ELECTRONICS DEVICES AND MATERIALS

Atsunori KAMEGAWA

kamegawa@material.tohoku.ac.jp

���� �� ������

4-1�

FERROELECTRIC DEVICES�

1. Capacitor ~high permittivity devices~ 2. Piezoelectric Devices 3. Pyroelectric Devices 4. Ferroelectric Memory Devices 5. Electrooptic Devices

4-2�

VARIOUS EFFECTS IN MATERIALS�

Charge Current�

Magnet-ization� Strain�

Temper-ature� Light�

Electric Field�

Permittivity Conductivity�

Elect.-Mag. efficient�

Converse piezo-effect�

Elec. Caloric effect�

Elec.-optic effect�

Magnetic Field�

Mgg.-elect. effect� Permeability� Magneto-

striction�Mag.caloric

effect�Mag.optic

effect�

Stress� Piezoelectric effect�

Piezomag. effect�

Elastic constant�

Photoelastic effect�

Heat� Pyroelectric effect�

Thermal expansion� Specific heat�

Light� Photovoltaic effect� Photostriction� Refractive

index�

Permittivity Conductivity

Mgg.-elect. effect

Piezoelectric effect

Pyroelectric effect

Photovoltaic effect

Converse piezo-effect

Magneto-striction

Elastic constant

Thermal expansion

Photostriction

Sensor� Actuator�

Input�Output�

Input� Material Device� Output�

4-3�CERAMIC CAPACITORS ~HIGH PERMITTIVITY DIELECTRICS~�

The basic Specifications required for capacitors: �! Small size, large capacitance

Materials with a large dielectric constant are desired.

! High frequency characteristics Ferroelectrics with a high dielectric constant are sometimes associated with dielectric dispersion, which must be taken into account for practical applications.

! Temperature characteristics We need to design materials to stabilize the temperature characteristics.�

4-4�

MULTILAYER CERAMIC CAPACITOR (MLCC)�

C=ε0εr =nε0εr A d

S L/n

External Electrode� Internal Electrode� Dielectric�

Dielectric Layer Thickness (µm)�

Number of Layers�

Die

lect

ric L

ayer

Thi

ckne

ss (µ

m)�

Num

ber o

f Lay

ers�

4-5�

TEMPERATURE CHARACTERISTICS�

Categories of dielectrics �thermal compensation type (TiO2) �high frequency, filtering, amplifier cir. �high permittivity type (BaTi03) �coupling or decoupling circuite�

-100

-80

-60

-40

-20

0

20

140120100806040200-20-40-60

Al-Electrolytic�

Ta-Electrolytic�

Ceramic(TiO2)

Ceramic(BaTiO3)

Temperature / �

Cha

nge

rate

of C

apac

itanc

e(%

)

BaTiO3

Shifter additive (decreasor: Sr, elevator: Pb� Depressor additive �MgTiO3, CaTiO3 ��

PMN

Combination of ferroelectrics with deferent composition (deferent Curie Temp.)�

Some approach to improve Temp. Characteristics�

Sr

MT

4-6�

PIEZOELECTRIC DEVICES�

Ps � ��

���

Piezoelectric Figures of Merit�Piezoelectric Strain Constant: d

x=dE External electric field: E, magnitude of he induced strain �x

Piezoelectric voltage constant: g

External stress: X, induced electric field: E

E=gX

Therfore� g= d ε0εr

An important figure of merit for actuator applications�

An important figure of merit for sensor applications�

Relationship of polarization and piezoelectric strain�

P=ε0ε(X) E

E= = P ε0ε(X)

dX ε0ε(X)

Fundamental Piezoelectric equations:�

x=c(E)X+dE P=dX+ε0ε(X)E

external E=0 P=dX

4-7�OTHER IMPORTANT FIGURE OF MERIT FOR PIEZOELECTRIC APPLICATIONS�

Electromechanical Coupling Factor: k

k2= Stored mechanical energy Input electrical energy�

Conversion rate between electrical energy and mechanical energy �

Mechanical Quality : Qm

Qm= ω0 2Δω ω0:resonance frequency�

The inverse of mechanical loss�

Acoustic Impedance : Z Evaluating the acoustic energy transfer between two materials �

4-8�

APPLICATIONS OF PIEZOELECTRICS�

Gas igniter Pressure sensor, Accelerometer,�

Piezoelectric transformer�

Piezo-actuator�Ultorasonic Motor�

Stress → Internal electrical field �

External electrical field →Stress

→ Internal electrical field �

External electrical field →Stress �

E1 E2

ω

Mass�

Acceleration�

Basic structure of an accelerometer�

Basic structure of a piezoelectric transformer�

4-9� PZT’S COMPOSITION AS PIEZOELECTRIC DEVICES�

PbZrO3 PbZrO3 [mol%] PbTiO3

Tem

pera

ture

/�

d15

d33

-d31

PbZrO3 [mol%]

Morphotropic phase

boudary

Dependence of several d constants on composition near morphotropic phase boundary in the PZT system�

4-10�

CERAMIC ACTUATOR MATERIALS ~PIEZOELECTRIC, ELECTROSTRICTIVE

AND PHASE CHANGE MATERIAL ~�

Piezoelectric material, (Pb, La)(Zr, Ti)O3, Ba(Sn, Ti)O3

Electrostrictive material, Pb(Mg1/3Nb2/3, Ti)O3

Phase change material, Pb (Zr, Sn, Ti)O3

Electrostriction �ΔyEy

2

Piezoelectric �ΔxEx

Field� Field� Field�

Field� Field�

stra

in�

stra

in�

4-11�

PYROELECTRIC DEVICES�

Ps

Principle of pyroelectric sensor�

Cur

rent�

Infrared irradiation�

A temperature increase due to the infrared irradiations (such as human body) �� Spontaneous polarization decrease ��� Variation in electric charge (or current)

