MICROFABRICATION OF PIEZOELECTRIC MEMS...prototypes: acoustic sensor, ultrasonic transducer By understanding the basic principles of piezoelectric device physics By the development

6 April 2004 1Kiel, 8 October 2004, jba

Microfabrication of piezoelectric MEMS

1st International Workshop on Smart Materials and Structures 7-8 October, 2004Kiel, Germany

Jacek BABOROWSKI

RF & Piezoelectric Components GroupCSEM SA, Neuchâtel, [email protected]

mailto:[email protected]

6 April 2004 ISIF 2004 jb INVITED TALK2

6 April 2004 ISIF 2004 jb INVITED TALK 2Kiel, 7. Oktober 2004, jba

csemSwiss Center of Electronicsand Microtechnologie

Microelectronics Division

RF & PIEZO COMPONENTS GROUP



csemSwiss Center for Electronics and Microtechnology

2003 :Turnover ~ 54 MCHF, employees ~ 290

+ SPINN-OFFS

Privately owned company with ~ 70 Shareholders, none profit

Long term contract with the Swiss Government for financing applied research

Core technologies: Micro- and Nano-technologies and System Engineering

41%

14%

45%

Government Contract Public Projects Industrial Income

RF & PIEZO COMPONENTS GROUPM.A.Dubois et al.



FACILITIES at EPFL

CENTER OF MICRO- and NANOTECHNOLOGY

CERAMIC LABORATORY(Prof. N.Setter Lab.)



From materials… to sensors…Reproducible and industrially exploitable microfabricationtechnology for piezoelectric MEMS based on PZT and AlN thin films

By demonstrating the microfabrication of

prototypes: acoustic sensor, ultrasonic

transducer

By understanding the basic principles

of piezoelectric device physics

By the development of dedicated

micromachining method for AlN, PZT, Pt in

combinaison with deep silicon etching of SOI

substrates

By integration of high performance piezoelectric

films• {100} PZT 53/47, e31,f = -12 C/m2, {111} • AlN, e31,f = -1.02 C/m2 , d33,f = 5.3 pm/V .



Today's challenges in piezoelectric MEMS

• Deposition and integration of high performance (e.g.high e31,f value)piezoelectric films on a wafer scale with thickness up to 10 µm.

Dedicated microfabrication methods, process flow

top electrode

bottom electrode

strain ε1

Q3, E3

x, 1

y, 2z, 3

External load or displacement

piezoelectric thin film

Si structure

Microfabrication and characterization of demonstrators

• Establishment of reliable microfabrication processes (mostly “dry” plasmaetching methods) to produce complex piezo MEMS with submicron resolution.

• Demonstration of new applications

Pb2+

O2-

Zr4+, Ti4+

Integration, processing,properties

N

Alc

a



Outline:

Introduction1. Overview of piezoelectric devices2. General Processing Issues3. Examples

• Vibrating membranes and thin plates (MUT, pressure sensors)

• Cantilevers and Beams• RF MEMSOutlook and Conclusions

Goal:• Present the existing devices• Comparison with our actual processing developments• Highlight the common problems and limitations



Overview of piezoelectric devicesPiezoelectric thin films in MEMSMicromachined flextensional actuators and transducers

Plate waves, SAWFlexural plate waves in membranes (Lamb waves) (Uozumi, Ohsone, White, 1983, ZnO)Signal, particle filtering, chemical sensors, fluidic systems (Lugienbühl, 1997 PZT)Ultrasonic micromotors for watches, (Udayakumar 1991 PZT, Flynn et al 1992 PZT, Racine et al 1994 ZnO, Muralt et al 1995 PZT)

Standing Waves• Micromachined Ultrasonic Transducers:

Bernstein, Cross 1997, PZTPercin, Khuri-Yakub, 1998, ZnOBaborowski, Muralt, 2002, PZTAkasheh, 2004, PZT

• Droplet Ejector Percin, 2001 ZnO• Intelligent can, Yamashita, 2002, PZT• Array microjet, Yuan, 2003, PZT• Microdegasing, Maeda 2001, bulk PZT.

Linear actuatorsPiezoelectric laminated cantilevers, AFM, surgery tools, optical phase shifters, relays, microvalves, pumps, micromirrors, switches .

