Plasma deposition of silicon nitride on silicon substrates in a microwave electron cyclotron resonance N2/Ar/SiH4 discharge was studied as a function of gas pressure (1–5 mTorr), gas composition, and discharge power (250–1000 W). The dependencies of deposition parameters on discharge characteristics obtained at 1 mTorr appear to be essentially different from those at higher pressures. Optical emission spectroscopy was used for plasma characterization. A high degree of ionization and dissociation of gas molecules was observed under present plasma conditions. It is shown that the contribution of ionized species to film deposition is comparable with that of neutral ones under high power and low pressure conditions. The best quality of films was obtained at a moderate rather than the highest available dissociation degree of silane. © 1997 American Vacuum Society.