FUNDAMENTALS OF THIN FILM PIEZOELECTRIC MATERIALS AND
PROCESSING DESIGN FOR A BETTER ENERGY HARVESTING MEMS
Kiyotaka Wasa, Isaku Kanno, and Hidetoshi Kotera
Microengineering, Kyoto University, Kyoto, Japan
Abstract: Piezoelectric materials are essential for harvesting energy from mechanical vibrations. Thin films of
piezoelectric materials including AlN and binary perovskite compounds PZT are widely studied for a fabrication
of energy harvesting MEMS. The piezoelectric materials have held a key position in the energy harvesting
devices. Varieties of bulk piezoelectric ceramic materials are studied and used in practice for the piezoelectric
sensors and/or actuators. However, piezoelectric properties of the thin film materials are different from bulk
properties due to their structural differences. The characters of the thin film piezoelectric materials are not fully
understood yet. For making a better energy harvesting device, a fundamental discussion will be necessary for the
thin film piezoelectric materials. In this paper, first the fundamentals of thin film piezoelectric materials and the
process design are discussed, and then novel piezoelectric thin films of PZT-based ternary perovskite compounds
are proposed as an example for making the improved energy harvesting MEMS.
Keywords: Power MEMS, Piezoelectric energy harvesting, PZT-based ternary perovskite, PMnN-PZT thin films
INTRODUCTION
compounds [6]. In this paper, first the fundamentals of
thin film piezoelectric materials and the process
design are discussed, and then the PZT-based ternary
compound piezoelectric thin films are shown in
relation to their potential application for making the
energy harvesting MEMS.
Thin films of AlN, ZnO, and Pb(Zr,Ti)O3 (PZT)
are widely studied for a fabrication of the
piezoelectric actuators and sensors in MEMS [1].
Thin films of AlN and ZnO are used for the
fabrication of the acoustic wave devices [2]. The thin
films of the ferroelectric PZT perovskite ceramics ,
binary compounds of PbTiO3(PT) and PbZrO3(PZ),
are used for the fabrication of gyro-sensors and ink-jet
printer heads due to their excellent piezoelectric
properties [3]. Recently much interests have been paid
to the energy harvesting power MEMS using
piezoelectric materials [4]. The piezoelectric materials
have held a key position for the power MEMS. The
power outputs are governed by the coupling factors k
and the dielectric constantsε, and are proportional to
the ratios k2/ε [5]. The PZT-based materials are
considered to be a suitable material for the energy
harvesting power MEMS due to their extremely high
material coupling factors k. However, at present the
ALN thin films with a small k value are also
considered as a candidate for the piezoelectric
materials in the power MEMS, since the values, k2/ε,
obtained in the AlN piezoelectric thin films are not
inferior to those of PZT thin films [5]. The quality
factors for the piezoelectric PZT-based thin films are
sensitive to the chemical composition and the
microstructure. Most of the reported PZT-based thin
films are intrinsic thin films. Further improvements of
their chemical compositions are still expected for the
PZT-based thin films. Recently the authors have
improved their chemical compositions and developed
thin films of the PZT-based ternary perovskite
0-9743611-5-1/PMEMS2009/$20©2009TRF
PIEZOELECTRIC THIN FILMS
Possible Materials
Non-ferroelectric AlN is well known hexagonal
piezoelectric materials with extremely high
mechanical quality factor Qm. The AlN thin films are
used as an acoustic device. However, the usage of the
non-ferroelectric materials is limited due to their low
coupling factors. Thin films of
ferroelectric PZT
perovskite ceramics are well known piezoelectric
materials of high piezoelectricity. However, the high
piezoelectricity could not be well utilized for the
energy harvesting MEMS because of their high
dielectric constants.
