Study of surface reactions during plasma enhanced chemical vapor deposition of SiO2 from SiH4, O-2, and Ar plasma

In situ attenuated total reflection Fourier transform infrared spectroscopy was employed in proposing possible surface reaction mechanisms during plasma enhanced chemical vapor deposition of SiO2 from a mixture of SiH4, O-2, and Ar in a helical resonator plasma reactor. Infrared spectra taken during the oxide deposition revealed that the oxide surface is covered with OH when the deposition is conducted with a low SiH4 to O-2 ratio (much less than 1). Both associated hydroxyls (SiOHassoc) and isolated hydroxyls (SiOHisol) were detected on the surface during growth. Two kinds of OH attached to Si exist on the surface: weakly bound OH that thermally desorb at 250 degrees C and strongly bound OH that desorb under ion bombardment. The thermal removal of weakly bound OH does not lead to SiO2 formation, but the ion-assisted removal of strongly bound OH by Ar+ ion bombardment results in oxide growth via the reaction 2SiOH((s))+Ar+-->Si-O-Si-(s)+H2O(g))+Ar+. Experiments undertaken to delineate the possible heterogeneous reaction paths between surface species and gas phase reactants showed that the molecular fragments of SiH4 (SiHx=1, 2, or 3) react with surface hydroxyl groups and produce surface hydrides such as HSiO3, H2SiO2, and H3SiO. The silane fragments also react with Si on the surface to give silicon hydrides whose Si constituent is backbonded to other Si atoms in addition to O [e.g., HSi(SiO2), H2Si(SiO), HSi(Si2O), H3SiSi, and H2SiSi2]. The latter reactions become more important as the OH on the surface is depleted, and the surface becomes silicon and silicon hydride rich. A subsequent exposure of the silicon-hydride-covered oxide surface to oxygen plasma creates SiOHassoc groups presumably by insertion of O into Si-H bonds of surface hydrides. The silicon hydrides are not observed during deposition using low SiH4 to O-2 flow rate ratios (much less than 1): silane fragments, upon adsorbing onto the surface, are promptly oxidized by O atoms, leading to a SiOH-covered surface at steady state conditions. Bombardment of the surface by ions such as Ar+ and O-2(+) assists H2O desorption from SiOH forming Si-O-Si bonds. (C) 1996 American Vacuum Society.