Micro/Nanosystems Technology - Technische Fakultät · 2019. 2. 4. · Piezoelectric coefficients...
Transcript of Micro/Nanosystems Technology - Technische Fakultät · 2019. 2. 4. · Piezoelectric coefficients...
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Micro/Nanosystems Technology Wagner / Meyners 1
Micro/Nanosystems Technology
Prof. Dr. Bernhard Wagner
Dr. Dirk Meyners
Piezoelectric
MEMS
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Piezoelectric vs. Capacitive MEMS
Piezo-MEMS of advantage due to:
• Sensors
• no power input required: Charges are generated by
external action
• Actuators
• higher energy density: Large force actuation at low
voltages
• plus:
• scales well with decreasing feature size
• can be used for energy harvesting
• less sensitive to environmental impacts:
no hermetic packaging required
Eom, Trolier-McKinstry, MRS Buletin, 37, 1007 (2012)
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Outline
Dielectricity and piezoelectricity
Piezoelectric thin films
Piezoelectric thin film materials
Piezo-MEMS
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mechanical
energy:
Stress, strain
electrical
energy:
Charge,
El. field
“Piezo” from Greek meaning “to press”
Conversion of mechanical into electrical energy and vice versa
Direct piezoelectric effect
sensing effect
Converse piezoelectric effect
actuation effect
Dielectric, piezoelectric, pyroelectric and
ferroelectric materials
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N. Setter Electroceramic-based MEMS, 2005
Dielectric, piezoelectric, pyroelectric and
ferroelectric materials
of 21 non-centrosymmetric groups
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Piezoelectric materials
- -
-
0
0
strained crystal
VP
Cm moment dipoleelectric
unstrained crystal
(change in) Polarisation
- -
-
F
01
21
non-pyroelectric
pyroelectric
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Ferroelectric materials
Barium titanate crystal: BaTiO3 in general: Perovskite: ABO3
Ti4+ Ion has 6 stable non-centric positions
dipole moment not fixed as in pyroelectric material
neighbouring dipols have strong interaction
domains with aligned polarisation
ferroelectrics have extremly high permittivity: r > 1000
ferroelectricity vanishes above Curie temperature Sauerstoff: O2-
Blei: Pb2+
Zirkon: Zr4+
Titan: Ti4+
O2-
Ba2+
Ti4+
Ferroelectric materials
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favorable aligned ferroelectric domains grow in electrical fields,
pinning during domain growth:
ferroelectric hysteresis
name in analogy to ferromagnetic materials
Ferroelectric materials Ferroelectric materials
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polar materials
Dielectric, pyroelectric and ferroelectric
materials
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EP
0electrical polarisation C/m2
dielectric displacement C/m2
electrical field V/m
electrical susceptibility -
dielectric constant, permittivity -
linear dielectrics (small field strength), P0 = 0
PEED r
00
D
1r
E
contribution from
external field
contribution from
polarized material
P
Vm
C120 1085.8
r
Dielectrics: Susceptibility and permittivity
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isotropic materials
EDEP
r
II and II
scalars are ,
anisotropic materials
parallel more no are , ,
tensors are
DEP
r
,
3
2
1
333231
232221
131211
3
2
1
E
E
E
D
D
D
= 0 r
Dielectrics: Susceptibility and permittivity
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Piezoelectric constants: d
EdD
ET sEdx
D: electrical displacement vector in C/m2
E: electrical field vector in V/m
: electrical permittivity matrix (3x3) in C/Vm = F/m
: stress vector (6x1) in N/m2
x: strain vector (6x1) in m/m
s: elastic compliance matrix (6x6) in m2/N; s=c-1
d: piezoelectric coupling matrix (3x6) in C/N = m/V
coupling between electrical and mechanical parameters
Direct effect
Converse effect
sE : stiffness at constant E
upper index T: transposed matrix
363534333231
262524232221
161514131211
dddddd
dddddd
dddddd
d
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EexD x
xcEe ET converse effect
E
jkijik cde
direct effect
c: elastic stiffness matrix (6x6) in N/m2; c = s-1
e: piezoelectric coupling matrix (3x6) in C/m2
Piezoelectric constants: e
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Piezoelectric Thin-Films
Piezoelectric bulk materials are well established:
Quartz, PZT, LiTaO3, LiNiO3, …
Piezoelectric thin films on silicon:
- only niche technology for long
