Exposure parameters for MeV proton beam writing on SU-8
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Proton beam writing was performed on a lithographic resist to determine the main parameters required to achieve the minimum feature size, maximum pattern lateral density and maximum aspect ratio. A 2.5 MeV proton beam focused to sizes between 1.5 and 2.5 mu m was used to expose SU-8 negative resist. The number of protons per pixel was varied in the exposure of SU-8 with thicknesses between 5 and 95 pm. Patterns consisting of single pixels, single-pixel lines and multi-pixel areas with different densities were fabricated. The smallest structures achieved were posts 1.5 pin in diameter with 4:1 structure-space ratio in 15 pm thick resist and the highest aspect ratio structures of 20:1 in 40 pm resist were produced. It was found that the minimum feature size depended only on the beam size, and +/- 10% post size accuracy could be achieved within 40-70% variation of the number of protons. MeV proton beam allows a direct fabrication of complex shapes without a mask in single-step irradiation and. in addition, no proximity correction is needed. We present examples of MeV proton beam written single and multi-pixel microstructures with easily reproducible high aspect ratios and densities. (c) 2006 Elsevier B.V. All rights reserved.