Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)
A FRUIT JELLY MEMS ELECTRET POWER GENERATOR
Jinwen Zhang, Zhiqiu Lv
National Key Laboratory of Nano/Micro Fabrication Technology
Institute of Microelectronics, Peking University, Beijing 100871, China
Abstract: In this paper, a novel MEMS electret power generator is presented. Fruit jelly with super flexibility
was utilized as spring supporting structure so that our electret MEMS power generator can scavenge energy
from vibration with very low frequency and small acceleration. On the other hand, silicon-based inorganic
electret, i.e. PECVD SiO2/Si3N4 double layers electrets, was also applied in MEMS electret power generator in
this paper. Our experimental results demonstrate that fruit jelly MEMS electret power generator with PECVD
SiO2/Si3N4 double layers electret has excellent sensitivity to the vibrations with frequencies lower than 10Hz
and accelerations lower than 0.1g, and exhibits good performance.
Key Words: MEMS power generator, Fruit jelly, PECVD SiO2/Si3N4 double layers electret
expected a wide application in miniaturized
transducers. Our experimental results have proved
good chargeability and charge stability of PECVD
prepared SiO2/Si3N4 double layers [7]. In this paper,
silicon-based inorganic electret, i.e. PECVD
SiO2/Si3N4 double layers electret, is applied in MEMS
power generators.
1. INTRODUCTION
Environmental energy scavenging has become
popular recently. Vibration is particularly attractive
because of its abundance. There have several
scavenging techniques based on piezoelectric,
electrostatic and electromagnetic transduction have
been reported [1-3]. MEMS Electret power generators
are purely electric, clean, long-lifetime and highpower-output in small scale. Most reported devices are
only capable of operating at several kHz, but at lower
frequencies they are ineffective. Several reported low
frequency electret power generators also work at few
decades’ Hz [4, 5]. However, it is at much lower
frequencies 1-10Hz where most ambient vibration
exists. In this paper, a novel MEMS electret power
generator based on fruit jelly is presented. Fruit jelly
with super flexibility was utilized as spring supporting
structure so that our MEMS electret power generator
can scavenge energy from vibration with very low
frequency and small acceleration.
In all reached papers, various organic electrets were
investigated and utilized. APCVD/LPCVD SiO2/Si3N4
double layers electrets on silicon substrates have been
well studied and proved good charge stability [6]. The
low deposition speed and high residual stress of these
techniques lead to difficulties to prepare thick layers
(>2um). However, thick electrets are necessary for
application in some micro devices, such as electret
power generators demanding electrets as thick as
possible. PECVD could be a better choice for its
relatively high deposition speed and low residual stress.
Further more, PECVD’s low deposition temperature
makes it feasible to prepare silicon-based inorganic
materials on non-silicon substrates with low melting
point like glasses, and on metal layers as lower
electrodes, such as Al, Au, and etc. So PECVD
prepared SiO2/Si3N4 double layers electrets can be
2. DEVICE
DESIGN
Our MEMS electret power generator mainly
consists of three parts, i.e. the rotor, the stator and the
spring supporting structure (as shown in figure 1). The
rotor has the metal strip with half of the spatial
frequency of the electrodes on the stator. The electret
is prepared on the rotor. The fruit jelly is used as the
spring supporting structure laid under the rotor for
sensing the ambient vibration. Because it is very soft
and flexible, the fruit jelly can be moved by very low
electret
Stator
Rotor
s
Fruit jelly
25μm
Au wire
2.4cm
Fig. 1 The schematic graph of fruit jelly EPG
and the photo of the rotor with fruit jelly
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Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)
corona charging whose condition is shown in table 1.
The surface potential measured by Trek 347 voltmeter
is about -100V.
(e)
(a)
Table 1 Negative corona charging conditions
Si
Glass
(b)
Metal
Electret
(c)
Fig. 2 Process flow of stator and rotor fabrication
frequency and small acceleration vibration. There are
two groups of electrodes on the stator forming
differential capacitor with the electrodes on the rotor.
The sizes of the device are according to reference [5].
The electrodes of stator are 2mm long and 1mm wide
with 50μm gap, and they are connected alternately and
those of the rotor are 2mm long and 1mm wide with
1mm gap.
