Lecture 8-1 - University of Pittsburghqiw4/Academic/MEMS1082/Lecture 8-1.pdfNote: ∆ l is independent of l! It only depends on the voltage V 3, and piezoelectric coefficient Stacked

Department of Mechanical Engineering

Piezoelectricity

Lecture 8-1


Piezoelectricity

Piezoelectricity– Discovered by Pierre Curie and Paul Jacques

in 1880– Generating an electric charge in a material

when subjecting it to applied stress, and conversely, generating a mechanical strain in response to an applied electrical field

Piezoelectric mat’l

Force or strain

Induced ∆Q, ∆V Piezoelectric mat’l

Induced force or strain

V



Piezoelectric Effect


For isotropic dielectric materials, [ε ] matrix reduced to scalar ε, Gauss’s law may be rewritten as

Dielectrics without center symmetry Piezoelectricity

∫ =⋅S

qdaDThis explain why D is often called the charge density

For dielectric materials, electric dipoles, i.e., closely coupled pairs of charges, will result in electric polarization, P, which is equal to the bound charge density.

Representation of total, free, and bound charge densities by field vector

PED o += ε



In some dielectric materials (crystals, ceramics, polymers) without center symmetry, an electric polarization can be generated by the application of mechanical stresses.----Piezo-electricity

– P = d σ, direct effect– ε = d E , converse effect

P: polarization (pC/m2)σ : stress (N/m2)ε :straind: piezoelectric coefficient (pC/N or m/V)


Piezoelectric constant & coupling coefficient

Piezoelectric constants– d [C/N] = (charge developed)/(applied stress) – g [V-m/N] = (Electric field developed)/(applied stress) – h [m/V]=(Strain developed)/(applied E-field)– e [N/V-m] = (Stress developed)/(applied E-field)

Electromechanical coupling coefficient (k)– Parameter used to compare different piezoelectric

materials– A measure of the interchange of electrical & mechanical

energy


3-D expression General expression

– Piezoelectric effect is orientation dependant– 1-D 3-D

∑=

=6

1kkiki dP σ

(E=0, no E-field)

1,2,3 axial stress, 4,5,6 shear stress σ1 = σx, …..

=

6

5

4

3

2

1

363534333231

262524232221

161514131211

3

2

1

σσσσσσ

dddddddddddddddddd

PPP

∑=

=3

1

~i

iikk Edε (σ=0, no stress)

=

3

2

1

362616

352515

342414

332313

322212

312111

6

5

4

3

2

1

EEE

dddddddddddddddddd

εεεεεε

polarization stress

strainField

i


For Piezoelectric PZT, BaTiO3, PbTiO3

∑=

=3

1

~i

iikk Edε

(σ=0, no stress)

=

3

2

1

15

15

33

31

31

6

5

4

3

2

1

0000000

000000

EEE

dd

ddd

εεεεεε

∑=

=6

1kkiki dP σ

(E=0, no E-field)

=

6

5

4

3

2

1

333131

15

15

3

2

1

0000000000000

σσσσσσ

dddd

d

PPP

Piezoelectric Coefficients



∆L=d31 ·V3 · L/t, ∆w=d31 ·V3 · w/t , ∆t=d33 ·V3

- When a voltage is applied across the thickness of the piezoelectric materials

- When a force F, is applied, in the length, width or thickness directionV3=d31 ·F1/(ε11 ·L), V3=d31 · F2/(ε11 ·w), V3=d33 · F3 · t/(ε33 ·L ·w)


P=d σ

Open circuit voltage



Change in length per unit applied voltage

3333 Ed=ε

lVd

ll 3

33=∆

( ) ( )nm

VVmVdl37.0

1/10370 12333 ××==∆ −

Note: ∆l is independent of l! It only depends on the voltage V3, and piezoelectric coefficient