- has recently emerged to highly recognized research field
- enables sensing and actuation in MEMS
- dominant materials: PZT, AlN, ZnO
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piezoelectric thin films are polycrystalline
in-plane directions 1 and 2 are equivalent
cylindrical symmetry around 3-axis
only 3 independent coefficients: d33, d31, d15
Convention: polarization axis has index 3:
usually normal to film surface
notation
P
Piezoelectric coefficient symmetry Piezoelectric coefficient symmetry
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000
00000
00000
333131
15
15
ddd
d
d
d
d33 longitudinal polarisation parallel to strain or stress
d31 transverse polarisation normal to strain or stress
d15 shear electric field normal to polarization,
Piezoelectric coefficient symmetry
stiffness (and compliance) for PZT, BaTiO3, AlN,…:
E
E
E
EEE
EEE
EEE
E
c
c
c
ccc
ccc
ccc
c
66
55
44
332313
232212
131211
00000
00000
00000
000
000
000
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Piezoelectric coefficients for thin films
Properties of piezoelectric films cannot be compared to bulk values
In coefficient measurement thin film is clamped to rigid substrate
in-plane strains stay zero: x1 = x2 = 0
In coefficient measurement thin film is free to relax normal to surface
out-off plane stress stays zero:
In film plane, polycrystalline material is isotropic:
0)( 331112111 Edssx
31,31 / Ee f
Set of effective piezoelectric
coefficients for thin films
which can be measured directly:
d33,f, e31,f, 33,f
Muralt, Integrated Ferroelectrics
17(1997) 297-307 Ess
03
21
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Absolute value of e31,f is always larger than bulk e31
e31,f measurement beam bending method: E3= f(1)
Y = Young‘s modulus
= Poisson‘s ratio
0)( 331112111 Edssx
31,31 / Ee f Ess
Piezoelectric coefficients for thin films
33
33
1331
31
1211
31,31
1e
c
ce
Yd
ss
de f
|e31,f |> |e31|
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3331133 2 Edsx
33,33 / Exd f
d33,f < d33 311211
1333,33
2d
ss
sdd f
d33,f measurement:
measure strain x3 = f(E3)
using laser interferometer
)(
2
12110
2
3133,33
ss
df
33,33 f
Piezoelectric coefficients for thin films
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Ledermann Sensors & Actuators A105 (2003) 162-170
Piezoelectric equations for thin films
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Longitudinal effect
3,333 fdD
Bulk mode actuation
El. input: E3
Mech. output: vertical strain x3
excitation of bulk vibrations
E3
Si
x3
D3
Si
3
Charge generation
Mech. input: 3
El. output: D3
3,333 Edx f
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El. input: E3
Mech. output: in-plane strain 1
beam bending
3,311 Ee f
Transverse effect in actuation application
Piezoelectric thin film E3
Si
1
Bidirectional
deflection
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Transverse effect in sensing application
Mech. input: load F
=> in-plane strain x1 El. output: el. displacement D3
1,313 xeD f
1
F
Piezoelectric
thin film
x1
D3
Si 3
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330
1,31
33
dxe
A
Qd
C
QV
f
o
A
QxeD f 1,313
Voltage response coefficient:
330
,31
fe
Signal-to-noise ratio:
(current and voltage) tan330
,31 fe
N
S
low power sensing principle
high sensitivity
static (d.c.) sensing not possible due to charge leakage: min ~1 Hz a.c
tan: loss tangent
Piezoelectric Sensing
Current response coefficient: fe ,31
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Properties of thinfilm piezoelectrics
ZnO AlN PZT
e31,f C/m2 -1.0 -1.3 -12 … -25
d33,f pm/V 5.9 5.2 60 …150
33 10.9 10.5 300…1300
e31,f /033 GV/m -10.3 -11.3 -2.2 … - 4.5
tan @1-10kHz 0.01…0.1 0.003…0.01 0.01 … 0.07
S/N 105 Pa1/2 3…10 24 8.8…13.5
c33 GPa 208 395 98
PZT is optimum for piezoelectric actuation
AlN is optimal material for piezoelectric sensing
Properties of thin-film piezoelectrics
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Piezoelectric thinfilms: PZT
Solid solution of lead zirconate and lead titanate
PZT shows highest piezoelectric d and e coefficients
disadvantage: lead-containing, non-IC-compatible, stoichiometry is critical
Lead zirconate titanate: Pb (Zrx Ti1-x) O3
Sauerstoff: O2-
Blei: Pb2+
Zirkon: Zr4+
Titan: Ti4+
tetragonal
phase
Perovskite structure:
ABO3
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Cubic phase above
Curie temperature Tc
is not piezoelectric,
but paraelectric,
Zr or Ti is in cell center
Morphotropic
phase boundary:
•At room temperature:
PbTiO3-content 48%
•competition between
