The physical and electrical properties of nitrogen-rich silicon nitride films deposited by electron-cyclotron resonance chemical vapor deposition with silane and molecular nitrogen have been investigated for pressures below 0.4 Pa. No Si-Si bonding or oxygen has been observed in the nitride films by Auger spectroscopy, and no SIH or NH2 groups have been observed by Fourier transform infrared spectroscopy, showing that the films have the composition SiNy-z(NH)(z). As the pressure was decreased by lowering the nitrogen flow, the stress in the films became more compressive while the amount of N-H bonding in the films increased. The electron temperature determined by Langmuir probe measurements increased at lower pressures as the plasma made a transition from overdense to underdense. Despite the increasing stress, electron temperature and NH concentration, capacitance-voltage (C-V) analysis of metal-nitride-Si(100) diodes showed that the SiN/Si interface improved with decreasing pressure. Using Al gates and 5 Omega cm p-type Si(100) substrates an interface state density of 5 x 10(10) eV(-1) cm(-2) was determined by the high-low frequency C-V measurement method for 30 nm thick films deposited at a substrate temperature of 300 degrees C and the lowest pressure of 0.055 Pa. Bulk conduction by the Frenkel-Poole mechanism dominated the current-voltage characteristics for negative gate potentials and breakdown voltages >9 MV/cm were obtained at this pressure. In situ single-wavelength ellipsometry showed that the interface is formed predominantly by nitridation of the Si substrate, and thus its high quality must be attributed to this nitridation rather than the chemical vapor deposition process. The results are discussed in terms of recent models for defects at the SiN/Si interface.