Schiller and Polla (1991)Lee, Itoh, Suga (1996)Nippon Denso,Y. Ohtuka, (1995)Chengkuo Lee, et al, (1996~1997)J. Tsaur et al, AIST, Tsukuba, (2002) etc..



Plate and Standing wavesFlexural plate waves in membranes (Lamb waves) (Uozumi, Ohsone, White, 1983, ZnO)



… Flextensional ultrasonic transducers




ZnO structure (0.3 µm) 100 µm diameter membrane

G. Percin et al, Micromachined two-dimensional array piezoelectrically actuated transducers, APL 72 (1998)




Cell size 30 - 80 micronsFrequency range: 3 – 12 MHzk2eff = 1.7 to 2.5 %



… Flextensional ultrasonic transducers Proximity Sensing



… Flextensional ultrasonic transducers Droplet Ejectors

G. Perçin and B.T. Khuri-Yakub,Micromachined droplet ejector arrays for controlled ink-jet printing and deposition, Rev.Sci.Instr. 73 (2002) 2193-96



Resonator shape and method to excite torsional vibration system with 2 degrees of freedom

Method to generate torqueDriving voltage characteristics

Frequency characteristics Scanning images

Piezoelectric In-Plane Scanning Mirror(Nippon Denso,Y. Ohtuka, 1995)

LINEAR ACTUATORS - Micromirrors



MicromirrorsPiezoelectric In-Plane Scanning Mirror(AIST, Tsukuba, Japan, Chengkuo Lee, et al, 1996~1997)

SiO2 substrate

PZT layer

a.a. b.b. Bimorph structure:Bimorph structure:• PZT +SiO2 (1.5µm)Actuation of 1 beam:Actuation of 1 beam:• Vin applied to the beam ---

bendingActuation of 2 beams:Actuation of 2 beams:• 2 Vin with 180° phase shift--

-1D rotation Actuation of 4 beams:Actuation of 4 beams:• additional 2 Vin with phase

shift---2D rotation

Upper electrode

Lower electrode

c.c.

V+

V-

V+ V-

Laser

1 beam

2 beams

4 beams

A. Schroth, C. Lee, S. Matsumoto, and R. Maeda," Application of Sol-gel Deposited Thin PZT Film for Actuation of 1D and 2D Scanners," Sensors & Actuators A, 73 (1999) 144-152.



MicromirrorsPiezoelectric In-Plane Scanning Mirror(AIST, Tsukuba, Japan, Chengkuo Lee, et al, 1996~1997)

C.v1= -v3=2.5V at 13.6kHzv2= -v4=2.5V at 14.6kHz

v4v1

v3

A.v1= -v3=2.5V at 13.6kHz

B.v2= -v4=2.5V at 14.6kHz

v2

v4 v1

v2 v3

A. Schroth, C. Lee, S. Matsumoto, and R. Maeda," Application of Sol-gel Deposited Thin PZT Film for Actuation of 1D and 2D Scanners," Sensors & Actuators A, 73 (1999) 144-152.



Micromirrors

~ 1 micron/V



2D Scanning Mirror Using Bi-layer PZT FilmsJ. Tsaur et al, AIST, Tsukuba, Japan, 2D Micro Scanner Actuated by sol-gel derived double layered PZT, MEMS 2002

Micromirrors



Cantilevers



Inertial systems

60 mic PZT40 mic Si membrane



RF MEMS Piezoelectric RF-switch

Piezoelectric (PZT) < 40 V 2 µs Gap 1 micron Penn State Univ.

d33 mode generated by IDT electrodes

d33= 120 pC/N



RF MEMS Piezoelectric RF-switch

Tunable parallel-plate varactor < 6 V Q = 210 2.5 – 3.5 µm LG, Park et al.

