EFFECT OF HIGH-ABSORBANCE GAS INTRODUCTION ON
SOFT-X-RAY CHARGING PROPERTIES FOR ELECTRET GENERATOR
K. Hagiwara1,2*, M. Goto1, Y. Iguchi1, T. Tajima3,
Y. Yasuno4, H. Kodama4, K. Kidokoro5, and Y. Suzuki2
1
Science & Technology Research Laboratories, NHK, Tokyo, Japan
2
Department of Mechanical Engineering, The University of Tokyo, Tokyo, Japan
3
Advanced Research and Development Division, NHK Engineering Services, Inc., Tokyo, Japan
4
Applied Piezoelectricity Division, Kobayasi Institute of Physical Research, Tokyo, Japan
5
Audiological Components, RION Co., Ltd., Tokyo, Japan
*Presenting Author: hagiwara.k-he@nhk.or.jp
Abstract: We examined that effect of introduced gas on charging rate of electrets with soft X-ray irradiation.
Photoionization current observed during soft X-ray irradiation was proportional to atomic number of the gases.
When the Ar and the Xe gas as the substitute for air was introduced to the gap between an electret sample and a
counter electrode, the charging rate of electrets was improved by up to 10 times higher than that under air. Even
when the gap distance of electrodes reduced to 100 !m, charging rate of the electret under Xe was -58 V/min.
Charging through a 450-!m-thick glass substrate under Ar atmosphere has also been demonstrated, where the
charging rate could be further increased by using high-energy X-ray with higher transmittance through the substrate.
Keywords: Energy harvesting, Soft X-ray, Electret, MEMS, Charging method, Photoionization
INTRODUCTION
Electret, that is a dielectric material has semipermanent charge, has been used as one of the key
parts for microphones, power generators, and so on,
because operating power supply of such devices can be
much saved or omitted [1]. Recently, various new
electret applications are proposed with the aid of
MEMS technologies (e.g., [2]). In energy harvesting
applications, vibration-driven generators with highperformance electrets attract much attention [3-5]. It is
expected that the generators would be helpful for
future green mobile electronics because the generators
can convert low-frequency vibration energy in the
environment to electric power.
For MEMS-based electret devices, however,
conventional charging methods of electrets such as
corona charging do not always have a good
compatibility with typical MEMS process. For
example, since corona ions cannot pass through
obstructive parts such as a counter electrode, electrets
should be formed prior to assembling processes of the
devices.
In order to overcome this problem, we developed a
soft X-ray charging method suitable for the MEMSbased electret devices [6-8]. Soft X-rays can generate a
large amount of ions from air by the photoionization
effect [9,10]. In addition, the radiation can even
penetrate a bulk substrate, so that electrets can be
formed after assembling/packaging process.
SOFT X-RAY CHARGING METHOD
Figure 1 shows the principle of the soft X-rays
charging method schematically. With soft X-ray
irradiation, positive/negative ions and electrons are
generated between a dielectric formed on a substrate
and a counter electrode formed on a substrate. These
charged particles are separated by applying an electric
field and are dragged to dielectric, and the charges are
transferred to the electret. The electrets charged using
this method have excellent retention properties as well
as the electret charged by corona discharge [7].
However, for soft X-ray charging, time to
complete charging is much longer than corona
discharge due to small amount of charged particles
generated by photoionization. This is because the
number of ions is proportional to path length of soft Xrays photon, which is the gap distance between
electrodes. For electret generators, however, small gap
distance on the order of 10 !m is necessary to obtain
high power output [11].
Here, in this paper, we will describe effects of
high-absorbance gases introduction on soft X-ray
charging properties. Figure 2 shows the transmission
spectrum of air, its components, and xenon (Xe) [12].
Fig. 1: Schematic of soft X-ray charging method.
After the X-ray photon ionize gas molecules, large
amount of charged particle is generated and
dragged toward to dielectric by electric field.
