Patent ID: 7580131

Claim:
An angularly resolved scatterometer comprising: an optical system comprising an objective lens including an object plane and a pupil plane, the optical system arranged to direct a first beam of radiation onto a target pattern of a substrate printed by a lithographic process, to collect radiation reflected or scattered by the target pattern, and to project a second beam of radiation to form an image of the pupil plane of the objective lens in an image plane; a detector located in the image plane and configured to convert radiation incident on a first portion of the detector into scatterometric spectra; an aperture member positioned in a path of the second beam of radiation at a location not congruent with the pupil plane of the objective lens, the aperture member defining at least one obscuration extending a predetermined distance into the second beam of radiation so as to form a dark area in the image of the pupil plane; and a reflecting device located in a path of a third beam of radiation and arranged to produce a fourth beam of radiation received at a second portion of the detector, the optical device being configured to direct the third beam of radiation to reflect from the reflecting device to produce the fourth beam of radiation.