A Piezoelectric Micro Machined Ultrasonic Transducer Based Hybrid-Morph AlScN Film

Abstract

Al1-xScxN has much higher piezoelectric coefficient compared with traditional AlN, thus is promising to be used in piezoelectric micro machined ultrasonic transducer (pMUT). However, high doping of Sc introduces the precipitation of the Sc at the grain boundaries, which makes it difficult to prepare AlScN membranes with good piezoelectric performance. This paper reports a method fabricating AlScN composite membranes on the 8-inch wafer. Sc with content of 20% and 9.6% are separately doped in the top and bottom of the composite membrane. The hybrid membrane shows better surface morphology and less Sc precipitations compared with pure Al0.8Sc0.2N film. Meanwhile, the process of the hybrid membrane is compatible with CMOS and facilitates the batch process. The pMUT prepared by the membrane has high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{k}_{\mathrm {t}}^{2}$ </tex-math></inline-formula> of 2.9%, which is about twice as large as that of pMUT made by Al0.904Sc0.096N (1.5%). The proposed hybrid-morph structure with highly doped AlScN thin film contributes a novel solution to new generation of high-quality film for both research and commercial application. [2022-0141]