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Piezoelectric Micromachined Ultrasonic Transducers
Philippe GAUCHER([email protected])
- 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