The plasma-enhanced chemical vapor deposition process for SiN=H~ films has been in use for over two decades, but the chemistry of the process has yet to be explained. In the Present work, the composition of a 13 MHz NH~-SiH4 parallel plate glow discharge plasma was analyzed by line-of-sight sampling from the film deposition plane into a triple-quadrupole mass spectrometer, which can resolve compositional ambiguities at a given mass number by utilizing collision-assisted secondary cracking. At low RF power, disilane was the main plasma product even when NHjSiH4 was 25/1, whereas at higher power (0.1 W/cm ~ of cross section) disilane was eliminated and tetra-aminosilane, Si(NH~)4, and the triaminosilane radical, Si(NH2)3, became dominant. The concentration of these aminosilanes closely tracked deposition rate, and they are believed to be the principal SiN=Hy film precursors. Films deposited with Si(NH2)~ maximized and disilane suppressed in the plasma were excess in N and contained no Si--H bonding, consistent with the precursor composition. Silane utilization was near unity. The composition and properties of films deposited under these "amino-saturated" plasma conditions were examined vs. substrate temperaiure, Ts. With increasing Ts, there occurred a densification, a loss of H and excess N in a 3/1 ratio, and an increase in tensile stress, suggesting surface and subsurface chemical condensation of the adsorbed precursors via 3Si(NH2)4 -~ Si3N4 + 8NH3 ~. Postdeposition flash desorption showed NH3, not H2, to be the main volatile product of condensation. These results demonstrate that plasma chemistry can be manipulated to control film properties in a predictable manner.