Anodized pore structural evolution of focused ion beam patterned Al: direct analysis of branched nanopores and nanosacks

In this work we describe three different trends of pore growth for anodic aluminum oxide nanopores based on their dependence on prepatterned interpore distances. Nanopatterned hexagonal concave arrays were formed by focused ion beam (FIB) lithography on aluminum foil with interpore distances in the range of 100 to 240 nm and the Al foil was anodized under the standard conditions known to result in a 100 nm interpore distance. This method allowed a systematic investigation of pore formation under the non-equilibrium conditions created by the FIB prepatterning. The pore diameter and the pore growth direction, which are affected by the interpore distance, were measured by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis with ion milling. When the interpore distance increases from 100 to 140 nm, the pore diameter becomes larger and nanopores are slightly tilted but maintained the interpore distance and straightness. As the interpore distance increases from 150 to 180 nm, the pore diameter becomes smaller and each nanopore starts to split into two nanopores. At interpore distances of over 190 nm, prepatterned concaves are developed into round flask-shaped nanosacks instead of one-dimensional tubes, and then these are split into three more sub-nanopores. The fundamental characteristics of anodic aluminum oxidation are discussed in accordance with various prepatterned concaves in the nanopore growth processes, providing a rational theory for the design of various complex 3-D AAO structures that can be controlled by prepatterning.