Initial Stages of Oxide Growth and Pore Initiation in the Porous Anodization of Aluminum

Abstract

Ellipsometry studies of evaporated aluminum films anodized in at constant current indicate that the anodic oxide produced is at first homogeneous with a refractive index of 1.62, as found for barrier anodic oxide of thin film and bulk Al. As the thickness increases, the optical properties change signifying the onset of pore formation. This occurs at increasing thicknesses with increasing current density, however, pore formation always occurs well before the peak in voltage is reached at constant current. Ellipsometry indicates that the mode of development of the oxide prior to pore initiation is similar to that of barrier anodic oxides. However, the efficiency of oxide growth and the electric field required to pass a given ionic current, , are generally lower and approach values for barrier oxides (of thin film and bulk Al) only at high current densities. The curvature of the curve is in the opposite sense to that normally observed for anodic oxides and it is interpreted in terms of cation and anion motion. After pore formation the efficiency of oxide growth increases and becomes near unity when steady‐state formation conditions are reached. These results appear consistent with the model that dissolution at the pore bases is an electric field‐aided process.