Investigation of Broadband Characteristics of Multi-Frequency Piezoelectric Micromachined Ultrasonic Transducer (MF-pMUT)

Multi-frequency ultrasonic transducers are highly promising devices in biomedical applications for providing both good imaging resolution and large detection depth. However, most conventional multi-frequency ultrasonic transducers are designed through integrating multiple single-frequency transducers, with complicated fabrication process, imperfect beam profiles, and limited bandwidths. In this paper, a single-element multi-frequency piezoelectric micromachined ultrasonic transducer (MF-pMUT) is proposed and investigated in terms of broadband property. Three rectangular-diaphragm MF-pMUTs with different length-to-width aspect ratios are fabricated with the microelectromechanical system (MEMS) technology and characterized both in air and underwater. All MF-pMUT samples exhibit at least two broad bands. After optimization of the aspect ratio, two smooth bands with comparable sound pressure level are achieved, providing -6 dB bandwidths of 112%/38% and corresponding central frequencies of 0.615 MHz/1.63 MHz. Benefited from such broadened multi-frequency property and simple structure design, the proposed MF-pMUT shows a great potential in ultrasonic detection, diagnosis, and imaging applications as well as wireless acoustic energy transmission.