Park et al., LG Electronics, 2001



2. Microfabrication of piezoelectric MEMS based on deflecting structures

Common tasks

• Silicon micromachining• Photolithography, 1 µm resolution is sufficient (now 0.13 µm in IC's)• Thin films deposition and etching processes for standard

SiO2, Si3N4, poly-Si and metal films.Madou (1997)

IC's

• Individual etching processes for PZT, AlN and its electrodes• Deep silicon etching (membrane definition, SOI substrate, …)• Surface micromachining (compatible sacrificial layers)• Stress compensation to obtain flat structures

+

Special tasks for pMEMSFor piezo MEMS, additionnal methods are required:



Piezoelectric multilayer, thin film structures

J.Baborowski,Journal of Electroceramics, 2004

Laminated PZT/Si deflecting structures used in piezoelectric MEMS:bridge, cantilever and suspended membrane.

AlN PZT

e31,f [C/m2] -1.33 -12

d33,f [pm/V] 5.15 65

ε33,f 10.2 1200

tgδ 0.002 0.03

Current resp. [C/m2] 1.33 12

Voltage resp. [GV/m2] 13 1

S/N [105 Pa0.5] 31.3 6.7

k2 0.22 0.15

Power efficiency 110 5

Actuators

Substrates: Si, SiO2/SiN membranes, SiC, diamond*)Electrodes: Pt/Ti, Mo, Al, Au/CrPiezoelectric layers: PZT, AlN, ZnO…

*) Shibata, Sensor and Actuators, 2004

Sensors

σ1,2 = −e31, f ⋅ E3 , x3 = d33, f ⋅ E3

D3 = e31, f ⋅(x1 + x2 )+ d33, f ⋅σ3

e 31 , f =d 31

s11E + s12

E ≡ e31 −c13

E

c 33E e 33 e 31 , f > e 31

d 33 , f =e 33c 33

E ≡ d 33 −2 s13

E

s11E + s12

E d31 < d 33

P. Muralt, Integrated Ferroelectrics,1997



Film bulk acoustic resonator (FBAR)Bulk micromachining

Surface micromachining

AlN

Si wafer

electrodes

BAW resonators are based on thelongitudinal thickness vibration mode of a piezoelectric thin film.

Acoustic insulation of the resonant structure performed by air (or vacuum)

+ Large achievable coupling coefficient+ Processing time+ May be used in front end filters, or in

VCO's for low power RF applications. + High Q factor (typically up to 1000)+ Resonance frequency: 2-10 GHz

– Structures sensitive to material stress, prone to buckling (1-2 µm thickmembranes or bridges)

– Fragile structures for manipulation, packaging, etc.

– Use of IC wafers difficult in the case ofbulk micromachined resonators

AlN

Si wafer

electrodes



Solidly mounted resonator (SMR)Acoustic insulation performed by a Bragg acoustic mirrorAlN

Si wafer

electrodes

acoustic reflector

f0 = 2.4 GHzStress compensated layers for a crack-free structureDense layers => low waveattenuationVery smooth interfaces => low wavediffraction

The acoustic reflector is a stackof alternating λ/4 layers made of

materials with very differentelastic properties.

M.A.Dubois, CSEM



2. Microfabrication of piezoelectric MEMS based on deflecting structures

DEPOSITION and growth controlPVD sputteringCSD (sol-gel)PLD

PATTERNINGWet chemical etchingHDP etchingSurface micromachining (Piekarski, 2001)

DIMENSIONAL CONTROLDefines miniaturization rulesDefines precision of elastic behavior and characteristics (resonance frequencies, coupling factor)Defines border conditions (accordance between simulation models and reality)

STRESS CONTROLTensile stresses present in most of the layers (up to +800 MPa Pt/Ti)Compressive stresses in SiO2 wetox (-300MPa)Very sensible to deposition conditions (AlN) and poling (e.g. PZT +/-60MPa)Incertitude in stress level in SOI substratesStress adjustment

(Lee,Itoh, 1996)



3) Optimized Sol-gel process PZT 53/47 (100) on PbTiO3 (PT) seeding layerThickness: about 0.25 µm / layer; total up to 4 µm

-e31,f = 12 ± 0.3 C/m2

Deposition and integration of {100}-textured sol-gel PZT films on silicon (Ceramics Lab., EPFL, 1996-2002)

2) Elaboration of PZT precursors

2 solutions with 10% and 30% Pb excess

Budd (1985)

1) Pt bottom electrode

100 nm Pt/TiO2/Ti + 10 nm PbTiO3 {100} seeding layer

Maeder (1998)Muralt (1998)Hiboux (1999)

