A high-overtone bulk acoustic resonator (HBAR) consisting of a piezoelectric film with two electrodes on a substrate exhibits a high quality factor (Q) and multi-mode resonance spectrum. By analyzing the influences of each layer's material and structure (thickness) parameters on the effective electromechanical coupling coefficient (Keff²), the resonance spectrum characteristics of Keff² have been investigated systematically, and the optimal design of HBAR has been provided. Besides, a device, corresponding to one of the theoretical cases studied, is fabricated and evaluated. The experimental results are basically consistent with the theoretical results. Finally, the effects of Keff² on the function of the crystal oscillators constructed with HBARs are proposed. The crystal oscillators can operate in more modes and have a larger frequency hopping bandwidth by using the HBARs with a larger Keff²·Q.
Keywords: effective electromechanical coupling coefficient; high-overtone bulk acoustic resonator; resonance spectrum.