By M. David Levenson
WaferNews Technical Editor
The International Microprocesses and Nanotechnology Conference is Asia’s premiere academic technology symposium, and this year’s version didn’t disappoint, with leading edge topics such as the limitations of CMOS, implantable electronics, DNA electronics, and supporting R&D efforts in nanotechnology headlining the sessions and papers.
Power dissipation and the limitations of CMOS dominated the plenary talk given by Prof. Tadahiro Kuroda of Keio U. Kuroda pointed out that CMOS replaced bipolar technology 20 years ago because high performance bipolar chips produced an untenable amount of heat. Today CMOS is in a similar situation, with some chips dissipating more heat per square centimeter than a hot plate does. However, there is no obvious low-power replacement technology coming on line. Thus one must adopt CMOS design methodologies to reduce power consumption and try to stretch the lifetime of CMOS for another six to 20 years. Kuroda advocated three power-reducing strategies: lowered voltage, reduced capacitance, and reduced switching activity. Variable threshold voltage CMOS technology (VTCMOS) can reduce power consumption at constant performance using multiple supply voltages and on-chip DC-DC converters. Embedded DRAM reduces capacitance by eliminating parasitics and drivers, and conditional flip-flop logic (where the internal clock is activated only when the function is needed) can reduce switching activity. Taken together, these innovations promise to reduce power consumption by 50 to 90%, extending the lifetime of CMOS by up to 20 years, according to Kuroda.
Prof. Wentai Lai of North Carolina State U. surveyed the opportunities and difficulties for developing electronic prostheses to correct blindness and other disabilities. The cochlear implant has been a tremendous success in curing deafness, but vision is more complex and delicate. He proposed an intra-ocular retinal prosthesis, with only the retinal transducer inside the eyeball and everything else outside communicating with it via wireless technology. Such a system minimized the power dissipation issues. which would be very significant inside a living eyeball. A low resolution prototype with a 30 x 30 array of electrodes was reported to be complete. The “vision” produced might be crude, but useful once the brain learns how to interpret the sensations it receives. Solving bio-engineering problems requires a multidisciplinary team, according to Lai, and that carries with it coordination and dominance challenges among the doctors, scientists, and engineers.
Carbon nanotubes have inspired nanotechnologists, but in spite of their evident promise, have been little used in silicon microelectronics. Dr. Wolfgang Hoenlein of Infineon Technologies AG, Munich, Germany, described their potential for conducting electricity and heat in ICs. Infineon has found a catalytic method of growing aligned nanotubes on a substrate to form interconnects and via conductors. Only contact resistance matters for metallic nanotubes; electrons move along them without scattering even from surfaces. Thus bundles of nanotubes can have lower resistance than copper for sub-100nm damascene interconnects if the density is sufficient. Other more visionary opportunities include silicon-free 3-dimensional ICs fabricated using metallic and semiconducting nanotubes.
Beyond nanotubes, nanotechnologists envision entire new “molectronics” technologies based on DNA and other biomimetic systems, fabricated by imprinting and self-assembly and which completely avoid the brick wall obstructing semiconductor progress in 2015. C.R.K. Marrian of the US DARPA outlined the National Nanotechnology Initiative (NNI) and its initial successes, perhaps in hopes of inspiring parallel efforts in Japan and other nations. The NNI has funneled most of its resources to academics like those attending the IMNC meeting, but part of the increase has resulted from diversion of funds from previously approved programs, according to Marrian.
Prof. Tomoji Kawai of Osaka U. explained some of the reasons for frustration among those working towards DNA electronics. While DNA is described as a “molecule” its electronic properties depend critically on the sequence of bases along the famous double-helix strands as well as the 3-dimensional configuration of the chain. Changing the helix pitch – which can happen easily when DNA lies on a surface or gets probed by an STM tip – alters the electronic behavior radically. The inconsistent results in the literature partly reflect actual differences among the individual molecules probed. In general, DNA is a wide-gap semiconductor, according to Kawai, with poly(G)- poly(C) dual chains being p type and poly(A)- poly(T) being n-type. Everything else is more complicated but perhaps more promising. In particular, 50-unit strands of single-chain DNA can be made to braid together into two-dimensional networks of 3-terminal nodes, potentially useful as neural-nets.
On a somewhat larger scale, Prof. K Hirao of Kyoto U. described how femtosecond pulse lasers can be used for debris-free machining and materials modification. Even stainless steel can be drilled cleanly by breaking the molecular bonds with short laser pulses! A femtosecond beam focused into conventional fused silica can precipitate the high-density amorphous phase, creating an embedded waveguide with complex 3-D geometry.
Other sessions addressed such seemingly esoteric topics as MEMS, microfluidics, photonic devices, nanoimprint lithography, metrology, and nanodevices. However, today’s academic topic becomes tomorrow’s leading-edge technology as soon as the students graduate and form their own start-ups. The vitality of the IMNC meeting demonstrated that neither persistent recession nor terrorism can stop innovation so long as brilliant people are willing to enter our field and pursue new useful knowledge.