THE COMPOSITION OF THE ALLOY FILM INTERFACE DURING ANODIC-OXIDATION OF AL-W ALLOYS/

The anodic oxidation at high faradaic efficiency of sputter-deposited Al-W alloys, containing 0.1 to 5 atom percent (a/o) W, has been examin ed using transmission electron microscopy and Rutherford backscatterin g spectroscopy. The oxidation is revealed to proceed in essentially tw o stages, with initial formation of relatively pure anodic alumina, ne glecting minor contamination from the electrolyte, and subsequent form ation of tungsten-contaminated alumina. In the initial stage, tungsten accumulates progressively in a layer of alloy, about 1.5 nm thick, ju st under the essentially pure anodic alumina film. The accumulation at tains levels corresponding to average compositions of the enriched lay er of 2.5, 25, and 30 a/o W for bulk alloy compositions of 0.1, 1.8, a nd 5 a/o W, respectively. The transition from accumulation of tungsten in the enriched alloy layer to incorporation of tungsten species into the anodic film is considered to be related to development of tungste n-rich clusters of critical size within the enriched layer. Subsequent ly, aluminum and tungsten atoms are incorporated into the anodic film in their alloy proportions in the presence of a steady-state enrichmen t of the alloy at the alloy/film interface. Due to the discrete nature of the incorporation process, tungsten is distributed nonuniformly, o n a fine scale, within the region of tungsten-contaminated alumina: Th e average migration rate of tungsten species in the film is about 0.38 that of Al3+ ions, thus resulting in films consisting of an outer lay er of relatively pure anodic alumina and an inner layer of tungsten-co ntaminated alumina.