High $k_{t}^{2}\cdot Q$ Waveguide-Based ScAlN-on-Si UHF and SHF Resonators

This paper reports, for the first time, on waveguide-based resonators implemented in scandium-doped aluminum nitride-on-silicon (ScAlN-on-Si) stack to simultaneously benefit from large piezoelectric constants of ScAlN and low acoustic dissipation in single crystal silicon. $1\mu\mathbf{m}$-thick ScAlN film with Sc content of 7% is reactively sputtered on silicon substrates using ac-powered dual-target S-gun magnetron with Al targets containing embedded pure Sc pellets. A Cl2/H2 based low-power plasma etching recipe is developed to pattern resonators with smooth vertical sidewalls. In-and out-of-plane waveguide-based resonator prototypes with large electromechanical coupling coefficient $(\mathbf{k}_{\mathbf{t}}^{2})$ and high quality-factor $(\mathbf{Q})$ are implemented over 80 MHz-3.5 GHz demonstrating $k_{t}^{2}$ of 0.7^%-2.9^% and $\mathbf{Q}$ of 2000–6400. Specifically, a high $\mathbf{f}_{0}\times \mathbf{Q}$ of $4.3\times 10^{12}$ is measured for a resonator at 3.5 GHz, and a high $\mathbf{k}_{\mathbf{t}}^{2}\times \mathbf{Q}$ of 51 is measured at 108 MHz. The large $\mathbf{k}_{\mathbf{t}}^{2}\times \mathbf{Q}$ of ScAlN-on-Si waveguide-based resonators along with lithographical frequency tailorability demonstrate their potential for realization of highly integrated front-end filters for multi-band 5G systems.