Rotational Capacitive Micromachined Ultrasonic Transducers

A rotational capacitive micromachined ultrasonic transducer (RCMUT) for airborne ultrasound applications is presented. The device is comprised of two conventional parallel-plate CMUTs mechanically coupled via a pivoting-beam structure. The cavity beneath each CMUT piston is vacuum-sealed to reduce thermal-mechanical noise and unwanted air-film dynamics. The pivoting structure is designed to provide high bending stiffness and low rotational stiffness. The beam therefore serves to reduce common-mode deflection of the structure under large omnipresent atmospheric pressure that statically loads the structure upon vacuum sealing, while enabling the structure to maintain sensitivity to small differential pressure between diaphragms. The coupling structure therefore permits larger, more compliant vacuum-sealed diaphragms to exist than would otherwise be possible without the coupling structure. Theoretically, this enables a design path towards highly-compliant, vacuum-sealed sensors to address relatively-low frequency airborne ultrasound applications. The paper summarizes the fabrication of prototypes and their dynamic characterization using electrostatic drive signals and laser Doppler vibrometry.