Ultra-thin porous anodic alumina films with self-ordered cylindrical vertical pores on a p-type silicon substrate

Ultra-thin alumina films with self-ordered cylindrical vertical pores were fabricated on a p-type silicon substrate by anodization of Al films with thickness in the range of 30–500 nm in sulfuric or oxalic acid aqueous solutions. In both cases the pores were arranged in hexagonal cells in a close-packed structure and their diameter and density depended on the electrochemical solution used. In the case of sulfuric acid both 30 and 500 nm Al films resulted in a similar uniform porous structure using exactly the same anodization conditions for both thicknesses, the pore diameter being in the range of 10–30 nm and their density of the order of 6–8 × 10 10 pores cm −2 . In the case of oxalic acid the 500 nm thick films resulted in a uniform porous structure with larger pores than in sulfuric acid, of diameter in the range of 20–40 nm and a density of the order of . On the other hand, with oxalic acid it was impossible to form a uniform porous structure from the 30 nm thick Al film at the same conditions as used for the 500 nm thick film. Plan-view and cross-sectional transmission electron microscopy was used to investigate systematically the structure and morphology of the alumina films. Cross-sectional TEM images showed that the alumina/Si interface was sharp, but a void was observed beneath each pore, separated from the pore by a thin alumina layer. The same structure was obtained with both electrolytes. The effect of pre-annealing of the Al films on the anodic alumina layers was also investigated in detail.