High-Performance ZnO-Based Piezoelectric Micromachined Ultrasonic Transducers with an I-Shaped Diaphragm: Design, Fabrication, and Characterization

In order to simultaneously improve the imaging resolution and sensitivity of piezoelectric micromachined ultrasonic transducers (pMUTs), a pMUT with an I-shaped composite diaphragm was developed in this work. Compared with the conventional rectangular diaphragm structure, the I-shaped structure is formed by removing the piezoelectric layer along the edges of the two long sides while retaining the remaining portion. By this design, the residual stress of the piezoelectric film is effectively reduced, and both the sensitivity and the resonant frequency of the pMUT are improved. Moreover, unlike existing residual-stress-releasing structures such as hole- or groove-etched structures, where the diaphragm is no longer a continuous membrane and through-thickness holes or grooves are present, only the piezoelectric layer is etched in the proposed design. Consequently, no additional packaging requirements are imposed on the composite diaphragm, and the intricate sealing issue associated with through-hole structures is circumvented. A comprehensive comparison from multiple aspects was performed between ZnO-based and AlN-based pMUTs. It was shown by simulation results that the ZnO-based pMUT was superior to the AlN-based counterpart in terms of transmitting sensitivity, receiving sensitivity, and loop gain. On this basis, the ZnO-based pMUT with an I-shaped diaphragm was successfully fabricated and characterized. The performance of the PMUT was evaluated in electronic fluorinated fluid (FC-3283). The results show that the device exhibits a resonant frequency of 10 MHz, with a transmitting sensitivity level of 173.5 dB (re 1 μPa/V), a receiving sensitivity level of −242.1 dB (re 1 V/μPa), a loop gain of −68.6 dB, and a −6 dB bandwidth of 10.65%. These results indicate that high sensitivity was achieved at a high operating frequency, providing a technical reference for the design and application of high-frequency, high-sensitivity pMUT arrays, and demonstrating considerable potential for applications in underwater detection, ultrasonic imaging, and beyond.