| 摘要 |
The present invention is directed to apparatus for detecting defocus which is particularly adapted, among other possible uses, for use with microlithography projection systems; said apparatus being characterized by a reticle, a source of illumination for illuminating said reticle, a beam splitter, a projection lens system, a wafer mounted in a primary image plane of the projection lens system, said beam splitter and projection lens system being arranged to project an image of the reticle on the wafer, a mask complementary to the reticle mounted in a secondary image plane, said beam splitter and said projection lens system being arranged to reimage light reflected from the wafer onto the complementary mask, and detector means mounted to collect light transmitted by the complementary mask. In one form of the invention the apparatus further includes a system for modulating the wafer and a phase-sensitive demodulation circuit for demodulating the output of the detector to obtain a bipolar electrical focus error signal.
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