Plasma oxidation mechanisms in tungsten silicide thin films

The oxidation of tungsten disilicide in a rf oxygen plasma at floating potential in the 300–900 °C temperature range has been investigated. The oxidation kinetics and the elemental depth distribution in the films have been analyzed by the complementary use of Rutherford backscattering spectrometry and nuclear reaction analysis. It has been found that the nature of the oxide largely depends on temperature. A mixture of WO3 and SiO2 is grown in the 300–650 °C range. The W:Si ratio in these oxides decreases monotonically with temperature reaching a minimum value at 650 °C. As a matter of fact, a SiO2 surface layer is formed when oxidation is carried out in the 650–900 °C range. Oxygen diffusion through the growing oxide seems to be the dominant rate controlling process. Despite the fact that oxygen diffusion is a process activated by temperature, oxygen diffusion through the mixture of WO3 and SiO2 proceeds more rapidly than through SiO2. This leads to an enhancement of the growth rate at temperatures near 450 °C. Finally, a mechanism of plasma oxidation of tungsten silicide films has been proposed.