+ All Categories
Home > Documents > A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter...

A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter...

Date post: 28-Jul-2015
Category:
Upload: raj-sahu
View: 60 times
Download: 0 times
Share this document with a friend
Popular Tags:
10
IEEE SENSORS JOURNAL, VOL. 7, NO. 7, JULY 2007 967 A Micromachined Piezoelectric Ultrasonic Transducer Operating in Mode Using Square Interdigital Electrodes Chao Wang, Zheyao Wang, Tian-Ling Ren, Senior Member, IEEE, Yiping Zhu, Yi Yang, Xiaoming Wu, Haining Wang, Huajun Fang, and Litian Liu Abstract—This paper presents the design, fabrication, and characterization of a piezoelectric micromachined ultrasonic transducer (pMUT) operating in d mode. A piezoelectric PZT thin film is polarized along the in-plane direction using top square interdigital electrodes (IDEs), and the in-plane polarization and IDEs enable the pMUT to operate in d mode, which allows the PZT to convert the ultrasonic pressure-induced in-plane stresses into in-plane direction charges. The square IDEs fully exploit the stresses of the diaphragm to improve sensitivity and increase device capacitance, and the pMUT is optimized by varying the IDE dimensions rather than the thickness of the PZT film. Theoretical and finite-element analyses are performed to optimize the diaphragm and the IDE dimensions in terms of resonance frequency and sensitivity, respectively. The ultrasonic directivity of the pMUTs with different packaging configurations is characterized, showing large ultrasound inlet areas beneficial to ultrasonic applications requiring high directivity. Index Terms—Directivity, d mode, interdigital electrode (IDE), in-plane polarization, PZT, ultrasonic transducer. I. INTRODUCTION D UE TO THE piezoelectricity, lead zirconate titanate (PZT) is capable of converting energies between mechanical and electrical domains mutually [1], [2], so PZT has been widely applied to microelectromechanical systems (MEMS) to extend their functions leading to the realization of a wide variety of piezoelectric microsensors and microactuators [3]–[9]. Silicon micromachining techniques provide a new approach to develop ultrasonic transducers [10]–[15]. The advantages of micromachined ultrasonic transducers (MUTs) include the increased and accurately controlled resonance frequency and bandwidth for air-coupled transducers [16], and the possibility to fabricate large 1- or 2-D arrays that would offer excellent performance than conventional bulk ceramic counterparts [10], Manuscript received October 18, 2006; revised January 8, 2007; accepted January 9, 2007. This work was supported in part by the National Natural Sci- ence Foundation of China under Grant 90407023, and “863” Program of China under Grant 2004AA404240. The associate editor coordinating the review of this paper and approving it for publication was Dr. Errol EerNisse. C. Wang was with the Institute of Microelectronics, Tsinghua University, Beijing 100084, China. He is now with the Department of Electrical Engi- neering, Princeton University, Princeton, NJ 08544 USA (e-mail: chaowang@ Princeton.edu). Z. Wang, T.-L. Ren, Y. Zhu, Y. Yang, X. Wu, H. Wang, H. Fang, and L. Liu are with the Institute of Microelectronics, Tsinghua University, Beijing 100084, China (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2007.896562 [11]. In addition, the utilization of PZT enables piezoelec- tric MUTs (pMUTs) to be realized by constructing a simple laminated diaphragm, which avoids the DC-biased voltage that is needed in capacitive MUTs [17]–[21], facilitating the fabrication and improving the reliability. Most pMUTs employ a laminated diaphragm with a PZT thin film sandwiched in-between two plate electrodes, and this configuration enables the PZT film to be polarized through the thickness direction by applying a DC-voltage to the two electrodes. This polarization makes the transducer to operate in mode by converting the stresses in the main plane of the PZT film that are induced by ultrasonic pressure into electrical charges collected by the two electrodes. Due to that the voltage sensitivity of a pMUT is roughly inversely proportional to the device capacitance, which in turn is inversely proportional to the PZT thickness, the sensitivity is rudimentarily linear with the PZT thickness. Therefore, thick PZT films are needed for high sensitivity. However, as it is difficult to deposit PZT films thicker than several micrometers without crack, the pMUTs operating in mode suffer from low sensitivity. To address this problem, transducers working in mode have been proposed [22]–[26]. The mode is achieved by fabricating interdigital electrodes (IDEs) on the top of PZT and polarizing the PZT by applying DC-voltage to the IDEs, so that the remaining polarization is parallel to the plane of the PZT film. The in-plane polarization enables the PZT to convert in-plane stresses to charges collected by the IDEs. In addition to PZT thickness, the configuration and the dimension of the IDEs determine the device capacitance, such that it is feasible to optimize the sensitivity of the mode pMUTs by varying the IDEs to avoid the need of thick PZT films. The stresses in the top surface of the PZT, which contributes dominantly to charges, change from tensile at the boundaries of the diaphragm to compressive at the central area [27]. As the piezoelectric coefficient has the same sign, the stress-in- duced electric charges at these two different areas have dif- ferent signs, resulting in counteractive contribution to the sen- sitivity. To fully exploit the high stresses, this paper proposes a mode pMUT which employs square IDEs surrounding the inside boundaries of the diaphragm. The IDEs locate only on the area with either tensile or compressive stresses, where the stresses have the same sign. To facilitate the measurement of small IDE capacitance, multiple-finger square IDEs are used to increase the IDE capacitance. Theoretical and finite-element method (FEM) analyses are performed in this paper to optimize 1530-437X/$25.00 © 2007 IEEE
Transcript
Page 1: A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter Digital Electrodes

