Impedance-Matched High-Overtone Thickness-Shear Bulk Acoustic Resonators With Scalable Mode Volume.

High-overtone bulk acoustic resonators (HBARs) are widely used in microwave signal processing due to their high quality factors and multimode resonances. However, conventional HBARs often suffer from impedance mismatch at metal interfaces, spurious modes, and limited flexibility in resonant mode volume design. Here we demonstrate a laterally excited high-overtone thickness-shear bulk acoustic resonator (X-HTBAR) based on a 3 µm 128° Y-cut LiNbO3 film on a high-resistivity Si substrate, enabling excitation of thickness-shear modes without a bottom electrode. The planar electrode configuration confines elastic energy within the electrode gap region and provides stable free spectral ranges. Experiments show comb-like phonon spectra from 0.1-1.8 GHz with quality factors of 103-105 and frequency-quality products exceeding 101 3. Gridded electrodes suppress spurious modes, while the strong electromechanical coupling of 128° Y-cut LiNbO3 enables tunable resonant mode volumes (0.008-0.064 mm3). These results demonstrate an alternative architecture for multimode acoustic resonators with potential applications in microwave and electro-acoustic systems.