tetragonal and rhombo-
hedral phase enhances
number of polarizable
directions to 14
nomenclature:
PZT 52/48 (PbTi0.48Zr0.52O3)
PZT phase diagramm
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Piezoelectric and dielectric coefficients strongly depend on stoichiometry
Maximum values close to morphotropic phase boundary composition
Coefficients are also PZT-texture dependent
Bottom electrode as nucleation layer to tune PZT texture:
e.g. (111) textured Platinum
Ledermann S&A 2003
PZT stoichiometry
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sol-gel deposition (chemical solution deposition, CSD):
multiple spin-on and curing process: ~ 0.1 µm per layer
low-cost equipment, very good uniformity and smoothness,
sensible to contamination
sol
gel
PZT thin film deposition methods
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PZT thin film deposition methods
Sputtering from Pb(ZrTi)O3 ceramic target or Pb, Zr, Ti metallic targets
good uniformity
stoichiometry critical and fixed by target composition
target composition has to account for e.g. lead loss from desorption
low rate: ~10nm/min
but: more promising for mass production
different processes, different
crystalinity
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wurtzite crystal structure
polar materials (no ferroelectric hysteresis)
quite similar piezoelectric properties
Sputter deposition: 1-2 µm
AlN is preferred:
fully IC compatible
high thermal stability and conductivity
chemically inert
highly uniform sputter process available
ZnO and AlN films
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2 µm thick AlN
on Pt-Electrode
Sputterprocess
Oerlikon-Clusterline 200
small columnar grains:
deposition in transition zone
due to high Tmelt
Strong c-axis orientation Ti/Pt
SiO2
AlN
Aluminum nitride layer
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Doping of aluminium nitride: Al1-xScxN
Akiyama et al., APL 95, 2009
e.g. reactive co-sputtering from pure Sc
and Al targets or AlSc compound
targets in N atmosphere
Sc partially substitutes Al while
preserving the piezoelectric wurtzite
structure
different preference in N-coordination
between Sc and Al leads to flatter ionic
potential
softening of crystal: decrease of
stiffness c
larger ionic displacements by same
electric field: increase of d31, d33
d3
3
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High force
High speed
Low power consumption
Membrane actuators
No counter electrode needed
Examples:
Inkjet printer f > 80 kHz
Micro mirror
Electrical switch
loudspeaker
membrane
piezoelectric layerelectrodes
nozzle
pump chamber
Piezoelectric (PZT) microactuators
feMeritOfFigure ,31
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PZT micro mirror
80 µm poly-Si + 2 µm PZT
mirror size: 1 mm
fres = 32 kHz
deflection angle: ± 10.5° @ 7V
feFOM ,31
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PZT electrical switch
device size e.g. 0.08mm2
contact force up to 2mN
PZT buckles with applied
voltage (contact closed)
buckling in open state
prohibited by electrostatic
clamping
feFOM ,31
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Silicon microphones:
20 Hz - 20 kHz
Single membrane or arrays
Ultrasonic transducers:
transmitting and receiving
20 kHz - 1 MHz
Phased arrays: electronic steering
Piezoelectric micromachined
ultrasonic transducer cell
(pMUT)
Piezoelectric microsensors
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R. Aigner, Infineon
AlN
2001:
FBAR introduction
Agilent Technologies
Membrane-type
Film Bulk Acoustic Wave
Resonator (FBAR)
FBAR RF-filter
D
f
c
eFOM
33330
,332
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PadsPZT
Elektrode
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Piezoelectric Energy Harvesting
Idea: Turn vibration/acceleration into
power through piezoelectric layer
Seismic mass
/AlN
Application:
energy-autonomous microsystems
Example:
Battery-less tire-pressure monitoring system
330
,312
feFOM
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Summary
piezoelectric coupling matrices: d and e
effective piezoelectric coefficents for thin films: d33,f, e31,f
dominant thinfilm materials: PZT, AlN, ZnO
PZT optimum for microactuation
AlN optimum for sensing
broad range of piezoelectric MEMS applications
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Literature
Madou pp. 551-560
Chang Liu Foundations of MEMS CH. 7
N. Setter Electroceramic-based MEMS, 2005
esp. Ch. 10 Thin film piezoelectrics for MEMS
by S. Trolier-McKinstry and P. Muralt