IEEE SENSORS JOURNAL, VOL. 7, NO. 7, JULY 2007 967

A Micromachined Piezoelectric UltrasonicTransducer Operating in d33 Mode Using

Square Interdigital ElectrodesChao Wang, Zheyao Wang, Tian-Ling Ren, Senior Member, IEEE, Yiping Zhu, Yi Yang, Xiaoming Wu,

Haining Wang, Huajun Fang, and Litian Liu

Abstract—This paper presents the design, fabrication, andcharacterization of a piezoelectric micromachined ultrasonictransducer (pMUT) operating in d33 mode. A piezoelectric PZTthin film is polarized along the in-plane direction using top squareinterdigital electrodes (IDEs), and the in-plane polarization andIDEs enable the pMUT to operate in d33 mode, which allowsthe PZT to convert the ultrasonic pressure-induced in-planestresses into in-plane direction charges. The square IDEs fullyexploit the stresses of the diaphragm to improve sensitivity andincrease device capacitance, and the pMUT is optimized byvarying the IDE dimensions rather than the thickness of thePZT film. Theoretical and finite-element analyses are performedto optimize the diaphragm and the IDE dimensions in terms ofresonance frequency and sensitivity, respectively. The ultrasonicdirectivity of the pMUTs with different packaging configurationsis characterized, showing large ultrasound inlet areas beneficial toultrasonic applications requiring high directivity.

Index Terms—Directivity, d33 mode, interdigital electrode(IDE), in-plane polarization, PZT, ultrasonic transducer.

I. INTRODUCTION

DUE TO THE piezoelectricity, lead zirconate titanate (PZT)is capable of converting energies between mechanical and

electrical domains mutually [1], [2], so PZT has been widelyapplied to microelectromechanical systems (MEMS) to extendtheir functions leading to the realization of a wide variety ofpiezoelectric microsensors and microactuators [3]–[9].

Silicon micromachining techniques provide a new approachto develop ultrasonic transducers [10]–[15]. The advantagesof micromachined ultrasonic transducers (MUTs) include theincreased and accurately controlled resonance frequency andbandwidth for air-coupled transducers [16], and the possibilityto fabricate large 1- or 2-D arrays that would offer excellentperformance than conventional bulk ceramic counterparts [10],

Manuscript received October 18, 2006; revised January 8, 2007; acceptedJanuary 9, 2007. This work was supported in part by the National Natural Sci-ence Foundation of China under Grant 90407023, and “863” Program of Chinaunder Grant 2004AA404240. The associate editor coordinating the review ofthis paper and approving it for publication was Dr. Errol EerNisse.

C. Wang was with the Institute of Microelectronics, Tsinghua University,Beijing 100084, China. He is now with the Department of Electrical Engi-neering, Princeton University, Princeton, NJ 08544 USA (e-mail: [email protected]).

Z. Wang, T.-L. Ren, Y. Zhu, Y. Yang, X. Wu, H. Wang, H. Fang, and L. Liuare with the Institute of Microelectronics, Tsinghua University, Beijing 100084,China (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2007.896562

[11]. In addition, the utilization of PZT enables piezoelec-tric MUTs (pMUTs) to be realized by constructing a simplelaminated diaphragm, which avoids the DC-biased voltagethat is needed in capacitive MUTs [17]–[21], facilitating thefabrication and improving the reliability.

Most pMUTs employ a laminated diaphragm with a PZTthin film sandwiched in-between two plate electrodes, and thisconfiguration enables the PZT film to be polarized throughthe thickness direction by applying a DC-voltage to the twoelectrodes. This polarization makes the transducer to operate in

mode by converting the stresses in the main plane of thePZT film that are induced by ultrasonic pressure into electricalcharges collected by the two electrodes. Due to that the voltagesensitivity of a pMUT is roughly inversely proportional to thedevice capacitance, which in turn is inversely proportional tothe PZT thickness, the sensitivity is rudimentarily linear withthe PZT thickness. Therefore, thick PZT films are needed forhigh sensitivity. However, as it is difficult to deposit PZT filmsthicker than several micrometers without crack, the pMUTsoperating in mode suffer from low sensitivity. To addressthis problem, transducers working in mode have beenproposed [22]–[26]. The mode is achieved by fabricatinginterdigital electrodes (IDEs) on the top of PZT and polarizingthe PZT by applying DC-voltage to the IDEs, so that theremaining polarization is parallel to the plane of the PZT film.The in-plane polarization enables the PZT to convert in-planestresses to charges collected by the IDEs. In addition to PZTthickness, the configuration and the dimension of the IDEsdetermine the device capacitance, such that it is feasible tooptimize the sensitivity of the mode pMUTs by varying theIDEs to avoid the need of thick PZT films.

The stresses in the top surface of the PZT, which contributesdominantly to charges, change from tensile at the boundaries ofthe diaphragm to compressive at the central area [27]. As thepiezoelectric coefficient has the same sign, the stress-in-duced electric charges at these two different areas have dif-ferent signs, resulting in counteractive contribution to the sen-sitivity. To fully exploit the high stresses, this paper proposesa mode pMUT which employs square IDEs surroundingthe inside boundaries of the diaphragm. The IDEs locate onlyon the area with either tensile or compressive stresses, wherethe stresses have the same sign. To facilitate the measurementof small IDE capacitance, multiple-finger square IDEs are usedto increase the IDE capacitance. Theoretical and finite-elementmethod (FEM) analyses are performed in this paper to optimize

1530-437X/$25.00 © 2007 IEEE

Page 2: A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter Digital Electrodes

968 IEEE SENSORS JOURNAL, VOL. 7, NO. 7, JULY 2007

Fig. 1. Schematic structure of the in-plane polarized pMUT. (a) Top view andcross-sectional view. (b) Electric field distribution.

the pMUT, and micromachining techniques are used to fabricatethe pMUT. Detailed ultrasonic directivity of different packagesis characterized, and comparison results are given for ultrasonicapplications.

