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An electron beam microfabrication tool for use in nanometer scale device research has been built incorporating a single bore objective lens. The sample resides in a micrometer-controlled in-lens stage which is an integral part of the boreless lower polepiece. This piece is removed to change samples, a straightforward operation due to the relaxed alignment tolerances between the two polepieces. The focal length of the lens, which has a 4 mm upper bore and a 4 mm gap, is 1.7 mm, with spherical and chromatic aberration coefficients taking on values of 1.0 and 1.2 mm, respectively. At the 40 kV operating voltage of the system with a tungsten hairpin source, 5 pA can be focused into a 50 Å diameter probe. Using ultrasonic liftoff with polymethyl methacrylate (PMMA) resist, we have obtained two-dimensional arrays of 200 Å gold:palladium particles on 500 Å centers on bulk GaAs. As a test of the overall system, which includes a computer-controlled pattern generator, a page of alphabetic characters was written in PMMA on a silicon nitride membrane at a data density of 1010 characters/cm2.
Newman et al. (Thu,) studied this question.