Patent Number: 
Section: claims

1. A lithographic projection apparatus comprising: a radiation system constructed and arranged to supply a projection beam of radiation;  a first object table for holding a mask;  a second object table for holding a substrate;  a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate; and  a gas structure constructed and arranged to supply a substantially laminar flow of flushing gas across at least a part of a path of said projection beam to displace ambient air therefrom, said flushing gas being substantially non-absorbent of said radiation,  wherein said substantially laminar flow of flushing gas traverses a substantially non-obstructed region in said projection apparatus. 2. Apparatus according to  claim 1  wherein said gas supply comprises a supply of flushing gas, a gas flow regulator and an evacuator constructed and arranged to remove flushing gas from said part of said beam path. claim 1 3. Apparatus according to  claim 2  wherein said flow regulator comprises a flow restrictor. claim 2 4. Apparatus according to  claim 2  wherein said flow regulator comprises a blowing unit. claim 2 5. Apparatus according to  claim 1 , wherein said at least a part of the path includes a plurality of parts of said beam path substantially separated from each other. claim 1 6. Apparatus according to  claim 1 , wherein said gas supply is arranged to provide separate laminar gas flows across a plurality of parts of said beam path, said parts of said beam path being substantially isolated from each other. claim 1 7. Apparatus according to  claim 5  further comprising at least one partition comprised of a material substantially transparent to said radiation and positioned parallel to the direction of said laminar flow to isolate said part of said beam path. claim 5 8. The apparatus according to  claim 7 , claim 7 wherein said partition closes a recess in said first object table in which said mask is mounted. 9. Apparatus according to  claim 1  further comprising at least one cover member formed of a material substantially transparent to said radiation, said cover member being substantially planar and provided substantially parallel to the direction of said laminar flow to cover a non-planar surface of a component of said lithographic apparatus in a region of said part of said beam path. claim 1 10. Apparatus according to  claim 9  wherein said cover member covers a non-planar surface of an element of one of said illumination system and said projection system. claim 9 11. Apparatus according to  claim 7  wherein said material substantially transparent to said radiation is selected from the group comprising: CaF 2 , SiO 2 , MgF 2  and BaF 2 . claim 7 12. Apparatus according to  claim 1  further comprising at least one flow control member provided in said part of said beam path. claim 1 13. Apparatus according to  claim 1  wherein a speed of said laminar flow is greater than a maximum speed of movement of any moving parts in a region of said part of said beam path. claim 1 14. Apparatus according to  claim 1  wherein a speed of said laminar flow is greater than a diffusion speed of air. claim 1 15. Apparatus according to  claim 1 , wherein said flushing gas comprises one or more gases selected from the group comprising: N 2 , He, Ar, Kr, and Ne. claim 1 16. Apparatus according to  claim 1  wherein said flushing gas in said part of said beam path has a contamination of air of less than 500 ppm. claim 1 17. Apparatus according to  claim 1  wherein said flushing gas has an extinction coefficient, k, less than 0.005 per cm. claim 1 18. Apparatus according to  claim 1  wherein said radiation of said projection beam has a wavelength less than 200 nm. claim 1 19. A method of manufacturing a device comprising: providing a substrate having a radiation-sensitive layer in a projection apparatus;  irradiating portions of a mask bearing a pattern with a projection beam of radiation and imaging said irradiated portions of the mask onto target portions of said substrate; and  providing flushing gas to flow in a substantially laminar flow across at least a part of a path of said projection beam to displace therefrom ambient air, said flushing gas being substantially non-absorbent of said radiation, and said laminar flow of flushing gas traversing a substantially non-obstructed region in said projection apparatus. 20. A device manufactured according to the method of  claim 19 . claim 19 21. Apparatus according to  claim 16  wherein said contamination of air is less than 100 ppm. claim 16 22. Apparatus according to  claim 16  wherein said contamination of air is less than 10 ppm. claim 16 23. Apparatus according to  claim 16  wherein said contamination of air is less than 1 ppm. claim 16 24. Apparatus according to  claim 17  wherein said extinction coefficient is less than 0.001 per cm. claim 17 25. Apparatus according to  claim 1  wherein said radiation of said projection beam has a wavelength selected from the group consisting of: between 152 nm and 162 nm and between 121 nm and 131 nm. claim 1 26. A lithographic projection apparatus comprising: a radiation system constructed and arranged to supply a projection beam of radiation;  a first object table for holding a mask;  a second object table for holding a substrate;  a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate; and  a gas structure constructed and arranged to supply a substantially laminar flow of flushing gas, in a vicinity of said first object table and said second object table, across at least a part of a path of said projection beam to displace ambient air therefrom, said flushing gas being substantially non-absorbent of said radiation. 