Silicon nitride (Si-N) and oxynitride (Si-O-N) thin films are obtained by low pressure rapid thermal chemical vapor deposition (RT-LPCVD) by using the reaction of diluted silane (SiH4/Ar=10%) with ammonia (NH3) or a mixture of ammonia and nitrous-oxide (N2O) at various gas ratios (R=NH3/SiH4 and R’=[N2O]/[NH3+N2O]). Deposition kinetics of silicon nitride films have been studied in the medium to high temperature range (700-850 degrees C) at fixed total pressure of 9.5 mbar and gas flow ratio (R=10). Oxynitride deposition rates have been also studied as a function of R’ at fixed temperature and total pressure parameters of 750 degrees C and 9.5 mbar respectively. Rutherford backscattering spectrometry results showed that the RT-LPCVD Si-N films are silicon-rich and that the composition stoichiometry is mainly controlled by the process parameters, particularly the gas flow ratios. Si3N4 silicon nitride stoichiometry can only be approached for R greater than or equal to 10. Fourier transform spectrometry measurements revealed that the Si-O-N films have a particular structure formed by a mixture of Si-O and Si-N bonds with no apparent separated phases. In addition, in contrast to the few 10 at.% usually reported for classical CVD processes, no significant traces of hydrogen bonds (SI-H or N-H) were detected for R greater than or equal to 10. Furthermore, secondary ion mass spectrometry analysis always indicated that Si-N and Si-O-N RT-LPCVD deposited layers are homogeneous along the whole film thickness.