From MEMS to Vacuum Microsystems

Silicon micro electromechanical systems (MEMS) is a well-established offshoot of the semiconductor industry, after a lengthy incubation period. To provide a frame of reference for what can be done in today's commercial foundries, I will review MEMS microphones, gyroscopes, and resonators for timing applications.

MEMS research has developed a variety of materials and processes for the fabrication of wafer-scale vacuum encapsulated devices. In addition, high-temperature materials such as silicon carbide have been integrated into micromachining processes. These developments provide a technology platform for vacuum microsystems, which are being explored in my group and others at Stanford. The first application is to achieve efficient sources at THz frequencies, by employing a microfabricated Barkhausen-Kurz oscillator architecture. A heated poly-SiC cathode is used to inject a sheet electron beam into the cavity. Thermionic energy converters operate by the collection of thermally emitted electrons from a heated cathode onto a cooler anode. Demonstrated by NASA and the Soviet space programs in the 1970s-1980s using vacuum-tube technology, these devices are also suitable for wafer-scale, MEMS-based materials and processes. Last year, researchers at Stanford and SLAC demonstrated a new version of this converter, in which a semiconducting photocathode was introduced. The photon-enhanced thermionic energy converter (PETE) appears highly promising, especially as a topping cycle in solar-thermal converters.

Finally, trends in nanotechnology research have motivated the construction of new, shared facilities at Stanford, as well as the renovation and expansion of the Stanford Nanofabrication Facility. I will briefly overview these developments.

Roger T. Howe is the William E. Ayer Professor of Engineering at Stanford and the Faculty Director of the Stanford Nanofabrication Facility. He received a B.S. degree in physics from Harvey Mudd College and the M.S. and Ph.D. in electrical engineering from Berkeley in 1981 and 1984. After teaching at CMU and MIT from 1984-1987, he taught at Berkeley until 2005. He was elected an IEEE Fellow in 1996 and to the National Academy of Engineering in 2005 for his contributions to MEMS. He co-founded Silicon Clocks in 2004, a start-up focused on MEMS resonators for timing applications, which was acquired by Silicon Labs in 2010.