52 GHz 35% Scandium Doped Aluminum Nitride Overmoded Bulk Acoustic Resonator

Scandium doped Aluminum Nitride (ScAlN) has become a piezoelectric material of interest for Bulk Acoustic Wave (BAW) resonators as it offers an intrinsically high electromechanical coupling $\left({k_t^2}\right)$. However, little is known about ScAlN at mm-wave frequencies. In this work, we demonstrate an innovative Overmoded Bulk Acoustic Resonator (OBAR) design that incorporates 35% Scandium doped Aluminum Nitride (Sc<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.35</inf>Al<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.65</inf>N) piezoelectric layer. The ScAlN OBAR presented herein is a BAW device that is excited in the 2<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> overtone by means of the ScAlN layer and a stack of metal electrodes that act simultaneously as acoustic cavity and as acoustic Bragg mirrors. The fabricated device demonstrates $k_t^2$ of 6.1% and Quality factor (Q) of 114 at 52 GHz – a substantial improvement in the $k_t^2\cdot{\text{Q}}$ figure of merit of ScAlN acoustic resonators at mm-wave frequencies.