piezoelectric coefficient affects the strain and displacement

respectively is shown in Figure 4.13 (a) and (b). Considering positive applied electric fields

the comparison of the undoped PLHT(0/45/55) film with the 10 % La doped film exhibits

some differences. The pure film shows a higher displacement, which is expected, because the

d33 is higher. One also observes minima at ±EC and a curved graph due to the decreasing

piezoelectric coefficient in the range of high applied electric fields. The measurement of the

10 % La doped film looks different. The displacement shows a quadratic dependence on the

applied electric field and is therefore more electrostrictive than piezoelectric. Further, the curve

shows a closed behavior instead of an opened butterfly loop.

The characterization can be concluded with the observation that PHT behaves similarly

to PZT and can be used for piezoelectric devices. Whether La doping is useful depends

on the request of the device. On one hand, pure PHT(45/55) exhibits the highest d33, but

the piezoelectric coefficient decreases with increasing fields and the strain is of non-linear

behavior. On the other hand PLHT(10/45/55) shows a smaller d33, but it develops linear and

remains constant in the high electric field regime. A device driven above 200 kV/cm will work

hysteresis free and exhibit a linear behavior. In addition, the electrical losses decrease linearly


with increasing La doping concentration. Hence, the use of PZT or PLHT depends strongly

on whether maximum displacement, linear movement or optimized power consumption is

required for the desired device.

4.5 Barium Strontium Titanate (BST)

BST is discussed with respect to its application in devices for infrared imaging.


The investigation starts with an elaborate characterization of the dependence of the dielectric

permittivity on the applied voltage. The measurement of a 120 nm thin BST(70/30) film

is presented in Figure 4.14. The square symbols depict a curve, which is taken at room

temperature. A strong dependence of εr on the applied voltage is observed with a maximum

of more than 400. Only one permittivity maximum is visible, since BST is in the paraelectric

phase above room temperature. Furthermore, the maximum is slightly shifted to negative

voltages. There is no exact explanation for this unbalanced behavior, but it is probably due to

stress variations of the top and bottom electrode.

To characterize the dependence of the permittivity on the temperature, various measurements

are carried out. The temperature range was chosen from -20 °C to 200 °C. With increasing

temperature the graph flattened and the maximum permittivity values decrease strongly. At

200 °C εr drops to 67 % of the initial value at room temperature. In Figure 4.15 the square

−6 −5 −4 −3 −2 −1 0 1 2 3 4 5 6150

200

250

300

350

400

ε r

Voltage [V]

BST(70/30) 25°C 60 °C130 °C200 °C

FIGURE 4.14:Dielectric permittivity vs.voltage of a BST(70/30)thin film (Vosc = 50 mV,fosc = 10 kHz). The graphsare monitored in the rangefrom room temperature to200 °C.

4.5 Barium Strontium Titanate (BST) 43

-30 0 30 60 90 120 150 180 210

200

240

280

320

360

400

ε r

Temperature [°C]

BST(70/30)BST(55/45)

FIGURE 4.15:Comparison of the dielectric permit-tivity (@ 0 V) vs. temperature ofBST(70/30) and BST(55/45).

symbols depict the progression of εr (at 0 V) with increasing temperature of the BST(70/30)

film. The permittivity decreases nearly linear from εr = 400 down to εr = 240. The round

symbols summarize the measurements for a BST(55/45) thin film showing a similar behavior

but generally BST(55/45) exhibits a smaller permittivity.

Concluding the electrical and electromechanical characterization presented in chapter 4

leads to the result that PZT(45/55) or PHT(45/55) with an amount of 10 % La doping are

good suited materials for the application in electromechanic driven MEMS. Both compositions

exhibit large piezoelectric coefficients whereby doping with La changes the material from

ferroelectric to relaxor behavior and therefore electrostrictive behavior. The effect of fatigue

can be successfully avoided, if devices using PZT on platinum electrodes are driven with

unipolar voltages.

For bolometer applications BST(70/30) is a suitable material, since at room temperature its

permittivity depends linear on temperature changes and the permittivity is higher than for

BST(55/45).


45

5 Technology

The following chapter describes various technologies, that are develped or improved during

the course of this thesis in order to integrate ferroelectric thin films into silicon based

microsystems. The spectrum enfolded lithography, silicon bulk- and surface micromachining,

sacrificial techniques, plasma etching and electroplating.

Two typical device designs are shown in Figure 5.1. A ferroelectric material is deposited

between two electrodes on a membrane (Figure 5.1 (a)). Therefore technologies for patterning

metals and ferroelectrics as well as silicon structuring are required. Another design bases on

cantilevers with an overlapping metal contact (Figure 5.1 (b)) and therefore additional to lateral

material deposition and structring also vertical deposition and structruring technologies are

required.

The basics of silicon MEMS microfabrication such as photolithography, pattern transfer with

dry etching, wet etching techniques and thin film deposition (e.g. SiO2, Si3N4, poly-Si, Al,

Au etc.) have been reviewed in the literature [136–140], but up to date the microfabrication

of microsystems using ceramic materials suffers from inadequate micromachining processes,

only few references have been published [141–143].

5.1 Photolithography

MEMS usually consit of a stack of different material layers, which are conformally deposited on

each other and photolithograhy is used for the structuring [144,145]. Since different structuring

methods are applied it is necessary to use various photoresists matching the requirements of

the process in terms of resist thickness, feature size, selectivity and temperature stability. The

Si

Contact

Si

Electrode

Ferroelectricfilm

(a)

SiO2

(b)

FIGURE 5.1:Basic structures of ferroelectric MEMS: (a) on membrane (b) cantilever with contact.

46 5 Technology

TABLE 5.4: Parameters of positive lithography processes

Resist Resist- Spinning- Prebake Exposure- Developmentthickness Speed C time time

@90 ° (AZ 826mif)[µm] [rpm.] [min.] [sec.] [sec.]

AZ 5214E 1.4 4000 5 7 40TI xlift 1.7 3000 5 40 60 diluted

3:1 PGMEAAZ 5214E 3.0 1000 7 16 60

TI xlift 4.8 2500 20 80 120AZ 4562 6.8 4000 7 40 90TI xlift 8.0 1200 40 160 180

process parameters for positive and negative lithography processes are summarized in Table 5.4

and Table 5.5.

The resists of the AZ series and the TI xlift resist vary mainly due to the maximum resist

thickness and sustain maximum temperatures around 130 °C, whereas the nlof resist withstands

temperatures up to 250 °C, but has a minimum thickness of 2 µm, which restricts the minimum

feature size.

TABLE 5.5: Parameters of negative lithography processes

Resist Resist- Spinning- Prebake Exposure- Reversal- Flood- Develop-thickness Speed time bake Exposure ment

[µm] [rpm.] [min.] [sec.] [min.] [sec.] [sec.]AZ 5214E 1.4 4000 5@90 °C 4.5 2@115 °C 14.5 35

TI xlift 1.7 3000 5@90 °C 30 2@130 °C 100 120nlof 2020 2 2500 5@20 °C 25 1@110 °C 30

5@90 °CAZ 5214E 3.0 1000 7@90 °C 12.5 2@115 °C 30 45nlof 2020 4 1000 5@20 °C 35 1@110 °C 35

5@90 °CTI xlift 4.8 2500 20@90 °C 40 2@130 °C 120 360TI xlift 8.0 1200 40@90 °C 55 2@130 °C 160 600

nlof 2070 8 2500 5@20 °C 45 1@110 °C 455@90 °C

5.2 Silicon Bulk Micromachining 47

5.2 Silicon Bulk Micromachining

Silicon bulk etching techniques are used to remove silicon selectively to metals, SiO2, Si3N4

etc. and have been broadly applied in the fabrication of micromachined sensors and actuators.

The available etching methods are categorized into three categories in terms of state of the

etchant: wet, vapor, and plasma. Silicon etching from the vapor phase is conducted using

xenon difluoride (XeF2) [146, 147] and more informations about plasma etching are given in

section 5.3.

Within silicon wet etching one distinguishes between isotropic and anisotropic etch reactants.

The first group consists of mixtures from hydrofluoric acid (HF), nitric acid (HNO3) and acetic

acid (CH3COOH) and etches independently of the silicon crystal orientation. To the second

group belong inorganic alkaline etchants like potassium hydroxide (KOH) and organic alkaline

etchants such as tetramethylammonium hydroxide (TMAH) [148–150]. Both chemical groups

exhibit unequal etch rates depending on the silicon crystal orientation and contain OH−-ions and

H2O-molecules, which are necessary for the reaction. In a first step the Si-atoms react with the

OH−-ions and deliver electrons to the conduction band (Eq. 5.1). In a second step the electrons

react with the H2O-molecules by creating H2 and OH−-ions, while the solid Si-complex forms

a soluble Si-complex with the aid of OH−-ions (Eq. 5.2).

Si + 2OH− −→ Si(OH)2+2 + 4e− (5.1)

Si(OH)2+2 + 4e− + 4H2O −→ Si(OH)2−

6 + 2H2 (5.2)

TMAH belongs to the anisotropic Si etchants, whose working principle is illustrated in Fig-

ure 5.2. If the process is applied to a (100) oriented wafer the vertical etch rate is much higher

than the lateral etch rate resulting in a typical angel of α = 54.74 between the surface and

the sidewall, which is an important integration aspect. In addition it is possible to etch per-

pendicular trenches, if the Si substrate is of (110) orientation. The reason for this behavior

lies in the different number of atomic bonds of the crystal directions. A (111) oriented plane

has one atomic bond, whereas a (110) or (100) plane exhibits two bonds per atom. Therefore,

more energy is necessary to release a Si atom from the (111) surface, resulting in the described

geometries.

The silicon substrates used within this work are of (100) orientation and have a thickness of

675 µm. Experiments with various TMAH concentrations and different temperatures proved an

etch rate of 32 µm/h, if a 20 % TMAH etch solution is used at 80 °C. A picture of such a cavity

is presented in Figure 5.3 (a), where SiO2 acts as hard mask. Alongside a sample is presented

48 5 Technology

α β

(100) (110)

FIGURE 5.2:Geometry of TMAH etching in (100) and (110) oriented wafers.

which is etched at the same temperature but with 10 % TMAH concentration (Figure 5.3 (b)).

A higher etch rate is observed, but the surface is rough which turns out to be regular pyramidic

shaped hillocks. An increase of the temperature generally results in an increase of the etch rate.

Due to a better surface quality and better process control the etch process with 20 % TMAH

solution at 80 °C is chosen for the device fabrication later on.

(a) (b)

FIGURE 5.3:SEM pictures of a silicon cavity etched at 80 °C with: (a) 20 % (b) 10 % TMAH solution.The inset in (b) indicates a rough surface formed by Si pyramides.

5.3 Surface Micromachining 49

5.3 Surface Micromachining

Wet etching techniques make use of hazardous chemicals, which are often of isotropic nature

and exhibit high etch rates, thereby making it difficult to control the process. Dry etching or

plasma etching techniques on the other hand enable a very good control of the process and

therefore they lead to improved etch profiles, e.g. deep trenches with steep edges, avoidance of

underetching and accurately etch stops.

Dry etching processes uses inert or reactive gases, which are ionized in a low pressure reactor

to form a plasma. For surface micromachining a dry etching system (Microsys from Roth und

Rau) is used, which is equipped with an electron cyclotron resonance (ECR) plasma source and

can be operated in two modes: (i) ECR mode and (ii) RF mode. Some technical data of the

system are given in Table 5.6.

In ECR-mode the plasma is generated by an electromagnetic field (2.4 GHz), which is coupled

into the chamber by an antenna (Figure 5.4) and ionize the gas, which is led in through the

upper gas inlet. Furthermore, the plasma is densified by a magnetic field. The electrical

impedance of the plasma, which depends on the process parameters (e.g. pressure, power) has

MicrowaveGeneratorTuner

Magnet

Plasma

Vacuum ChamberGas Ring

Wafer

Gas Inlet

He-Cooling

Load-Lock

VacuumPump

Antenna

RF-Generator

FIGURE 5.4:Schematic drawing of the plasma etching system used for structuring of thin films anddifferent materials.

50 5 Technology

to be matched to the internal impedance of the microwave source. By changing the geometry of

the waveguide using tuner stubs, it is possible to optimize the induced power. To yield physical

or chemical etching the plasma is directed to the wafer by an RF-field. The wafer forms one

electrode and the reaction chamber itself the opposite electrode. By applying the RF-field

(13.54 MHz) the ions and the electrons are accelerated in opposite directions during the first

half of the period and a potential difference arises. This potential difference maintains in the

second half of the periode due to the different masses of the electrons compared to the ions and

due to the different areas of the electrodes. Hence, the wafer is charged more negatively than

the reaction chamber [151]. Positive charged ions from the plasma accelerates to the wafer,

where they attack the surface due to physical impact or by chemical reaction with the material

and the formation of volatile products. The magnitude of the physical impact is controlled by

the applied RF-power. The ion flow in direction of the wafer represents a plasma themselves

and therefore a matching network is necessary to adjust the induced RF-power. Strong physical

ion impact on the wafer surface leads to strong heating, which easily exceeds the temperature

to which photoresist is stable (≈ 120 C). Therefore, the wafer is cooled from the backside

with helium.

In RF-mode, the gas or gas mixture respectively enters the chamber through the gas ring

nearby the wafer and the plasma is ignited and maintained by the RF-field applied to the

chamber and wafer. Since, the distance between plasma and wafer is reduced the ions have less

kinetic energy and therefore the physical impact of the ions is reduced. Furthermore, the ion

impact is not strongly perpendicular to the wafer surface. Hence, RF generated plasmas are

suitable for isotropic, reactive etch processes.

TABLE 5.6: Technical data: MicroSys plasma etching system.

max. microwave power 1200 Wmax. RF power 600 W

typ. pressure 1 - 100 µbarcooling -30 °C to 20 °C

provided gases ArO2

CHF3 (Trifluoromethane)CF4 (Tetrafluoromethane)SF6 (Sulfur Hexafluoride)

He (only for cooling)


Pt bottom

trench

SiO2 residues ofphotoresist

Pt/PZT/Pt

FIGURE 5.5:Trench formation during plasmaetching due to a inhomogeneous dis-turbance of the electric potential.

To appraise the quality of an etch process some characteristic aspects are considered. Selectivity

compares the etch rate of the thin film that is etched with the etch rate of the masking layer or

surrounding materials.

selectivity =etch rate of the material to be etched

etch rate of the masking layer(5.3)

A second criteria is the aspect ratio. It describes the ratio between the vertical etch depth to the

undercut of the masking layer.

aspect ratio =etch depth

undercut(5.4)

The aspect ratio characterizes the shape of a trench: The higher the aspect ratio the steeper the

sidewalls of a trench.

The third important parameter is the etch homogeneity of a wafer. The etch rate in the center of

a wafer may be different from the etch rate at the border. Supplementary, the etch process can

be inhomogeneous in the range of the structure size due to varying conductance of the involved

materials and densification of the electric field at edges. Figure 5.5 shows a SEM picture of a

Pt/PZT/Pt stack, which is protected by photoresist. Close to the border of this stack the lower

Pt is removed, which means the etch rate is higher in this area. The inset identifies the strong

impact at the corner, which is etched in a way, that after the processing two tips are formed.

The appearance of fences is another problem in combination with dry etching processes. Fences

arise if removed material redeposits on the sidewalls of the protecting photoresist instead of

being pumped outside the reaction chamber together with the reactive gas (Figure 5.6 (a)). The

52 5 Technology

(a)

substrate

photoresist

thin film

+

+

+

+

+

+

+

(b)

resist

PZT

fence

FIGURE 5.6:(a) Formation of fences during dry etching processes. (b) Fences formed at the sidewall ofphotoresist.

fences in Figure 5.6 (b) have been emerged during PZT etching with non-optimized parameters.

On the right hand side one notices the photoresist, which is also partly removed during the

etching.

Special attention has to be paid to the temperature during the etch process. High physical ion

impact is combined with high energy transfer from the ions to the substrate. Whereas most

of the involved materials like Pt, Si or PZT withstand temperatures of several hundred °C, the

photoresist is mostly stable up to 120 °C. Temperatures above this point leads to melting of

the resist combined with loos in shape. The pattern to be transfered into the substrate becomes

larger than designed on the original photomask. Furthermore, the resist may be burned and

therefore it is impossible to remove it with conventional dissolvents or in an oxygen plasma

dependent on the degree of combustion. In Figure 5.5 still some residues of the photoresist

are on the structure and therefore further processing is not wise. To avoid this problem an

automated chopping technique is induced consisting of etch steps and cooling steps with pure

He backside cooling.

5.3.1 Plasma Etching of Pt and BST

Dry etching of Pt and BST can be performed in chlorine (Cl2), CF4 or Ar containing plas-

mas [152–158].

Pt layers are structured using a pure Ar plama, which is generated by the ECR source (parame-


TABLE 5.7: Parameter for Pt and BST etch process

Pressure ECR RF Bias Gas Etch Selectivitypower power voltage flow rate to resist

[µbar] [W] [W] [V] [sccm] [nm/min]Pt 5.5 600 50 110 35 Ar 9.4 0.3

BST 5.5 600 50 110 35 Ar 4.5 0.15

ters are given in Table 5.7), whereby the milling is due to the physical impact of the ions. A

relatively low RF-power is chosen in order to keep the temperature low, concerning the tem-

perature budget of the masking photoresist. The process exhibit an etch rate of 9.4 nm/min.

Calculating the total etch time from this etch rate enables to stop the etching precisely on the

underlying layer. The etch rate of the resist is around 30 nm/min, which is noncritical since the

resist is at least 1.4 µm thick.

The etching of BST thin films is performed with the same process and exhibits an etch rate of

4.5 nm/min.

5.3.2 Plasma Etching of PZT

For plasma etching of PZT various gas combinations are applied, such as

Cl2, CCl4, C2F6, HBr, CF4, CF2Cl2, SF6 [159–162]. Alternative processes uses CF4 in

combination with Ar [163, 164]. Thereby the Ar+ ions break the metal-oxide bonds releasing

Pb, Zr and Ti into the crystal. XPS measurements reveal that the metals then form volatile

halide compounds [165–167].

Experiments using CF4 and Ar turn out the parameters given in Table 5.8 are the best in terms

of etch rate and selectivity to Pt. Higher etch rates can easily be obtained by increasing the

power, but this induces high wafer temperatures, which exceed the temperature budget of the

photoresist. The etch rate of Pt is 4.2 nm/min and the etch rate of photoresist is in the range

of 148 nm/min to 210 nm/min, depending on the photoresist. Therefore a resit thickness of

5.4 µm is sufficient to transfer the lithography mask into a 255 nm PZT layer.

Reduction of spontaneous polarization, induced during thin film etching due to pinning of do-

mains and remaining of metal halides on the surface [168–170] is curred by heating the film

above the crystallization temperature.

54 5 Technology

TABLE 5.8: Parameter for PZT etch process

Pressure ECR RF Bias Gas Etch Selectivity Selectivitypower power voltage flow rate to Pt to Resist

[µbar] [W] [W] [V] [sccm] [nm/min]PZT 3.5 800 50 197 12 Ar 7 1.6 0.06

38 CF4

5.3.3 Plasma Etching of SiO2 and Si3N4

SiO2 and Si3N4 are two key materials in modern MEMS fabrication. Since these materials are

modifications from Si and utilized in conventional CMOS technology it is just a small step, to

employ them in MEMS fabrication. Nevertheless, the electrical and mechanical properties vary

from those of Si and therefore these materials are used as dielectric layer, as mechanical support

layer or as encapsulation layers. Furthermore, the chemical properties are different from those

of Si and SiO2 and Si3N4 are widely used as hard mask requiring highly selective etching over

Si [171, 172].

Because fluorine atoms etch Si faster than SiO2, it is difficult to achieve etch selectivity of silicon

dioxide over silicon in a pure CF4 or SF6 discharge. An unsaturated, polymer forming gas feed

is usually employed to pattern oxide on silicon layers, while an unsaturated halocarbon gas (e.g.

CHF3) discharge results in the formation of a fluorocarbon layer on exposed silicon during

oxide etching. Since there is no available reaction with silicon that can gasify the polymer

film, a fairly thick layer (≈ 15 nm) accumulates on the silicon surface and prevents chemical

etching [173]. Here, fast selective etching of SiO2 over Si is desired and therefore a low RF-

power process with 50 % CHF3 and 50 % CF4 is applied (Table 5.9).

Si3N4 is etched by both fluorine-atom-containing plasma and CF4 unsaturated gas plasma and

shows etching characteristics intermediate between those of silicon and SiO2 [174]. Since Si3N4

TABLE 5.9: Parameters for SiO2 and Si3N4 etch processes


[µbar] [W] [W] [V] [sccm] [nm/min]SiO2 10 1000 30 180 30 CHF3 38 0.4

30 CF4

Si3N4 7 1200 40 177 30 CF4 30


is widely used as hard mask for silicon bulk micromachining (e.g. wet etching using TMAH) a

formation of a protecting layer on top of the silicon has to be avoided and therefore pure CF4 is

used at low RF-power to maintain a smooth silicon surface (Table 5.9).

5.3.4 Anisotropic Silicon Etching

For on-chip integration, the sensing or actuated elements are typically defined after circuit fab-

rication. This has benefits in permitting the use of standard technology and also in minimizing

the wafer processing after the fabrication of the microsystems. For the processing of anisotropic

structures several approaches are used to inhibit lateral etching during the process and thereby

maximize the aspect ratio. The most successful techniques are all based on sidewall passivation

and a wide range of chemistries, mostly fluorine based are known [175–180].

