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Mastering Semiconductor Doping Technology
Extending Its Applications to Life Sciences

Takahiro Shinada
Associate Professor, Waseda Institute for Advanced Study

Single-Ion Implantation Method That Supports the Nano-Age

My specialization is in semiconductor integrated circuit technology that is built into computers and smart-phones, particularly technology known as impurity doping that utilizes ion implantation methods. Silicon is a core material of semiconductor integrated circuits, and although electric current doesn't actually flow through it in its pure state, , and by intentionally implanting impurity atoms (dopants) such as phosphorus at a proportion of 1 silicon atom per 10 million, it is possible to control the ease of the current flow. With doping technology, semiconductors are, for the first time, able to provide transistors with an on-off switching function.

It was Professor Emeritus Iwao Ohdomari of Waseda University who devised the "Single-ion implantation method" (Diagram 1), that will lead the nanotech age, for achiving precise doping controls required in next generation devices. From about the time I joined his laboratory, I have participated in the development of this technology, and until now, aiming for the realization of single-ion implantation methods and their refinement, have been engaged in developmental research at the cutting-edge of doping technology.

Diagram 1. Single-ion implantation method: a method that Waseda University leads the world in developing for chopping an ion-beam cross-directionally at high velocity (ion-beam traverse time of only 20 nanoseconds), and implanting individual ions through micro-aperture.

Within advancements of miniaturization of transistors, what has particularly become an issue is technology that delicately controls the quantity and location of dopants. Anticipating low power consumption through miniaturization, as well as including the numerous applications within confined spaces, possibilities for semiconductor devices are extremely far reaching, however, their implementation demands delicate control of electric resistance in miniscule spaces.

The largest problem facing us is fluctuation of the number of ions to be implanted. In case of the device that highly integrates the extremely minute elements whose dimensions are less than 30 nanometers, electric resistance are strongly affected by each individual atom. In order to avoid this problem, a proposal has been to push forward technological innovation and apply new technology that does not use doping but which uses new semiconductor materials. Moving toward realizing these next generation devices means technology from now will engage in fierce competition, and until their implementation, each method faces a mountain of difficult issues (Diagram 2). Free of fluctuation in the number of ions and the idea of completely controlling the quantity, namely the single-ion implantation method, is increasing in importance.

Diagram 2 Developmental trend of advanced devices

Observation of Quantum Mechanical Behavior of Single Dopants

Diagram 3 Observation of quantum transport phenomenon through single dopants (Shinada, IEDM 2011): Engineered reproduction of the phenomenon at discrete levels (Coulomb potential) that reveal two peaks for each dopant. Through success of this world's first proven experiment, we opened the road to the development of quantum devices through dopants.
*Through research assistance from the Semiconductor Research Corporation, USA, MEXT Research Fund Foundation (S), innovative research (A), and Italian Ministry of Foreign Affairs' Japan Italy Executive Program

Within this trend, several notable results appeared. The first result was the realization of the single-ion method, an essential issue for breakthroughs toward next generation devices. Since 1993, with Professor Emeritus Iwao Ohdomari and other seniors and juniors in the laboratory, and through an accumulation of trials and errors, we have been successful in controlling and implantation of individual dopant atoms. By around the year 2000, this technology had almost been perfected.

The second result was that we were able to implement a semiconductor that ordered arranged dopants. Further increasing the accuracy of the position, we developed semiconductors that implanted ions one by one at decided intervals of 100 nano meters. Compared with random positioning of the past, we demonstrated that we could stably control electrical resistance of ions one by one through exceedingly low voltage. This result was presented in the magazine Nature in 2005, drawing worldwide attention.

The third result was from research on increasing the accuracy of quantity and position and arranging dopants into single lines, which has only just been presented at an authoritative international conference (International Electron Device Meeting: IEDM) in December, 2011. We were successful in observing quantum mechanical behavior that controls the characteristics of each individual dopant. Although until now we have observed this accidentally, we were, for the first time in the world, able to make this phenomenon manifest with engineering control (Diagram 3). Through this, it can be said that the increased efficiency of semiconductor devices has opened the road to treating dopants as quantum computing devices.

One section of these research achievements progressed as contracted research with an American company. There would not be many examples of young Japanese researchers forming industry-academic collaboration with an overseas industry. Since 2000, in the field of semiconductors, globalized industry and academic collaborative research has become very popular. To participate, research capability and proactively presenting at international conferences and in journals is necessary. As you would expect, being published in a worldwide magazine such as Nature and actively presenting at conferences overseas, in my experience, are all tied in with international industry-academic collaboration.

Success in Doping Tests on Live Cells

For five years after 2004, as a strategic research center of Waseda University, and taking the opportunity of being assigned to the Consolidated Research Institute for Advanced Science and Medical Care, I also worked on applications for Life Sciences and related fields.

