The oxidation of polycrystalline tantalum at low pressures and low temperatures

The oxidation of polycrystalline tantalum foil was investigated at pressures between 10-8 and 10-4 Torr and temperatures in the range 300-800 K by means of Auger electron spectroscopy coupled with sputter depth profiling. It was found that when a chemically clean tantalum surface is subjected to a given oxygen exposure a diffusion of the oxygen into the bulk occurs simultaneously with the chemisorption on the surface. The kinetics of this chemisorption-diffusion process were determined. The kinetics for oxide growth on polycrystalline tantalum were also investigated. After an initial oxidation stage which depends on the temperature, the oxidation kinetics follow a temperature-independent logarithmic type of law of the form z=a+b[ln(L+1)] where z is the oxide thickness, L the oxygen exposure and a and b are constants. This result is interpreted in terms of the Mott theory, based on electronic tunnelling, for the growth of very thin oxide films. (Auth.)