Patent ID: 7349072

Claim:
A lithographic projection apparatus comprising: an illumination system configured to condition a beam of radiation; a support structure configured to support a patterning device, the patterning device serving to pattern the beam of radiation according to a desired pattern; a substrate table configured to hold a substrate; a projection system configured to project the patterned beam of radiation onto a target portion of the substrate; and an interferometer measurement system configured to determine a reflection angle of a beam of interferometer radiation reflected by a reflective portion of an object so as to determine a flatness of said reflective portion, the interferometer measurement system including: a plate having first and second parallel surfaces, the first surface comprising a beam-splitter and the second surface comprising a reflector; and a detector configured to detect a signal resulting from the interference between two beam portions derived from said beam of interferometer radiation reflected by said reflective portion and subsequently incident on said first surface, wherein the two beam portions include a first beam portion reflected by the beam-splitter of the first surface and a second beam portion reflected by the reflector of the second surface.