Fast and blister-free irradiation conditions for cross-linking of PMMA induced by 2 MeV protons

For soft lithography, the conventional negative tone resists, such as SU-8, that are used to create the mold have a number of drawbacks. PMMA, which is normally used as a positive tone resist, can be used as a negative resist by using high-fluence irradiation conditions. In this report, we outline optimization of the irradiation conditions for PMMA thin films using 2 MeV H+ ions to exploit their ability to work as a negative tone resist at ion fluences above 1.0 × 1015 ions cm−2. The main aim was to induce cross-linking while maintaining the exposed regions free of blisters and maintaining short irradiation times. We found that by using a two-step process with a low-flux irradiation, followed by a high-flux irradiation, the exposure time could be shortened by ∼50%. We also found that ion fluences greater than 5.0 × 1015 ions cm−2 minimized the distortion in stitched regions.

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