Microfabrication of piezoelectric MEMS

Transcription

1 Microfabrication of piezoelectric MEMS 1 st International Workshop on Smart Materials and Structures 7-8 October, 2004 Kiel, Germany Jacek BABOROWSKI RF & Piezoelectric Components Group CSEM SA, Neuchâtel, Switzerland jacek.baborowski@csem.ch 6 April Kiel, 8 October 2004, jba

2 csem Swiss Center of Electronics and Microtechnologie Microelectronics Division RF & PIEZO COMPONENTS GROUP 6 April 2004 ISIF 2004 jb INVITED TALK 2 2

3 csem Swiss Center for Electronics and Microtechnology Privately owned company with ~ 70 Shareholders, none profit 45% 41% Long term contract with the Swiss Government for financing applied research 14% Government Contract Public Projects Industrial Income 2003 : Turnover ~ 54 MCHF, employees ~ SPINN-OFFS Core technologies: Micro- and Nanotechnologies and System Engineering RF & PIEZO COMPONENTS GROUP M.A.Dubois et al. 6 April 2004 ISIF 2004 jb INVITED TALK 3 3

4 FACILITIES at EPFL CENTER OF MICRO- and NANOTECHNOLOGY CERAMIC LABORATORY (Prof. N.Setter Lab.) 6 April 2004 ISIF 2004 jb INVITED TALK 4 4

5 From materials to sensors Reproducible and industrially exploitable microfabrication technology for piezoelectric MEMS based on PZT and AlN thin films By understanding the basic principles of piezoelectric device physics By demonstrating the microfabrication of prototypes: acoustic sensor, ultrasonic transducer 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, e 31,f = -12 C/m 2, {111} AlN, e 31,f = C/m 2, d 33,f = 5.3 pm/v. 6 April 2004 ISIF 2004 jb INVITED TALK 5 5

6 Today's challenges in piezoelectric MEMS Deposition and integration of high performance (e.g.high e 31,f value) piezoelectric films on a wafer scale with thickness up to 10 µm. Establishment of reliable microfabrication processes (mostly dry plasma etching methods) to produce complex piezo MEMS with submicron resolution. Demonstration of new applications Pb 2+ O 2- c Zr 4+, Ti 4+ N Al Q 3, E 3 Microfabrication and characterization of demonstrators top electrode piezoelectric thin film bottom electrode Integration, processing, properties a Si structure strain ε 1 z, 3 y, 2 x, 1 External load or displacement Dedicated microfabrication methods, process flow 6 April 2004 ISIF 2004 jb INVITED TALK 6 6

7 Outline: Introduction 1. Overview of piezoelectric devices 2. General Processing Issues 3. Examples Vibrating membranes and thin plates (MUT, pressure sensors) Cantilevers and Beams RF MEMS Outlook and Conclusions Goal: Present the existing devices Comparison with our actual processing developments Highlight the common problems and limitations 6 April 2004 ISIF 2004 jb INVITED TALK 7 7

8 Overview of piezoelectric devices Piezoelectric thin films in MEMS Micromachined flextensional actuators and transducers Plate waves, SAW Flexural 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) Linear actuators Standing Waves Micromachined Ultrasonic Transducers: Bernstein, Cross 1997, PZT Percin, Khuri-Yakub, 1998, ZnO Baborowski, Muralt, 2002, PZT Akasheh, 2004, PZT Droplet Ejector Percin, 2001 ZnO Intelligent can, Yamashita, 2002, PZT Array microjet, Yuan, 2003, PZT Microdegasing, Maeda 2001, bulk PZT. Piezoelectric 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.. 6 April 2004 ISIF 2004 jb INVITED TALK 8 8

9 Plate and Standing waves Flexural plate waves in membranes (Lamb waves) (Uozumi, Ohsone, White, 1983, ZnO) 6 April 2004 ISIF 2004 jb INVITED TALK 9 9

10 Flextensional ultrasonic transducers 6 April 2004 ISIF 2004 jb INVITED TALK 10 10

11 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) 6 April 2004 ISIF 2004 jb INVITED TALK 11 11

12 Flextensional ultrasonic transducers Cell size microns Frequency range: 3 12 MHz k 2 eff = 1.7 to 2.5 % 6 April 2004 ISIF 2004 jb INVITED TALK 12 12

