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Sensors and Materials, Vol. 32, No. 7 (2020) 2493–2501
MYU Tokyo
S & M 2277

Experimental Evaluation of Bipolar Surface Potential
with Corona Charging for Electret Activation
Koki Yamamoto,1* Takayuki Fujita,2 Adrien Badel,3Fabien Formosa,3 Kensuke Kanda,2 and Kazusuke Maenaka2
1

Fundamental Technologies Research Department, Technologies and R&D Division, Ebara Corporation,
4-2-1 Honfujisawa, Fujisawa, Kanagawa 251-8502, Japan
2
Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan
3
SYMME Laboratory, Université Savoie Mont Blanc, 11 Chemin de bellevue, Annecy-le-vieux 74940, France(Received April 8, 2020; accepted June 15, 2020)

Keywords: vibration energy harvester, MEMS, bipolar charging, electret, experimental evaluation

In this work, we focus on the experimental evaluation of the bipolar surface potential in a
MEMS electret vibration energy harvester. Bipolar charging increases the absolute electricalenergy of an electret when the electrostatic force is kept low. Measuring the practical surface
potential is necessary for the design and optimization of the harvester. However, it is difficult toestimate the potential after assembly because the typical approach using surface potentiometers
is not applicable. In this work, an evaluation method for bipolar charging in conjunction withan external voltage source was developed. To evaluate the proposed method, a typical MEMS
electret vibration energy harvester was fabricated. In the experiment, the harvested voltage
and displacement at a low acceleration of 0.2 mg (gravitational acceleration, g = 9.81 m/s2)
were measured. Since the proposed method is applied at harvesting, the harvested voltage canbe changed with an external voltage. The proposed method shows that factors including therelative responses of the harvested voltage can be estimated using the practical potential. In thiswork, the surface potential in the electret was explicitly considered in the harvesting experimentusing a practical bipolar MEMS electret-based vibration energy harvester and an external
voltage source.

1.

Introduction

Over the past decade, applications of Internet of Things (IoT) technology have been strongly
anticipated in consumer activities, specifically industrial applications that reduce personal
expenses, develop smart factories, and reduce risk by accident prevention.(1–3) This technology
will be a novel strategy to drive the fourth industrial revolution in manufacturing [Industry 4.0(4)].
A semipermanent battery is essential for this technology to expand its industrial applications.
Autonomous batteries, which collect environmental energy and convert it into electricalenergy, are one of the leading candidates that dispel the concerns about the lifetime of primary
batteries. Typical energy harvesting (EH) techniques to capture environmental energies, suchas solar, vibrational, thermal, and radio frequency energies, into mesoscale devices have been*

Corresponding author: e-mail: yamamoto.koki@ebara.com
https://doi.org/10.18494/SAM.2020.2901
ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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Sensors and Materials, Vol. 32, No. 7 (2020)studied.(5) In conjunction with MEMS technology, miniaturized electrical elements can be used
to increase the harvesting power density. The vibrating energy harvester (VEH) structure iseasy to apply to MEMS because its mechanical and electrical components can be completely
processed by micromachining. The VEH system is classified into the transducer type, which
is piezoelectric,(6,7) electromagnetic,(8) or electrostatic.(9) The electrostatic VEH (ES-VEH)
has attracted much attention for its application to low-frequency vibration sources(10) and ischaracterized by its small volume and high power density compared with other transducers. We
have proposed a bipolar charging ES-VEH structure for industrial applications.(11) In this work,
the ES-VEH device is based on an electret material that can keep the surface potential within
itself.

2.

