The plasma enhanced chemical vapor deposition of silicon nitride films from SiH4 and N-2 gases was investigated below 450 K in a helical resonator plasma reactor using in situ spectroscopic ellipsometry and in situ attenuated total reflection Fourier transform infrared spectroscopy. Silicon nitride growth proceeds through nitridation of a thin Si-rich region at the film surface and the effects of the externally controlled parameters, such as plasma power and pressure, on the film composition and structure can be understood in terms of this mechanism. The key factor that affects the film composition and structure is the active nitrogen (N-2*) flux arriving at the surface which determines the nitridation rate of Si-Si bonds. Silicon nitride films deposited using low plasma power and high pressure, conditions that produce low N-2* flux, contains a relatively high concentration of I-I in the form of Si-H and a-Si inclusions. Increasing the power or decreasing the pressure results in higher N-2* flux and nitridation rate. The films deposited under such conditions have less Sill and a-Si incorporation but higher NHx, (x = 1,2) which disrupts the interconnectedness of the nitride network resulting in films that contain voids. Film properties can be tailored and optimized between these two extremes by manipulating power and pressure.