Epitaxial Al0.77Sc0.23N SAW and Lamb Wave Resonators

This work reports on surface acoustic wave (SAW) and the super high frequency (SHF) Lamb wave resonators (LWR) with reflective gratings based on 400 nm-thick epitaxial Aluminum Scandium Nitride (AlScN) piezoelectric films. The films are grown on silicon substrates by molecular beam epitaxy (MBE), with $\text{Sc}/(\mathrm{A}1+\text{Sc})$ ratio of 23%, which is the highest Sc concentration, reported to date for BAW/SAW resonators based on AlN epitaxial films. We first demonstrate a SAW resonator, with reflective gratings. The Si substrate of the SAW resonator is then removed from the backside, to form a suspended 400 nm-thick plate. A floating bottom metal electrode is deposited from the backside to enhance the effective electromechanical coefficient (${k_{eff}}^{2}$) of the Lamb wave resonators. A high ${k_{eff}}^{2}$ value of 7.45% at a resonant frequency of 4.92 GHz is reported in this work, yielding ${k_{eff}}^{2}\times Q_{m}$ of 7.3. To the authors' knowledge, this work marks the highest ${k_{eff}}^{2}$ achieved for >3GHz AlN-based Lamb wave resonators to date.