Method and apparatuses for optical pupil symmetrization
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United States of America Patent
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Feb 27, 2018
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N/A
app pub date -
Dec 15, 2015
filing date -
Dec 15, 2014
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Abstract
An inspection apparatus may determine precise OV measurements of a target on a substrate using an optical pupil symmetrizer to reduce the inspection apparatus's sensitivity to asymmetry and non-uniformity of the illumination beam in the pupil plane. The inspection apparatus includes an illumination system that forms a symmetrical illumination pupil by (1) splitting an illumination beam into sub-beams, (2) directing the sub-beams along different optical branches, (3) inverting or rotating at least one of the sub-beams in two dimensions, and recombining the sub-beams along the illumination path to symmetrize the intensity distribution. The illumination system is further configured such that the first and second sub-beams have an optical path difference that is greater than a temporal coherence length of the at least one beam and less than a depth of focus in the pupil plane of the objective optical system.
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