RF MEMS seminar at Tohoku University

- Summary -

Global Emerging Technology Institute

A seminar on the current status and future of RF MEMS technology was held at Tohoku University on November 22, 2002. The seminar was organized by Tohoku University Venture Business Laboratory and was the 28th event in the "Micro & Nanomachining Seminar" series. The seminar featured a comprehensive overview of RF MEMS switches for use in phase shifters by Prof. Gabriel Rebeiz of the University of Michigan and a discussion on RF micro magnetic devices by Prof. Masahiro Yamaguchi of Tohoku University. Their talks were followed by two shorter presentations on a bimorph RF MEMS switch and a 2GHz resonator.

Dr. Rebeiz started his talk by stating that RF MEMS development has been done mostly by microwave engineers, not electrical or mechanical engineers. Typical MEMS devices are made of polysilicon, while RF MEMS use aluminum and gold that have the stress and dielectric characteristics very different from those of polysilicon. The process for fabricating RF MEMS is "low technology" compared to the processes used for more complex MEMS structures. However, designing microwave circuits is just like an art, requiring many years of experience, mastery of dedicated design tools, and the understanding of application system requirements ranging from electronic warfare systems, automotive radars, satellite and wireless communications and instrumentation. The development of RF MEMS switches has accelerated considerably over the past several years, and currently there are several switches which have been tested to 50-100 billion cycles with no failures. The US DARPA has supported phase shifter development since the mid-1990s. As a result, RF MEMS switch R&D is, in his view, 90% complete. However, it is still hard to package these switches, and fundamental questions regarding the need of a hermetic package and the failure modes of RF MEMS switches under high power conditions are not well understood. Regarding the commercialization of RF MEMS devices for use in cellular handsets, he told that the cost would be a major bottleneck because the cellular handset is a very price-sensitive application unlike the radar systems for defense applications.

Dr. Yamaguchi presented his work on sandwich-type ferromagnetic RF integrated spiral inductor for the 2GHz range. The inductor showed better characteristics (19% higher inductance and 23% higher Q factor) compared to an air core of the same coil size. Dr. Kiyoto Nakamura of Advantest Laboratories Ltd. presented an outline of the development of a bimorph RF MEMS switch for LSI test system applications. Preliminary data show that the RF MEMS switch offers better RF performance compared with mechanical relays and photo-MOS relays. The company will commercialize a chip-size packaged (CSP) bimorph RF MEMS switch in the 15GHz range for use in test systems. Finally, M. Hara of Tohoku University presented work on aluminum nitride (AlN) thin film 2GHz resonator using germanium (Ge) sacrificial layer etching. The fabricated resonator achieved a Q factor of 780. The use of AlN and Ge makes the fabrication process compatible with CMOS process and may prove effective in the development of CMOS chip integrated resonators for wireless communication and sensing systems.