Thermal oxidation of the silicides CoSi 2, CrSi 2, NiSi 2, PtSi, TiSi 2 and ZrSi 2

Abstract

The formation of SiO 2 layers on the silicides CoSi 2, CrSi 2, NiSi 2, PtSi, TiSi 2 and ZrSi 2 during thermal oxidation was studied by using Rutherford backscattering of 2 MeV α particles. Oxidation was carried out between 750 and 1100°C in a quartz tube furnace through which steam or dry oxygen flowed at atmospheric pressure. Pure SiO 2 layers could be grown on all the silicides except ZrSi 2, in which case a mixed SiO 2-zirconium oxide layer formed. It is also shown how the chemical stability of a silicide can be predicted from thermodynamic considerations. The silicides NiSi 2 and PtSi were found to be structurally very unstable at temperatures near or above the lowest eutectic temperatures of 966°C and 830°C in the respective binary systems. These two silicides, however, are structurally stable when oxidized on SiO 2 substrates. During oxidation under such conditions good SiO 2 layers could be formed on CoSi 2, CrSi 2, NiSi 2 and PtSi, the silicides transforming to the more metal-rich phases. In the case of TiSi 2 and ZrSi 2, metal oxidation took place. Kinetic measurements show that SiO 2 formation on the silicides is much larger during steam oxidation than during oxidation in dry oxygen. Tracer experiments using radioactive silicon show that silicon is very mobile in the silicide during oxidation and that SiO 2 formation takes place at the silicide-SiO 2 interface, the oxidant diffusing through the growing SiO 2 layer.