Patent ID: 7242454

Claim:
A lithographic apparatus comprising: an illumination system configured to condition a radiation beam; a patterning support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; a substrate support 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 apparatus configured to measure a position of one of said supports in a medium, the position measuring apparatus comprising: (a) a barometer configured to measure a pressure of the medium and to provide a pressure signal; (b) a distance measuring device configured to measure a reference distance and to provide a reference distance signal; and (c) a processor configured to: (c1) convert the reference distance signal into a pressure-change signal; (c2) process the pressure-change signal in a similar way as the pressure signal and to provide a processed pressure-change signal; (c3) determine a difference between the processed pressure-change signal and the pressure signal to provide a drift signal; (c4) determine a difference between the pressure-change signal and the drift signal to provide an absolute pressure signal; and (c5) correct a position measurement based on the absolute pressure signal.