Micro/Nanosystems Technology - Uni Kiel

Micro/Nanosystems TechnologyWagner / Meyners 1

Micro/Nanosystems Technology

Prof. Dr. Bernhard Wagner

Dr. Dirk Meyners

Piezoelectric

MEMS


Micro/Nanosystems Technology

Prof. Dr. Bernhard Wagner

Dr. Dirk Meyners

Piezoelectric

MEMS


Piezoelectric vs. Capacitive MEMS

Piezo-MEMS of advantage due to:

• Sensors

• no power input required: Charges are generated by

external action

• Actuators

• higher energy density: Large force actuation at low

voltages

• plus:

• scales well with decreasing feature size

• can be used for energy harvesting

• less sensitive to environmental impacts:

no hermetic packaging required


Outline

Dielectricity and piezoelectricity

Polarization

Piezoelectric thin films

Piezoelectric thin film materials

Piezo-MEMS


mechanical

energy:

Stress, strain

electrical

energy:

Charge,

El. field

“Piezo” from Greek meaning “to press”

Conversion of mechanical into electrical energy and vice versa

Direct piezoelectric effect

sensing effect

Converse piezoelectric effect

actuation effect

Dielectric, piezoelectric, pyroelectric and

ferroelectric materials


Piezoelectricity and Free Energy

Heckman diagram of free

energy

Piezoelectricity is related to a change in free energy F



crystalline dielectrics:

dielectrics: 32 point groups

Dielectric

strain





Piezoelectric

strain

piezoelectrics: 20 non-

centrosymmetric point groups





Pyroelectric

strain



pyroelectrics: 10 polar point

groups

strain





Ferroelectric

strain




groups

Ferroelectric

strain





Ferroelectric

strain




groups

Ferroelectric


polar materials

Dielectric, pyroelectric and ferroelectric

materials


polar materials

Dielectric, pyroelectric and ferroelectric

materials

not incorrect, but not always/never observed!


Piezoelectric materials

- -

-

strained crystal

Cm moment dipoleelectric

unstrained crystal

- -

-

F

VP


Piezoelectric materials

- -

-

strained crystal


unstrained crystal

- -

-

F

VP

wrong


So what is Polarization?

VP

wrong



VP

wrong



VP

wrong

no unique

Definition for



VP

wrong

no unique

Definition for



VP

wrong

no unique

Definition for



VP

wrong

no unique

Definition for

well defined



- -

-

strained crystal


unstrained crystal

- -

-

F

only changes in polarization are uniquely defined



- -

-

strained crystal


unstrained crystal

- -

-

F

only changes in polarization are uniquely defined correct



How do we measure polarization?

AGAIN: We measure only changes as P0 unknown

Changes with respect to what?

…a question that really matters…

Wurtzite Polarization constants:

in units of C/m²



How do we measure polarization?

AGAIN: We measure only changes as P0 unknown

Changes with respect to what?

…a question that really matters…

Wurtzite Polarization constants:

in units of C/m²

different reference structures

were used here…


favorable aligned ferroelectric domains grow in electrical fields,

pinning during domain growth:

ferroelectric hysteresis

name in analogy to ferromagnetic materials

Ferroelectric materialsFerroelectric materials


favorable aligned ferroelectric domains grow in electrical fields,

pinning during domain growth:

ferroelectric hysteresis

name in analogy to ferromagnetic materials

Ferroelectric materialsFerroelectric materials

sometimes, true

unfortunately, not a very

good analogy

short range vs. long range

interaction

dipoles vs. monopoles

etc…


EP

0

electrical polarisation C/m2

dielectric displacement C/m2

electrical field V/m

electrical susceptibility -

dielectric constant, permittivity -

linear dielectrics (small field strength), P0 = 0

PEED r

00

D

1r

E

contribution from

external field

contribution from

polarized material

P

Vm

C12

0 1085.8

r

Dielectrics: Susceptibility and permittivity


EP

0

electrical polarisation C/m2

dielectric displacement C/m2

electrical field V/m

electrical susceptibility -

dielectric constant, permittivity -

linear dielectrics (small field strength), P0 = 0

PEED r

00

D

1r

E

contribution from

external field

contribution from

polarized material

P

Vm

C12

0 1085.8

r


here we think of


isotropic materials

EDEP

r

II and II

scalars are ,

anisotropic materials

parallel more no are , ,

tensors are

DEP

r

,

3

2

1

333231

232221

131211

3

2

1

E

E

E

D

D

D

= 0 r



Piezoelectric constants: d

EdD

ET sEdx

D: electrical displacement vector in C/m2

E: electrical field vector in V/m

: electrical permittivity matrix (3x3) in C/Vm = F/m

: stress vector (6x1) in N/m2

x: strain vector (6x1) in m/m

s: elastic compliance matrix (6x6) in m2/N; s=c-1

d: piezoelectric coupling matrix (3x6) in C/N = m/V

coupling between electrical and mechanical parameters

Direct effect

Converse effect

sE : stiffness at constant E

upper index T: transposed matrix

363534333231

262524232221

161514131211

dddddd

dddddd

dddddd

d


EexD x

xcEe ET converse effect

E

jkijik cde

direct effect

c: elastic stiffness matrix (6x6) in N/m2; c = s-1

e: piezoelectric coupling matrix (3x6) in C/m2

Piezoelectric constants: e


Piezoelectric Thin-Films

Piezoelectric bulk materials are well established:

Quartz, PZT, LiTaO3, LiNiO3, …

Piezoelectric thin films on silicon:

- only niche technology for long

- has recently emerged to highly recognized research field

- enables sensing and actuation in MEMS

- dominant materials: PZT, AlN, ZnO


piezoelectric thin films are polycrystalline

in-plane directions 1 and 2 are equivalent

cylindrical symmetry around 3-axis

only 3 independent coefficients: d33, d31, d15

Convention: polarization axis has index 3:

usually normal to film surface

notation

P

Piezoelectric coefficient symmetryPiezoelectric coefficient symmetry


000

00000

00000

333131

15

15

ddd

d

d

d

d33 longitudinal polarisation parallel to strain or stress

d31 transverse polarisation normal to strain or stress

d15 shear electric field normal to polarization,

Piezoelectric coefficient symmetry

stiffness (and compliance) for PZT, BaTiO3, AlN,…:

E

E

E

EEE

EEE

EEE

E

c

c

c

ccc

ccc

ccc

c

66

55

44

332313

232212

131211

00000

00000

00000

000

000

000


Piezoelectric coefficients for thin films

Properties of piezoelectric films cannot be compared to bulk values

In coefficient measurement thin film is clamped to rigid substrate

in-plane strains stay zero: x1 = x2 = 0

In coefficient measurement thin film is free to relax normal to surface

out-off plane stress stays zero:

In film plane, polycrystalline material is isotropic:

0)( 331112111 Edssx

31,31 / Ee f

Set of effective piezoelectric

coefficients for thin films

which can be measured directly:

d33,f, e31,f, 33,f

Muralt, Integrated Ferroelectrics

17(1997) 297-307Ess

03

21


Absolute value of e31,f is always larger than bulk e31

e31,f measurement beam bending method: E3= f(1)

Y = Young‘s modulus

= Poisson‘s ratio

0)( 331112111 Edssx

31,31 / Ee f Ess


33

33

1331

31

1211

31,31

1e

c

ce

Yd

ss

de f

|e31,f |> |e31|


3331133 2 Edsx

33,33 / Exd f

d33,f < d3331

1211

1333,33

2d

ss

sdd f

d33,f measurement:

measure strain x3 = f(E3)

using laser interferometer

)(

2

12110

2

3133,33

ss

df

33,33 f



Ledermann Sensors & Actuators A105 (2003) 162-170

Piezoelectric equations for thin films


Longitudinal effect

3,333 fdD

Bulk mode actuation

El. input: E3

Mech. output: vertical strain x3

excitation of bulk vibrations

E3

Si

x3

D3

Si

3

Charge generation

Mech. input: 3

El. output: D3

3,333 Edx f


El. input: E3

Mech. output: in-plane strain 1

beam bending

3,311 Ee f

Transverse effect in actuation application

Piezoelectric thin filmE3

Si

1

Bidirectional

deflection


Transverse effect in sensing application

Mech. input: load F

=> in-plane strain x1

El. output: el. displacement D3

1,313 xeD f

1

F

Piezoelectric

thin film

x1

D3

Si 3


330

1,31

33

dxe

A

Qd

C

QV

f

o

A

QxeD f 1,313

Voltage response coefficient:

330

,31

fe

Signal-to-noise ratio:

(current and voltage) tan330

,31 fe

N

S

low power sensing principle

high sensitivity

static (d.c.) sensing not possible due to charge leakage: min ~1 Hz a.c

tan: loss tangent

Piezoelectric Sensing

Current response coefficient: fe ,31


Properties of thinfilm piezoelectrics

ZnO AlN PZT

e31,fC/m2 -1.0 -1.3 -12 … -25

d33,fpm/V 5.9 5.2 60 …150

3310.9 10.5 300…1300

e31,f /033GV/m -10.3 -11.3 -2.2 … - 4.5

tan @1-10kHz 0.01…0.1 0.003…0.01 0.01 … 0.07

S/N 105 Pa1/2 3…10 24 8.8…13.5

c33GPa 208 395 98

PZT is optimum for piezoelectric actuation

AlN is optimal material for piezoelectric sensing

Properties of thin-film piezoelectrics


Piezoelectric thinfilms: PZT

Solid solution of lead zirconate and lead titanate

PZT shows highest piezoelectric d and e coefficients

disadvantage: lead-containing, non-IC-compatible, stoichiometry is critical

Lead zirconate titanate: Pb (Zrx Ti1-x) O3

Sauerstoff: O2-

Blei: Pb2+

Zirkon: Zr4+

Titan: Ti4+

tetragonal

phase

Perovskite structure:

ABO3


Cubic phase above

Curie temperature Tc

is not piezoelectric,

but paraelectric,

Zr or Ti is in cell center

Morphotropic

phase boundary:

•At room temperature:

PbTiO3-content 48%

•competition between

tetragonal and rhombo-

hedral phase enhances

number of polarizable

directions to 14

nomenclature:

PZT 52/48 (PbTi0.48Zr0.52O3)

PZT phase diagramm


Piezoelectric and dielectric coefficients strongly depend on stoichiometry

Maximum values close to morphotropic phase boundary composition

Coefficients are also PZT-texture dependent

Bottom electrode as nucleation layer to tune PZT texture:

e.g. (111) textured Platinum

Ledermann S&A 2003

PZT stoichiometry


sol-gel deposition (chemical solution deposition, CSD):

multiple spin-on and curing process: ~ 0.1 µm per layer

low-cost equipment, very good uniformity and smoothness,

sensible to contamination

sol

gel

PZT thin film deposition methods


PZT thin film deposition methods

Sputtering from Pb(ZrTi)O3 ceramic target or Pb, Zr, Ti metallic targets

good uniformity

stoichiometry critical and fixed by target composition

target composition has to account for e.g. lead loss from desorption

low rate: ~10nm/min

but: more promising for mass production

different processes, different

crystalinity


wurzite crystal structure

polar materials (no ferroelectric hysteresis)

quite similar piezoelectric properties

Sputter deposition: 1-2 µm

AlN is preferred:

fully IC compatible

high thermal stability and conductivity

chemically inert

highly uniform sputter process available

ZnO and AlN films


wurzite crystal structure

polar materials (no ferroelectric hysteresis)

quite similar piezoelectric properties

Sputter deposition: 1-2 µm

AlN is preferred:

fully IC compatible

high thermal stability and conductivity

chemically inert

highly uniform sputter process available

ZnO and AlN films

true until last year…


2 µm thick AlN

on Pt-Electrode

Sputterprocess

Oerlikon-Clusterline 200

small Columnar grains:

deposition in transition zone

due to high Tmelt

Strong c-axis orientation Ti/Pt

SiO2

AlN

Aluminum nitride layer


Doping of aluminium nitride: Al1-xScxN

Akiyama et al., APL 95, 2009

e.g. reactive co-sputtering from pure Sc

and Al targets or AlSc compound

targets in N atmosphere

Sc partially substitutes Al while

preserving the piezoelectric wurzite

structure

different preference in N-coordination

between Sc and Al leads to flatter ionic

potential

softening of crystal: decrease of

stiffness c

larger ionic displacements by same

electric field: increase of d31, d33

d3

3


Ferroelectric Al1-xScxN

AlN

flatter ionic potential:

• lower energy barrier

• material becomes

ferroelectric

• …and a ferroelectric

like no other

• huge coercive fields

• huge polarization

• CMOS compatible



AlScN




ferroelectric


like no other



• CMOS compatible



AlScN




ferroelectric


like no other



• CMOS compatible






ferroelectric


like no other



• CMOS compatible


High force

High speed

Low power consumption

Membrane actuators

No counter electrode needed

Examples:

Inkjet printer f > 80 kHz

Micro mirror

Electrical switch

loudspeaker

membrane

piezoelectric layerelectrodes

nozzle

pump chamber

Piezoelectric (PZT) microactuators

feMeritOfFigure ,31


PZT micro mirror

80 µm poly-Si + 2 µm PZT

mirror size: 1 mm

fres = 32 kHz

deflection angle: ± 10.5° @ 7V

feFOM ,31


PZT electrical switch

device size e.g. 0.08mm2

contact force up to 2mN

PZT buckles with applied

voltage (contact closed)

buckling in open state

prohibited by electrostatic

clamping

feFOM ,31


Silicon microphones:

20 Hz- 20 kHz

Single membrane or arrays

Ultrasonic transducers:

transmitting and receiving

20 kHz - 1 MHz

Phased arrays: electronic steering

Piezoelectric micromachined

ultrasonic transducer cell

(pMUT)

Piezoelectric microsensors


R. Aigner, Infineon

AlN

largest commercial MEMS

success

~50 devices in each modern

smartphone

Film Bulk Acoustic Wave

Resonator

or

Surface Accoustiv Wave

(SAW) Resonator

BAW RF-filter

D

f

c

eFOM

33330

,332

or acoustic

mirror

PadsPZT

Elektrode


Piezoelectric Energy Harvesting

Idea: Turn vibration/acceleration into

power through piezoelectric layer

Seismic mass

/AlN

Application:

energy-autonomous

microsystems

Example:

Battery-less tire-pressure

monitoring system

330

,312

feFOM


Summary

only changes in polarization matter

piezoelectric coupling matrices: d and e

effective piezoelectric coefficents for thin films: d33,f, e31,f

dominant thinfilm materials: PZT, AlN, AlScN

PZT optimum for microactuation

AlN optimum for sensing

AlScN can combine advantages of both AlN and PZT

broad range of piezoelectric MEMS applications


Literature

Madou pp. 551-560

Chang Liu Foundations of MEMS CH. 7

N. Setter Electroceramic-based MEMS, 2005

esp. Ch. 10 Thin film piezoelectrics for MEMS

by S. Trolier-McKinstry and P. Muralt

Documents

Micro/Nanosystems Technology - Uni Kiel