Development of an Air-Coupled Piezoelectric Micromachined Ultrasonic Transducer Using Sol-Gel PZT Thin Film for Fast-Prototyping

This study demonstrated the first air-coupled pMUT using sol-gel PZT thin film that could deliver ultrasonic waves to mid-air. First, the deposition conditions for making PZT thin film with high remanent polarization were determined. Then, air-coupled pMUTs with resonance frequencies close to 40 kHz were designed using the circular plate model. According to the design, pMUTs with radii measuring $600~\mu $ m to $775~\mu $ m were fabricated to evaluate the acoustic output pressure. Among these, the pMUT with the $725~\mu $ m radius achieved a maximum sound pressure output of 4.42 Pa at 3 cm above when driven with 10 Vpp, and the resonance frequency was 40.48 kHz. Finally, the output pressure of a phased array consisting of sol-gel PZT-based pMUTs with a $725~\mu $ m radius was calculated using the k-Wave toolbox. The output pressure of the $11\times 11$ pMUT array reached 365.62 Pa when focused at 3 cm above it. This result revealed that the output pressure of the proposed pMUT array could fulfill the requirement for most mid-air ultrasound applications.