エアロゾル堆積法によるPZT厚膜

エアロゾル堆積法によるPZT厚膜
Lead Zirconate Titanate Thick Films Fabricated by the Aerosol Deposition Method
秋山 善一
Yoshikazu AKIYAMA
マキシム レベデフ* 明渡 純*
Maxim LEBEDEV
Jun AKEDO
要
旨
音速の加速微粒子を基板に衝突させ，粒子の運動エネルギーを粒子と基板及び粒子間の接合エ
ネルギーに変換する成膜方法であるエアロゾル堆積法(ADM)を用いて，Si基板上にPZT圧電セラ
ミックス厚膜(10μm以上)を作製した．下部電極にPt/Ir/Ta膜を用いて作製した積層アクチュエー
タ(形状：Siダイアフラム6mm×6mm×65μm，PZT膜：4.5mm×4.5mm×13μm)は，直流50V印加
時に1.5μm，共振状態(周波数22.4kHz，振幅8V印加)で22μmなる極めて大きな変位を示す．この
時のPZT厚膜の圧電係数(d31)は，バルク燒結体の約30％の値に相当する．
ABSTRACT
The lead zirconate titanate (PZT) thick films were fabricated by the aerosol deposition method (ADM),
which is based on the impact phenomena of ultrafine particles on the substrate. The actuation properties
of PZT on the Si membrane were investigated. For a 6 x 6 mm2, 65-µm-thick Si membrane driven by a
4.5 x 4.5 mm2, 13- µm-thick PZT layer, the deflections, which were 1.5 µm upon applying 50 V at nonresonance frequency and 22 µm upon applying 8 V at resonance frequency, were measured. The
piezoelectric coefficient (d31) of PZT thick film is approximately 30% of that of the bulk material.
*
研究開発本部 中央研究所
Research and Development Center,
*
Research and Development Group
独立行政法人 産業技術総合研究所
National Institute of Advanced Industrial Science and Technology
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PZT thick films is more than 95% of bulk material. The
1．Introduction
temperature in ADM during deposition and annealing processes
For new micro electromechanical systems (MEMS), such as
does not exceed 600°C. This low process temperature reduces
microactuators, micropumps, ink-jet printer heads, flapper-
the chemical reaction between PZT film and the substrate
actuators for high density hard disk drive, ultrasonic devices and
material, reducing damages of the structure.
others, which need large strain and high speed response, it is
In this paper, we reported on the results of PZT thick film on a
necessary to produce dense piezoelectric films with thickness in
Si membrane by AMD. The actuation properties of such a
the 1 to 50 µm range structured on a Si substrate. There are
membrane as it relates to micropumps and micromixer fabrication
some reports of the fabrication of PZT (lead zirconate titanate)
are presented as well.
1)
2)
thick films by the sol-gel,
3)
sputtering
or hydrothermal
synthesis4) methods. However, PZT thick films produced by these
2．Experimental Procedure
methods usually have cracks, may easily peel from the substrates
and fabrication takes a long process time. The etching of thick
The apparatus and details of our ADM deposition technique
5)
have been reported elsewhere.13)
ceramics films by plasma etching , inductively coupling plasma
etching6), or reactive ion etching7) is also difficult. PZT thick films
The success of fabrication of a piezo on Si (POS) structure
8)
fabricated by the screen printing method have low density and
depends on many factors. The construction of a bottom electrode
PZT/Pt/Si structures can be damaged because of the long time of
of the Si membrane is very important. This bottom electrode
firing at temperatures higher than 800°C.
Recently, the
should: 1) have good adhesion force with the Si substrate and
improvement of the screen printing method with the development
PZT during deposition, annealing and post treatment, and 2)
of low-temperature sintering and a high-resistance electrode was
function as an effective barrier layer for thermal diffusion between
demonstrated in ref. 9, but the piezoelectric properties of films
the PZT layer and the Si substrate. During deposition, some Si
derived by this method were not reported.
substrate damage take place.13) Thus, the bottom electrode
should: 3) prevent damage to the Si membrane as well.
