Sealed-Cavity Bulk Acoustic Resonator for Subsequent Fabrication and Higher Order Mode

This focuses on developing a new platform for the thin-film bulk acoustic wave (BAW) resonator, which features predefined sealed cavities, self-formed acoustic boundaries, and compatibility with subsequent fabrication. Different from conventional BAW resonator fabrication methods, this work has simplified fabrication by using silicon migration technology: building freely predefined cavities with self-formed acoustic boundaries without patterning the piezoelectric layer in only two steps (etching and annealing). Additionally, the sealed cavity is sturdy enough to be compatible with subsequent hetero-integrating with other devices. For higher frequency and better electromechanical coupling ( $K^{2}$ ), the proposed platform can excite the second-order asymmetric Lamb wave mode (A2) in scandium-doped aluminum nitride (Al1–xScxN) film with an optimized stress field. The fabricated devices demonstrate S1 and A2 resonant modes at 1.58 GHz and 3.52 GHz with electromechanical coupling coefficients of 1.47% and 5.12%, respectively. [2023-0185]