Direct observation of anodic film growth on superimposed aluminium and tantalum metallic layers

Abstract

A direct electron optical study has been made of anodic film growth on substrates comprising a tantalum layer superimposed on aluminium and the reverse situation (i.e. an aluminium layer superimposed on tantalum). To achieve the latter, the system comprised an ion-plated aluminium layer superimposed above a tantalum layer which had been ion-plated initially onto an electropolished aluminium substrate. For the tantalum-above-aluminium situation, observations of ultramicrotomed anodic film sections revealed a dark band in the outer region of film, consistent with the initial development of an anodic tantalum oxide followed by uniform anodizing of aluminium to form the usual anodic alumina film material. In the case of aluminium superimposed above tantalum, direct observation clearly showed the development of tantalum oxide fingers which penetrated the initially formed anodic alumina to different extents. The non-uniform tantalum oxide film growth may ultimately short-circuit the alumina and result in the development of a tantalum oxide film overlying the anodic alumina film. However, in the present situation, the ion-plated tantalum layer was thin and its local consumption through anodic oxidation allowed anodizing of the underlying aluminium. In this case, the tantalum oxide fingers continued to penetrate the the initially formed anodic alumina, with the development of anodic alumina between the tantalum oxide band and the aluminium substrate. The behaviour observed when the initially formed anodic oxide is of higher resistivity than the subsequently formed anodic oxide of the buried metal (for aluminium above tantalum) clearly points to a microscopic mechanism of finger penetration rather than a macroscopic mechanism based solely on increased probabilities of movement of the cation of the buried metal in the initially formed anodic oxide.