Stacked actuator



HereT: thicknessF: applied force




Piezoelectricity

Piezoelectric Materials– Ceramics

Pb(ZrTi)O3 (PZT), PbTiO3 (PT), etc.– Single crystals

Quartz, LiTaO3, LiNbO3, PZN-PT,etc– Polymers

PVDF and copolymers, nylon, etc.– Composites

PZT-polymer 0-3, 2-2, 1-3 composites, etc.– Thin/thick films

PZT, PT, ZnO and AlN films

Important parameters for piezo- materials– piezoelectric strain coefficient d (m/V)– piezoelectric voltage coefficient g(Vm/N)– electromechanical coupling k33, k31, kt

– dielectric constant K– dielectric loss tangent tanδ– mechanical quality factor Q– acoustic impedance ρc

Property Unit PZTceramic

PVDF ZnOfilm

PZT film(4 µm on Si)

d33 (10-12)C/N 220 -33 12 246d31 (10-12)C/N -93 23 -4.7 -105d15 (10-12)C/N 694 -12 ?Κ3 ε33/εo 730 12 8.2 1400

tanδ 0.004 0.02 0.03k31 0.31 0.12Q 400ρc (106)kg/m2-

sec30 2.7

Typical properties of PZT, PVDF, ZnO

Crystalline quartz

0000000000

000

2625

141211

ddddd

d11 = -d12 = -d26/2 = 2.31 pC/N, d14 = -d25 = 0.73 pC/N


Piezoelectricity

Piezoelectric Composites for transducer applications


Piezoelectric longitudinal andtransverse effect

Piezoelectric multilayer andbimorph actuators

Piezoelectric Shear mode actuator

Longitudinal multilayer actuatorLarge output force, low displacement

Shear mode actuatorMedium force and displacement

Bending mode actuatorLow force, large displacement

Piezoelectric actuators and sensors




PZT film deposition - Sol-gel method

Well Studied and widely used for PZT films

Organometallic compounds (such as metal alkoxide) as precursors

All chemicals dissolved in solvent to form a solution (sol)

Polymerization to produce a gel with a continuous network

AdvantagesHomogeneity, mixing in molecular

levelLow processing temperaturePrecise control of stoichiometric


PZT film deposition by sol-gel processing

-----Solution preparation

Refluxed/dehydrated150oC

Mixed in N2 atm.PZT composition

Pb-excess

Titaniumisopropoxide

Ti[OCH(CH3)2]4

Zirconiumn-propoxideZr[OC3H7]4

Mixed/refluxed80oC

De-ionizedH2O

Ethylene glycolCH2OHCH2OH

Lead acetate trihydratePb(CH3COO)23H2O

Acetic acidCH3COOH

0.9 M solution Stable in air

For film spin-on coating


PZT film deposition by sol-gel Processing- Thin/thick film deposition

O.9 M solution

Pre-annealing600oC

400oC to remove residual organics

Hot plate

Spin-on coating7500 rpm, 30 sec.

Pt/Ti/SiO2/Si substrate

Dried at 105oC

To densify the layer

Annealing700oC

Film with desired thickness

Multilayercoating

Formation of PZT films


P-E Hysteresis Loop

• Sol-gel PZT• 5.0 µm• High P


Piezoelectric Accelerometer



Piezo Micro-accelerometer: (a) the front side with the interdigitated electrodes (see inset), and (b) shows the proof mass and the accelerometer frame.



The setup for the frequency response measurement, A and B are charge amplifiers. The reference accelerometer and the test accelerometer are mounted on top of each other. Frequency response of an accelerometer

with a resonance frequency of 24.1 kHz and sensitivity of 0.53 pC/g



Schematic view of the mass deflection of a DRIE (a) and KOH (b) etched mass for perpendicular (a) and parallel (b) acceleration, respectively.



Fabrication process of the triaxial accelerometer.

