HIGH-SPEED ELECTRET CHARGING METHOD USING VACUUM UV
IRRADIATION
1
M. Honzumi1, 2*, K. Hagiwara1, 3, Y. Iguchi3 and Y. Suzuki1, 2
Department of Mechanical Engineering, the University of Tokyo, Tokyo, Japan
2
G Device Center BEANS Project, Tsukuba, Japan
3
NHK Science & Technical Research Laboratories, Tokyo, Japan
*Presenting Author: honzumi@mesl.t.u-tokyo.ac.jp
Abstract: A novel high-speed charging method for electrets using vacuum UV irradiation has been
proposed for the first time, and its optimal condition is investigated. The charging time is as short as
1 s, 2 orders of magnitudes faster than corona discharge/soft X-ray charging methods. When 15-µmthick CYTOP (CTL-M) film is charged by the present method, the surface potential of -900 V has
been obtained. No charge decay is found at least up to 2,000 hours at 20 ºC and 60%RH, indicating
that the implanted charges are as stable as those by other charging methods.
Keywords: Electret, Vacuum UV, Photoionization, Charging method, CYTOP
INTRODUCTION
VACUUM UV CHARGING METHOD
Recently, energy harvesting from environmental
vibration attracts significant attention for low power
applications such as RFIDs and automotive sensors [12]. Since the frequency range of vibration existing in
the environment is below 100 Hz, electret power
generators [3-9] should have higher performance than
electro-magnetic counterparts.
Recently, we developed a new high-performance
electret material based on amorphous perfluorinated
polymer CYTOP (Asahi Glass) [10], and prototyped a
low-frequency large-amplitude electret generator with
nonlinear polymer springs [9, 11]. Corona charging is
usually used for electrets, but it is somewhat tricky to
optimize various parameters. In addition, corona ions
cannot penetrate narrow gaps due to charge build-up
near the opening. This restricts the use of electrets only
to simple two-dimensional patterns, and the substrate
with electrets should be assembled with the other
substrate after charging. Thus, in order to obtain stable
high surface charge density in a well-controlled
manner, alternative reliable charging method is also
indispensable.
Mescheder et al. [12] employed ion implantation
to charge 500-nm-thick SiO2 electrets, and obtained
stable surface charge density of 7.5 mC/m2. Hagiwara
et al. [13, 14] developed a new charging method using
soft X-rays, and found that charge stability of soft-Xray-charged CYTOP electrets is almost the same as
those charged with corona discharge. Since soft X-rays
can ionize gas molecules inside narrow gaps, one can
also develop MEMS comb-drive transducers with
vertical electrets [15, 16], which enables single-wafer
approach.
In the present study, we propose a new high-speed
charging method using vacuum UV (VUV) irradiation,
which could enable 2 orders of magnitudes faster
charging time than corona/soft X-ray charging
methods.
In corona discharge, corona ions emitted from a
high-voltage needle transfer their charges onto the
electret film when they arrive at the surface. Usually,
charging for at least 2 minutes is required to get stable
charges [10, 17]. On the other hand, since the
ionization efficiency of soft X-ray is very low, time
required for soft X-ray charging is long; for charging a
few µm narrow gaps, irradiation up to 30 minutes is
required.
Our new idea for high-speed charging is to use
vacuum UV (wavelength < 200 nm) for ionization of
gas molecules. Figure 1 schematically shows the
present charging method. Although VUV is rapidly
absorbed at the atmospheric pressure, it can travel in
low-pressure environment.
Under irradiation of VUV on nitrogen, positive
ions and electrons are generated by photoionization,
i.e.,
*
 N 2 + hν1 → N 2
(1)
 *
+
−
 N 2 + hν 2 → N 2 + e
Upon photoionization, positive ions and electrons
Figure 1. A conceptual diagram of charging method
using vacuum UV irradiation.
8
!!N2!2!sccm
Ion!Current![µA]
generated can be separated with the electric field
between a bottom electrode of electret and a grounded
electrode or chamber wall. When positive bias voltage
is applied to the bottom electrode, electrons are
dragged toward the electret surface. The surface
potential can be controlled by the applied voltage to
the bottom electrode.
Note that, when O2 molecules exist, quenching
mechanisms shown in Eq. (2) coexiss; O2 and O3
molecules collide with excited-state N2 molecules and
absorb energy of the N2 molecules, preventing
generation of electrons [18].
O2 + N 2* → N 2 + O + O

(2)
O2 + O → O3
O + N * → O + O + N
2
2
2
 3
N2!
UV!
Figure 2. Vacuum UV charging system.
4
2
0
0.1
2
3 4 5 6
2
1
3 4 5 6
10
2
3 4 5 6
2
100
Pressure![Pa]
Figure 3. Ion current versus the chamber pressure
for different bias voltage.