This technique can be understood by considering a model of a SF6/O2 dry etching process.

Within the plasma, free radicals of SxFy and O are created:

SF6 + e− −→ SxFy+ + SxFy + F + 2e− (5.5)

O2 + e− −→ O+ + O + 2e− (5.6)

The free oxygen radicals react with the silicon on the surface and create a passivation layer in

form of an oxide film:

O + Si(s) −→ SiOn(sf) (5.7)

where (s) and (sf) indicate surface and surface film respectively. The surface passivation layer

is then removed by the plasma prior to the etching of the Si with fluorine:

SiOn(sf) + F −→ SiOn(sf) − F (5.8)

SiOn(sf) − nF −→ ion energy −→ SiFx(ads) + SiOxFy(ads) (5.9)

where the fluorine adsorbs (Fads) onto the surface and the ion bombardment plays the critical

role in the removal of the passivation film by enhancing the adsorption, reaction and desorption.

Now, the fluorine can proceed with silicon etching and desorption as a gas (g):

Si + F −→ Si − nF (5.10)

Si − nF −→ ion energy −→ SiFx(ads) (5.11)

Si − nF −→ SiFx(g) (5.12)

56 5 Technology

TABLE 5.10: Parameters for anisotropic Si etch processes


[µbar] [W] [W] [V] [sccm] [nm/min]Si 10 1000 30 170 100 SF6 614 6.1

10 O2

The addition of 10 % O2 in a SF6 plasma is suggested to promote the forming of vertical

trenches with high etch rates, due to sidewall pasivation [181]. Experiments turn out the process

parameters given in Table 5.10 are a good compromise between etch rate and anisotropy.

Figure 5.7 shows the cross section of a sample etched by this process. A smooth surface and

a silicon etch rate of 614 nm/min are observed. The aspect ratio amounts to 1.5, whereby the

masking material is Pt with photoresist on top. The photoresit is not burned and exhibits an

etch rate of 100 nm/min. This process enables deep silicon etching and is applied to etch deep

cavities or creating membranes.

5.3.5 Isotropic Silicon Etching

Front side micromachining can also be applied to produce membranes or to release cantilevers

from the silicon substrate. It is achieved by means of lateral, nearly isotropic dry etching of

the silicon. Figure 5.8 gives a schematic overview. First, a oxide or nitride layer is deposited

on the silicon, which later serves as substrate (Figure 5.8 (a)). Photoresist is patterned onto

the substrate to define an etch window before the oxide layer is removed by dry etching (Fig-

ure 5.8 (b)). In the last step, the cantilever is released by isotropic dry etching of the silicon

Si

Resist Pt

FIGURE 5.7:Cross section of anisotropic etchedsilicon with process parametersgiven in Table 5.10.


Silicon Substrate

Silicon Substrate

Resist

Silicon Substrate

Resist

Silicon oxide or nitride film

(a)

(b)

(c)

FIGURE 5.8:Schematics of the process flow forthe fabrication of suspended sili-con oxide and nitride membranes:(a) deposition of the oxide/nitridefilm (b) deposition of photoresist (c)isotropic dry etching of silicon sub-strate to provide release of the struc-ture.

under the structure (Figure 5.8 (c)). Therefore, the process has to be optimized with respect to

the lateral etch rate, the selectivity to the photoresist on the front side and SiO2 membrane.

Isotropic silicon etching is reported using xenon difluoride (XeF2) gas [182] or SF6 based

plasma etching [183], whereby isotropic etch profiles are achieved with high pressures and

chemical dry etching (> 53 µbar) [184]. Due to some technical features of the dry etching

system microwave generated plasmas are avoided in this pressure range. Hence, the following

(a)

20 40 60 80 1000

100200300400500600700800900

100011001200

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Etc

h ra

te [n

m/m

in]

Pressure [µbar]

vertical horizontal

Aspe

ct ra

tio

(b)

SiO2

Si

FIGURE 5.9:(a) Etch rate and aspect ratio vs. pressure (Bias voltage adjusted to 60 V, 100 sccm SF6)(b)Cross section of an isotropic etched silicon substrate at 100 µbar.

58 5 Technology

(a)

Pt

SiO2Si

(b)

Pt

SiSiO2

FIGURE 5.10:Pt surface and SiO2 cross section of a sample etched at (a) 100 µbar (bias voltage 60 V)and (b) 120 µbar (bias voltage 85 V).

experiments are conducted using the RF-mode.

The first result presented here, concentrates on the etch rate and the aspect ratio depending on

the pressure. At low pressures (20 µbar) the vertical etch rate is around 300 nm/min and the

horizontal etch rate 80 nm/min, which gives an aspect ratio of 3.5 and therefore an anisotropic

behavior (Figure 5.9 (a)). The vertical etch rate increases nearly by a factor of four, if the

pressure is at 100 µbar. In the same way the lateral etch rate is enhanced, but the increase is

not so strong, resulting in a lowered aspect ratio of 1.5. Figure 5.9 (b) shows a cross section

of an under-etched SiO2 membrane with a round profile and a silicon surface of good quality.

The SiO2 layer is usually covered by a stack of Pt/ferroelectric film/Pt. Long term etching is

neccessary to achieve the required undercuts up to 50 µm. Therefore, the experiment is repeated

with a Pt electrode on the SiO2 to investigate the resistance of Pt to the etch parameter. In

Figure 5.10 (a) one can see the result: No etching of the Pt layer is visible. This is confirmed by

experiments with etch times of several hours. Hence, the selectivity is very high and satisfactory

for device fabrication. Beyond, the initial thickness of the SiO2 layer is 450 nm and remains

TABLE 5.11: Parameter for isotropic Si etch processes

Pressure ECR RF Bias Gas Etch Selectivity Selectivitypower power voltage flow rate to Pt to resist

[µbar] [W] [W] [V] [sccm] [nm/min]Si 100 - 110 60 50 SF6 580 → ∞ 10

5.4 HF-Vapor Etching 59

after the process. For some reason, the vertical etch rate is reduced to 580 nm/min. To make

the process faster, the pressure and the RF power are slightly increased. A sample can be seen

in Figure 5.10 (b). Indeed, the etch rate increases but the Pt surface has become very rough

indicating physical impact. For this reason the process working at 100 µbar is found to be the

optimum. All parameters are summarized in Table 5.11.

5.4 HF-Vapor Etching

Additional to isotropic silicon surface micromachining and anisotropic silicon bulk microma-

chining a technique is developed to release free standing structures, which is based on a SiO2

sacrificial layer.

SiO2 is a well known material for sacrificial etching [136, 139, 185]. Common methods use

hydrofluoric acid (HF) to remove the SiO2 under the structure, but surface tension while drying

often causes permanent sticking of the microstructures.

Plasma etching can overcome this drawback, but another problem is observed. The lateral size

of a typical cantilever is in the range of hundreds of micrometers, but the sacrificial SiO2 is less

than 1 µm thick. Due to shielding effects, the plasma etch technique is not suitable to remove

the SiO2 from such a small horizontal slit.

Si

SiO2

gaseous: HF + H O2

(a)gaseous: HF + H O2

(b)

~~~~~~~~~~~~~~~~~~~~~~~~~ liquid: H O + HF2

Si

SiO2

gaseous: HF + H O2

(c)

~~~~~~~~~~~~~~~~~~~~~~~~~ liquid: H O + HF + H SiF2

Si

SiO2

SiF4

(d)

2 6 gaseous: HF + H O~~~~~~~~~~~~~~~~~~~~~~~~~

liquid: H O + HF + H SiF2

Si

SiF4

2 6

2

FIGURE 5.11:Chemical reaction of HF in vapor phase with SiO2.

60 5 Technology

The use of HF in the vapor phase has the potential to yield high etch rates, high lateral un-

deretching, a very good selectivity between Si and SiO2, and avoids stiction if properly ap-

plied [186, 187].

Etching in HF vapor phase follows the same chemical reactions than etching in the liquid phase:

SiO2 + 4HF −→ SiF4 + 2H2O (5.13)

or

SiO2 + 6HF −→ H2SiF6 + 2H2O (5.14)

A schematic of this process is shown in Figure 5.11. Initially, a SiO2 surface is exposed to a

vapor phase mixture of H2O and HF (Figure 5.11 (a)). If the partial pressures are sufficient

then a condensed film of HF and H2O will form and continue to grow on the SiO2 surface

(Figure 5.11 (b)). Etching proceeds with the formation of H2SiF6 and additional water (Fig-

ure 5.11 (c)). Although the partial pressure of SiF4 over diluted H2SiF6 : HF : H2O solutions

is relatively low, some SiF4 is formed during the etching process. When the etching is complete

and the HF and H2O reactants are switched off, the liquid H2SiF6 : HF : H2O film can be

evaporated and a released structure remains (Figure 5.11 (d)).

The experiments are performed with silicon-on-insulator substrates (SOI). The wafer consists

of a 625 µm thick handle silicon layer, a 1 µm thick buried oxide layer (BOX) and a 3 µm

thick device silicon layer. The device silicon, which later serves as substrate, is structured down

(a)

1 µm SiO2

3 µmDevice - Si

Si

Si

(b)

IR-heater

~~~~~~~~~~~~~~

~~~~~~~~~~~~

set

act.