Bulk PZT-based ternary perovskite ceramics
show a variety of dielectric and piezoelectric
properties. The coupling factor k, mechanical quality
factor Qm ,and the dielectric constants ε are controlled
by the doping of relaxor ferroelectric materials such
as Pb(Mg,Nb)O3 (PMN) and Pb(Mn,Nb)O3 (PMnN)
into the binary compounds PZT [7-8]. Among the
ternary perovskite compounds, the hard ferroelectric
materials PMnN-PZT will be useful for making the
piezoelectric energy harvesting MEMS. Typical phase
diagram of the PMnN-PZT is shown in Fig.1 [9].
61
PowerMEMS 2009, Washington DC, USA, December 1-4, 2009
ferroelectric perovskite thin films without the postannealing. Polycrystalline thin films are deposited on
the glass substrates. Single crystal thin films are
deposited on the single crystal substrates using the
heteroepitaxial growth. The polycrystalline films
show the fiber structure and/or grained structure. The
epitaxial films show strained and/or dislocated
structure due to the thermal and lattice mismatch
between the PZT-based materials and the substrates as
shown in Fig.2. These structural properties reduce the
coupling factor k and/or the mechanical quality factor
Qm. One of the ideal structures of the PZT-based thin
films is the relaxed single crystal structure as shown
in Fig.2. The crystal quality including interface is
improved by the buffer layers. The single crystal
substrates and the buffer layers are shown in Tab.2.
Among the several deposition processes described
above, sputtering is one of the useful deposition
processes, since the porosity of the sputtered thin
films is extremely small.
Recently we have found the sputtered epitaxial
thin films show bulk single crystal-like relaxed
structure, i.e. one of the ideal structures, when the
epitaxial PZT-based thin films are quenched after the
deposition [12].
Fig. 1. Phase diagram of PZT-based ternary
compounds: xPMnN-(1-x)PZT, 0<x<0.2 [9].
Basic Processing for PZT-Based Thin Films
Thin films of perovskite materials are fabricated
by sputtering, pulsed laser ablation (PLD), MOCVD,
and sol-gel deposition [10]. The ferroelectric
properties of these perovskite thin films are different
from bulk ceramic properties probably due to the
difference of microstructure and the presence of the
growth stress in the thin films [11].
The PZT-based thin films of perovskite phase are
fabricated by two different deposition conditions as
shown in Tab.1. One is deposition at room
temperature
followed
by
post-annealing
at
crystallizing temperature Tcr of perovskite phase (low
temperature process), the other is deposition at the
crystallizing temperature Tcr of perovskite phase
(high temperature process). The sintering process of
the bulk ceramics is also shown in the Tab. 1. The
piezoelectric properties of the thin films are governed
by the material compositions and their structures. The
low temperature process is similar to the sintering
process. The high temperature process provides the
substrate
Single domain
Strained single
crystal PZT Tetra
Multi domain
Polycrystal
PZT Tetra -
Absence of phase
transition at MPB
Rhombo at MPB
Single domain
relaxed single
crystal PZT
Tetra Absence of
Phase transition
at MPB
Fig. 2. Lattice images for heteroepitaxial thin films.
Table 1. Basic fabrication processes for PZT- based thin
films
Table 2. Cubic single crystal substrates.
Process
Ceramics
mixing
Structure
sintering
Substrates:
Sapphire (a=0.4763nm), SrTiO3(ST)(a=0.3905nm),
MgO(a=0.4203nm), LaAlO3 (a=0.3792nm)
Si (a=0.5431nm),GaAs (a=0.5654nm)
polycrystalline
Thin (1)*1 deposition post anneal polycrystalline
films
at RT
at Tcr*3
(2)
*2
deposition
at Tcr
Buffer layers : YSZ ( a=0.516nm) , SrRuO3 ( a=0.393nm)
Epitaxial relations:
(111)PT//(0001)sapphire, (001)PT//(001)LaAlO3,
(001)PT//(001)ST,
(101)SRO //PT //(001)ST,
(001)PT//(001)MgO, (101)SRO// PT// (001)MgO,
(111)PT//YSZ//(100)Si
polycrystalline
single crystal*4
*1 low temperature process, *2 high temperature process
*3
Tcr: 500-650oC *4 deposition on single crystal substrates
62
PMnN-PZT THIN FILMS
Fig.3 shows the cross sectional TEM image with
electron diffraction patterns. The TEM image
suggested the sputtered films showed high density
structure without grains and/or interfacial layers
between thin films and substrates. The electron
diffraction patterns describe the sputtered PMnN-PZT
thin films were single c-domain/ single crystal thin
films epitaxially grown on the SRO/Pt/MgO substrates.
