Capacitive Micromachined Transducers With Out-of-Plane Repulsive Actuation for Enhancing Ultrasound Transmission in Air

This paper presents a novel approach to enhance ultrasound transmission using capacitive micromachined ultrasonic transducers (CMUTs). This is achieved by increasing the cavity height through the use of electrostatic repulsion. Conventional CMUTs based on attractive forces have promising electroacoustic characteristics but limited output pressure, compared to piezoelectric transducers due to the limited motion ranges for CMUTs imposed by the capacitive transduction gap. Therefore, we propose here an electrostatic repulsive CMUT design with three fixed electrodes and one movable electrode that displaces out-of-plane. Simulation results demonstrate the design’s effectiveness in increasing the transducer’s range of motion, thus enhancing transmission sound pressure. Prototypes were fabricated using MEMSCAP’s PolyMUMPs process. Repulsive actuation allows for more than an order of magnitude (11x) improvement in the allowable motion range and therefore an improvement in the acoustic output by a factor up to 25.42 dB. Experimental tests using a vibrometer and an ultrasonic microphone confirm the effectiveness of the proposed approach. The CMUT array operates over a wide band of frequencies from 150 kHz to 650 kHz, which opens the doors for several applications such as ranging, gesture recognition, and non-destructive testing, with the potential for further improvements in ultrasound transmission. [2023-0158]