In a lithographic apparatus, such as an extreme ultra-violet (EUV) lithographic apparatus, a beam of radiation is generated by a radiation source and directed to a reticle, which imparts a pattern to the beam. The patterned beam is then directed to a substrate by plurality of optical components (e.g. mirrors), The plurality of optical components may be contained within a projection system. The projection system may be enclosed within a housing, such that the environment surrounding the projection system can be controlled.When the lithographic apparatus is in operation, silane (SiH_4) may be produced at the reticle. This silane may enter into the projection system housing, where it can cause significant degradation to the optical components. The presence of silane can be reduced by introducing a small amount of oxygen (O_2) into the projection system housing, because oxygen oxidises silane into silicon oxide (SiO_2). However, oxygen itself can cause degradation to the mirrors through oxidation.
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