Tab. 4 shows the lattice parameters of
(001)PMnN-PZT thin films for different compositions
of the PZT. The in plane lattice parameters were
almost the same to bulk values independent of the
lattice parameters of the SRO. The sputtered films
were stress free and/or relaxed.
The PMnN-PZT ternary ceramics comprise PZT
with donor additive Nb and acceptor additive Mn.
However, it is known the ternary ceramics show hard
ferroelectric response. Fig. 4 shows a typical P-E
hysteresis curve for the single c-domain/ single crystal
thin films
of 0.06PMnN-0.94PZT(45/55). The
hysteresis curve shows typical hard ferroelectric
properties of square shaped loops with high Ec and
large Pr (2Ec≅230kV/cm and 2Pr≅120μC/cm2). The
PMnN-PZT thin film showed dielectric anomaly at
Tc=560oC, Curie temperature, as shown in Fig.4. The
Tc is higher than bulk ceramic values. The higher Tc
will be caused by growth stress during the sputtering
deposition. The dielectric constant decreases with the
increase of Ti similar to the bulk ceramics. The
observed relative dielectric constants were 150-200
with tanδ=0.01-0.02 at 1kHz. The dielectric constant
was much lower than bulk non-doped intrinsic PZT (
bulk values≅700).
The low dielectric constant reveals the sputtered
thin films consist of highly (001) oriented crystal
structure.
Deposition: Relaxed Single Crystal Thin Films
A planar rf-magnetron sputtering was used for
the heteroepitaxial growth of the PZT-based ternary
compounds. Thin films of PMnN-PZT were directly
sputtered from PMnN-PZT powder target on
(001)MgO substrates. The powder target was
composed of the mixture of PT, PZT, PbO, Nb2 O5，
MnO2, ZrO2, and TiO2. The high temperature process
shown in Tab.1 was used for the deposition. The
chemical composition of the sputtered thin films was
easily controlled by the composition of the mixed
powder. The epitaxial temperature was 500~650oC.
Typical sputtering conditions are shown in Tab. 1[13].
The sputtered films were quenched after the
deposition to make the single crystal-like relaxed
structure.
Table 3. Sputtering conditions.
RF-magnetron
Sputter up( Balance mag.field)
Target*1
mixed powder: PT, PZT,PbO, Nb2O5
Substrates*2
Sputtering gas
Growth temp
Surface
Growth rate
Film thickness
Quenching rate
MnO2,ZrO2, TiO2
(001)MgO
0.5 Pa (Ar/O2 =20/1)
500-650oC
(101)SRO/(001)Pt
5-15 nm/min
200-2000 nm
100oC/min. in air
*1 Composition: xPMnN-(1-x)PZT + 10%PbO x-=0-0.2
*2 Conductive base electrode: (001)Pt on (001)MgO
Crystal Structure and Piezoelectric Properties
XRD measurements suggested the PMnN-PZT
thin films deposited on the SRO/Pt/MgO substrates
show highly (001) orientation.
002022
Tab. 4. Lattice parameters of sputtered PMnN-PZT
thin films epitaxially grown on (101)SRO/ (001)Pt/
(001)MgO for different compositions of PZT.
000020
PMnN-PZT
[100]
Composition
MgO
SRO/Pt
001
011
000
010
[100]
Fig. 3. Cross sectional TEM images of sputtered PMnNPZT thin films epitaxially grown on(110)SRO/(001)Pt
(001)MgO: Composition, 0.06PMnN-0.94PZT(45/55),
film thickness, 1.3μm.
