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

Abstract

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 × 1010 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.

Nanotemplate alumina films on a silicon substrate fabricated by electrochemistry

“…In (a) and (c) the thickness of the Al film before anodization was 500 nm, while in (b) and (d) it was 30 nm. The circle around each pore that we observe in images from thinner samples reflects a void of larger diameter below the pore; identified in cross sectional TEM images [12] (see also figure 3a). We have to note here that by changing the anodization conditions, it is possible to find conditions for which the resulting films are identical for both thicker and thinner films.…”

“…Otherwise, phases II and III should coincide. If we stop anodization when the current density starts to fall to zero (end of phase III), the interface of alumina with Si shows the typical form illustrated in figure 3a, where we identify a void between a thin alumina barrier layer at the bottom of the pore and the silicon substrate [12][13]. The barrier layer is easily dissolved if the sample is immersed in a 5% in weight phosphoric acid aqueous solution for 2-3 minutes and then the pore is connected to the silicon substrate.…”

“…Two different Al thicknesses were used, 30 and 500nm. Details on the electrolyte concentration and the voltage used are given in table I [12]. For each electrolyte we used two different anodization conditions, in the range of optimum conditions in the literature for the most homogeneous structures [3][4][5].…”

The influence of aluminum grain size on alumina nanoporous structure

“…The native silicon oxide of p-type Si was removed using 48% HF acid solution for 30 s prior to deposition of the Al film. The samples were annealed at 500°C for 3,6,15,18, and 24 h in a conventional furnace at a pressure of 3 ϫ 10 −4 Pa to relax the internal stress and to promote grain growth of the polycrystalline Al thin films.…”

“…Kokonou et al 18 reported that for Al films with thicknesses of 500 nm anodized with electrolytes, oxalic, or sulfuric acid solutions, D p values around 30-40 nm were obtained. This is in good agreement with the expected values calculated from the proportionality constant, 1.29 nm V −1 .…”