Patent Number: 
Section: claims

1. A uniformity correction module to improve the uniformity of a radiation distribution in a rectangular illumination slit having two longer sides and two shorter sides, comprising:a plurality of movable blades arranged along each longer side of the illumination slit; anda chamber configured to contain a fluid;wherein, in use of the uniformity correction module, said movable blades are at least partly immersed in said fluid, and the difference between a refractive index of each blade and a refractive index of said fluid is sufficiently small to prevent significant reflection and refraction at a surface of each blade. 2. A uniformity correction module as recited in claim 1, wherein a difference between the refractive index of each blade and the refractive index of said fluid is 0.15 or less. 3. A uniformity correction module as recited in claim 1, wherein the refractive index of said fluid and the refractive index of said blades are substantially equal at a wavelength of said radiation. 4. A uniformity correction module as recited in claim 1, wherein said blades comprise quartz, fused silica, or calcium fluoride. 5. A uniformity correction module as recited in claim 1, wherein said fluid is water. 6. A uniformity correction module as recited in claim 1, wherein each blade further comprises a radiation absorbing coating on at least one surface. 7. A uniformity correction module as recited in claim 6, wherein said radiation absorbing coating varies in a gradual way, so that a degree of absorption increases with increasing distance from a center of the illumination slit. 8. A uniformity correction module as recited in claim 6, wherein a degree of absorption at the end of each blade nearest to a center of the illumination slit is 0%. 9. A uniformity correction module as recited in claim 1, wherein each blade is triangular when viewed in a direction of an optical axis of the module. 10. A uniformity correction module as recited in claim 1, wherein each blade has the shape of an elongate rectangle. 11. A uniformity correction module as recited in claim 10, wherein each blade is arranged along an axis perpendicular to a longitudinal axis of the illumination slit. 12. A uniformity correction module as recited in claim 10, wherein each blade is arranged along an axis that is slanted with respect to a direction perpendicular to a longitudinal axis of the illumination slit. 13. A uniformity correction module as recited in claim 12, wherein an end of each blade nearest the center of the illumination slit is substantially parallel with a longitudinal axis of the illumination slit. 14. A uniformity correction module as recited in claim 1, further comprising a transparent top cover located upstream of said blades, and a transparent bottom cover located downstream of said blades. 15. A uniformity correction module as recited in claim 14, wherein an absorptive coating is applied to the top or bottom cover at locations between adjacent blades, so as to reduce radiation leakage between the blades. 16. A uniformity correction module as recited in claim 1, wherein said liquid is more absorptive than said blades. 17. A uniformity correction module as recited in claim 16, which is provided with a transparent top cover located upstream of said blades, and a transparent bottom cover located downstream of said blades, and wherein a distance between each blade and the top cover, and between each blade and the bottom cover, is between about 0.05 mm and about 0.1 mm. 18. A uniformity correction module as recited in claim 16, wherein said blades are transparent. 19. A uniformity correction module as recited in claim 16, wherein said liquid has an absorption coefficient of about 0.2/cm. 20. A uniformity correction module as recited in claim 16, wherein the blades are slanted at an angle relative to a direction perpendicular to a longitudinal axis of the illumination slit, and wherein an end of each blade nearest a center of the illumination slit is substantially parallel to the longitudinal axis of the illumination slit. 21. A uniformity correction module as recited in claim 16, wherein the blades are provided with complementary shaped protrusions and grooves along their sides, each protrusion fitting into a respective groove, so as to prevent radiation from passing through the uniformity correction module without also passing through the blades. 22. A uniformity correction module as recited in claim 16, wherein edges of adjacent blades on a same side of the illumination slit have a V-shaped profile, and wherein a V-shaped projection along the edge of each blade fits into a V-shaped groove along the edge of an adjacent blade, so as to prevent radiation from passing through the uniformity correction module without also passing through the blades. 23. A uniformity correction module as recited in claim 16, wherein a thickness of each blade, from its upstream edge to its downstream edge, is between about 1 mm and about 2 mm. 24. A uniformity correction module as recited in claim 16, wherein edges of each blade are polished. 25. A uniformity correction module as recited in claim 1, which further comprises a circulation system configured to introduce liquid into said chamber and remove liquid from said chamber during use. 26. A uniformity correction module as recited in claim 1, wherein each blade comprises an opaque member supported by a quartz substrate. 27. A uniformity correction module as recited in claim 26, wherein said opaque member comprises a metallic foil. 28. A uniformity correction module as recited in claim 1, wherein said liquid contains at least one additive which affects a level of absorption of radiation by the liquid. 29. A lithographic apparatus comprising:an illumination system configured to condition beam of radiation;a support structure configured to support a patterning device, the patterning device configured to impart the beam with a pattern in its cross-section;a substrate table configured to hold a substrate;a projection system configured to project the patterned beam onto a target portion of the substrate; anda uniformity correction module including:a plurality of movable blades arranged along each side of the beam; anda chamber configured to contain a fluid;wherein, in use of the uniformity correction module, said movable blades are at least partly immersed in said fluid, and the difference between a refractive index of each blade and a refractive index of said fluid is sufficiently small to prevent significant reflection and refraction at a surface of each blade. 30. A lithographic apparatus as recited in claim 29, wherein said uniformity correction module is located before said patterning device in the path of the beam. 31. A method of increasing uniformity of an illumination beam of radiation comprising:producing the illumination beam; andpositioning a plurality of moveable blades at least partially immersed in a liquid or pressurized gas along each side of the illumination beam such that the blades block at least a portion of the radiation of the illumination beam thereby increasing a uniformity of the illumination beam. 32. A device manufacturing method comprising:supplying a beam of a radiation;patterning the beam of radiation with a pattern in its cross-section;projecting the beam of radiation as patterned onto a target portion of a substrate;positioning a plurality of moveable blades at least partially immersed in a liquid or pressurized gas along each side of the beam such that the blades block at least a portion of the radiation of the beam thereby increasing a uniformity of the beam.