Mobility of copper ions in anodic alumina films

Transmission electron microscopy of ultramicrotomed sections and Rutherford backscattering spectroscopy have been used to quantify the significantly faster mobility of copper ions compared with Al3+ ions in anodic alumina films. To determine the migration rate of copper ions, an Al-0.4 at % Cu alloy film of ca. 35 nm thickness has been sputter deposited onto an electropolished superpure aluminium substrate; the specimen comprising the alloy layer superimposed on aluminium was then anodized at a constant current density to various voltages at high current efficiency. The anodic oxidation of the alloy film results in the prior oxidation of aluminium and the accumulation of copper in a layer of alloy, ca. 2 nm thick, just beneath the anodic film; consequently, no copper is incorporated into the alumina film during anodizing of the alloy. At ca. 49 V for the particular alloy thickness, the alloy film is totally consumed by anodizing and the copper-enriched layer beneath the anodic film is incorporated abruptly into the film, because of the presence of an air-modified electropolishing film sandwiched between the alloy and aluminium regions. With further anodizing, the incorporated copper ions migrate outwards at a rate ca. 3.2 times that of Al3+ ions. The method employed provides a novel approach to determining the precise mobility of foreign ions incorporated into anodic alumina. (C) 1997 Elsevier Science Ltd.