Self-organised nanopore structures in anodic alumina films have attracted great attention due to their high pore density and their potential use for masking or information storage. When the pores are filled with metals or semiconductors these films can be fabricated into interesting magnetic recording, electronic and electrooptical devices.

The objectives are to generate experimental data on the formation of dense arrays of pores on aluminium foils and to correlate the structural and functional characteristics of such foils to the electrochemical parameters (composition of solution, temperature, cell voltage ...) used for their synthesis. As templates with known pore densities become available they can be used to deposit metallic materials within the pores used pulse electroplating (deposition of Co, Ni, Cu, multilayers,…). On the other hand, demonstration material will be elaborated like filters with pores of controlled size.

• Selection, synthesis and characterisation of aluminium foils, and development of a methodology for preserving the base material integrity during subsequent manipulations will be done,
• Determination of a practical annealing and electropolishing procedure with emphasis on an ideal macro and micro flatness of the substrate material,
• Research onto the initial pore density generated by hard anodising aluminium will be performed,
• Manufacturing of fine membranes will be worked out,
• Finally, electrodeposition of metals into the arrays of pores on aluminium will be investigated.

The quality of aluminium substrates and their surface pre-treatment have a large influence on the surface ordering by electrochemical processing. A characterisation of the initial surface conditions and the subsequent chemical and/or electrochemical pre-treatments of the base materials were done. Development of a methodology for preserving the base material integrity during subsequent manipulation - was done on the base of experiments with commercially available aluminium foil. Procedures for the pre-treatment of aluminium foil (annealing and electropolishing) which result in a smooth and flat surface of the starting material, were successfully worked out.

One-, two-, and three-step anodising procedures were used. In the one-step process, aluminium foils were anodised at different cell potentials, but the formation of well-ordered nanopore arrays in anodic alumina was not successful. The pore structure on such anodised aluminium is rather disordered. In the two-step anodising process, the process was started with a short first anodising step at a pre-selected cell potential. Subsequently, the oxide layer was chemically etched away. After chemical removal of the oxide layer, the aluminium was anodised again for a long duration at the same cell potential as used during the first anodising step. A top view of the pores formed after the second anodising step at is shown in Fig. 1. Hexagonally arranged pores are seen over a large surface area of samples produced at both cell potentials. The pores grow as parallel channels as shown in Fig. 2. In the case of a three-step process, the oxide layers obtained after the first and the second anodising step were chemically removed. Finally, the aluminium was anodised for a third time for a long duration at a pre-selected cell potential. A third anodising step does not significantly improve the ordering of the pores and the size of the well-ordered domains.

The manufacturing of fine membranes by anodising aluminium foils was also developed. Membranes can be obtained from anodised aluminium after removing of the remaining Al base and chemical opening of the pores. A SEM image of the bottom part of the membrane anodised at 23 V and after chemical opening of pores is presented in Fig. 3.

The problem of depositing in a controlled and progressive way magnetic material in the pores resulting from the anodising step is under investigation.

Fig. 1. SEM micrographs of the self-ordered hole configuration of anodic porous alumina obtained in sulphuric acid solution after two-step anodization.

Fig. 2 Cross-sectional SEM image of oxide layer after two-step anodising at 23 V.

Fig. 3 SEM bottom view of porous alumina membrane obtained in two-step anodising at 23 V.

G.D. Sulka, S. Stroobants, V. Moshchalkov, G. Borghs, J-P. Celis, 'Synthesis of well ordered nanopores by anodising aluminium foils in sulphuric acid', J. Electrochem. Soc., 2002 in press.

This research is partially supported by DWTC (Diensten van de Eerste Minister Wetenschappelijke Technische en Culturele Aangelegenheden, Belgium) and FWO - Vlaanderen (Flemish Science Foundation, contract G. 0299.99).