Suppression Method of Higher Order Mode Spurious Responses in Laterally Excited Shear Mode Bulk Acoustic Wave Resonators

This paper discusses a suppression method of higher order modes in laterally excited shear mode bulk acoustic wave resonators (XBARs), which have been widely studied for radio frequency filters for next generation mobile communication systems. In XBAR structures, various types of unwanted resonances can be excited, resulting in many spurious responses. Among these resonances, higher order modes in the thickness direction often deteriorate high frequency out-of-band rejections. In order to analyze the behavior of the higher order modes, a three-layer structure, SiO2/LiNbO3/SiO2, is assumed as an XBAR stack, and the resonance properties such as resonance frequency and coupling coefficient are calculated using the admittance formula for XBARs. The relationship between the resonance properties of each mode and the thickness ratio of the stack are illustrated, and the suppression mechanism is discussed using the finite element method simulations, which demonstrate the validity of the analysis. Finally, resonators and a ladder type filter with optimized stacks are fabricated, and not only excellent in-band characteristics but also clean out-of-band rejection are realized.