Anodic oxide films on tantalum: Incorporation and mobilities of electrolyte-derived species

A detailed study has been undertaken of the distribution of species incorporated into anodic oxide films on tantalum from various electrolytes using secondary ion mass spectroscopy sputter depth profiling. The proportion of the outer layer containing incorporated electrolyte species to the total film thickness is strongly dependent upon the incorporated species resulting from the particular electrolyte employed. Thus, whilst the transport numbers for anodic tantalum oxide formation do not change during film growth in the different electrolytes, the incorporated species may be immobile (silicon species) and mobile inwards (phosphate and sulphate anions) or mobile outwards (boron species). The behaviour and mobilities of incorporated electrolyte species are explained by reference to a 'liquid' model of film growth and comparison of single metal-oxygen bond energies of the incorporated species in the anodic tantalum oxide.