DEVELOPMENT OF A MEMS ENERGY HARVESTER WITH
HIGH-PERFOMANCE POLYMER ELECTRETS
Yuji Suzuki*
Department of Mechanical Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, Japan
*Presenting Author: ysuzuki@mesl.t.u-tokyo.ac.jp
Abstract: Recent progress on high-performance polymer electrets based on CYTOP, novel charging
methods using soft X-ray and vacuum UV, and development of MEMS energy harvesters are discussed.
With the parylene high-aspect-ratio nonlinear springs and high-performance polymer electrets, we have
successfully developed a MEMS electret generator for energy harvesting applications. With an acceleration
of 1 G, about 4 µW has been obtained for a broad frequency range of 40-70 Hz.
Keywords: Energy harvesting, Polymer electret, Parylene springs, CYTOP
INTRODUCTION
Energy harvesting is a method to capture energy
naturally present in the environment for powering lowpower electronics [1-6]. As shown in Fig. 1, body heat,
human motion, mechanical/structural vibration, radio
frequency, and ambient light could be the energy
source. Unlike solar or wind power plants for grid
systems, energy harvesting devices provide electricity
with high-added-value lasting for extremely long time
period. Possible applications include wireless sensors
for structural health monitoring, home/building energy
management system (HEMS/BEMS), wireless ID tag
for livestock industry, and automotive sensor such as
tire pressure monitoring systems (TPMS). It is
believed that energy harvesting devices could make
contribution to reduce button battery consumption
leading to reduction of hazardous waste in batteries.
Among the energy sources shown in Fig. 1,
structural vibration is widely available in various
applications. Since the vibration frequency range
existing in the environment is below 100 Hz,
electrostatic/electret power generators should have
higher power output than electromagnetic ones. In this
paper, our recent progress of electret materials and
electret generators for energy harvesting applications
are discussed.
As energy conversion principles from kinetic
energy of mass to electricity for energy harvesting,
electromagnetic
induction,
piezoelectric,
and
electrostatic induction are used. Electromagnetic
induction [7-9] prevails in macro scale, but it is not
always the case in energy harvesting; since induction
voltage is proportional to the area of coil and the
relative speed between permanent magnet and coil, the
power output is proportional to square values of those.
Thus, the power output is decreased drastically for
small-scale generators operating at low frequencies. In
contrast, power output of piezoelectric and
electrostatic induction is proportional to the frequency,
which makes those principles are superior to
electromagnetic counterpart.
Recently, various kinds of piezoelectric generators
are prototyped using bulk PZT, thin-film PZT and AlN
thin film [10-14]. In electrostatic induction generators
in a narrow sense, electricity charged with an external
circuit is magnified using variable capacitors [15-18].
The other type of electrostatic generators is the
electret generator with self-bias voltage using electrets.
Figure 2 shows three different configurations of
electret generators. In this paper, our recent progress
on high-performance polymer electrets, new charging
methods, and MEMS generators are described with
some brief review of the field.
Figure 1 Principles of energy harvesting.
Figure 2 Different configurations of electret
generators (Top: gap closing type, middle: inplane type, bottom: in-plane type with oscillating
dielectric ).
HIGH-PERFORMANCE ELECTRET AND
CHARGING TECHNOLOGIES
Electret material can be divided into two groups,
i.e., SiO2-based inorganic electret and polymer-based
organic electret [19]. SiO2 offers extremely high
surface charge density, but long-term stability of
charges is of concern. To solve this problem, multilayer structures using SiO2 and Si3N4 are often used
[20, 21]. On the other hand, fluorinated polymer
materials such as PTFE and FEP provide better longterm stability. However, they are not compatible with
MEMS processes, since they are insoluble in solvents.
In addition, the surface charge density is insufficient
for generators with high power output.
Hsieh et al. [22] and Boland et al. [23] employed
Teflon AF (Du Pont Inc.) as their electret film, which
is perfluorinated amorphous polymer. Lo and Tai [24]
found that fluorinated parylene (parylene HT, SCS)
provide very high surface charge density of 3.7 mC/m2
with a 7.3-µm-thick film.