Seifert,, Ledermann (2001)Ledermann, Baborowski, Muralt (2002)

0 100

5 104

1 105

1.5 105

2 105

2.5 105

20 25 30 35 40 45 50

inte

nsity

(cou

nts)

angle 2Θ (deg)

PZT

(100

)/(00

1)

PZT

(110

)

PZT

(111

)P

t (11

1)

PZT

(200

)/(00

2)P(100) = 98%

{100}

Revealed lateral underetch:- Global (several microns),- In each crystalised layer during each

RTA step (below 100nm),

- Heterogeneity in each crystalisedlayer due to migration of Zr and Ti(Baborowski 2002, Cantoni 2003)

TiZr Pb

% at

ZrTi



PVD DEPOSITIONSpider: a versatile sputtering equipment

Cluster architecture: loadlock, wafer transfermodule, 4 process chambers

Adapted for 100 or 150 mm wafers

DC, pulsed DC, and RF power sources for metal andceramic films

1 chamber designed andreserved for AlN thin films



Sputtering of AlN thin films

Reactive sputtering from a pure Al targetin a nitrogen atmosphere

Very smooth surfaceDense columnar microstructure~3% uniformity on 4" waferc-axis orientation (FWHM[002] = 1.58°), induced by the hexagonal plane of Pt (111)d33,f=3.7±0.3 pmV-1

Single run for deposition of :Bottom electrode: Ti , PtPiezoelectric layer : AlNTop electrode Pt or AlSi

M.A.Dubois, CSEM

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

30 40 50 60 70 80 90

2theta [°]

inte

nsity

AlN

(002

)

AlN

(004

)

Si (

200)

Si (

400)

Pt (

111)

Pt (

222)



Deposition and integration of AlN films on SOI substrates(CSEM 2004)

Ti sputtering 10nm

Pt sputtering 100nm

AlN sputtering1500 nm

pvacuum 7*10-8 mbar 3*10-7mbar

5*10-3mbar

300°C

15 sccm Ar

4 nm/sec

DC 1000W

5*10-8 mbar

pwork 3*10-3mbar 4*10-3 mbar

T 300°C 300°C

gas 9 sccm Ar 50 sccm N2Deposition rate 1 nm/sec 0.72nm/s

Field source DC 1000W Pulsed DC 1500W

1. SiO2 compensation layer : 1500 nm thermal wet oxidation of Si followed by HFdip2. Ti adhesion layer : 10nm 3. Pt seed layer : 100nm 4. AlN piezoelectric layer : 1500 nm 5. Pt top-electrode : 100nm

AlN

30 40 50 60 70 80 902θ (degree)

Cou

nts(

a.u)

Ti (0002)

Ti (0004)

Pt (111)

Pt (200)

Pt (311)

Pt (222)

AlN(0002)

AlN(0004)

Oxygen presence in Ti layer

AES profile of SOI_5_1through VIA of bottom-elec

0

5000

10000

15000

20000

25000

30000

35000

0 5 10 15 20 25 30 35

etch time [min]

I [a.

u.]

Oxygen

Titanium

Platinum

Platinum 100 nmTitanium 10 nm



PZT {100}, 1 µm thin film

0

2

4

6

8

10

12

14

-200

-100

0

100

200

300

25 30 35 40 45 50 55 60 65

Piezoelectric coeff.

Film stress

Pie

zoel

ectri

c co

effic

iet-

e 31,

f(C

/m2 )

Composition [Zr]/([Zr]+[Ti])

Poling:+ 64 MPa

PZT 53/47

STRESS COMPENSATED BEAMS/ MEMBRANES

-20

-15

-10

-5

0

5

10

15

20

0 500 1000 1500 2000

after poling

before poling

z-de

flect

ion

(µm

)

distance from the attachement (µm)

Poling of PZT effect on residual stress :Deflection + 20 µm = + 60 MPaσδ = 3 MPa/µm

Ledermann et al., Sensor and Actuators, 2003

-300

-200

-100

0

100

200

300

400

0.00 0.50 1.00 1.50 2.00 2.50

AlN thickness [µm]

