Piezoelectric Micromachined Ultrasonic Transducers

Philippe GAUCHER(philippe.gaucher@ecp.fr)

- Ecole Centrale ParisLaboratoire Structures, Propriétés, Modélisation des Solides

- Consultant at THALES Research & Technology

Outline

• Physical principles• Technology• Natural frequencies• Bandwidth and Admittance• Amplitude and Acoustic Power• Conclusion

Physical principles

Single Element Pulse-Echo acoustic transducer

in

out target

switch

We need:- Power- Bandwidth- Sensitivity

Propagation medium

SONAR principle delay

Multi-elements Ultrasonic Transducers : towards 3D imaging

1.5D probe 2D probe1D probeElectronic Beam steering

Ultrasonic Transducers: integration into Silicon technology

baking PZT

Adaptation

Psu.s

E

E

Si

Si n++

oxyde

Al

SiNx

u.s u.s

PZT

Present:Piezo in thickness mode

C-MUT Concept:Capacitive

P-MUT Concept:Piezo in Flexion (bimorph)

C-MUT and p-MUT

Piezoelectric materialSmall air gap

Electrical power : Pe = 1/2ε0εE2.ω

2

2acDC

0e

)vV.(S.21F +ε=Force = Cte+k.EDC.eac(t)

In piezo-electricity : EDC=Pr/ε0 = local field in the material

Force

C-MUT:Design coupling

Force

P-MUT:Material coupling

VVDC+vac Vac

Piezoelectric material specifications for apiezo-electric transducer

• High coupling coefficient ---> high band width• High dielectric constant ----> low electrical impedance (coaxial: Ze=50Ω)• High d coefficients ---> large strain• Low elastic stiffness -----> acoustic impedance (ρ.c) matching (water: Zm=20 Mrayl)• High mechanical and electrical Q for materials ---> low losses• High speed of sound ---> high resonance frequency

Technology

Generic structure of a Piezoelectric MEMS

Electrodes

(Cross section)

Piezoelectric materialMembrane

Barrier

Etch stop

Downscaling technologies for piezoelectric bimorphs

STICKING (BULK CERAMICS)

SCREEN PRINTING

THIN FILMS DEPOSITION

Lateral dimensions > 1cm > 1mm > 1µm

Thickness > 100µm > 10µm > 0.1µm

Voltage > 100V > 10V > 1V

Problems low coupling with substrate

High temperature processing Pb diffusion in Si

Solution Optimise glue layer noble metal substrate barrier

PARMENIDE ProjectMEMS technology: Bulk Silicon Micromachining

Back side:

Membrane machined by DRIE

Front side:

Si wafer with ferroelectric structures

Thales TRT and Thales Microsonics, Cranfield Univ, EPFL, FhG IBMT, Protavic

SOI (Silicon On Insulator) substrate

SiO2 0.5 µm

SOI 5µm

SiO2 0.2 µm

TiPt-PZT-Ptstack 2.3µm

Silicium

SEM Cross section of the layer stacking

Interfacial stresses :Delaminations and cracking of layers

τ=σ .t4

LInterfacial Shear stress τ Layer stress σ

L

tcouche

substrat

σ

ττσ

•films with τ>0 Delamination

•films with τ<0 Cracks

Sensors and Actuators A89 (2001)

Interfacial stress between film and substrate

Thermal stress Intrinsic stress

αfilm <α

substrat

α film>α su

bstra

t

Tamb

Tension, σ > 0 compression, σ < 0

Td

Interfacial stress cancellationResultant stress :substrate

• σ < 0 compressive stress

• σ > 0 tensile stress

σ2>0

≡σ

∑

∑

=

=

⋅= n

1ii

n

1iii

e

eσσ

σ1<0

Buckling of structures due to compressive stresses

2

32

22

LYwt.

3LI.Y.F π=π>

Dimension ÷ 100 => F ÷ 104

L

t

FF

Euler force:

Example:L=w=1mmt=10µmY=100GPa

σ = 30MPa only!

Feynman: Lectures on Physics, Vol 2

Buckled Piezoelectric diaphragms

Acoustic array of piezoelectric transducers

Packaging Anodic bonding for via through interconnexions

connectic waferVia holes: 70 µmHole isolation: PECVD oxide SOG 3.2 µm

Acoustic waferPECVD oxideTiPt electrodesbarrier oxide

Poling of PZT film

Sol-gel films:

Permittivity (1 kHz): 800

tanδ (1Vrms-1 kHz) : 1-3 %

Surface capacitance : 12 nF/mm2

Coercive field: 11 V/µm

Breakdown field: > 50 V/µm.

