Piezoelectric Micromachined Ultrasonic Transducers

Transcription

1 Piezoelectric Micromachined Ultrasonic Transducers Philippe GAUCHER - Ecole Centrale Paris Laboratoire Structures, Propriétés, Modélisation des Solides - Consultant at THALES Research & Technology

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

3 Physical principles

4 Single Element Pulse-Echo acoustic transducer in We need: - Power - Bandwidth - Sensitivity switch out Propagation medium target SONAR principle delay

5 Multi-elements Ultrasonic Transducers : towards 3D imaging 1D probe 1.5D probe 2D probe Electronic Beam steering

6 Ultrasonic Transducers: integration into Silicon technology E Ps u.s oxyde SiNx u.s E u.s baking Adaptation PZT Si n++ Al Si PZT Present: Piezo in thickness mode C-MUT Concept: Capacitive P-MUT Concept: Piezo in Flexion (bimorph)

7 C-MUT and p-mut Small air gap Piezoelectric material Force V DC +v ac Force V ac C-MUT: Design coupling P-MUT: Material coupling Electrical power : P e = 1/2ε 0 εe 2.ω Force F= 1ε 2 S.(VDC+ v. 2 e 0 ac ) 2 = Cte+k.E DC.e ac (t) In piezo-electricity : E DC =Pr/ε 0 = local field in the material

8 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

9 Technology

10 Generic structure of a Piezoelectric MEMS Membrane Electrodes Piezoelectric material Barrier Etch stop (Cross section)

11 Downscaling technologies for piezoelectric bimorphs Lateral dimensions STICKING (BULK CERAMICS) SCREEN PRINTING > 1cm > 1mm > 1µm THIN FILMS DEPOSITION Thickness > 100µm > 10µm > 0.1µm Voltage > 100V > 10V > 1V Problems Solution low coupling with substrate Optimise glue layer High temperature processing noble metal substrate Pb diffusion in Si barrier

12 PARMENIDE Project MEMS technology: Bulk Silicon Micromachining Front side: Si wafer with ferroelectric structures Back side: Membrane machined by DRIE Thales TRT and Thales Microsonics, Cranfield Univ, EPFL, FhG IBMT, Protavic

13 SOI (Silicon On Insulator) substrate TiPt-PZT-Pt stack 2.3µm SiO µm SOI 5µm SiO µm Silicium SEM Cross section of the layer stacking

14 Interfacial stresses : Delaminations and cracking of layers Interfacial Shear stress τ Layer stress σ L L σ =. τ 4t σ τ couche substrat τ σ t films with τ>0 Delamination films with τ<0 Cracks Sensors and Actuators A89 (2001)

15 Interfacial stress between film and substrate Thermal stress Intrinsic stress T d α film >α substrat αfilm <α substrat T amb Tension, σ > 0 compression, σ < 0

16 Interfacial stress cancellation substrate Resultant stress : σ 1 <0 σ 2 >0 σ n i= 1 = n σ e i= 1 i e i i σ σ < 0 compressive stress σ > 0 tensile stress

17 Buckling of structures due to compressive stresses F L F t Euler force: F > π 2 Y.I. 2 L = π 3 2 Ywt. 2 L 3 Dimension 100 => F 10 4 Example: L=w=1mm t=10µm Y=100GPa σ = 30MPa only! Feynman: Lectures on Physics, Vol 2

18 Buckled Piezoelectric diaphragms

19 Acoustic array of piezoelectric transducers

20 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

21 Poling of PZT film Sol-gel films: Permittivity (1 khz): 800 tanδ (1V rms -1 khz) : 1-3 % Surface capacitance : 12 nf/mm 2 Coercive field: 11 V/µm Breakdown field: > 50 V/µm. Effective d 33 up to 100pC/N Ferroelectric Hysteresis at 115 Hz for a 1.8 µm film

22 Resonant frequencies

23 Natural frequencies of generic structures f = L t. 2 Y ρ f = Y L t 2 ρ(1 ν 2) f = Y L t 2 ρ(1 ν 2) Composite membranes: Y Y D and r ρ equ

24 Finite Elements Modelling for complex designs Coventorware Intellisense ANSYS, ATTILA

25 Modal analysis (polytec laser interferometer) 39 khz 98 khz 108 khz 57 khz 58 khz 153 khz

26 First natural frequency : experimental and modelling 1,E+07 Résonance f0 (Hz) 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

27 Testing with water loading Sealing P-MUT Glass tube PCB

28 Resonance frequency In air : 520 khz One side water loaded : 181 khz f =0.243 a t ² EEQ ρ EQ f = a t ² EEQ ρh2 ρeq ρ O EQ a t

29 Electrical Admittance and Bandwidth

30 Parametric analysis Z r =ρ.c resonance 2 Coupling k: 2 k 0 N = 2 C C m1 k Mechanical Q factor: Q m = Lω Zr Resonance: L(C 0 +C m )ω r2 =1 Antiresonance: LC m ω a2 =1 R Impedance: Z= Zr+ jc 1 0ωr Electrical Q factor (at ω r ): Z r Q N C0 ωr e= Mechanical Q factor: Q m = L Z 2 ωr r

31 Bandwidth and impedances matching Admittance Yr(f0) Y i -3 db B Yi(f0) Yr fr f0 fa Frequency f = 0 Yr Q m 1 Q e = 1 B Yi (with Yi=50Ω)

32 Admittance in air : membrane 250 µm fr=2 MHz, Qm=80

33 Parasitic Capacitance reduces bandwidth R 1 C 0 C1 Co C1 R1 Ro R C Element R C L C1 Ro Co Value 1.5E5 ohm 4.236E-14 F 1.24 H 8.5E-11 F ohm 7.511E-11 F L

34 Bandwidth and acoustic impedance: Admittance measurement in air and in fluorinert 2.50E E E E E E E E E E E E E E-05 G (S) B (S) G (S) 6.00E-06 B (S) 1.00E E E E E E E E E E E E E E E E E E+05 Frequency 0.00E E E E E E E E E E E E E+05 Frequency Disc transducer in air: fr=570 khz Qm=142 Disc transducer loaded with Flurinert (front side): fr= 315 khz Qm=1.26

35 Amplitude and Acoustic Power

36 Analytical modelling of cantilever Modal response and force t/2 z x x+l electrodes PZT 0 -t s /2 z n neutral plane Substrate Φ Mode 1: 1.62 khz Mode 2: 62.8 khz Mode 3: 496 khz 0 x (mm) 2.5 Sensor response: Actuator force: x2 (t+ tp).y 2 Vs=δ..g.tp. 2.dx 2l Φ x x1 x2 (t+ tp).y 2 F=.d.V. 2.dx 6 Φ +Ψ x x1

37 Amplitude at resonance Déplacement (microns) Tension (V) Membrane 30.9 khz

38 Acoustic power - Displacement speed : V = A.ω = 3 m/s - Acoustic pressure in air : Za= Rayl P= Z a.v = 6 kpa - Acoustic pressure in water : Za= Rayl but attenuation by a factor of 100 P = 3 MPa

39 Testing in reception (in air) HV Acoustic burst generator (labo GPS Univ Paris VI) Parmenide sensor 1.2 ms Reception signal of a X mm 2 p-mut at a distance d=40cm. The oscillations are at 30 khz. 400 µs Reception signal of a 250 X 250 µm 2 p-mut at a distance d=10cm. The oscillations are at 700 khz

40 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