Air-coupled piezoelectric micromachined ultrasonic transducer array based on low-cost and large remnant polarization PZT thin film

Due to the development of semiconductor technology, control and driving circuits for large phased arrays have become cheaper and more compact, enabling air-coupled ultrasonic transducer arrays to be applied in areas such as ultrasonic haptic feedback and acoustic levitation. However, current air-coupled ultrasonic transducers are quite bulky, which prevents the commercialization of these applications. Hence, piezoelectric micromachining ultrasonic transducers (pMUT) have been proposed to replace conventional ultrasound transducers. Currently, PZT, or lead zirconate titanate, thin film is the most reliable and, consequently, most popular piezoelectric layer for pMUT, but existing PZT fabrication methods are too expensive or require rare metals. To counter this problem, this study proposed a cost-effective and reliable PZT thin film fabrication method. By depositing a commercial sol-gel solution as a seed layer followed by radio frequency sputtering, a PZT thin film with remnant polarization of 113.35 µC/cm² and a coercive field of 211.6 kV/cm was demonstrated. Moreover, high-throughput air-coupled pMUTs and pMUT arrays were made based on the proposed PZT thin film. Finite element method simulations were conducted to optimize the structure parameters for the 40 kHz pMUT. A standard microphone was used to measure the acoustic pressure at 3 cm above the pMUT and the pMUT array to evaluate their performance. The pMUT generated an acoustic pressure of 0.764 Pa/V with a resonance frequency of 52.2 kHz, and the pMUT array's maximum acoustic pressure was 87.4 Pa. The device demonstrated the highest acoustic output pressure among existing sub-100 kHz air-coupled pMUT arrays using PZT as the actuation layer, highlighting the potential of using the proposed PZT thin film deposition method for high-pressure mid-air ultrasonic applications.