Highly sensitive piezoelectric micromachined ultrasonic transducer operated in air

Abstract

Piezoelectric micromachined ultrasonic transducer (pMUT) is gaining increasing research interest. It overcomes the inherent shortcomings of conventional bulk ultrasonic transducers such as acoustic impedance mismatching and poor sensitivity. In addition, pMUT does not require the extremely large input voltage as capacitive micromachined ultrasonic transducer. Therefore the pMUT is potential to be integrated into portable electronics. A pMUT array operated in air using the lead zirconate titanate (PZT) thin film polled by high voltage pulses is realised and fully investigated. The high voltage pulses are found to be more effective for PZT poling, where the piezoelectric constant d 31 is elevated to 105 pm/V. Benefited from such high performance PZT thin film and optimised structure, the fabricated pMUT (500 × 300 μm) achieves a displacement sensitivity of 807 nm/V at its resonant frequency (482 kHz) without DC offset. Compared with previously reported PZT pMUTs, the sensitivity is superior to them even with a smaller membrane. The in-air transmitting performance is also evaluated. A single pMUT element is able to generate 63.7 dB sound pressure level at 10 mm in air with only 2 V input. The proposed high performance pMUT shows its promise for practical applications in portable electronics.