US Patent 7525640 Lithographic apparatus and device manufacturing method

A lithographic apparatus is disclosed that includes an optical arrangement having an array of optical elements arranged in a plane perpendicular to the radiation beam. Each optical element comprises an electrical heating device to change an optical path length of the radiation beam. By selectively actuating the electrical heating devices, a position dependent change in optical path length can be achieved in order to correct for irradiation-induced optical path length errors.