40% HF (l)

~~~~~~~~ ~~~~~~~~

ultrasonic water quench

~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~

HF/H O (g)2

sample

EuroTherm

temperaturecontroller

thermo couple

temperaturemeasurement

FIGURE 5.12:(a) Sample for HF vapor experiments. (b) Schematic drawing of the experimental setup forHF vapor etching.

5.4 HF-Vapor Etching 61

thermo couple

hotplate

temperaturecontroller

ultrasonicwater quench

IR-heaterbox withliquid HF

and sample

FIGURE 5.13:Picture of the experimentalsetup for HF vapor etching.

to the BOX, which is therefore exposed to the HF-vapor (Figure 5.12(a)). The experimental

setup is shown in Figure 5.12(b). A box filled with HF (concentration of 40 %) is placed into

a temperature controlled ultrasonic waterbath while the sample is mounted into a teflon holder

upside down approximately 5 cm over the HF. The temperature of the sample is independently

controlled by a thermocouple, a control unit (EuroTherm) and an infrared radiator. Infrared

heating is chosen for safety reasons because the container has to be closed so that thermal

radiation avoids additional cable feedthroughs. Figure 5.13 shows a picture of the experimental

setup. The samples are placed on a hotplate after HF treatment to volatilize the remaining water

and HF on the sample.

Since a chemical reaction without plasma assistance is responsible for removing the SiO2, no

shielding effects limit the underetching. Nevertheless, attention has to be paid to adhesion

issues because liquid H2O and HF are formed during etching in the horizontal slit and leads to

stiction. Therefore, an equilibrium state between the necessary amount of water at the surface

for the reaction and avoidance of a liquid film between the device silicon and the silicon handle

layer needs to be established. Heating the water bath in a range between room temperature and

50 °C and therefore heating the HF controls the density of the HF vapor in the chamber. Thus,

the HF is transported to the SiO2 and forms volatile products. Thereby heating of the sample

establishes the equilibrium state and prevents adhesion due to the formation of a liquid film.

Figure 5.14 (a) shows the cross section of a sacrificial etch process. One can clearly distinguish

62 5 Technology

(a)

device Si

handle Si

SiO 2

(b)

device Si

handle Si

SiO2

FIGURE 5.14:(a) SEM picture of an under etched structure. (b) Magnification of the buried SiO2 layer.

between the handle Si, the device Si and the BOX layer. The device layer is structured with

an isotropic dry etching process before etcing in HF vapor for two hours, which results in an

undercut of 39 µm. Figure 5.14 (a) shows the magnification of the etch border. There are no

residues on the Si surface and no damage of the silicon is visible resulting in a very good surface

quality.

The achievable lateral etch rate is in the range between 4 and 12 µm/h, if the temperature

of the liquid HF is 50 °C and the sample temperature varies from 65 to 80 °C (square dots

in Figure 5.15). A decrease of the etch rate is observed for higher temperatures, because the

reaction speed is reduced due to a reduction of liquid HF and water in the slit. On the other

hand, experiments below 65 °C often exhibit stiction problems due to the generation of to much

65 70 75 800

5

10

15

20

25

30

Hor

izon

tal e

tch

rate

[µm

/h]

Sample temperature [°C]

HF 50 °C HF 50 °C + ultra sonic assisted

FIGURE 5.15:Etch rate vs. sample temperature(square dots: without ultra sonic as-sistance; round dots: with ultrasonicassistance).

5.5 Stress Compensation 63

liquid H2O and HF within the sacrificial slit. However, vapor generation assisted by ultra sonic

excitation results in a higher vapor density and the etch rate is doubled at the same sample

temperature (round dots in Figure 5.15). The maximum lateral etch rate is 20 µm/h resulting

in a very good surface quality, qualifying this technique to release movable structures from the

substrate.

5.5 Stress Compensation

While the deposition of thin films on top of a substrate mechanical stress is induced into the sys-

tem. This is a serious problem in the processing of MEMS, since it leads to an initial deflection

of fragile, freestanding structures like membranes and cantilevers. Thereby tensile stress leads

to a concave deflection and compressive stress results in a convex deflection of membranes.

Stress arises from different sources. Intrinsic stress Tint depends on parameters like layer thick-

ness or the microstructure. Extrinsic stress Text occurs during the processing e.g. the lattice

parameters of PZT changes with the temperature and therefore stress is induced during the

crystallization process. The third part is determined by different thermal expansion coefficients

and is call thermal stress Ttherm [23]. However, since the entire stress defines the deflection of

the structures a detailed investigation has been undertaken to determine the stress and achieve

an appropriate stress compensation.

Stress compensation is achieved by the final deposition of a material, which induces opposite

stress to balance the overall stress in the system. Low stress LPCVD-Si3N4 can be used for this

task. Alternative SiO2 also works for stress compensation and since it mostly severs as device

substrate it is part of the device anyway and one process step is saved.

0 30 60 90 120 150 180 210 2400

100

200

300

400

500

600

700

800

900

1000

Oxidation time [min]

Oxi

de th

ickn

ess

[nm

]

FIGURE 5.16:SiO2 thickness vs. oxidation time.Oxidation is undertaken in O2/H2

atmosphere at 1050 °C.

64 5 Technology

Because the internal stress depends on the layer thickness [188], investigations are performed

with different oxide thicknesses. SiO2 is deposited on Si in a oxygen and hydrogen containing

atmosphere at 1050 °C (wet oxidizing). Figure 5.16 gives the correlation between oxidation

time and SiO2 thickness. The Si reacts very fast with the oxygen at the beginning and a high

deposition rate is observed. The reaction slows down with increasing oxidation time and even-

tually a point of saturation is reached after 180 minutes resulting in a layer thickness of 980 nm.

A widely used method to determine stress within thin films is to measure the induced curvature

of the substrate before and after film deposition [189]. The stress is then calculated with the aid

of the Stoney formula [190, 191]

Tfilm =ESi t2Si

6 (1 − νSi) tfilm

(1

r− 1

r0

) (5.15)

where σfilm is the stress of the thin film, ESi is the Young’s modulus of the silicon substrate,

νSi the Poisson’s ratio of the silicon substrate, hSi the thickness of the wafer, hfilm the thickness

of the thin film, r0 denotes the initial substrate radius before film deposition and r is the radius

after film deposition, respectively.

Measurements of the substrate bow are undertaken with a DEKTAK surface profilometer on

various combinations of PZT film thickness and SiO2 thickness. Figure 5.17 and Figure 5.18

presents measurements taken on two different systems performed on one inch substrates. In Fig-

ure 5.17 the first measurement is performed after the deposition of 75 nm SiO2. The curvature

becomes a slight convex shape, which indicates that the deposition of SiO2 induces compressive

stress. The next measurement is undertaken after the deposition of the 110 nm thin platinum

bottom electrode with the TiO2 adhesion layer. The curvature of the substrate has strongly

changed from convex to concave and therefore high tensile stress is induced by the deposition

of Pt. Next, six coatings PZT (255 nm) are deposited resulting in a slight increase of the tensile

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

-1200

-1000

-800

-600

-400

-200

0

200

75 nm SiO2

110 nm Pt/TiO2

255 nm PZT 100 nm Pt

Def

lect

ion

[nm

]

Probe position [mm]

FIGURE 5.17:Curvature of the wafer af-ter different process steps:75 nm SiO2, 125 nm PZTand 100 nm top and bottomPt.

5.5 Stress Compensation 65

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12-200

0

200

400

600

800

1000

1200

1400

780 nm SiO2

110 nm Pt/TiO2

125 nm PZT 100 nm Pt

Def

lect

ion

[nm

]

Probe position [mm]

FIGURE 5.18:Curvature of the wafer af-ter different process steps:780 nm SiO2, 255 nm PZTand 100 nm top and bottomPt.

stress. It seems that the deposition of PZT on Pt do not affect the system very strong. The final

measurement, after Pt top electrode sputtering reveal again a strong increase of the concave

deflection to a maximum of 1150 nm.

The next system presented in Figure 5.18 takes into account that 75 nm SiO2 do not induce suf-

ficient compressive stress to balance the strong tensile stress mainly induced by the Pt layers.

Therefore the system is build up on a 780 nm SiO2 film turning out a maximum convex deflec-

tion of 1.3 µm, which equals a compressive stress of 342 MPa, if ESi is assumed to 131GPa, νSi

= 0,23 [136] and hSi = 675 µm. Sputtering of the bottom electrode results in a flattening of the

wafer deflection, which means that the tensile stress induced by the Pt reduces the compressive

stress of the SiO2. To calculate the stress in the entire system hfilm is formed by the addition of

the thickness of SiO2 and Pt. The overall stress is reduced to 164 MPa. Since PZT has only a

minor contribution to the stress this sample is processed with a 125 nm film reducing the overall

stress to 123 MPa. After the final deposition of the top electrode the deflection measurement

turns out a almost flat profile and the calculation reveal a remaining stress of 0.6 MPa. In other

words, wet oxidized SiO2 with a thickness of 780 nm is suitable to compensate tensile stress,

which is induced during the fabrication mainly by the deposition of Pt.

66 5 Technology

5.6 Electroplating of Freestanding Copper Contacts

The next process contributes the aim of the fabrication of integrated micro switches for mi-

crowave applications. In order to keep the ohmic losses low and therefore the microwave losses

small, the signal circuits and contacts needs to be fabricated from a very good conducting ma-

terial with a thickness in micrometer range to avoide the skin effect. Hence, copper is a good

choice, since it posses a high electric conductance (σ = 56 m Ω−1 mm−2) and can be electro-

plated with high deposition rates and a large thickness at low costs compared to other deposition

methods e.g. evaporation. Additionally, thick copper ensures mechanical stability, which is nec-

essary since the contacts are freestanding.

The fabrication process of such high frequency contacts is described in Figure 5.19. First, pho-

toresist is spread over the substrate and patterned defining the overlap of the contacts (Fig-

ure 5.19 (a)). Thereby a postbake is important to round the edges of the resist, so that a

(a)

Substrate

Resist

Substrate

Resist

Copper platingbase(b) Resist

(c) electoplated copper

(d)

(e) (f)

Substrate

Resist

Substrate

Resist

Substrate

Resist

Substrate

FIGURE 5.19:Fabrication scheme of freestanding copper contacts by electroplating.

5.6 Electroplating of Freestanding Copper Contacts 67

thin copper layer (100 nm), working as plating base and deposited by evaporation, covers the

whole wafer without intersection. A second photoresist is patterned on the plating base (Fig-

ure 5.19 (b)). Within the next step, copper is grown inside the photoresist trenches by electro-

plating in a bath of coppersulphatepentahydrate (CuSO4 · 5H2O), sulfuric acid (H2SO4) and

deionized water (Figure 5.19 (c)). The growth rate depends on the current density and is around

38 nm/min, if the current density is ≈ 2 mA/cm2. Low deposition rates support the growth of

stress free Cu with a good surface quality. After electroplating several micrometer of copper

the photoresist is removed using acetone (Figure 5.19 (d)). The contacts are still shorted by the

connecting platingbase and for removing it, the whole sample is exposed to an inert Ar plasma.

Thereby, the electroplated copper is also affected, but since the contacts are several micrometer

thick and only 100 nm needs to be removed this is an allowable process step (Figure 5.19 (e)).

In the final step, the photoresist underneath the electroplated copper needs to be removed.

Photoresist can be removed in oxygen and fluorine containing plasmas [192]. Therefore a

process with CF4 and O2 is developed (Table 5.12). To yield an acceptable lateral etch rate

for removing the photoresist under the bridge a pressure of 400 µbar is established. The low

coppercontact

underlyingmetal

silicon

silicon

FIGURE 5.20:Picture of electroplated copper contact on Pt base. The magnifications depict the end ofthe freestanding contact and the transmission from base to freestanding part respectively.

68 5 Technology

TABLE 5.12: Parameters for isotropic photoresist etch processes

Pressure ECR RF Bias Gas Etchpower power voltage flow rate

[µbar] [W] [W] [V] [sccm] [nm/min]Photoresist 400 - 15 25 60 O2 12 - 25

30 CF4

bias voltage prevents physical sputtering so that the underlying and surounding layer are not

affected when the resist is completely removed (Figure 5.19 (f)).

Figure 5.20 shows SEM pictures of an electroplated copper contact. The upper picture shows

the whole contact with the copper base attacht to a metalization on the silicon substrate (right

side) and the overhanging end (left side). Thereby the contact overlap a metallization on the

silicon. The magnification to the left side enlarges the end of the freestanding part. Clearly

to see is the separation between contact and metalization. The right magnification shows the

transition from the copper base to the overhanging part. One can see the rounded shape, due to

the rounded sacrificial photoresist.

69

6 Piezoelectric Cantilever Structures on Silicon Substrates

Piezoelectric cantilevers are composed of a sandwich structure with a piezoelectric layer

between two electrodes on top of a substrate (Figure 6.1). An electric field applied to the

electrodes leads to an enlargement of the film in direction of the electric field and a contraction

perpendicular to the applied field due to the converse piezoelectric effect. The enlargement par-

allel to the electric field is neglected since the film thickness is in the range of nanometers and

therefore the displacement is in the range of picometers. Here, more interesting is the piezo-

electric effect in horizontal direction. Indeed d31 is only around half of d33, but the effect works

in lateral direction along the cantilever dimensions and therefore the contraction is applied to

an large area in the range of several hundred micrometers, which induces bending in the system

since the piezoelectric film is clamped on top of the substrate. Therefore, the direction of the

movement is controlled by the sequence of substrate and actuated layer, and is upwards, if the

substrate is beneath the piezoelectric film.

6.1 Cantilevers Fabricated by Silicon Bulk Micromachining

This subsection reports on the development and processing of cantilevers as movable part in

integrated micro-relays. Basically the idea consists of a separation of the piezoelectric activation

and the signal circuit. A schematic drawing is shown in Figure 6.2 indicating the piezoelectric

thin film between two Pt electrodes on a Si/SiO2 substrate, the signal contact formed by a

metalization at the end of the beam and two overlapping contacts on the surrounding substrate.

substratepiezoelectric layer

d31

d33

P

electrodes

+ –

FIGURE 6.1:Working principle of piezoelectric cantilever due to clamping on the substrate.

70 6 Piezoelectric Cantilever Structures on Silicon Substrates

Pt electrodes

PZT

SiO2Si

metal

signalcontact

FIGURE 6.2:Concept of an integrated piezoelectric driven micro switch with separation of actuation andsignal line fabricated using silicon bulk micromachining.

The cantilever is released from the silicon substrate by a cavity etched from the back side. One

main advantage of this concept is the separation between the switch actuation and the signal

circuit which promise good contact properties, the prevention of crosstalk due to the electrically

isolation and offers the possibility for the use in RF-circuits since thick and highly conductive

metals like copper and gold can be used for the signal contact.

The process flow is illustrated in Figure 6.3 and is based on the technologies described in chap-

ter 5. The fabrication starts with the wet oxidation of a (100) oriented silicon wafer on front and

back side, followed by the deposition of 10 nm TiO2 as adhesion layer and 100 nm Pt on top

(Figure 6.3 (a)). The SiO2 on the backside is structured with a photoresist mask and ammonium

fluoride etching mixture. The 880 nm thick SiO2 layer will act as hard mask for the next step.

Here, the main part of the silicon is etched with 20 % TMAH. Usually this is the last step in

cantilever processing, whereas the SiO2 at the front side works as etch stop. It has proved that

this is a very sensitive step, since TMAH attacks also PZT and therefore a sample protection

mount is needed. This is very delicate, since leakage of the mount or cracking of the remaining

membrane due to internal or external stress damage the front side. Most important during this

step is a remaining membrane thickness of around 100 µm (Figure 6.3 (b)). This ensures the

mechanical stability for the ongoing processing, e.g. clamping on the vacuum chuck to spread

6.1 Cantilevers Fabricated by Silicon Bulk Micromachining 71

Pt/TiO2(a)

(b)

Si

Si

(c)

Si

(d)

Si

(e)

Si

(f)

Si

SiO2

PZT

FIGURE 6.3:Schematic drawing of the process flow of a cantilever beam using silicon bulk microma-chining and silicon surface micromachining (cross section).

photoresist over the wafer. Next, the top side is coated with 255 nm PZT by CSD. Double side

lithography is used to align the Pt top contacts to the cavities on the backside whereby the Pt

is structured by lift-off (Figure 6.3 (c)). The PZT film is protected by a photoresist mask and

then patterned by dry etching. Due to the etch selectivity between PZT and resist during this

process the mask has to be at least 4.3 µm thick (Figure 6.3 (d)). Figure 6.3 (e) clarifies the

first part of the release step. After etching the platinum layer around the structure and removing

of the surrounding silicon dioxide, a trench is etched 5.6 µm deep into the silicon. A second

silicon etch step is applied from the backside of the wafer to release the structure, by etching

the remaining membrane until the cavity from the backside reaches the trench at the front side

(Figure 6.3 (f)). If a high frequency suitable metal is needed at the perpendicular tip, it can be

deposited with one additional mask and process step after Pt top electrode structuring.

This method is very flexible for the fabrication of devices with different substrate thicknesses,

since the thickness is determined by the etch process and no further deposition step or additional

lithography mask is needed. Furthermore, cantilevers basing on SiO2 substrates are limited to

a substrate thickness of a few micrometers due to the limit of wet oxidation (compare Fig-


silicon window

base forrelay-contacts

150

50

FIGURE 6.4:Picture of a 478 µm longand 50 µm wide releasedcantilever.

ure 5.16). The fact is important, because the substrate affects strongly the cantilever properties.

Figure 6.4 shows a picture of a 475 µm long cantilever with a breadth of 50 µm. The

piezoelectric part is 350 µm long and the remaining 125 µm are consumed by the metalization

for the signal contact and the gap between the two electrodes. The dark area around the

structure is the shadow of the silicon window and the two structures on both sides are the bases

for the overlapping contacts. Internal stress is successfully avoided with this concept.

Quasi-static results

The cantilever performance is tested using a laser vibrometer (POLYTEC OFV 3001 vibrom-

eter controller, POLYTEC OFV 512 fiber interferometer) in combination with a function

generator (HP3314A) and a digital oscilloscope (TDS684).

Figure 6.5 (a) depicts a displacement measurement of a cantilever with a length of 450 µm.

The curves have the typical symmetric shape, when using a ferroelectric material. Either with

positive or negative applied voltages the deflection is positive, due to the change of the poling

direction. The measurement is taken with 10 V (10 Hz) and reveals a maximum deflection

of 5 µm. Figure 6.5 (b) shows the displacement results of three cantilevers with a length

of 300 µm and widths of 50 µm, 100 µm and 200 µm, respectively. The measured curves

are identical and the measurements exhibit a deflection of 2.5 µm. Therefore the deflection

depends strongly on the beam length, but is totally independent on the beam width.

The measurements are supported by FEM simulations using the program ANSYS (material pa-

rameters are given in Table 6.13). The cantilever geometry is reduced to a two dimensional


(a)

-10 -8 -6 -4 -2 0 2 4 6 8 10

0

1

2

3

4

5

Dis

plac

emen

t [µm

]

Voltage [V]

(b)

-10 -8 -6 -4 -2 0 2 4 6 8 10

0.0

0.5

1.0

1.5

2.0

2.5

3.0 width 200 µm width 100 µm width 50 µm

Dis

plac

emen

t [µm

]

Voltage [V]

FIGURE 6.5:(a) Displacement measurement of a cantilever (overall length 450 µm, width 200 µm,active PZT length 325 µm). (b) Displacement measurements of 300 µm long cantileverswith different width (10 Hz).

model, because the structure width has no impact on the result. However the length of the

piezoelectric inactive, perpendicular contact at the cantilever end needs to be taken into ac-

count. Therefore, the material density of those elements is triplicated. Furthermore, the model

is simplified by merging the silicon dioxide layer and the silicon layer to one layer and the

simulations take into account that the measurements are performed in the middle of the signal

contact. Figure 6.6 compares computed displacements with measured deflections and shows

that the calculations matches the measurements very well. The simulated displacements and

the measured deflections are almost identical for a 350 µm and a 450 µm long structure, which

are supported by 6.5 µm substrate. Simulations for different beam length show a continuous

increase of the deflection with increasing length. Furthermore, the impact of the substrate thick-

ness is investigated and shows that thicker substrates leads to a decrease of the deflection. At the

TABLE 6.13: Material parameter used for FEM simulations [24, 74, 90, 193].

ρ E ν e31 e33 e15

[g/cm3] [GPa] [C/m2] [C/m2] [C/m2]Si 2.33 131 0.23 - - -

SiO2 2.33 131 0.28 - - -Pt 21.45 173 0.39 - - -

PZT 7.5 95 0.3 -3.6 13.3 13.2


300 350 400 450 500 550 6000

5

10

15

20

25

30

35

40

452 µm

9 µm6.5µmD

ispl

acem

ent [

µm]

Length [µm]

FEM simulation measurement

FIGURE 6.6:Comparison of FEM simulationswith displacement measurementsfor different cantilever length byvariation of the substrate thickness(10 V, 100 nm Pt, 255 nm PZT).

other hand, 2 µm thin substrates and a beam length of 550 µm leads to a deflection of 40 µm.

Dynamic results

The dynamic properties of the cantilevers are investigated. Figure 6.7 (a) shows the displace-

ment as a function of the frequency of the applied field. The measurements are taken from a

350 µm long structure with a unipolar voltage between 0 V and 1 V. Each rounded symbol

