VIBRATION-DRIVEN MEMS ENERGY HARVESTER WITH
VERTICAL ELECTRETS
K. Yamashita1,2*, M. Honzumi1,2, K. Hagiwara1,3, Y. Iguchi3, and Y. Suzuki1,2
1
Dept. of Mechanical Engineering, The University of Tokyo,
2
G Device Center, BEANS Project, and
3
NHK Science & Technology Research Laboratories
*Presenting Author: yamasita@mesl.t.u-tokyo.ac.jp
Abstract: A MEMS electret generator with vertical electrets has been developed with the aid of novel
soft X-ray charging. Fluorinated parylene deposited on the comb drive is employed as the electret. The
resonance frequency and the quality factor of the present early prototype are respectively 6.1 kHz and
10.2. Under external oscillation at the resonant frequency and 15 G, 0.4 Vp-p output has been obtained,
which corresponds to 2 nW.
Keywords: Energy harvesting, Vertical electrets, Comb drive, Soft X-ray charging, diX-F
INTRODUCTION
Electret is a dielectric material with quasipermanent charges. Recently, electrets are applied to
various micro devices including MEMS microphone
[1], low-voltage droplet manipulation [2], and
vibration-driven energy harvester [3-7]. However,
since corona discharge is usually employed for poling
the electrets, assembling is necessary after charging.
Recently, we have developed a new charging
method using soft X-ray irradiation [8], and found that
surface potential and thermal stability of electrets
charged with the soft X-ray are as good as those with
corona discharge [9]. Since soft X-rays can penetrate
into narrow gaps and generate ions by photoionization,
vertical electrets on the sidewall of high-aspect-ratio
trenches such as MEMS comb fingers can be realized
[10], which enables fabrication of electret energy
harvesters with a single wafer approach.
In the present study, we design a comb-drive
energy harvester with vertical electrets, and examine
its performance using our early prototype.
electrets.
Figure 2 shows surface potential of vertical
electrets charged through narrow gaps versus the
distance from the opening. 2-µm-thick parylene-C is
deposited on one side of copper substrates. Soft X-ray
tube of 9.5 keV is used for irradiation. The bias
voltage and the irradiation time are respectively chosen
as 100 V and 10 minutes. The surface potential is
measured with an electrostatic voltmeter (Monroe
Electronics, model 244A). The surface potential is
almost constant up to the distance 20 30 times of the
gap opening. Therefore, almost uniform surface charge
distribution on vertical electrets should be available up
to the depth of 100 µm for gap opening of a few µm.
VERTICAL ELECTRETS CHARGED WITH
SOFT X-RAYS
Corona discharge is a conventional charging
method for electrets, in which colona ions emitted
from a high-voltage needle transfer their charges onto
the electret film when they arrive at the surface. Since
corona ions cannot penetrate narrow gaps due to
charge build-up near the opening, electret films should
be exposed during charging. Therefore, with electrets,
one could not employ single-wafer approaches such as
comb-drive capacitive generators [11, 12].
Recently, we have developed a new electret
charging method using soft X-rays [8, 9], and
successfully microfabricated vertical electrets. As
shown in Fig. 1, with soft X-rays irradiation, positive
and negative ions equally generated in the air gap. The
ions are driven toward the sidewall with the electric
field across the gap, transferring the charges to the
Figure 1: In-situ charging method for vertical electrets
using soft X-ray irradiation.
Figure 2: Soft X-ray charging tests in narrow gaps. 2µm-thick parylene-C is used as the electret film.
into the 50-µm-thick device layer with DRIE (Fig. 4b).
The handle layer is etched from the backside with
DRIE to release the structure and to avoid stiction of
the proof mass during parylene deposition (Fig. 4cd).
Figure 3: Time history of the surface potential for 1.7mm-thick fluorinated parylene electrets (diX-F,
KISCO). Data for different charging temperatures are
plotted.
For electret materials on vertical walls, we employ
parylene, since electret films with uniform thickness
can be deposited after fabrication of the structure. Lo
and Tai [5] found that perfluoro parylene (parylene HT,
SCS) has very high surface charge density. In this
study, we evaluated fluorinated parylene (diX-F,
KISCO), in which two out of four hydrogen atoms of
parylene-N are replaced with fluorine as shown in the
inset of Fig. 3. Figure 3 shows the surface charge
density of 1.7-µm-thick diX-F films charged with
corona discharge. The surface charge density is 1.4
mC/m2, which is much lower than that of parylene HT
with no hydrogen atoms. However, the surface
potential of about 100 V is reasonably high for initial
tests using our early prototype. Note that the surface
potential with charging temperature of 90 C° is higher
than the other cases.
