Resonant frequency method for monitoring MEMS fabrication

Abstract

MEMS surface-micromachining fabrication requires the use of many different tools to deposit thin-films, precisely define patterns using typical photolithography, and perform etching processes. As with any fabrication process there is inherent variation, which is acceptable when controlled within suitable limits. The ability to monitor and respond to this variation is paramount in maintaining a viable fabrication process. Electrostatic comb-drive resonators are candidate test structures used to validate uniformity in the MEMS fabrication process. Although directly dependent on mass and spring constant, a measure of their resonant frequencies generally provides a good indicator of both process repeatability and geometric variation. In this study, sets of five graduated comb-drive resonator structures, located at each die on a ¼ wafer, were stimulated to resonant frequency using the “blur envelope” technique. This technique facilitates fast, straightforward, and repeatable resonant frequency measurements usually with a resolution of approximately 50-100 Hz. Wafer maps of resonant frequency versus die position for a ¼ wafer reveal a pattern with comb-drive resonator devices exhibiting highest resonant frequencies at the center and lowest at the perimeter of the wafer. Using a numerical model, coupled with discrete geometric measurements, a method was developed which links resonant frequency to fabrication parameters.