63
a-lattice
0.06PMnN-0.94PZT(45/55)
0.06PMnN-0.94PZT(48/52)
0.06PMnN-0.94PZT(55/45)
PZT(52/48)
Bulk PZT: Tetra (50/50)
SRO (101)
nm
0.4024
0.4074
0.4095
0.4054
c-lattice
nm
0.4140
0.4138
0.4133
0.4138
c/a
1.029
1.016
1.009
1.021
a=0.4031nm c=0.4139nm 1.0268
a=0.393nm
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Fig. 4. P-E hysteresis curve and temperature
variations of dielectric constants: (001)PMnN-PZT
thin films epitaxially grown on (101)SRO/ Pt/MgO
substrates. Composition, 0.06PMnN-0.94PZT (45/55),
film thickness,1.9μm.
Piezoelectric
Properties
PMN-PZT
Impedance Z (db !
FBAR
Fig. 5. Cantilever beam and FBAR structures.
60
1.5
50
1
40
0.5
30
Z
Φ
0
20
-0.5
10
-1
Φ
0
Fig. 5 shows the construction of the cantilevers
and the FBARs for the measurements of piezoelectric
properties. The effective piezoelectric coefficients for
thin films e31f are estimated by using following
relation [1]: e31f =*e31x [s11/ (s11 +s12)], where
-1.5
3.5
3.7
3.9
4.1
Frequency (GHz)
4.3
4.5
Phase _ (rad)
-.*+,&
'##
*e31=d31 / *s11=‐（h2/3s11L2 ）δ/V, h and s11 are
thickness and elastic compliance of MgO substrates,
*s11is the compliance of PMnN-PZT thin films, L is
length of the cantilever, δ shows the deflection of the
cantilever at the applied voltage V [14]. Taking
typical experimental values, δ =840nm at 10V,
h=0.3mm, L=10mm, 1/s11≅248x109 N/m2(MgO),
1/*s11≅92x109N/m2(PMnN-PZT), the e31f and d31 for
the PMnN-PZT thin films were e31f= ‐ 10.8C/m2,
d31= ‐ 83x10-12m/V, respectively. The observed
transverse piezoelectric constants are almost the same
to bulk PZT ceramic values.
Bulk PMnN-PZT ceramics show the doping of
PMnN into PZT remarkably enhances the Qm . In
order to confirm the effect of the PMnN doping on the
Qm, PMnN-PZT thin film FBAR structure was
fabricated and evaluated their Qm values. The
(001)MgO substrates of 0.3 mm in film thickness
were used for the fabrication of the FBAR structure.
The FBAR structure comprised film thickness of the
PMnN-PZT thin films of 280 to 320nm thick, SRO/Pt
base conductive electrodes, and Al top electrodes.
Size of the Al top electrodes were 500nmx500nm.
The effective coupling factor keff was evaluated by the
relation keff=[(fp2-fs2)/fp2]1/2, where fp and
fs
denote the parallel and series resonant frequency. The
kt was evaluated by the relation kt2= (π/8)2(keff2)/(1keff2). The Qm was obtained by the phase change of the
impedance at the antiresonant frequency fp using the
relation Qm =1/2 ω (dΦ/dω). These impedance
properties were evaluated by the network analyzer [6].
Typical impedance properties are shown in Fig.6
for the PMnN-PZT FBAR. From these impedance
measurements we have investigated the effect of the
doping of PMnN into PZT on kt and Qm at GHz
FBAR structure. Typical thin films of PMnN-PZT,
0.1PMnN -0.9PZT(55/45), 300nm in film thickness
showed fs=3.373 GHz and fp=3.870GHz. Taking the
observed fp and fs values, the keff=0.487 and the kt =
0.689. From the phase properties Qm=185. Thin films
Fig. 6. Typical impedance properties of PMnN-PZT
thin film FBAR.
64
coupling factor k312 and/or the ratio, e31f2/ε, where ε is
the dielectric constants of the piezoelectric thin films.
The ratio e31f2/ε is tentatively defined as a power
generation factor.