Res

idua

l stre

ss [M

Pa]

Residual stresses in wetox SiO2

-400

-300

-200

-100

0

100

200

300

400

0 100 200 300 400 500 600 700 800 900 1000

Thickness of SiO2 (nm)

σ (M

Pa)

0°90°

Residual stresses as a function of film thickness in:

SiO2 wetox AlN




Mechanical system at equilibrium Equilibration of the structure

z

-ts

hSiO2hTi/PthAlN

hPttPt tAlN tTi/Pt

tSiO2

-tb0

0=RM0)()( =∆−+∆−= ∑i iiisssu

R tTcEtTcEF αα

( )0

)(

1

0

=−

+−

= ∑ ∫∫−− i

h

h

bi

t

bsbR

i

is

dzr

tzEdz

rtz

Fσ

( ) ∑ ∫∫−

−+−=− i

h

hbi

tbSR

i

is

dztzdztzM1

)(0

σσ

(1)

(2)

(3)

Membrane definition

ρ [103 Kg/m3] E [GPa] α [ppm] Tf [K] ν

Si 2.33 180 2.8 + 0.003*T --- 0.278

SiO2 2.2 70.4 0.058 --- 0.3

Ti 4.5 100 8.6 1941 0.33

Pt 21.45 136 8.8 2041 0.42

AlN 3.26 310 4.2 3273 0.24

Materials constants

Setting (3) to zero and resolving (1),(2),(3)

Determination of the wetox SiO2 layer thickness for deflection-free membrane

Determination of residual stress



PZT

Pt /Ti

SiO2 burried SiO2 wetSi bulk

Au/Cr

σres =E

1 − υ⋅

ts2

3L2t f⋅ d ts = substrate thickness (10 µm) tf = thin films thickness (1 µm)d = cantilever tip deflection (µm), L = cantilever length (2 mm)

E, ν = Young modulus and Poisson’s coefficient of Silicon

0.12 µm (PVD) Pt/TiO2 = + 839 ± 72 MPa1.00 µm (sol-gel) PZT = + 113 ± 8 MPa (unpoled)1.00 µm (250 kV/cm, 150°C, 10’) PZT = + 64 ± 5 MPa (poling)0.2 µm (evaporation) Au/Cr = + 282 ± 11 MPaX ? (wetox) SiO2 = - 297 ± 3 MPa

E.g. for 2 µm PZT XSiO2 = 1200 ± 40 nm or ± 12MPa

STRESS COMPENSATED BEAMS/ MEMBRANESEXAMPLE of sol – gel PZT / Si

For SOI substrates the stress of buried oxide is not well known




PZT on 5 µm SOI

Bending B1003-26

192123252729313335

0.5 1 1.5 2distance [mm]

heig

ht [ µ

m]

cross 1cross 2

~10 - 20 µm

AlN on 5 and 10 µm SOI

1 x 1mm25 µm thick

0.3 mm2 to 1 mm2 SQUARE MEMBRANE :

2 - 4 micron of PZT

5 micron of Si

Well stress compensatedEven if the stress value of buried oxide is uknown

Bending B1002-27

-2

-1

0

1

2

3

4

0.5 0.7 0.9 1.1 1.3 1.5 1.7

distance [mm]

heig

ht [ µ

m] less than 1 µm

1 x 1mm210 µm thick



PZT

Pt /Ti


SiO2 PECVD

Au/Cr

MICROFABRICATION: PROCESSING WITH SOI WAFERS (Ceramics Lab., EPFL, 1999-2002)

AlAlNPtAir gapSi

Bridge of suspended membraneBridge of suspended membrane

PlasmaEtch (ICP, Cl2, Ar)

Wet etch (HCl:HF)

PlasmaEtch (ICP, Cl2, Ar)

Deep Reactive Ion Etching(Plasma Etch, ICP, RT, SiF6, C4 F8 )

Carazzetti et al.,EPFL 2002

Baborowski et al.,EPFL 2001



Reactive Ion Beam Etching of PZT and Pt thin films

PZT : RIBE, CCl4/CF4/Ar, low bias70 nm/min, SPR = 0.5, SPt = 1.6

Pt : RIBE (CCl4) or RIE/ICP (Cl2)60 nm/min, SPR = 0.5

Major problem with PZT, platinum thin films dry etching:

• limited volatility of reactive etch by-products, need energy !!• the processes are more physical than chemical,• low selectivity with respect to PR.