II. THEORETICAL ANALYSIS

The axes are defined following the “IEEE Standard on Piezo-electricity” [28], which requires the coordinate axis to be par-allel to the crystallographic axis of PZT crystal, i.e., the po-larization direction, to make the piezoelectric coefficientpositive. Therefore, the axis is set to parallel to the PZT filmplane, the axis is perpendicular to the PZT film plane, and the

axis is thus determined by the right-handed coordinate rule.Fig. 1(a) shows a generic pMUT operated in mode.

The transducer consists of a laminated square diaphragm, in which the PZT thin film de-

posited on the supporting layer acts as the functionalmaterial, and Pt/Ti on the top of the PZT serves as the IDE.Square configuration is chosen instead of circular one due toKOH etching, which etches pyramidal cavities from waferbackside to form the thin diaphragm. The zirconia or titania[22], [23], [25], [26] acts as a barrier layer and replaces the Ptbottom plate electrode in mode transducers to prevent Pbfrom diffusion during annealing.

The piezoelectricity is achieved by polarizing the PZT witha DC-voltage applied to the IDE, generating a complex electricfield and remaining polarization vectors, as shown in Fig. 1(b).When an external ultrasonic wave pressure is perpendicularlyapplied on the laminated diaphragm, stresses are induced inthe PZT. As the lateral dimensions of the diaphragm is muchgreater than the thickness, the stresses are mainly parallel to the

Fig. 2. A simplified model of in-plane polarized microsensor.

diaphragm and along the direction. In case that the film thick-ness is much smaller than the interdigital electrode gap , theelectric field , the electric displacement , and the polariza-tion vector between two adjacent fingers are roughly parallelto the main-plane and along the axis, whereas those under thefingers are radial. Therefore, the stresses in the axis lead tocharges in the axis, making the pMUT working in mode.The piezoelectric constitutive equation is

(1)

where the subscript denotes the components in the axis, andis the piezoelectric coefficient of PZT.

Due to the complexity of the IDE, the electric displacement inthe PZT film is neither homogeneous in magnitude nor in direc-tion, as shown in Fig. 1(b). On condition that the thickness of thePZT film is smaller than the finger gap, the vertical componentof the electric displacement in the PZT beneath the electrodefingers contribute much less than the horizontal component ofthe electric displacement between two fingers. Thus, for simpli-fication, the electric displacement in the PZT between adjacentelectrode fingers are assumed homogeneous and parallel to themain plane, whereas the electric displacement beneath the elec-trodes is neglected and the potential in these regions is uniform,as shown in Fig. 2. As the electric field dose exist beneath theelectrodes of the IDE, the neglect of the capacitance underneaththe electrodes makes the calculated capacitance smaller than theactual capacitance.

The capacitance of the IDE can be calculated using Gevor-gian’s model [29], which is, however, complicated and incon-venient. By simplification, the device capacitance can be cal-culated as a parallel plate capacitor by introducing an “effectiveelectrode area” , which is defined as the equipotential area onwhich the electric field vectors act perpendicularly. Accordingto the definition, equals the product of the total electrodelength and the PZT thickness , and the device capacitance isapproximated by

(2)

where denotes the permittivity of PZT. The electrical chargesinduced on the electrodes can be obtained from the electric

displacement and the effective electrode area

(3)

Consequently, the induced voltage is given by

(4)

Page 3: A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter Digital Electrodes

WANG et al.: A MICROMACHINED PIEZOELECTRIC ULTRASONIC TRANSDUCER OPERATING IN MODE 969

TABLE IPARAMETERS OF THE DIAPHRAGM AND THE IDE

and energy stored in a capacitor is

(5)

It can be concluded from (4) that the voltage output and en-ergy depends strongly on the electrode gap . As it is easierto change the electrode configuration than to deposit thick PZTfilms, mode provides more design flexibility with respect tooptimization of the performance. Besides, as is twice thatof , the mode pMUTs are capable of achieving highersensitivity.

Despite of the imperfection of the simplification, the resultingsimplified model provides clear insight into the dependencyof the transducer specifications on the IDE dimensions, and ittherefore can be used for FEM simulation.

III. FINITE-ELEMENT ANALYSIS

Finite-element analysis has been extensively used to developpiezoelectric microdevices [9], [11], [30], [31], since strongcoupling among multi-energy domains imposes great difficultyfor theoretical analysis. As the electric displacement in thePZT film is stress dependent, the voltage output can be solvedfrom (4) provided that the stress distribution in the PZT filmis known. The simplified model shown in Fig. 2 is used forfinite-element analysis so that the stress distribution and, inturn, the changes of the sensitivity versus the IDE dimensionscan be obtained.

A. FEM Modeling

A model of the laminated diaphragm is setup using ANSYS, and the solid model is transferred to finite-el-ement model by mapped meshing. An external pressure of 1 Pais applied on the top surface of the diaphragm. The dimensionsof the diaphragm and the IDE are listed in Table I.

Different element types are assigned for different materials.The buffer layer is very thin, so SHELL41 element,which is a 3-D element with in-plane stiffness, is assignedfor it. Since Si and are almost isotropic, a 3-D elementwith isotropic properties SOLID45 is assigned for them. Twoelements types are used for modeling the PZT film, i.e., a 3-Dcoupled-field solid element SOLID5 to model piezoelectricity

TABLE IIPIEZOELECTRIC CONSTANTS AND STIFFNESS CONSTANTS FOR PZT [33]

TABLE IIIMATERIAL PARAMETERS USED IN MODELING

and an anisotropic solid element SOLID64 to model elas-ticity [32]. SOLID5 is assigned to the PZT between adjacentelectrodes, which are well in-plane polarized and thus piezo-electric. Elastic SOLID64 is assigned to other PZT regions,including those beneath the electrodes and those unpolarized.The elastic PZT regions have the same material properties asthe piezoelectric regions except for without piezoelectricity.The piezoelectric coefficients and the elastic compliances ofPZT are listed in Table II, and the other material properties arelisted in Table III.