Table 4 shows the piezoelectric properties for the
PZT-based thin films and the power generation factor,
e31f2/ε, in comparison with the AlN thin films.
of intrinsic PZT(48/52) showed fs= 3724MHz and
fp=4451Mhz. Their keff=0.547 and kt =0.726. The Qm
was 114. The kt of the intrinsic PZT thin films was
slightly higher than the PMnN doped PZT thin films.
The doping increases the Qm almost two times in
magnitude. Typical variations of Qm and kt with the
doping of PMnN to PZT are shown in Fig.7.
Qm
200
kt
0.8
Table 4. Typical dielectric and piezoelectric properties
of PZT-based thin films and AlN thin films.
kt
0.7
150
PZT
0.6
Qm
xPMnN -(1-x)PZT(45/55)
100
0.4
0
5
x (%)
10
xPMnN-(1-x)PZT
Fig. 7. Typical variations of Qm and kt with doping of
PMnN into PZT thin films for xPMnN-(1x)PZT(45/55) thin films: PZT near MPB composition.
PMnN-PZT
Substrate
Structure
ε/ε0
2Ec(kV/cm)
2Pr (µC/cm2)
*e31 (C/m2)
e31f (C/m2)
d31 (pC/N)
e31f2/ε (GPa)
(001)MgO
(001) MgO
Epi
200
200
100
- 4.8
- 6.2
20.5
Epi
155
230
120
-7.7
-10.8
-83
85
Ref
[14-15]*1
[13]*2
AlN
Sapphire
Epi
9.5
-1.37
-2.65
22.3
[2]
*1 Conventional sputtering. PZT(53/47). e31f ≅*e31 x 1.3.
*2 Sputtering and quenching. High Q m（＝185）,high kt (=0.7).
0.06PMnN-0.94PZT(45/55)
DISCUSSION
It is seen the power generation factor, e31f2/ε, for
the AlN thin films are comparable to those of
conventional PZT thin films [14-15]. This is mainly
caused by the small dielectric constants of AlN. Thin
films of single c-domain PMnN-PZT have a high
potential for the better energy harvesting devices. The
output powers of the present ternary PZT-based thin
films will be one order in magnitude higher than those
of the conventional PZT thin films and/or AlN thin
films.
For the better energy harvesting MEMS, the
selection of the substrate materials are essential for
the optimum design of the mechanical vibration
portion. The piezoelectric single crystal thin films are
deposited on a single crystal substrate. The substrate
materials are selected for epitaxy. It is noted the
resultant single crystal thin films could be transferred
onto another substrate having an optimum elastic
constant which achieves high mechanical Q in the
mechanical vibration port. The transfer process does
not affect on their structural and piezoelectric
properties of the epitaxial single crystal thin films [16].
In this paper the sputtered single crystal
piezoelectric thin films are described as an example.
Single c-domain/single crystal thin films of the
PZT-based ternary compounds were successfully
synthesized by the sputtering. The resultant films
show the hard ferroelectric properties with high kt and
high Qm. Their piezoelectric properties are superior to
the conventional binary PZT-based thin films due to
the doping effects and the structural perfection.
Porous thin films of PZT show small Qm values. The
high Qm is achieved at the bulk-like pore free high
density structure. The high Qm values of the
piezoelectric thin films are essential for the long term
stability of the piezoelectric devices.
The energy harvesting system consists of a
mechanical vibration portion and the piezoelectric
power generating portion. If the piezoelectric power
generator is actuated by the transverse piezoelectric
mode, the maximum power generated by the
mechanical vibration operated at resonance is given
by Pmax=k312mQ2 A2/4ω, where k31 is the
electromechanical coupling coefficient, m is the mass
of the cantilever, Q is the mechanical quality factor of
the vibration portion, A is the acceleration magnitude
of the input vibrations and ω is the resonance
frequency [5]. Under a given mechanical vibration
condition, the output powers are proportional to the
65
Numbers of the piezoelectric PZT thin films studied
for making the energy harvesting MEMS are
polycrystalline thin films deposited by the low
temperature process, for instance, the sol-gel method.
