Improving Pmut Transmit Performance Via Sub-Micron Thickness Scaling

Analytical models for micromachined ultrasonic transducers (MUTs) show that the transmit voltage sensitivity (Pa/V) has an inverse dependence on thickness. However, the ability to scale capacitive MUTs (CMUTs) to submicron thickness is limited by electrostatic pull-in instability. PMUTs are not subject to pull-in instability, making them an ideal vehicle to explore the benefits of thickness scaling. Using analytical model and experiment measurements, we demonstrated individual PMUT's transmit voltage sensitivity at a given frequency is inversely proportional to the device thickness, while the quality factor dominated by the acoustic radiation loss linearly scales with the device thickness. AlN PMUTs with 500 nm and 1 gm thick piezoelectric layer were fabricated to demonstrate an improved transmit voltage sensitivity via sub-micro thickness scaling.