MICROFABRICATION OF PIEZOELECTRIC MEMS - fh-kiel.de · 6 April 2004 1 Kiel, 8 October 2004, jba...
Transcript of MICROFABRICATION OF PIEZOELECTRIC MEMS - fh-kiel.de · 6 April 2004 1 Kiel, 8 October 2004, jba...
6 April 2004 1Kiel, 8 October 2004, jba
Microfabrication of piezoelectric MEMS
1st International Workshop on Smart Materials and Structures 7-8 October, 2004Kiel, Germany
Jacek BABOROWSKI
RF & Piezoelectric Components GroupCSEM SA, Neuchâtel, [email protected]
6 April 2004 ISIF 2004 jb INVITED TALK2
6 April 2004 ISIF 2004 jb INVITED TALK 2Kiel, 7. Oktober 2004, jba
csemSwiss Center of Electronicsand Microtechnologie
Microelectronics Division
RF & PIEZO COMPONENTS GROUP
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6 April 2004 ISIF 2004 jb INVITED TALK 3Kiel, 7. Oktober 2004, jba
csemSwiss Center for Electronics and Microtechnology
2003 :Turnover ~ 54 MCHF, employees ~ 290
+ SPINN-OFFS
Privately owned company with ~ 70 Shareholders, none profit
Long term contract with the Swiss Government for financing applied research
Core technologies: Micro- and Nano-technologies and System Engineering
41%
14%
45%
Government Contract Public Projects Industrial Income
RF & PIEZO COMPONENTS GROUPM.A.Dubois et al.
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6 April 2004 ISIF 2004 jb INVITED TALK 4Kiel, 7. Oktober 2004, jba
FACILITIES at EPFL
CENTER OF MICRO- and NANOTECHNOLOGY
CERAMIC LABORATORY(Prof. N.Setter Lab.)
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6 April 2004 ISIF 2004 jb INVITED TALK 5Kiel, 7. Oktober 2004, jba
From materials… to sensors…Reproducible and industrially exploitable microfabricationtechnology for piezoelectric MEMS based on PZT and AlN thin films
By demonstrating the microfabrication of
prototypes: acoustic sensor, ultrasonic
transducer
By understanding the basic principles
of piezoelectric device physics
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, e31,f = -12 C/m2, {111} • AlN, e31,f = -1.02 C/m2 , d33,f = 5.3 pm/V .
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6 April 2004 ISIF 2004 jb INVITED TALK 6Kiel, 7. Oktober 2004, jba
Today's challenges in piezoelectric MEMS
• Deposition and integration of high performance (e.g.high e31,f value)piezoelectric films on a wafer scale with thickness up to 10 µm.
Dedicated microfabrication methods, process flow
top electrode
bottom electrode
strain ε1
Q3, E3
x, 1
y, 2z, 3
External load or displacement
piezoelectric thin film
Si structure
Microfabrication and characterization of demonstrators
• Establishment of reliable microfabrication processes (mostly “dry” plasmaetching methods) to produce complex piezo MEMS with submicron resolution.
• Demonstration of new applications
Pb2+
O2-
Zr4+, Ti4+
Integration, processing,properties
N
Alc
a
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6 April 2004 ISIF 2004 jb INVITED TALK 7Kiel, 7. Oktober 2004, jba
Outline:
Introduction1. Overview of piezoelectric devices2. General Processing Issues3. Examples
• Vibrating membranes and thin plates (MUT, pressure sensors)
• Cantilevers and Beams• RF MEMSOutlook and Conclusions
Goal:• Present the existing devices• Comparison with our actual processing developments• Highlight the common problems and limitations
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6 April 2004 ISIF 2004 jb INVITED TALK 8Kiel, 7. Oktober 2004, jba
Overview of piezoelectric devicesPiezoelectric thin films in MEMSMicromachined flextensional actuators and transducers
Plate waves, SAWFlexural 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)
Standing Waves• Micromachined Ultrasonic Transducers:
Bernstein, Cross 1997, PZTPercin, Khuri-Yakub, 1998, ZnOBaborowski, Muralt, 2002, PZTAkasheh, 2004, PZT
• Droplet Ejector Percin, 2001 ZnO• Intelligent can, Yamashita, 2002, PZT• Array microjet, Yuan, 2003, PZT• Microdegasing, Maeda 2001, bulk PZT.
Linear actuatorsPiezoelectric 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..
