Determination of longitudinal wave and shear wave acoustic reflectivity of Bragg reflector in the GHz range

Solidly mounted resonator (SMR) consists of a piezoelectric layer and a Bragg reflector. However, isolating and extracting the acoustic (mechanical) properties of the Bragg reflector alone has proven challenging. We propose a method for evaluating the acoustic properties of the Bragg reflector using the pulse-echo technique with an SiO2 delay line. Moreover, evaluation of shear acoustic properties of the reflector is crucial, as the Q-value of SMR improves when the transmission leakage of both longitudinal and shear waves is suppressed within the same frequency range. In this study, the transmission loss of longitudinal and shear waves was measured for three types of Bragg reflectors, i.e., longitudinal wave Bragg reflector, shear wave Bragg reflector, and a Bragg reflector, which reflects both longitudinal and shear waves in the same frequency range. The experimental transmittance of the isolated reflector part demonstrated a strong correlation with the theoretical values calculated using the mechanical circuit model. In addition, with the increasing demand for high-frequency filters, we evaluated the longitudinal wave transmission loss of Bragg reflectors for 5 and 6 GHz SMRs. The method successfully detected transmission loss at -30 dB for 4 GHz, and the experimental and theoretical properties accords well in the 7-9 GHz range, demonstrating that the present method can evaluate acoustic properties of Bragg reflectors in the higher-frequency range as well.