High-G and High-Bandwidth Bulk Acoustic Wave (BAW) Accelerometers Using a Metal-Less AlN-HARPSS Process With 95 nm Gaps

We present a 4.4-MHz resonant accelerometer based on the electrostatic softening effect in a bulk acoustic wave (BAW) resonator, fabricated using a metal-less AlN-HARPSS process with combined piezoelectric and capacitive transduction. BAW accelerometers can be used in high-g and high-dynamic range applications with a very wide bandwidth. The fabrication process supports both piezoelectric and nanogap capacitive transduction in a micro/nanoelectromechanical device, primarily applicable to stiff high-frequency devices that use a nanoscale (<; 200 nm) capacitive gap for actuation or frequency tuning purposes. It allows for high-temperature processing of substrates containing a sputtered AlN piezoelectric film, which is not typically possible for piezoelectric films sandwiched between metal electrodes. As a demonstration of the process, a metal-less AlN-on-Si BAW accelerometer with a 95-nm gap-changing electrode has been fabricated and experimentally characterized.