1.000
Transmittance
0.995
0.990
Air
N2
CO2
O2
Ar
Xe
0.985
0.980
0.975
0.970
3
4
5
6
7
8
9
10
Photon Energy (keV)
Since absorbance of gases increase with their atomic
number, larger photoionization current would appear
for argon (Ar) or Xe atmosphere. Thus, Ar and Xe was
introduced to the electrode gap, in order to improve
charging rate.
EXPERIMENTAL SETUP
To examine effects of the substitute gases
introduction on charging properties, a vacuum
chamber with gas flow control systems was used.
Figure 3 shows a photo of the experimental setup for
charging electrets under various gas atmospheric
conditions. The vacuum chamber has a 50-µm-thick
polyimide (Kapton) window, which can transmit over
90% of 6 keV X-rays. After the vacuum chamber was
evacuated to below 1"10-2 Pa by a turbo molecular
pump, dry air, N2, Ar, and Xe were introduced to keep
the chamber pressure at 100 kPa.
As a dielectric sample, 20-µm-thick CYTOP
(Asahi Glass, CTL-M) film formed on a Si substrate
by spincoating was prepared. A 100-µm-thick Al foil
is used for the counter electrode, by which about 5% of
6 keV X-ray can pass through. The electrode was fixed
with a certain gap distance from the dielectric samples.
A source measure unit was used to apply a bias voltage
Vb between the counter electrode and the dielectric
samples, and to measure the photoionization current.
The tube current and the acceleration voltage of X-ray
tube were fixed at 300 µA and 9.5 keV, of which
spectral peak of X-ray is about 6 keV. Before charging,
all dielectric samples were neutralized with soft X-ray
irradiation without applying bias voltage.
RESULTS AND DISCUSSION
Photoionization Current
Firstly, photoionization current observed during
soft X-ray irradiation was examined. The current is
proportional to the charging rate of electrets [7]. As
shown in Fig. 4, photoionization current 8 times and 9
times larger than air was observed respectively for Ar
and Xe. By contrast, the current under N2 atmosphere
is 90% of air. It is now clear that photoionization
current is increased with Ar and Xe.
Fig. 3: A photo of experimental setup for soft X-ray
charging of electret under various gas conditions.
Distance between X-ray tube and samples was about
20 mm.
Photoionzation Current (µA)
Fig. 2: Transmission spectra of air, its components,
and Xe. (100 !m path length, 100 kPa)
-0.1 6
-0.1 2
-0.0 8
-0.0 4
0.00
N2
Air
Ar
Xe
Fig. 4: Photoionization current for each gas. A nonCYTOP coating metal substrate was used to measure
the current. (400 !m gap, Vb = -200 V)
Note that improvement with Xe is lower than the
value expected from the transmission spectra shown in
Fig. 2. We anticipate that the possible reason is the
difference of photoionization mechanism between Ar
and Xe, such as the energy for photoelectron emission,
the probability of secondary ionization of neutral gas
molecules by the emitted auger electron, and so on.
Charging Rate
Figure 5 shows the surface potential of the
CYTOP electret film under each gas condition. After
the surface potential reaches closer to the bias voltage,
the potential becomes constant. This is because the
surface potential of electrets neutralizes the electric
field. To examine the charging rate, measured data
points except saturated potential were fitted by a
straight line from the origin.
When the bias voltage is -200 V, the charging rate
of electret in air was about -0.44 V/s (Fig. 5(a)). With
the introduction of Ar and Xe, the charging rate was
increased to about -2.59 V/s and -3.83 V/s, which are 6
and 9 times larger than that of air respectively. On the
other hand, the charging rate under N2 was -0.37 V/s,
which is about 90% of air. From these results, it was
found that the ratio of the charging rate under different
gas conditions to that of air almost agree with the ratio
-200
-150
Air
N2
Ar
Xe
-100
-50
0
0
200
400
Surface Potential (V)
Surface Potential (V)
-200
Air
N2
Ar
Xe
-150
-100
-50
0
600
200
Irradiation Time (s)
Surface Potential (V)
Surface Potential (V)
-500
Air
N2
Ar
Xe
-200
-100
0
0
200
800
Fig. 6: Surface potential vs. gap distance between
electrodes under each gas atmosphere. (Vb = -200
V, 1 min irradiation)
-500
-300
600
Gap Distance (!m)
(a): Vb = -200 V
-400
400
400
Ar
-400
-300
-200
-100
600
0
0
Irradiation Time (s)
20
(b): Vb = -500 V
of photoionization current. Similar tendency of the
charging rate was obtained when the bias voltage of 500 V is used (Fig. 5(b)). Under Xe atmosphere, the
charging rate was 10 times larger than air, so that soft
X-ray charging rate can be improved considerably with
the introduction of high-absorbance gas.
