Patent ID: 7424699

Claim:
A method of modifying a plurality of sub-resolution assist features, comprising: receiving a mask pattern for a photolithographic mask, the mask pattern comprising one or more main features, the photolithographic mask operable to pattern a wafer pattern for a semiconductor wafer; estimating placement of one or more sub-resolution assist features for the one or more main features; then repeating the following for one or more iterations: correcting the one or more main features using a wafer pattern model, the wafer pattern model operable to estimate the wafer pattern; evaluating the one or more sub-resolution assist features according to the wafer pattern model and comprising: establishing a first derivative of a model image of a main feature having an associated sub-resolution assist feature; determining that the first derivative of the model image is less than a minimum slope threshold; and selecting the associated sub-resolution assist feature as the at least one sub-resolution assist feature to modify; and modifying at least one sub-resolution assist feature of the one or more sub-resolution assist features in accordance with the evaluation.