Pop-down FBAR Structure for Thermal Stability without Sacrificing Coupling

This work proposes a novel pop-down film bulk acoustic resonator (FBAR) structure to improve thermal stability without sacrificing electromechanical coupling. The pop-down structure is achieved using a Si cavity formed by silicon migration technology (SiMiT), which simultaneously provides a self-defined acoustic boundary and induces initial tensile stress in the sputtered AlScN piezoelectric layer. Over the temperature range of −100 ¼C to 100 ¼C, the measured temperature coefficients of frequency (TCFs) are −12.98 ppm/¼C for the pop-down sealed-cavity bulk acoustic resonator (S-BAR) and −7.28 ppm/¼C for the pop-down FBAR, significantly better than the −30 ppm/¼C typical of uncompensated AlScN FBARs. The improvement is attributed to residual tensile stress arising from the concave pop-down geometry, lattice mismatch, and coefficient of thermal expansion (CTE) mismatch. These results demonstrate that the pop-down FBAR offers a promising pathway for thermally stable resonators while preserving strong electromechanical coupling, avoiding the coupling penalties typical of passive SiO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>-based compensation.