A High-sensitivity Capacitive Micromachined Ultrasonic Transducer (CMUT) with Large Radius-to-thickness Ratio

Capacitive Micromachined Ultrasonic Transducers (CMUTs) demonstrate highly competitive advantages in fields such as underwater near-field communication, short-to-medium range precision imaging, and target detection. However, compared to traditional piezoelectric transducers, their lower sensitivity limits the application of CMUTs in these areas. This paper designs, simulates, fabricates, and experimentally validates CMUTs with membranes having a large radius-to- thickness ratio. It systematically studies the relationship between CMUT membrane sizes and both resonant frequency and transmission sensitivity, and discusses the effects of the stress stiffening effect under large deflections on the resonant frequency and the dynamic velocity of the membrane. A larger radius-to-thickness ratio helps to improve sensitivity, but the accompanying stress stiffening effect leads to an increase in frequency. It is necessary to ensure that the ratio of membrane deflection to thickness does not exceed 40%, in which case the frequency deviation due to stress stiffening effects does not exceed 10%. These design concepts are well validated through finite element method (FEM) simulation and experimental results. The acoustic characterization experiments in water show that the CMUT with a membrane thickness of 5 μm and a radius-to-thickness ratio of 44:1, has a center frequency of 112.4 kHz, a transmitting voltage response (TVR) of 148.15 dB (μPa·m/V), and a -6 dB fractional bandwidth of 138.4%. The outstanding transmitting voltage response and large fractional bandwidth make the CMUT designed in this paper highly competitive in areas such as underwater target detection, imaging, and near-field communication.