Patent ID: 8751979

Claim:
A computer-implemented method for improving a design of mask patterns to be imaged using a lithographic process by illuminating the mask patterns with an illumination source in a lithographic apparatus, the method comprising: receiving an initial design layout of the mask patterns, wherein the mask patterns include a known critical dimension for which a calculated value of an image log slope (ILS) at one or more evaluation points on an image plane is below a predefined threshold value; calculating a second order partial derivative of the ILS at the one or more evaluation points with respect to a first variable and a second variable, the first variable being associated with mask transmission, and, the second variable being associated with an intensity at a point on the illumination source; and using the calculated second order partial derivative to determine a correction amount by which to adjust the critical dimension to improve the value of the ILS at the one or more evaluation points, wherein the steps of the method are performed by the computer.