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Interview with Professor Gen Hashiguchi (Shizuoka University)

Jan 1, 2012

Prof_Hashiguchi.bmp (SAMCO Deep Reactive Ion Etching System, RIE-800iPB and Reactive Ion Etching System, RIE-10NR User)

2007 - Present    Professor, Research Institute of Electronics
Shizuoka University
2005 - 2007Professor, Faculty of Engineering
Kagawa University
1999 - 2005Associate Professor, Faculty of Engineering
Kagawa University
1998 - 1999Researcher, Japan Science and Technology Agency
1996Ph. D., Tokyo University
1988 - 1998Nippon Steel Corporation
1988MS of Electrical Engineering
Chuo University
1986BS of Electrical Engineering
Chuo University

For this issue of SAMCO Interview we visited Professor Hashiguchi at the Research Institute of Electronics Laboratory, Shizuoka University, to learn about his research in the field of MEMS (Micro Electro Mechanical System) technology.

Could you tell us about your research?

     I study the MEMS (Micro Electro Mechanical System) produced through the use of the semiconductor microscopic processing technique. My particular field is the development of functional microscopic probes and physical sensors, which are beneficial to research in nanotechnology. More specifically, my research pertains to devices that are often referred to as "MEMS using electret (electrically charged) silicon (Si)," although that expression might not be sufficiently clear. Each of these devices contains a self-generating sensor having an ordinary electrostatic actuator with an electret film formed on it, as well as a device used for environmental-vibration power generation, the latter of which has drawn considerable attention lately. The procedure for forming an electret is very simple. First, an aqueous solution of potassium hydroxide (KOH), which is used for the wet etching of Si, is bubbled with nitrogen (N2) to create water vapor containing KOH and then Si is exposed to KOH in the water vapor and oxidized. If the oxide film contains alkali ions, MEMS will operate in the electrical field created by the ions. Alkali ions are impurities that must be avoided in the semiconductor manufacturing process, since the presence of alkali ions results in an electrically charged MEMS. I published a thesis illustrating this effect. The electrically charged MEMS generates approximately 40 volts, so it's useful in the creation of various devices.
     Another subject of my research is a national project called Bio-Electromechanical Autonomous Nano Systems, or simply, BEANS. It pertains to a next-generation inter-field amalgam device that functions autonomously as a combination of the conventional MEMS and nano-biological techniques. My research in this area concerns the device by which transistors are integrated on an electrostatic MEMS. Mounting a transistor on the side wall of the MEMS allows us to detect the displacement of the MEMS directly. Ultimately, this research will lead to electronic elements called, "Surface Acoustic Wave (SAW)" or the elements used to extract surface acoustic waves or electric signals within a given frequency band. This research includes physical sensors such as acceleration sensors and gyroscopes as well as SAW filters.

What opened the way to your research on MEMS? What's the story behind that?