Baborowski, Ledermann, Muralt (1999, 2000)

ECR/RF Reactive Ion Beam Etching

ECR ion gun

ions

gaz inlet RF, 2.45 GHz

wafer

water cooledsubstrate holder 12.56 MHzRF bias

pumping2 µm

PtPZT

+ PZT



Advanced patterning for fundamental studies on ferroelectrics

ECRPlasma

RF chuck

grids

gasAr, CCl4, CF4

100 mm

2.45 GHz

RF 13.56 MHzWater cooled

STOPZTPMMA

E-beam

-20 -10 0 10 20-800

-600

-400

-200

0

200

400

600

800

Am

plitu

de o

f pie

zore

spon

se (a

.u.)

DC bias to tip (V)

S.Bühlman, J.Baborowski, P.Muralt, 2002S.Bühlman PhD Thesis EPFL, 2004



PATTERNING OF GROOVES and SLIT OPENINGS THROUGH PZT/Pt/SiO2/Si(e.g.unclamped membranes, cantilevers)

STANDARD: Photolithography & Dry etch:

• Standard photolithogaphy• 100 nm Pt etch (ICP STS)• 1200 nm wetox SiO2 etch (Alcatel, 300 sec) • up to 50 µm Si (SOI) (Alcatel601E) – ETCH STOP on

SiO2buried

PZT

Pt /Ti


SiO2 PVD

Au/Cr


PtSiO2

Si

SiO2 burried

Below 5 micron ?



PATTERNING OF SUBMICRON SLIT OPENINGS THROUGH AlN/Pt/SiO2/Si(e.g.limited air conductivity in pressure sensors with minimum roughness of sidewalls)

HIGH DEFINITION: Direct Laser Writing & HDP Dry etchHMDS & Shipley 1805, 1.75 µm, 90°C

Direct writing DWL: 4.04.W Development : Standard Rite Track 1818 SerieEtching:

Pt: STS; Pt_etch (150W bias), 380sec. (photoresist removed)SiO2: Alcatel 601E; SiO2 stand; DFA, 150 secSi: Alcatel 601E; Si_ambiant_2 (optimized for smalll openings); 360

secFinal clean (solvant & SDR), plasma O2 ashing

AlN or PZT

Pt /Ti


SiO2 PVD

Au/Cr


724 nm

Bow < 50 nm

947 nm

100nm Pt/Ti

1200 nm SiO2



Definition of membrane thickness (Ceramics Lab., EPFL, 1999-2004)Standard vs. SOI wafers

Backside view;2µm large slit traversing the 10 µm thick membrane



4. EXAMPLES (2001-2004)

Suspended Membranes:

3.1. MICROMACHINED ULTRASONIC TRANSDUCERS (PZT)3.2. MICROPHONES FOR PHOTOACOUSTIC SENSING (PZT, AlN)

RF filters and resonators based on AlN thin films

RF Switch



3.1. MICROMACHINED ULTRASONIC TRANSDUCERS

Micromachined ultrasonic transducers (MUT) are investigated for phased arrays in high frequency acoustic imaging

The basic element consists of a micromachined membrane that is driven by piezoelectric actuation (pMUT)

We deal with piezoelectric MUT’s using Pb(ZrxTi1-x)O3 (PZT) thin films of 2 µm thickness deposited by sol-gel on 5 micron Si membranes .

