Study of surface reactions during plasma enhanced chemical vapor deposition of SiO₂ from SiH₄, O₂, 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, O₂, 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 O₂ ratio (≪1). Both associated hydroxyls (SiOH_assoc) and isolated hydroxyls (SiOH_isot) 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 °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)+H ₂O_(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 (SiH_x, x=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 siliconhydride-covered oxide surface to oxygen plasma creates SiOH_assoc. 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 O₂ flow rate ratios (≪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 O₂⁺ assists H₂O desorption from SiOH forming Si-O-Si bonds.