depicts the maximum value of a complete displacement measurement at the certain frequency.

Since the applied voltage is only 1 V, the deflection is small, nevertheless at 46.3 kHz a large

resonance peak occurs. It becomes so huge that the measurement setup reach its limit and

therefore it is not possible to get the correct displacement value. However, a measurement error

can be excluded, because in resonance the shape of the displacement becomes elliptic due to the

phase shift of π/2. Thereby, the longer axis of the ellipse is in vertical position. In Figure 6.7 (b)

one can see a measurement at 45.2 kHz, just below resonance. The curve has already the shape

of an ellipse with its longer axis from bottom left to top right, which means that the phase shift

is smaller than π/2. A measurement taken slightly above resonance (Figure 6.7 (c)) shows an

ellipse with the longer axis from bottom right to top left, corresponding to a phase shift larger

than π/2. Furthermore, in Figure 6.7 (a) one can obtain a second peak, which appears at half of

the resonance frequency at 23.1 kHz.

These results are confirmed by FEM simulations of the eigenfrequencies (Figure 6.7 (d)). The

resonance frequencies increase steadily if the structures become shorter. A maximum frequency

of 65.8 kHz can be found for the 300 µm long cantilever and is going down to 20.6 kHz for

the 550 µm long structure, if a substrate thickness of 6.5 µm is simulated. Higher resonance

frequencies can be reached, if the substrate thickness is changed, e.g. a 300 µm long cantilever

with a substrate thickness of 9 µm has an eigenfrequency of around 100 kHz.


(a)

20 25 30 35 40 45 50 55

0

2

4

6

8

10

Dis

plac

emen

t [µm

]

Frequency [kHz]

(b)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Dis

plac

emen

t [µm

]

Voltage [V]

45263 Hz

(c)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Dis

plac

emen

t [µm

]

Voltage [V]

47368 Hz

(d)

300 350 400 450 500 550 6000

102030405060708090

100 9 µm

6.5 µm

Res

onan

t fre

quen

cy [k

Hz]

Length [µm]

FEM Simulation Measurement

2 µm

FIGURE 6.7:Various displacement measurements of a 350 µm long cantilever. The measurements aretaken with unipolar voltages between 0 V and 1 V. (a) Maximum displacement vs. fre-quency of the applied electric field. (b) Displacement measurement just below resonancefrequency. (c) Displacement measurement just above resonance frequency. (d) FEM simu-lations and frequency measurements vs. cantilever length depending on different substratethicknesses.

Mechanical results

Because of the large peak at the resonance frequency only 1 V is applied for the dynamic mea-

surements. If higher electric fields are used, the deflection becomes so large that the mechanical

stress in the material, especially in the silicon, exceeds the breakage stress and causes the de-

struction of the device. Simulations reveal that a breakage stress of around 300 to 350 MPa

corresponds to a displacement of 30 µm. Figure 6.8 shows a simulated stress distribution in the

area around the fixed support on the silicon. The maximum stress appears at the border between


FIGURE 6.8:Simulated stress distribution. Mag-nified area around the transitionfrom the silicon bulk to the can-tilever (length 350 µm).

the freestanding beam and the silicon wafer. Experiments prove that the cantilever is destroyed

exactly in this area, if the device operates in resonance frequency at 2 V, which corresponds to

a displacement larger than 30 µm.

Contact forces calculated with FEM show that a switch contact located in a distance of 3 µm

will be closed with a contact force of 31 µN, if the cantilever is 450 µm long and 100 µm

wide. This is in the range, which is reported with electrostatic switch concepts, but the applied

voltages are significantly reduced [40].

6.2 Cantilevers Fabricated by Silicon Surface Micromachining

The release of cantilevers by wet backside etching limits the integration density since the dis-

tance between the structures is determined by the thickness of the wafer and the etch angle

of the side wall. Structure release from the front side avoids this problem and waives on wet

chemicals, since dry etching with XeF2 [194] or isotropic plasma etching is applied [195].

The concept is similar to the concept proposed in the section before (Figure 6.9), where a T-

structure separates the contact actuation from the contact itself. However, no large hole is etched

into the wafer instead only the part of the silicon underneath the cantilever is removed with the

silicon dioxide serving as substrate for the Pt/PZT/Pt stack.

The fabrication sequence for this cantilever is sketched in Figure 6.10. The silicon is wet ox-

idized to a thickness of 780 nm, a 100 nm Pt bottom electrode with a TiO2 adhesion layer is

sputtered and a 255 nm PZT layer is deposited by CSD (Figure 6.10 (a)). In the next step, a

100 nm Pt top electrode is deposited and structured using lift-off technique (Figure 6.10 (b)).

Figure 6.10 (c) describes the dry etching patterning. The PZT around the top electrode is re-

moved, followed by the dry etching of the bottom electrode and the SiO2 in a window around

6.2 Cantilevers Fabricated by Silicon Surface Micromachining 77

SiO2

Si

electrodes

PZT

signal contactFIGURE 6.9:Concept of an integrated piezoelectrically driven micro relay fabricated by silicon surfacemicromachining.

the cantilever. Thereby, the photoresist has to withstand both etch processes. Finally, the release

is performed, which is done by an isotropic silicon etch process (see section 5.3.5). The silicon

under the cantilever is removed from both sides (Figure 6.10 (d)) and the remaining SiO2 serves

as substrate.

Figure 6.11 (a) shows a SEM picture which denotes the progress of underetching during the

process (this example does not include the T-structure for RF-switches). One can see four

cantilevers, 50 µm in width, still supported by concave shaped silicon. During the long term

process the Pt layer is also working as a hard mask to protect the silicon from the top. Photoresist

cannot be successfully used since the release process takes several hours dependent on the width

of the structures. Figure 6.11 (b) shows a set of cantilevers ranging from 50 to 1000 µm in

length and 50 µm in width. The deflection of the 1000 µm cantilever is due to a lack of stress

compensation, since this structures are fabricated on a 450 nm thick SiO2 substrate whereby the

whole stack is 900 nm thin [196].


Si

(a) SiO2 PZT (b) Pt

(c) (d)

FIGURE 6.10:Schematic drawing of the process flow of a cantilever beam using isotropic dry etchingtechniques.

However, the T-cantilever in Figure 6.12 (a) is supported from a 780 nm SiO2 substrate and

therefore the stress compensation is improved. The overall length of the structure is 625 µm

whereby the perpendicular contact bar is 100 µm wide, 300 µm long and the gap to the the

actuation part is 25 µm. Therefore, an active piezoelectric length of 500 µm contributes to

the actuation. Figure 6.12 (b) shows a displacement measurement, which exhibits a symmetric

behavior and a maximum deflection of 22 µm at 8 V. The displacement is larger than found for

(a)

Si Si support

top Pt/PZT

bottom Pt/SiO2

(b)

FIGURE 6.11:(a) Picture of cantilevers during the release process. (b) Released cantilevers with lengthup to 1000 µm and 50 µm in width.

6.3 Cantilevers Fabricated on Silicon On Insulator Substrates 79

(a) (b)

-10 -8 -6 -4 -2 0 2 4 6 8 10

0.0

5.0

10.0

15.0

20.0

25.0

Dis

plac

emen

t [µm

]

Voltage [V]

FIGURE 6.12:(a) Freestanding T-cantilever with a length of 625 µm and 100 µm width. (b) Displacementmeasurement.

the example in section 6.1, because here the structure is longer and the substrate is thinner.

6.3 Cantilevers Fabricated on Silicon On Insulator Substrates

Silicon on insulator (SOI) substrates consist of a silicon wafer (referred as handle silicon) with a

buried SiO2 layer (BOX) and a second Si layer on top (referred as device silicon). Therefore the

buried SiO2 can be used as a sacrificial material. The application of SOI substrates is a recent

approach for the fabrication of MEMS. Nevertheless, some studies are undertaken [197–199]

and several devices based on silicon are demonstrated e.g. optical silicon accelerometers [200],

a comb-drive actuator [201] and micromirror arrays [202].

The combination of SOI substrates with the integration of ferroelectric films enables new fabri-

cation methods and functionalities of MEMS.

The technique is demonstrated using a classic cantilever design corresponding to Figure 6.13,

where Figure 6.13 (a) displays the top view of the final device and Figure 6.13 (b) -(e) show

the cross view. The used SOI substrate has a 1 µm thick BOX layer and a 3 µm silicon device

layer, which is wet oxidized in a first step to enable adhesion between the bottom electrode

(TiO2/Pt) and the silicon. Next, the deposition of 255 nm PZT and the structuring of a 100 nm

Pt top electrode by lift-off follows (Figure 6.13 (b)). Within the next steps, the PZT, the bottom

electrode and the SiO2 layer are structured. Each of those dry etching steps require a unique

photoresist mask while over-etching into the device silicon is desired (Figure 6.13 (c)). Before

the release, the remaining silicon needs to be etched down to the BOX. Therefore, a photoresist


(a) (b)

(c)

(d) (e)

(b)-(e) (b)-(e)

PZTPt

SiO2

device Si

handle Si

resist

FIGURE 6.13:Cantilever processing by sacrificial technique (a) top view (b)-(e) cross section.

mask is applied, which will not be removed after this step (Figure 6.13 (d)) to protect the

cantilever, while releasing the structure from the silicon handle layer by removing the BOX

layer in HF vapor (Figure 6.13 (e)).

Figure 6.14 shows the cross section through a cantilever during the release process. One

can distinguish between the handle silicon, the device silicon and the BOX in the middle of

the cantilever, which is not fully removed at this time. On top of the device silicon the Pt

electrodes, the PZT layer and the photoresist are visible. The steps, visible at the left and right

side of the device silicon are due to the over-etching into the silicon with the SiO2 etch step

and the following trench etching down to the BOX, whereby the edges are not symmetric since

the final mask is not properly aligned. The protection by photoresist is sometimes insufficient

because sometimes the wet oxidized silicon is removed with the HF vapor etching. In the


FIGURE 6.14:SEM picture of a cross section dur-ing the HF vapor release step.

magnified area (upper right corner) no SiO2 is visible between the Pt/PZT/Pt stack and the

device silicon. It is assumed that HF ions diffuse through the resist during the long term vapor

etching, since the photoresist is not dense enough.

Cantilevers up to 400 µm in width have been demonstrated with this technique (Fig-

ure 6.15 (a)) and stiction is successfully avoided. The very long structures are slightly

predeflected due to small stress and a large ratio between cantilever length and thickness. Light

reflections are visible in the etch window, indicating a very high surface quality in terms of

roughness. This result is supported by Figure 6.15 (b) where the bottom side of a cantilever is

presented. The silicon is not attacked at all and no residual SiO2 is observed indicating a very

high selectivity between Si and SiO2.

(a)(b)

FIGURE 6.15:(a) A set of cantilevers 400 µm in width. (b) Bottom side of a fully released cantilever.


(a)

−8 −6 −4 −2 0 2 4 6 8

0

5

10

15

20

25

30

35Cantilever dimensions

800 µm x 50 µm

Dis

plac

emen

t [µm

]

Voltage [µm]

(b)

0 1 2 3 4 5 6 7 8 9

0

5

10

15

20

25

30

35

40 max. Displacement FEM Simulation

max

. Dis

plac

emen

t [µm

]

Voltage [V]

(c)

1 2 3 4 5 6 7 8

0102030405060708090

100110

max

. Dis

plac

emen

t [µm

]

Frequency [kHz]

(d)

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

0

10

20

30

40

50

60

70

80

90Cantilever dimensions

1600 µm x 160 µmD

ispl

acem

ent [

µm]

Voltage [V]

FIGURE 6.16:Measurements taken at a cantilever of 800 µm length and 50 µm width with a 255 nmPZT layer (a)-(c). (a) Displacement vs. voltage (measured at 100 Hz). (b) Maximumdisplacement vs. applied signal (100 Hz) compared to FEM simulations. (c) Maximumdisplacement measured at certain frequencies with 3 V alternating signal. (d) Displacementvs. voltage characteristic of a 1600 µm long cantilever at reduced driving voltage of 5 V(100 Hz).

Measurements are performed at cantilevers with different length and width. Figure 6.16 (a)

shows the displacement of an 800 µm long cantilever. The butterfly-shaped curve is slightly

shifted to positive voltages and reveals a maximum deflection of 31 µm at 8 V. The maximum

achievable displacement depends on the amplitude of the applied signal and shows a linear

increase. Furthermore, the measurements are in very good agreement with FEM simulations

(Figure 6.16 (b)). In Figure 6.16 (c) a plot of the maximum displacement at certain frequencies

is shown, while the applied signal is 3 V. The maximum deflection of 104 µm is measured at

the resonance frequency of 5 kHz, which is around five times more than measured at lower


(a)

0 200 400 600 800 1000 1200 1400 1600

0

20

40

60

80

100

120

140

6

7

8

9

10

11

12

13

max

. Dis

plac

emen

t [µm

]

Length [µm]

Con

tact

forc

e [µ

N]

(b)

0 200 400 600 800 1000 1200 1400 16000

102030405060708090

100110

Res

onan

ce fr

eque

ncy

[kH

z]

Length [µm]

FIGURE 6.17:(a) FEM simulation of maximum displacement and contact force in a distance of 10 µm, if8 V are applied to cantilevers in the range from 200 µm to 1600 µm and 100 µm in width.(b) Simulated resonance frequency dependent on cantilever length.

frequencies.

Figure 6.16 (d) shows the displacement of a 1600 µm long structure (160 µm wide, 3.4 µm

thick, 255 nm PZT). The driving voltage is reduced to 5 V, nevertheless the displacement in-

creases to 79 µm, due the large cantilever dimensions. A resonance is obtained at 1.5 kHz

and both measurement results are in very good agreement with simulations, which turn out a

maximum displacement of 85 µm and a resonance frequency of fr = 1.5 kHz).

Further simulations reveal the potential of this technique since large areas can be under-etched

and the processing of very long structures is possible, if an appropriate substrate thickness is

given. Figure 6.17 (a) shows calculations of the maximum displacement dependent on the

cantilever length if a driving voltage of 8 V is applied. A 200 µm long structure reveals a

displacement of only 1.8 µm, but a high resonance frequency of 114.5 kHz (Figure 6.17 (b)).

From the measurements it is known that the displacement in resonance increases by a factor of

five compared to lower frequencies, hence fast switching with a deflection of 9 µm is possible.

Increasing the cantilever length leads to a quadratically increase of the maximum deflection,

whereas the resonance frequency decreases. The maximum achievable deflection is calculated

to 137 µm for a cantilever with a length of 1600 µm .

Additionally calculations are performed on the contact force, if the contact is assumed to be

in a distance of 10 µm to achieve a satisfying open resistance [23]. In case of quasi-static

operation 600 µm long structures produce a force of 12.2 µN (shorter cantilevers do not reach

a deflection of 10 µm) and the contact force is reduced, if the cantilever becomes longer


(Figure 6.17 (a)).

Concluding chapter 6, three methods for integrating ferroelectric thin films into MEMS

are presented. Thereby, the main difference is defined by the release of the structures since this

is one challenging process, if PZT is used in combination with silicon micromachining.

The first process bases on silicon bulk micromachining and releases the cantilever from the

back side. Therefore, double side lithography is needed. The integration density is limited by

the thickness of the wafer since anisotropic wet etching using TMAH leads to hopper shaped

cavities. On the other hand, the mechanical properties can easily be changed with the same

set of lithography masks by varying the substrate thickness via adjusting the time of silicon

plasma etching and additionally this technique does not limit the cantilever width due to a limit

of silicon under-etching.

The second process uses isotropic dry etching to release the cantilever structures. The plasma

is applied from the front side and little space around the cantilever is needed for the release

process. Hence, the integration density is much higher. Characteristically to the process is

the use of a SiO2 substrate, whose thickness is adjusted to yield a proper mechanical stress

compensation.

The third release process focuses on SOI substrates. Thereby, the substrate is defined by the

thickness of the device silicon layer, while HF vapor is used to etch the buried oxide in order

to release the cantilever. This process posses the capability of a high integration density due

to the application of surface micromachining, gives a very good surface quality and allows the

fabrication of structures up to 400 µm in width. However, etching of the SiO2 layer between

the bottom electrode and the device silicon is still a problem and further investigations need

to be done on this task. Nevertheless, various cantilevers are fabricated and characterized.

Operation voltages below 10 V cause a deflection of 80 µm and resonance frequencies of

46 kHz are observed. A contact force of 12 µN is predicted, if the contact is in a distance of

10 µm.

85

7 Devices

This chapter presents the integration of ferroelectric thin films into a variety of MEMS.

Thereby, the devices takes advantage of the special material characteristics presented in

chapter 4 and are fabricated with the technologies described in chapter 5.

7.1 Integrated Piezoelectric Driven Micro-Relays

This section demonstrates a piezoelectric driven micro-relay with integrated PZT thin films.

The concept follows the proposed design described in chapter 6.2 and is enhanced in the way

that it is changed from a single side suspended cantilever to a double side suspended bridge

in order to ensure highly mechanical stability and sufficiently control of the mechanical stress.

The contact is formed by a perpendicular metalization and two overlapping copper bridges

(Figure 7.1). The device can be used in two ways: (i) The contact is closed and therefore a

micro-relay is realized. (ii) The configuration is used as variable air capacitor, whereby the total

capacitance is composed of two serial air capacitances and its value is changed by alternating

the distance h between the perpendicular contact and the copper contact. The total capacitance

C is

C(V ) =1

2ε0

A

h(V )(7.1)

where A describes the overlapping area between the copper contacts and the perpendicular met-

alization.

To ensure high integration density the micro-relay is designed using silicon surface microma-

chining to release the cantilever from the substrate.

Si

SiO2

Pt

copper contact

metalization

PZT

FIGURE 7.1:Bridge concept of a piezo-electric driven micro-relay.

86 7 Devices

(a)

Pt bottom layer

Pt top

PZT

(b)

SiO2

(c)

Si

(d)

copper bridge

FIGURE 7.2:Processing of the micro-relay: (a) Plasma etching of the Pt top electrode and the PZTlayer. (b) Plasma etching of the Pt bottom electrode. (c) Opening of the SiO2 window. (d)Electroplating of the copper bridges using sacrificial technique.

The processing starts with the deposition of 255 nm PZT by CSD on a Pt/TiO2/SiO2/Si sub-

strate followed by sputter deposition of a 100 nm Pt bottom electrode layer. Next, the top

electrode, the switch contact and the PZT are structured by dry etching (Figure 7.2 (a)) fol-

lowed by a new mask to texture the bottom electrode (Figure 7.2 (b)). The following mask

defines the areas where the SiO2 substrate is etched while the bridge will be released later on

(Figure 7.2 (c)). Before releasing the bridge structure, two overlapping copper contacts are

formed by electroplating. Thereby, the insertion of vias is required to enable the oxygen plasma

to remove the sacrificial photoresist under the whole contact. Finally, the bridge is released by

isotropic underetching using SF6 plasma (Figure 7.1).

Figure 7.3 shows the double side suspended cantilever. The SiO2 windows indicate the area

from where the bridge is under etched by dry etching to release the structure from the silicon.

The dimensions are of the same size than the cantilever itself and ensure the potential of a high

7.1 Integrated Piezoelectric Driven Micro-Relays 87

(a) (b)

SiO2

copper contactwith vias

Si

top electrode

relaycontact

FIGURE 7.3:(a) Photo of a double side suspended bridge with copper bridges. (b) Enlargement of thecentral part.

integration density. In Figure 7.3 (b) one can clearly see the electroplated copper contacts with

vias. The copper is around 3 µm thick, which gives a good mechanical stability and low ohmic

resistance. Also visible is the separation of the actuation part and the signal line perpendicular

in the middle. Piezoelectric bridges from 400 µm to 1400 µm length and a width of 40 µm have

been fabricated.

The fabricated bridge structures are characterized by laser vibrometer measurement, whereby

the laser spot is adjusted to the middle of the bridge and the device is driven with a frequency

of 10 Hz. To avoid displacement limitation by the copper contacts the characterization is done

(a)

-10 -8 -6 -4 -2 0 2 4 6 8 10 12

-9-8-7-6-5-4-3-2-10

Dis

plac

emen

t [µm

]

Voltage [V]

(b)

0 1 2 3 4 5 6 7 8 9 100123456789

1011

length 1000 µm; width 40 µm Laser measurement @ 10 Hz FEM simulation

max

. Dis

plac

emen

t [µm

]

Voltage [V]FIGURE 7.4:(a) Displacement vs. voltage measurement of a 1000 µm long bridge (40 µm width) mea-sured at 10 Hz and (b) maximum displacement at different voltages compared with FEMcalculation.

88 7 Devices

400 600 800 1000 1200 14000

2

4

6

8

10

12

14 9 V 8 V 7 V 6 V 5 V 4 V

max

. Dis

plac

emen

t [µm

]

Cantilever length [µm]

FIGURE 7.5:Maximum displacement of bridgeswith different lengths dependent onthe applied voltage (10 Hz).

without them. One of the measurements recorded for a 1000 µm long bridge is presented in

Figure 7.4 (a). The overall length of the freestanding part is 1000 µm, resulting in an actu-

ated length of 470 µm per side. The displacement vs. voltage characteristic shows the typical

butterfly loop related to PZT. Special attention has to be paid to the reversed shape of the mea-

surement. After processing, the bridges are slightly bent upwards due to mechanical stress. By

applying an electric field to the Pt electrodes, the PZT contracts on both sides so that the SiO2

substrate contracts and becomes shorter, resulting in a downward movement of the bridge. For

this reason, the displacement is negative, but since the total distance between the copper con-

tacts and the bridge determines the insulation characteristic, it is only important that the switch

is initially closed and opens if an electric field is applied. The 1000 µm long structure possess

a deflection of 8.5 µm if 10 V are applied.

Figure 7.4 (b) compares the maximum deflection of the bridge for different amplitudes deter-

mined by laser measurements and calculated by FEM. The plot depicts a very good agreement

