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 cm2. The main aim was to induce crosslinking
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 cm2 minimized the distortion in stitched regions.