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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 second 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 ($k_{t}^{2}$) of 1.7%, and series resonance quality factor ($Q_{s}$) of 110 at 33 GHz. [ABSTRACT FROM AUTHOR]

In this letter we present the first demonstration of an ultra-thin (100 nm) lithium niobate (LN) flexural resonator for airborne ultrasound transduction. Due to the aggressively scaled thickness of the monocrystalline X-cut LN layer, the trasnsducers exhibit a low resonance frequency of 32 kHz while occupying an area of only 120 μm × 120 μm. An innovative electrode design converts the applied electric field to out-of-plane deflections excited by the e21 piezoelectric coefficient. The displacement per unit of voltage at different in-plane rotation angles scales consistently with the theoretical e21 coefficients of X-cut LN. Overall, the transducer is capable of sensitivity per unit area that is significantly larger than the state-of-the-art. [2020-0340] [ABSTRACT FROM AUTHOR]