Effects of DC Bias Tuning on Air-Coupled PZT Piezoelectric Micromachined Ultrasonic Transducers

In this paper, we report an air-coupled piezoelectric ultrasonic micromachined transducer (PMUT) using a lead-zirconate-titanate (PZT) piezoelectric layer. A dc bias voltage applied to the PZT film controls its polarization and intrinsic stress, tuning the frequencies of two closely-spaced resonance modes of the rectangular shaped PMUT. At an optimal bias voltage of approximately 5 V, the modes nearly overlap at 230 kHz, increasing the bandwidth by a factor of 8 relative to the zero dc biased state. Measurements of the electromechanical coupling coefficient of the PZT film show that it is maximized at 5-6 V, agreeing with device performance experiments. Acoustic transmission and reception were demonstrated using two identical PMUTs by adding the optimum dc bias of 6 V to the 1.4 V peak-to-peak ac voltage which is maximum within the linear displacement regime. The signal detectable range of the transmit and receive measurement with optimum dc bias tuning was 4 cm to 19 cm in air.