Nanofabrication Using Self-Assembled Alumina Templates
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Nanofabrication Using Self-Assembled Alumina Templates Oded Rabin1, Paul R. Herz2, Stephen B. Cronin3, Yu-Ming Lin2, Akintunde I. Akinwande2 and Mildred S. Dresselhaus2,3 Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. 1 Dept. of Chemistry. 2 Dept. of Electrical Engineering and Computer Science. 3 Dept. of Physics. ABSTRACT A new approach for the use of porous alumina films as a template for nanofabrication is presented. In this process the porous films are prepared on silicon substrates, simplifying both the template fabrication and subsequent processing, and improving the quality of the films and their surfaces. Structural analysis of the film was carried out. Bismuth and bismuth telluride nanowires were prepared by pressure injection and electrochemical deposition, respectively, in alumina films 5-10 µm thick with parallel ordered pores 40 nm in diameter. The films were also patterned by lithography, offering new opportunities for area-selective anodization of non-planar structures. The new approach offers a straightforward method for the fabrication of arrays of nanostructures and their incorporation into electronic and optical devices. INTRODUCTION Porous anodic alumina (PAA) has received considerable attention as a template for the fabrication of nanostructures [1,2]. The ordered triangular array of pores of high aspect ratio, whose dimensions can be accurately tuned by the process parameters [3], has made PAA a suitable host for the fabrication of nanowires of a wide range of materials. Applications of these arrays of nanowires include dense magnetic storage [4], field emission [5], nano-electrodes [6], and the study of low-dimensional quantum effects [7]. Several researchers have used PAA as a mask for etching or deposition processes [8]. Thus, the nanometer-scale hexagonal pattern that was formed in the alumina by self-assembly was transferred to another substrate. More recently, it was found that the optical properties of alumina together with the proper positioning of the voids in the film result in a 2-dimensional photonic crystal with a bandgap which can be controlled in the wavelength range of 520-600 nm (for certain polarizations and propagation directions of the light) [9]. The conventional way of fabricating the PAA films [10,11] starts with an aluminum sheet that goes through several steps of mechanical and electrochemical polishing. Once the surface roughness is down to the sub-micron level, the metal is anodized in an acidic bath and the porous oxide is obtained. The quality of the starting oxide is usually low in terms of the ordering and uniformity of the pores. Therefore, this initial film is etched away and a new PAA film is grown under the same anodization conditions. The growing porous film is separated from the underlying metallic aluminum by a scalloped layer of oxide, known as the barrier layer. In order to obtain a PAA membrane in which the pores run through the film and are open (and accessible) on both sides, it is necessary to etch away the metallic aluminum sustai
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