     I studied crystal growth as part of my Master's degree program, and later I joined Nippon Steel Corporation. At that time, Japanese steel companies were entering the semiconductor industry as a result of diversification, and I was assigned to Nippon Steel's sensor department. My boss directed me to investigate the field of semiconductor micromachining to develop new market. This investigation opened the way to research on MEMS. First, I visited Dr. Esashi at Tohoku University and received instruction from him. Later, I earned a doctorate under the tutelage of Dr. Fujita at the University of Tokyo.
     Initially, my research concerned the fabrication process. Since then, the focus of my research has changed somewhat, given the fact that I also study the theoretical aspects of MEMS technology. When I first moved to Shizuoka University, I started to study the electret elements and integration of transistors on MEMS, and continue to conduct my current research on the same theme. It would be a while before I could accrue experience in the fabrication process, so in the meantime I studied theory of MEMS from scratch. The understanding of analytical dynamics and the theory of stochastic processes gave me the chance to see the essence of MEMS in a totally new light. Gradually, I gained an understanding of MEMS in terms of analytical dynamics. The theory of stochastic processes also benefited my understanding of noise. I joined a national project devoted to the MEMS equivalent of circuit generators, which is the design platform that could enhance the design efficiency of CMOS-MEMS. The equivalent circuit generator allows a design engineer to simulate the circuitry of the entire system, including detection circuits and drive circuits. This characteristic OR phenomenon is based on the equivalent circuit's modeling of MEMS devices. As I tackled that research, I became aware of the essence of MEMS, thanks to my theoretical studies, even though I hadn't known much about it during the time I was fabricating MEMS. As a result, I found that the electrical field used for energy conversion was vital to the electrostatic MEMS. In the modeling process, the Si used in MEMS is handled as if it were a metal. Unlike metals, though, Si as a semiconductor has an electrical field for energy conversion, and the electrical field exists within the Si. The fact that Si is a semiconductor hadn't been considered in the previous models. In a thesis, I published a theory that gave consideration to this fact. It inspired me to make calculations of how deeply the electrical field penetrates Si, and as a result I found that a transistor is created when the gap is narrowed. This research had already started in Europe in 2007. We successfully modeled Si transistors, applying the semiconductor theory mentioned above. Now we are striving to create devices based on the calculated characteristics of transistors according to our theoretical model. That's how I began to study devices in which transistors are integrated on an electrostatic MEMS.
     I started my research on MEMS as an electret when I was about to view, theoretically, the damage that occurs on the etching interface of the MEMS as a semiconductor. The question had to do with how much damage would be done to the etching interface, for example in Si deep etching in the Bosch process by a SAMCO system. In fact, while such damage usually isn't a problem in a general MEMS, an interface is, theoretically considered to include large numbers of defects that trap electrons. I study what that phenomenon or problem will incur and in the process I will refer to previous semiconductor-related documents. In the past I had found that the defects on an interface have impurities, which, in fact, include alkali ions. According to these documents, alkali ions generate an electrical field that's harmful to the transistor. Consequently, my idea was to feed a large volume of alkali ions into the MEMS, and that idea led to the current study. When I fed a huge volume of alkali ions into the MEMS, I obtained an interesting result. So, all this bit of background supports the theme of my current research.

What points do you keep in mind as you go about your work?

     MEMS must be commercialized, but something that lacks theory can't be commercialized. While know-how alone might be sufficient for commercialization, it's necessary for us to achieve the proper modeling of our goal and the identification of its characteristics. In other words, the understanding obtained through experiments is modeled so that we can understand the phenomenon. That might be a fundamental principle of research, but it's very important to my work at Shizuoka University, and I make it a rule to create a model.

How do you use the SAMCO equipment in your work?

     I study the conventional electrostatic MEMS, so the deep etching equipment for Si--the "RIE-800iPB"--is a key piece of manufacturing equipment. Regrettably, research on comb-drive actuators became obsolete and hasn't been pursued for the past few years. However, we're striving to create new devices and products on the basis of deep etching equipment, using the SAMCO RIE-800iPB system. We often use reactive ion etching equipment, the SAMCO RIE-10NR, which is easy to operate, for the etching of SiN (silicon nitride film) or SiO2 (silicon dioxide film).

What are the prospects for your research?

     The MEMS, as a semiconductor, has been fully commercialized in acceleration sensors and physical sensors. However, the Japanese camp seems to have been defeated by foreign camps. While we might remain competitive in the field of microphones, the foreign camp is becoming more dominant. My idea is to create a MEMS that intrinsically includes an electrical field for energy conversion by using the electrostatic MEMS as an electret. This will lead to a product in a category that hasn't previously been commercialized. It would further increase the need for MEMS semiconductors. While the biological MEMS is an area rich in promise, with great interest and various challenges, our semiconductor MEMS is also something to be exploited. I know that only mass-produced devices can create a new industry, but I believe that MEMS as an electret or MEMS, including the integration of MEMS and transistors, will someday be used as part of an electronic component instead of merely being part of a sensor.

Do you have any hobbies?

     Yes, I have a lot of hobbies. I've always played baseball, and I love the game. Before I moved to Shizuoka University, I was the manager of the baseball team at Kagawa University. I've always been a Yomiuri Giants fan, but my friend Wada, who will manage the Hanshin Tigers next season, was my senior in the baseball club in high school. So, I'll be a temporary Hanshin Tigers fan while he manages the Tigers. I love programming, too. On my days off, I enjoy creating prediction software for horse racing, by statistically analyzing downloaded horse-racing data from the JRA (Japan Racing Association) website.

Please offer a brief comment for everyone at SAMCO.

     SAMCO's engineers are very earnest in regard to the technology. They're friendly and sociable too, so we always feel comfortable consulting SAMCO. That approach to business is what's needed most of all.

Thank you very much for sharing your time with us!

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