Method, computer readable storage medium and computer system for creating a layout of a photomask

A method, comprising:

• providing a layout of at least a portion of a photomask in a computer system, the layout comprising a plurality of target features, each target feature having a shape in accordance with a corresponding one of at least one target shape;
• for each of the at least one target shape, providing a local map in the computer system 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, wherein the local SRAF usefulness for a position relative to the target shape is a quantity that is indicative of a change of a process window obtainable in a photolithography process wherein a photoresist is patterned with only the target shape by placing an SRAF element at the position relative to the target shape;
• for each of the plurality of target features, providing an assignment of each of at least a part of the values of the local SRAF usefulness of the local map for the target shape corresponding to the target feature to a respective one of a plurality of positions relative to the at least a portion of a photomask in the computer system;
• providing a global map in the computer system specifying a respective global SRAF usefulness for each of the plurality of positions relative to the at least a portion of the photomask on the basis of the assignment of the values of the local SRAF usefulness, wherein providing the global map comprises calculating, for each of the plurality of positions relative to the at least a portion of the photomask, a sum of the values of the local SRAF usefulness assigned to the respective position relative to the at least a portion of the photomask;
• placing one or more SRAFs in the layout on the basis of the global map;
• and
• manufacturing a photomask comprising the plurality of target features and the plurality of SRAFs.