B. FEM Results

To solve the stress distribution in the PZT film, the staticstructural analysis is carried out using ANSYS. Fig. 3 showsthe vertical displacement of the diaphragm. Although the di-aphragm displaces downwards, it experiences different stressesin different areas. Fig. 4 shows the in-plane and the out-of-planecomponents of the stress. From Fig. 4(a), it can be seen that thein-plane components change from tensile at diaphragm bound-aries to compressive at the central area. The relative small valueof the out-of-plane components verifies that it is reasonable toneglect the out-of-plane components. To obtain high sensitivity,the thickness of the supporting layers, such as or Si, shouldbe carefully designed so that the PZT sit on above the neu-tral plane of the diaphragm to ensure identical stress polaritythrough the thickness of the PZT.

To explore the boundaries between the tensile and compres-sive stress areas, the stresses along the midlines and diagonalson the top surface of the PZT film are plotted in Fig. 5. The in-terfaces where the stress changes from negative to positive cor-respond to the stress boundaries. These boundaries form a smallsquare with size of around 65% of the diaphragm. It can also befound that the maximum stress occurs at the diaphragm bound-aries, in accordance with the electric field distribution, as shownin Fig. 6. Since the PZT has the same piezoelectric coefficient

over the whole diaphragm, the electric displacement and

Page 4: A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter Digital Electrodes

970 IEEE SENSORS JOURNAL, VOL. 7, NO. 7, JULY 2007

Fig. 3. Vertical displacement of the laminated diaphragm.

Fig. 4. Stress distributions in the laminated diaphragm. (a) In-plane compo-nent. (b) Out-of-plane component.

the induced charge change signs from positive in the area withtensile stress to negative in the area with compressive stress.

Fig. 5. Stress distribution in the top surface of the PZT film. (a) Along themidline. (b) Along the diagonal.

Fig. 6. Electric field distribution in the PZT film.

Therefore, the IDEs should be within the area with only ten-sile or compressive stress to avoid the counteraction of chargeand the resulting decrease of sensitivity caused by electrodes

Page 5: A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter Digital Electrodes

WANG et al.: A MICROMACHINED PIEZOELECTRIC ULTRASONIC TRANSDUCER OPERATING IN MODE 971

Fig. 7. In-plane polarized sensor with square interdigital electrodes.

across both the areas. In addition, the symmetry of the stress tothe orthogonal midlines implies that using square IDE instead ofconventional comb IDE to surround the inside boundaries of thediaphragm could increase the device capacitance. Accordingly,a square IDE is adopted in the area with either pure tensile orcompressive stress, as shown in Fig. 7. The configuration is op-timized with respect to large device capacitance to reduce elec-tric noise and obtain high sensitivity. The IDE in tensile stressarea experiences larger stress and have a larger size than com-pressive stress area, further increasing the capacitance and thesensitivity.

Besides the voltage sensitivity, the resonance frequency isanother important consideration to pMUTs. The resonancefrequency, calculated using ANSYS modal analysis, dependson the size and thickness of each layer of the diaphragm forgiven materials, and the IDE is neglected in modal analysisconsidering its small area and small thickness. Fig. 8 showsthe changes of the resonance frequency versus the width ofthe diaphragm and the thickness of the PZT layer. Other pa-rameters except for these variables are listed in Tables I–III.As expected, large diaphragm size and small thickness lead tolow resonance frequency. The supporting layers such asor silicon, though not shown, change the frequency with thesame manner as the PZT. Hence, the resonance frequency canbe increased at the cost of low sensitivity by increasing layerthickness or decreasing layer dimensions.

Fig. 8. Resonance frequency versus the diaphragm dimensions. (a) Diaphragmlength. (b) Thickness of PZT film.

Fig. 9. Fabrication process of in-plane polarized sensor. (a) Silicon wafercleaning. (b) Silicon thermal oxidation and silicon nitride deposition. (c) Se-lective removal of SiO and Si N . (d) Backside silicon etching. (e) Si Nand SiO removal. (f) Silicon thermal oxidation. (g) Backside SiO removal.(h) Titanium deposition and oxidation. (i) PZT thick film deposition. (j) Topinterdigital electrodes (IDE) deposition and patterning. (k) Backside silicondry etching.

IV. FABRICATION PROCESS

The pMUTs were fabricated using micromachining tech-niques; the main processes are shown in Fig. 9. The fabricationstarted from double-sided polished (100) silicon wafers withthermal dioxide and silicon nitride deposition. The siliconnitride on the backside was selectively removed using reactiveion etching (RIE), and the silicon dioxide was removed by

Page 6: A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter Digital Electrodes

972 IEEE SENSORS JOURNAL, VOL. 7, NO. 7, JULY 2007

Fig. 10. (a) X-ray diffraction pattern of the 15-layer PZT film. (b) SEM cross section of the 15-layer PZT film.

buffered hydrofluoric acid (BHF). Then, the silicon wafer wasetched with KOH from the backside to form square cavitiesuntil m silicon was left. After silicon wet etching,all the silicon dioxide and silicon nitride were removed byhydrofluoric acid.

