Dual-layer acoustic matching for sensitivity and bandwidth enhancement in self-focusing high-frequency ultrasonic sensors

High-frequency ultrasonic sensors exhibit outstanding spatial resolution in biomedical imaging and micro-scale, non-destructive testing. However, their performance is constrained by severe acoustic impedance mismatch between the piezoelectric layer and the propagation medium, resulting in low sensitivity and limited operating bandwidth. The quarter-wavelength theory is the most traditional method for acoustic impedance matching, but it is difficult to achieve both a specific acoustic impedance and precise thickness control at the same time. This paper proposes and experimentally validates a dual-layer acoustic impedance matching strategy for self-focusing ZnO high-frequency ultrasonic sensors. The matching structure comprises a low-impedance parylene layer and a high-impedance molybdenum metal layer. Their thicknesses were jointly designed using a mass-spring model and microwave transmission line theory and then optimized through finite element simulation. Using chemical vapor deposition and ion beam sputtering, the bilayer matching film was conformally deposited onto the sensor's focusing surface, demonstrating the feasibility of this bilayer structure on non-planar high-frequency devices. Pulse-echo testing revealed that the introduction of the dual-layer matching structure increased the sensor's peak-to-peak echo voltage from 320 to 751 mV (a 135% improvement) and expanded the −6 dB bandwidth from 60.49% to 87.27%. The results demonstrate that the synergistic effect of impedance gradient transition and mass-spring resonance enhancement simultaneously improves sensitivity and bandwidth. This study provides a novel acoustic matching scheme for high-performance self-focusing high-frequency ultrasonic sensors and, for the first time, extends multilayer matching technology from planar devices to self-focusing high-frequency devices.