In situ wafer-level polarization of electret films in MEMS acoustic sensor arrays

MEMS-based electret transduction has been reported for both acoustic sensors and energy harvesters. Common techniques employed in MEMS electret formation include corona discharge and backlighted thyratron. This paper reports a method for post-release in situ polarization of electret films within a MEMS device. The method utilizes microplasma discharges with self-aligned charging grids integrated within the device to charge fluoropolymer films in a fashion similar to the corona discharge technique. This in situ approach enables the integration of uncharged electret films into MEMS and subsequent post-fabrication charging, simultaneously enabling the formation of buried or encapsulated electrets as well as eliminating the need to restrict fabrication processes that might otherwise discharge pre-charged electret materials. The method is applied to a single-chip array of acoustic sensors designed to capture and analyze waveforms from impacts.