Non-lithographic Nanofabrication Using Porous Alumina Membranes
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Non- lithographic Nanofabrication Using Porous Alumina Membranes K. Bhargava Ram1, L. Tian2, Z. Wu and L. Menon Department of Physics, Northeastern University, Boston, MA 02115 1 Department of Electrical Engineering, Virginia Commonwealth University, 2 Department of Physics, Texas Tech University, Lubbock, TX 79409 ABSTRACT We describe a fabrication method to prepare highly ordered Si nanopore arrays. A nanoporous alumina template of thickness ~1µm is prepared by means of anodization of an aluminum film. The template has a highly ordered hexagonal array of pores of diameter ~50nm. The template is detached from the aluminum layer and placed on a Si substrate. The nanoporous pattern is transferred onto silicon substrate by means of a dry plasma etch process. This produces an array of nanopores in silicon with a diameter of ~50nm and depth of ~300nm. We have used such an array to prepare Fe nanopillars inside the pores by means of thermal evaporation. Magnetization versus applied magnetic field measurements for the Fe nanoarrays, demonstrate large perpendicular anisotropy typical of high aspect ratio magnetic nanopillars. The value of coercivity is about 500Oe in the perpendicular direction and 40Oe in the parallel direction. INTRODUCTION With continued miniaturization of devices for advanced performance, extensive research is underway to develop novel nanofabrication methods. In particular, it is important to develop nanofabrication methods compatible with Si-based structures, since at present the semiconductor industry relies mostly on Si-based devices. In this regard, nanoporous alumina (aluminum oxide) template is a versatile template and may be used in several different ways to prepare a variety of nanostructures on different substrates [1]. These are membranes consisting of an array of cylindrical pores in an aluminum oxide film arranged in an ordered hexagonal array. The pores are vertical to the film surface and are arranged parallel to each other. The pores are open at the top and are closed at the bottom by a thin U-shaped barrier layer of aluminum oxide and a layer of aluminum left behind after anodization. Such templates are prepared electrochemically by anodizing an Al surface in the presence of an acid under dc conditions. The diameter and density of the pores in the template are largely determined by anodization conditions, in particular the acid used and the voltage applied. Typical pore diameters range from about 10-150nm. Pore densities are of the order of 1010-1011 pores/cm2. In this paper, we report the use of nanoporous alumina templates to prepare a nanoporous pattern on a Si wafer, by means of dry plasma etching through the pores. Fe nanopillars are thermally evaporated inside the Si pores. Magnetic properties of these Fe nanopillars are investigated by means of an alternating gradient force magnetometer. The nanopillars are found to exhibit large perpendicular anisotropy, resulting from shape anisotropy. EXPERIMENTAL RESULTS AND DISCUSSION Nanoporous alumina templates are prepared
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