Types of Infrared-sensors � semiconductor type ��Imaging and its processing � pyroelectric type (Pyrosensor) ��Infrared light sensors, ��Temperature sensors The merits of pyrosensors � wide range of response frequency � use at room temperature � quick response in comparison with other temperature sensors � low costs�

4-12�

PYROELECTRIC RESPONSE�

Light Intensity�

Temperature rise�

Pyroelectric current�

Time�

Time�

Time�

Pyroelectric response to chopped IR irradiation�

Current responsivity�ri

ri= ηp ρCph

η: transmittance of incident radiation p�pyroelectric coefficient ρ: density of pyro-material Cp :specific heat of detector h :thickness of detector�

Example of pyro-material for IR detector: �

(Ba, Sr)TiO3�

4-13�

FERROELECTRIC MEMORY DEVICES�

FeRAM (FRAM)

1

0

E

P

Ferroelectric for memory-cell (FeRAM)

1

0 E

P

Paraelectrics for memory-cell (DRAM)

Word Line�

Plate Line�

Bit Liine�

Ferroelectric cell�

Non-volatile memory device �(cf. MRAM, PRAM)

Pr

-Pr

When External Field is Zero, memorized information is non-volatile!�

-Pr

1 1

4-14�

EQUIVALENT CIRCUITS AND SCHEMATIC STRUCTURES OF TYPICAL MEMORY DEVICES �

Equivalent Circuit�

Memory State�

1

0

Word Line�

Bit Line�

Word Line�

Bit Line�

Word Line�

Plate Line�

Bit

Line�

Sou

rce�

Word Line�

Dra

in�Floating Gate�

Bit

Line�

Word Line�

Plate Line�

Crystalline�

Amorphous�

Insulator�Electrode�

Electrode�

+ + + +

- - - -

Control Gate�

Si substrate�Floating Gate�

- - - -

Electrode�

Electrode�

Ferroelectric Film�

+ + + + - - - -

+ + + + - - - -

Insulator�

Ferromagnetic Films�

DRAM Flash EEPROM FeRAM MRAM PRAM

4-15� PROBLEMS OF FERROELECTRICS FOR FERAM APPLICATIONS�

Leak-current� Dielectric breakdown�

Fatigue�

E

Q

Imprint�

E

Q

Retention�

E

Q

E

I

t

Q

4-16�

FERROELECTRIC MATERIALS FOR FERAM�

PZT (Pb(Zr,Ti)O3) SBT (SrBi2Ta2O9) Large Pr Relatively easy fabrication Relatively large Ec Large Fatigue Preparation conditions �Pb-composition� La addition → leakage & Ec Ca, Sr additions

→ Imprint and Retention Oxide Electrode, Buffer �����→ Fatigue Properties Film Orientation → Pr, Fatigue �����reduction of thickness�

Low Ec�low operation Voltage� Good tolerance for Fatigue Low Pr Difficult preparation conditions �High Temp., Bi-composion� Nb addition → Increment of Pr SBT-BT composite

→ dielectric properties�

Material

Merits

Demerits

Improvement Approach

Ramtron, Fujitsu, … Symetrix, Panasonic, NEC, …

4-17�

ELECTROOPTIC DEVICES�

Fundamental Construction of an electrooptic light shutter�

Δn=- Rn3E2 1 2

Second-order electrooptic effect (Kerr effect)�

Birefringence: Δn

R: quadratic electrooptic coefficient n: original refractive index (E=0) E: electric field�

L

-45��45��

Polarizer� Analyzer�

PLZT

Electric Field: E

Light Shutter, Light Switch, Waveguide Modulator�Typical Application of Electrooptic Devices�

First-order electrooptic effect (Pockels effect)

○ high speed, high contrast, gradation × high cost, high operation voltage�

4-18�

Birefringence�

RELATION BETWEEN PLZT COMPOSITION AND STRUCTURE AND ELECTROOPTIC APPLICATION�

PbZrO3 PbZrO3 [mol%] PbTiO3

La [m

ol%

]

Rhombohedral, Ferro� Tetragonal , Ferroelectric Phase�

Antiferroelectric�

Cubic, Parraelectric�

1st EO effect�Memory�

PLZT:(Pb1-xLax)(Zr1-yTiy)1-x/4O3�

4-19�POLARIZATION AND BIREFRINGENCE AS A FUNCTION OF ELECTRIC FIELD FOR PLZT�

Polarization P and Birefringence ∆n as a Function of Electric Field E for some PLZT ceramics�

PLZT 9/65/35 PLZT 7/38/62

2nd order EO effect� 1st order EO effect�P

-E

Δn-

E

Field E[kV/cm]

Field E[kV/cm]

Δn(

x10-

3 )

P P

4-20�

DIELECTRIC MATERIALS OF ELECTROOPIC DEVICES�

Pockels (1st) and Kerr (2nd) electrooptic coefficients for various materials�

Material�

Primary electrooptic coefficient�

Secondary electrooptic coefficient�

4-21� REQUIRED PROPERTIES FOR ELECTROOPTOIC DEVICES�

Required Properties for electrooptoic devices � high electrooptic coefficient � good mechanical properties�especially high toughness� � high transmittance � Δn close to zero, when external electric field is zero � low porosity, high sintered density�

(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3�

Tran

dmitt

ance

(%)

Ref

ract

ive

Inde

x n

Ele

ctro

optic

coe

ffici

ent

R/x

10-1

6 m2 V

2

Composition x

(1-x)PMN-xPT

Composition x Composition x

4-22