Plasma Etching of Aluminum Nitride and Molybdenum in Multidimensional MEMS Structures

ABSTRACT The transition to 3D device architectures is a rapidly advancing trend in microelectronics. As a result, several manufacturing technologies have has to rapidly evolve to meet the challenge of more complex structures, whilst maintaining the high precision needed for functional devices. Currently, plasma etching of thin films on high aspect ratio 3D structures, like multidimensional piezoelectric microelectromechanical systems (piezoMEMS), has not been sufficiently studied. This work studies plasma etching of multidimensional structures using aluminum nitride (AlN) and molybdenum (Mo) thin films as the etched material, to determine the isotropicity of the etching and damage to exposed vertical thin films. The etching process is characterized through horizontal and vertical etch rate, and the surface roughness of the films. Chemical etching and ion milling are> found to contribute to the isotropic etching of Mo, however, low volatility products appeared to act as a mask, partially protecting sidewall films from ion milling. Conversely, etching of AlN appeared to be entirely anisotropic, further showing that low volatility ions prevent isotropic ion milling. This work provides the first look into patterning multidimensional piezoMEMS structures and acts as a base for future optimization of similar processes.