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

Polymer electrets, polymers with a quasi-permanent electrostatic polarization, are commonly employed in a macro-scale form factor within transducers such as the electret condenser microphone. In addition, MEMS-based electret transduction has been reported for both acoustic sensors and energy harvesters. In these micro-devices, the polymer film is typically polarized prior to assembly into the device. Common techniques employed in MEMS electret polarization include corona discharge and backlighted thyratron, with wafer-bonding and simple stacked assemblies being employed to perform the actual assembly and integration. In contrast, 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 ultrasonic sensors designed to capture and analyze waveforms from impacts.