Then, m thermal dioxide was grown on the wafer, and70 nm Ti was sputtered on , followed by annealing at

600 C and oxygen ambient for 3 h to oxide the titanium intotitania. Afterward, a 15-layer film wasdeposited by repetitive spin-coating and annealing. The PZTsolution was spun at 3000 rpm for 30 s, pyrolyzed on a hot-plate at 380 C for 10 min, and then annealed at 700 C inoxygen atmosphere. The Pb proportion in the PZT precursoris a little higher than the stoichiometry in the final annealedfilm to compensate the loss during annealing. After PZT depo-sition, 10 nm Ti and 150 nm Pt were sequentially sputtered onthe PZT film, and were patterned into square IDE configurationusing liftoff technique. Finally, the remained silicon was etchedaway using inductively coupled plasma (ICP) etcher to furtherimprove the device sensitivity.

The pMUTs were polarized for 5 min with a 10 m ex-ternal DC-voltage applied on the IDE. Then, the pMUT dieswere diced from the wafers, glued on a printed circuit board(PCB), and electrically connected with the circuits on the PCBwith wire bonding. After that, the pMUT die together with thePCB was packaged in a metal cap with circular inlet holes toprotect the dies and provide desired ultrasonic directivity fea-tures. The dimensions of the cap are 4 mm 6 mm, and thediameters of the circular holes are 0.9, 1.5, and 2.5 mm, respec-tively. A small outlet hole with a diameter of 0.2 mm is openedon the backside of the PCB to vent air in the rear of the packageand eliminate the influence of the airflow.

The X-ray diffraction of the PZT film was measured using anX-ray diffractometer (Rigaku D/max-RB, Japan), and the pat-tern is shown in Fig. 10(a). The (100)/(200) orientations are thehighest peaks, indicating the PZT film is well crystallized and(100)/(200) preferred. Fig. 10(b) shows the SEM photo (JEOLJSM-6301F, Japan) of the cross section of the 15-layer PZT.The total thickness of the 15-layer PZT is about 1.8 m, i.e.,120 nm per layer. Although the thickness of each layer could beincreased by using dense precursors, thick PZT film is prone tocrack during annealing because of residual stress. Fig. 11 shows

Fig. 11. Optical photo of the in-plane polarized pMUTs with square IDE andPMUTs with different packages.

the optical photo of a as-fabricated pMUT wafer and three pack-aged pMUTs with different packages. Since the diaphragm isvery thin, it is transparent after the remaining 50 m silicon isremoved.

V. ULTRASONIC CHARACTERIZATION

The resonant behaviors of the pMUTs were characterizedusing Agilent 4294A impedance/gain-phase analyzer. Thetop electrodes of the test pMUTs are in the central area ofthe diaphragm, where the stress is compressive, as shown inFig. 7(b). Fig. 12 shows the measured results of the impedance,

value, capacitance, and the phase angle versus the resonancefrequency. The first-order resonance frequency is 44 kHz,which is about twice the resonance frequency predicted bythe FEM, as shown in Fig. 8. This deviation could be mainlyattributed to the residual stress in the diaphragm. Due to themismatch in the thermoelastic properties and lattice parametersbetween the deposited films and the substrate, thermal residualstresses are induced in the films during the cooling process[36]. The residual stresses cause the variations of the structurestiffness, and thus influence both the devices sensitivity and theresonance frequency [37].

The ultrasonic response of the pMUTs is characterized usingthe testing system shown in Fig. 13. Ultrasonic sinusoidal sig-nals are generated using a function generator and transmittedto a speaker, which emits ultrasonic waves and applied perpen-dicularly to the diaphragm of a pMUT 28.5 cm away from thespeaker. The pMUT locates at the center of a revolving platform,which can rotate for 360 with a step of 5 . The voltage output

Page 7: A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter Digital Electrodes

WANG et al.: A MICROMACHINED PIEZOELECTRIC ULTRASONIC TRANSDUCER OPERATING IN MODE 973

Fig. 12. Resonance characteristics of the pMUTs. (a) Measured frequencyin 1 kHz�100 kHz. (b) Capacitance in-series and Q value (Cs-Q) at around44 kHz. (c) Impedance magnitude and phase (Z� �) at around 44 kHz.

Fig. 13. Testing system for ultrasonic directivity.

of the pMUT is displayed on an oscilloscope, and the magni-tude is recorded by a dynamic signal analyzer (Agilent 35670A).After the measurement at the position where the pMUT directlyfaces the speaker is finished, the pMUT is rotated by an angleof 5 , and the measurement described above is repeated to ob-tain the ultrasonic response at the new position. By revolving

the pMUT for 360 with a step of 5 and repeating the mea-surement, the directivity pattern of the pMUT is obtained. Thesame measurement is performed for different pMUTs with dif-ferent packages.

Fig. 14 shows the directivity patterns of different pMUTs.The angles 0 and 180 in the polar coordinates correspondto the positions where the front side and the backside of thepMUT face the speaker, respectively. It can be found that pack-aging exerts great influences on the directivity patterns. The pat-tern of a pMUT without package has multiple lobes at whichthe responses are comparatively stronger than other positions,as shown in Fig. 14(a). The main lobe appears at around 0 ,and several side lobes are distributed at the angles of 100 ,180 , 240 , and 300 , respectively. For a pMUT sealed in acap without holes, as shown in Fig. 14(b), the response at 0is quite small, because the ultrasonic waves are mostly isolatedby the cap when the pMUT faces the speaker. On the otherhand, the response is obvious at 180 , which means the ultra-sonic waves pass through the outlet hole on the PCB and act onthe diaphragm from the backside.

