Immersion PMUTs Fabricated with a Low Thermal-Budget Surface Micromachining Process

This study presents encapsulated piezoelectric micromachined ultrasonic transducers (PMUTs) fabricated by surface micro machining, forming high fill-factor arrays for high transmitting pressure output. The structures were made by the low-temperature processes, and in-situ sealed by Si02 PECVD reactor in vacuum, and coated by a PDMS layer to have good protection and acoustic impedance matching, resulting in a high mechanical displacement and wide bandwidth for immersion applications. The displacement-frequency measurement shows the bandwidth improvement by a quarter wavelength thickness PDMS coating, which agrees with the simulations. By pulse-echo measurements, a 6x6 PMUT array of 0.81 mm2 is characterized by 20Vp_p continuous-wave excitation, resulting in 1.3kPa echo receiving and 60% fractional bandwidth with 0.7 mm fluid depth. This study achieves 5X greater transmission efficiency and 2X better receiving bandwidth than the previous work.