These polycrystalline PZT-based ternary perovskite
thin films also show high coupling [17]. However, the
polycrystalline thin films exhibit high dielectric
constants due to the presence of (111) oriented grains.
The (111) oriented grains should be reduced for the
better energy harvesting MEMS.
[6] Yamauchi N, Shirai T, Matsushima T, Matsunaga
T, Wasa K, Kanno I, Kotera H 2009 High
coupling piezoelectric thin films of PZT-based
ternary perovskite compounds for GHz FBAR
Appl. Phys. Lett. 94172903-05.
[7] Cross L.E 1993 Ferroelectric Ceramics Setter N
and Colla E.L ed. (Birkhäuser-Verlag, Basel) 185.
[8]Takahashi M, Tsubouchi N, Ohno T 1971
Piezoelectric properties of the ternary and
quaternary systems containing PbTiO3-PbZrO3 IEC
Report Japan 1971CPM71-22 1-17,.
[9] Zhang T, Wasa K, Kanno I, Zhang S-Y 2008
Ferroelectric properties of Pb(Mn1/3Nb2/3)O3Pb(Zr,Ti)O3 thin films epitaxially grown on (001)
MgO substrates J. Vac. Sci. Technol. A26(4) 985990.
[10] K. Wasa, H. Adachi, M. Kitabatake 2004 Thin
Film Materials Technology (Springer, William
Andrew Pub., New York ) p.408.
[11] V. Nagarajan,C.S. Ganpule, B. K. Nagaraj, S.
Aggarwai, S. P. Alpay, A.L. Roytburd, E.D.
Williams, R. Ramesh 1999 Effect of mechanical
constraint on the dielectric and piezoelectric
behaviour of epitaxial Pb(Mg1/3 Nb2/3) O3(90%)PbTiO3(10%) Appl. Phys. Lett. vol.75 41834185 .
[12] Wasa K, Nakamura K, Matsunaga T, Kanno I,
Suzuki T, Okino H, Yamamoto T, Seo S.H, Noh
D.Y 2006 Electromechanical coupling factors of
single-domain 0.67Pb (Mg1/3 Nb2/3)3-0.33PbTiO3
single-crystal thin films Appl. Phys. Lett. 88
122903.
[13] Wasa K, Kanno I, Kotera H, Yamauchi N,
Matsushima T 2008 Thin films of PZT-based
ternary perovskite compounds for MEMS Proc.
of 2008 IEEE International Ultrasonics Symp.
p.213-216.
[14] Kanno I, Kotera H, Wasa K 2003 Measurement
of transverse piezoelectric properties of PZT thin
films Sensors and Actuators A 107 68-74.
[15] Kanno I, Fujii S, Kamada T, Takayama R 1997
Piezoelectric characteristics of c-axis oriented
Pb(Zr,Ti)O3 thin films Appl. Phys. Lett. 7013781380.
[16] Terada K, Suzuki T, Kanno I, Kotera H 2007
Fabrication of single crystal PZT thin films on
glass substrates Vacuum 81 571-578.
[17] Zhang T, Wasa K, Zhang S-Y, Chen Z.J, Zhou F,
Zhang Z, Yang Y 2009 High piezoelectricity of
Pb(Zr,Ti)O3-based ternary compound thin films
on silicon substrates Appl. Phys. Lett. 94 122909.
CONCLUSION
If the conventional binary PZT thin films are
replaced by the ternary PZT thin films of single cdomain/single crystal structure, the output powers of
the energy harvesting system will surely increase. The
high Qm values and the high Curie temperature of the
ternary perovskite will improve the temperature
stability and/or the long term stable operation. The
transfer technology of epitaxial piezoelectric thin
films onto the vibrating beam is useful for the total
material design of the power MEMS.
ACKNOWLEDGEMENTS
This study is a part of Advancing Technology
Excellence “Nano-Medicine” project, which is under
Kyoto City Collaboration of Regional Entities
assigned by JST.
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