Hydrogenated amorphous silicon nitride (a-SiNx:H) films fabricated by plasma-enhanced chemical vapor deposition at low temperatures (<150 degrees C) tend to oxidize within 1 month because of their porous structure. Therefore, these low temperature a-SiNx:H films are not suitable for application in the passivation of microelectronic devices. It is found, however, that low temperature amorphous silicon (a-Si:H) films prepared by plasma-enhanced chemical vapor deposition resist oxidation, indicating a relatively less porous structure capable of resisting the percolation of H2O molecules. Therefore, if a thin layer (6.4 nm) a-Si:H film is deposited at 100 degrees C on a 100 degrees C deposited a-SiNx:H film, the double film structure resists oxidation for more than 14 months of air exposure. This indicates that the composite film structure has the potential to be utilized for device passivation of thin film transistors when a low temperature (<150 degrees C) process is required.