For bulk PZT adhered to the Si membrane, it is difficult to
ensure sufficient mechanical and electrical coupling between films,
Taking into account the requirements mentioned above, to
and to avoid the complexity of assembling. Thus, we can conclude
fabricate the bottom electrode Pt, Ir and Ta layers were
that the structuring of thick (over 10 µm) PZT films on Si-based
sputtered on 525 µm-thick Si substrate using a DC-magnetron
substrates by conventional methods is difficult.
sputtering system. The sputtering conditions were as follows:
1)
The principle of the aerosol deposition method (ADM) , which
10)
is a variant of the gas deposition method (GDM)
sputtering gas Ar 100%, gas pressure 3.0 Pa, input power density
1.5 W/cm2. The Si substrates’ temperature during spattering was
without
vaporization of the material, is based on impact phenomena of
460°C.
ultrafine particles on the substrate. Submicron particles form an
A square based (5x5 mm2) PZT layer was deposited by ADM
aerosol flow by mixing with carrier gas in an aerosol chamber. The
on this Pt/Ir/Ta/SiO2/Si substrate at substrates temperature
aerosol flow is transported through the tube to a nozzle. This flow
550ºC. The experimental parameters in ADM process are listed
is accelerated and ejected from the nozzle into a deposition
in Table I. The deposition rate of PZT film was 5-20 µm/min for
chamber. Ultrafine particles speed up to 300 m/s,11) bombard the
an area of 5x5 mm2. Sample was annealed at 600ºC for 1 h in air.
substrate and form the film. The structuring12), ferroelectrical13)
Next, to obtain a membrane with sufficient thickness, part of the
14)
15)
of PZT
Si layer was etched by the wet-etching process using a potassium
thick films deposited by ADM have been previously reported. It is
hydroxide aqueous solution. The base of the membrane had
possible to form structures without etching. The density of such
squared shape and was 7x7 mm2. The Au electrode was
and piezoelectrical
properties, and microstructure
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sputtered on the surface of the PZT thick film. The PZT thick
treatment to prevent any chemical reaction between PZT and Si.
film was poled by applying a 60 kV/cm electrical field at 250°C
The Ta layer (50nm thick) ensured the adhesion between the Ir
for 30 min. Next, the membrane was fixed at a metal base plate to
and SiO2 layer (1 µm thick) which was brought about by the
eliminate any undesired vibrations.
thermal oxidation of the Si substrate. When the bottom electrode
consists of Pt/Ta layers, The PZT thick film has some defects,
Table I Process parameters
which result from the chemical reaction of the lead element and
Pressure in deposition chamber
2 Torr
the Si substrate. This substrate structure (Pt/Ir/Ta/SiO2/Si)
Size of nozzle orifice
5×0.3 mm2
ensures good adhesion during deposition and annealing. A total
Carrier gas
He
Pt/Ir/Ta layer thickness of more than 0.6 µm prevents damage
Consumption of carrier gas
2～6 l/min
to the Si substrate during the ADM deposition.
Average size of PZT particles
0.3 µｍ
Substrate temperature
550 ℃
Annealing conditions
600℃×1h
3-2
Actuation properties of POS structure
The polarization-electrical field (P-E) hysteresis loop for
annealed 14-µm-thick PZT film is presented in Fig.2.
The membrane displacements were measured by a laser
Doppler interferometer (LV-1610 & LV-0120, Ono-Sokki Co)
with resolution 0.01 µm and by a laser displacement sensor (LA2420, Keyence Co.) with a resolution a 0.02 µm.
3．Results and Discussion
3-1
Formation of PZT layer on substrate
PZT thick films were successfully deposited on the platinumcoated Si substrate. A schematic and view of the POS structure
are presented in Fig.1.
Fig.2
Fig.1
Schematic of the POS structure.
Hysteresis loops of 14-mm-thick PZT thick film on
Pt/Ir/Ta/Si substrate after annealing at 600o C for 1
h.
