High-velocity and large-coupling surface wave mode using acoustic Bragg reflector stack with matched excitation condition

Abstract

High-velocity, large-coupling acoustic mode is the eternal pursuit of microacoustic research. Surface mode on piezoelectric films supported by an acoustic Bragg reflector (ABR) stack is one possible solution for realizing such a goal. However, some acoustic modes exhibit lower couplings in experiments than theoretically predicted, especially when metal layers are used as high-impedance layers in the ABR. The mechanism of this performance deterioration is still unclear. In this work, the electric field distribution in a piezoelectric plate mounted to an ABR stack is analyzed by considering the inductive current effect in conductive ABR layers. The field distribution required by one mode is found to be distorted by the effect, which results in a reduction in the excitation efficiency for the mode. We propose the use of another acoustic mode with a concentrated electric field in the top layers. As analyzed in comparison, the impact of the inductive effect on the mode is negligible. On an optimized crystal cut of lithium niobate, the selected mode exhibits 22.64% coupling and an acoustic velocity of 6006 m/s.