Formation of Al Nanodot Array by the Combination of Nano-Indentation and Anodic Oxidation
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Formation of Al Nanodot Array by the Combination of Nano-Indentation and Anodic Oxidation S. Shingubara, Y. Murakami, K. Morimoto, H. Sakaue, and T. Takahagi Graduate School of ADSM, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8530, Japan e-mail:[email protected] ABSTRACT The control of nanoholes formed by anodic oxidation of aluminum (Al) was investigated using AFM nano-indentation on Al film prior to the anodic oxidation. It is well known that ordered trigonal nanohole arrays are formed under certain voltage conditions of anodic oxidation of Al. We succeeded in forming both tetragonal and trigonal arrays of alumina nanoholes on a SiO2/Si- substrate by using nano-indentation on the surface of sputtered pure Al film. The ordered array of nanoholes was obtained at indentation intervals that were close to the nearest neighbor distance of nanoholes in the self-organization condition. Furthermore, we fabricated tetragonal and hexagonal Al nanodot arrays by the selective removal of porous alumina film. INTRODUCTION Porous alumina films formed by anodic oxidation of aluminum (Al) have been intensively studied for use as molds to form nano wires or dots by depositing various metals or semiconductors in them, since highly ordered nanohole arrays can be formed [1-12]. By the use of oxalic acid, sulfuric acid, or phosphoric acid, trigonal lattices of alumina nanoholes with distances of several tens of nm can be formed under the self-organization conditions of anodic oxidation. We recently succeeded in forming an Al dot hexagonal array on a SiO2/Si(100) substrate through adequate conditions of anodic oxidation and the subsequent selective removal of porous alumina film [13,14]. Al dots measuring a few tens of nm were formed at an interface between the porous alumina film and the SiO2 film by a break-up of Al film due to completion of anodic oxidation. These Al dots are useful for application to quantum effect devices such as single electron memory nodes or quantum cellular automata devices based on tunneling phenomena within two-dimensional quantum dot arrays [15]. However, the size and geometrical arrangement of the ordered porous alumina nanohole array are restricted by the self-organization conditions of each acid species. For example, the nearest neighbor distances of nanoholes at the self-organization condition are 475 nm, 90 nm, and 60 nm for phosphoric acid (anodization voltage Va=195V) [10], oxalic acid (Va=40V) [7,8], and sulfuric acid (Va=28V) [9], respectively. Furthermore, the arrangement of nanoholes is limited to trigonal lattices only. To overcome these limitations, Asoh et al. proposed modifying the surface of Al prior to anodization by pressing the Al surface with a SiC substrate that has periodical convex cones [16,17]. In this study, we tried to control the arrangement and the nearest neighbor distance of nanoholes, as well as of Al dots, by the use of AFM (atomic force microscopy) nano-indentation, and by the following anodic oxidation and selective wet chemical etching. EXPERIMENTS
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