Piezoelectric Micromachined Ultrasonic Transducers P. GAUCHER

Piezoelectric Micromachined Ultrasonic Transducers P. GAUCHER

Piezoelectric Micromachined
Ultrasonic Transducers
Philippe GAUCHER
([email protected])
- Ecole Centrale Paris
Laboratoire 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
We need:
- Power
- Bandwidth
- Sensitivity
in
switch
Propagation
medium
out
SONAR principle
delay
target
Multi-elements Ultrasonic Transducers :
towards 3D imaging
1D probe
1.5D probe
Electronic Beam steering
2D probe
Ultrasonic Transducers: integration into
Silicon technology
u.s
u.s
PZT
Al
Present:
Piezo in thickness mode
E
Si n++
Adaptation
PZT
baking
u.s
SiNx
Ps
oxyde
E
Si
C-MUT Concept:
Capacitive
P-MUT Concept:
Piezo in Flexion (bimorph)
C-MUT and p-MUT
Piezoelectric material
Small air gap
VDC+vac
V
Force
Force
P-MUT:
Material coupling
C-MUT:
Design coupling
Electrical power : Pe = 1/2ε0εE2.ω
Force
S.(VDC+vac)2
1
F= ε0.
2
e2
= Cte+k.EDC.eac(t)
In piezo-electricity : EDC=Pr/ε0 = local field in the material
Vac
Piezoelectric material specifications for a
piezo-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
Membrane
Piezoelectric material
Barrier
Etch stop
(Cross section)
Downscaling technologies for piezoelectric
bimorphs
Lateral
dimensions
Thickness
Voltage
Problems
Solution
STICKING
(BULK CERAMICS)
SCREEN
PRINTING
THIN FILMS
DEPOSITION
> 1cm
> 1mm
> 1µm
> 100µm
> 10µm
> 0.1µm
> 100V
low coupling with
substrate
> 10V
High temperature
processing
noble metal
substrate
> 1V
Optimise glue layer
Pb diffusion in Si
barrier
PARMENIDE Project
MEMS technology: Bulk Silicon Micromachining
Front side:
Back side:
Si wafer with ferroelectric
structures
Membrane machined by DRIE
Thales TRT and Thales Microsonics, Cranfield Univ, EPFL, FhG IBMT, Protavic
SOI (Silicon On Insulator) substrate
TiPt-PZT-Pt
stack 2.3µm
SiO2 0.2 µm
SOI 5µm
SiO2 0.5 µm
Silicium
SEM Cross section of the layer stacking
Interfacial stresses :
Delaminations and cracking of layers
Interfacial Shear stress τ
L
σ = .τ
4t
Layer stress σ
L
σ
σ
couche
τ
substrat
t
τ
•films with τ>0
Delamination
•films with τ<0
Cracks
Sensors and Actuators A89 (2001)
Interfacial stress between film and substrate
Intrinsic stress
Thermal stress
Td
t
tra
s
b
α su
α
>
f ilm
α
f ilm
<α
su
bs
tra
t
Tamb
Tension, σ > 0
compression, σ < 0
Interfacial stress cancellation
substrate
Resultant stress :
σ1<0
n
σ=
σ2>0
≡σ
∑σ
i =1
i
⋅ ei
n
∑e
i =1
i
• σ < 0 compressive stress
• σ > 0 tensile stress
Buckling of structures due to compressive
stresses
L
F
F
t
Euler force:
Y.I π 2 Ywt 3
F>π . 2 =
.
3 L2
L
2
Dimension ÷ 100 => F ÷ 104
Example:
L=w=1mm
t=10µm
Y=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 wafer
Via holes: 70 µm
Hole isolation: PECVD
oxide
SOG 3.2 µm
Acoustic wafer
PECVD oxide
TiPt electrodes
barrier 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
f =0.162. t2 . Y
L ρ
Y
f =0.615. t2 .
L ρ(1−ν 2)
Composite membranes: YÆ YD and rÆ ρequ
Y
f =1.654. t2 .
L ρ(1−ν 2)
Finite Elements Modelling for complex designs
•Coventorware
•Intellisense
•ANSYS, ATTILA …
Modal analysis (polytec laser interferometer)
108 kHz
39 kHz
98 kHz
57 kHz
58 kHz
153 kHz
First natural frequency : experimental and modelling
Résonance f0 (Hz)
1,E+07
1,E+06
1,E+05
1,E+04
1,E+03
0,10
1,00
10,00
Membrane dimension (mm)
analytical model
memcad model
experimental
Differences dues due non ideal geometry of the clamping
Testing with water loading
P-MUT
Glass tube
Sealing
PCB
Resonance frequency
In air : 520 kHz
f =0.243 t EEQ
a² ρEQ
One side water loaded : 181 kHz
f =0.243 t
a²
EEQ
ρEQ1+0.77 ρH O a 
ρEQ t 

