Fabrication of Sub-250nm High Aspect Ratio Apertures by Focused Ion Beam Lithography
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Fabrication of Sub-250nm High Aspect Ratio Apertures by Focused Ion Beam Lithography Todd Simpson and Ian V. Mitchell Physics & Astronomy, University of Western Ontario, 1151 Richmond St, London, N6A3K7, Canada ABSTRACT Aperture arrays were fabricated in 1.0µm thick gold films supported on 20nm thick silicon nitride membranes. Lithographic milling strategies in gold were evaluated through the use of in-situ sectioning and high resolution SEM imaging with the UWO CrossBeam FIB/SEM. A successful strategy for producing a 250nm diameter hole with sidewalls approaching vertical is summarized. INTRODUCTION Focused Ion Beam (FIB) milling is an extremely flexible and efficient route to prototyping and fabricating sub-micron scale features. Modern focused ion beam instruments are capable of sub-10nm imaging resolution. The minimum feature size that can be produced depends on the depth of feature and is typically limited to an aspect ratio of about 3:1. The interaction volume of a 30keV gallium beam is tens of nanometers in diameter in typical inorganic solids and this limits the size of features that can be produced. The smallest holes will be produced on very thin membranes, typically silicon nitride [1]. Redeposition of sputtered material is the primary limitation to achieving high aspect ratios in sub-micron scale features[2]. Unlike patterning via resist exposure, the order and rate of beam sweeping in FIB milling plays a crucial role in determining pattern fidelity[3]. We report on the application of lithographic control on the profile and determine the achievable aspect ratios of small apertures produced in 1um thick gold membranes. EXPERIMENT Silicon nitride membranes 1mm square and 20nm thick in a 5mm square silicon frame were purchased from SPI supplies. The silicon nitride was coated with a 1.0um thick layer of gold by electron beam deposition. The samples were mounted with the nitride-side facing the beam for FIB milling. Hole diameters were measured by SEM in plan view from both sides of the sample. All FIB milling and SEM imaging was performed with the Zeiss 1540XB FIB/SEM at the University of Western Ontario Nanofabrication Laboratory. A 37pA gallium ion beam of 30keV energy was focused and stigmated using ion induced secondary electron image quality. Focus conditions were then refined by investigating, via SEM imaging in plan view, the size and shape of holes ‘drilled’ by the beam as a function of focus conditions. The optimum conditions were determined at the coincidence point of the FIB and SEM beams so that samples could be repeatably positioned to the focus point. Beam current was measured periodically by a picoammeter with the beam directed into a Faraday cup. The FWHM imaging resolution of this ion beam was measured at the edge of the Faraday cup aperture to be less than 20nm. These beam conditions were used throughout the experiment. Lithographic patterning of the ion beam was controlled by a NPGS v9.0 lithography system. Holes were milled with either a stationary beam or by sweeping the b
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