We have studied:MUT fundamentals, thin film properties and processing, fabrication and characterization of single elements,FEA simulations, experimental characterization of single transducers and linear arrays in air and liquid

ParmenideEU project



3.1. MICROMACHINED ULTRASONIC TRANSDUCERSFabricated devices: 2 micron PZT on 5 micron Si SOI substrate

Frequency range (air loaded):

50 to 150 kHz

400 to 600 kHz

750 to 1200 kHz

Membrane size:

1000 µm

450 µm

300 µm



Single element:Comparison between the simulation and measurements

Basic Mode@ 55,2 kHz

Second Mode@ 106 kHz

Simulated by D.Schmidt (IBMT, Fraunhofer Institut)

Measured with a stroboscopic interferometric microscope (A. Bosseboef CNRS, Orsay)

LCLC

EPFLEPFL



Suspended disc 1 mm2BASIC Mode 1 - Vertical deflection @ 55,2 kHz, 0.5 V AC , 50kV/cm DC bias

A

A

B B

C

C

Deflection of res ≈ 60 kHz

distance x (µm)-1000

-800

-600

-400

-200

0

200

0 800 1500

device #13, RND GR - wafer 01

coupe A-Acoupe B-Bcoupe C-C

Def

lect

ion

Z (n

m) -1.3µm

/V

• Observed

• Simulated

-2000

-1500

-1000

-500

0

500

0 200 400 600 800 1000

Deflection vs. distance - simulation

Def

lect

ion

z (m

icro

n)

Distance x (micron)

1.7 mic/V

The shape is close to the desired piston movement. The displacement is uniform around 360°.



HIGHEST COUPLING COEFFICIENT FOR HIGH FREQUENCY TRANSDUCERS…

-0.002

-0.001

0

0.001

0.002

0.003

0.004

7.2 105 7.3 105 7.4 105 7.5 105 7.6 105 7.7 105 7.8 105 7.9 105

a1-df-res1-20dc

real measimag meas

real fit

imag fitA

dmitt

ance

Frequency (Hz)

k2 = 5.3 %k = 23 %Q = 135C

0 = 90 pF

Cpara

=0

SOI wafer: 3.5 µm of SiPZT (001) 2 µm

f res = 753 kHz

k2 = 5.3 %Q =135

k=23%

DC bias = 100kV/cm



LOW FREQUENCY SINGLE DEVICESin Air and in FLUORINERT TM (3M)

/22Q/5

k2=const

AIR:

fres = 97.5 kHz

k2 = 1.0%

Q = 75

Fluorinert:

fres = 19 kHz

k2 = 1.0%

Q= 16

k2 = constQ / 5



APPLICATIONSPresence and precise positioning sensor; transmission

Paper

Plastic

Receiver

Emitter

-4

-3

-2

-1

0

1

2

3

4

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 200 400 600 800 1000

Air transmission, distance = 10 mm

actuator DISC F 1.1

sensor DISC F 3.1

actu

ator

vol

tage

(V) sensor voltage (V

)

time (µs)

Charge amplifier: 10 pCExcitation frequency = 98 kHz, 10 V AC, 5 V offset

-4

-3

-2

-1

0

1

2

3

4

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0 200 400 600 800 1000

Air transmission, distance = 10 mm

emitter DISC F 1.1

receiver DISC F 3.1

emitt

er v

olta

ge (V

) receiver voltage (V)

time (µs)

Liquid



3.2 Piezoelectric microphones for photoacoustic detection

Specificity of the applicationLow frequency operation up to100HzLow acoustic pressure level order of mPa

Key pointsBuild a MEMS device based on partially unclamped, deflecting piezoelectric structuresBridge or cantilever structures have been focusedUse of PZT or AlN as piezoelectric layerUse of SOI wafers perfectly well defined Si membranePatterning of very narrow slits advanced ICP dry etchPossibility to build arrays of devices for complementary propertiesIntegration of both amplification unit and device on same PCB

3mm

B B

A

A1,2 mm

1,2,3 µm slit

1,2,3 µm slitPt Top electrode

Pt Bottom electrode

1,2 mm

AlN



3.2. Acoustic sensors based on PZT sol-gel filmsThe cantilever concept for an audio microphone/microspeaker was proposed by White & al in 1998. (J. Micromech. Microeng. 8 (1998) 230-238)4.5 µm thick cantilever ZnO/SiN/SiO2: 10 µm slit, response at 100 Hz = 38 mV/Pa