13 Flextensional ultrasonic transducers Proximity Sensing 6 April 2004 ISIF 2004 jb INVITED TALK 13 13

14 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) April 2004 ISIF 2004 jb INVITED TALK 14 14

15 LINEAR ACTUATORS - Micromirrors Piezoelectric In-Plane Scanning Mirror (Nippon Denso,Y. Ohtuka, 1995) Frequency characteristics Scanning images Resonator shape and method to excite torsional vibration system with 2 degrees of freedom Method to generate torque Driving voltage characteristics 6 April 2004 ISIF 2004 jb INVITED TALK 15 15

16 Micromirrors Piezoelectric In-Plane Scanning Mirror (AIST, Tsukuba, Japan, Chengkuo Lee, et al, 1996~1997) a. b. 1 beam Bimorph structure: Upper electrode Lower electrode c. V+ 4 beams PZT layer SiO 2 substrate 2 beams Laser PZT +SiO 2 (1.5µm) Actuation of 1 beam: V in applied to the beam --- bending Actuation of 2 beams: 2 V in with 180 phase shift-- -1D rotation Actuation of 4 beams: additional 2 V in with phase shift---2d rotation V- V+ V- 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) April 2004 ISIF 2004 jb INVITED TALK 16 16

17 Micromirrors Piezoelectric In-Plane Scanning Mirror (AIST, Tsukuba, Japan, Chengkuo Lee, et al, 1996~1997) A. B. C. v1= -v3=2.5v at 13.6kHz v1= -v3=2.5v at 13.6kHz v2= -v4=2.5v at 14.6kHz v2= -v4=2.5v at 14.6kHz v1 v3 v2 v4 v1 v2 v4 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) April 2004 ISIF 2004 jb INVITED TALK 17 17

18 Micromirrors ~ 1 micron/v 6 April 2004 ISIF 2004 jb INVITED TALK 18 18

19 Micromirrors 2D Scanning Mirror Using Bi-layer PZT Films J. Tsaur et al, AIST, Tsukuba, Japan, 2D Micro Scanner Actuated by sol-gel derived double layered PZT, MEMS April 2004 ISIF 2004 jb INVITED TALK 19 19

20 Cantilevers 6 April 2004 ISIF 2004 jb INVITED TALK 20 20

21 Inertial systems 60 mic PZT 40 mic Si membrane 6 April 2004 ISIF 2004 jb INVITED TALK 21 21

22 RF MEMS Piezoelectric RF-switch d 33 mode generated by IDT electrodes d 33 = 120 pc/n Piezoelectric (PZT) < 40 V 2 µs Gap 1 micron Penn State Univ. 6 April 2004 ISIF 2004 jb INVITED TALK 22 22

23 RF MEMS Piezoelectric RF-switch Tunable parallel-plate varactor < 6 V Q = µm LG, Park et al. Park et al., LG Electronics, April 2004 ISIF 2004 jb INVITED TALK 23 23

24 2. Microfabrication of piezoelectric MEMS based on deflecting structures Common tasks IC's Silicon micromachining Photolithography, 1 µm resolution is sufficient (now 0.13 µm in IC's) Thin films deposition and etching processes for standard SiO 2, Si 3 N 4, poly-si and metal films. Madou (1997) + Special tasks for pmems For piezo MEMS, additionnal methods are required: 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 6 April 2004 ISIF 2004 jb INVITED TALK 24 24

25 Piezoelectric multilayer, thin film structures Substrates: Si, SiO 2 /SiN membranes, SiC, diamond *) Electrodes: Pt/Ti, Mo, Al, Au/Cr Piezoelectric layers: PZT, AlN, ZnO *) Shibata, Sensor and Actuators, 2004 Laminated PZT/Si deflecting structures used in piezoelectric MEMS: bridge, cantilever and suspended membrane. J.Baborowski, Journal of Electroceramics, 2004 D 3 = e 31, f (x 1 + x 2 )+ d 33, f σ 3 Sensors σ 1,2 = e 31, f E 3, x 3 = d 33, f E 3 e 31, f = d 31 s E E e 11 + s 31 c 13 E 12 c e 33 e 31, f > e d 33, f = e 33 E d 33 c 33 E 2 s 13 E s 11 E + s 12 E d 31 < d 33 P. Muralt, Integrated Ferroelectrics,1997 Actuators 6 April 2004 ISIF 2004 jb INVITED TALK 25 AlN PZT e 31,f [C/m2] d 33,f [pm/v] ε 33,f tgδ Current resp. [C/m 2 ] Voltage resp. [GV/m 2 ] 13 1 S/N [10 5 Pa 0.5 ] k Power efficiency