If an electrical field is applied between the top (Au) and bottom
(Pt) electrodes of the PZT thick film presented in Fig.1, the PZT
To ensure the good deposition of PZT on the Si substrate
will shrink or expend in the horizontal direction. This movement
coated with Pt, a buffer Ir/Ta layer was introduced between Pt
of PZT leads to the deflection of the Si membrane in vertical
and Si. A 300-nm-thick Pt film was used as a bottom electrode.
direction. The responses of central point of the 7x7 mm2, 170-
It provided good adhesion force with PZT thick films. The Ir layer
µm -Si membrane driven by the 5x5 mm2, 40-µm-PZT layer with
(300 nm thick) was used as a thermal barrier layer during heat
unipolar rectangular electric pulses at different driven frequencies
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are shown in Fig.3. The amplitude of membrane displacement of
0 - 50 V, 100 Hz. The recorded displacement signals measured
membrane is 1.6 µm applying 0-+100 V and is constant in the
from the PZT side (2) and the Si side (3) have a phase shift of
frequency range up to 1 kHz. There is no phase shift between
exactly 180°, the shapes of the signals are the same and the
the excitation pulse and the membrane response. The throw rate
amplitudes are 0.7 µm for both. This result shows that the PZT
of the membrane is about 5 µs/µm and the deflection is
thick film does not peel from the substrate during deflections.
proportional to the excitation impulse. The damped oscillation
shown at the bottom signals in Fig.3 correspond to the resonance
frequency of the membrane, 27 kHz. The decay time for the
damped oscillation for this membrane is 0.35 ms.
Fig.4
Deflection of 7x7 mm2 , 100-mm-thick Si membrane
driven by 10-mm-thick PZT layer (area 5 x 5 mm2):
1) Sine wave drive signal 0-50 V, 100 Hz; 2)
displacement measurement from PZT side; 3)
displacement measurement from Si side.
To evaluate the dynamic response in the wider frequency and
excitation voltage ranges, measurements were carried out using
sine-wave driving signal. The results of this measurement for a
6.0 x6.0 mm2, 65-µm –thick Si membrane driven by 4.5 x 4.5
Fig.3
2
Dynamic response (without liquid) of 7x7 mm , 170mm-thick Si membrane driven by 5x5 mm2 , 40-mmthick PZT thick film; displacements were measured
at the central point.
mm2, 13-µm-thick PZT thick film are presented in Fig.5. The
amplitude of the deflection is proportional to applied voltage for
the unipolar drive and is 1.5 µm when applying 0-52V at a
nonresonance frequency of 1 Hz. The frequency dependence of
To confirm the existence of good electrical and mechanical
the displacement for this membrane is presented in Fig.5(a). The
contacts among the PZT thick film, Pt electrode and Si membrane,
phase shift between the excitation signal and the deflection was
measurements of the deflection from the PZT side and the Si side
not observed up to 1 kHz. The decrease of the peak amplitude
were carried out. If some peeling occurred inside the PZT/Si
deflection in the frequency range from 0.01 Hz-10 kHz is less
structure, the PZT thick film and the membrane will not move
than 10%. The buckling of the membrane before the first
together in the same direction, and thus the values of the
resonance mode is shown in Fig.5(b). All membrane points are
displacements measured from the PZT side and the Si side will be
moving together in the same direction. The buckling shape is
different and recording signals will probably have a phase shift
symmetrical and can be approximated to a cosine shape.
2
different from 180°. In Fig.4, the deflections of a 7x7 mm , 100µm-thick Si membrane driven by a 5x5 mm2, 10 µm PZT layer
are shown. The excitation signal (indicated as 1 in the Fig.4) was
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sufficient to successfully realize micropump and/or micromoxer
device driving with low (around 50 V) voltage. The high deflection
amplitude of about 20 µm in the membrane at the resonance
frequency of 22.4 kHz by applying only 8 V (shown in Fig.5(a)
indicates the good possibility of using the membrane for
micromixers and other microactuators.
3-3
Comparison with simulation code
A piezoelectric analysis model is fitted for a mechanical
analysis model of a finite-element method (FEM) computer
simulation using the ANSYSS Rev. 5 code. As the initial input
parameters, the geometrical dimensions of the POS structure, the
driven electrical field and the piezomechanical coefficients (i.e.,
piezoelectric coefficients dij, mechanical compliance Sij and
Young's modulus Y11) of the PZT-5A bulk sample were used
(Table II).
Table II FEM simulation parameters.
