A Solidly Mounted 55 GHz Overmoded Bulk Acoustic Resonator

Abstract

In this work we present a solidly mounted overmoded bulk acoustic resonator (OBAR) designed to operate at millimeter wave (mmWave) frequencies. This device uses a combination of overmoded operation, a layer transfer based fabrication process, all metal Bragg mirrors, and series arrays to allow acoustic resonator frequency scaling through V band. The solidly mounted OBAR can have up to 2/3 the $k_t^2$ of a fundamental mode (~4% for AlN, >10% for ScAlN or LN), reasonable piezoelectric film thicknesses (>100 nm at 50 GHz for AlN), arbitrarily thick electrodes to minimize ohmic loss, and be made in series arrays to allow 50 Ω matched devices with reasonable area to perimeter ratios. We demonstrate a proof of concept device using a 110 nm AlN piezoelectric layer operating at 55 GHz with an electromechanical-coupling coefficient $\left( {k_t^2} \right)$ of 2.2%, and a series resonance quality factor (Qs) of 90.