Recently, plasma-CVD silicon nitride has been used for LSI passivation films. In this study, to deposit SiN films by the plasma-CVD method, SiH4 and N2 are used as reactant gases. The correlations between SiH4/N2 gas ratio and properties of SiN films were investigated. When SiH4/N2 gas ratio is varied in a range from 1.0% to 2.5%, the properties of SiN films are varied as follows; deposition rate 200-350 Å/min. refractive index 1.77-2.15, Si/N ratio 0.5-2.0. When SiH4/N2 gas ratio becomes smaller, the oxygen content is remarkably increased, and some properties of SiN as passivation films deteriorate with the oxygen content in the films. The optimum SiH4/N2 gas ratio in this study was 2.0%, at which the buffered HF etch rate was the minimum value of 60 Å/min., the electric field strength at the step region was the maximum value of 4.8 MV/cm and the Na+ blocking effect was the strongest.