The directivity patterns depend strongly on the dimensionsof the inlet holes. As shown in Fig. 14(c), when there is asmall hole on the package, the lobe at angle 0 is the main lobeand becomes dominant as the diameter increases, as shown inFig. 14(d) and (e). This can be expected because more ultra-sonic energies reach the pMUT for large diameters, and evenmaintains when the pMUT rotates way from the direct facingposition. This concludes that the ultrasonic energy reaching thepMUT does not change remarkably with rotation of the pMUTin a large angle range, which expands as the diameter of theinlet holes increases. The side lobe at 180 is also dependent onpackages. When the backside of the pMUT faces the speakerand ultrasonic wave applies on the diaphragm from the back-side via the outlet hole, the air sealed in the chamber betweenthe diaphragm and the cap induces stress in the diaphragm.For completely sealed packages, the pressure that applied onthe cap compresses the air sealed in the chamber and vibratesthe diaphragm, and thus results in output. However, for thepackages with a large inlet hole, the pressures those applieson the diaphragm via the outlet hole and via the inlet hole arebalanced to some extent, because the air escapes the chambervia the inlet hole as the diaphragm vibrates. As a result, thestress induced in the diaphragm is not large, especially for largeinlet holes.

The directivity patterns depend on the area of the inletholes, as shown in Fig. 14(f) and (g). The side lobes at 180reduce with the increase of the inlet hole number. For one-holepackage, the side lobe at 180 is still evident compared withthe main lobe at 0 , and decrease rapidly to a negligible levelfor four-hole and six-hole packages. The reason responsiblefor the variation is as aforementioned, which is further con-firmed by the similarity of the directivity patterns shown inFig. 14(e) and (f), indicating that the area of the holes has crit-ical influence on the directivity patterns. The small differencein the patterns of the six-hole and the four-hole packages is dueto that the size and the location of the holes are not identical.

Since the main lobe at 0 is dominant for the package withlarge total area of holes, e.g., four-hole package or one-large-

Page 8: A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter Digital Electrodes

974 IEEE SENSORS JOURNAL, VOL. 7, NO. 7, JULY 2007

Fig. 14. Directivity patterns of the pMUTs with different packages. (a) No cap. (b) No holes. (c) One hole with a diameter of 0.9 mm. (d) One hole with a diameterof 1.5 mm. (e) One hole with a diameter of 2.5 mm. (f) Four holes with a diameter of 0.9 mm. (g) Six holes with a diameter of 0.9 mm.

hole package, inlet holes with large areas are beneficial tothe applications to receive signals in a specific direction andsuppress noise from other directions. Accordingly, pMUTsintegrated into array configurations [14] can be used in hand-writing recognition system [12], positioning reconstruction

system [10], and so on. For example, in handwriting recognitionsystems, two pMUTs are used to measure the distances betweenthe pMUTs and a handheld speaker. Then, the position of thespeaker, which is determined by the distances, are recorded andprocessed to recognize the handwriting as the speaker moves.

Page 9: A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter Digital Electrodes

WANG et al.: A MICROMACHINED PIEZOELECTRIC ULTRASONIC TRANSDUCER OPERATING IN MODE 975

VI. CONCLUSION

A mode pMUT operating in mode has been devel-oped using a laminated diaphragmstructure. The PZT thin film used as the sensing element was de-posited using multiple repetitive sol-gel method, and polarizedwith top IDEs to obtain in-plane polarization and operationmode. The diaphragm was optimized by changing its dimen-sions to achieve the desired frequency, and the electrode config-uration, e.g., the electrode gap, electrode length, and electrodewidth, were optimized to obtain high sensitivity. This avoidsthe thick PZT film needed for high sensitivity and demonstratesthe design flexibility of mode pMUTs. The pMUTs werefabricated using silicon micromachining technology, and squareIDEs were fabricated in the area with only either tensile or com-pressive stresses to ensure the sensitivity. Ultrasonic characteri-zation was performed to obtain directivity patterns for differentpackages, and the measurement results show that the directivitypatterns depend strongly on the number and the location of theinlet holes. Large area of inlet holes is beneficial to array config-uration of pMUTs for applications to receive signals in a specificdirection and suppress noise from other directions.

ACKNOWLEDGMENT

The authors would like to acknowledge Dr. Q. Zhang’s helpconcerning PZT film deposition, and wish to thank Prof. J. Hanfor her valuable suggestions in processing. They also acknowl-edge the reviewers for their constructive suggestions.

REFERENCES

[1] P. Muralt, “Ferroelectric thin films for micro-sensors and actuators: Areview,” J. Micromech. Microeng., vol. 10, pp. 136–146, 2000.

[2] D. Damjanovic, P. Muralt, and N. Setter, “Ferroelectric sensors,” IEEESensors J., vol. 1, no. 3, pp. 191–206, 2001.

[3] L. Du, G. Kwon, and F. Arai et al., “Structure design of micro touchsensor array,” Sens. Actuators A, vol. 107, pp. 7–13, 2003.

[4] V. Ferrari, A. Ghisla, D. Marioli, and A. Taroni, “Array of PZT py-roelectric thick-film sensors for contactless measurement of XY posi-tion,” IEEE Sensors J., vol. 3, no. 2, pp. 212–217, Apr., 2003.

[5] G. M. Krishna and K. Rajanna, “Tactile sensor based on piezoelectricresonance,” IEEE Sensors J., vol. 4, no. 5, pp. 691–697, Oct. 2004.

[6] M. Zhu, P. Kirby, and M. Y. Lim, “Lagrange’s formalism for modelingof a triaxial microaccelerometer with piezoelectric thin-film sensing,”IEEE Sensors J., vol. 4, pp. 455–463, Aug. 2004.

[7] M. S. Weinberg, “Working equations for piezoelectric actuators andsensors,” IEEE J. Microelectromech. Syst., vol. 8, no. 4, pp. 529–533,De. 1999.

[8] Y. Nemirovsky, P. Muralt, and N. Setter et al., “Design of a novelthin-film piezoelectric accelerometer,” Sens. Actuators A., vol. 56, pp.239–249, 1996.

