Fabrication of Ordered Arrays of Anodic Aluminum Oxide Pores with Interpore Distance Smaller than the Pitch of Nano-pits formed by Ion Beam Etching

We investigated a method for preparation of ordered nanopore arrays with the interpore distance of 60 nm by guided self-organization of anodic aluminum oxide with a prepatterned array of pits in the starting Al film. An ordered triangular array of 100 nm-pitch pits was formed on Al film by ion beam etching through an electron beam lithography fabricated mask, and then it was used as a guide for formation of anodic aluminum oxide pores. We found it was possible to reduce the interpore distance to 1/√3 of the pitch of the pits by the appropriate choice of the parameters of ion beam etching and anodization voltage.

Anodic aluminum oxide (AAO) has ordered honeycomb structure of pores obtained by self-organization; these pores are perpendicular to the surface and parallel to each other [1-3]. The interpore distance of AAO can be controlled from several tens of nanometers to hundreds of nanometers depending on the anodic voltage and the type of aqueous acid solution used as the electrolyte [4-9]. Moreover, the diameter and the depth of the pores are also controllable by growth conditions, such as anodic voltage and duration of anodization, respectively. The regularity of AAO pore arrangement is one of the important issues for applications such as ultra-high density patterned media using magnetic nanowire arrays [10-16], and the patterning of surfaces for sensors [17-19], optically active surfaces [20,21], and for substrates for controlled living cell cultures [22-24], and AAO nanopore arrays are considered as the suitable structure for preparation of nanowire and nanodot of various materials [25], such as magnetic and semiconductor materials. However, it is known that the ordered AAO pore arrays can be obtained by several appropriate growth conditions [26], and the inter-pore distances of ordered AAO pore arrays are limited. Moreover, preparation of fine pitch arrays is getting more difficult, especially below 100 nm.
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