DESIGN
OF
HARVESTER
ELECTRET
Figure 4: Fabrication process of the comb-drive
MEMS electret energy harvester prototype.
a)
b)
ENERGY
To demonstrate the performance of vertical
electrets, an electret energy harvester prototype with
comb drives was designed. An SOI wafer with a 50µm-thick device layer was used. The length of
suspension beam is 400 µm with a cross section of 50
µm x 6 µm. The comb length and width are
respectively 30 µm and 5 µm, while the initial gap
between facing combs is 5 µm. After deposition of 1.5µm-thick diX-F electrets, the gap becomes 3 µm. The
proof mass is 3.2 mm x 2.1 mm. Designed values of
the resonant frequency and the capacitance of the
comb drives are 2 kHz and 2.6 pF, respectively. A
numerical model based on one-dimensional electrostatic field [13] is employed to design parameters.
c)
MEMS FABRICATION
Fabrication process of the comb-drive electret
generator is shown in Fig. 4. Process starts with EB
lithography on a 4” SOI wafer using thick EB resist
(Fig. 4a). Springs and comb fingers are then etched
Figure 5: SEM images of the comb-drive MEMS
energy harvester prototype with vertical electrets. a)
Overview, b) Close-up view of spring suspension, c)
Close-up view of comb fingers with diX-F electrets.
Fig. 6: Photograph of the electret generator fixed on a
PC board for wire connection.
Figure 8 shows the output voltage. Radio noise
below 100 Hz is removed with a high-pass Fourier
numerical filter. Output voltage of 0.4 Vp-p has been
obtained, which corresponds to 2 nW power output.
Figure 9 shows the output voltage amplitude versus
external vibration amplitude. The output voltage is
almost linearly increased with the vibration amplitude.
Surface potential estimated by the curve fitting is 95 V,
showing that diX-F verticacl electrets work properly.
Figure 10 shows simulation results of the output
power versus external load at the resonant frequency.
The power output is maximized at a 34 MΩ external
load, and the maximum power is 0.17 µW at 10G.
After the SiO2 layer is released with vapor HF (Fig.
4e), 1.5-µm-thick diX-F film is deposited by CVD on
the whole structure forming electret films on both
sides of the comb fingers (Fig. 4f). The device is fixed
on a PC board and wired for experiments.
Figure 5 shows SEM images of the fabricated
device with a proof mass supported with the 6-µmwide Si beams. Figure 5c shows the close-up view of
the comb fingers with diX-F electrets. The device
fixed on a PC board is shown in Fig. 6.
EXPERIMENTAL RESULTS
Soft X-ray of 9.5 keV is used to charge diX-F
electret with the bias voltage of 150 V for 45 minutes.
The device is then fixed onto an electromagnetic
shaker (ET-126B-1, Labworks), and in-plane
oscillation is applied to the device. The amplitude is
measured with a laser displacement meter (LC-2430,
Keyence). The resonant frequency is measured with a
vibration analyzer (Polytec, MSA-500). As shown in
Fig. 7, the resonance frequency is 6.1 kHz, which is
much higher than our designed value of 2 kHz. The Qfactor is 10.2.
For some limitation of our experimental set-up,
experiments were made at 500 Hz, which is much
lower than the resonant frequency. A 10MΩ resister is
used as the external load.
Figure 8: Output voltage across a 10 MΩ load
resistor at 500 Hz and 15 G acceleration.
Figure 9: Output voltage versus amplitude of external
vibration at the resonant frequency of 6.1 kHz.
Figure 7: Frequency response of the MEMS electret
generator prototype.
Figure 10: Simulated output power versus external
load at the resonant frequency 6.1 kHz.
[3]
[4]
[5]
Figure 11: Simulated output power versus external
acceleration at the resonance frequency of 6.1 kHz.
Simulated output power at the resonance frequency
versus external acceleration for the mamtched
impedance of 34 MΩ is shown in Fig. 11. The output
power is estimated to be 0.67 µW at 20 G, where the
estmates of the amplitude and the capacitance change
are respectively 1.4 µm and 0.17 pF. Although the
power output of the present early prototype is small
even at high frequency oscillations, the present
approach of using vertical electrets can be easily
applied to capacitive generators with comb drives and
make contribution to development of cost-effective
efficient capacitive energy harvesters.
[6]
[7]
[8]
[9]
CONCLUSIONS
Early prototype of MEMS electret energy harvester
with vertical electrets on comb drive has been designed
and successfully microfabricated. The resonant
frequency and the quality factor of the present
prototype device are respectively 6.1 kHz and 10.2. By
using novel soft X-ray charging, diX-F electret on the
side wall of the comb drive is charged to 95 V. At 500
Hz and 15 G acceleration, voltage amplitude of 0.2 V
has been obtained, which corresponds to 2 nW power
generation.
[10]
ACKNOWLEDGEMENTS
This work was supported by New Energy and
Industrial Technology Development Organization
(NEDO). Photomasks are made using the University of
Tokyo VLSI Design and Education Center (VDEC)’s
8-inch EB writer F5112+VD01 donated by
ADVANTEST Corporation.
[11]
[12]
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