2
Electrical Admittance and Bandwidth
Parametric analysis
resonance
Zr=ρ.c
2
k
• Coupling k: N2= C0
Cm 1−k2
• Mechanical Q factor: Qm= LωR
Zr
• Resonance: L(C 0+Cm)ωr2=1
• Antiresonance: LCmωa2=1
• Impedance: Z=Zr + 1
jC0ωr
• Electrical Q factor (at ωr):
Qe= ZrC02ωr
N
• Mechanical Q factor:
Qm= Lωr
Zr
Bandwidth and impedances matching
Admittance
Yi
Yr(f0)
Yi(f0)
B
-3 dB
Yr
fr
f0
Qm =
≈1
B
f0
fa
Yr
Qe =
≈1
Yi
(with Yi=50Ω)
Frequency
Admittance in air : membrane 250 µm
fr=2 MHz, Qm=80
Parasitic Capacitance reduces bandwidth
C0
C1
R1
Co
C1
R
R1
Ro
C
L
Element
R
C
L
C1
Ro
Co
Value
1.5E5 ohm
4.236E-14 F
1.24 H
8.5E-11 F
352.5 ohm
7.511E-11 F
Bandwidth and acoustic impedance:
Admittance measurement in air and in fluorinert
1.20E-05
1.40E-04
2.50E-05
1.20E-04
1.40E-04
1.20E-04
1.00E-05
2.00E-05
1.00E-04
1.00E-04
8.00E-06
1.50E-05
8.00E-05
B (S)
G (S)
B (S)
G (S)
8.00E-05
6.00E-06
6.00E-05
6.00E-05
1.00E-05
4.00E-06
4.00E-05
4.00E-05
5.00E-06
2.00E-05
0.00E+00
4.00E+05
4.50E+05
5.00E+05
5.50E+05
6.00E+05
Frequency
Disc transducer in air:
fr=570 kHz
Qm=142
0.00E+00
6.50E+05
2.00E-06
0.00E+00
1.00E+05
2.00E-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
Frequency
Disc transducer loaded
with Flurinert (front side):
fr= 315 kHz
Qm=1.26
5.50E+05
0.00E+00
6.00E+05
Amplitude and Acoustic Power
Analytical modelling of cantilever
Modal response and force
z
electrodes
x
PZT
x+l
t/2
zn
0
-ts /2
neutral plane
Substrate
Mode 1: 1.62 kHz
Sensor response:
Φ
x2 2
(t+tp).Y
Vs=δ.
.g.tp.∫ ∂2Φ.dx
2l
∂x
x1
Mode 2: 62.8 kHz
Actuator force:
Mode 3: 496 kHz
0
x (mm)
2.5
x2 2
(t+tp).Y
F=
.d.V.∫ ∂2Φ.dx
6+Ψ
∂x
x1
Amplitude at resonance
Déplacement (microns)
18
16
14
12
10
8
6
4
2
0
0
2
4
6
8
Tension (V)
Membrane 4X4mm2 @ 30.9 kHz
10
12
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)
HV
Acoustic burst generator
(labo GPS Univ Paris VI)
1.2 ms
Parmenide sensor
400 µs
Reception signal of a 1.350 X 1.350 Reception signal of a 250 X 250 µm2 p-mut
mm2 p-mut at a distance d=40cm. The at a distance d=10cm. The oscillations are at
700 kHz
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 Æ low frequency
• c-MUTs Æ high frequency
Work supported by 5th PCRDT of the European Commission: contract PARMENIDE