Gap Distance
A plot of surface potential versus the electrode gap
distance under different gas conditions for 1-min soft
X-ray irradiation is shown in Fig. 6. Charging rate is
proportional to the electrode gap. The larger the mass
of introduced gas molecule, the larger the slope of
fitted line. Under the Xe atmosphere, surface potential
was reached to -200 V within 1 min at the gap of 400
!m. Even when the gap distance of electrodes reduced
to 100 !m, which roughly corresponds to the gap of
electret generators, charging rate of the electret under
Xe was -58 V/min.
For Ar and N2, the slope of fitted line was -0.37
V/!m and -0.035 V/!m, which were 9 and 0.8 times of
air respectively. These results were also consistent
with the tendency of the photoionization current data.
Charging through Thick Substrate
To examine feasibility of applying the soft X-ray
60
80
100
120
Fig. 7: Charging property through a 450-!mthick glass substrate under Ar atmosphere (400
!m gap, Vb = -500 V).
-1
10
-2
Transmittance
Fig. 5: Surface potential vs. irradiation time under
each gas. (400 !m gap, 1 atm)
40
Irradiation Time (m)
10
-3
10
450!m-SiO2
100!m-Al
-4
10
-5
10
-6
10
5
6
7
8
9
Photon Energy (keV)
10
Fig. 8: Transmission spectra of a 450-!m-thick
glass substrate and a 100-!m-Al foil.
charging method to practical electret generators after
assembling, charging rate through thick glass substrate
was investigated. Figure 7 shows the surface potential
of CYTOP electrets charged through a 450-!m-thick
glass substrate with 50-nm Al electrode under Ar
atmosphere. Electrode gap distance was set to 400 !m.
Irradiation time of about 120 min was necessary to
complete charging, which is about 50 times longer
than that through the 100-!m-thick Al counter
electrode. This is because, as shown in Fig. 8, soft Xray transmittance of 450-!m-thick glass is much lower
than that of 100-!m Al at 6 keV.
However, if the spectral peak of X-ray were
shifted to higher energy range, transmittance of X-ray
photon would be much increased. The transmittance of
450-!m-thick glass at 10 keV is about 16%, which is
about 500 times larger than that at 6 keV. Although the
absorbance of Ar gas would be slightly reduced at 10
keV as shown in Fig. 2, the estimate of charging time
through a 450-!m-thick glass substrate is only 4 min
even with the electrode gap distance of 100 !m.
Therefore, soft X-ray charging of MEMS electret
devices after assembling/packaging is promising.
[4]
[5]
[6]
CONCLUSION
In order to investigate the possibility of throughwafer charging of electret generators after
assembling/packaging, we examined effects of highabsorbance gas introduction on soft X-ray charging
properties. When the high-absorbance gas such as Ar
or Xe is introduced, photoionization current and
charging rate of CYTOP electrets are increased by 8~9
times larger if compared with air atmosphere. Even
with the electrode gap distance of 100 !m, the
charging rate under Xe is as large as -58 V/min.
Charging through a 450-!m-thick glass substrate under
Ar atmosphere has also been demonstrated, where the
charging rate could be further increased by using highenergy X-ray with higher transmittance through the
substrate.
We believe that the present charging method
improves compatibility of electrets with various
MEMS devices including electret generators and also
contributes to enhancement of the device performance.
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