When speaking of semiconductor technology application to and Life Sciences, the first thing that comes to mind is applications for miniaturized technology toward biochips, however this will come later. Because it was our mission to attempt something that no one else has tried in different fields, taking a straight-on approach, I thought about trying to introduce doping technology into the Life Sciences.

Diagram 4 Survey of doping effects on living cells.

Together with researchers in Medicine and the Life Sciences, we managed through trial and error to gather some ideas while searching for needs in these fields. In one of the fundamental tools of cell biology, there is an experimental method for implanting substances in cells by micropipette known as the microinjection method, and it occurred to that we should perhaps apply this method (Diagram 4). It took almost two years before we could earnestly embark on experimentation, but once we began, something that surprised us was how well we were able to change the nature of cells. For example, when we implanted metal ions into myoblast cells, which will become future muscle tissue, we learned that energy that produces living cells (adenosine triphosphate) increases by approximately 20%. On the other hand, when arsenic is implanted the energy decreased.

Focusing on applications for drug development, we also tried tests on cancer cells. We then carried out comparative experiments by implanting silicon, arsenic and metals into both regular cells and irregular cells. Our hypothesis was silicon = innocuous, arsenic = toxicity, metals = benign. By implanting arsenic, we were able to confirm a decrease in cancer cells by 10 to 20%, and understood the tendency of cancer cells being even stronger against toxicity than regular cells.

During the testing, we wondered if this technology wouldn't also be useful in the field of semiconductors. This is because we are entering an age in that we are now unable to manufacture without using not only silicon, but most of the chemical elements that appear in the periodic table. The problem with this is safety toward the human body. By applying a safety evaluation to each material used in cell doping, we anticipate being able to contribute to environmental preservation in the field of nanoelectronics.

Leading the next-generation doping technology

Photo 1 Deterministic Doping Workshop (November, 2010, Berkley, California, USA) planned and organized by Associate Professor Shinada. 31 researchers from Japan, USA, Europe, Taiwan, Australia assembled.

A distinctive feature of the semiconductor industry is that the whole business world is sharing in the drawing up as a roadmap a vision for the next and following generations of future technology. Looking carefully over the roadmap, we can see the stages before entering the open market, referred to as precompetitive joint research, are unfolding all over the world. I myself am a Secretary and committee member of Japan's working group of this international road map. Also, from 2009, I have been carrying out surveys and analysis, and making proposals on research development trends as a Specially Appointed Fellow of the Center for Research and Development Strategy, Japan Science and Technology Agency (JST) and as a specialist in the field of nanotechnology materials.

In November, 2010, I proposed a new concept called "Deterministic Doping" to the International Technology Roadmap for Semiconductors (ITRS), and chaired a workshop at Berkley, California, USA. When I attempted to compile device development related technology, researchers related to this field from around the world assembled and discussed it and collected ideas on device processes investigated to the utmost limit of dopant controls (Photo 1). The report from this workshop will be published on-line in January 2012.

In tackling my research, while referring to the road map so as not to become smug in my research, I think of it as my mission as a university researcher to address challenges, which companies would not be able to do. While I am not sure whether I can explain clearly about my visions for the future, I aspire at all times to tackle any original ideas that may arise.

Photo 2. Associate Professor Shinada is continuously involved in outreach activities to impart the interest and fun of science and technology to elementary and junior high school students (To the left is at Waseda University in 2010, to the right is a lecture scene at his home school that was printed in the Hokkaido Newspaper in 2008)

Takahiro Shinada
Associate Professor, Waseda Institute for Advanced Study

Born in Kushiro City, Hokkaido. Graduated from the Doctoral Program of the Department of Electronics, Information andCommunication, Graduate School Science and Engineering, Waseda University. Earned a Ph.D in Engineering in March 2000.
Graduated from the Master's Program of the Department of Technology Management, Waseda Business School, Waseda University. Earned his MBA degree in Technology Management in September 2007. After serving as an assistant professor in the Department of Science and Engineering at Waseda University in 2000, a lecturer in the Waseda University Biomedical Engineering Institute in 2004 and an associate professor in the same Institute in 2006, he got his present post in 2009. He specializes in Semiconductor Engineering.
Specially Appointed Fellow at the Nanotechnology and Materials Unit, Center for Research and Development Strategy, Japan Science and Technology Agency (JST) (from 2009)
Secretary for the Semiconductor Technology Roadmap Committee of Japan (STRJ) WG12 (Emerging Research Devices: ERD) (from 2010) and Committee Member for the WG13(Emerging Research Materials: ERM)(from 2010)
Secretary for the 165th Committee on Ultra Integrated Silicon Systems, Japan Society for the Promotion of Science (JSPS) (from 2011)