PZT 5A
Electric permitivity
Piezoelectric constant
Symbol
Unit
33
T/
0
-
1700
11
T/
0
-
1730
10-12m/V
-171
d31
d33
Fig.5
a):Frequency dependence (measured at 40 V) and
b) buckling shape (measured at 52 V, 100 Hz) of Si
membrane (thickness is 65 mm, area 6.0 x 6.0 mm2)
driven by PZT thick film (thickness is 13 mm, area is
4.5 mm x 4.5 mm2).
Elastic compliance
-12
374
-12
10 m/V
d15
10 m/V
584
S11E
10-12m2/N
16.4
S12E
10-12m2/N
-5.74
S13E
-12
-7.22
-12
10 m2/N
S33E
10 m2/N
18.8
The amplitude of the membranes deflection is about 1-2 µm
S44E
10-12m2/N
47.5
driven by voltage of 50 V in a wide frequency range and the high
S66E
10-12m2/N
44.3
-
0.31
Poisson's ratio
response indicate the possibility of using ADM in the fabrication
of micropumps and micromixers for liquids. The actuation
density
properties of our membrane are compatible with those of the
Silicon
same scale micropump16-18) and micromixer18) based on the bulk
Young's modulus
PZT material. In refs. 16 and17, the micropump has a deflection
Poisson's ratio
density
at a center point 0.3-0.8µm applying 170 V at nonresonance
E
3
Y
3
10 kg/m
7.75
1010N/m2
19.6
-
0.32
3
3
10 kg/m
2.34
frequency. In ref. 18, this deflection is 1.3 µm when applying 120
V, 870 Hz. Our membrane has a deflection 1.5 µm at 52 V in the
frequency range 0.01 Hz-10 kHz. This deflection value is
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31 (1992) L1260.
8)
H. D. Chen, K. R. Udayakumar, L. E. Cross, J. J. Bernstein and L.
C. Niles: J. Appl. Phys., 77 (1995) 3349.
9)
Y. Akiyama, K. Yamanaka, E. Fujisawa and Y. Kowata: Jpn. J. Appl.
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10) S.Kasyu, E. Fuchita, T. Manabe and C. Hayashi: Jpn. J. Appl. Phys.
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11) M. Lebedev, J. Akedo, K. Mori, and T. Eiju: J. Vac. Sci. Technol. A.
Fig.6
Calculation result by FEM code.
18 (2000) No 2, 563.
12) J. Akedo: J. Micro System Technology (in printing).
Figure 6 shows the simulation of membrane buckling. The
13) J. Akedo, N. Minami, K. Fukuda, M. Ichiki and R. Maeda:
calculated displacement value was higher than the measured value.
Ferroelectrics 231 (1999) No 7, 289.
It appears that the PZT thick film fabricated by ADM has a
14) J. Akedo and M. Lebedev: Appll. Phys. Lett. (submitted)
piezoelectric coefficient d31 about 30% of the bulk material and it
15) J. Akedo and M. Lebedev: Jpn. J. Appl. Phys. 38 (1999) No. 9b,
has mechanical properties closed to that of the bulk material
5397.
PZT-5A.
16) F. Forster, R. Badell, M. Afromowitz, M. Sharma and A Blanchard:
Proceeding of the ASME Fluids Engineering Division, 1995, (ASME
IMECE, 1995) Vol. 234, p. 39.
4．Conclusions
17) L. Jang, C. Morris, N. Sharma, R. Bardell and F. Forster..
1) PZT thick film can be directly deposited on the structured
Proceedings of the ASME winter annual meeting 1999, MEMS-
Si membrane by ADM. 2) The deposited PZT thick film can drive
ASME 1999, Vol.1 p. 503.
a thick (over 60 µm) Si membrane with symmetrical buckling. 3)
18) H. Goto, Z. Yang, M. Matsumoto and Y. Yada: Proceedings of Int.
The deflection amplitude is constant over a wide frequency range.
Workshop on Microfactories, 1998, Japan (MEL 1999) p.250.
The optimization of the structure of the bottom electrode should
be studied in the future. The results indicate a possibility of
fabricating micropump and micromixers devices using ADM. The
aerosol deposition method will become a promising method for
the fabrication of ferroelectric-coupled Si microactuators.
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