Patent ID: 6891598

Claim:
A lithographic apparatus comprising: a radiation system for providing a beam of radiation; a patterning device to impart a desired pattern onto said beam of radiation along its cross-section; a substrate holder for holding a substrate; a projection system for projecting said patterned beam of radiation onto a target portion of said substrate; and a substrate alignment system for detecting a position of said substrate relative to a position of said patterning device, said substrate alignment system comprising: a source configured to generate an incoming optical beam, at least one grating, provided on said substrate, having a diffraction length, said at least one grating configured to generate at least one diffraction order of constituent diffracted beams based on an interaction with said incoming optical beam over said diffracting length, and an optical device configured to image said at least one diffracted order on a sensor device, wherein said optical device includes an aperture at a predetermined location to allow said constituent diffracted beams to pass through, and wherein said optical device is arranged to broaden said constituent diffracted beams such that a beam diameter of said constituent diffracted beams is larger than a diameter of said aperture.