Patent ID: 7535578

Claim:
A lithographic apparatus, comprising: a substrate holder configured to hold a substrate; an illuminator configured to condition a beam of radiation; a support structure configured to support a patterning device that imparts a desired pattern to the beam of radiation; a projection system configured to project the patterned beam onto a target portion of the substrate; and a unitary measurement and zeroing interferometer system configured to measure an absolute position of an object to assist a singularly driven interferometer device in positioning the object, the system including a modulation system configured to modulate a frequency of a radiation beam to form a modulated radiation beam, the modulated radiation beam split within the unitary measurement and zeroing interferometer system into a first modulated radiation beam that propagates along a measurement path that extends between the unitary measurement and zeroing interferometer system and the object and a second modulated radiation beam that propagates along a reference path, a response monitoring system configured to measure an interferometry signal obtained by interference between the first and second modulated radiation beams, and a processing unit configured to determine an absolute path length difference between the measurement and the reference paths based on the interferometry signal and the frequency modulation of the modulation system, the absolute path length difference being proportional to a ratio between an amplitude of the interferometry signal and an amplitude of the frequency modulation.