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6 April 2004 ISIF 2004 jb INVITED TALK 9Kiel, 7. Oktober 2004, jba
Plate and Standing wavesFlexural plate waves in membranes (Lamb waves) (Uozumi, Ohsone, White, 1983, ZnO)
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… Flextensional ultrasonic transducers
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6 April 2004 ISIF 2004 jb INVITED TALK 11Kiel, 7. Oktober 2004, jba
… 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)
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6 April 2004 ISIF 2004 jb INVITED TALK 12Kiel, 7. Oktober 2004, jba
… Flextensional ultrasonic transducers
Cell size 30 - 80 micronsFrequency range: 3 – 12 MHzk2
eff = 1.7 to 2.5 %
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6 April 2004 ISIF 2004 jb INVITED TALK 13Kiel, 7. Oktober 2004, jba
… Flextensional ultrasonic transducers Proximity Sensing
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6 April 2004 ISIF 2004 jb INVITED TALK 14Kiel, 7. Oktober 2004, jba
… 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) 2193-96
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6 April 2004 ISIF 2004 jb INVITED TALK 15Kiel, 7. Oktober 2004, jba
Resonator shape and method to excite torsional vibration system with 2 degrees of freedom
Method to generate torqueDriving voltage characteristics
Frequency characteristics Scanning images
Piezoelectric In-Plane Scanning Mirror(Nippon Denso,Y. Ohtuka, 1995)
LINEAR ACTUATORS - Micromirrors
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6 April 2004 ISIF 2004 jb INVITED TALK 16Kiel, 7. Oktober 2004, jba
MicromirrorsPiezoelectric In-Plane Scanning Mirror(AIST, Tsukuba, Japan, Chengkuo Lee, et al, 1996~1997)
SiO2 substrate
PZT layer
a.a. b.b. Bimorph structure:Bimorph structure:• PZT +SiO2 (1.5µm)Actuation of 1 beam:Actuation of 1 beam:• Vin applied to the beam ---
bendingActuation of 2 beams:Actuation of 2 beams:• 2 Vin with 180° phase shift--
-1D rotation Actuation of 4 beams:Actuation of 4 beams:• additional 2 Vin with phase
shift---2D rotation
Upper electrode
Lower electrode
c.c.
V+
V-
V+ V-
Laser
1 beam
2 beams
4 beams
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) 144-152.
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6 April 2004 ISIF 2004 jb INVITED TALK 17Kiel, 7. Oktober 2004, jba
MicromirrorsPiezoelectric In-Plane Scanning Mirror(AIST, Tsukuba, Japan, Chengkuo Lee, et al, 1996~1997)
C.v1= -v3=2.5V at 13.6kHzv2= -v4=2.5V at 14.6kHz
v4v1
v3
A.v1= -v3=2.5V at 13.6kHz
B.v2= -v4=2.5V at 14.6kHz
v2
v4 v1
v2 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) 144-152.
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6 April 2004 ISIF 2004 jb INVITED TALK 18Kiel, 7. Oktober 2004, jba
Micromirrors
~ 1 micron/V
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6 April 2004 ISIF 2004 jb INVITED TALK 19Kiel, 7. Oktober 2004, jba
2D Scanning Mirror Using Bi-layer PZT FilmsJ. Tsaur et al, AIST, Tsukuba, Japan, 2D Micro Scanner Actuated by sol-gel derived double layered PZT, MEMS 2002
Micromirrors
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6 April 2004 ISIF 2004 jb INVITED TALK 20Kiel, 7. Oktober 2004, jba
Cantilevers
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6 April 2004 ISIF 2004 jb INVITED TALK 21Kiel, 7. Oktober 2004, jba
Inertial systems
60 mic PZT40 mic Si membrane
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6 April 2004 ISIF 2004 jb INVITED TALK 22Kiel, 7. Oktober 2004, jba
RF MEMS Piezoelectric RF-switch
Piezoelectric (PZT) < 40 V 2 µs Gap 1 micron Penn State Univ.
d33 mode generated by IDT electrodes
d33= 120 pC/N
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6 April 2004 ISIF 2004 jb INVITED TALK 23Kiel, 7. Oktober 2004, jba
RF MEMS Piezoelectric RF-switch
Tunable parallel-plate varactor < 6 V Q = 210 2.5 – 3.5 µm LG, Park et al.
Park et al., LG Electronics, 2001
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6 April 2004 ISIF 2004 jb INVITED TALK 24Kiel, 7. Oktober 2004, jba
2. Microfabrication of piezoelectric MEMS based on deflecting structures
Common tasks
• Silicon micromachining• Photolithography, 1 µm resolution is sufficient (now 0.13 µm in IC's)• Thin films deposition and etching processes for standard
SiO2, Si3N4, poly-Si and metal films.Madou (1997)
IC's
• 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
+
Special tasks for pMEMSFor piezo MEMS, additionnal methods are required:
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6 April 2004 ISIF 2004 jb INVITED TALK 25Kiel, 7. Oktober 2004, jba
Piezoelectric multilayer, thin film structures
J.Baborowski,Journal of Electroceramics, 2004
Laminated PZT/Si deflecting structures used in piezoelectric MEMS:bridge, cantilever and suspended membrane.