26 Film bulk acoustic resonator (FBAR) Bulk micromachining Surface micromachining electrodes AlN Si wafer electrodes BAW resonators are based on the longitudinal 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 AlN Si wafer Structures sensitive to material stress, prone to buckling (1-2 µm thick membranes or bridges) Fragile structures for manipulation, packaging, etc. Use of IC wafers difficult in the case of bulk micromachined resonators 6 April 2004 ISIF 2004 jb INVITED TALK 26 26

27 Solidly mounted resonator (SMR) electrodes AlN acoustic reflector Acoustic insulation performed by a Bragg acoustic mirror Si wafer The acoustic reflector is a stack of alternating λ/4 layers made of materials with very different elastic properties. f 0 = 2.4 GHz Stress compensated layers for a crack-free structure Dense layers => low wave attenuation Very smooth interfaces => low wave diffraction M.A.Dubois, CSEM 6 April 2004 ISIF 2004 jb INVITED TALK 27 27

28 2. Microfabrication of piezoelectric MEMS based on deflecting structures DEPOSITION and growth control PVD sputtering CSD (sol-gel) PLD STRESS CONTROL Tensile stresses present in most of the layers (up to +800 MPa Pt/Ti) Compressive stresses in SiO 2 wetox (- 300MPa) Very sensible to deposition conditions (AlN) and poling (e.g. PZT +/-60MPa) Incertitude in stress level in SOI substrates Stress adjustment PATTERNING Wet chemical etching HDP etching Surface micromachining (Piekarski, 2001) DIMENSIONAL CONTROL Defines miniaturization rules Defines precision of elastic behavior and characteristics (resonance frequencies, coupling factor) Defines border conditions (accordance between simulation models and reality) (Lee,Itoh, 1996) 6 April 2004 ISIF 2004 jb INVITED TALK 28 28

29 Deposition and integration of {100}-textured sol-gel PZT films on silicon (Ceramics Lab., EPFL, ) 1) Pt bottom electrode 100 nm Pt/TiO 2 /Ti + 10 nm PbTiO 3 {100} seeding layer 2) Elaboration of PZT precursors Maeder (1998) Muralt (1998) Hiboux (1999) intensity (counts) Budd (1985) PZT (100)/(001) P(100) = 98% PZT (110) PZT (111) Pt (111) PZT (200)/(002) angle 2Θ (deg) {100} 2 solutions with 10% and 30% Pb excess Seifert,, Ledermann (2001) Ledermann, Baborowski, Muralt (2002) 3) Optimized Sol-gel process PZT 53/47 (100) on PbTiO 3 (PT) seeding layer Thickness: about 0.25 µm / layer; total up to 4 µm -e 31,f = 12 ± 0.3 C/m 2 Ti Zr Zr % at Ti Revealed lateral underetch: - Global (several microns), - In each crystalised layer during each RTA step (below 100nm), - Heterogeneity in each crystalised layer due to migration of Zr and Ti (Baborowski 2002, Cantoni 2003) 6 April 2004 ISIF 2004 jb INVITED TALK 29 Pb 29

30 PVD DEPOSITION Spider: a versatile sputtering equipment Cluster architecture: loadlock, wafer transfer module, 4 process chambers Adapted for 100 or 150 mm wafers DC, pulsed DC, and RF power sources for metal and ceramic films 1 chamber designed and reserved for AlN thin films 6 April 2004 ISIF 2004 jb INVITED TALK 30 30