(a) Depositing all layers, silicon dioxide, platinum bottom electrode, PZT, platinum top electrode and gold bond pads;

(b) patterning of top electrode, PZT and bottom electrode;

(c) silicon (DRIE) and silicon dioxide (RIE) dry etching of the front using the bottom electrode as a mask;

(d) DRIE of the back.


Piezoelectric Accelerometer Schematic of fabrication process.

Typical accelerometer impact response


Microactuators– ink droplet ejectors (printhead)– piezoelectric transformers– piezoelectric scanning tunneling microscope tip

Microsensors – accelerometers– micro-resonators– surface acoustic wave (SAW) devices– underwater acoustic imaging sensors


Performance Criteria – Actuators

generative force/momentumdisplacement frequency response

– Sensors sensitivity frequency response stability or repeatability


Piezoelectric MEMS Devices

Silicon

Silicon NitridePZT

Passivation layerElectrodesConnection pads

Piezoelectric PZT-on-Si cantilever resonantor

Cantilever


Piezoelectric Micropumps Pump


Photoresist patternOxide etching

Boron-diffused layer5-10 m, as etch stopµ

oxide removal

LTO deposition

Pt/Ti metal layersputtering

PZT layer depositionby sol-gel spin-oncoating

EDP or KOH etchingto form cavity

PZT patterning bywet chemical etching

Top Pt/Ti metal layersputtering & patterning

Acoustic Imaging Sensors

n-Si (100) waferOxdized


Piezoelectric Printhead


Piezo- actuator deforms when electrical pulseapplied

Example: Piezo Ink Jet

XeroxLexmark...


Example: Piezo Ink Jet Printheads

Three Types:

Using multilayer piezoelectric (PZT) ceramic actuator arrays

– Rod type.



– Chip type

Using bending mode PZT ceramic actuators arrays

substrate

nozzle

actuatorInk chamber

nozzle plate



– Shear mode

Using diced PZT ceramic shear mode actuators as ink chamber walls

Cover plate

Cover plate Fluid manifold


Example: Thermal Ink Jet Printheads Power FETs immersed in

caustic ink

Nozzle Plate

Protective OvercoatsConductorResistive FilmThermal Barrier

Ink Drop

BubbleThermalRegion

To Paper

Nozzle


STEP1: Initial conditions

STEP2: Resistor heated upon command and liquid vaporizes instantly causing a vapor bubble to form.

STEP3: Vapor bubble grows to maximum size and ink ejected out of nozzle.

STEP4: The bubble collapses and breaks off. Nozzle returns to initial condition.

1 2

3 4

Example: Thermal Ink Jet Printheads


Nozzles

Electrical contacts

Example: Thermal Ink Jet Printheads


Monochrome cartridge1020, 1000, 1100, 2030, 3000, 2050Black = 56 nozzlesColor = 48 nozzles

2070 Black = 104 nozzlesColor = 96 nozzles

7000, 7200, 3200, 5000, 5700, Z51 (Excimer tech)Black = 208 nozzlesColor = 192 nozzles

Example: Inkjet Printhead---Nozzle Plates

Color Cartridge


5700 Laser Crafted Nozzles (600x)

5700 Laser Crafted Nozzles (50x)

Nozzles Micro Photo


Layers

Example: Inkjet Printhead--- A Semiconductor Technology


Thermal Ink Jet vs Piezo Ink Jet Thermal

– Higher power required– High nozzle density– Ejectors very small,

approximately same size as drops

– Inexpensive to make Piezoelectric

– Low power consumption– Ejectors are large due to low

strain rates– Expensive to make Why Piezo- Printhead ?

Reliability♦ no excessive heating problem

Fast frequency response Low energy consumption Scalability Drop modulation Ink compatibility

Piezo-electric transducers

Thermal Heating

Documents

Lecture 8-1 - University of Pittsburghqiw4/Academic/MEMS1082/Lecture 8-1.pdfNote: ∆ l is independent of l! It only depends on the voltage V 3, and piezoelectric coefficient Stacked