1. A method of generating photolithographic mask data for a mask, the method comprising: identifying a target image, the target image corresponding to an image to be printed onto a substrate through an optical lithographic process; generating a set of mask data for an attenuated phase shift mark, the mask comprising: a main feature having a transmission value of 100%, the main feature comprising an isolated pattern and corresponding to a feature on the target image, a plurality of assist features surrounding the main feature and having a linewidth substantially smaller than a linewidth of the main feature, the assist features having a transmission value of 100%, and a plurality of phase shift areas having a transmission value of greater than 6%; simulating by a computer an intensity on a substrate generated by employment of the set of mask data in the optical lithographic process; identifying an objective function that compares the simulation of the intensity to an ideal image intensity corresponding to the target image; and minimizing an intensity on the substrate outside the main feature based on the objective function to determine a placement for the plurality of assist features in the set of mask data.