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

Abstract

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 2150 th 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 Sc 0.33 Al 0.67 N (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, f×Q > 10 14 Hz, and k 2 eff ×Q BVD > 0.22 at cryogenic temperatures for frequencies as high as 40 GHz (>500, >6 × 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.

Micromachined piezoelectric Lamb wave resonators: a review

“…Prior studies on ultra-thin AlN films have been performed in this research direction. An ultra-thin piezoelectric thin film grown by RF-plasma-assisted MBE on SiC substrate has been exploited in a high overtone mode BAW resonator, which consists of 150 nm Al 0.67 Sc 0.33 N, 50 nm AlN, and 50 nm NbN [161]. In another work of thermal detectors, sputtering deposited AlN ultra-thin plates with thickness down to 50 nm is applied to fabricate utilized S0 mode Lamb wave resonators [162].…”

Single crystal ferroelectric AlScN nanowires

“…AlScN ferroelectrics show significant promise for use in wideranging device applications, including random-access memory [1][2][3] and acoustic resonators. [4][5][6] Epitaxial growth of AlScN thin films on a wide range of substrates and buffer layers, including Si(111), 7 Al 2 O 3 (0001), 8,9 Al 2 O 3 (0001)/ Mo(110), 10 SiC, 4,11 and GaN(0001) [12][13][14] have been reported. More commonly, columnar polycrystalline AlScN thin films are grown on a variety of textured metallic bottom electrodes deposited onto Si wafers.…”