Characterization and Optimization of PZT-Based PMUTs With Wide Range Frequency Tuning

In this paper, we present air-coupled lead-zirconate-titanate (PZT) piezoelectric micromachined ultrasonic transducers (PMUTs) which demonstrate ultra-wide frequency tuning range via controllable in-plane stress generated by DC bias voltage. A PMUT designed to have a resonant frequency of similar to 200kHz generates a 223kHz total frequency shift from 182.5kHz to 405.5kHz with +/- 35V DC bias (corresponding to 97.8% variation referring to the resonant frequency without bias), and a 124kHz frequency shift from 188kHz to 312kHz with +/- 10V DC bias (corresponding to 54.4% variation). The effects of DC bias tuning were further characterized by both impedance analyzer and laser Doppler vibrometer (LDV), the fluctuation of electromechanical coupling coefficient (k(t)(2)) and the change of the direction of polarization of the piezoelectric layer were successfully observed. Frequency tuning along different curves of the hysteresis loop was studied, and given both reasonably good k(t)(2) (> 5%) and frequency tuning sensitivity (>10kHz/V), a 188.5kHz to 254kHz frequency tuning with 1 similar to 6V DC bias is chosen from the best option of the hysteresis loop. Furthermore, characteristics of PMUTs with different top electrode thickness were studied and summarized, and thinner top electrode was considered as an optimization method to achieve better performance for PMUT under DC bias in terms of frequency tuning. PMUTs with various frequencies were evaluated, and measurement results show a smaller tuning range of PMUTs with higher resonant frequency than those with lower resonant frequency due to the different membrane modulus, and, therefore different contributions of intrinsic stress generated by DC bias to the overall membrane modulus.