Post exposure modification of critical dimensions in mask fabrication

A system and method are described for modifying an exposure image in a radiation sensitive layer by treating the exposure image with a heterogeneous and non-uniform post exposure thermal treatment. The treatment may comprise providing different portions of the exposure feature, such as different exposure features or critical dimensions, with different thermal fluxes from a thermal modification system, such as a post exposure bake oven or hot plate configured to provide different thermal fluxes. The thermal modification system may comprise one or more adjustable spacers to adjust a radiant energy flux from a thermal energy source to the radiation sensitive layer by adjusting a separation distance between the source and the layer.