31 Sputtering of AlN thin films Reactive sputtering from a pure Al target in a nitrogen atmosphere Very smooth surface Dense columnar microstructure ~3% uniformity on 4" wafer c-axis orientation (FWHM [002] = 1.58 ), induced by the hexagonal plane of Pt (111) d 33,f =3.7±0.3 pmv E E+05 Single run for deposition of : Bottom electrode: Ti, Pt Piezoelectric layer : AlN Top electrode Pt or AlSi intensity 1.0E E E E+01 Si (200) AlN (002) Pt (111) Si (400) AlN (004) Pt (222) M.A.Dubois, CSEM 1.0E theta [ ] 6 April 2004 ISIF 2004 jb INVITED TALK 31 31

32 Deposition and integration of AlN films on SOI substrates (CSEM 2004) AlN (0002) Ti (0002) Pt (111) Pt (200) AlN (0004) Pt (311) Ti (0004) Pt (222) AlN Counts(a.u) θ (degree) I [a.u.] AES profile of SOI_5_1 through VIA of bottom-elec Platinum 100 nm etch time [min] Oxygen Titanium Platinum Titanium 10 nm Oxygen presence in Ti layer 1. SiO 2 compensation layer : 1500 nm thermal wet oxidation of Si followed by HFdip 2. Ti adhesion layer : 10nm 3. Pt seed layer : 100nm 4. AlN piezoelectric layer : 1500 nm 5. Pt top-electrode : 100nm Ti sputtering 10nm Pt sputtering 100nm p vacuum 7*10-8 mbar 3*10-7 mbar AlN sputtering 1500 nm 5*10-8 mbar p 3*10-3 mbar 5*10-3 mbar 4*10-3 mbar work T 300 C 300 C 300 C gas 9 sccm Ar 15 sccm Ar 50 sccm N 2 Deposition rate 1 nm/sec 4 nm/sec 0.72nm/s Field source DC 1000W DC 1000W Pulsed DC 1500W 6 April 2004 ISIF 2004 jb INVITED TALK 32 32

33 STRESS COMPENSATED BEAMS/ MEMBRANES Piezoelectric coefficiet -e 31,f (C/m 2 ) PZT {100}, 1 µm thin film Piezoelectric coeff PZT 53/ Poling:+ 64 MPa Film stress Composition [Zr]/([Zr]+[Ti]) z-deflection (µm) after poling before poling distance from the attachement (µm) Poling of PZT effect on residual stress : Deflection + 20 µm = + 60 MPa σ δ = 3 MPa/µm Ledermann et al., Sensor and Actuators, Residual stresses as a function of film thickness in: SiO 2 wetox Residual stresses in wetox SiO AlN σ (MPa) Residual stress [MPa] Thickness of SiO 2 (nm) AlN thickness [µm] 6 April 2004 ISIF 2004 jb INVITED TALK 33 33

34 STRESS COMPENSATED BEAMS/ MEMBRANES Mechanical system at equilibrium u FR = Es ( c α s T ) ts + Ei ( c α i T ) ti = 0 F b R M R ( z t ) 0 i σ s b = dz + r = ts 0 t σ s S i i h h i 1 i ( z tb ) dz + i h Ei ( z t r h i 1 b σ ( z t i ) dz = 0 b ) dz (1) (2) (3) Equilibration of the structure Setting (3) to zero and resolving (1),(2),(3) M R = 0 Determination of residual stress Membrane definition z Materials constants Determination of the wetox SiO 2 layer thickness for deflection-free membrane t Pt t AlN t Ti/Pt t SiO2 h Pt h AlN h Ti/Pt h 0 SiO2 -t b ρ [10 3 Kg/m 3 ] E [GPa] α [ppm] T f [K] ν Si *T SiO Ti Pt t s AlN April 2004 ISIF 2004 jb INVITED TALK 34 34