AlN PZT
e31,f [C/m2] -1.33 -12
d33,f [pm/V] 5.15 65
ε33,f 10.2 1200
tgδ 0.002 0.03
Current resp. [C/m2] 1.33 12
Voltage resp. [GV/m2] 13 1
S/N [105 Pa0.5] 31.3 6.7
k2 0.22 0.15
Power efficiency 110 5
Actuators
Substrates: Si, SiO2/SiN membranes, SiC, diamond*)
Electrodes: Pt/Ti, Mo, Al, Au/CrPiezoelectric layers: PZT, AlN, ZnO…
*) Shibata, Sensor and Actuators, 2004
Sensors
σ1,2 = −e31, f ⋅ E3 , x3 = d33, f ⋅ E3
D3 = e31, f ⋅(x1 + x2 )+ d33, f ⋅σ3
e 31 , f = d 31s11
E + s12E ≡ e31 − c13
E
c 33E e 33 e 31 , f > e 31
d 33 , f = e 33c 33
E ≡ d 33 − 2 s13E
s11E + s12
E d31 < d 33
P. Muralt, Integrated Ferroelectrics,1997
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6 April 2004 ISIF 2004 jb INVITED TALK 26Kiel, 7. Oktober 2004, jba
Film bulk acoustic resonator (FBAR)Bulk micromachining
Surface micromachining
AlN
Si wafer
electrodes
BAW resonators are based on thelongitudinal 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
– Structures sensitive to material stress, prone to buckling (1-2 µm thickmembranes or bridges)
– Fragile structures for manipulation, packaging, etc.
– Use of IC wafers difficult in the case ofbulk micromachined resonators
AlN
Si wafer
electrodes
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6 April 2004 ISIF 2004 jb INVITED TALK 27Kiel, 7. Oktober 2004, jba
Solidly mounted resonator (SMR)Acoustic insulation performed by a Bragg acoustic mirrorAlN
Si wafer
electrodes
acoustic reflector
f0 = 2.4 GHzStress compensated layers for a crack-free structureDense layers => low waveattenuationVery smooth interfaces => low wavediffraction
The acoustic reflector is a stackof alternating λ/4 layers made of
materials with very differentelastic properties.
M.A.Dubois, CSEM
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6 April 2004 ISIF 2004 jb INVITED TALK 28Kiel, 7. Oktober 2004, jba
2. Microfabrication of piezoelectric MEMS based on deflecting structures
DEPOSITION and growth control
PVD sputteringCSD (sol-gel)PLD
PATTERNINGWet chemical etchingHDP etchingSurface micromachining (Piekarski, 2001)
DIMENSIONAL CONTROLDefines miniaturization rulesDefines precision of elastic behavior and characteristics (resonance frequencies, coupling factor)Defines border conditions (accordance between simulation models and reality)
STRESS CONTROLTensile stresses present in most of the layers (up to +800 MPa Pt/Ti)Compressive stresses in SiO2 wetox (-300MPa)Very sensible to deposition conditions (AlN) and poling (e.g. PZT +/-60MPa)Incertitude in stress level in SOI substratesStress adjustment
(Lee,Itoh, 1996)
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6 April 2004 ISIF 2004 jb INVITED TALK 29Kiel, 7. Oktober 2004, jba
3) Optimized Sol-gel process PZT 53/47 (100) on PbTiO3 (PT) seeding layerThickness: about 0.25 µm / layer; total up to 4 µm
-e31,f = 12 ± 0.3 C/m2
Deposition and integration of {100}-textured sol-gel PZT films on silicon (Ceramics Lab., EPFL, 1996-2002)
2) Elaboration of PZT precursors
