THE MIGRATION OF FLUORIDE IONS IN GROWING ANODIC OXIDE-FILMS ON TANTALUM

Film growth conditions were selected to enable the extent of migration of fluoride ions in anodic oxides on tantalum to be determined precis ely. By using a preformed film formed in sodium orthosilicate electrol yte in which the incorporated silicon species are immobile, a marker i s generated to which mobilities may be referenced. Thus, by reanodizin g the preformed film in fluoride electrolyte, secondary ion mass spect roscopy depth profiling allows precise determination of the extent of migration of the inwardly mobile fluoride ions. Fluoride ions migrate inward at a rate 1.85 times faster than that of O2- ions, leading to t he development of a fluoride enriched region of TaF5 between the tanta lum substrate and the outer anodic tantala oxide. From this base, cons ideration is given to the varied processes involved in ionic transport in amorphous anodic oxides and the requirements for enhanced understa nding.