The deposition mechanism of Si3N4 films from dichlorosilane (DCS) and ammonia, formed by a conventional low-pressure chemical vapor deposition system, was studied, combining mass spectrometric analysis and the "macrocavity method." The dominant neutral species in the gas phase are NH3 and aminochlorosilane (ACS), which was produced by the gas-phase reaction of DCS, in a high NH3 (a large NH3/DCS ratio) condition, while DCS dominantly exists in a low NH3 condition. Contrary to the mass spectrometric results, there is not a major dependence of the kinetic constants of the deposition precursors on the gas-mixture ratio. By fitting the growth-rate profiles to an optimal theoretical profile, both of the two kinds of precursor are estimated to have low activity. The conformal step coverage quality agrees with the small sticking probabilities estimated as 10(-4) and 10(-6). Those precursors can be assigned to DCS and ACS, so that the mass spectrometric data would be consistent with the growth profiles. The conversion to ACS is calculated based on an assumed simple kinetic form and leads to a qualitative interpretation for gas-mixture ratio-dependence of conversion. A kinetic model including precursor molecules is presented to correlate the mass spectrometric results, the growth-rate profiles, and the step coverage quality.