2 solutions with 10% and 30% Pb excess
Budd (1985)
1) Pt bottom electrode
100 nm Pt/TiO2/Ti + 10 nm PbTiO3 {100} seeding layer
Maeder (1998)Muralt (1998)Hiboux (1999)
Seifert,, Ledermann (2001)Ledermann, Baborowski, Muralt (2002)
0 100
5 104
1 105
1.5 105
2 105
2.5 105
20 25 30 35 40 45 50
inte
nsity
(cou
nts)
angle 2Θ (deg)
PZT
(100
)/(00
1)
PZT
(110
)
PZT
(111
)P
t (11
1)
PZT
(200
)/(00
2)P(100) = 98%
{100}
Revealed lateral underetch:- Global (several microns),- In each crystalised layer during each
RTA step (below 100nm),
- Heterogeneity in each crystalisedlayer due to migration of Zr and Ti(Baborowski 2002, Cantoni 2003)
TiZr Pb
% at
ZrTi
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6 April 2004 ISIF 2004 jb INVITED TALK 30Kiel, 7. Oktober 2004, jba
PVD DEPOSITIONSpider: a versatile sputtering equipment
Cluster architecture: loadlock, wafer transfermodule, 4 process chambers
Adapted for 100 or 150 mm wafers
DC, pulsed DC, and RF power sources for metal andceramic films
1 chamber designed andreserved for AlN thin films
6 April 2004 ISIF 2004 jb INVITED TALK31
6 April 2004 ISIF 2004 jb INVITED TALK 31Kiel, 7. Oktober 2004, jba
Sputtering of AlN thin films
Reactive sputtering from a pure Al targetin a nitrogen atmosphere
Very smooth surfaceDense columnar microstructure~3% uniformity on 4" waferc-axis orientation (FWHM[002] = 1.58°), induced by the hexagonal plane of Pt (111)d33,f=3.7±0.3 pmV-1
Single run for deposition of :Bottom electrode: Ti , PtPiezoelectric layer : AlNTop electrode Pt or AlSi
M.A.Dubois, CSEM
1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
30 40 50 60 70 80 90
2theta [°]
inte
nsity
AlN
(002
)
AlN
(004
)
Si (
200)
Si (
400)
Pt (
111)
Pt (
222)
6 April 2004 ISIF 2004 jb INVITED TALK32
6 April 2004 ISIF 2004 jb INVITED TALK 32Kiel, 7. Oktober 2004, jba
Deposition and integration of AlN films on SOI substrates(CSEM 2004)
Ti sputtering 10nm
Pt sputtering 100nm
AlN sputtering1500 nm
pvacuum7*10-8 mbar 3*10-7mbar
5*10-3mbar
300°C
15 sccm Ar
4 nm/sec
DC 1000W
5*10-8 mbar
pwork3*10-3mbar 4*10-3 mbar
T 300°C 300°C
gas 9 sccm Ar 50 sccm N2
Deposition rate 1 nm/sec 0.72nm/s
Field source DC 1000W Pulsed DC 1500W
1. SiO2 compensation layer : 1500 nm thermal wet oxidation of Si followed by HFdip2. Ti adhesion layer : 10nm 3. Pt seed layer : 100nm 4. AlN piezoelectric layer : 1500 nm 5. Pt top-electrode : 100nm
AlN
30 40 50 60 70 80 902θ (degree)
Cou
nts(
a.u)
Ti (0002)
Ti (0004)
Pt (111)
Pt (200)
Pt (311)
Pt (222)
AlN(0002)
AlN(0004)
Oxygen presence in Ti layer
AES profile of SOI_5_1through VIA of bottom-elec
0
5000
10000
15000
20000
25000
30000
35000
0 5 10 15 20 25 30 35
etch time [min]
I [a.
u.]
Oxygen
Titanium
Platinum
Platinum 100 nmTitanium 10 nm
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6 April 2004 ISIF 2004 jb INVITED TALK 33Kiel, 7. Oktober 2004, jba
PZT {100}, 1 µm thin film
0
2
4
6
8
10
12
14
-200
-100
0
100
200
300
25 30 35 40 45 50 55 60 65
Piezoelectric coeff.