in the low voltage regime whereas for voltages larger than 5 V a deviation of the measured

displacement from the calculated result occurs. The calculated deflection increases linearly

since the piezoelectric properties are assumed to be constant. In fact, the d33 decreases with

increasing electric fields (section 2.3) and therefore the deflection increases not linearly.

Figure 7.5 compares the deflection of bridges with different length and applied signals. As ex-

pected, extending the bridges results in an increase of the amplitude with a linear behavior. The

maximum is obtained for a 1400 µm long cantilever, which shows a displacement of 13.3 µm.

The dynamic properties are also investigated. Therefore, displacement measurements with

1.5 V oscillation level are accomplished in a wide frequency range. The maximum ampli-

tude of each measurement is plotted in Figure 7.6 (a) for a 800 µm long structure. A large

7.1 Integrated Piezoelectric Driven Micro-Relays 89

(a)

11 12 13 14 15 160.0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

Dis

plac

emen

t [µm

]

Driving frequency [kHz]

length 800 µmwitdh 40 µmoszillation level 1.5 V

(b)

0 1 2 3

0.0

0.5

1.0

1.5

2.0

2.5

Driv

ing

volta

ge [V

]

Time [ms]

Dis

plac

emen

t [µm

]

FIGURE 7.6:Frequency measurements taken with a 800 µm long bridge: (a) Displacement vs. electricalexcitation frequency measurement. (b) Step response, if unipolar voltage pulse with a pulsewidth of 1 ms are applied.

peak is obtained at 15.1 kHz, where the amplitude increases about a factor of six. However,

this is the electrical excitation frequency. The mechanical resonance frequency is two times the

electrical frequency because during one electrical period the bridge deflects two times in the

same direction. A second method to determine the eigenfrequencies of such structures is a step

response measurement (Figure 7.6 (b)). Thereby, voltage pulses with a pulse width of 1 ms are

applied to the structure and the oscillation of the bridge is independently recorded. Therefrom

the resonance frequency can be determined. Note, that these measurements are undertaken with

unipolar voltages to measure the eigenfrequency. A comparison of the two measurement meth-

ods with FEM calculations is presented in Figure 7.7, where the resonance frequency is plotted

vs. the bridge length. Measurements and simulation are in good agreement, giving a maximum

resonance frequency of more than 100 kHz for 400 µm long structures and an exponential decay

400 600 800 1000 1200 14000

102030405060708090

100110120

Frequency sweep measurement Step response measurement FEM Simulation

Res

onan

t fre

quen

cy [k

Hz]

Cantilever length [µm]

FIGURE 7.7:Resonance frequency depending onthe bridge length measured by fre-quency sweep and step responsemeasurement compared to FEMsimulations.

90 7 Devices

for longer structures down to 13 kHz if the structure is 1400 µm long.

7.2 Piezoelectric Driven Microwave Switches 91

7.2 Piezoelectric Driven Microwave Switches

The micro-relay described in the section before offers the possibility to realize an integrated

switch in a microwave circuit. The bridge deflection is large enough to by-pass a distance of

several micrometers and therefore to achieve a sufficient insulation between the copper contacts

and the bridge, if the relay is open. Furthermore, the application of surface micromachining

enables a high integration density and the possibility to place the micro-relay within unused

areas between microwave signal lines.

One commonly used microwave test design has the structure of a coplanar wave guide (CPW).

This is a planar design which exhibit low dispersion. Thereby, a signal line is flanked by paral-

lel ground planes on each side and the microwave propagates in the substrate as well as in the air.

The design is presented in Figure 7.8. At a first glance, one notices the three signal

lines on a Si/SiO2 substrate, whereby the tapering at the ends of the middle line and the

broadening of the two outer signal lines matches the requirements of the probe tips used for

microwave measurements. In the area between those signal lines, a micro-relay is placed on

each side. The relay design is changed in the way that the perpendicular contact of the bridge

signal lines

piezoelectric bridge

Si window

SiO2

Pt/PZT/Pt

bridgecontact

FIGURE 7.8:Concept of a MV-switch in coplanar waveguide configuration based on the loaded lineprinciple.

92 7 Devices

is turned by 90° to increase the integration density (compare to Figure 7.1). This goes to the

expense of the length of the piezoelectric active layer, if the overall bridge length remains

constant. However, the characterization in section 7.1 prooved that the deflection is still large

enough. The copper contacts are directly attached to the signal lines and not opposite to each

other. Instead, they are shifted to the side overlapping the whole bridge assuring a large contact

area and good contact properties.

In the state of open micro-relays, a microwave launched at one side of the structure will

propagate along the gaps between center and outer signal lines and at the other side released.

Thereby, ideally all energy is transmitted from one side to the other, if no ohmic losses occur

within the structure and the CPW matches adequately the requirements of the measurement

probe tips. If the piezoelectric bridge shortens the copper contacts, the switch is closed and

therefore the CPW transmission characteristics will change dramatically. In closed state, the

signal lines are shortened in the middle, which blocks the signal propagation and yields high

reflection. Thus, the microwave signal is highly attenuated. Such MEMS switches can be

employed in various microwave applications, e.g. filters and multiplexers.

Figure 7.9 displays the electric field distribution within the cavities under the piezoelectric

bridges, the silicon substrate, and the air in cross-sectional view. The electric field is simulated

at 25 GHz by using the software CST Microwave Studio. Due to some simulation constraints,

Si substrate

middle line

ground plane

Pt/PZT/Pt/SiO2cavity

FIGURE 7.9:Vector representation of the electric field distribution in the cross section of the CPW withintegrated piezoelectric bridges and air cavities.


the PZT film is modeled by a 2 µm thick dielectric layer with a permittivity of εr = 800 en-

closed between the Pt bottom and top electrodes. This model is close to reality, and additional

simulations prove that the thickness and permittivity of the PZT film has only minor impact on

the device performance. This is important, since a change of permittivity of the PZT comes

along with a DC electric field to be applied to the PZT film in order to activate the micro-

relay due to converse piezoelectric effect (compare Figure 4.3). The electric field is mainly

concentrated in the cavities between the middle line and the ground plane, where the vector

orientation appears almost parallel to the substrate surface. At the middle line and the ground

plane, the field is normally oriented to boundary surfaces of metalization. As seen in the figure,

the Pt/PZT/Pt/SiO2 stack does not significantly disturb the field distribution. No fields can

invade the PZT film, even if this film is very thick. This has two reasons: First, the boundary

conditions at the air/film boundaries do not allow the occurrence of high field amplitudes in

the dielectric film due to the enormous difference of the two dielectric constants (air and PZT).

Second, the electric field is shortened across the Pt top and bottom electrodes.

The fabrication of such a MW-switch is similar to the fabrication of the micro-relay. First, the

piezoelectric bridges are formed by the deposition of PZT on a Pt/SiO2/Si substrate and the

forming of Pt top electrodes. After the structuring of the PZT, the bottom Pt and the definition

of the Si window by etching the SiO2 in this area, the signal lines are formed by electroplating.

In a first step the structures of the signal lines are defined by lithography, which is followed

by the deposition of a copper plating base. In the second step the copper bridges are defined

using a second layer of photoresist and then thick copper is grown by electroplating (see section

Figure 5.19). After removal of the photoresist and the plating base the piezoelectric bridges are

released by isotropic underetching.

Figure 7.10 shows the top view of a fabricated CPW with two integrated micro-relays. This

example possesses a distance of 200 µm between the middle and the ground lines, whereby

the gap narrows at the end to 50 µm. The piezoelectric bridges are 60 µm in width and the

silicon window is 1400 µm long. The picture on the right hand side depicts a magnification

of the switch contacts. It shows the separated contact bar on the piezoelectric bridge and the

copper contacts with vias. Since one can see the contact bar through the vias it is ensured that

all sacrificial photoresist is removed.

The transmission characteristic of the device is shown in Figure 7.11 (a). If the switches are

open, the insertion loss is nearly 0 dB in the range from 0 to 35 GHz and the RF signal can

pass the structure without any significant loss, independent of the frequency. If the switches are

94 7 Devices

middle lineground plane

SiO substrate2

Si window

Cu contact bridgecontact bar

cantilever top electrode

cantilever bottom electrode

FIGURE 7.10:Top view of a MW design with two embedded cantilever bridges and magnification of thecenter part around the copper contacts.

closed and therefore the middle line and ground plane are shortened, the insertion loss is very

high at frequencies below 2 GHz. Above this frequency the insertion loss ranges from -35 dB to

-10 dB, which still provides highly attenuation of the passing microwave signal. Figure 7.11 (b)

shows the return loss of the structure. In opened state, the return loss is very high at low

frequencies and decreases with higher frequencies reaching a minimum at ≈ 20 GHz. If the

contacts are shortened the return loss is nearly 0 dB at low frequencies and increases slightly

with increasing frequencies. However, the difference between open and closed state is at least

10 dB.


(a)

0 5 10 15 20 25 30 35

-80

-70

-60

-50

-40

-30

-20

-10

0open

closed

Inse

rtion

loss

[dB

]

Frequency [GHz]

S21

(b)

0 5 10 15 20 25 30 35

-40

-35

-30

-25

-20

-15

-10

-5

0

open

closed

Ret

urn

loss

[dB

]

Frequency [GHz]

S11

FIGURE 7.11:(a) Transmission coefficient (S21) and (b) reflection coefficient (S11) of MW circuit withembedded 1400 µm long micro-switches (simulations).

Further miniaturization requires to shorten the length of the piezoelectric bridges. Therefore

the microwave characteristics are simulated for bridge length shortened down to 600 µm.

In this case the contact distance is still sufficient, since the deflection of the bridge is 4 µm

(compare Figure 7.5). For the reason of the abbreviate piezoelectric bridges the whole CPW

and the cavities become shorter. Figure 7.12 (a) presents the transmission characteristic of

the CPW in open configuration. The insertion loss is slightly improved, if the cavity size

is reduced to a length of 600 µm. Obviously, the air cavities cause some slight impedance

(a)

0 5 10 15 20 25 30 35

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

1400 µm

600 µm

Inse

rtion

loss

[dB

]

Frequency [GHz]

S21

(b)

0 5 10 15 20 25 30 35

-40

-35

-30

-25

-20

-15

-10

-5

0

600 µm

1400 µm

Ret

urn

loss

[dB

]

Frequency [GHz]

S11

FIGURE 7.12:Comparison of (a) transmission coefficient (S21) and (b) reflection coefficient (S11) of MWcircuits with micro-relay bridges of different length (600 µm, 1400 µm) in open configu-ration.

96 7 Devices

mismatch due to the change of the characteristic impedance of the CPW along the cavities.

At shorter cavity lengths, the influence on the transmission characteristic is reduced. The

reflection characteristics for open relay contacts are shown in Figure 7.12 (b). In the range up

to 20 GHz the return loss is improved by around 8 dB.

The transmission and reflection characteristics for the MW-structure with closed micro-relays

exhibit no significant difference between the employment of 1400 µm or 600 µm long piezo-

electric bridges. Hence, the reduction of the length of the micro-relays leads to a reduction of

device size and furthermore improves the microwave switching characteristics.

This section presented a novel concept of piezoelectric actuated MW-switches. Piezoelectric

activation leads to a significant reduction of the driving voltage compared to electrostatic

actuation. Thereby, good switching properties are verified by simulations showing a switching

level better than 20 dB for frequencies below 20 GHz.

7.3 Thin Film Bulk Acoustic Wave Resonators 97

7.3 Thin Film Bulk Acoustic Wave Resonators

Beside micro-relays, TFBARs have recently attracted the attention of research labs and industry

in the field of mobile communication.

In principle, a TFBAR consist of a piezoelectric film with a thickness of about λ/2, if λ being

the acoustic wave length in the piezoelectric material. An alternating electric field applied to the

piezoelectric film generates acoustic waves, which propagates within the material and excite the

fundamental thickness mode resonance. Besides good piezoelectric properties, a good acoustic

insulation is requested in order to keep the losses low. There are two principles: The first

one uses an acoustic Bragg reflector consisting of n-pairs of high and low acoustic impedance

layers of AlN and SiO2 [203]. The second principle bases on membrane structures to achieve

isolation from the substrate. The membranes are fabricated either by removing the silicon from

the backside [204] or by sacrificial etching [205].

A TFBAR can be calculated with the resonant spectrum method proposed by Zhang et al. [113].

Thereby the piezoelectric layer represents an electric impedance, which is terminated by two

acoustic impedances (Figure 7.13 (b)). One acoustic impedance is comprised by the top elec-

trode and the other by the combination of bottom electrode and substrate. The electric im-

pedance is denoted by

Zin =V

I=

1

jωC0

[1 − k2

t

γ

(z1 + z2) sin γ + j2 (1 − cos γ)

(z1 + z2) cos γ + j (1 + z1z2) sin γ

], (7.2)

where C0 = εs33

Al

is the static capacitance of the resonator. A is the area of the electrodes, εs33

(a)

RF

electrode piezoelectric film

(b)

Z

topelectrode

bottomelectrode

piezoelectricfilm substrate

Z

ZI

1 2

in

V

FIGURE 7.13:(a) Schematic drawing of a bulk acoustic resonance in a piezoelectric plate. (b) Networkof elements contributing to the impedance of a TFBAR.

98 7 Devices

2 3 4 5 6 7 8 9 10 11

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

200 nm

120 nm

160 nm

80 nm

40 nm

Cap

acita

nce

[pF]

Frequency [GHz]

FIGURE 7.14:Calculation of the resonance behav-ior of TFBARs using different BSTfilm thicknesses (capacity area 6 x 6µm, bias voltage 8 V).

and l are the permittivity and thickness of the piezoelectric layer; k2t is the electromechanical

coupling coefficient of the piezoelectric layer; γ = ωt/v is the phase delay of the longitudinal

acoustic wave with the sound velocity v of the piezoelectric layer and ω is the angular frequency.

Furthermore, z1 is the normalized acoustic impedance of the top electrode

z1 =Z1

Z0

(7.3)

and z2 is the normalized acoustic impedance of the bottom electrode and the substrate

z2 =Z2

Z0

, (7.4)

with the acoustic impedance Z0 = Aρv of the piezoelectric material incorporating the specific

material density ρ. The acoustic impedance of the top electrode is denoted by

Z1 = jZe1 tan γe1 with Ze1 = Aρe1ve1 and γe1 =ωte1ve1

, (7.5)

and the combined acoustic impedance of bottom electrode and substrate by

Z2 = jZsb tan γsb + Ze2 tan γe2

1 − (Zsb/Ze2) tan γe2 tan γsb, (7.6)

thereby

Ze2 = Aρe2ve2 and Zsb = Aρsbvsb (7.7)

denotes the acoustic resistance and

γe2 =ωle2ve2

and γsb =ωtsbvsb

(7.8)

the phase shift.

Calculations of TFBARs based on the electrostrictive behavior of BST are presented in Fig-

ure 7.14. Thereby the permittivity corresponds to an applied bias voltage of 8 V, due to the


dependence of the dielectric and electrostrictive properties on the applied electric field (Fig-

ure 4.14 and [206]). The simulations are performed on thin films with different thicknesses and

turn out a strong variations of the resonance frequency. The use of 40 nm thin films leads to a

distinctive resonance peak at 7.7 GHz. With increasing film thickness the resonance frequency

decreases down to 4.2 GHz, if the film is 200 nm thick. Besides a decrease of the resonance

frequency the resonance peak becomes smaller and further the capacitance becomes smaller.

For the characterization of high frequency structures a device design is needed, which matches

the requirements of the signal analyzer. In this work the concept of a CPW in combination

with a sacrificial technique is chosen to achieve a satisfactorily acoustic isolation. A schematic

drawing of the design is shown in Figure 7.15, whereby the distance between top- and bottom

electrode (50 µm) is determined by the microwave probe tips. The capacitor is formed by

overlapping the top and bottom electrode with the piezoelectric material in between. Since

it is essential that the impedance of the device matches the impedance of the analyzer and

ferroelectric materials exhibit a large permittivity the dimensions of the capacitors have to be

small and decreases down to 2 x 2 µm. Hence, to avoid shorting of the electrodes due to

alignment errors (the maximum resolution of the available optical lithography is ≈ 1 µm),

the area of the piezoelectric material exceeds the area of the bottom electrode and the total

capacitance is defined by the size of the top electrode.

The fabrication starts on a 450 nm wet oxidized silicon wafer with a 100 nm thin platinum

layer. In a first step, the platinum is structured by dry etching to form the bottom electrode

and then covered with 80 nm BST(70/30) by CSD. After the deposition, the BST is patterned

with dry etching and the Pt top electrode is structured by lift-off to form the capacitor. Finally

bottom electrode

top electrode

piezoelectricmaterial

SiO

window forunderetching

2FIGURE 7.15:Schematic drawing of TFBAR de-sign using membrane technologyfor acoustic isolation.

100 7 Devices

bottom electrode

topelectrode

BST

windows for underetching

SiO2

FIGURE 7.16:Picture of TFBAR device using BST as active material.

the membrane needs to be fabricated by underetching. Therefore photoresist is spread over

the wafer and two windows nearby the capacitor are formed. The HF-vapor process is applied

to remove the SiO2 underneath the bottom electrode, so that the PT/BST/PT stack is fully

released, forming a membrane without any substrate. Photoresist is working for protection

since the area to be under-etched is very small and therefore the processing takes only a couple

of minutes.

Figure 7.16 shows a picture of a TFBAR structure. It is taken before the final release

step and shows the CPW concept with a center line (top electrode) and the surrounding ground

line (bottom electrode). On the right hand side, the magnification of the parallel plate capacitor

structure shows the bottom electrode, covered with BST and the overlapping top electrode (20

x 20 µm), whereby this stack is flanked by windows for the underetching.

4.04.0 4.5 5.05.0 5.5 6.06.0 6.5 7.07.0 7.5 8.08.0 8.5 9.09.00.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

Frequency [GHz]

on Si/SiO2 substrate without substrate

6.2 GHz

Cap

acita

nce

[pF]

FIGURE 7.17:Measurement of BST based TFBAR(capacity area 6 x 6 µm, bias volt-age 8 V).


(a)

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.00.25

0.30

0.35

0.40

0.45

0.50

measurement

simulation

Cap

acita

nce

[pF]

Frequency [GHz]

(b)

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

0.2

0.3

0.4

0.5

0.6

0.7

measurement

simulation

Cap

acita

nce

[pF]

Frequency [GHz]

FIGURE 7.18:Comparison of measurements with the simulation of a 6 x 6 µm small TFBAR structure.(a) On Si/SiO2 substrate (b) Acoustically isolated (bias voltage 8 V).

Figure 7.17 (a) shows two capacitance measurements taken on a 6 x 6 µm small structure with

an applied bias voltage of 8 V in order to activate the electrostrictive behavior of the BST. The

first curve is taken without acoustic isolation on the Si/SiO2 substrate and shows a resonance

around 6 GHz, whereby the resonance peak is distributed over the range from 5 GHz to 7 GHz.

The second measurement is performed on an under-etched structure and exhibits a large distinc-

tive resonance peak at 6.2 GHz. The bandwidth is much smaller due to the enhanced acoustic

insulation.

Calculations performed for the same capacitor configuration are in good agreement with the

measurements. Figure 7.18 (a) compares the measurement with the calculation of a TFBAR

structure with underlying substrate, if a bias voltage of 8 V is applied. The simulation shows

slightly higher capacity values, but the resonance peak appears at the same frequency and is

of the same broadened shape. Figure 7.18 (b) shows the comparison of calculation and mea-

surement, if the structure is acoustically isolated. The simulation shows exactly the measured

capacity and the resonance peak appears at the same frequency and is of the same narrow shape.

Since TFABRs using BST base on electrostriction and furthermore the permittivity of BST

is strongly dependent on the applied electric field, the performance of a TFBAR can be

additionally alternated by the bias voltage. Figure 7.19 summarizes TFBAR measurements

performed with bias voltages between 0 V and 10 V. Without an applied electric field BST

exhibits the highest permittivity, but on the other hand the electrostrictive effect is low and

therefore no resonance caused by acoustic waves is visible. If a bias voltage of 2 V is applied

102 7 Devices

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

bias

vol

tage

10 V

8 V6 V

4 V

2 V

0 V

Cap

acita

nce

[pF]

Frequency [GHz]

FIGURE 7.19:Measurements indicating the influ-ence of the bias voltage on the TF-BAR performance (capacity area 6 x6 µm).

to the structure the capacitance decreases, but also a resonance starts to form at 6.2 GHz. By

further increase of the bias voltage, the capacitance decreases, whereby the capacitance change

becomes continuously smaller with higher voltages. Additionally the resonance peak becomes

more and more pronounced, but appears still at the same frequency. This behavior enables one

additionally degree of freedom in RF circuits. The total capacity can be changed whereby the

resonance remains constant.

In conclusion, the integration of BST thin films in surface micromachined TFBAR is

successfully demonstrated. Through the application of the HF vapor etch technique it is

possible to remove the substrate completely and therefore to achieve an acoustically full

isolated membrane. Thereby the working principle takes advantage of the electrostrictive

behavior of BST and the measurements are in good agreement with the performed simulations.

7.4 Dielectric Bolometers for Infrared Detection 103

7.4 Dielectric Bolometers for Infrared Detection

