High kt2 Q Lamb-wave ScAlN-on-Silicon UHF and SHF Resonators
Abstract – 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 μm-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 (kt2) and high quality-factor (Q) are implemented over 80 MHz – 3.5 GHz demonstrating kt2 of 0.7%-2.9% and Q of 2000-6400. Specifically, a high f0 x Q of 4.3 x 1012 is measured for a resonator at 3.5 GHz, and a high kt2 x Q of 51 is measured at 108 MHz. The large kt2 x 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.