Sakane et al. [25, 26] recently reported that
amorphous perfluorinated polymer CYTOP (Asahi
Glass Co., Ltd.) can provide up to 4 times higher
surface charge density than Teflon AF. They found
that the end group of CYTOP has large effect on the
charging characteristics. Figure 3 shows surface charge
density of 15-µm-thick corona-charged electret
samples. The needle voltage is -8 kV. During the
corona charging, the samples were kept at 120 ˚C,
which is slightly higher than the glass transition
temperature (Tg=108˚C) of CYTOP. Surface potential
was measured with a surface voltmeter (Model 279,
Monroe Electronics). The surface charge density of
CTL-M (amidosilyl end group) is as high as 1.3
mC/m2, and is kept for more than 4,000 hrs. Implanted
charges in CTL-A (carboxyl end group) are also stable,
but the surface charge density is lower than that of
CTL-M. On the other hand, CYTOP with no
functional end group (CTL-S) and Teflon AF give low
surface charge density. Also shown in Fig. 3 is the
surface charge density of 15-µm-thick CTL-A with 3%
aminosilane (3-aminopropyl (diethoxyl) methylsilane)
coupling reagents in order to introduce larger number
of amidosilyl molecules into the CYTOP structure. By
doping only 3 % of aminosilane into CTL-A, the
surface charge density has been almost doubled;
surface charge density as high as 1.5 mC/m2 has been
achieved.
Kashiwagi et al. [27] also developed aminosilanedoped CYTOP electrets, in which aminosilane-based
nano clusters are formed. They have obtained
extremely-high surface charge density more than 2
mC/m2 with 15-µm-thick films (Fig. 4). It is also noted
that additives to CYTOP also give better thermal
stability of charges. Figure 5 shows the thermal
stimulated discharge (TSD) spectra of aminosilanedoped CTL-A compared with that of CTL-S, CTL-A,
CTL-M, and Teflon AF. The peak location of
aminosilane-doped CYTOP is 184°C, which is much
higher than other CYTOP materials and Teflon AF
[26].
Corona charging, thermal poling, contact charging,
and electron-beam irradiation are the methods for
poling electrets [19]. Among them, corona charging is
mostly widely used. By attracting corona ions with the
electrostatic field between the grid and the substrate,
charges are transferred from the ions to the electret
surface. The surface potential of electrets after
charging is usually the same as the grid voltage. It is
Figure 3 Surface charge density for CYTOP with
different end group and Teflon AF [26].
Figure 4 Surface charge density of nano-clusterenhanced CYTOP versus the grid voltage [27].
Figure 5 TSD spectra for CYTOP and Teflon AF
[26].
MEMS ELECTRET GENERATOR USING
PARYLENE SPRINGS
In in-plane electret generators, it is challenging to
design a low-resonant-frequency spring system that
enables large in-plane amplitude. In addition, the
spring should have large out-of-plane spring constant
in order to avoid stiction due to electrostatic force
between electrets and electrodes.
High-aspect-ratio Si springs microfabricated on a
Si or SOI substrate are often employed [35, 36].
Advantage of Si springs is that large Q factor can be
obtained with relatively simple fabrication process.
However, since Si is brittle, maximum amplitudes of
those Si springs are on the order of 10 µm. Since
power output is proportional to the oscillation
amplitude, spring systems for larger amplitudes are
required for large power output.
Suzuki and Tai [37] developed high-aspect-ratio
parylene springs. Using parylene-C deposition onto a
deep trench etched into a Si substrate as a mold,
parylene springs with the aspect ratio up to 20 can be
microfabricated. Since the Young’s modulus and the
Figure 6 Schematic of through-substrate soft-Xray
charging [29-31].
Figure 7 a) Conventional fabrication method of
electret transducers with assembling after
charging, b) In-situ charging method for ‘vertical’
electrets using soft X-ray irradiation enabling
single-wafer electret transducers.
-800
Surface Potential [V]
noted that optimization of various parameters in
corona charging such as needle voltage, needle-grid
and grid-sample distances is not a straightforward
process and sometimes tricky.
Hsieh et al. [22] employed a back-lighted thyratron
(BLT), in which a pulsed electron beam several
millimeters in diameter and several keV in energy is
formed. They charged 1.2-µm-thick Teflon AF for
MEMS electret microphone, and obtained 0.6 mC/m2.
Mescheder et al. [28] employed ion implantation using
phosphorous/boron to charge 500-nm-thick SiO2
electrets, and obtained stable surface charge density of
7.5 mC/m2.
Hagiwara et al. [29] recently developed a novel
charging method using soft X-rays, in which X-rays
ionize gas molecules and the ions are attracted to the
electret surface by imposed electric field (Fig. 6).
Since soft X-rays can penetrate substrates, even
electrets can be charged after packaging the device.
Hagiwara et al. [30, 31] also examined soft-X-raycharged CYTOP electrets, and found that their stability
is almost the same as those charged with corona
discharge.
With the soft X-ray charging, Honzumi et al. [32]
have developed vertical electrets in narrow gaps.
