The Formation Mechanism of 3D Porous Anodized Aluminum Oxide Templates from an Aluminum Film with Copper Impurities

Abstract

This paper describes the fundamental mechanism for the formation of a 3-dimensional porous template during the anodization of Al with less than 1 atom % Cu percentages. It is known that the presence of Cu impurities in an Al film introduces horizontal pores interconnecting the vertically aligned porous structure of the anodized aluminum oxide (AAO) template. We show that the formation of these horizontal pores is accompanied by current density oscillations when the anodization is performed at a constant voltage. The frequency of these oscillations is directly related to the horizontal interpore distance. We propose a mechanism that links the current density oscillations to the Cu accumulation at the metal/oxide interface through the cyclic change in anode potential. The distance between the horizontal pores is found independent of the current density, temperature, and electrolyte concentration. Instead, it was found that the spacing between the vertical pores, and thus, the anodization voltage determines the spacing between the horizontal pores. A model based on the plastic flow of the alumina barrier layer was suggested to link the spacing between the horizontal and the vertical pores. These results provide important insights in the formation of 3D AAO templates. In addition, we show the fabrication of rigid 3D metal nanomeshes by electrochemical deposition into these 3D porous templates.