A method includes receiving a layout for fabricating a mask, determining a first target contour corresponding to a first set of process conditions, determining a second target contour corresponding to a second set of process conditions, simulating a first potential modification to the layout under the first set of process conditions to generate a first simulated contour, simulating a second potential modification to the layout under the second set of process conditions to generate a second simulated contour, evaluating costs of the first and second potential modifications based on comparing the first and second simulated contours to the first and second target contours, respectively, and providing the layout and one of the first and second potential modifications having a lower cost for fabricating the mask. The first set of process conditions is different from the second set of process conditions.