35 STRESS COMPENSATED BEAMS/ MEMBRANES EXAMPLE of sol gel PZT / Si σ res = E 2 1 υ t s d t s = substrate thickness (10 µm) t f = thin films thickness (1 µm) 3L 2 d = cantilever tip deflection (µm), L = cantilever length (2 mm) t f Au/Cr E, ν = Young modulus and Poisson s coefficient of Silicon PZT Pt /Ti SiO 2 burried Si bulk SiO 2 wet 0.12 µm (PVD) Pt/TiO 2 = ± 72 MPa 1.00 µm (sol-gel) PZT = ± 8 MPa (unpoled) 1.00 µm (250 kv/cm, 150 C, 10 ) PZT = + 64 ± 5 MPa (poling) 0.2 µm (evaporation) Au/Cr = ± 11 MPa X? (wetox) SiO 2 = ± 3 MPa E.g. for 2 µm PZT X SiO2 = 1200 ± 40 nm or ± 12MPa For SOI substrates the stress of buried oxide is not well known 6 April 2004 ISIF 2004 jb INVITED TALK 35 35

36 STRESS COMPENSATED BEAMS/ MEMBRANES PZT on 5 µm SOI 0.3 mm 2 to 1 mm 2 SQUARE MEMBRANE : 2-4 micron of PZT 5 micron of Si Well stress compensated Even if the stress value of buried oxide is uknown height [µm] height [µm] AlN on 5 and 10 µm SOI Bending B cross 1 cross 2 ~10-20 µm distance [mm] Bending B less than 1 µm distance [mm] 1 x 1mm 2 5 µm thick 1 x 1mm 2 10 µm thick 6 April 2004 ISIF 2004 jb INVITED TALK 36 36

37 MICROFABRICATION: PROCESSING WITH SOI WAFERS (Ceramics Lab., EPFL, ) Au/Cr SiO 2 PECVD PZT Pt /Ti SiO 2 burried Si bulk SiO 2 wet Plasma Etch (ICP, Cl 2, Ar) Wet etch (HCl:HF) Plasma Etch (ICP, Cl 2, Ar) Al AlN Pt Air gap Si Deep Reactive Ion Etching (Plasma Etch, ICP, RT, SiF 6, C 4 F 8 ) Baborowski et al.,epfl 2001 Bridge of suspended membrane Carazzetti et al.,epfl April 2004 ISIF 2004 jb INVITED TALK 37 37

38 Reactive Ion Beam Etching of PZT and Pt thin films 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. ECR/RF Reactive Ion Beam Etching gaz inlet RF, 2.45 GHz PZT : RIBE, CCl 4 /CF 4 /Ar, low bias 70 nm/min, S PR = 0.5, S Pt = 1.6 Pt : RIBE (CCl 4 ) or RIE/ICP (Cl 2 ) 60 nm/min, S PR = 0.5 ECR ion gun ions pumping 2 µm + PZT wafer water cooled substrate holder MHz RF bias Baborowski, Ledermann, Muralt (1999, 2000) Pt PZT 6 April 2004 ISIF 2004 jb INVITED TALK 38 38

39 Advanced patterning for fundamental studies on ferroelectrics E-beam gas Ar, CCl 4, CF GHz PMMA PZT STO grids ECR Plasma mm RF MHz Water cooled RF chuck S.Bühlman, J.Baborowski, P.Muralt, 2002 S.Bühlman PhD Thesis EPFL, April 2004 ISIF 2004 jb INVITED TALK 39 Amplitude of piezoresponse (a.u.) DC bias to tip (V) 39

40 MICROFABRICATION: PROCESSING WITH SOI WAFERS (Ceramics Lab., EPFL, ) PATTERNING OF GROOVES and SLIT OPENINGS THROUGH PZT/Pt/SiO 2 /Si (e.g.unclamped membranes, cantilevers) STANDARD: Photolithography & Dry etch: Standard photolithogaphy 100 nm Pt etch (ICP STS) 1200 nm wetox SiO 2 etch (Alcatel, 300 sec) up to 50 µm Si (SOI) (Alcatel601E) ETCH STOP on SiO 2 buried Pt SiO 2 Si SiO 2 burried Au/Cr SiO 2 PVD PZT Pt /Ti SiO 2 burried Si bulk SiO 2 wet Below 5 micron? 6 April 2004 ISIF 2004 jb INVITED TALK 40 40