The devices presented before utilizes the piezoelectric effect. Another very interesting effect

exhibited by ferroelectric materials is their strong permittivity dependence on the temperature.

This effect can be used to detect infrared (IR) radiation and therefore to fabricate infrared imag-

ing sensors.

This section presents the fabrication and characterization of a ferroelectric thin film based inte-

grated dielectric bolometer working at room temperature.

Figure 7.20 shows the schematic of an one pixel dielectric bolometer. Two ferroelectric parallel

plate capacitors are fabricated whereby a top electrode connect them. One capacitor is placed

on a membrane and the other one on the silicon bulk.

The two bottom electrodes are connected to an alternating signal of ± 2.5 V with a phase shift

of 180° (Figure 7.21). The two capacitors Cf and Cref are identical at the same temperature

and therefore by applying the alternating signal to the bottom electrodes the voltage at the inter-

section node equals zero. If the temperature changes due to infrared radiation, the permittivity

of the ferroelectric film on the membrane changes immediately because the membrane is ther-

mally insulated and therefore the heat cannot dissipate. On the other hand, the film on the bulk

is radiated with the same intensity of IR radiation, but the generated heat dissipates into the

silicon and the temperature of the film remains constant. Therefore the two capacitors have a

different capacitance, which leads to an electrical potential V = 0 at the intersection node. The

potential is amplified by a lock-in amplifier and displayed on an oscilloscope.

In the dielectric operation mode, materials exhibiting a permittivity change due to temperature

changes around room temperature are desired and therefore, based on the material characteri-

zation in chapter 4, BST seems to be a good candidate [207, 208].

The fabrication starts with a silicon wafer, which is covered with 450 nm SiO2 on both sides

and has a 50 nm Pt layer on the front side. In a first step, the bottom electrode is structured by

Dielectriclayer

Si - bulkSiO2

membrane

CfCref

Top electrode+ 2.5 V20 kHz

- 2.5 V20 kHz

FIGURE 7.20:Schematic drawing of a one pixel di-electric bolometer.

104 7 Devices

Lock-in Oscilloscope

+ 2.5 V20 kHz

- 2.5 V20 kHz

Chopper

IR-s

ourc

e

Cf on membrane

Cref on bulk

FIGURE 7.21:Measurement principle of a dielectric bolometer.

dry etching (Figure 7.22 (a)). In the second step, the first part of the membrane is etched using

double side photolithography and dry etching of the SiO2 hardmask on the backside followed by

a silicon wet etching step using TMAH. Thereby, for further processing a remaining membrane

thickness of around 100 µm is necessary (Figure 7.22 (b)). On the so prepared substrate a BST

thin film is deposited by CSD. Special attention is paid to a sufficient covering of the bottom

electrode flange to assure a conformal deposition. Furthermore, the top electrode is deposited

by lift-off, generating two parallel plate capacitors by the overlapping area and a contact pad

Si - bulk

Pt electrodes(a) (b)

Dielectriclayer

Top electrode(c)

SiO2membrane

(d)

FIGURE 7.22:Fabrication flow of a dielectric thin film bolometer.


(a)(b)

Contact pad

Capacitor on membrane

Capacitoron bulk

Bottom electrode covered with BST

FIGURE 7.23:(a) Picture of a sample with different bolometer structures. (b) Magnified view to theparallel plate capacitors on bulk and membrane.

in some distance to the membrane. This ensures mechanical stability during measurement with

probe needles (Figure 7.22 (c)). Since now the bottom electrode is covered by the ferroelectric

film the layer is removed in the area of the bottom contacts either by dry or wet etching using

photoresist as a mask. In the final step the remaining silicon membrane is etched from the

backside by dry etching (Figure 7.22 (d)). The processed bolometers use a 100 nm BST(70/30)

thin film with Pt top and bottom electrodes.

Figure 7.23 (a) shows a picture of a sample with various bolometer structures. A complete

structure has the form of a "T" with contact pads at each end. In Figure 7.23 (b) the area

around the parallel plate capacitors is magnified. The length of the capacitor boarders ranges

from 10 µm to 600 µm. The membrane is not visible from the front side but the dotted square

signifies the area.

For characterization of the bolometers a measurement setup is developed (Figure 7.24). The

picture shows the open measurement setup with a sample placed in the middle of a hotplate

and contacted with three probe needles. To reduce the noise arising from the wiring between

probe needle and lock-in amplifier an additional operational amplifier is inserted directly at

the probe needle. The hotplate is used to simulate different ambient temperatures besides room

temperature. In closed state the sample is radiated from the IR-radiator trough an aperture in the

lid. Thereby, the distance of the IR source is variable to perform measurements with a variable

radiation power density. The aperture is periodically opened and shutted with a chopper in the

range from 1 Hz to 100 Hz.

Figure 7.25 (a) shows one measurement result. The chopper opens and closes the aperture

106 7 Devices

IR source

Chopper

Sample

Hotplate

Aperture

FIGURE 7.24:Picture of theopen bolometermeasurementsetup with IR-radiator, chopperand sample ona hotplate. Inclosed state theIR-radiator andchopper areplaced directlyover the bolome-ter sample.

periodically, whereby the chopper position is recorded by a photo sensor. When the chopper

opens the aperture, the temperature of the sample surface rises due to IR radiation and the

permittivity of the BST film decreases, which causes a capacitance change of the capacitor on

the membrane and therefore a potential arises at the bolometer top contact. After rising, the

output signal remains constant for the same period as the chopper signal opens the aperture and

the capacitor on the membrane is heated to equilibrium state. When the IR radiation is shutted

the output signal decreases immediately. The detected output signal is within the range of mV,

which is assumed to be sufficient for signal processing. In Figure 7.25 (b) two measurements

are taken before and after the final membrane etching step from the back side. The bolometer

(a)

0.0 0.5 1.0 1.5

0

10

20

30

40

50

60

70

Cho

pper

sig

nal

Time [ms]

Out

put s

igna

l [m

V]

(b)

0 10 20 30 40 50 60 70 80 90 100

0

20

40

60

80

100thin membrane

thickmembraneSi

gnal

[mV]

Time [ms]

BST 70/30

FIGURE 7.25:(a) Chopper signal and response of bolometer structure. (b) Two measurements performedon the same bolometer structure before and after final membrane etching.


(a)

2 3 4 5 6 7 8

1.5

2.0

2.5

3.0

3.5

4.0

Out

put s

igna

l [m

V]

Radiated power density [mW/cm2]

(b)

0.0 0.1 0.2 0.3 0.4 0.5

-4

-2

0

2

4

Ir source Room lighting Daylight

Out

put s

igna

l [m

V]

Time [s]

FIGURE 7.26:(a) Bolometer signal vs. IR power density. (b) Output signal dependent on different IR-radiation: With IR radiator, with room light and with daylight. (Measurements taken with-out an additional operational amplifier.)

with a membrane thickness of around 100 µm already shows an output signal. It shows a steep

rising slope, but the falling slope is less steep due to the large thermal capacitance of the silicon

substrate. A second measurement performed at a bolometer structure with a thinner membrane

shows a signal whose amplitude is doubled. Compared to the structure on the thick membrane

the rising and falling slopes are more steep.

To characterize the sensibility of the device the distance between the bolometer and the IR ra-

diator is continuously increased. (The measurement is taken without the additional operational

amplifier close to the contact needle). From the technical data of the IR source one knows that

the radiated power at room temperature is P = 2.76 W. If spherical radiation is assumed, the

power density L in a distance r can be calculated by:

L =P

4πr2. (7.9)

Figure 7.26 (a) presents the output signal depending on the radiated power density. First, the

output signal increases continuously with an increase of the power density, but at a certain value

the output signal remains constant. From this is concluded that the capacitor on the membrane

is in an equilibrium state. Figure 7.26 (b) compares three measurements taken at different

conditions. The first one is performed with the IR source in a distance of 5.3 cm generating

a distinctive signal. The second measurement is done without any IR source, but only the

radiation from the room lightning passes the aperture, if the chopper opens. The signal is much

smaller but still detectable. The third measurement is undertaken only with the daylight in

108 7 Devices

0 20 40 60 80 10030

40

50

60

70

80

90

100

Out

put s

igna

l [m

V]

Chopper frequency [Hz]

FIGURE 7.27:Output signal vs. chopper fre-quency.

the laboratory and although the signal has decreased it is clearly detectable. This proves that

the bolometer exhibits a high sensitivity and can be used for infrared imaging, if the fact is

considered that the measurement is taken without the additional amplifier.

The chopper frequency is also an important property for infrared imaging is. Image processing

requires a certain number of pictures per second and therefore the bolometer signal is investi-

gated with regard to the chopper frequency. Figure 7.27 shows a linear decrease of the output

signal, if the period of radiation and non radiation becomes shorter. However, at a frequency of

100 Hz the signal is still 35 mV and therefore detectable. The final characterizations deals with

the dependence of the bolometer signal on the ambient temperature (Figure 7.28). The result

shows that an increased ambient temperature leads to a continuous loss of the signal amplitude

in the range up to 120 °C, but there is still a detectable signal, even without the operational

amplifier.

0.0 0.1 0.2 0.3 0.4 0.5

−6

−4

−2

0

2

4

6

Out

put s

igna

l [m

V]

Time [s]

40 °C 60 °C100 °C120 °C FIGURE 7.28:

Bolometer signal depen-dent on the ambient tem-perature.


Concluding this section, dielectric bolometers have been successfully fabricated by sur-

face and silicon bulk micromachining using BST thin films. The characterization turns out

detectable signals even if the bolometer is radiated only with daylight and the devices work at

least up to a frequency of 100 Hz. Therefore dielectric bolometers based on BST(70/30) are

suitable for the use in infrared imaging.

110 7 Devices

7.5 Piezoelectric Cantilevers on Metal Foil Substrates

This section describes the deposition of PZT thin films on flexible metal foils in order to benefit

from the piezoelectric properties in a wide range of macro devices by maintaining low operation

voltages.

The experiments in the following section concentrate on stainless steel foils with a thickness

of 50 µm, whereby different bottom electrode configurations are investigated: Deposition of

PZT directly on stainless steel, deposition of PZT on platinum coated stainless steel to provide

a similar surface to standard deposition on silicon substrates and finally the deposition of PZT

on stainless steel with a lanthanum nickel oxide (LNO) diffusion barrier [209, 210]. The PZT

precursor solution and the deposition procedure is the same as described in section 3.2.1. Since

the focus is on piezoelectric devices, PZT(45/55) is chosen for this task.

Figure 7.29 (a) shows the top view of a 500 nm PZT film on Pt coated stainless steel and reveals

a dense film with a grain size between 50 - 100 nm. XRD measurements reveal the crystal-

lographic orientation of PZT on different bottom electrode configurations (Figure 7.30 (b)).

Films prepared directly on stainless steel exhibit a random orientation with amounts of (100),

(101) and (111) orientation. PZT deposited on a LNO layer is less oriented and exhibits only

(100) and (101) orientation, whereas films deposited on Pt covered foils show (100) and (101)

orientation, but also a strong (111) orientation, which is adapted from the Pt layer.

Polarization measurements up to E = 400 kV/cm on these films are presented in Figure 7.31.

The film directly deposited on the foil shows a hysteresis curve with a remanent polarization

of 33 µC/cm2, which is more than on Pt/Si substrates with PZT(45/55), where usually values

around 26 µC/cm2 are achieved. One possible explanation is a reduced stress due to clamping

FIGURE 7.29:Top view of a 500 nm PZT film onstainless steel with a Pt layer.

7.5 Piezoelectric Cantilevers on Metal Foil Substrates 111

20 25 30 35 40 45

PZT(101)

500 nm PZT on: stainless steel with Pt stainless steel with LaNiO

3 stainless steel

Inte

nsity

[a.u

.]

θ-2θ [deg.]

PZT(100)

PZT(111) Pt(111)

FIGURE 7.30:XRD measurements of PZTfilms (500 nm) on metal foilswith various bottom elec-trode configurations (directlyon stainless steel, stainlesssteel with an LaNiO3 barrier,stainless steel with platinum).

of the film since the metal foils are more flexible than usually used silicon substrates. The us-

age of an LNO diffusion barrier leads also to a Pr = 33 C/cm2, but the hysteresis behavior is

less pronounced and the curve is not closed in the high voltage regime, instead the polarization

decreases. This indicates high electric losses. A further advanced result is achieved with Pt

coated stainless steel foils. Indeed the remanent polarization shows no further increase, but the

coercive field decreases by one half to 4 V (80 kV/cm), similar to the coercive field obtained

on Pt/Si substrates. Therefore, this hysteresis curve shows a steep drop around the coercive

field, an increasing polarization in the high voltage regime and a closed behavior in saturation.

Furthermore, the polarization values are higher than on LNO and PZT directly deposited on

stainless steel. The results of the experiments suggest that PZT films with 12 or more coatings

-25 -20 -15 -10 -5 0 5 10 15 20 25-60

-40

-20

0

20

40

60

Pol

ariz

atio

n [µ

C/c

m2 ]

Voltage [V]

500 nm PZT on: stainless steel stainless steel with LaNiO3 stainless steel with Pt

FIGURE 7.31:Polarization measurementstaken at 500 nm PZT filmsdeposited on stainless steelwith various bottom electrodeconfigurations.

112 7 Devices

-25 -20 -15 -10 -5 0 5 10 15 20 25

-50-40-30-20-10

01020304050

Pol

ariz

atio

n [µ

C/c

m2 ]

Voltage [V]

FIGURE 7.32:Polarization of a 750 nm thickPZT film deposited on 200 nmPt on top of polished stainlesssteel.

on Pt coated stainless steel are a good candidates for the fabrication of piezoelectric devices

using metal foils.

However, due to the relatively thick substrate of 50 µm and leakage currents due to the sur-

face roughness, thicker films are desired to increase the piezoelectric force and to reduce the

electrically losses. Figure 7.32 shows the polarization measurement performed on a sample

with 750 nm PZT deposited on polished stainless steel covered with a 200 nm Pt electrode.

The hysteresis curve is symmetric and reveals an improvement of the remanent polarization to

38 µC/cm2.

After material characterization piezoelectric cantilevers are fabricated using a method which

goes without complicated and cost extensive technologies. A one inch piece of metal foil is

polished and then sputtered with a Pt electrode. Afterwards 750 nm PZT are deposited by CSD

Cantilevers

Probeneedles

FIGURE 7.33:Picture of two cantilevers contactedwith probe needles in the laser mea-surement setup.


(a)

-30 -20 -10 0 10 20 30

0

2

4

6

8

10

Dis

plac

emen

t [µm

]

Voltage [V]

(b)

0 2000 4000 6000 8000

0

2

4

6

8

10

1230 V

20 V

10 V

Dis

plac

emen

t [µm

]

Length [µm]

(c)

500 600 700 800 90005

10152025303540455055

Dis

plac

emen

t [µm

]

Frequency [Hz]

(d)

0 2000 4000 6000 80000

1000

2000

3000

4000

5000R

eson

ance

freq

uenc

y [H

z]

Length [µm]

FIGURE 7.34:Measurements of cantilevers coated with 750 nm PZT on top of a Pt coated 50 µm thickstainless steel foil (all cantilevers are 500 µm in width). (a) Displacement vs. voltagemeasurement of an 8000 µm long structure. (b) Displacement vs. cantilever length atdifferent driving voltages. (c) Displacement vs. frequency sweep of a 6000 µm longstructure. (d) Resonance frequency vs. cantilever length.

and finally a number of Pt strips serving as top contact are deposited by lift-off technology.

The length of those top electrodes is between 600 to 8000 µm and the width between 100 to

500 µm. Finally, the foil is cutted with a wafer saw into cantilevers. No further micromachining

is necessary. Figure 7.33 shows a picture with two of those cantilevers. For displacement

measurements the structures are mounted in the laser measurement setup and contacted with

probe needles.

Measurements of the cantilevers reveal the typical butterfly shaped displacement curves, if PZT

is used for the piezoelectric activation (Figure 7.34 (a)). Measurements with different ampli-

114 7 Devices

(a)

0 2000 4000 6000 8000

0

5

10

15

20

25

30

35

3x

stainless steel substrate 50 µm 35 µm 10 V 20 V 30 V

Dis

plac

emen

t [µm

]

Length [µm]

(b)

2000 3000 4000 5000 6000 7000 80000

500

1000

1500

2000 50 µm stainless steel 35 µm stainless steel

Res

onan

ce fr

eque

ncy

[Hz]

Length [µm]

FIGURE 7.35:Influence of the substrate thickness on cantilevers with various length. (a) Displacementvs. cantilever length (b) Resonance frequency vs. cantilever length.

tudes are undertaken and show that an 8000 µm long structure exhibits a maximum displace-

ment of 10 µm. Note, here the substrate is 50 µm and the PZT is clearly below 1 µm thick,

so that the ratio between the active layer and the substrate is very large. Further displacement

measurements are performed on structures with different length and the maximum deflection

at 10 V, 20 V and 30 V are displayed in Figure 7.34 (b). Thereby, the deflection increases

steadily with a quadratic behavior. Besides deflection measurements, resonance frequencies are

of special interest in such long structures. Frequency dependent displacement measurements at

unipolar voltages with an amplitude of 5 V are undertaken and one example is plotted in Fig-

ure 7.34 (c), whereby each point depicts the maximum displacement at the certain frequency.

The cantilever is 6000 µm long and at low frequencies the displacement is small due to the

low driving voltage but at 683 Hz the displacement increases to more than 50 µm. Thereby the

displacement measurement reveals a circle as expected in resonance (compare section 6.1). In

Figure 7.34 (d) the resonance frequencies of all cantilevers are plotted and reveal a decreasing

behavior with increasing cantilever length, while the maximum of 4760 Hz is observed for a

600 µm long structure.

In the next paragraph the influence of the substrate thickness is investigated in detail. A 750 nm

PZT film is deposited on a 35 µm thick stainless steel substrate from which cantilevers are

fabricated as described before. Comparing the results of displacement measurements turns out

an increase of the deflection by a factor of three. The maximum observed deflection from a

8000 µm long structure is 32 µm , if 30 V are applied (Figure 7.35 (a)). The comparison of the


(a)

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

-50-40-30-20-10

01020304050

Pol

ariz

atio

n [µ

C/c

m2 ]

Voltage [V]

(b)

3000 4000 5000 6000 7000 8000 9000

02468

101214161820

6.6 V10 V

10 V15 V

Dis

plac

emen

t [µm

]

Length [µm]

35 µm stainless steel 750 nm PZT 500 nm PZT

FIGURE 7.36:(a) Polarization measurement of a 500 nm PZT film on a Pt coated polished steel foil.(b) Displacement vs. cantilever length of cantilevers with different PZT thicknesses.

resonance frequencies reveals that cantilevers fabricated on thinner substrates exhibit a lower

resonance frequency (Figure 7.35 (b)), due to their weaker structure.

After enhancing the total displacement by reducing the substrate thickness the next aim is the

reduction of the driving voltage. Since the polished stainless steel substrates exhibit a lower

surface roughness also the problem with spikes penetrating the PZT and shorting the structures

is reduced. Therefore, the deposition of thinner PZT is possible and the same electric fields

are reached at lower operation voltages. Figure 7.36 (a) shows a polarization measurement,

which is undertaken at a 500 nm PZT film on a Pt coated stainless steel substrate (35 µm).

The top electrode has a size of 2.4 mm2 and the hysteresis shape of the curve proves that the

film is dense over this large area. Figure 7.36 (b) shows the comparison of the displacement

of structures with 500 nm PZT to those coated with 750 nm PZT. Applying the same electric

field to the cantilevers results in almost the same displacement, but the total driving voltage is

reduced by 2/3 from 15 V to 10 V and from 10 V to 6.6 V respectively.

116 7 Devices

117

8 Conclusions

8.1 Summary

The present work deals with the integration of ferroelectric thin films into microsystems

based on silicon and the use of this material class on low cost metal foils. The special

piezoelectric and dielectric properties of those functional materials are utilized for the fabri-

cation of integrated microwave and infrared applications in order to replace commonly used

components and to enhance the performance.

Ferroelectric thin films are deposited and characterized in the first part of this work:

Due to the piezoelectric properties lead zirconate titanate (PZT) and lead hafnate titanate

(PHT) films are deposited using chemical solution deposition (CSD) mainly in a thickness

range between 100 nm and 700 nm. Performing electromechanical measurements it could be

shown that PZT with a ratio of 45 % Zr and 55 % Ti, close to the morphotropic phase boundary,

exhibits the highest piezoelectric coefficient d33=115 pm/V compared to other compositions

and is therefore intensively investigated. Further measurements prove that the piezoelectric

properties are independent on the film thickness until the microstructure is affected.

Reliability is a major concern for the integration of ferroelectric thin films. For the first time

the electromechanical fatigue is measured by double beam laser interferomtry and compared to

electrical fatigue. It is found that the electrical polarization loss and the loss of d33 are identical

in thick films, but the loss of d33 becomes less than the loss of polarization with decreasing film

thicknesses, if the material is operated with bipolar voltage pulses. The drawback of fatigue

is successfully avoided by applying unipolar (non-switching) voltage pulses, which is possible

in MEMS, since the electromechanical strain is always directed in the positive direction. The

piezoelectric coefficient remains constant until at least 109 cycles and therefore PZT(45/55) is

chosen for the application in MEMS.

PZT(45/55) films deposited on platinum covered stainless steel foils grow with preferential

(111) orientation and exhibit a remanent polarization of Pr = 38 µC/cm2, which is 1/3 more

than observed on platinum coated silicon substrates. Since the polarization and the piezoelectric

effect are coupled it can be concluded that the piezoelectric coefficient is also improved and

