Patent ID: 7515245

Claim:
A lithographic apparatus comprising: an illumination system configured to provide a beam of radiation; a support structure configured to support a patterning device, the patterning device serving to impart the beam of radiation with a pattern in its cross-section; a patterning device handling apparatus constructed and arranged to supply said patterning device to said support structure; a substrate table configured to hold a substrate; a substrate handling apparatus constructed and arranged to supply said substrate to said substrate table; a projection system configured to project the patterned beam onto a target portion of the substrate; an infrared radiation source configured to provide infrared radiation into a measurement zone, said measurement zone being defined by at least two surfaces of and located within said projection system, at said patterning device handling apparatus, or at said substrate handling apparatus, wherein the infrared radiation propagates along a distance corresponding to at least a length of a dimension of said projection system, a dimension of said patterning device handling apparatus, or a dimension of said substrate handling apparatus; and a detector configured to receive said infrared radiation from said infrared radiation source after said infrared radiation has passed through said measurement zone, said detector outputting a signal indicative of the presence of a gas within said measurement zone.