The chemistry of silicon nitride deposition from ammonia-silane and nitrogensilane mixtures in a glow-discharge plasma was analyzed using special mass spectrometric techniques involving line-of-sight sampling through an orifice in the wafer plane of a parallel-plate reactor. It was found that high rf power is needed to activate the nitrogen source sufficiently, and that silane flow must be held below a certain critical value to ensure an excess of activated nitrogen. This critical value is detectable by the appearance of disilane in the plasma, which is indicative of silane reacting with itself. At low power, silane reacts only with itself, and disilane is the dominant gaseous product. Conversely, disilane-free plasmas in either gas mixture result in films that are nitrogen-rich and free of the Si-H bonding that has been correlated with poor dielectric behavior. Utilization of silane to form SiNxHy is near unity. The aminosilanes, SiH4-n(NH2)n, are the sole gas-phase Si-N products of the ammonia mixture, and the triaminosilane radical, Si(NH2)3, is the key deposition precursor. The precursor is transformed into SiNxHy by way of a temperature-driven chemical condensation reaction at the surface, with the evolution of ammonia. The nitrogen mixture produces no gas-phase Si-N products, and deposition proceeds by the reaction of SiHn radicals and N atoms at the surface. These basic differences in deposition mechanism result in significant and logical differences in the physical and electrical properties of films deposited from the two mixtures.