Needle voltage
-6kV
Grid voltage
-500V
Needle height
10mm
Grid height
1.5mm
Substrate temperature
80℃
Charging time
30min
4. MEASUREMENTS AND RESULTS
Special equipment was designed and machined for
mounting our device on the shaker. There includes two
plate-forms for fixing the rotor and stator, i.e. upper
plate-form for the stator and lower plate-form for the
rotor. The distances between these two plate-forms can
be adjusted at X, Y and Z directions. The fruit jelly
was directly laid on the lower plate-form and then the
rotor was placed on the top of the jelly. Because of
jelly’s wet surface, there was no glue used for sticking
jelly with the plate-form and the rotor. The average
distance between the rotor and stator is about 0.9mm.
All necessary wires were soldered. It must announce
here that a 25um diameter gold wire, which is too thin
to influence the motion of the rotor, was used to
connect the lower electrode of the rotor with the
grounded pad on the lower plate-form. The whole
equipment with our device was mounted to ES-025-S
electrodynamic shaker (as shown in figure 4), which
was driven sinusoidally by HP3652A dynamic signal
analyzer through a power amplifier. The acceleration
3. Fabrication
Both the rotor and stator started with Pyrex7740
glass wafers. Then, Cr/Au (30nm/100nm) were
sputtered and patterned by wet-etching forming the
electrodes of the rotor and stator (as shown in figure 2
(a) and (e)). Thirdly, SiO2/Si3N4 (20um/50nm) double
layers were both prepared by PECVD at 300°C on the
rotor (as shown in figure 2 (b)). Fourthly, SiO2/Si3N4
(20um/50nm) double layers were patterned by AOE
opening the pad of the lower electrodes of the rotor (as
shown in figure 2 (c)). Figure 3 shows the photos of
the rotor and stator. Afterward, SiO2/Si3N4
(20um/50nm) double layers were charged by negative
Gap=50μm
Upper plate-form
Stator
Fruit jelly
Rotor
Width =1mm
Length=2cm
Accelerometer
Lower plate-form
Gap=1mm
Rotor
Fig. 4 Testing setup Fruit jelly EPG
Fig. 3 Photos of prepared rotor and stator
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Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)
increases the power output increases However, 50Hz
outside noise coupling increases too. It is necessary to
add a shielded box for avoiding this phenomenon. The
output power can increase by leveling the top surface
of the fruit jelly so as to decreasing the distance
between the rotor and stator, and improving the
performance of the PECVD SiO2/Si3N4 double layers
electrets.
Charge (nC)
10
5
0
-5
5. CONCLUSION
-10
0.0
0.2
0.4
0.6
0.8
This paper designed and fabricated a novel MEMS
electret power generator. The rotor and stator were
prepared by micromachining technology. Fruit jelly
with super flexibility was utilized as spring supporting
structure so that electret MEMS power generator can
scavenge energy from vibration with very low
frequency and small acceleration. Silicon-based
inorganic electret, i.e. PECVD SiO2/Si3N4 double
layers electrets, was also applied in our electret MEMS
power generator. The experimental results show that
several decades’ mg acceleration can make the rotor
moving amplitude up to about 2mmp-p at 5-20Hz and
the total charge output is about 12nC one cycle at 9Hz.
Therefore, fruit jelly electret MEMS power generator
is very sensitive to the vibrations with frequencies
lower than 10Hz and accelerations smaller than 0.1g.
The output power can increase by leveling the top
surface of the fruit jelly and improving the
performance of the PECVD SiO2/Si3N4 double layers
electrets.
1.0
Time (s)
Fig. 5 Time traces at 9Hz, 2mmp-p
of the equipment was measured using a CA-YD-122
piezoelectric accelerometer. The lowest frequency of
our shaker is just 5Hz so our shaking frequency was
varied from 5Hz to 20Hz. The electret power generator
was connected to a 10Ω resistive load and the charge
across the load was measured from a YE5852A charge
amplifier whose output signal displayed by Tektronix
TDS2014B digital storage oscilloscope.
Our fruit jelly electret power generator is very
sensitive to very low frequency and small acceleration
vibration. Our experiment shows that just several
decades’ mg acceleration can make the rotor moving
amplitude up to about 2mmp-p at 5-20Hz, ex. about
0.03g acceleration at 9Hz. The time trace at 9Hz and
about 2mmp-p is shown in figure 5 and the total charge
output is about 12nC one cycle. It is supposed that
there is no resistive load from charge amplifier, and
then the average power output can be calculated to
0.13pW (where R=10Ω). At about 2mmp-p, the total
charge output one cycle increases as the frequency
increases (as shown in figure 6). As the resistance
ACKNOWLEDGEMENT
This work was partly supported by the National
Science Foundation of China (No. 60676042) and
National Basic Research Program of China (No.
2009CB320305).
0.16
power (pW)
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Fig. 6 Power versus frequency at 2mmp-p
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Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)
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