EXPERIMENTAL SETUP
2.5
Ion!Current![µA]
Figure 2 shows a photograph of the VUV charging
system prototype we developed. Before charging, the
chamber is evacuated to 5 x 10-3 Pa. Then, N2 gas
(99.9995%) is introduced into the vacuum chamber at
a constant flow rate with a mass flow controller (SECE40, Horiba TEC). The chamber pressure is measured
with the ionization gauge (GI-M2, ULVAC). The
chamber pressure is regulated with a butterfly valve
connecting to a turbo molecular pump. A deuterium
lamp (C9935, Hamamatsu Photonics), of which
wavelength is from 120 to 160 nm, is used as the VUV
source. Electret samples are fixed on a Teflon tube for
applying a bias voltage to the bottom electrode. A
source measure unit (Model 2410, Keithley
Instruments) is used as a voltage source and an
ammeter for the ion current measurement. We employ
15 µm-thick CYTOP CTL-M electrets, which are spun
on a low-resistance Si substrate and cured at 185 ºC for
1.5 hours. After charging, the surface potential of the
electret sample is measured with an electrostatic
voltmeter (Model 279, Monroe Electronics).
bias!voltage:
!+1=>!kV
!+>=2!kV
!+>=A!kV
!+>=2!kV
!+>=1!kV
6
2.0
1.5
1.0
N2!7ressure!:!1!Pa
0.5
bias!voltage!:!+1kV
0.0
1
2
3
4
5
6 7 8 9
10
2
3
4
5
-lo/!0ate![sccm]
Figure 4. Ion current versus the N2 flow rate for the
bias voltage of 1 kV and the chamber pressure of 1
Pa.
RESULTS
Figure 3 shows the ion current under VUV
irradiation for different chamber pressure. The N2 flow
rate is 5 sccm. The ion current is increased with the
chamber pressure because of higher number density of
ionized molecules, while it decreases at much higher
pressure. This is because excited-state N2 molecules
are quenched by collisions with O2 or O3 molecules.
The peak ion current is at around 20-40 Pa. The
maximum ion current obtained in the present
experiment is 6 µA, which is already 20 times as large
as the ion current with soft X-ray irradiation [14]. It is
noted that, when the chamber pressure is over 4 Pa and
the bias voltage is above 0.5 kV, discharge occurs. We
could expect one order of magnitude higher ion current
with the bias voltage of 1 kV in a chamber that can
suppress discharge at higher voltage.
The ion current for different N2 flow rate is shown
in Fig. 4. It is found that the ion current is independent
on the flow rate. Thus, we keep the N2 flow rate at 5
sccm throughout this paper.
Figure 5 shows the surface potential versus the
chamber pressure. The bias voltage is +1 kV, and the
irradiation time is 3 seconds. Up to -900 V has been
obtained with 3 seconds irradiation. Due to large ion
current, surface potential is saturated at the pressure of
0.1 Pa.
Figure 6 shows the surface potential versus the
Surface!Potential![V]
-1000
-800
-600
-400
0
0.01
CONCLUSIONS
bias!voltage!:!+1!kV
irradiation!time!:!3!s
-200
2
3
4 5 6 7
2
0.1
3
4 5 6 7
2
1
Pressure![Pa]
Figure 5. Surface potential of CYTOP film versus
the chamber pressure for the bias voltage of 1 kV.
Irradiation time is 3 s.
-800
Surface!Potential![V]
charging time. Unlike time-consuming corona/soft Xray charging methods, only irradiation of 0.3 seconds
is necessary to get high surface voltage of about -700
V. Therefore, the present charging method is at least 2
orders of magnitudes faster than the corona and the
soft X-ray charging methods. The surface potential
becomes saturated for charging time over 2 seconds.
Figure 7 shows time trace of the surface potential
after charging. Samples are kept at 20 ºC and 60%RH.
The surface potential shows no distinct change for
2,000 hours. Therefore, the charge retention property
with the present charging method should be as good as
the one charged by corona discharge/soft X-ray
irradiation.
-600
-400
We have proposed a new high-speed charging
method with VUV irradiation for the first time.
Optimum pressure is around 20-40 Pa under the
present condition examined, and the maximum ion
current is 20 times that of soft X-ray irradiation. With
CYTOP electrets, surface potential up to -900 V is
obtained for the bias voltage of +1 kV. The present
method is at least 2 orders of magnitudes faster than
existing charging methods, and only a few seconds is
enough to get high surface potential. In addition, we
demonstrate that the surface potential is retained at
least for 2,000 hours, indicating sufficient stability of
the implanted charges. The present VUV charging
method should be a useful tool for high-throughput
fabrication of electret generators.
ACKNOWLEDGEMENT
-200
0
This work is supported by the New Energy and
Industrial Technology Development Organization
(NEDO) of Japan.
bias!voltage!:!+1!kV
N2!pressure!:!1.0!Pa
0
1
2
3
4
5
6
Irradiation!Time![s]
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Figure 6. Surface potential of CYTOP electrets
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the bias voltage are 1 Pa and 1 kV, respectively
.
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!"#$a&'!()*'+*,a-!./0
-1000
[3]
-800
-600
[4]
-400
p#'55"#'67,a5!8)-*ag'6,##a:,a*,)+!*,m'
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[5]
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1,m'!.30
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