Embedded-Compatible Low-Voltage Poling of PMUTs

Abstract

Piezoelectric micromachined ultrasonic transducers (PMUTs) have the potential to be integrated into future applications of fingerprint sensors, ultrasonic communication, and implantable devices, where micrometer footprints are crucial and conventional bulk transducers’ dimensions are unsuitable. These PMUTs feature such millimeter to micrometer sizes, while their resonance frequencies still fall within the desired upper kHz to lower MHz range. Their working principle is based on the piezoelectric effect, while lead zirconate titanate (PZT) often serves as the material of choice. However, the piezoelectric response of the material strongly depends on its polarization state. This contribution investigates polarization techniques that can be applied within an embedded device and compares them with conventional DC poling at a higher electric field. The experiment comprised five PNZT (niobium-doped PZT) PMUTs fabricated in-house through sputtering instead of sol-gel deposition. Both the current and voltage profiles for each polarization procedure were recorded while laser Doppler vibrometer measurements captured the decrease in the PMUTs’ deflection over time. In addition, changes in resonance frequency and apparent power during PMUT excitation were monitored, while the quality factor $Q$ was determined. The results indicate that the application of low-voltage poling can significantly enhance the PMUTs’ deflection by more than 125 %, while an internal bias field can influence the aging rate of the polarization state. In comparison, our conventional poling technique yielded a deflection increase of 928 %, and a decreased aging rate. Small changes in the apparent power and in the resonance frequency were partially observed during the depolarization process.