Film stress
Pie
zoel
ectri
c co
effic
iet-
e 31,
f(C
/m2 )
Composition [Zr]/([Zr]+[Ti])
Poling:+ 64 MPa
PZT 53/47
STRESS COMPENSATED BEAMS/ MEMBRANES
-20
-15
-10
-5
0
5
10
15
20
0 500 1000 1500 2000
after poling
before poling
z-de
flect
ion
(µm
)
distance from the attachement (µm)
Poling of PZT effect on residual stress :Deflection + 20 µm = + 60 MPaσδ = 3 MPa/µm
Ledermann et al., Sensor and Actuators, 2003
-300
-200
-100
0
100
200
300
400
0.00 0.50 1.00 1.50 2.00 2.50
AlN thickness [µm]
Res
idua
l stre
ss [M
Pa]
Residual stresses in wetox SiO2
-400
-300
-200
-100
0
100
200
300
400
0 100 200 300 400 500 600 700 800 900 1000
Thickness of SiO2 (nm)
σ (M
Pa)
0°90°
Residual stresses as a function of film thickness in:
SiO2 wetox AlN
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6 April 2004 ISIF 2004 jb INVITED TALK 34Kiel, 7. Oktober 2004, jba
STRESS COMPENSATED BEAMS/ MEMBRANES
Mechanical system at equilibrium Equilibration of the structure
z
-ts
hSiO2
hTi/Pt
hAlN
hPttPt tAlN tTi/Pt
tSiO2
-tb
0
0=RM0)()( =∆−+∆−= ∑i
iiisssu
R tTcEtTcEF αα
( )0
)(
1
0
=−
+−
= ∑ ∫∫−− i
h
h
bi
t
bsbR
i
is
dzr
tzEdz
rtz
Fσ
( ) ∑ ∫∫−
−+−=− i
h
hbi
tbSR
i
is
dztzdztzM1
)(0
σσ
(1)
(2)
(3)
Membrane definition
ρ [103 Kg/m3] E [GPa] α [ppm] Tf [K] ν
Si 2.33 180 2.8 + 0.003*T --- 0.278
SiO2 2.2 70.4 0.058 --- 0.3
Ti 4.5 100 8.6 1941 0.33
Pt 21.45 136 8.8 2041 0.42
AlN 3.26 310 4.2 3273 0.24
Materials constants
Setting (3) to zero and resolving (1),(2),(3)
Determination of the wetox SiO2 layer thickness for deflection-free membrane
Determination of residual stress
6 April 2004 ISIF 2004 jb INVITED TALK35
6 April 2004 ISIF 2004 jb INVITED TALK 35Kiel, 7. Oktober 2004, jba
PZT
Pt /Ti
SiO2 burried SiO2 wetSi bulk
Au/Cr
σres =E
1 − υ⋅
ts2
3L2t f
⋅ d ts = substrate thickness (10 µm) tf = thin films thickness (1 µm)d = cantilever tip deflection (µm), L = cantilever length (2 mm)E, ν = Young modulus and Poisson’s coefficient of Silicon
0.12 µm (PVD) Pt/TiO2 = + 839 ± 72 MPa1.00 µm (sol-gel) PZT = + 113 ± 8 MPa (unpoled)1.00 µm (250 kV/cm, 150°C, 10’) PZT = + 64 ± 5 MPa (poling)0.2 µm (evaporation) Au/Cr = + 282 ± 11 MPaX ? (wetox) SiO2 = - 297 ± 3 MPa
E.g. for 2 µm PZT XSiO2 = 1200 ± 40 nm or ± 12MPa
STRESS COMPENSATED BEAMS/ MEMBRANESEXAMPLE of sol – gel PZT / Si
For SOI substrates the stress of buried oxide is not well known
6 April 2004 ISIF 2004 jb INVITED TALK36
6 April 2004 ISIF 2004 jb INVITED TALK 36Kiel, 7. Oktober 2004, jba
STRESS COMPENSATED BEAMS/ MEMBRANES
PZT on 5 µm SOI
Bending B1003-26
192123252729313335
0.5 1 1.5 2distance [mm]
heig
ht [ µ
m]
cross 1cross 2
~10 - 20 µm
AlN on 5 and 10 µm SOI
1 x 1mm2
5 µm thick0.3 mm2 to 1 mm2 SQUARE MEMBRANE :
2 - 4 micron of PZT
5 micron of Si
Well stress compensatedEven if the stress value of buried oxide is uknown
Bending B1002-27
-2
-1
0
1
2
3
4
0.5 0.7 0.9 1.1 1.3 1.5 1.7
distance [mm]
heig
ht [ µ
m] less than 1 µm
1 x 1mm2
10 µm thick
6 April 2004 ISIF 2004 jb INVITED TALK37
6 April 2004 ISIF 2004 jb INVITED TALK 37Kiel, 7. Oktober 2004, jba
PZT
Pt /Ti
SiO2 burried SiO2 wetSi bulk
SiO2 PECVD
Au/Cr
MICROFABRICATION: PROCESSING WITH SOI WAFERS (Ceramics Lab., EPFL, 1999-2002)
AlAlNPtAir gapSi
Bridge of suspended membraneBridge of suspended membrane
PlasmaEtch (ICP, Cl2, Ar)
Wet etch (HCl:HF)
PlasmaEtch (ICP, Cl2, Ar)
Deep Reactive Ion Etching(Plasma Etch, ICP, RT, SiF6, C4 F8 )
Carazzetti et al.,EPFL 2002
Baborowski et al.,EPFL 2001
6 April 2004 ISIF 2004 jb INVITED TALK38
6 April 2004 ISIF 2004 jb INVITED TALK 38Kiel, 7. Oktober 2004, jba
Reactive Ion Beam Etching of PZT and Pt thin films
PZT : RIBE, CCl4/CF4/Ar, low bias70 nm/min, SPR = 0.5, SPt = 1.6
Pt : RIBE (CCl4) or RIE/ICP (Cl2)60 nm/min, SPR = 0.5
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.
