Novel pre-oxidation pattering on thin aluminum film generating ordered nanopores through anodization
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Novel pre-oxidation pattering on thin aluminum film generating ordered nanopores through anodization Giovanni Fois1, Ciara T. Bolger1,2 ,Justin D. Holmes1,2 and Graham L. W. Cross1,3 1 Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland. 2 Materials and Supercritical Fluids Group, Department of Chemistry and the Tyndall National Institute, University College Cork, Cork, Ireland. 3 School of Physics, Trinity College Dublin, Dublin 2, Ireland.
ABSTRACT Anodic Aluminum Oxide (AAO) is widely employed as a template for fabrication of nanowires and nanotubes due to its ability to generate self organized (SO), well ordered pore structures. We have developed a new aluminum pre-patterning technique to create well ordered nanopore arrays on thin films deposited on silicon substrates. We form patterns of thicker oxide on the surface via local oxidation process using a conducting Atomic Force Microscope (AFM) tip working in contact mode. Pores are forced to nucleate between the pre-oxidized regions during the anodization process. The relation between applied voltage and ordered interpore distance has been found to be linear for these supported thin films. However, the pore spacing is highly reduced compared to free standing foils. A new empiric law has been confirmed for a wide range of voltages, solution concentrations and different electrolytes, including oxalic and phosphoric acid. Our results show that pre-oxidation patterning is an alternative technique to achieve an ordered nanoporous template through the anodization process. INTRODUCTION Porous anodic alumina (PAA) has long been considered a promising material for templated growth of nanostructured materials for application in electronics, magnetics and optics[1-3], as well as mask for pattern transfer through wet and dry etching[4]. Since Masuda et al.[5] reported the formation of highly ordered porous layers of aluminum oxide with a hexagonal configuration, an increasing emphasis has been put on PAA capabilities to give easy access to a self-organized ordered array of hexagonal pores with high aspect ratio and tunable pore size, with exceptional thermal stability over a wide range of temperature. Anodization is an electric field-enhanced process during which a pore develops in a sequence of three steps: Random nucleation on the surface; self organization, in which pores rearrange under the surface to achieve a close packed, ordered configuration; and steady state pore growth. While self-ordering in PAA has been studied for more than 50 years, a comprehensive theoretical model explaining pore formation yet to be developed. Most proposed models examine the self organization process and the steady state pore growth. In 1998, Jessensky et al.[6] proposed that self organization was driven by mechanical stress at the metal/oxide interface due to the volume expansion of the aluminium during oxidation. This induced stress creates an upward flow of the oxide from the pore bottom, driving pore rearrangem
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