Patent Number: 
Section: claims

1. An apparatus for generating extreme ultraviolet light, the apparatus comprising:a reference member;a chamber fixed to the reference member, the chamber including at least one window;a laser beam introduction optical system configured to introduce an externally supplied laser beam into the chamber through the at least one window; anda positioning mechanism configured to position the laser beam introduction optical system to the reference member, the positioning member including:three legs configured to support the laser beam introduction optical system;three mounts fixed to the reference member, the three mounts being configured to respectively support the three legs so as to position the laser beam introduction optical system on a predetermined plane; andtwo stoppers fixed to the reference member, the two stoppers being configured to position the laser beam introduction optical system in the predetermined plane while the three mounts respectively support the three legs. 2. The apparatus according to claim 1, further comprising a moving mechanism configured to move the laser beam introduction optical system and the three legs relative to the reference member such that the three legs respectively reach the three mounts. 3. The apparatus according to claim 2, wherein the moving mechanism includes:a rail anda wheel moving along the rail, andthe positioning system is configured to position the laser beam introduction optical system while the wheel is distance from the rail. 4. The apparatus according to claim 1, wherein the positioning mechanism includes a pressing member configured to bias the laser beam introduction optical system against both of the two stoppers. 5. The apparatus according to claim 1, further comprising two biasing members attached to the laser beam introduction optical system, one of the two biasing members having a groove formed in a direction of gravity, another one of the two biasing members having a planer surface parallel to the direction of gravity, the groove and the planer surface being respectively biased against the two stoppers so as to position the laser beam introduction optical system. 6. The apparatus according to claim 1, wherein each of the two stoppers has a columnar shape and is fixed such that an axis of each of the two stoppers coincides with a direction of gravity. 7. The apparatus according to claim 1, wherein each of the three legs has a hemispherical bottom. 8. The apparatus according to claim 1, wherein each of the three mounts has a planar upper surface. 9. The apparatus according to claim 1, further comprising two biasing members attached to the laser beam introduction optical system, one of the two biasing members having a groove formed in a direction of gravity, another one of the two biasing members having a planer surface parallel to the direction of gravity, the groove and the planer surface being respectively biased against the two stoppers so as to position the laser beam introduction optical system, whereineach of the two stoppers has a columnar shape and is fixed such that an axis of each of the two stoppers coincides with the direction of gravity. 10. The apparatus according to claim 1, wherein each of the three legs has a hemispherical bottom and each of the three mounts has a planar upper surface. 11. The apparatus according to claim 1, further comprising two biasing members attached to the laser beam introduction optical system, one of the two biasing members having a groove formed in a direction of gravity, another one of the two biasing members having a planer surface parallel to the direction of gravity, the groove and the planer surface being respectively biased against the two stoppers so as to position the laser beam introduction optical system, whereineach of the two stoppers has a columnar shape and is fixed such that an axis of each of the two stoppers coincides with the direction of gravity, andeach of the three legs has a hemispherical bottom and each of the three mounts has a planar upper surface. 12. An apparatus for generating extreme ultraviolet light, the apparatus comprising:a reference member;a chamber fixed to the reference member, the chamber including at least one window;a laser beam introduction optical system including a plurality of optical elements, the laser beam introduction optical system being configured to introduce at least one laser beam into the chamber through the at least one window; anda positioning mechanism including a single plate configured to support the laser beam introduction optical system, the positioning mechanism being configured to position the single plate so as to position the plurality of optical elements to the reference member. 13. The apparatus according to claim 12, wherein the moving mechanism includes:a rail provided on the reference member; anda wheel attached to the positioning mechanism to move along the rail. 14. The apparatus according to claim 12, whereinthe positioning mechanism includes an engagement unit attached to the interior of the reference member for suspending the laser beam introduction optical system. 15. The apparatus according to claim 12, wherein the plurality of optical elements include:a beam splitter for splitting the at least one laser beam into first and second beam paths, the second beam path leading to the chamber; anda laser beam measuring unit provided in the first beam path to receive the at least one laser beam traveling through the first beam path. 16. The apparatus according to claim 12, whereinthe at least one laser beam includes a pre-pulse laser beam output from a first laser apparatus and a main pulse laser beam output from a second laser apparatus, andthe plurality of optical elements includes:a beam combiner configured to control a direction of the pre-pulse laser beam and a direction of the main pulse laser beam to coincide with each other; anda laser beam measuring unit configured to receive a part of the pre-pulse laser beam output from the beam combiner and a part of the main pulse laser beam output from the beam combiner.