A method for extracting sole mechanical properties of Bragg reflector by GHz ultrasonic pulse-echo technique

Abstract

Solidly mounted resonators (SMRs) with an acoustic Bragg reflector should be more suitable for high power RF filters than a self-standing structure because heat can be transferred to the supporting substrate. Thus, even though the demand for SMRs is expected to increase, the evaluation of the properties of the Bragg reflector that comprise them cannot be separated from the properties of the resonator as a whole. A method to evaluate the mechanical reflection coefficient of the Bragg reflector alone before the piezoelectric layer is grown would, therefore, be attractive, as it would be useful in optimizing the Bragg reflector. In this study, a nondestructive evaluation method of the reflector using a GHz ultrasonic pulse-echo technique is proposed. The Sc0.40Al0.60N film ultrasonic transducers with electromechanical coupling coefficient kt2 of 15.8% and longitudinal wave insertion loss of 7.0 dB with 77% fractional bandwidth were employed for the measurement system. Mo/SiO2 Bragg reflector test samples with various number of pairs were evaluated. The experimental mechanical reflectance of the Bragg reflector agreed well with theoretical results simulated by a mechanical equivalent circuit model.