A GHz range, single structure, multi-mode ZnO solidly-mounted bulk acoustic resonator

Multiple modes excited in a single solidly mounted ZnO bulk acoustic resonator would provide a multi band resonator operating at non-harmonically-related frequencies in the GHz range. Ordinarily, separate devices with different electrode and reflector designs are needed to excite the thickness shear (TSM) and thickness-extensional (TE) bulk acoustic modes, each device optimized for a single mode. In this work, we present a solidly mounted, c-axis oriented, ZnO bulk acoustic resonator with a single electrode geometry that excites both TSM and TE modes within the same structure. The acoustic reflector stack is specifically designed to efficiently reflect both modes. In addition, the resonator footprint is only 400μm × 400μm making it ideal for on-chip integration. Successful fabrication and testing of approximately 50 multi-mode devices was demonstrated. The resonant frequencies are located at 1.2GHz and 2.3GHz for the TSM and TE modes, respectively. The quality factors of the devices are between 400-500 and 800 1000, respectively. In addition, a hybrid mode was observed, non-harmonically-related to the TSM and TE, with a resonant frequency of about 1.5GHz and a Q of 700. It is also shown that the resonant frequency of each mode changes accordingly with mass-loading on the center electrode, indicative of mode trapping under the electrode.