Activated reactive evaporation deposited silicon nitride as a masking material for MEMS fabrication

Abstract

Silicon nitride films were deposited in a nitrogen gas plasma by an ‘activated reactive evaporation’ (ARE) process on silicon substrates maintained at room temperature. Silicon powder was evaporated by e-beam evaporation in the presence of the nitrogen gas plasma excited by a radio frequency (RF) power source (13.56 MHz). The films deposited at 80 W RF plasma power for 30 min deposition time were characterized by x-ray diffraction, x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy and ellipsometry techniques. The x-ray diffraction pattern reveals the polycrystalline nature of the films with characteristics of hexagonal structure even at room temperature. X-ray photoelectron spectroscopy showed formation of silicon-rich silicon nitride. The FTIR spectrum shows Si–N bond formation and absence of Si–H bond, indicating hydrogen-free films. Scanning electron microscope shows pinhole and crack-free surface of the films without any particulates of silicon on the surface. The refractive index (η) and the thickness (d) are observed to be 1.99 and 1800 Å, respectively. The present work also describes the results on application of ‘ARE’ silicon nitride thin films as a masking material for the anisotropic etching of silicon in aqueous KOH solution. A pattern was lithographically transferred through the masking material on the silicon substrate. The etch rate of the films ∼4 Å min−1 was calculated from the masking time data of the films. The activation energy of 0.52 eV was determined by the relationship between temperature and the rate constant, which is in good agreement with the results reported in the literature. The etch rate value is low as compared to the etch rate of the conventional masking materials (60 Å min−1 for SiO2 and 8 Å min−1 for LPCVD Si3N4). Thus, ‘ARE’ silicon nitride films withstand higher concentrations of KOH for larger time periods compared to the conventional masking materials and act as a good masking material for MEMS.