[9] L. P. Wang and R. A. Wolf, Jr. et al., “Design, fabrication, and mea-surement of high-sensitivity piezoelectric microelectromechanical sys-tems accelerometers,” IEEE J. Microelectromech. Syst., vol. 12, pp.433–439, 2003.

[10] K. Yamashita, L. Chansomphou, and H. Murakami et al., “Ultrasonicmicro array sensors using piezoelectric thin films and resonant fre-quency tuning,” Sens. Actuators A., vol. 114, pp. 147–153, 2004.

[11] P. Muralt, N. Ledermann, and A. Barzegar et al., “Piezoelectric micro-machined ultrasonic transducers based on PZT thin films,” IEEE Trans.Ultrason. Ferroelectr. Freq. Control, vol. 52, no. 12, pp. 2276–2288,Dec. 2005.

[12] Y. P. Zhu, T. L. Ren, and Y. Yang et al., “Novel ferroelectrics-basedmicro-acoustic devices and their ultrasonic applications,” in Proc.IEEE Int. Electron Dev. Meeting, 15–13, 2004, pp. 51–54.

[13] Z. Wang, W. Zhu, and H. Zhu et al., “Fabrication and characterizationof piezoelectric micromachined ultrasonic transducers with thick com-posite PZT films,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control,vol. 52, no. 12, pp. 2289–2297, Dec. 2005.

[14] K. Yamashita, H. Katata, and M. Okuyama et al., “Arrayed ultrasonicmicrosensors with high directivity for in-air use using PZT thin film onsilicon diaphragms,” Sens. Actuators A, vol. 97–98, pp. 302–307, 2002.

[15] J. J. Bernstein, S. L. Finberg, and K. Houston et al., “Micromachinedhigh frequency ferroelectric sonar transducers,” IEEE Trans. Ultrason.Ferroelectr. Freq. Control, vol. 44, no. 5, pp. 960–969, Sep. 1997.

[16] M. Torndahl, M. Almqvist, and L. Wallman et al., “Characterizationand comparison of a cMUT versus a piezoelectric transducer for airapplications,” in Proc. IEEE Ultrason. Symp., 2002, pp. 997–1000.

[17] I. Ladabaum, X. C. Jin, and H. T. Soh et al., “Surface micromachinedcapacitive ultrasonic transducers,” IEEE Trans. Ultrason. Ferroelectr.Freq Control, vol. 45, no. 3, pp. 678–690, May 1998.

[18] A. S. Ergun, B. Temelkuran, and E. Ozbay et al., “A new detectionmethod for capacitive micromachined ultrasonic transducers,” IEEETrans. Ultrason. Ferroelect. Freq. Control, vol. 48, no. 4, pp. 932–942,Jul. 2001.

[19] D. W. Schindel, D. A. Hutchins, and L. Zou et al., “The design andcharacterization of micromachined air coupled capacitance trans-ducers,” IEEE Trans. Ultrason. Ferroelect. Freq. Control, vol. 42, no.1, pp. 42–50, Jan. 1995.

[20] A. G. Bashford, D. W. Schindel, and D. A. Hutchins, “Micromachinedultrasonic capacitance transducers for immersion applications,” IEEETrans. Ultrason. Ferroelect. Freq. Control, vol. 45, no. 2, pp. 367–375,Mar. 1998.

[21] O. Oralkan, A. S. Ergun, and J. A. Johnson et al., “Capacitive micro-machined ultrasonic transducers: Next-generation arrays for acousticimaging?,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 49,pp. 1596–1610, 2002.

[22] B. Xu, R. G. Polcawich, and S. Trolier-McKinstry et al., “Sensing char-acteristics of in-plane polarized lead zirconate titanate thin films,” Appl.Phys. Lett., vol. 75, pp. 4180–4182, 1999.

[23] B. Xu, L. E. Cross, and J. J. Bernstein, “Ferroelectric and antiferroelec-tric films for microelectromechanical systems applications,” Thin SolidFilms, vol. 377–378, pp. 712–718, 2000.

[24] S. J. Gross, “Micromachined switches and cantilever actuators basedon piezoelectric lead zirconate titanate (PZT),” Ph.D. dissertation, ThePennsylvania State Univ., Philadelphia, PA, 2004.

[25] E. Hong, S. V. Krishnaswamy, and C. B. Freidhoff et al., “Microma-chined piezoelectric diaphragms actuated by ring shaped interdigitatedtransducer electrodes,” Sens. Actuators A, vol. 119, pp. 520–526, 2005.

[26] H. G. Yu, L. Zou, and K. Deng et al., “Lead zirconate titanate MEMSaccelerometer using interdigitated electrodes,” Sens. Actuators A., vol.107, pp. 26–35, 2003.

[27] K. Yamashita and M. Okuyama, “Sensitivity improvement of di-aphragm type ultrasonic sensors by complementary piezoelectricpolarization,” Sens. Actuators A., vol. 127, pp. 119–122, 2006.

[28] “ANSI/IEEE Standard 176-1987 on piezoelectricity,” IEEE Trans. Ul-trason. Ferroelectr. Freq. Contr., vol. 43, no. 5, pp. 717–772, Sep. 1996.

[29] S. S. Gevorgian, T. Martinsson, and P. L. J. Linnkr et al., “CADmodels for multilayered substrate interdigital capacitors,” IEEE Trans.Microw. Theory, vol. 44, no. 6, pp. 896–904, Jun. 1996.

[30] Q. M. Wang, Z. Yang, and F. Li et al., “Analysis of thin film piezoelec-tric microaccelerometer using analytical and finite element modeling,”Sens. Actuat A, vol. 113, pp. 1–11, 2004.

