Lithographic projection apparatus, device manufacturing method, device manufactured thereby and gas composition

摘要

A lithographic projection apparatus which uses one, and optionally also two or more color interferometric devices to accurately determine the position of a movable table within the apparatus. The apparatus comprises a purge gas source to supply purge gas to a space accommodating at least a part of said movable table, the purge gas being selected such that leakage of the purge gas into the area of operation of the interferometric devices does not cause significant error in the interferometric measurements.