Baborowski, Ledermann, Muralt (1999, 2000)
ECR/RF Reactive Ion Beam Etching
ECR ion gun
ions
gaz inlet RF, 2.45 GHz
wafer
water cooledsubstrate holder 12.56 MHz
RF bias
pumping2 µm
PtPZT
+ PZT
6 April 2004 ISIF 2004 jb INVITED TALK39
6 April 2004 ISIF 2004 jb INVITED TALK 39Kiel, 7. Oktober 2004, jba
Advanced patterning for fundamental studies on ferroelectrics
ECRPlasma
RF chuck
grids
gasAr, CCl4, CF4
100 mm
2.45 GHz
RF 13.56 MHzWater cooled
STOPZTPMMA
E-beam
-20 -10 0 10 20-800
-600
-400
-200
0
200
400
600
800
Am
plitu
de o
f pie
zore
spon
se (a
.u.)
DC bias to tip (V)
S.Bühlman, J.Baborowski, P.Muralt, 2002S.Bühlman PhD Thesis EPFL, 2004
6 April 2004 ISIF 2004 jb INVITED TALK40
6 April 2004 ISIF 2004 jb INVITED TALK 40Kiel, 7. Oktober 2004, jba
PATTERNING OF GROOVES and SLIT OPENINGS THROUGH PZT/Pt/SiO2/Si(e.g.unclamped membranes, cantilevers)
STANDARD: Photolithography & Dry etch:
• Standard photolithogaphy• 100 nm Pt etch (ICP STS)• 1200 nm wetox SiO2 etch (Alcatel, 300 sec) • up to 50 µm Si (SOI) (Alcatel601E) – ETCH STOP on
SiO2buried
PZT
Pt /Ti
SiO2 burried SiO2 wetSi bulk
SiO2 PVD
Au/Cr
MICROFABRICATION: PROCESSING WITH SOI WAFERS (Ceramics Lab., EPFL, 1999-2002)
PtSiO2
Si
SiO2 burried
Below 5 micron ?
6 April 2004 ISIF 2004 jb INVITED TALK41
6 April 2004 ISIF 2004 jb INVITED TALK 41Kiel, 7. Oktober 2004, jba
PATTERNING OF SUBMICRON SLIT OPENINGS THROUGH AlN/Pt/SiO2/Si(e.g.limited air conductivity in pressure sensors with minimum roughness of sidewalls)
HIGH DEFINITION: Direct Laser Writing & HDP Dry etchHMDS & Shipley 1805, 1.75 µm, 90°C
Direct writing DWL: 4.04.W Development : Standard Rite Track 1818 SerieEtching:
Pt: STS; Pt_etch (150W bias), 380sec. (photoresist removed)SiO2: Alcatel 601E; SiO2 stand; DFA, 150 secSi: Alcatel 601E; Si_ambiant_2 (optimized for smalll openings); 360
secFinal clean (solvant & SDR), plasma O2 ashing
AlN or PZT
Pt /Ti
SiO2 burried SiO2 wetSi bulk
SiO2 PVD
Au/Cr
MICROFABRICATION: PROCESSING WITH SOI WAFERS (Ceramics Lab., EPFL, 1999-2002)
724 nm
Bow < 50 nm
947 nm
100nm Pt/Ti
1200 nm SiO2
<10 µm Si SOI
100nm Pt/Ti
1200 nm SiO2
<10 µm Si SOI
6 April 2004 ISIF 2004 jb INVITED TALK42
6 April 2004 ISIF 2004 jb INVITED TALK 42Kiel, 7. Oktober 2004, jba
Definition of membrane thickness (Ceramics Lab., EPFL, 1999-2004)
Standard vs. SOI wafers
Backside view;2µm large slit traversing the 10 µm thick membrane
6 April 2004 ISIF 2004 jb INVITED TALK43
6 April 2004 ISIF 2004 jb INVITED TALK 43Kiel, 7. Oktober 2004, jba
4. EXAMPLES (2001-2004)
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 TALK44
6 April 2004 ISIF 2004 jb INVITED TALK 44Kiel, 7. Oktober 2004, jba
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(ZrxTi1-x)O3 (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
ParmenideEU project
6 April 2004 ISIF 2004 jb INVITED TALK45
6 April 2004 ISIF 2004 jb INVITED TALK 45Kiel, 7. Oktober 2004, jba
3.1. MICROMACHINED ULTRASONIC TRANSDUCERSFabricated devices: 2 micron PZT on 5 micron Si SOI substrate
Frequency range (air loaded):
50 to 150 kHz
400 to 600 kHz
750 to 1200 kHz
Membrane size:
1000 µm
450 µm
300 µm
6 April 2004 ISIF 2004 jb INVITED TALK46