the deposition of PZT on flexibel stainless steel foils opens a new field of applications for

electromechanical operated microsystems.

118 8 Conclusions

For the first time PHT thin films are characterized with regard to the application in MEMS. The

experiments are concentrated on the composition of 45 % Ha and 55 % Ti, since similar elec-

trical and electromechanical properties compared to PZT are expected. Additional lanthanum

doping leads to slim hystreresis curves during polarization switching and electromechanical

measurements confirm this behavior for the piezoelectric coefficient, causing a change from

piezoelectric to electrostrictive behavior with increasing La amount. Films with 10 % La

doping turn out an electric field independent piezoelectric coefficient of d33=60 pm/V and

strain measurements exhibit a nearly linear behavior above 200 kV/cm, which is advantageous

for the application in MEMS.

In a second task, a technology platform is developed for the integration of ferroelectric

thin films into silicon based microsystems.

Plasma etching with Ar and CF4 in combination with photolithography is applied for the

pattering of PZT, BST and Pt layers. To release cantilever and membrane structures from the

substrate silicon bulk micromachining and silicon surface micromachining are applied. Liquid

TMAH etches silicon anisotropic with an etch rate of 32 µm/h, if it is diluted to 20 % at 80 °C.

Two newly developed processes belong to the group of surface micromachining processes and

uses either a SF6 containing plasma or HF vapor. In the first case the cantilever structures

are released by an isotropic etch process, which removes the silicon under the stuctures from

both sides until the cantilever is free. The release of cantilever structures by HF vapor etching

implies the use of silicon on insulator (SOI) substrates. The burried oxide layer is removed

by HF vapor, seperating the silicon device layer which serves as cantilever substrate from the

silicon handle layer. The advantage of HF vapor compared to liquid HF is the avoidance of

adhesion by maintaining a good surface quality. A further advantage of SOI wafers is the

exactly defined device layer, which can be thicker than achievable with SiO2 substrate layers.

The application of those surface micromachining technologies leads to a several times higher

integration density.

Cantilevers with length between 50 µm and 1600 µm and overall thicknesses below 1 µm

are fabricated using the described technologies. The width of the cantilevers are in the range

between 40 µm and 400 µm, whereby the width is not limited, if backside etching with TMAH

is used. A maximum quasi-static deflection of 79 µm is obtained from the 1600 µm long

structure if a voltage of only 5 V is applied. This is the highest deflection ever observed at

such low voltages and several times more than can be achieved with electrostatic actuation

principles. A maximum resonance frequency of 46.3 kHz is measured for single side suspended

8.1 Summary 119

cantilevers (350 µm long, 6.5 µm thick), which is an important fact, since in resonance, the

driving voltage reduces by a factor of six while maintaining high deflection.

All displacement and resonance frequency measurements are in good agreement with FEM

simulations and therefore the model is used to calculate a sufficient contact force of 12.2 µN ,

for a contact clearance of 10 µm.

To form integrated micro-switches from piezoelectric actuated cantilevers, for the first time

freestanding metal contacts are integrated on the same substrate. The complex process is based

on the application of sacrificial photoresist in combination with electroplating of copper.

The combination of ferroelectric thin films together with the application of the devel-

oped technologies and freestanding copper contacts enables the fabrication of a variety of

devices:

1. A novel concept of a piezoelectric driven micro-relay with a galvanic separation be-

tween driving circuit and signal contact is demonstrated by extending the concept of a

single side suspended cantilever to a double side suspended bridge. This is advantageous

in terms of mechanical stability and stress compensation. A deflection of 13.3 µm and a

maximum resonance frequency of 100 kHz are measured and confirmed by FEM simula-

tions.

2. For the first time a piezoelectric driven MW-switch is processed by embedding two

micro-relays in a complex microwave test design in form of a coplanar wave guide

(CPW). Thereby, the application of the newly invented silicon surface micromachin-

ing technologies yield the possibility to placed the micro-relays between the center and

ground lines to short the CPW signal lines in order to change the microwave transmission

characteristics. The switching level is determined to be better than 20 dB up to 20 GHz.

3. Further devices working in microwave range are thin film bulk acoustic resonators

(TFBARs). A novel design in combination with electrostrictive BST, which is used for

the first time in TFBARs, shows a capacitance characteristic with a pronounced peak at

6.2 GHz. Acoustically isolation is achieved by underetching the membrane, which solely

consists of the electrode and the BST film. The measured results are supported by theo-

retical considerations.

This novel process avoids the deposition of a reflector stack or a membrane etched with

complex silicon bulk micromachining. By the application of BST films one additional de-

120 8 Conclusions

gree of freedom is achieved since the permittivity of BST depends on the applied electric

field (tunability) and therefore the overall capacitance of the TFBAR can be changed.

4. BST thin films are also applied for the processing of a dielectric bolometer for infrared

applications. Measurements up to 100 Hz using a chopper technique prove a high sensi-

tivity by detecting the infrared radiation of day light.

5. For the first time flexible stainless steel foils are coated with PZT using the CSD methode

in order to fabricate piezoelectric cantilevers on metal foil substrates. A number of

cantilevers are processed and the structures are up to 8 mm long and 0.5 mm in width.

Measurements show a maximum displacement of 32 µm on 35 µm thick substrates. A

reduction of the driving voltage by 1/3 is achieved by the reduction of the PZT film

thickness due to an improved substrate surface.

The experiments opens a new field for the application of piezoelectric thin films, since

the device processing is not reliant on expensive micromachining technologies but rather

the use of fine mechanical techniques is sufficient.

8.2 Outlook

The present thesis provides the essential technologies for the design and fabrication of

future microsystems. Thereby, the approach to use piezoelectric micro-relays in microwave

applications is especially encouraging, since it promise low operating voltages, low power

consumption and high reliability. By using a number of micro-relays in an appropriate

microwave circuit design the fabrication of numerous MW-devices is possible e.g. multi-bit

reflecting phase shifters. Thereby, several micro-switches are in parallel configuration inserted

in a microwave signal line. By closing single switches the transmission characteristic changes

and therefore the signal phase shifts by a certain degree.

The deposition of PZT thin films on metall foils is also an encouraging idea. Detailed

investigations are suggested to confirme the explanation for the improved ferroelectric proper-

ties on stainless steel substrates. A further goal is the reduction of the PZT film thickness by

improving the substrate surface in order to reduce the operation voltage.

Manifold devices are possible e.g. controllable micromirrors, moveable spirals or cantilevers,

which bends in both directions, since the front and backside can be coated with PZT and

electrodes.

REFERENCES 121

References

[1] C. P. de Araujo, L. McMillan, B. Melnick, J. Cuchiaro, and J. Scott, Ferroelectric memo-

ries, Ferroelectrics 104, 241-256 (1990)

[2] J. Scott, L. McMillan, and C. P. de Araujo, Ferroelectric memories: A comparison with

other high-speed digital devices, Ferroelectrics 116, 147-155 (1991)

[3] R. Bruchhaus, B. K. Moon, A. Hilliger, N. Nagel, Y. Yamada, H. Itokawa, K. Yamakawa,

I. Kunishima, and G. Beitel, Comparison of materials for the ferroelectric thin film to be

integrated into high density FeRAMs, Integr. Ferroelectr. 68, 115-124 (2004)

[4] Y. Arimoto, H. Ishiwara, Current status of ferroelectric random-access memory, MRS Bul-

letin 29, 823-828 (2004)

[5] Y. Park, J.-H. Lee, J.-M. Koo, S.-P. Kim, S. Shin, C.-R. Cho, and J.-K. Lee, Preparation

of Pb(ZrxTi1−x)O3 films on trench structure for high-density ferroelectric random access

memory, Integr. Ferroelectr. 66, 85-95 (2004)

[6] A. K. Tagantsev, V. O. Sherman, K. F. Astafiev, J. Venkatesh, and N. Setter, Ferroelectric

materials for microwave tunable applications, J. Electroceramics 11, 5-66 (2003)

[7] R. Bruchhaus, D. Pitzer, M. Schreiter, and W. Wersing, Optimized PZT thin films for

pyroelectric IR detector arrays, J. of Electroceramics 3, 151-162 (1999)

[8] D. L. Polla and L. F. Francis, Ferroelectric thin films in microelectromechanical systems

applications, MRS Bulletin 21, 59-65 (1996)

[9] P. Muralt, M. Kohli, T. Maeder, A. Kholkin, K. Brooks, N. Setter, and R. Luthier, Fabrica-

tion and characterization of PZT thin-film vibrators for micromotors, Sens. Actuat. A 48,

157-165 (1995)

[10] P. Muralt, A. Kholkin, M. Kohli, T. Maeder, K. G. Brooks, and R. Luthier, Fabrication

and characterization of PZT thin films for micromotors, Integr. Ferroelectr. 11, 213-220

(1995)

[11] M.-A. Dubois and P. Muralt, PZT thin film actuated elastic fin micromotor, IEEE Trans.

Ultrason., Ferroelect., Freq. Contr. 45, 1169-1177 (1998)

[12] W. P. Robbins, Ferroelectric-based microactuators, Integr. Ferroel. 11, 179-190 (1995)

122 REFERENCES

[13] D. L. Polla, Application of PZT thin films in microelectromechanical systems, SPIE 2046,

24-27 (1997)

[14] M. Arik, S. M. Zurn, A. Bar-Cohen, Y. Nam, D. Markus, and D. Polla, Development of

CAD model for MEMS micropumps, Proc. of Int. Conf. on Modelling and Simulation of

Microsystems, Semiconductors, Sensors and Actuators, 651-654 (1999)

[15] Y. Nemirovsky, A. Nemirovsky, P. Muralt, and N. Setter, Design of a novel thin-film

piezoelectric accelerometer, Sens. Actuat. A 56, 239-249 (1996)

[16] N. Ledermann, J. Baborowski, A. Seifert, B. Willing, S. Hiboux, P. Muralt, N. Setter, and

M. Forster, Piezoelectric cantilever microphone for photoacoustic GAS detector, Integr.

Ferroelectr. 35, 177-184 (2001)

[17] K. M. Johnson, Variation of dielectric constant with voltage in ferroelectrics and its appli-

cation to parametric devices, J. Appl. Phys. 33, 2826 (1962)

[18] L. M. Sheppard, Advances in processing of ferroelectric thin films, Ceram. Bulletin 71,

85 (1992)

[19] F. De Flaviis, N. G. Alexpopoulos, and M. Staffsud, Planar microwave integrated phase-

shifter design with high purity ferroelctric material, IEEE Transac. Microwave Theory

Tech. 45, 963-969 (1997)

[20] P. Padmini, T. R. Taylor, M. J. Lefevre, A. S. Nagra, R. A. York, and J. S. Speck, Real-

ization of high tunability barium strontium titanate thin films by RF magnetron sputtering

Appl. Phys. Lett. 75, 3186-3188 (1999)

[21] S. Tappe, U. Böttger, and R. Waser, Investigation of the high frequency properties of BST

thin films-a comparison of three different commonly used methods, Integr. Ferroelectr. 53,

455-464 (2003)

[22] J. Bouchaud and H. Wicht, RF MEMS analysis, forecasts and technology review, CHIP,

the Business and Technical News from Unaxis semiconductros 9, 26-29 (2003)

[23] M. Hoffmann, Untersuchungen zur Integration piezoelektrischer und elektrostriktiver

Dünnschichtkeramiken in einen elektromechanischen Mikrorelaisantrieb, Ph.D Thesis,

Rheinisch-Westfälische Technische-Hochschule Aachen, Germany (2003)

REFERENCES 123

[24] C. Lee, T. Itoh, R. Maeda, and T. Suga, Characterization of micromachined piezoelectric

PZT force sensors for dynamic scanning force microscopy, Rev. Sci. Instrum. 68, no. 5,

2091-2100 (1997)

[25] H.-J. Nam, S.-M. Cho, Y. Yee, H.-M. Lee, D.-C. Kim, J.-U. Bu, and J. Hong, Fabrication

and characteristics of piezoelectric PZT cantilever for high speed atomic force microscopy,

Integr. Ferroelectr. 35, no. 1-4, 185-197 (2001)

[26] A. I. Kingon and S. Srinivasan, Lead zirconate titanate thin films directly on copper elec-

trodes for ferroelectric, dielectric and piezoelectric applications, nature materials 4, 233-

237 (2005)

[27] K. Saegusa, Preparation and electrical properties of sol-gel derived lead zirconate titanate

glass-ceramic thin films on metal foil substrates, Jpn. J. Appl. Phys. 36, 6888-6893 (1997)

[28] Q. Zou, H. E. Ruda, and B. G. Yacobi, Dielectric properties of lead zirconate titanate thin

films deposited on metal foils, Appl. Phys. Lett. 77, 1038-1040 (2000)

[29] G. Zhang, Q. Zou, P. Sun, X. Mei, and H. E. Ruda, Influence of nitrogen annealing on

electrical properties of lead zirconate titanate thin film deposited on titanium metal foil,

Mat. Lett. 58, 706-710 (2004)

[30] S. Seifert, D. Sporn, T. Hauke, G. Müller, and H. Beige, Dielectric and electromechanical

properties of sol-gel prepared PZT thin films on metallic substrates, J. Europ. Ceram. Soc.

24, 2553-2566 (2004)

[31] K. Li, H. L. W. Chan, K. W. Lee, and C. L. Choy, Preparation of thick PZT films on

stainless steel substrates, Integr. Ferroelectr. 30, 253-260 (2000)

[32] H.-C. Lee, J.-H. Park, J.-Y. Park, H.-J. Nam, and J.-U. Bu, Design, fabrication and RF

performances of two different types of piezoelectrically actuated ohmic MEMS switches,

J. Micromech. Microeng. 15, 2098-2104 (2005)

[33] J. Y. Park and H. C. Lee, Comparison of ultra-low voltage operated RF MEMS in-line

switches with serial and shunt configurations, Integr. Ferroelectr. (2005)

[34] B. McCarthy, G. G. Adams, N. E. McGruer, and D. Potter, A dynamic model, including

contact bounce, of an electrostatically actuated microswitch, J. Microelectromech. Syst.

11, no. 3, 276-283 (2002)

124 REFERENCES

[35] J. J. Yao, RF MEMS from a device perspective, J. Micromech. Microeng. 10, R9-R38

(2000)

[36] S. Melle, F. Flourens, D. Dubuc, K. Grenier, P. Pons, F. Pressecq, J. Kuchenbecker, J. L.

Muraro, L. Bary, and R. Plana, Reliability overview of RF MEMS devices and circuits,

33rd Eur. Microw. Conf., 37-40 (2003)

[37] J. Y. Park, G. H. Kim, K. W. Chung, and J. U. Bu, Monolithically integrated microma-

chined RF MEMS capacitive switches, Sens. Actuat. A 89, 88-94 (2001)

[38] Z. Haixia, H. Yilong, X. Zhiyong, L. Dongmei, N. Finch, J. Marchetti, D. Keating, V.

Narasimha, Design of a novel bulk micro-machined RF MEMS switch, International Jour-

nal of Nonlinear Sciences and Numerical Simulation 3, 369-372 (2002)

[39] J. Oberhammer and G. Stemme, Design and fabrication aspects of an S-shaped film actu-

ator based DC to RF MEMS switch, J. Microelectromech. Syst. 13, no. 3, 421-428 (2004)

[40] H. Lee, R. A. Coutu Jr., S. Mall, and P. E. Kladitis, Nanoindentation technique for charac-

terizing cantilever beam style RF microelectromechanical systems (MEMS) switches, J.

Micromech. Microeng. 15, 1230-1235 (2005)

[41] N. Setter, V. Sherman, K. Astafiev, and A. K. Tagantsev, Polar ceramics in RF-MEMS and

microwave reconfigurable electronics: A brief review on recent issues, J. Electrocer. 13,

215-222 (2004)

[42] J. Baborowski, P. Muralt, N. Ledermann, S. Petitgrand, A. Bosseboeuf, N. Setter, and

P. Gaucher, PZT coated membrane structures for micromachined ultrasonic transducers,

Proceedings of the ISAF, 483-486 (2002)

[43] N. Ledermann, P. Muralt, J. Baborowski, M. Forster and J.-P. Pellaux, Piezoelectric

Pb(Zrx, Ti1−x)O3 thin film cantilever and bridge acoustic sensors for miniaturized pho-

toacoustic gas detectors, J. Micromech. Microeng. 14, 1650-1658 (2004)

[44] J. D. Larson III, S. R. Gilbert, and B. Xu, PZT material properties at UHF and microwave

frequencies derived from FBAR measurements, IEEE Ultrasonics Symposium 1 173-177

(2004)

[45] P. Muralt, N. Ledermann, J. Paborowski, A. Barzegar, S. Gentil, B. Belgacem, S. Petit-

grand, A. Bosseboeuf, and N. Setter, Piezoelectric micromachined ultrasonic transducers

REFERENCES 125

based on PZT thin films, IEEE Trans. Ultrason., Ferroelect., Freq. Contr.. 52, 2276-2288

(2005)

[46] D. C. Dube, J. Baborowski, P. Muralt, and N. Setter, The effect of bottom electrode on the

performance of thin film based capacitors in the gigahertz region, Appl. Phys. Lett. 74, no.

23, 3546-3548 (1999)

[47] M.-A. Dubois and P. Muralt, Measurement of the effective transverse piezoelectric coeffi-

cient e31,f of AlN and Pb(Zrx, Ti1−x)O3 thin films, Sens. Actuat. A 77, 106-112 (1999)

[48] M.-A. Dubois and P. Muralt, Properties of aluminium nitride thin films for piezoelectric

transducers and microwave filter applications, Appl. Phys. Lett. 74, 3032-3034 (1999)

[49] R. Lanz and P. Muralt, 8 GHz Microwave Filters Based on Bulk Acoustic Waves in Piezo-

electric AlN Thin Films, Proc. of the 13th IEEE International Symposium on Applications

of Ferroelectrics, 137-140 (2002)

[50] R. Lanz and P. Muralt, Solidly mounted BAW filters for 8 GHz based on AlN thin films,

IEEE Ultrasonics Symposium, 178-181 (2003)

[51] F. Martin and P. Muralt, Thickness dependence of the properties of highly c-axis textured

AlN thin films, J. Vac. Sci. Tech. A 22, 361-365 (2004)

[52] E. Mottin, A. Bain, J.L. Martin, J.L. Ouvrier-Buffet, S. Bisotto, J.J Yon, and J.L. Tissot,

Uncooled amorphous silicon technology enhancement for 25 µm pixel pitch achievement,

SPIE 4820, 200-207 (2003)

[53] A. Crastes, J.L. Tissot, Y. Guimond, P.C. Antonello, J. Leleve, H.J. Lenz, P. Potet, and J.J.

Yon, Low cost uncooled IRFPA and molded IR lenses for enhanced driver vision, SPIE

5251, 272-279 (2003)

[54] B. Fieque, A. Crastes, J.L. Tissot, J.P. Chatard, and S. Tinnes, 320x240 uncooled mi-

crobolometer 2D array for radiometric and process control applications, SPIE 5251, 114-

120 (2003)

[55] R. W. Whatmore, Pyroelectric arrays: Ceramics and thin films, J. Electrocer. 13, 139-147

(2004)

[56] A. Sharma, Z.-G. Ban, S. P. Alpay, and J. V. Mantese, Pyroelectric response of ferroelectric

thin films, J. Appl. Phys. 95, no. 7, 3618-3625 (2004)

126 REFERENCES

[57] S. O. Ryu, and S. M. Cho, Surface micromachining and characterization of a pyroelectric

infrared ray focal plane array utilizing SiO2 as the IR absorbing layer, J. Micromech.

Microeng. 15, 104-108 (2005)

[58] S. Chopra, S. Sharma, T. C. Goel, and R. G. Mendiratta, Ca substituted PbTiO3 thin films

for infrared detectors, J. Electroceram. 13, 155-158 (2004)

[59] M. Schreiter, R. Bruchhaus, D. Pitzer, and W. Wersing, Sputtering of self-polarized PZT

films for IR-detector arrays, IEEE Int. Symp. Appl. Ferroel. 11, 181-185 (1998)

[60] P. Muralt, A. Kholkin, M. Kohli, and T. Maeder, Piezoelectric actuation of PZT thin-film

diaphragms at static and resonant conditions, Sens. Actuat. A 53, 398-404 (1996)

[61] P. Muralt, Ferroelectric thin films for micro-sensors and actuators: A review, J. Mi-

cromech. Microeng. 10, 136-146 (2000)

[62] D. Damjanovic, P. Muralt, and N. Setter, Ferroelectric sensors, IEEE Sens. J. 1, 191-206

(2001)

[63] A. I. Kingon, S. K. Streiffer, C. Basceri, and S. R. Summerfelt, High permittivity per-

ovskite thin films for dynamic random access memories, Mat. Res. Soc. Bulletin 21, 46

(1996)

[64] J. F. Scott, High dielectric constant thin films for dynamic random access memories

(DRAM), Annu. Rev. Mater. Sci. 28, 79-100 (1998)

[65] J.-G. Cheng, J. Tang, A.-J. Zhang, X.-J. Meng, and J.-H. Chu, Sol-gel derived pyroelectric

barium strontium titanate thin films for infrared detector applications, Appl. Phys. A 71,

667-670 (2000)

[66] D. L. Polla and P. J. Schiller, Integrated ferroelectric microelectromechanical systems

(MEMS), Integr. Ferroelectr. 7, 359-370 (1995)

[67] D. L. Polla, Microelectromechanical systems based on ferroelectric thin films, Microel.

Eng. 29, 51-58 (1995)

[68] M. Sayer, M. Lukacs, T. Olding, G. Pang, L. Zou, and Y. Chen, Piezoelectric films and

coatings for device purposes, Mat. Res. Soc. Symp. Proc. 541, 599-610 (1999)

[69] P. Muralt, PZT thin films for microsensors and actuators: Where do we stand?, IEEE

Trans. Ultrason., Ferroelect., Freq. Contr. 47, no. 4, 903-915 (2000)

REFERENCES 127

[70] R. Waser (Ed.), Nanoelectronics and Information Technology, Weinheim, Wiley-VCH,

2003

[71] F. Jona and G. Shirane, Ferroelectric Crystals, New York, Dover Publikations, 1993

[72] M. E. Lines and A. M. Glass, Principles and Applications of Ferroelectrics and Related

Materials, Clarendon Press, Oxford, 1977

[73] B. A. Strukov and A. P. Levanyuk, Ferroelectric Phenomena in Crystals, Springer-Verlag

Berlin, Heidelberg, New York, 1998

[74] B. Jaffe, W. Cook, and H. Jaffe, Piezoelectric Ceramics, Academic Press, London, 1971

[75] D. Bolten, Reversible and irreversible polarization processes in ferroelectric ceramics

and thin films, Ph.D. thesis, Rheinisch-Westfälische Technische-Hochschule Aachen, Ger-

many (2002)

[76] D.-J. Kim, J.-P. Maria, and A. I. Kingon, Evaluation of intrinsic and extrinsic contributions

to the piezoelectric properties of Pb(Zr1−xTix)O3 thin films as a function of composition,

J. Appl. Phys. 93, 5568-5575 (2003)

[77] D. Damjanovic, Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin

films and ceramics, Rep. Prog. Phys. 61, 1267-1324 (1998)

[78] A. F. Devonshire, Theory of barium titanate, part I, Phil. Mag. 40, 1040-1063 (1949)

[79] A. F. Devonshire, Theory of barium titanate, part II, Phil. Mag. 42, 1065-1079 (1951)

[80] M. J. Haun, E. Furman, S. J. Jang, and L. E. Cross, Thermodynamic theory of the lead

zirconate-titanate solid solution system. V. Theoretical calculations, Ferroelectrics 99, 63-

86 (1989)

[81] M. Tuynina, J. Levoska, A. Sternberg, and S. Leppävuori, Relaxor behavior of pulsed

laser deposited ferroelectric (Pb1−xLax)(Zr0.65Ti0.35)O3 films, J. Appl. Phys. 84, 6800-

6810 (1998)

[82] R. Thomas, S. Mochizuki, T. Mihara, and T. Ishida, PZT(65/35) and PLZT(8/65/35) thin

films by sol-gel process: A comparative study on the structural, microstructural and elec-

trical properties, Thin Solid Films 443, 14-22 (2003)

128 REFERENCES

[83] J. Frantti, Y. Fujioka, S. Ericsson, S. Hull, and M. Kakihana, Neutron powder diffraction

study of Pb(HfxTi1−x)O3 ceramics (0.10 ≤ x ≤ 0.50), Inorg. Chem. 44, 9267-9278

(2005)

[84] C. Muller, J.-L. Baudour, V. Madigou, F. Bouree, J.-M. Kiat, C. Favotto, and M. Roubin,

Temperature-dependent neutron powder diffraction evidence for splitting of the cationic

sites in ferroelectric PbHf0.4Ti0.6O3, Acta Cryst. B55, 8-16 (1999)

[85] C. Heremans and H. L. Tuller, Lead hafnate zirconate titanate-based perovskite materials

for actuation, J. Europ. Ceram. Soc. 19, 1133-1137 (1999)

[86] P. J. Schorn, T. Schneller, U. Böttger, and R. Waser, Characterization of chemical solution

deposition-derived lead hafnate titanate thin films, J. Am. Ceram. Soc. 88, 1312-1314

(2005)

[87] A. F. Hollemann and E. Wiberg, Lehrbuch der anorganischen Chemie, Walter de Gruyter

Verlag, Berlin, New York, 1985

[88] S. Hoffmann, Modifizierte Erdalkalititanat-Dünnschichten für integrierte Bauele-

mente: Morphologie, dielektrische Eigenschaften und Ladungstransportmechanismen,