Electret Material Activation of MEMS ES-VEH Structure

ES-VEHs must retain the ions stored to drive itself. The harvester structure, which
combines a variable capacitor and an electrical energy source, can generate output power. Mostof the ES-VEHs are based on electrets,(12) whereas others generate electrical energy through anexternal power source.(13) We combine an electret-based ES structure with a simple structure
without an external driving circuit. To activate the electret material, charging is necessary.
The corona discharge method is one of the typical and simplest methods known to realize anelectret-based ES-VEH.(14) In this study, the corona discharge method is used to selectively
charge the bipolar charging structure. A typical MEMS ES-VEH structure often uses amonopolar charger that can only keep negative or positive ions inside.(15) This method is the
traditional way of providing an ES transducer in a component. However, owing to the pull-in
effect of both electrodes, the electrodes may suddenly become fixed owing to the electrostaticforce between them, which can prevent them from storing a large potential. The ES-VEH
structure is also necessary to increase the potential for a large output, which is proportionallysquared as a variable parameter. Bipolar charging reduces the electrostatic force in the presenceof an equivalent monopolar charging. The applied potential is set as a bias voltage at which thesurface potential can be determined.(16) To optimize the ES structure, it is important to estimate
the practical surface potential and capacitance. In this work, we focus on evaluating the surface
potential after assembly. Since the ES-VEH is already monolithically packaged, it is difficult
to directly measure the surface potential of bipolar charging after its assembly. Typically,
the surface potential is measured using a noncontact voltage potentiometer for monopolarcharging.(17) Its ideal operation for measuring the surface potential before assembly is shownin Fig. 1(a). The electrodes for monopolar charging are shown and consist of a potentiometerand a noncontact probe. This method can easily be applied to large-width patterns, and if theelectrode pattern is much smaller than the resolution of the probe, the potential is not correct, so
the measured voltage reaches 1/2 Vcp, which is approximately half the amplitude of the actual
surface potential. Figure 1(b) shows a schematic diagram of bipolar charging. The noncontact
method could not be applied to bipolar charging because the average voltage amplitude would
be zero in the bipolar ES-VEH owing to electrical symmetry, so the method does not include
any effects of charges. Thus, a novel strategy is proposed in this study; it combines an external
voltage source that can indicate the various amplitude responses of the harvesting voltage with

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(b)

Fig. 1. (Color online) Schematic diagram of surface potential measurement for electret charge model.(17)(a) Monopolar charge. (b) Bipolar charge.

the increase and decrease in electrical bias using voltage sources for measuring the potential.
This method can be applied to an assembled bipolar ES-VEH for evaluating the charge conditioneven when the device is operated in the harvesting mode.

3.

Bipolar Electret-based MEMS VEH Structure

Figure 2 shows a schematic diagram of the bipolar charging electret-based ES-VEH structure
and the equivalent circuit.(18,19) The cross section of the bipolar charging electret-based ESVEH is shown in Fig. 2(a). The simulation model consists of a supported substrate, baseelectrodes (BE1 and BE2), electrets, and variable capacitors. Four of the variable capacitors are
air-gap-dependent, Cn1, Cn2, Cp1, and Cp2, and can generate harvesting energy, and the parasitic
capacitor in the counter electrode is denoted as C12. The other parasitic capacitors between CEs
and BEs are written as Cpar_n and Cpar_ p. The positive and negative surface potentials are placedon the electret film on the base electrode, which is located about 1 µm from the top surface.(20)The capacitors of the electret material are denoted as Ccyt_n and Ccyt_ p. The counter electrodes(CE1 and CE2) are electrically isolated and can be driven separately. This structure generates
electrical energy supplied by two types of variable capacitance (Cn1,2 and Cp1,2) that depend on
the seismic mass and the relative displacement of the opposing electrode. Current is generated
through the CEs and BEs. The practical device consists of 50 pairs of comb structures. In the
structure, the load resistances are connected between the CEs and the ground so that the energy
is taken from two electrically separated wires.
Figure 2(b) shows a diagram illustrating the equivalent circuit of the harvester describedwith respect to Fig. 2(a). The surface potentials consisting of negative and positive biases forachieving the bipolar potential are rewritten as Vcn and Vcp, respectively.

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(a)

(b)

Fig. 2. (Color online) Bipolar charged electret-based ES-VEH structure: (a) cross-sectional drawing and (b)equivalent circuit.

4.