41 MICROFABRICATION: PROCESSING WITH SOI WAFERS (Ceramics Lab., EPFL, ) PATTERNING OF SUBMICRON SLIT OPENINGS THROUGH AlN/Pt/SiO 2 /Si (e.g.limited air conductivity in pressure sensors with minimum roughness of sidewalls) SiO 2 PVD Au/Cr AlN or PZT Pt /Ti 100nm Pt/Ti 1200 nm SiO nm <10 µm Si SOI 100nm Pt/Ti 1200 nm SiO 2 SiO 2 burried Si bulk SiO 2 wet <10 µm Si SOI 724 nm HIGH DEFINITION: Direct Laser Writing & HDP Dry etch HMDS & Shipley 1805, 1.75 µm, 90 C Direct writing DWL: 4.04.W Development : Standard Rite Track 1818 Serie Etching: Pt: STS; Pt_etch (150W bias), 380sec. (photoresist removed) SiO 2 : Alcatel 601E; SiO 2 stand; DFA, 150 sec Si: Alcatel 601E; Si_ambiant_2 (optimized for smalll openings); 360 sec Final clean (solvant & SDR), plasma O 2 ashing Bow < 50 nm 6 April 2004 ISIF 2004 jb INVITED TALK 41 41

42 Definition of membrane thickness (Ceramics Lab., EPFL, ) Standard vs. SOI wafers Backside view; 2µm large slit traversing the 10 µm thick membrane 6 April 2004 ISIF 2004 jb INVITED TALK 42 42

43 4. EXAMPLES ( ) 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 6 April 2004 ISIF 2004 jb INVITED TALK 43 43

44 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(Zr x Ti 1-x )O 3 (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 Parmenide EU project 6 April 2004 ISIF 2004 jb INVITED TALK 44 44

45 3.1. MICROMACHINED ULTRASONIC TRANSDUCERS Fabricated devices: 2 micron PZT on 5 micron Si SOI substrate Frequency range (air loaded): Membrane size: 50 to 150 khz 1000 µm 400 to 600 khz 450 µm 750 to 1200 khz 300 µm 6 April 2004 ISIF 2004 jb INVITED TALK 45 45

46 Single element: Comparison between the simulation and measurements Basic 55,2 khz Second 106 khz LC Simulated by D.Schmidt (IBMT, Fraunhofer Institut) Measured with a stroboscopic interferometric microscope (A. Bosseboef CNRS, Orsay) EPFL 6 April 2004 ISIF 2004 jb INVITED TALK 46 46

47 Suspended disc 1 mm 2 BASIC Mode 1 - Vertical 55,2 khz, 0.5 V AC, 50kV/cm DC bias A C 200 Deflection of res 60 khz device #13, RND GR - wafer 01 0 B B Observed Deflection Z (nm) µm/V -600 C A distance x (µm) Deflection vs. distance - simulation coupe A-A coupe B-B coupe C-C 0 Simulated Deflection z (micron) mic/v The shape is close to the desired piston movement. The displacement is uniform around Distance x (micron) 6 April 2004 ISIF 2004 jb INVITED TALK 47 47

48 HIGHEST COUPLING COEFFICIENT FOR HIGH FREQUENCY TRANSDUCERS a1-df-res1-20dc SOI wafer: 3.5 µm of Si PZT (001) 2 µm f res = 753 khz k 2 = 5.3 % k = 23 % Q = 135 C 0 = 90 pf real meas imag meas real fit imag fit k 2 = 5.3 % Q =135 Admittance C para =0 DC bias = 100kV/cm k=23% Frequency (Hz) 6 April 2004 ISIF 2004 jb INVITED TALK 48 48

49 LOW FREQUENCY SINGLE DEVICES in Air and in FLUORINERT TM (3M) /22 Q/5 k 2 =const AIR: f res = 97.5 khz k 2 = 1.0% Q = 75 Fluorinert: f res = 19 khz k 2 = 1.0% Q= 16 k 2 = const Q / 5 6 April 2004 ISIF 2004 jb INVITED TALK 49 49

50 APPLICATIONS Presence and precise positioning sensor; transmission Paper Receiver Plastic Emitter actuator voltage (V) Air transmission, distance = 10 mm actuator DISC F 1.1 sensor DISC F 3.1 Charge amplifier: 10 pc Excitation frequency = 98 khz, 10 V AC, 5 V offset time (µs) sensor voltage (V) emitter voltage (V) 4 Air transmission, distance = 10 mm Liquid emitter DISC F 1.1 receiver DISC F time (µs) receiver voltage (V) 6 April 2004 ISIF 2004 jb INVITED TALK 50 50

