ANODIC OXIDE-FILMS ON TANTALUM - INCORPORATION AND MOBILITIES OF ELECTROLYTE-DERIVED SPECIES

A detailed study has been undertaken of the distribution of species in corporated into anodic oxide films on tantalum from various electrolyt es using secondary ion mass spectroscopy sputter depth profiling. The proportion of the outer layer containing incorporated electrolyte spec ies to the total film thickness is strongly dependent upon the incorpo rated 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 incor porated 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 expl ained by reference to a 'liquid' model of film growth and comparison o f single metal-oxygen bond energies of the incorporated species in the anodic tantalum oxide.