Epitaxial Aluminum Scandium Nitride Super High Frequency Acoustic Resonators

This paper demonstrates super high frequency (SHF) Lamb and surface acoustic wave resonators based on single-crystal orientation Aluminum Scandium Nitride (AlScN) thin films grown on silicon substrates by molecular beam epitaxy (MBE). We report on the experimental frequency response and electromechanical properties of 400 nm-thick crystalline AlScN acoustic resonators with up to 12% Sc/(Sc+Al) ratio. The film thickness is optimized for operation at the SHF range, targeting emerging wireless communication standards, such as 4G LTE/5G. We report on high-performance acoustic devices that take advantage of the crystallinity, and high piezoelectric properties of 400 nm-thick epitaxial AlScN films. Our work presents enhanced effective electromechanical coupling coefficients (keff2) up to 5.3% and unloaded quality factors (Qm) of 192 at 3-10 GHz. However, fabrication challenges due to the high-stress levels of sub-micron AlScN epi-layers grown on Si substrates remain challenging and will be discussed in this paper.