Method and system of lithography using masks having gray-tone features

A method forms patterns on a substrate by exposing the substrate a first time and exposing the substrate a second time using a mask containing gray-tone features. The gray-tone features locally adjust an exposure dose in regions corresponding to features defined in the primary exposure. Moreover, the gray-tone features enable the forming of features having different critical dimensions on a substrate. The gray-tone features may be implemented as sub-resolution features formed by pixellation. The gray-tone features may also be realized by the local size bias of trim features on the trim mask that have dimensions near the resolution limit of the exposure system. The trim mask containing gray-tone features may have regions with different transmissivities or generate varying illumination intensities.