Effective d33 up to 100pC/N

Ferroelectric Hysteresis at 115 Hz for a 1.8 µm film

Resonant frequencies

Natural frequencies of generic structures

)1(..615.0 22 νρ −= YLtf )1(..654.1 22 νρ −= Y

Ltfρ

YLtf ..162.0 2=

Composite membranes: Y YD and r ρequ

Finite Elements Modelling for complex designs

•Coventorware•Intellisense•ANSYS, ATTILA …

Modal analysis (polytec laser interferometer)

39 kHz

57 kHz 58 kHz

98 kHz108 kHz

153 kHz

First natural frequency : experimental and modelling

1,E+03

1,E+04

1,E+05

1,E+06

1,E+07

0,10 1,00 10,00

Membrane dimension (mm)

Rés

onan

ce f0

(Hz)

analytical model memcad model experimental

Differences dues due non ideal geometry of the clamping

Testing with water loading

P-MUT Glass tube

PCBPCB

Glass tubeSealingSealing

Resonance frequency

One side water loaded : 181 kHzIn air : 520 kHz

+

=

ta

Eatf

EQ

OHEQ

EQ

ρρρ 277.01²243.0

EQ

EQEatf ρ²243.0=

Electrical Admittance and Bandwidth

Parametric analysis

• Impedance: r0

r jC1ZZ ω+=

Zr=ρ.cresonance

• Coupling k:2

2

m02

k1k

CCN

−=

• Mechanical Q factor: rRm Z

LQ ω=• Electrical Q factor (at ωr):

20

NCZQ rre ω=

• Mechanical Q factor:• Resonance: L(C 0+Cm)ωr2=1

• Antiresonance: LCmωa2=1

r

rm ZLQ ω=

Bandwidth and impedances matching

Yr

Yi

f0

Yr(f0)

Yi(f0)B-3 dB

Admittance

Frequencyfr fa

10 ≈=BfQm 1≈=

YiYrQe

(with Yi=50Ω)

Admittance in air : membrane 250 µm

fr=2 MHz, Qm=80

Parasitic Capacitance reduces bandwidth

C0

C1R1

R

C

L

C1

Ro

Co

Element ValueR 1.5E5 ohmC 4.236E-14 FL 1.24 HC1 8.5E-11 FRo 352.5 ohmCo 7.511E-11 FR1

Bandwidth and acoustic impedance:Admittance measurement in air and in fluorinert

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.00E+05 1.50E+05 2.00E+05 2.50E+05 3.00E+05 3.50E+05 4.00E+05 4.50E+05 5.00E+05 5.50E+05 6.00E+05

Frequency

G (S

)

0.00E+00

2.00E-05

4.00E-05

6.00E-05

8.00E-05

1.00E-04

1.20E-04

1.40E-04

0.00E+00

5.00E-06

1.00E-05

1.50E-05

2.00E-05

2.50E-05

4.00E+05 4.50E+05 5.00E+05 5.50E+05 6.00E+05 6.50E+05

Frequency

G (S

)

0.00E+00

2.00E-05

4.00E-05

6.00E-05

8.00E-05

1.00E-04

1.20E-04

1.40E-04

B (S

)

B (S

)

Disc transducer loadedwith Flurinert (front side):

fr= 315 kHzQm=1.26

Disc transducer in air:fr=570 kHzQm=142

Amplitude and Acoustic Power

Analytical modelling of cantileverModal response and force

Actuator force:

Sensor response:

PZT

Substrate

neutral plane

electrodesz

0-ts/2

zn

t/2x x+l

2.50 x (mm)

Φ

Mode 1: 1.62 kHz

Mode 2: 62.8 kHz

Mode 3: 496 kHz

dx.x

.tp.g.l2Y).tpt(.Vs

2x

1x2

2

∫∂Φ∂+δ=

dx.x

.V.d.6Y).tpt(F

2x

1x2

2

∫∂Φ∂

Ψ++=

Amplitude at resonance

0

2

46

8

10

1214

16

18

0 2 4 6 8 10 12

Tension (V)

Dép

lace

men

t (m

icro

ns)

Membrane 4X4mm2 @ 30.9 kHz

Acoustic power

- Displacement speed : V = A.ω = 3 m/s

- Acoustic pressure in air : Za=0.2 103

Rayl

P = Za.v = 6 kPa

- Acoustic pressure in water : Za=1.5 106 Rayl but attenuation by a factor of 100

P = 3 MPa

Testing in reception (in air)

Parmenide sensor

HV

Acoustic burst generator(labo GPS Univ Paris VI)

400 µs

Reception signal of a 250 X 250 µm2 p-mut at a distance d=10cm. The oscillations are at 700 kHz

1.2 ms

Reception signal of a 1.350 X 1.350 mm2 p-mut at a distance d=40cm. The

oscillations are at 30 kHz.

Conclusion

• p-MUTs technology is an alternative to c-MUTs for air or underwater acoustic transducers• Thin film PZT bimorphs can support high electric fields, but have low coupling factor• Large vibration amplitude is possible with p-MUTs • A compromise between bandwidth and sensitivity has to be found,by putting adaptive layers onto p-MUTs

• p-MUTs low frequency• c-MUTs high frequency

Work supported by 5th PCRDT of the European Commission: contract PARMENIDE