6 April 2004 ISIF 2004 jb INVITED TALK 46Kiel, 7. Oktober 2004, jba
Single element:Comparison between the simulation and measurements
Basic Mode@ 55,2 kHz
Second Mode@ 106 kHz
Simulated by D.Schmidt (IBMT, Fraunhofer Institut)
Measured with a stroboscopic interferometric microscope (A. Bosseboef CNRS, Orsay)
LCLC
EPFLEPFL
6 April 2004 ISIF 2004 jb INVITED TALK47
6 April 2004 ISIF 2004 jb INVITED TALK 47Kiel, 7. Oktober 2004, jba
Suspended disc 1 mm2
BASIC Mode 1 - Vertical deflection @ 55,2 kHz, 0.5 V AC , 50kV/cm DC bias
A
A
B B
C
C
Deflection of res ≈ 60 kHz
distance x (µm)-1000
-800
-600
-400
-200
0
200
0 800 1500
device #13, RND GR - wafer 01
coupe A-Acoupe B-Bcoupe C-C
Def
lect
ion
Z (n
m) -1.3µm
/V
• Observed
• Simulated
-2000
-1500
-1000
-500
0
500
0 200 400 600 800 1000
Deflection vs. distance - simulation
Def
lect
ion
z (m
icro
n)
Distance x (micron)
1.7 mic/V
The shape is close to the desired piston movement. The displacement is uniform around 360°.
6 April 2004 ISIF 2004 jb INVITED TALK48
6 April 2004 ISIF 2004 jb INVITED TALK 48Kiel, 7. Oktober 2004, jba
HIGHEST COUPLING COEFFICIENT FOR HIGH FREQUENCY TRANSDUCERS…
-0.002
-0.001
0
0.001
0.002
0.003
0.004
7.2 105 7.3 105 7.4 105 7.5 105 7.6 105 7.7 105 7.8 105 7.9 105
a1-df-res1-20dc
real measimag meas
real fit
imag fitA
dmitt
ance
Frequency (Hz)
k2 = 5.3 %k = 23 %Q = 135C
0 = 90 pF
Cpara
=0
SOI wafer: 3.5 µm of SiPZT (001) 2 µm
f res = 753 kHz
k2 = 5.3 %Q =135
k=23%
DC bias = 100kV/cm
6 April 2004 ISIF 2004 jb INVITED TALK49
6 April 2004 ISIF 2004 jb INVITED TALK 49Kiel, 7. Oktober 2004, jba
LOW FREQUENCY SINGLE DEVICESin Air and in FLUORINERT TM (3M)
/22Q/5
k2=const
AIR:
fres = 97.5 kHz
k2 = 1.0%
Q = 75
Fluorinert:
fres = 19 kHz
k2 = 1.0%
Q= 16
k2 = constQ / 5
6 April 2004 ISIF 2004 jb INVITED TALK50
6 April 2004 ISIF 2004 jb INVITED TALK 50Kiel, 7. Oktober 2004, jba
APPLICATIONSPresence and precise positioning sensor; transmission
Paper
Plastic
Receiver
Emitter
-4
-3
-2
-1
0
1
2
3
4
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
0 200 400 600 800 1000
Air transmission, distance = 10 mm
actuator DISC F 1.1
sensor DISC F 3.1
actu
ator
vol
tage
(V) sensor voltage (V
)
time (µs)
Charge amplifier: 10 pCExcitation frequency = 98 kHz, 10 V AC, 5 V offset
-4
-3
-2
-1
0
1
2
3
4
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 200 400 600 800 1000
Air transmission, distance = 10 mm
emitter DISC F 1.1
receiver DISC F 3.1
emitt
er v
olta
ge (V
) receiver voltage (V)
time (µs)
Liquid
6 April 2004 ISIF 2004 jb INVITED TALK51
6 April 2004 ISIF 2004 jb INVITED TALK 51Kiel, 7. Oktober 2004, jba
3.2 Piezoelectric microphones for photoacoustic detection
Specificity of the applicationLow frequency operation up to100HzLow acoustic pressure level order of mPa
Key pointsBuild a MEMS device based on partially unclamped, deflecting piezoelectric structuresBridge or cantilever structures have been focusedUse of PZT or AlN as piezoelectric layerUse of SOI wafers perfectly well defined Si membranePatterning of very narrow slits advanced ICP dry etchPossibility to build arrays of devices for complementary propertiesIntegration of both amplification unit and device on same PCB
