| 摘要 |
Ellipticity in an illumination beam may be corrected by measuring an ellipticity of the illumination beam and substantially eliminating the ellipticity using a light attenuating filter at a defocus position of the illumination beam, wherein the light attenuating filter has a two-dimensional pattern that compensates for ellipticity variations in the illumination beam. The light attenuating filter may stand alone, or the filter may be combined with a uniformity correction system, such that it corrects both uniformity and ellipticity. In one embodiment, the light attenuating filter is printed with an assembly of microscopic dots according to the two-dimensional pattern.
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