Thermal oxidation of β-Ta below 500 °C

Abstract Results are presented for the thermal oxidation behaviour of two types of β-Ta thin films below 500 °C. The thin films investigated were composed of the pure phase with no other observable phases or of mixtures containing b.c.c. tantalum and TaO z phases. Their crystalline structures were investigated by X-ray diffraction, and their compositions were determined by Auger electron spectroscopy combined with in situ ion sputtering. We showed that the allowable cell expansion of β-Ta due to the incorporation of interstitial oxygen at elevated temperature is less than 3.5% irrespective of the contents of the mixed phases. The oxidation proceeds via the precipitation of the suboxide related to the cubic oxide with cell constants a of 3.95 A and 7.90 A respectively with the subsequent formation of amorphous Ta 2 O 5 . The effects of annealing of the heat-treated thin films show that the suboxide has a very stable structure. The TaO z phase is not observed during the “normal” oxidation process in air of β-Ta films with thicknesses between 1000 and 4000 A, but it is observed when thin films incorporating certain amounts of oxygen are treated above 500 °C with no supply of oxygen from the atmosphere.