Investigation of Bragg Reflectors for Polarity-Inverted AlN BAW Resonators Operating at Super High Frequency

Solidly mounted resonator (SMR) structures are promising candidates for bulk acoustic wave (BAW) devices operating in the super high frequency (SHF) range, owing to their superior mechanical stability and power handling capabilities. In this study, we designed and fabricated Bragg reflector (BR) layers using two high acoustic impedance materials, W and Mo. The Mo-based BR demonstrated superior performance in terms of wafer stress management and thickness control. Utilizing Mo-SiO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> BRs, we successfully fabricated single-crystal polarity-inverted AlN resonators using novel two-step piezoelectric film deposition and cavity-embedded processes. These resonators exhibited resonance frequencies of 12.79 GHz (Q ~ 696) and 12.88 GHz (Q ~ 460), with a $k_{eff}^2$ value of 1.7%, which is nearly double that of the first-order mode. This work demonstrates the significant potential of SMR structures for acoustic filter applications in the SHF bands.