ColumnMerger of Integrated Circuits and MEMS
- At the Toyohashi University of Technology -Makoto Ishida, Toyohashi University of Technology
Although sensor and MEMS technology are derived from
integrated circuit (IC) technology, it is not a simple matter tofabricate sensors or MEMS and ICs on the same chip. Because
the microfabrication of ICs has developed to the nm level, thebarriers to this kind of merger are high. This is because ICs
involve ultra high precision engineering, while sensors andMEMS involve precision engineering. Ultraprecision ICs
basically are not amenable to further processing besides themanufacturing process. On the other hand, the requirements ofsensors and MEMS include materials, structures, andfabrication processes that enable a range of further processing.
Fulfilling these contradictory requirements is the key tomerging the two. One method is to fabricate the sensors andMEMS after fabricating the IC. Another is to constrain thefabrication of the sensors and MEMS to the extent possible withIC processes. However, it is difficult to fabricate sensors andMEMS with the fullest capability, so compromises must bemade somewhere. We faced the same conflicts at Toyohashi
University in merging sensors and MEMS with integrated
circuits, but we worked to ensure that both were satisfactory as
far as possible. Here I will explain how we built a research anddevelopment environment at the University with the goal ofmerging integrated circuits with sensors and MEMS, and I willoffer an example of smart chip development.
Twenty-six years ago now, there were very few universities
in Japan that were actually producing integrated circuits fromthe design stage, while seriously pursuing teaching andresearch. The University of Technology was established inToyohashi as the 88th national university, and the current
President Tatau Nishinaga, the late Tetsuro Nakamura (fromNEC), and Yukio Yasuda currently at Kochi University ofTechnology (from Toshiba) took the lead in starting unified
semiconductor-related courses with the aim of building anenvironment where serious teaching and research, andfabrication of integrated circuits could be pursued in one place.
The main equipment at first was a 2-inch line from the NEC
Tamagawa Factory, and in the autumn of 1979, as the 4th year
laboratory experiment (a 3-month experiment called the bigexperiment), students succeeded in fabricating a simple npn
bipolar transistor, whereupon a great cheer went up in theclean room. This was the start of it all. The radio produced fromthese six transistors is still running in the VBL exhibition room.
Roughly speaking, if you can make a basic transistor, you canmake however complicated an IC. Then we received a requestfrom Genyo Mitarai of the Research Institute of Environmental
Medicine in Nagoya to develop an EEG IC chip for thecerebellum of carp for exploring space sickness in a spaceexperiment, for which we pursued development as a Master's
research theme. This chip was used in experiments on theEndeavor in 1992, when Mamoru Mohri went into space andwhen this chip first left the earth.
In the meantime, up to this year, a course with the samecontent as the big experiment has been held 25 times (25years) for 1 week continuously during the summer holiday.
(More than 400 people have taken part.) All the teaching staff
participated and it was a tough business getting through the hotsummer with the students, but besides maintaining theequipment in operation, it is an excellent opportunity forchecking the machinery (since one member of the staff is anengineering official).
Currently a 4-inch CMOS, 1 micron process is possible usinga stepper. In 1994, the department moved to the Electron
Device Research Center from the original building where it wasestablished, and in 2002 it was incorporated in the VBL(Venture Business Laboratory) facilities. A clean room wheremicro and nano processing is possible was set up on the firstfloor of the VBL, and MEMS processing was enhanced. We havefabricated many kinds of smart chips that merge sensors andMEMS with integrated circuits, but MEMS manufacture
requires special process other than the CMOS process, so it hasoften become necessary to use the equipment separately. Howto organize and manage this is a difficult matter, and success
rests upon being able to adjust for the interests of both. It isnecessary to have a relationship where both can allow
compromise as far as possible. It is also necessary to design
hitherto unknown processes.

Si microprobe
electrode array
2 µm
60 µm

100 µmOn-chip signal processors

SourceGateDrain

50 µm

Figure 1 Conceptual diagram of a nerve potential
sensor integrating a miniature electrode arrayand signal processing circuit, and electron
microscope photograph of a prototype chip
My proposal for a nerve potential measuring smart chip witha silicon microprobe formed by crystal growth on an IC chipshown in Figure 1 is clearly one such example. This chip which
was at first thought to be completely impossible probably met
with many objections in the laboratory too. That's because if
you give a reason why something cannot be done, any numberof others can also be found. With this background, separate
groups worked on the crystal growth and integrated circuit
processes so that a system was achieved for assessing theproblems and limitations of each process, and for pursuingsolutions by pooling the knowledge of each group. As a result
we were able to develop the chip to a state where it detected asignal from living cells which had been our ambition at the startof last year. 1) In this way we have proposed or prototyped manysmart microchips that incorporate sensors and MEMS with ICs,but in order to raise the level of these chips further, we areproceeding through discussion of the problems involved inmerging ICs with sensors and MEMS.
1) T. Kawano et.al “Three-dimensional multichannel Si
microprobe electrode array chip for analysis of the nervoussystem”, IEDM 2004, pp.1013-10167