Synergistic design of an SU-8-based CMUT array for low-frequency ultrasonic detection under liquid loading

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) operating under liquid loading conditions suffer from significant added-mass effects and radiation damping, which degrade low-frequency vibration response and acoustic output performance. To address this issue, a synergistically optimized SU-8-based CMUT structure is proposed by combining a slotted membrane with an inverted trapezoidal stepped cavity. The slotted membrane reduces the effective bending stiffness to enhance mechanical compliance, while the stepped cavity improves electric-field distribution and displacement space. A coupled electromechanical-acoustic model is established in COMSOL Multiphysics to investigate the effects of membrane geometry, cavity configuration, and array topology. Simulation results show that, compared with a conventional flat-cavity CMUT, the proposed structure under water loading achieves a 29.8% reduction in resonant frequency, together with increases of 225.8% in membrane displacement and 40.3% in acoustic pressure. Furthermore, a 30 × 30 offset open hollow-square array is designed to improve directivity performance while reducing the number of active elements. The optimized array achieves a main-lobe width of 17.15° and a sidelobe level of −2.95 dB using only 81.8% of the array elements. These results demonstrate that the proposed membrane–cavity–array codesign provides a physically interpretable strategy for improving low-frequency CMUT performance under liquid loading.