51 3.2 Piezoelectric microphones for photoacoustic detection Pt Bottom electrode A 1,2,3 µm slit Specificity of the application Low frequency operation up to100hz Low acoustic pressure level order of mpa 3mm 1,2 mm B AlN B Key points Build a MEMS device based on partially unclamped, deflecting piezoelectric structures Bridge or cantilever structures have been focused Use of PZT or AlN as piezoelectric layer Use of SOI wafers perfectly well defined Si membrane Patterning of very narrow slits advanced ICP dry etch Possibility to build arrays of devices for complementary properties Integration of both amplification unit and device on same PCB Pt Top electrode A 1,2 mm 1,2,3 µm slit 6 April 2004 ISIF 2004 jb INVITED TALK 51 51

52 3.2. Acoustic sensors based on PZT sol-gel films The cantilever concept for an audio microphone/microspeaker was proposed by White & al in (J. Micromech. Microeng. 8 (1998) ) 4.5 µm thick cantilever ZnO/SiN/SiO 2 : 10 µm slit, response at 100 Hz = 38 mv/pa 2x2 mm beam 1 micron thick PZT on 10 micron Si membrane; Ledermann et al., 2002 Slit 5 and 10 µm 2x2.5 mm beam 2x2 mm bridge Electrode for pyro effect compensation (static). via Cantilever top electrode 6 April 2004 ISIF 2004 jb INVITED TALK 52 52

53 3.2. Acoustic sensors based on PZT sol-gel films N.Ledermann, J.Baborowski et al., JMM, Cantilever C5-3 - w Bridge B w Sensitivity (mv/pa) frequency (Hz) V = 4 cm 3 V = 2 cm 3 V = 1 cm 3 V = 0.5 cm 3 S 0, theo = 3.21 pc/pa or 321 mv/pa S 0, C5-3 = 226 mv/pa 70 % S 0, theo sensitivity (mv/pa) V = 4 3 cm V = 2 3 cm V = 1 3 cm V = cm frequency (Hz) S 0, theo = 1.72 pc/pa or 172 mv/pa S 0, B3-10 = 68.3 mv/pa 39 % S 0, theo 6 April 2004 ISIF 2004 jb INVITED TALK 53 53

54 3.2. Acoustic sensors based on AlN films Advantages of AlN based sensors: Full IC compatible Reduced Thermal Budget No poling required Good selectivity for micromachining Low losses Future integration SoC (System on Chip) possible Ground Phase Cantilever backside view Si SOI membrane 2 µm slit AlN on membrane Etched AlN for slit opening Re[Y] 4.5E E E E E E E E E E x1 mm CANTILEVER 6'000 6'500 7'000 7'500 8'000 8'500 9'000 Frequency [Hz] J.BABOROWSKI, CSEM April 2004 ISIF 2004 jb INVITED TALK 54 54

55 Microhotplate as processing device for local thin film growth Backside view 2 mm After PZT crystallization Frontside view Pt SiO 2 Ta 5 Si 3 Si 3 N 4 SiO nm 150 nm 200 nm 200 nm 650 nm Cross-section PZT Pt SiO 2 Ta 5 Si 3 Si 3 N 4 SiO 2 Diploma work of F. Calame; with J. Baborowski 6 April 2004 ISIF 2004 jb INVITED TALK 55 55

56 RF MEMS for wireless communication and Ambient Intelligence (EC IP Mimosa) CSEM involved in: BAW resonators Filters RF PIEZOELECTRIC Switch 6 April 2004 ISIF 2004 jb INVITED TALK 56 56

57 Conclusions Piezoelectric 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 design Actuators, 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 6 April 2004 ISIF 2004 jb INVITED TALK 57 57

58 ACKNOWLEDGEMENTS TEAMS of Ceramic Lab, EPFL, Center of Micro- Nanotechnology, EPFL Center of Microscopy, EPFL LMARC, Besancon, France, CSEM, Neuchatel HachUltra Analytics, Geneva OFES and EU Commission Thank you for your kind attention And for investing your time 6 April 2004 ISIF 2004 jb INVITED TALK 58 58