Low thermal budget surface micromachining process for piezoelectric micromachined ultrasonic transducer arrays with in-situ vacuum sealed cavities

Abstract

This paper presents a low thermal budget surface micromachining process for piezoelectric micromachined ultrasonic transducer (PMUT) arrays with in-situ vacuum sealed cavities and high fill-factor. Low-temperature PECVD amorphous silicon (a-Si) and CMOS compatible aluminum nitride (AlN) are used in this process, making it possible to fabricate PMUT arrays on top of CMOS wafers directly at a low cost. The presented square PMUT array has a high fill-factor of 79% with a resonant frequency at 5.5 MHz in Fluorinert (FC-70, 3M), suitable for medical imaging applications. The transmitting surface pressure of this array is 2.93 kPa/V, 1.25x higher than previously reported PMUT arrays for fingerprint sensors and the receiving sensitivity is 0.5 V/kPa with a 10 V/pC charge amplifier. The presented fabrication process also showed tunability of PMUT 3-dimension (3D) profile by controlling the AlN residual stress in the deposition process and the possibility to increase the dynamic responses. Meanwhile, the reliability of the PMUTs was tested for 21 hours continuous driving and was tested for seal quality when immersed in Fluorinert.