Low-voltage electron beam lithography

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The use of low-voltage electron beam lithography to reduce proximity effects, improve throughput, and reduce substrate damage caused to underlying materials has been investigated. Various films of PMMA were exposed with a field emission scanning electron microscope adapted with blanking capability and a 16 bit resolution beam control package. The exposure voltages used were from 1 to 15 keV with probe sizes of ≤150 to ≤70 Å, respectively. The dose latitude or working dose range was determined for each voltage on film thicknesses of 0.05, 0.18, and 0.38 μm poly(methylmethacrylate). Optimum beam voltage for a particular thickness which maximizes the energy deposited within the resist has been approximated using Monte Carlo modeling and verified experimentally. Atomic force microscopy showed that at lower voltages the dose required to properly expose a feature is relatively low and as beam voltage increases, the dose required to expose a given area increases. This data also verifies the fact that if the exposing voltage is too low, the resist will not be fully exposed. Because low energy electrons have a smaller interaction volume, there is a significant decrease in the proximity effect that is evident when high energy electrons are used for exposure. Comparison of low energy and higher energy electron beam exposures in this study reveal this decrease. Results indicate that by using low energy electrons for resist exposure the dose can be lowered and thus the throughput increased; the proximity effect is significantly decreased; and because of the limited penetration depth of low energy electrons, the substrate damage is potentially minimized.