METHOD, COMPUTER READABLE STORAGE MEDIUM AND COMPUTER SYSTEM FOR CREATING A LAYOUT OF A PHOTOMASK

A method includes providing a layout of a portion of a photomask. The layout includes a plurality of target features having a shape in accordance with a corresponding one of a target shape. For each of the target shapes, a local map specifying a respective value of a local sub-resolution assist feature (SRAF) usefulness for each of a plurality of positions relative to the target shape is provided. For each of the target features, an assignment of a part of the values of the local SRAF usefulness of the local map for the target shape corresponding to a target feature to a position relative to the portion of the photomask is provided. A global map specifying a global SRAF usefulness for each of the positions relative to the portion of the photomask is provided on the basis of the assignment of the values of the local SRAF usefulness.