Enhanced Vertical Sidewall Quality for Functional AlN Films in 3D Piezo MEMS Applications

Abstract

Integrating piezoelectric materials onto the vertical surfaces of microelectromechanical systems (MEMS) microstructures enables three-dimensional piezoelectric (3D piezoMEMS) devices, providing multi-axis sensing and actuation capabilities with a reduced device footprint. Metal–organic chemical vapor deposition (MOCVD) is the preferred method for depositing highly crystalline, c-axis oriented aluminium nitride (AlN). However, achieving optimal film quality requires vertical surfaces with minimal roughness and uniformity. Traditional etching techniques, such as wet and plasma etching, often result in rough and irregular surfaces, which challenge the crystal quality of the deposited AlN. In this study, cryogenic deep reactive ion etching (cryo-DRIE) is used to fabricate vertical sidewalls with a root mean square roughness of 26 nm and waviness of 131 nm, followed by hydrogen annealing to further enhance surface quality. Hydrogen annealing reduces the roughness to 7 nm, but the waviness varies depending on the pre-annealing surface conditions. When AlN is deposited on these treated surfaces using MOCVD, the films exhibit high crystal quality comparable to those grown on wet-etched surfaces with extremely low roughness. A slight misalignment of the AlN c-axis orientation with the Si (111) plane is observed, with localized surface irregularities impacting grain alignment in specific areas.