3mm
B B
A
A1,2 mm
1,2,3 µm slit
1,2,3 µm slitPt Top electrode
Pt Bottom electrode
1,2 mm
AlN
6 April 2004 ISIF 2004 jb INVITED TALK52
6 April 2004 ISIF 2004 jb INVITED TALK 52Kiel, 7. Oktober 2004, jba
3.2. Acoustic sensors based on PZT sol-gel filmsThe cantilever concept for an audio microphone/microspeaker was proposed by White & al in 1998. (J. Micromech. Microeng. 8 (1998) 230-238)4.5 µm thick cantilever ZnO/SiN/SiO2: 10 µm slit, response at 100 Hz = 38 mV/Pa
2x2 mm beam
2x2.5 mm beam2x2 mm bridge
Electrode for pyroeffect compensation(static). via
Cantilever top electrode
Slit 5 and 10 µm
1 micron thick PZT on 10 micron Si membrane;Ledermann et al., 2002
6 April 2004 ISIF 2004 jb INVITED TALK53
6 April 2004 ISIF 2004 jb INVITED TALK 53Kiel, 7. Oktober 2004, jba
3.2. Acoustic sensors based on PZT sol-gel filmsN.Ledermann, J.Baborowski et al., JMM, 2004
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50
Bridge B3-10 - w.1202
V = 4 cm3
V = 2 cm3
V = 1 cm3
V = 0.5 cm3
sens
itivi
ty (m
V/Pa
)
frequency (Hz)
0
50
100
150
200
0 10 20 30 40 50
Cantilever C5-3 - w.1202
Sens
itivi
ty (m
V/P
a)
frequency (Hz)
V = 1 cm3
V = 4 cm3
V = 0.5 cm3
V = 2 cm3
S0, theo = 1.72 pC/Pa or 172 mV/PaS0, B3-10 = 68.3 mV/Pa ≈ 39 % S0, theo
S0, theo = 3.21 pC/Pa or 321 mV/PaS0, C5-3 = 226 mV/Pa ≈ 70 % S0, theo
6 April 2004 ISIF 2004 jb INVITED TALK54
6 April 2004 ISIF 2004 jb INVITED TALK 54Kiel, 7. Oktober 2004, jba
3.2. Acoustic sensors based on AlN filmsAdvantages of AlN based sensors:
Full IC compatibleReduced Thermal BudgetNo poling requiredGood selectivity for micromachiningLow losses
Future integration SoC (System on Chip) possible
Ground
Phase
Cantilever – backside view
Si SOI membrane
2 µm slit
1.5x1 mm CANTILEVER
0.0E+00
5.0E-08
1.0E-07
1.5E-07
2.0E-07
2.5E-07
3.0E-07
3.5E-07
4.0E-07
4.5E-07
6'000 6'500 7'000 7'500 8'000 8'500 9'000
Frequency [Hz]
Re[
Y]
AlN on membrane
Etched AlN forslit opening
J.BABOROWSKI, CSEM 2004
6 April 2004 ISIF 2004 jb INVITED TALK55
6 April 2004 ISIF 2004 jb INVITED TALK 55Kiel, 7. Oktober 2004, jba
Ta5Si3 200 nmSiO2 150 nm
Si3N4 200 nmSiO2 650 nm
Pt 100 nm
Cross-section
Frontside view
Backside view
2 mm
Microhotplate as processing device for local thin film growth
PZT
Ta5Si3
SiO2
Pt
SiO2
Si3N4
After PZT crystallization
Diploma work of F. Calame; with J. Baborowski
6 April 2004 ISIF 2004 jb INVITED TALK56
6 April 2004 ISIF 2004 jb INVITED TALK 56Kiel, 7. Oktober 2004, jba
RF MEMS for wireless communication andAmbient Intelligence (EC IP Mimosa)
CSEM involved in:
BAW resonatorsFiltersRF PIEZOELECTRIC Switch
6 April 2004 ISIF 2004 jb INVITED TALK57
6 April 2004 ISIF 2004 jb INVITED TALK 57Kiel, 7. Oktober 2004, jba
ConclusionsPiezoelectric 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 designActuators, 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 TALK58
6 April 2004 ISIF 2004 jb INVITED TALK 58Kiel, 7. Oktober 2004, jba
ACKNOWLEDGEMENTS
TEAMS of Ceramic Lab, EPFL, Center of Micro- Nanotechnology, EPFLCenter of Microscopy, EPFLLMARC, Besancon, France,CSEM, NeuchatelHachUltra Analytics, Geneva
OFES and EU Commission
Thank you for your kind attentionAnd for investing your time