Variations in the physico-chemical properties of near-stoichiometric silica deposited from SiH4-N2O and SiH4-N2O-He radiofrequency discharges

Silicon oxide thin films were deposited at low temperatures by Plasma-Enhanced Chemical Vapour Deposition (PECVD) from silane-nitrousoxide (SiH4-N2O) and silane-nitrous oxide-helium (SiH4-N2O-He) mixtures. The film structure is shown to vary with the position in the reactor by means of several physico-chemical analyses (infrared, Raman, X photon spectroscopy). The main features are an increase of both the constrain of the Si-O-Si bridges and film compactness, whereas the amount of neighboring-OH groups decreases. The appearance of those specific OH groups is favored by a progressive post-venting hydrolysis. By comparing the experimental results with modeling results, the film property modifications are put in relation with the film precursors and the deposition rate. In some conditions and especially in helium-diluted mixtures, stoichiometric and dense films but different from thermal silica are obtained. A low deposition rate seems to be a determining factor in order to obtain such films, but the helium is also shown to play a special role.