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

This focuses on developing a new platform for the thinfilm 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]