Fabrication of MEMS ES-VEH

The fabrication process that realizes the harvester structure using silicon MEMS is based ona previous work.(19) The prototype consists of three layers: a counter electrode, a transducer,
and a cover glass. The assembled prototype is shown in Fig. 3. The spring and transducer
components include a linear resonator based on an electret material (CYTOP CTL-809M,
AGC Corporation), a seismic mass, and a transducer. The CE components are based on lowresistance silicon wafers and aluminum electrodes connected to electrical load. As shown in
Fig. 3(a), the cover glass wafer is pyrex prepared by glass wet etching with hydrofluoric acidto fabricate a spacer about 30 μm higher than the center of the device. After fabrication, thechips are assembled with epoxy resin (KMPR1025, MicroChem) and heating. An overview of
the completed device is shown in Fig. 3(b). The overall size is 12 × 12 × 1.2 mm3. The surface
potential for activating the electret material is set to ±150 V. The device is connected to thesubstrate and receives electrical energy through the wire by using an ultrasonic wire bonder.
The electrical energy is output through a variable capacitor for load resistance. The mechanicalresonance frequency is chosen to be 300 Hz, but the practical resonance frequency has beenmeasured to be 313 Hz because of the fabrication accuracy. The electrode width is chosen to
be 75 µm, which was optimized in a previous work. For variable capacitor designs, the air
gap should be set to 30 µm. The practical variable capacitances Cn1, Cn2, Cp1, Cp2, and C12
measured after charging are 19.3, 28.2, 28.3, 27.6, and 53.1 pF, respectively, including initial
position fabrication errors and position errors. The capacitance Cn1 is shown to be smaller thanthe other capacitances, because it includes a small parasitic resistance. The capacitances of the
CYTOP material, Ccyt_n and Ccyt_ p, were estimated to be both 185 pF, with a thickness of 3 µm
and a relative permittivity of 2.1.

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(b)

Fig. 3. (Color online) Assembled MEMS ES-VEH structure: (a) individual components and (b) overview ofassembled structure.

5.

Typical Harvesting Experiments and Results

The main experiments involved driving and measuring the dynamic motion and electricalvibration of the harvested voltage using a combination of a laser Doppler vibrometer, a shaker,
an amplifier, and an accelerometer. The measured oscillation was captured using several
harvested voltages from CE1 and CE2. An overview of the experimental setup is shown in
Fig. 4(a). The experimental apparatus consisted of a laser to capture the dynamic motion anda shaker to generate the in-plane motion. For practical oscillation measurements, a pick-up
accelerometer was applied. The electrical oscillation was 0.2 mg p-p at 40 Hz, the electricalload R L was 1 MΩ, and the capacitance of the probe for the oscilloscope was 32 pF, as shownin Fig. 4(b). The measured relative motion was 50 µm and the voltage was 0.1 V. The harvested
voltage was small when the relative motion was large because R L was much smaller than the
optimum impedance. To evaluate the surface potential, the acceleration and frequency were
kept the same in the following experiments.

6.

Experimental Evaluation of Surface Potential by Proposed Method

After setting up the experimental apparatus, a bias voltage was applied to evaluate thenegative surface potential. The bias voltage was set at Vbs located at the bottom of Vcn, as
shown in Fig. 5. CE2 was connected to the ground to cancel its voltage. The load resistanceR L was connected to CE1 to measure its voltage. The average RMS calculated using themeasured voltage oscillation at the time domain is shown in Fig. 6. The lowest measured
voltage was around 130 V. It is assumed that the external bias was balanced to the potential of
the electret; thus, the measured voltage was smallest at small electrical biases for the variablecapacitors in ES-VEH. The voltage curve shows the shape of a valley. In contrast to thenegative bias response that indicates a large electrical bias of the capacitor, the voltage curve2498

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(a)

(b)

Fig. 4. (Color online) (a) Overview of experimental setup and (b) time-scale oscillations (acceleration,
displacement, and harvested voltage).

(a)

(b)

Fig. 5. (Color online) Experimental evaluation of surface potential: (a) schematic diagram of proposed method and (b)equivalent circuit.

Fig. 6.

(Color online) RMS voltages of harvested power.

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increases with the applied voltage. The responses to the applied external biases indicate thatthe biases controlled the electrical potential in the electret. At the smallest voltage captured,
the variable capacitors did not include sufficient voltage for harvesting. These results show thatthe proposed method can be used to evaluate the practical surface potential in the electret fromvariable harvested voltages with external biases.

7.

Discussion

Evaluation results of the surface potential are described above. When external biases wereapplied in the positive DC direction, the measured voltage curve showed a valley shape witha minimum point of around 130 V, which is smaller than the surface potential of 150 V. The
practical ES-VEH is evaluated after charging, which took a long time, so the actual voltage
decreased because the charged ions were excluded.

8.

Conclusions

In this study, the surface potential of the MEMS ES-VEH was evaluated by the biasing
method. The experimental technique was proposed and a complete prototype was fabricatedby silicon-MEMS processing. We conclude that it is possible to experimentally evaluate thesurface potential of bipolar charging by using an external voltage approach.

