33 GHz Overmoded Bulk Acoustic Resonator

In this work we present a new class of thickness extensional microelectromechanical resonator, the overmoded bulk acoustic resonator (OBAR) for filtering applications in the 5G millimeter wave (mm-wave) spectrum. This resonator operates in a 2nd overtone thickness mode with approximately equal thickness electrodes and piezoelectric layer so that acoustic energy is distributed evenly between the different layers. Compared to a fundamental mode at a fixed frequency, the OBAR possess a 3 times thicker piezo layer and 5-10 times thicker metal electrodes, enabling manufacturable 30-60 GHz devices. We demonstrate the OBAR experimentally through fabrication of a Pt-AlN-Al device with electromechanical-coupling coefficient (kt2) of 1.7%, and series resonance quality factor (Qs) of 110 at 33 GHz.