It is possible to prepare high-density, moisture-resistive and optically transparent SiN
x films by a catalytic chemical vapor deposition (Cat-CVD, often called Hot-Wire CVD) technique at low substrate temperatures such as 353K. The key point is the addition of a large amount of H
2 to the SiH
4/NH
3 system. Gas-phase diagnoses show that, in Cat-CVD processes, the H-atom density in the gas phase is typically more than one order of magnitude higher than that in plasma processes; these H atoms abstract atomic hydrogen on the growing surface and also contribute to the local heating of the substrate surface due to their recombination reactions. In addition, H atoms re-activate the catalyzer surfaces poisoned by SiH
4 to increase the decomposition efficiency of NH
3. Pressure cooker tests combined with FTIR measurements show that SiN
x films thus prepared are highly moisture-resistive even when the substrate temperature is as low as 293K. The water vapor transmission rate of plastic substrates covered with thin SiN
x films is less than the detection limits of a MOCON or a cup method. The transmission rate was decreased by more than two orders of magnitude by the SiN
x coating. The stress of the SiN
x films was typically less than 100 MPa and can be controlled from compressive to tensile by choosing the appropriate deposition conditions. It is concluded that SiN
x films prepared by Cat-CVD from SiH
4, NH
3 and an excess amount of H
2 can be used as passivation films for organic materials, including organic light-emitting diodes.
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