In this paper, a flexible piezoelectric micromachined ultrasonic transducer (PMUT) based on Scandium (Sc)-doped Aluminum Nitride (AlN) film was designed and modeled by the three-dimensional finite element method (3D-FEM). The resonant frequency of 218.1 kHz was reported. It was noticeable that a high effective electromechanical coupling coefficient (k²_eff) of 1.45% was obtained when a combination of a flexible PI and a thin Si layer was used as the PMUT supporting structure layer. Compared with a pure Si supporting layer counterpart, the coupling coefficient had been improved by 110.68%. Additionally, the increase of Sc doping concentration in AlN film further enhanced the device electromechanical coupling coefficient and resulted in an improvement for transmitting/receiving sensitivity of the proposed flexible PMUT. When the doping concentration of Sc reached 30%, the emission sensitivity was as large as 1.721 μm/V, which was 2.86 times greater than that of conventional AlN film-based PMUT. The receiving sensitivity was found to be 2.11 V/KPa, which was as high as 1.23 times the performance of an undoped device. Furthermore, the bending simulation result showed that the proposed flexible PMUT device can maintain a good mechanical stability when the bending radius is greater than 1.5 mm. The simulation of sound field characteristics demonstrated that the flexible PMUT based on AlScN could receive stable sound pressure signals under the bending radius of 1.5 cm.

Al $_{\mathrm{1-x}}$ Sc $_{\mathrm{x}}$ N 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 Al $_{0.8}$ Sc $_{0.2}$ N 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 k $_{\mathrm{t}}^{2}$ of 2.9%, which is about twice as large as that of pMUT made by Al $_{0.904}$ Sc $_{0.096}$ N (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