Fortschritt-Berichte VDI 9, no. 288, VDI Verlag, Düsseldorf (1998)

[89] S. Hoffmann and R. Waser, Control of the morphology of CSD-prepared (Ba, Sr)TiO3

thin films, J. Europ. Ceram. Soc. 19, 1339-1343 (1999)

[90] Y. Xu, Ferroelectric materials and their applications, North-Holland Publishing, Amster-

dam (1991)

[91] Landolt-Börnstein, Numerical Data and Functional Relationships in Science and Tech-

nology: Ferroelectric Oxides III/16a New Series Springer Verlag Berlin (1981)

[92] P. Erhard, in R. Waser (Ed.), Nanoelectronics and Information Technology, Weinheim,

Wiley-VCH, 2003

[93] S.-G. Yoon, J.-C. Lee, and A. Safari, Preparation of thin films (Ba0.5Sr0.5)TiO3 by laser

ablation technique and electrical properties, J. Appl. Phys. 76, 2999-3003 (1994)

[94] S. Yamamihi, H. Yabuta, T. Sakuma, and Y. Miyasaka, (Ba + Sr)/Ti ratio dependence of

the dielectric properties of (Ba0.5Sr0.5)TiO3 thin films prepared by ion beam sputtering,

Appl. Phys. Lett. 64, 1644-1646 (1994)

REFERENCES 129

[95] K. Abe and S. Komatsu, Dielectric constant and leakage current of epitaxial grown and

polycrystalline SrTiO3 thin films Jpn. J. Appl. Phys. 32, 4186-4189 (1993)

[96] H. Tabata, H. Tanaka, and T. Kawai, Thin film formation of perovskite type dielectric and

ferroelectric materials (MTiO3; M = Ca, Sr, Ba, Pb) by pulsed laser deposition Mater.

Res. Soc. Symp. Proc. 361, 533-538 (1995)

[97] O. Auciello, A. I. Kingon, and S. B. Krupanidhi, Sputter synthesis of ferroelectric films

and heterostructures, Mat. Res. Soc. Bulletin 6, 25-30 (1996)

[98] O. Auciello and R. Ramesh, Laser ablation deposition and characterization of ferroelectric

capacitors for nonvolatile memories, Mat. Res. Soc. Bulletin 6, 31-36 (1996)

[99] T. Morimoto, O. Hidaka, K. Yamakawa, O. Arisumi, H. Kanaya, T. Iwamotot, Y. Kumura,

I. Kunishima, and S. Tanaka, Ferroelectric properties of Pb(Zr, Ti)O3 capacitor with thin

SrRuO3 films within both electrodes, Jpn. J. Appl. Phys. 39, 2110 (2000)

[100] A. Seifert, N. Ledermann, S. Hiboux, J. Baborowski, P. Muralt, and N. Setter, Processing

optimization of solution derived PbZr1−xTixO3 thin films for piezoelectric applications,

Integr. Ferroelectr. 35, 159-166 (2001)

[101] B. Malic, M. Kosec, I. Arcon, A. Kodre, S. Hiboux, and P. Muralt, PZT thin films pre-

pared from modified zirconium alkoxide, Integr. Ferroelectr. 30, 81-89 (2000)

[102] Y.-C. Hsu, C.-C. Wu, C.-C. Lee, G. Z. Cao, and I. Y. Shen, Demonstration and charac-

terization of PZT thin-films sensors and actuators for meso- and micro-structures, Sens.

Actuat. A, Article in Press (2004)

[103] H. Kirschner, Einführung in die Röntgenfeinstrukturanalyse, Vieweg, Braunschweig

(1990)

[104] C. M. Foster, G.-R. Bai, R. Csencsitis, J. Vetrone, R. Jammy, L. Wills, E. Carr, and J.

Amano, Single-crystal Pb(Zr, Ti)O3 thin films prepared by metal-organic chemical va-

por deposition: Systematic compositional variation of electronic and optical properties, J.

Appl. Phys. 81, 2349-2357 (1997)

[105] S. Hiboux, P. Muralt, and T. Maeder, Domain and lattice contributions to dielectric

and piezoelectric properties of Pb(Zrx, Ti1−x)O3 thin films as function of composition,

J. Mater. Res. 14, 4307-4318 (1999)

130 REFERENCES

[106] C. Kügeler, M. Hoffmann, U. Böttger, and Rainer Waser, Integration, electrical and

electromechanical properties of PZT and PMN-PT thin films for MEMS applications,

Proc. of SPIE 4699, 114-123 (2002)

[107] M.-A. Dubois, P. Muralt, D. V. Tylor, and S. Hiboux, Which PZT thin films for piezo-

electric microactuator applications?, Integr. Ferroelectr. 22, 535-543 (1998)

[108] C. H. Peng, and S. B. Desu, Investigation of structure development in MOD PZT films

by an optical method, Ferroelectr. Thin Films II Symp., Mater. Res. Soc. 335-340 (1992)

[109] N. Ledermann, P. Muralt, J. Baborowski, S. Gentil, K. Mukati, M. Cantoni, A. Seifert,

and N. Setter, 100-textured, piezoelectric Pb(Zrx, Ti1−x)O3 thin films for MEMS: In-

tegration, deposition and properties, Sens. Actuat. A 105 162-170 (2003)

[110] C. Kügeler, R. Liedtke and R. Waser, Leakage current properties of Ba0.7Sr0.3TiO3 thin

films depending on the film thickness, Proc. of the 13th IEEE International Symposium on

the Applications of Ferroelectrics; IEEE pp.215-218 (2002)

[111] aixACCT Systems GmbH, Aachen Germany, Manual of the TF Analyzer 2000 Fe Mod-

ule

[112] A. K. Tagantsev, I. Stolichnov, E. L. Colla, and N. Setter, Polarization fatigue in ferro-

electric films: Basic experimental findings, phenomenological scenarios, and microscopic

features, J. Appl. Phys. 90, 1387-1402 (2001)

[113] Y. Zhang, Z. Wang, and J. D. N. Cheeke, Resonant spectrum method to characterize

piezoelectric films in composite resonators, IEEE Trans. Ultrason., Ferroelect., Freq.

Contr. 50, 321-333 (2003)

[114] K. Lefki and G. J. M. Dormans, Measurement of piezoelectric coefficients of ferroelectric

thin films, J. Appl. Phys. 76, 1764-1767 (1994)

[115] D.-G. Kim, I.-D. Kim, C.-H. Lee, J.-H. Park, K.-P. Choi, and H.-G. Kim, Evaluation

method of longitudinal and transverse piezoelectric d-coefficients for thin films, Integr.

Ferroelectr. 35, no. 1-4, 299-312 (2001)

[116] G.-T. Park, J-J. Choi, J. Ryu, H. Fan, and H-E. Kim, Measurement of piezoelectric coeffi-

cients of lead zirconate titanate thin films by strain-monitoring pneumatic loading method,

Appl. Phys. Lett. 80, 4606-4608 (2002)

REFERENCES 131

[117] A. Barzegar, D. Damjanovic, and N. Setter, The effect of boundary conditions and sam-

ple aspect ratio on apparent d33 piezoelectric coefficient determined by direct quasistatic

method, IEEE Trans. Ultrason., Ferroelect., Freq. Contr. 51, 262-270 (2004)

[118] A. Barzegar, D. Damjanovic, N. Ledermann, and P. Muralt, Piezoelectric response of

thin films determined by charge integration technique: Substrate bending effects, J. Appl.

Phys. 93, 4756-4760 (2003)

[119] R. A. Wolf and S. Trolier-McKinstry, Temperature dependence of the piezoelectric re-

sponse in lead zirconate titanate films, J. Appl. Phys. 95, 1397-1406 (2004)

[120] Q. M. Zhang, W. Y. Pan, and L. E. Cross, Laser interferometer for the study of piezo-

electric and electrostrictive strains, J. Appl. Phys. 63, 2492-2496 (1988)

[121] M. Sulc and D. Barosová, Laser interferometer for piezoelectric material study - possi-

bilities and limits, Ferroelectrics 224, 129-136 (1999)

[122] M. Sulc, L. Burianová, and J. Nosek, On the precision of a new laser interferometer in a

wide temperatur range, Ann. Chim. Sci. Mat. 26, 43-48 (2001)

[123] L. Lian and N. R. Sottos, Effects of thickness on the piezoelectric and dielectric proper-

ties of lead zirconate titanate thin films, J. Appl. Phys. 87, 3941-3949 (2000)

[124] P. Gerber, Aufbau und Erprobung eines Doppelstrahl-Laser-Interferometers zur Charak-

terisierung von ferroelektrischen Dünnschichten, Diplomarbeit, Rheinisch-Westfälische

Technische-Hochschule Aachen, Germany (2001)

[125] P. Gerber, A. Roelofs, O. Lohse, C. Kügeler, S. Tiedke, U. Böttger, and R. Waser, Short-

time piezoelectric measurements in ferroelectric thin films using a double-beam laser in-

terferometer, Rev. Sci. Instrum. 74, 2613-2615 (2003)

[126] A. L. Kholkin, Ch. Wütchrich, D. V. Taylor, and N. Setter, Interferometric measurements

of electric field-induced displacements in piezoelectric thin films, Rev. Sci. Instrum. 67,

1935-1941 (1996)

[127] H. Maiwa, J. A. Christman, S.-H. Kim, D.-J. Kim, J.-P. Maria, B. Chen, S. K. Streiffer,

and A. I. Kingon, Measurement of piezoelectric displacements of Pb(Zr, T i)O3 thin films

using a double-beam interferometer, Jpn. J. Appl. Phys. 38, 5402-5405 (1999)

132 REFERENCES

[128] U. Böttger, in R. Waser (Ed.), Nanoelectronics and Information Technology, Weinheim,

Wiley-VCH, 2003

[129] T. Abe and M. L. Reed, RF-magnetron sputtering of piezoelectric lead-zirconate-titanate

actuator films using composite targets, IEEE. Proc. Micro Electro Mechanical Systems,

164-169 (1994)

[130] A. Kholkin, E. Colla, P. Muralt, M. Kohli, T. Maeder, D. Taylor, and N. Setter, Inter-

ferometric study of piezoelectric degradation in ferroelectric thin films, Microel. Eng. 29,

261-264 (1995)

[131] E. L. Colla, A. L. Kholkin, D. Taylor, A. K. Tagantsev, K. G. Brooks, and N. Setter,

Characterisation of the fatigued state of ferroelectric PZT thin-film capacitors, Microel.

Eng. 29, 145-148 (1995)

[132] A. L. Kholkin, E. L. Colla, A. K. Tagantsev, D. V. Taylor, and N. Setter, Fatigue of

piezoelectric properties in Pb(Zr, Ti)O3 films, Appl. Phys. Lett. 68, no. 18, 2577-2579

(1995)

[133] E. L. Colla, S. Hong, D. V. Taylor, A. K. Tagantsev, N. Setter, and K. No, Direct

observation of region by region suppression of the switchable polarization (fatigue) in

Pb(Zr, Ti)O3 thin film capacitors with Pt electrodes, Appl. Phys. Lett. 72, no. 21, 2763-

2765 (1998)

[134] C. Kügeler, P. Gerber, U. Böttger, and R. Waser, Thickness dependence of piezoelectric

properties for PZT thin films with regard to MEMS applications, Integr. Ferroelectr. 54,

527-535 (2003)

[135] P. Gerber, C. Kügeler, U. Ellerkmann, P. Schorn, U. Böttger, and R. Waser, Effects of fer-

roelectric fatigue on the piezoelectric properties (d33) of tetragonal lead zirconate titanate

thin films, Appl. Phys. Let. 8654, 112908 (2005)

[136] P. Rai-Choudhury (Ed.), MEMS and MOEMS Technology and Application, SPIE Press,

Bellingham, Washington USA (2001)

[137] K. Schade, Mikroelektroniktechnologie, Verlag Technik GmbH, Berlin, Deutschland

(1991)

[138] C. Linder, L. Paratte, M.-A. Grétillat, V. P. Jaecklin, and N. F. Rooij, Surface microma-

chining, J. Micromech. Microeng. 2, 122-132 (1992)

REFERENCES 133

[139] K. R. Williams and R. S. Muller, Etch rates for micromachining processing, J. Micro-

electromech. Syst. 5, 256-269 (1996)

[140] K. E. Petersen, Silicon as a mechanical material, Proceedings of the IEEE 70, Vol. 5,

420-444 (1982)

[141] J. Baborowski, Microfabrication of piezoelectric MEMS, J. Electroceram. 12, 33-51

(2004)

[142] M. Arik, S. M. Zurn, A. Bar-Cohen, and D. L. Polla, Design, fabrication, and character-

ization of thin film PZT membranes for high flux electronics cooling applications, Smart

Mater. Struct. 14, 1239-1249 (2005)

[143] Y. S. Yoon, J. H. Kim, M. T. Hsieh, and D. L. Polla, Fabrication and characteristics of

microelectromechanical system device based on PZT films and surface micromachining,

J. Kor. Phys. Soc. 32, S1760-S1762 (1998)

[144] S. Okazaki and J. Moers, in R. Waser (Ed.), Nanoelectronics and Information Technol-

ogy, Weinheim, Wiley-VCH, 2003

[145] H. Beneking, Halbleiter-Technologie - Eine Einführung in die Prozeßtechnologie von Si

und III-V-Verbindungen, Stuttgart, Teubner, 1991

[146] G. T. A. Kovacs, N. I. Maluf, and K. E. Petersen, Bulk micromachining of silicon, Proc.

IEEE 86, 1536-1551 (1998)

[147] S. J. Gross, S. Tadigadapa, T. N. Jackson, S. Trolier-McKinstry, and Q.Q. Zhang, Lead-

zirconate-titanate-based piezoelectric micromachined switch, Appl. Phys. Lett. 83, no. 1,

174-176 (2003)

[148] U. Schnakenberg, IC-Prozeßkompatible anisotrop wirkende Ätzlösung zur Herstellung

integrierter Mikrosysteme in Silizium, Ph. D. Thesis, Technische Universität Berlin,

Berlin, Germany (1994)

[149] O. Tabata, Anisotropic etching of silicon in TMAH solutions, Sensors Materials 13, 271-

283 (2001)

[150] J. T. L. Thong, W. K. Choi, and C. W. Chong, TMAH etching of silicon and the interac-

tion of etching parameters, Sens. Actuat. A 63, 243-249 (1997)

134 REFERENCES

[151] M.Sugawara, Plasma Etching - Fundamentals and Applications, Oxford University

Press, New York Inc., USA (1998)

[152] K. Nishikawa, Y. Kusumi, T. Oomori, M. Hanazaki, and K. Namba, Platinum etching

and plasma characteristics in RF magnetron and electron cyclotron resonance plasmas,

Jpn. Appl. Phys. 32, 6102-6108 (1993)

[153] T. Shibano, K. Nakamura, T. Takenaga, and K. Ono, Platinum etching in Ar/Cl2 plasmas

with a photoresist mask, J. Vac. Sci. Technol. A 17, 799-804 (1999)

[154] W. J. Yoo, J. H. Hahm, H. W. Kim, C. O. Jung, Y. B. Koh, and M. Y. Lee, Control of

etch slope during etching of Pt in Ar/Cl2/O2 plasmas, Jpn. J. Appl. Phys. 35, 2501-2504

(1996)

[155] C. H. Chou and J. Phillips, Platinum metal etching in a microwave oxygen plasma, J.

Appl. Phys. 68, 2415-2423 (1990)

[156] T. Shibano, K. Nakamura, and T. Oomori, Platinum etching in Ar/O2 mixed gas plasma

with a thin SiO2 etching mask, J. Vac. Sci. Technol. A 16, 502-508 (1998)

[157] S.-B. Kim, C.-I. Kim, E.-G. Chang, and G.-Y. Yeom, Study on surface reaction of

(Ba, Sr)TiO3 thin films by high density plasma etching, J. Vac. Sci. Tech. A 17, 2156-

2161 (1999)

[158] P.-S. Kang, K.-T. Kim, D.-P. Kim, and C.-I. Kim, Study of damage reduction of

(Ba0.6, Sr0.4)TiO3 thin films etched in Ar/CF4 plasmas, J. Vac. Sci. Tech. A 21, 1469-

1474 (2003)

[159] H. Macé. H. Achard, and L. Peccoud, Reactive ion etching of Pt/PZT/Pt ferroelectric thin

film capacitor in high density DECR plasma, Microelectr. Eng. 29, 45-48 (1995)

[160] M. Bale and R. E. Palmer, Reactive ion etching of piezoelectric Pb(ZrxTi1−x)O3 in an

SF6 plasma, J. Vac. Sci. Technol. A 17, 2467-2469 (1999)

[161] C. W. Chung, W. I. Lee, and J. K. Lee, Dry etching of Pt/PbZrxTi1−xO3/Pt thin film

capacitors in an inductively coupled plasma (ICP), Integr. Ferroel. 11, 259-267 (1995)

[162] B. Charlet and K. E. Davies, Dry etching of PZT films in an ECR plasma, Mat. Res.Soc.

Sym. Proc. 310, 363-368 (1993)

REFERENCES 135

[163] A. Cofer, P. Rajora, S. DeOrnellas, and D. Keil, Plasma etch processing of advanced

ferroelectric devices, Integr. Ferroelectr. 16, 53-61 (1997)

[164] J. Baborowski, P. Muralt, N. Ledermann, E. Colla, A. Seifert, S. Gentil, and N. Setter,

Mechanisms of Pb(Zr0.53Ti0.47)O3 thin films etching with ECR/RF reactor, Integr. Ferro-

electr. 31, 261-271 (2000)

[165] S. Yokoyama, Y. Ito, K. Hamada, S. Ohnishi, J. Kudo, and K. Sakiyama, High-

temperature etching of PZT/Pt/TiN structure by high-density ECR plasma, J. Appl. Phys.

34, 767-770 (1995)

[166] N. Ikegami, T. Matsui, and J. Kanamori, Dry-etching mechanism of sputtered

Pb(Zr1−xTix)O3 film, Jpn. J. Appl. Phys. 35, 2505-2511 (1996)

[167] K. Saito, J. H. Choi, T. Fukuda, and M. Ohue, Reactive ion etching of sputtered

PbZr1−xTixO3 thin films, Jpn. J. Appl. Phys. 31, L1260-L1262 (1992)

[168] J. K. Lee, T.-Y. Kim, I. Chung, and S. B. Desu, Characterization and elimination of dry

etching damaged layer in Pt/Pb(Zr0.53Ti0.47)O3/Pt ferroelectric capacitor, Appl. Phys.

Lett. 75, 334-336 (1999)

[169] C. Soyer, E. Cattan, D. Rèmiens, and M. Guilloux-Viry, Ion beam etching of lead-

zirconate-titanate thin films: Correlation between etching parameters and electrical prop-

erties evolution, J. Appl. Phys. 92, 1048-1055 (2002)

[170] K.-T. Lim, K.-T. Kim, D.-P. Kim, and C.-I. Kim, Reduction of etching damage in lead-

zirconate-titanate thin films with inductively coupled plasma, J. Vac. Sci. Technol. A 21(4),

1563-1567 (2003)

[171] C. J. Mogab, A. C. Adams, and D. L. Flamm, Plasma etching of Si and SiO2 - The effect

of oxygen additions to CF4 plasmas, J. Appl. Phys. 49, 3796-3803 (1978)

[172] B. E. E. Kastenmeier, P. J. Matsuo, and G. S. Oehrlein, Highly selective etching of silicon

nitride over silicon and silicon dioxide, J. Vac. Sci. Technol. A 17, 3179-3184 (1999)

[173] M. Schaepkens, T. E. F. M. Standaert, N. R. Rueger, P. G. M. Sebel, and J. M. Cook,

Study of the SiO2 -to- Si3N4 etch selectivity mechanism in inductively coupled fluorocar-

bon plasmas and a comparison with the SiO2 -to- Si mechanism, J. Vac. Sci. Technol. A

17, 26-37 (1999)

136 REFERENCES

[174] S. K. Ray, C. K. Maiti, and N. B. Chakraborti, Rapid plasma etching of silicon, silicon

dioxide and silicon nitride using microwave discharges, Semicond. Sci. Technol. 8, 599-

604 (1993)

[175] B. Mahi, Y. Arnal, and C. Pomot, The etching of silicon in diluted SF6 plasmas: Corre-

lation between the flux of incident species and the etching kinetics, J. Vac. Sci. Technol. B

5, 657-666 (1987)

[176] H. Shindo, Y. Sawa, and Y. Horiike, Silicon etching employing negative ion in SF6

plasma, Jpn. J. Appl. Phys. 34, L925-L928 (1995)

[177] M. Esashi, M. Takinami, Y. Wakabayashi, and K. Minami, High-rate directional deep dry

etching for bulk silicon micromachining, J. Micromech. Microeng. 5, 5-10 (1995)

[178] G. Grigoropoulos, E. Gogolides, A. D. Tserepi, and A. G. Nassipoulos, Highly

anisotropic silicon reactive ion etching for nanofabrication using mixtures of SF6/CHF3

gases, J. Vac. Sci. Technol. B 15, 640-645 (1997)

[179] P. Nallan, A. H. Khan, A. Kumar, and D. Podlesnik, Speeding high aspect ratio etch,

Europ. Semicond., 41-45 (2002)

[180] R. Dussart, M. Boufnichel, G. Marcos, P. Lefaucheux, A. Basillais, R. Benoit, T.

Tillocher, X. Mellhaoui, H. Estrade-Szwarckopf, and P. Ranson, Passivation mechanisms

in cryogenic SF6/O2 etching process, J. Micromech. Microeng. 14, 190-196 (2004)

[181] C. P. D’Emic, K. K. Chan, and J. Blum, Deep trench plasma etching of single crystal

silicon using SF6/O2 gas mixtures, J. Vac. Sci. Technol. B 10, 1105-1110 (1992)

[182] J-S. Park, H-D. Park, and S-G. Kang, Fabrication and properties of PZT micro cantilevers

using isotropic silicon dry etching process by XeF2 gas for release process, Sens. Actuat.

A 117, 1-7 (2005)

[183] A. Tserepi, C. Tsamis, E. Gogolides, and A. G. Nassiopolou, Dry etching of porous

silicon in high density plasmas, Phys. Stat. Sol. 197, 163-167 (2003)

[184] A. Tserepi, C. Tsamis, G. Kokkoris, E. Gogolides, and A. G. Nassiopolou, Fabrication

of suspended thermally insulating membranes using frontside micromachining of the Si

substrate: Characterization of the etching process, J. Micromech. Microeng. 13, 323-329

(2003)

REFERENCES 137

[185] H. Baltes, O. Brand, G. K. Fedder, C. Hierold, J. G. Korvink, and O. Tabata (Eds.),

Enabling Technology for MEMS and Nanodevices, Weinheim, Wiley-VCH, 2004

[186] C. R. Helms and B. E. Deal, Mechanisms of HF/H2O vapor phase etching of SiO2, J.

Vac. Sci. Technol. A 10, 806-811 (1992)

[187] Y. Fukuta, H. Fujita, and H. Toshiyoshi, Vapor hydrofluoric acid sacrificial release tech-

nique for micro electro mechanical systems using labware, Jpn. J. Appl. Phys. 42, 3690-

3694 (2003)

[188] A. Szekeres, and P. Danesh, Mechanical stress in SiO2/Si structures formed by thermal

oxidation of amorphous and crystalline silicon, Semicond. Sci. Technol. 11, 1225-1230

(1996)

[189] M. W. Denhoff, A measurement of Young’s modulus and residual stress in MEMS

bridges using a surface profiler, J. Micromech. Microeng. 13, 686-692 (2003)

[190] K.-S. Chen, and K.-S. Ou, Modification of curvature-based thin-film residual stress mea-

surement for MEMS applications, J. Micromech. Microeng. 12, 917-924 (2002)

[191] T. Cui, D. Markus, S. Zurn, and D. L. Polla, Piezoelectric thin films formed by MOD

on cantilever beams for micro sensors and actuators, Microsystem Technol. 10, 137-141

(2004)

[192] S. Schneider and S. McClatchie, in R. Waser (Ed.), Nanoelectronics and Information

Technology, Weinheim, Wiley-VCH, 2003

[193] A. Schroth, Modelle für Balken und Platten in der Mikromechanik, Ph. D. thesis, TU

Dresden (1996)

[194] S. J. Gross, Q. Q. Zhang, S. Tadigadapa, S. Trolier-McKinstry, T. N. Jackson, and F.

Djuth, Reliable integration of piezoelectric lead zirconate titanate with MEMS fabrication

processes, Proceedings of the SPIE 4558, 72-80 (2001)

[195] C. Kügeler, S. Tappe, U. Böttger, and R. Waser, A novel design for integrated RF-MEM

switches using ferroelectric thin films, Integr. Ferroelectr. 76, 59-67 (2005)

[196] C. Kügeler, U. Böttger, and R. Waser, Piezoelectric MEMS actuators fabricated by

chemical solution deposition and surface micromachining technologies, Electrochem. Soc.

Proc. 2004-9, 278-287 (2004)

138 REFERENCES

[197] S. Zurn, M. Hsieh, G. Smith, D. Markus, M. Zang, G. Hughes, Y. Nam, M. Arik, and D.

Polla, Fabrication and structural characterization of a resonant frequency PZT microcan-

tilever, Smart Mater. Struct. 10, 252-263 (2001)

[198] V. Milanovic, Multilevel beam SOI-MEMS fabrication and applications, J. Micromech.

Sys. 13, 19-30 (2004)

[199] J. Li, Q. X. Zhang, A. Q. Liu, W. L. Goh, and J. Ahn, Technique for preventing stiction

and notching effect on silicon-on-insulator microstructure, J. Vac. Sci. Technol. B 21(6),

2530-2539 (2003)

[200] J. A. Plaza, A. Lobera, C. Dominguez, J. Esteve, I. Salinas, J. Garcia and J. Berganzo,

BESOI-based integrated optical silicon accelerometer, J. Micromech. Sys. 13, 19-30

(2004)

[201] D. Hah, C.-A. Coi, C.-K. Kim, and C.-H. Jun, A self-aligned vertical comb-drive actuator

on an SOI wafer for a 2D scanning micromirror, J. Micromech. Microeng. 14, 1148-1156

(2004)

[202] T. D. Kudrle, C. C. Wang, M. G. Bancu, J. C. Hsiao, A. Pareek, M. Waelti, G. A. Kirkos,

T. Shone, C. D. Fung, and C. H. Mastrangelo, Single-crystal silicon micromirror array

with polysilicon flexures, Sens. Actuat. A 119, 559-566 (2005)

[203] R. Lanz, and P. Muralt, Bandpass filters for 8 GHz using solidly mounted bulk acoustic

wave resonators, IEEE Trans. Ultrason., Ferroelect., Freq. Contr. 52, 936-946 (2005)

[204] W. Pang, H. Zhang, and E. S. Kim, Micromachined acoustic wave resonator isolator from

substrate IEEE Trans. Ultrason., Ferroelect., Freq. Contr. 52, no. 8, 1239-1246 (2005)

[205] B. Belgacem, D. Alquier, P. Muralt, J. Baborowski, S. Lucas, and R. Jerisian, Optimiza-

tion of the fabrication of sealed capacitive transducers using surface micromachining, J.

Micromech. Microeng. 14, 299-304 (2004)

[206] S. Tappe, U. Böttger, and R. Waser, Electrostrictive resonances in (Ba0.7Sr0.3)TiO3 thin

films at microwave frequencies, Appl. Phys. Lett. 85, 1-3 (2004)

[207] M. Noda, K. Hashimoto, R. Kubo, H. Tanaka, T. Mukaigawa, H. Xu, and M. Okuyama,

A new type of dielectric bolometer mode of detector pixel using ferroelectric thin film

capacitors for infrared image sensor, Sens. Actuat. A 77, 39-44 (1999)

REFERENCES 139

[208] K. Hashimoto, H. Xu, T. Mukaigawa, R. Kubo, H. Zhu, M. Noda, and M. Okuyama, Si

monolithic microbolometers of ferroelectric BST thin film combined with readout FET

for uncooled infrared image sensor, Sens. Actuat. A 88, 10-19 (2001)

[209] Q. Zou, H. E. Ruda, and B. G. Yacobi, Improved dielectric properties of lead zirconate

titanate thin films deposited on metal foils with LaNiO3 buffer layers, Appl. Phys. Lett.

78, 1282-1284 (2001)

[210] J.-G. Cheng, R. Gabl, D. Pitzer, R. Primig, M. Schreiter, and W. Wersing, Chemical solu-

tion deposition of columnar-grained metallic lanthanum nitrate thin films, J. Am. Ceram.

Soc. 86, 1786-88 (2003)

140 REFERENCES

Lebenslauf

Persönliche Daten

Name: Carsten Kügeler

Geburtsdatum: 20. August 1973

Geburtsort: Rheda-Wiedenbrück

Familienstand: ledig

Schulbildung

08/80 – 07/84 Grundschule Rheda-Wiedenbrück

08/84 – 06/90 Realschule Rheda-Wiedenbrück

08/90 – 06/93 Städtische Kollegschule für Metall- und Elektroberufe

Bielefeld

Wehrdienst

07/93 – 06/94 Panzergrenadierbataillon 212, Augustdorf

Studium

10/94 – 06/01 Studium der Elektrotechnik an der Rheinisch-

Westfälischen Technischen Hochschule Aachen

Berufliche Tätigkeit 07/01 - 07/06 Wissenschaftlicher Mitarbeiter am Institut für

Werkstoffe der Elektrotechnik der RWTH-Aachen seit 08/06 Wissenschaftlicher Mitarbeiter am Forschungszentrum

Jülich

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Thesis 1 - Integration of Ferroelectric and Piezoelectric Thin Films

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