Acknowledgments
This work was partially supported by the Japan Society for the Promotion of Science (JSPS)
KAKENHI Grant-in-Aid for Scientific Research (C) Number 19K04533 for the scientific workin the University of Hyogo.

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About the Authors

		Koki Yamamoto received his B.E. and M.E. degrees from the University of
Hyogo, Japan, in 2014 and 2016, respectively. He received his Ph.D. degreefrom École doctorale SISEO of Université Grenoble Alpes, Grenoble, France,
in 2019. From 2016 to 2019, he worked as a Contrat doctoral in SYMME
Laboratory, Université Savoie Mont Blanc, Annecy, France. Since 2019, hehas been a researcher of the Mechanical Elements, Vibration and Acoustic
Section of Ebara Corporation, Kanagawa, Japan. His research interests are in
MEMS, energy harvesting, and nonlinear dynamical systems.
(yamamoto.koki@ebara.com)
Takayuki Fujita received his B.E., M.E., and Ph.D. degrees from the
Himeji Institute of Technology, Japan in 1995, 1997, and 2000, respectively.
From 2001 to 2007, he was a research associate at the Himeji Institute ofTechnology. From 2008 to 2013, he was the Power group leader of theMaenaka Human-Sensing Fusion Project supported by the Japan Science andTechnology Agency. From 2013 to 2014, he worked as a visiting researcher
at the IMEC Holst Centre, the Netherlands. He is currently an associate
professor in the Department of Electronics and Computer Science, Universityof Hyogo. His research interests are in MEMS and vibration energy
harvesters. (fujita@eng.u-hyogo.ac.jp)

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Adrien Badel received his M.S. degree in electrical engineering from
Institute National des Sciences Appliquées de Lyon (INSA), Lyon, France
in 2002, and his Ph.D. degree in 2005 while working at the ElectricalEngineering and Ferroelectricity Laboratory of INSA Lyon. He received his
Ph.D. degree on vibration control and energy harvesting. From November
2005 to November 2007, he was a Japan Society for the Promotion of
Science (JSPS) postdoctoral fellow at the Institute of Fluid Science of Tohoku
University, Sendai, Japan. He is now a professor at the Laboratory of Systems
and Materials for Mechatronics of the Université Savoie Mont Blanc, Annecy,
France. His research interests include energy harvesting, vibration damping,and piezoelectric actuator modeling and control. (adrien.badel@univ-smb.fr)
Fabien Formosa received his M.S. degree from the École Normale Supérieurede Cachan, Cachan, France, in 1999, and his Ph.D. degree from Université
Paris VI in 2002. He is currently an associate professor at Université Savoie
Mont Blanc, Annecy, France. His expertise is on the design of miniaturesystems for energy conversion, such as microheat engines (microStirling)
and vibration energy harvesting. His activities range from the analyticalmodeling of multiphysics systems to the technological integration of smallscale devices. (fabien.formosa@univ-smb.fr)

		Kensuke Kanda received his B.E., M.E., and Ph.D. degrees in mechanicalengineering from Tokyo Metropolitan University, Japan in 2000, 2003, and2006, respectively. From 2006 to 2008, he worked for the Advanced Software
Technology and Mechatronics Research Institute of Kyoto, Japan. From 2008
to 2010, he worked for the Maenaka Human-Sensing Fusion Project of JST.
Since 2010, he has been an assistant professor at the University of Hyogo.
His research interests are in microfabrication technology, piezoelectric
microdevices (piezo-MEMS), power harvesters, and their integration.
(kanda@eng.u-hyogo.ac.jp)
Kazusuke Maenaka received his B.E., M.E., and Ph.D. degrees from
Toyohashi University of Technology, Japan, in 1982, 1984, and 1990,
respectively. Since 1993, he has been with the Department of Electronics
of Himeji Institute of Technology. After the unification of the universities
in Hyogo Prefecture in April 2004, he joined the University of Hyogo asa professor. From 2008 to 2013, he was the project leader of the Maenaka
Human-Sensing Fusion Project supported by the Japan Science andTechnology Agency. His research interests are in MEMS devices andtechnology, particularly silicon mechanical sensors, and their integration.
(maenaka@eng.u-hyogo.ac.jp)