In this study, we proposed an annular-shaped piezoelectric micromachined ultrasonic transducer (pMUT) based on a Pb(Zr,Ti)O3-based monocrystalline thin film. This pMUT is expected to increase the resonance frequency while maintaining displacement sensitivity, making it superior to an island-shaped pMUT, which is a conventional design. To demonstrate the validity of this assumption, annular- and island-shaped pMUTs with a 60 μm-diameter diaphragm were prototyped and characterized. As a result, the annular-shaped pMUT exhibited a resonance frequency of 11.9 MHz, a static displacement sensitivity of 2.35 nm V-1 and a transmitting figure-of-merit (FOM) of 28 nm MHz V-1. On the other hand, the island-shaped pMUT exhibited a resonance frequency of 9.6 MHz and a static displacement of 2.5 nm V-1 and an FOM of 24 nm MHz V-1. Therefore, the annular-shaped pMUT was experimentally demonstrated to provide a higher FOM compared to the island-shaped pMUT. In addition, the annular-shaped pMUT with the optimal dimensions is found to be able to keep a relatively large fabrication margin. This is an advantageous point for the practical device fabrication. We believe this design has a potential to become a standard design for high-performance pMUT devices.
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