Patent ID: 8111377

Claim:
A lithographic apparatus comprising: an illumination system configured to condition a first beam of radiation; a support constructed to support a patterning device, the patterning device being capable of imparting the first beam of radiation with a pattern in its cross-section to form a patterned radiation beam; a substrate table constructed to hold a substrate; a projection system configured to project the patterned radiation beam onto a target portion of the substrate; a position measuring system configured to measure a position of a moveable object with respect to a reference frame of the lithographic apparatus, in at least one direction of an orthogonal x-y-z coordinate system of the moveable object, the position measuring system comprising an optical x-z-encoder configured to measure a displacement of a radiation source, a first grating, and a detector with respect to a second grating of the encoder, the radiation source, the detector and the second grating mounted on one of the moveable object or the reference frame, and the first grating arranged on the other one of the moveable object or the reference frame, wherein the encoder is configured to, during scanning along the first grating, split a second beam of radiation coming from the radiation source into at least a first positive and a first negative order of the second beam of radiation, each of the first positive and the first negative order having an angle with respect to the first and the second grating and resulting in a response on the detector which is diffracted by the first and the second grating, wherein the first grating comprises an alignment marker; and a controller configured to define a zero level of the moveable object with respect to the reference frame in at least one of the x- and z-direction by performing the scanning along the first grating, the alignment marker during the scanning causing a change in a phase of the response of both the first positive and negative orders.