Patent ID: 7714984

Claim:
A method of correcting for pupil asymmetry in a lithography system, comprising: measuring a first pupil asymmetry of a uniformity profile of an illumination beam; determining positions of first and second attenuating elements of a plurality of attenuating elements arranged for compensating for the first pupil asymmetry, including referencing a map of pupil asymmetry caused by insertion of each element of the plurality of attenuating elements; identifying a second pupil asymmetry that substantially nulls the first pupil asymmetry; and determining, from the map, positions of the first and second attenuating elements that cause the second pupil asymmetry; moving the first attenuating element into the illumination beam, wherein the plurality of attenuating elements is located at a defocus position of the illumination beam; and moving the second attenuating element out of the illumination beam, the second attenuating element being located across the path of the illumination beam from the first attenuating element; wherein the first attenuating element and the second attenuating element are moved to positions that compensate for the first pupil asymmetry by introducing the second pupil asymmetry.