Absorption, discharge, and internal partitioning behavior of hydrogen in the tantalum and tantalum oxide system investigated by in situ oxidation SIMS and ab initio calculations

Abstract

Hydrogen behavior in tantalum and tantalum oxide thin films was examined using the in situ oxidation secondary ion mass spectrometry (in situ oxidation SIMS) method previously developed by the authors. Oxidation of Ta films by the introduction of O2 into the sputter deposition chamber immediately after film growth was found to reduce the amount of H absorbed in the Ta films by 2.7 times for samples exposed to lab air at ambient temperature; the difference increased to 4.8 times for samples exposed to air at 300 °C. From these results, it is apparent that Ta absorbs H from H2 or through reaction with H2O in air and that an oxide film “cap” largely stymies H absorption. To investigate the redistribution of hydrogen during oxidation of Ta, sputtered Ta films were implanted with deuterium, and some were subsequently anodized. In situ oxidation SIMS analysis of Ta2O5/Ta bilayer films created by anodization of deuterium (D)-implanted Ta films revealed no deuterium in the upper Ta2O5 portion; however, the total amount of deuterium detected in the underlying Ta layer of the anodized samples was close to the total amount of deuterium measured in the Ta layer of a nonanodized, D-implanted Ta film. These results indicate that during anodization, D is concentrated in the residual metal region as it is excluded from the growing oxide film. Ab initio calculations of H interstitial defects in Ta and Ta2O5 revealed that the heat of formation, ΔH, for H interstitial defects in Ta is 1.31 eV lower than that of Ta2O5; this result is consistent with the observed H blocking property of oxide films and the observed redistribution of D from oxide to metal during anodization.