While assembling is necessary for corona-charged
electrets, comb-drive electret transducers can be
developed with a single wafer approach as shown in
Fig. 7. Yamashita et al. [33] has obtained 4 nW at 500
Hz and 15 G acceleration with their early prototype.
Honzumi et al. [34] has developed a high-speed
charging method using vacuum UV. Since ionization
rate of vacuum UV is much higher than that of soft Xray, only a few seconds is necessary to achieve
sufficiently-high stable surface potential of electrets as
shown in Fig. 8.
-600
-400
-200
0
bias voltage : +1 kV
N2 pressure : 1.0 Pa
0
1
2
3
4
5
6
Irradiation Time [s]
Figure 8 Surface potential of CYTOP electrets
versus irradiation time. The chamber pressure and
the bias voltage are 1 Pa and 1 kV, respectively
[34].
yield strain of parylene are respectively 4 GPa and 3 %,
soft but robust springs can be realized.
Kamezawa et al. [38] examined thermal stability
and fatigue strength of parylene high-aspect-ratio
springs. As shown in Fig. 9, whereas parylene-C
degrades in oxygen environment over 100 ºC, diX-HR
(KISCO), which is a thermally-stable-grade parylene,
has better thermal tolerance over 140 ºC. Note that, in
terms of thermal stability, fluorinated parylene such as
parylene HT is superior to diX-HR. However, unlike
parylene HT, diX-HR has almost the same deposition
speed and large yield strain as parylene-C, so that diXHR is suitable for mechanical elements. Kamezawa et
al. also examined fatigue characteristics of the
parylene beams at room temperature through cyclic
load tests. They found no mechanical degradation up
to 107 cycles at 1 % strain, indicating robustness of the
paryelne springs.
Various gap control methods have been
investigated in order to avoid stiction. For instance,
Naruse et al. [39] adopted the micro ball bearing [40]
to realize large amplitude oscillation in their electret
generator. Tsurumi et al. [41] and Suzuki et al. [42]
developed a non-contact gap control method based on
electrets as shown in Fig. 10. Both substrates have
patterned electret strips with bottom electrodes. With
the fringe field around the patterned electrets, repulsive
force can be obtained between the substrates.
Figure 11 shows a schematic of our MEMS
electret generator [42, 43]. The dimension of device is
18.5 x 16.5 mm2. The top substrate consists of a Si
proof mass supported with 20-µm-wide parylene highaspect-ratio springs. Patterned electrets and electrodes
in the dual-phase arrangement [44] are formed both on
the Si mass and the bottom Pyrex substrate. The gap
between the substrates is defined with micro beads.
The size of the mass is 11.6 x 10.2 mm2, of which
weight is 0.1 g. The width of the patterned electret and
electrode is 480 µm. Power output estimated with the
present numerical simulation is 17 µW at the surface
voltage of –600 V.
Figures 12 show photographs of the generator
prototype. The resonant frequency without added mass
is 63 Hz with a quality factor of 8.6. In-plane
amplitude at the resonance is as large as 1.8 mmp-p.
Another important feature of the present MEMS
generator is the nonlinearity of the spring system for
broadband response [45]. When the amplitude
becomes large, the secondary spring is engaged and
the effective spring constant is increased. Thus, the
spring system behaves as a hardening spring, which
broadens the frequency range of resonance.
Figure 13 shows the frequency response with 0.4 g
mass at 1G, where large amplitude is obtained for a
broad frequency range between 40 – 70 Hz when the
Figure 11 In-plane electret generator with parylene
high- aspect-ratio spring.
Figure 9 Free-standing parylene-C (left) and diXHR (right) beams after thermal annealing at 140 ºC
for 5 hours [38]. Beam width and height are
respectively 30 µm and 350 µm.
Figure 10 Principle of electrostatic micro-bearing
using polymer electret strips on both sides of the
substrates [41, 42].
b)
Figure 12 MEMS electret generator [41,42]. a)
Overview of the Si mass suspended with parylene
springs. b) Integrated device in a ceramic package.
Figure 13 Power output of the MEMS electret
generator for different frequencies at 1G [43].
external vibration is large [43]. The maximum power
output is 3.8 µW.
CONCLUSIONS
Our recent progress on high-performance electret
materials, novel charging methods and MEMS energy
harvesters are discussed. Applications of electret
transducers should be enlarged by the new electret
materials and the novel charging methods. With the
parylene high-aspect-ratio nonlinear springs and highperformance polymer electrets, we have successfully
developed a MEMS electret generator for energy
harvesting applications. With an acceleration of 1 G,
about 4 µW has been obtained for a broad frequency
range of 40-70 Hz.
This work is partially supported by the New
Energy and Industrial Technology Development
Organization (NEDO) of Japan.
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