2x2 mm beam

2x2.5 mm beam2x2 mm bridge

Electrode for pyroeffect compensation(static). via

Cantilever top electrode

Slit 5 and 10 µm

1 micron thick PZT on 10 micron Si membrane;Ledermann et al., 2002



3.2. Acoustic sensors based on PZT sol-gel filmsN.Ledermann, J.Baborowski et al., JMM, 2004

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50

Bridge B3-10 - w.1202

V = 4 cm3

V = 2 cm3

V = 1 cm3

V = 0.5 cm3

sens

itivi

ty (m

V/Pa

)

frequency (Hz)

0

50

100

150

200

0 10 20 30 40 50

Cantilever C5-3 - w.1202

Sens

itivi

ty (m

V/P

a)

frequency (Hz)

V = 1 cm3

V = 4 cm3

V = 0.5 cm3

V = 2 cm3

S0, theo = 1.72 pC/Pa or 172 mV/PaS0, B3-10 = 68.3 mV/Pa ≈ 39 % S0, theo

S0, theo = 3.21 pC/Pa or 321 mV/PaS0, C5-3 = 226 mV/Pa ≈ 70 % S0, theo



3.2. Acoustic sensors based on AlN filmsAdvantages of AlN based sensors:

Full IC compatibleReduced Thermal BudgetNo poling requiredGood selectivity for micromachiningLow losses

Future integration SoC (System on Chip) possible

Ground

Phase

Cantilever – backside view

Si SOI membrane

2 µm slit

1.5x1 mm CANTILEVER

0.0E+00

5.0E-08

1.0E-07

1.5E-07

2.0E-07

2.5E-07

3.0E-07

3.5E-07

4.0E-07

4.5E-07

6'000 6'500 7'000 7'500 8'000 8'500 9'000

Frequency [Hz]

Re[

Y]

AlN on membrane

Etched AlN forslit opening

J.BABOROWSKI, CSEM 2004



Ta5Si3 200 nmSiO2 150 nm

Si3N4 200 nmSiO2 650 nm

Pt 100 nm

Cross-section

Frontside view

Backside view

2 mm

Microhotplate as processing device for local thin film growth

PZT

Ta5Si3

SiO2Pt

SiO2Si3N4

After PZT crystallization

Diploma work of F. Calame; with J. Baborowski



RF MEMS for wireless communication andAmbient Intelligence (EC IP Mimosa)

CSEM involved in:

BAW resonatorsFiltersRF PIEZOELECTRIC Switch



ConclusionsPiezoelectric Micro-Electro-Mechanical Systems (pMEMS) are efficient for monitoring of pressure, vibration and positioning and for RF applications.Technology for thin PZT / SOI MEMS and AlN / SOI MEMS demonstrated and actuators & sensors has been fabricated with accordance to simulations and designActuators, vibrating membranes, deflecting systems, sensing devices in large range of frequency (few Hz to few GHz) can be fabricated in mass quantity with high yield (the use of silicon on insulator wafers in combination with deep silicon dry etching ICP)Stress control of the multilayer thin film structures is key factor for achieving high and uniform response.Submicron patterning of complex piezoelectric structures achieved.

OPEN QUESTIONS:• Yield and reproducibility in fabrication of piezoelectric layers

(PZT, AlN)• Post processing (poling) and Packaging = Price• Reliability and life time• Low cost mass production facilities• Special facilities• Competitive new applications



ACKNOWLEDGEMENTS

TEAMS of Ceramic Lab, EPFL, Center of Micro- Nanotechnology, EPFLCenter of Microscopy, EPFLLMARC, Besancon, France,CSEM, NeuchatelHachUltra Analytics, Geneva

OFES and EU Commission

Thank you for your kind attentionAnd for investing your time

Microfabrication of piezoelectric MEMScsemSwiss Center of Electronics and MicrotechnologieMicroelectronics DivisionRF & PIEZO COMPONENTS GROUPcsem Swiss Center for Electronics and MicrotechnologyFACILITIES at EPFLFrom materials… to sensors…Reproducible and industrially exploitable microfabricationtechnology for piezoelectric MEMS basedToday's challenges in piezoelectric MEMSOverview of piezoelectric devices Piezoelectric thin films in MEMS Micromachined flextensional actuators and transducersPlate and Standing waves

MICROFABRICATION OF PIEZOELECTRIC MEMS...prototypes: acoustic sensor, ultrasonic transducer By understanding the basic principles of piezoelectric device physics By the development

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