This paper describes the fundamental mechanism for the formation of a 3 dimensional porous template
during the anodization of Al with less than 1at. % 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 on 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.