27. The apparatus according to  claim 26 , wherein said gas structure comprises a gas flow regulator and an evacuator constructed and arranged to remove flushing gas from said part of said beam path. claim 26 28. The apparatus according to  claim 26 , wherein said at least a part of the path includes a plurality of parts of said beam path substantially separated from each other. claim 26 29. The apparatus according to  claim 26 , wherein said gas structure is arranged to provide separate laminar gas flows across a plurality of parts of said beam path, said parts of said beam path being substantially isolated from each other. claim 26 30. The apparatus according to  claim 26  further comprising: claim 26 at least one cover member formed of a material substantially transparent to said radiation, said cover member being substantially planar and provided substantially parallel to the direction of said laminar flow to cover a non-planar surface of a component of said lithographic apparatus in a region of said part of said beam path. 31. The apparatus according to  claim 26 , wherein a speed of said laminar flow is greater than a maximum speed of movement of any moving parts in a region of said part of said beam path. claim 26 32. The apparatus according to  claim 26 , wherein a speed of said laminar flow is greater than a diffusion speed of air. claim 26 33. A lithographic projection apparatus comprising: a radiation system constructed and arranged to supply a projection beam of radiation;  a first object table for holding a mask;  a second object table for holding a substrate;  a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate; and  a gas structure constructed and arranged to supply a substantially laminar flow of flushing gas, in a vicinity of said first object table, across at least a part of a path of said projection beam to displace ambient air therefrom, said flushing gas being substantially non-absorbent of said radiation. 34. The apparatus according to  claim 33 , wherein said gas structure comprises a gas flow regulator and an evacuator constructed and arranged to remove flushing gas from said part of said beam path. claim 33 35. The apparatus according to  claim 33 , wherein said at least a part of the path includes a plurality of parts of said beam path substantially separated from each other. claim 33 36. The apparatus according to  claim 33 , wherein said gas structure is arranged to provide separate laminar gas flows across a plurality of parts of said beam path, said parts of said beam path being substantially isolated from each other. claim 33 37. The apparatus according to  claim 33  further comprising: claim 33 at least one cover member formed of a material substantially transparent to said radiation, said cover member being substantially planar and provided substantially parallel to the direction of said laminar flow to cover a non-planar surface of a component of said lithographic apparatus in a region of said part of said beam path. 38. The apparatus according to  claim 33 , wherein a speed of said laminar flow is greater than a maximum speed of movement of any moving parts in a region of said part of said beam path. claim 33 39. The apparatus according to  claim 33 , wherein a speed of said laminar flow is greater than a diffusion speed of air. claim 33 40. A lithographic projection apparatus comprising: a radiation system constructed and arranged to supply a projection beam of radiation;  a first object table for holding a mask;  a second object table for holding a substrate;  a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate; and  a gas structure mounted to an extremity of at least one of said radiation system and said projection system, said gas structure is constructed and arranged to supply a laminar flow of flushing gas across at least a portion of a path of said projection beam, said flushing gas being substantially non-absorbent of said radiation. 41. The apparatus according to  claim 40 , wherein said gas structure comprises an evacuator constructed and arranged to remove flushing gas from said part of said beam path in a vicinity of said extremity of said at least one of said radiation system and said projection system. claim 40 42. The apparatus according to  claim 41 , wherein the structure of flushing gas is provided through an outlet mounted to an extremity of said at least one of said radiation system and said projection system and said evacuator is provided with an inlet facing said outlet and mounted to an extremity of said at least one of said radiation system and said projection system. claim 41 43. The apparatus according to  claim 42 , wherein at least one of said inlet and said outlet comprises vanes configured and arranged to control the flow of said flushing gas. claim 42 44. The apparatus according to  claim 40 , wherein a speed of said laminar flow is greater than a maximum speed of movement of any moving parts in a region of said part of said beam path. claim 40 45. The apparatus according to  claim 40 , wherein a speed of said laminar flow is greater than a diffusion speed of air. claim 40 46. The method according to  claim 19 , wherein said flushing gas is provided in a vicinity of at least one of the mask and the substrate. claim 19 47. The method according to  claim 19 , wherein said flushing gas flows across at least one substantially transparent cover member covering a non-planar surface of a component of the projection apparatus in a region of said part of said beam path, the cover member being substantially planar and substantially parallel to the direction of said laminar flow. claim 19