Patent ID: 8232537

Claim:
A radiation source configured to generate extreme ultraviolet radiation, the radiation source comprising: a plasma formation site located at a position in which a fuel is contacted by a beam of radiation to form a plasma; a collector constructed and arranged to collect extreme ultraviolet radiation formed at the plasma formation site, the collector comprising a mirror to reflect the extreme ultraviolet radiation to a second focal point, wherein the position of the plasma formation site is at a first focal point of the collector at a focal distance F from the collector measured along an optical axis defined by the first focal point and the second focal point; and a contamination barrier having a periphery and located between the plasma formation site and the collector, wherein the periphery of the contamination barrier does not occlude the solid angle subtended by the mirror at the second focal point so that the extreme ultraviolet radiation reflected by the mirror to the second focal point does not pass through the contamination barrier.