Patent ID: 8189203

Claim:
A reticle inspection system, comprising: a coherent illumination source configured to illuminate respective portions of an inspection reticle and a reference reticle; an inspection interferometer branch having a first microscope objective system configured to apply a first Fourier transform to a first light beam propagated from the illuminated portion of the inspection reticle; a reference interferometer branch having a second microscope objective system configured to apply a second Fourier transform to a second light beam propagated from the illuminated portion of the reference reticle; a beam splitter configured to receive and combine the first and second light beams; an interferometer element in the inspection interferometer branch, the interferometer element configured to induce a 180-degree phase shift between the inspection interferometer branch and the reference interferometer branch and to direct the first light beam from the first microscope objective system to the beam splitter; a Fourier lens configured to provide an inverse Fourier transform to the combined first and second light beams whereby an image is formed; and a detector configured to detect the image, the image representing a difference in amplitude and phase distributions of the inspection reticle and the reference reticle, wherein the inspection interferometer branch has an optical path between the illumination source and the detector and the reference interferometer branch has an optical path between the illumination source and the detector, and wherein an optical path length difference between the inspection interferometer branch and the reference interferometer branch is less than a coherence length of the illumination source.