Patent ID: 7126671

Claim:
A lithographic apparatus, comprising: a radiation system configured to provide a beam of radiation; a support structure configured to support a patterning device that imparts said beam of radiation with a desired pattern; a substrate holder configured to hold a substrate; a projection system configured to project said patterned beam of radiation onto a target portion of said substrate; and a positioning system configured to maintain at least a first optical element and a second optical element in predetermined positions, said at least first and second optical elements being disposed in said radiation system and/or said projection system, said positioning system comprising: at least one position sensor that directly measures a relative position of at least said first optical element relative to said second optical element, and at least one interferometric displacement measuring device for measuring at least the relative position and orientation of said first and second optical elements, wherein the at least one of the first and second optical elements includes a rigid extension carrying a beam directing element for an interferometer beam of the interferometric displacement measuring device.