X-Ka Band Epitaxial ScAlN/AlN/NbN/SiC High-Overtone Bulk Acoustic Resonators

This letter presents the first demonstration of epitaxial scandium aluminum nitride (ScAlN) based high-overtone bulk acoustic resonators (epi-HBARs) with over 1600 acoustic cavity resonance modes spanning the X - Ka bands (8 GHz - 40 GHz). We present data up to the 2150th overtone (39.99 GHz). This is an unprecedented result even for HBARs, which often exhibit hundreds of overtones. The measurements demonstrate the successful combination of multiple innovations, namely: a) the growth of ultra-thin, high quality Sc0.33Al0.67N (high Sc fraction), b) on an epitaxial heterostructure with lattice-and acoustic impedance-matched layers, and c) the selective etching of ScAlN on NbN. Key metrics for the ScAlN epi-HBARs include Q > 7000, fxQ > 10(14) Hz, and k(eff)(2) xQ(BVD) > 0.22 at cryogenic temperatures for frequencies as high as 40 GHz (>500, >6 x 10(12) Hz, >0.1 at room temperature). Temperature trends motivate future investigation of unquantified high frequency acoustic loss mechanisms. Such robust RF MEMS epi-HBARs with piezoelectric drive and readout are promising candidates for compact microwave/millimeter wave signal processing elements.