1. A method of forming a pattern of fine features on a substrate using an exposure system, comprising: (a) forming, using a mask during a primary exposure, a pattern of fine features of a critical dimension on a substrate; and (b) trimming the pattern of fine features formed on the substrate by exposing the substrate using a trim mask containing features to modify the critical dimension of the fine features formed by the mask during the primary exposure, the features on the trim mask having dimensions near a resolution limit that corresponds to a transition to graytone behavior.