[31] H. L. Li, J. H. Hu, and H. L. Chan, “Finite element analysis on piezo-electric ring transformer,” IEEE Trans. Ultrason. Ferroelectr. Freq.Control, vol. 51, no. 10, pp. 1247–1254, Oct. 2004.

[32] ANSYS I, ANSYS Release 8. 0 Documentation, 2003.[33] W. P. Mason and R. N. Thurston, Physical Acoustics: Principles and

Methods. New York: Academic, 1964.[34] D. R. Lide, Handbook of Chemistry and Physics. Boca Raton, FL:

CRC Press, 2003.[35] Titanium Dioxide—Titania. [Online]. Available: http://www.azom.

com/details.asp?ArticleID=1179#_Background[36] L. Lian and N. R. Sottos, “Stress effects in sol-gel derived ferroelectric

thin films,” J. Appl. Phy., vol. 95, pp. 629–634, 2004.[37] S. Lee, T. Tanaka, and K. Inoue et al., “Stress influences on the ultra-

sonic transducers,” Sens. Actuators A, vol. 119, pp. 405–411, 2005.

Chao Wang was born in China in 1981. He received the B.S. degree from theDepartment of Electrical Engineering and the M.Sc degree from the Institute ofMicroelectronics both from Tsinghua University, Beijing, China, in 2003 and2006, respectively. He is currently working towards the Ph.D. degree at theDepartment of Electrical Engineering, Princeton University, Princeton, NJ.

His research interests are micro/nanosensors, micro and nanofabrication tech-nologies, and biosensors.

Page 10: A Micro Machined Piezoelectric Ultrasonic Transducer Operating in d33 Mode Using Square Inter Digital Electrodes

976 IEEE SENSORS JOURNAL, VOL. 7, NO. 7, JULY 2007

Zheyao Wang was born in China in 1972. He received the B.S. degree inmechanical engineering and the Ph.D. degree in mechatronics from TsinghuaUniversity, Beijing, China, in 1995 and 2000, respectively.

From 1995 to 2000, he was with the Department of Precision Instruments,Tsinghua University, where he developed digital force sensors using piezoelec-tric materials. From 2000 to 2002, he was a Postdoctoral Research Fellow atthe Institute of Microelectronics, Tsinghua University, where he worked on sil-icon micromachining for RF applications with a special focus on ferroelectricthin films on silicon substrates. In 2002, he joined DIMES, Delft University ofTechnology, Delft, The Netherlands, as a Postdoctoral Researcher, and workedon silicon micromachined structures and components for 3-D packaging. Hecurrently is an Associate Professor at Tsinghua University. His research inter-ests include piezoelectric materials for microsensors, silicon micromachiningtechniques, MEMS, and biosensors.

Tian-Ling Ren (M’02–SM’05) was born in Ji’nan, Shandong Province, China,in 1971. He received the Ph.D. degree from the Department of Modern AppliedPhysics, Tsinghua University, Beijing, China, in 1997.

He is a Full Professor and Director of Micor/Nano Devices and Systems Divi-sion of the Institute of Microelectronics, Tsinghua University. He has publishedmore than 150 papers and 20 patents. His present research is focused on novelmicro/nano devices and systems, such as MEMS, nonvolatile memories, ferro-electric devices, and magnetic devices.

Prof. Ren is an Academic Committee Member of the Chinese Institute ofElectronics (CIE).

Yiping Zhu was born in China in 1981. He received the B.S. degree fromthe Department of Information and Control Engineering, Shanghai Jiao TongUniversity, Shanghai, China, in 2002. He currently working towards the Ph.D.degree at the Institute of Microelectronics, Tsinghua University, Beijing, China.

His research interests include silicon-based ferroelectric materials and de-vices and microacoustic devices and systems.

Yi Yang was born in Shandong Province, China, in 1978. He received theB.S. degree from the Department of Electrical Engineering, Fudan University,Shanghai, China, in 2001 and the Ph.D. degree from the Institute of Microelec-tronics, Tsinghua University, Beijing, China, in 2006.

He is currently a Research Assistant at Tsinghua University. His researchinterests include silicon-based piezoelectric devices and microacoustic devices.

Xiaoming Wu was born in China in 1973. He received the B.S. degree fromthe Department of Automatic Control, Beijing Institute of Technology, Beijing,China, in 1994 and the Ph.D. degree from the Department of Precision Instru-ments, Tsinghua University, Beijing, in 2002.

He is currently a Research Assistant at Tsinghua University. His re-search interests are MEMS sensors and actuators, BioMEMS, and integratedferroelectrics.

Haining Wang was born in China in 1978. She received the B.S. degree from theDepartment of Information Technology, Tianjian University, Tianjian, China,in 2001, and the M.S. degree from the Department of Electronic Engineering,Tsinghua University, Beijing, China, in 2004.

Huajun Fang was born in China in 1972. He received the Ph.D. degree fromthe Institute of Microelectronics, Tsinghua University, Beijing, China, in 2006.

He is currently an Assistant Researcher at Tsinghua University. His researchinterests are in micro/nano devices and systems.

Litian Liu was born in China in 1947. He received the B.S. degree from theDepartment of Electrical Engineering, Tsinghua University, Beijing, China, in1970.

Since 1970, he has been working on the research and development of semi-conductor devices and ICs. He is currently a Full Professor at the Institute ofMicroelectronics, Tsinghua University. He is the Vice Director of the Micro/Nanometer Technology Research Center, Tsinghua University. He has authoredor coauthored more than 150 technical papers on peer-reviewed journals. His re-search fields include MEMS, smart sensors, biochips, and novel semiconductordevices.

Prof. Liu is also a Senior Member of the China Institute of Electronics (CIE).


Recommended