Design, Characterization, and Analysis of PZT Micromachined Piezoelectric Ultrasonic Transducers With Good Coupling to Solids

This article presents the piezoelectric micromachined ultrasonic transducer (PMUT) and its arrays that were based on a sputtered PZT/Si diaphragm structure and prototyped from an SOI substrate. Due to the high piezoelectric coefficient of PZT, polarization tuning pretreatment, and membrane thickness optimization, the PMUT shows high transmitting sensitivity in air and good coupling capability to liquid and solid. The PMUT transmitter exhibited a high sensitivity of 809, 190, and 135 nm/V at a resonant frequency of 0.450, 0.887, and 1.689 MHz, respectively, in air. The ${5} \times {5}$ array of 0.5-MHz PMUTs’ acoustic output in water was measured as 42.4 Pa at a distance of 3 cm with a 10.0- $\text{V}_{\text {pp}}$ input. Thickness-measuring ability in solids was evaluated with an ${8} \times {8}$ array of 1-MHz PMUTs as transmitter providing 8.0- $\text{V}_{\text {pp}}$ input and another single PMUT of identical frequency response as receiver showing 0.2 $\text{V}_{\text {pp}}$ (after 20 times magnification) output when the acoustic wave was transmitted through a 5-cm-thick graphite plate. Meanwhile, the time response of the receiver through different thicknesses of graphite plates is in reasonable agreement with predication from the analytical calculation. This high-performance PMUT with good coupling to solids will be utilized in various applications for solid-state sensing and detecting or as an alternative to the bulk piezoelectric ceramic transducers in the near future.