Density multiplication of pores and their propagation in a thin layer of nanoporous alumina on silicon substrates

International audienceIn this work, perfectly organized triangular arrays of vertical nanopores are formed in an alumina matrix by combining a pre-patterning technique with the natural ability of alumina to form a triangular unit cell. More precisely, we imprinted a triangular array of indents on a thin layer of aluminum deposited on silicon substrates using nano-imprint lithography. During the anodization process, we forced the growth of pores in and in-between the indents obtaining a larger number of pores in the final alumina array than the initial number of indents patterned on the aluminum. Adapting the anodization conditions, a density multiplication by three was successfully achieved with a very good surface organization. The experimental details of the process are described in this paper. We studied in details the inner organization of the pores and we identified differences in their propagation between oxalic and orthophosphoric acid. The former showed a good surface propagation until 1500 nm in depth. On the contrary, the latter showed a perturbation in the organization at 450 nm: at this depth, the induced pores stopped whereas the indented ones rearranged into two or three. A longer shift in the initiation of the induced pores seemed to causes this poor propagation. A systematic study was performed to investigate the effect of the anodization conditions on the pores' propagation. We demonstrate that the optimization of the orthophosphoric acid concentration and the applied voltage towards harder anodization conditions, i.e. to higher values, allows a better control of this self-assembling process and deeper order propagation until more than 1000 nm. (C) 2015 Elsevier B.V. All rights reserved