51.3 GHz Overmoded Bulk Acoustic Resonator Using 35% Scandium Doped Aluminum Nitride

This work demonstrates an Overmoded Bulk Acoustic Resonator (OBAR) design that incorporates 35% Scandium doped Aluminum Nitride (Sc_0.35Al_0.65N) as the piezoelectric layer. The ScAlN OBAR presented here is a Bulk Acoustic Wave (BAW) resonator that excites a second overtone within a stack formed by a ScAlN layer and a set of alternating metallic layers. The metal electrodes act simultaneously as the acoustic cavity and as acoustic Bragg mirrors. Individual resonators are connected to each other by thick floating electrodes and top interconnects to form the devices demonstrated herein. The fabricated ScAlN OBAR with best performance exhibits a series resonant frequency of 51.3 GHz, electromechanical coupling ( $k_{t}^{2}$ ) of 6.1% and a Quality factor (Q) at series resonance of 108. The measurements of various ScAlN OBAR devices with different geometries show that Q is increasing as the perimeter and area of the individual resonator increases and $k_{t}^{2}$ is increasing as the number of resonators in series increases. Material losses and surface roughness with associated acoustic energy leakage are discussed as possible sources of damping in these mmWave resonators. The investigations trace a path for further technological improvement and show that the ScAlN OBAR is a promising device for mmWave acoustics and filtering applications. [2025-0071]