The catalytic decomposition conditions of O-2, N2O and NO were investigated to avoid oxidization of heated tungsten filaments. It was confirmed that no oxidization takes place in the presence of an excess amount of H-2 or NH3: for example, when the catalyzer temperature is 1990 K and the H-2/O-2 flow rate ratio is more than 18. These results are consistent with the recent results that no oxidization is observed in the SiH4/ NH3/H-2/O-2/He system to deposit SiOxNy films as long as the O-2 flow rate is low. In addition, it was revealed that in this SiH4/NH3/H-2/O-2/He system, O-2 is not only decomposed on the catalyzer surfaces but also is consumed in homogeneous and/or heterogeneous reactions. One of the consumption paths in the gas phase should be the reaction with SiH3 to produce SiO. (c) 2007 Elsevier B.V. All rights reserved.