Method for manufacturing a lens of synthetic quartz glass with increased h2 content

Abstract

A method for manufacturing at least one lens of synthetic quartz glass with increased H 2 content for an optical system with an operating wavelength of less than 250 nm. The method involves: (1) providing at least one precursor product of synthetic quartz glass with a first H 2 content, the precursor product having a circumferential border surface and two mutually opposed base surfaces, wherein at least one partial surface of at least one of the base surfaces has a curvature; (2) determining at least one target value for at least one treatment parameter for treating the precursor product in an H 2 -containing inert gas atmosphere; (3) treating the precursor product in the H 2 -containing atmosphere in accordance with the target value of the treatment parameter, thereby producing at least one precursor product of synthetic quartz glass with a second H 2 content that is greater than the first H 2 content; and (4) working at least part of at least one of the base surfaces of the treated precursor product, thereby producing at least one final lens shape.

1 . A method for manufacturing at least one lens of synthetic quartz glass with increased H 2  content for an optical system with an operating wavelength of less than 250 nm, comprising:
 (1) providing at least one precursor product of synthetic quartz glass with a first H 2  content, the precursor product having a circumferential border surface and two mutually opposed base surfaces, wherein at least one partial surface of at least one of the base surfaces has a curvature,   (2) determining at least one target value for at least one treatment parameter for treating the precursor product in an H 2 -containing inert gas atmosphere,   (3) treating the precursor product in the H 2 -containing atmosphere in accordance with the target value of the treatment parameter, thereby producing at least one precursor product of synthetic quartz glass with a second H 2  content that is greater than the first H 2  content, and   (4) working at least part of at least one of the base surfaces of the treated precursor product, thereby producing at least one final lens shape.   
     
     
         2 . The method according to  claim 1 , wherein providing the precursor product comprises cutting the precursor product from a cylindrical blank. 
     
     
         3 . The method according to  claim 2 , wherein cutting the precursor product comprises cutting the cylindrical blank along at least one spherical surface, thereby producing at least two precursor products from the one cylindrical blank. 
     
     
         4 . The method according to  claim 1 , wherein providing the precursor product comprises forming the synthetic quartz glass in a heated state by action of a force, selected from casting, pressing, gravity, pressure imparted by a ram, or a force exerted by a stream of gas. 
     
     
         5 . The method according to  claim 1 , wherein providing the precursor product comprises selecting a shape of the precursor product to match substantially the final lens shape. 
     
     
         6 . The method according to  claim 1 , wherein providing the precursor product comprises selecting a shape for which extrapolation of the treating of the precursor product in the H 2 -containing atmosphere produces a specified distribution of the H 2  concentration in the precursor product. 
     
     
         7 . The method according to  claim 1 , wherein providing the precursor product comprises providing the circumferential border surface of the precursor product with at least one of bevels and grooves prior to treating the precursor product in the H 2 -containing atmosphere. 
     
     
         8 . The method according to  claim 1 , wherein the precursor product has an OH content of less than 150 ppm in proportion to weight, and has a decadic extinction coefficient of less than 2·10 -3 /cm. 
     
     
         9 . The method according to  claim 8 , wherein the precursor product has an OH content of less than 70 ppm in proportion to weight. 
     
     
         10 . The method according to  claim 1 , wherein determining the target value comprises experimentation with a sample precursor product. 
     
     
         11 . The method according to  claim 1 , wherein determining the target value comprises a theoretical determination for an idealized precursor product. 
     
     
         12 . The method according to  claim 1 , wherein determining the target value comprises simulating at least one of an H 2  diffusion and a formation of SiH for a representative precursor product. 
     
     
         13 . The method according to  claim 1 , wherein determining the target value comprises extrapolating from at least one known value for an analogous precursor product. 
     
     
         14 . The method according to  claim 1 , wherein treating the precursor product is performed in an H 2 -containing inert gas atmosphere, wherein the H 2  content of the H 2 -containing inert gas atmosphere is at least 5%. 
     
     
         15 . The method according to  claim 1 , wherein treating the precursor product is performed at a temperature of less than 600° C. and at a pressure of at least 1 bar. 
     
     
         16 . The method according to  claim 1 , wherein the H 2  content of the precursor product prior to treating the precursor product in the H 2 -containing atmosphere is less than 2·10 15  molecules/cm 3 , and wherein the H 2  content of the precursor product after treating the precursor product in the H 2 -containing atmosphere it is at least 5·10 15  molecules/cm 3 . 
     
     
         17 . The method according to  claim 1 , wherein treating the precursor product comprises a hydrogen-charging process and the treatment parameter comprises at least one of a temperature variation and a hydrogen partial pressure variation over time. 
     
     
         18 . The method according to  claim 1 , wherein determining the target value comprises selecting a placement for the lens in an optical system, determining at least one lens parameter for the lens in the selected placement, and determining a threshold H 2  content for the lens with increased H 2  content in accordance with the lens parameter. 
     
     
         19 . The method according to  claim 18 , wherein the lens parameter comprises a setpoint operational radiation energy for the lens with increased H 2  content. 
     
     
         20 . The method according to  claim 18 , wherein determining the lens parameter comprises utilizing an H 2  consumption model that produces an H 2  concentration greater than zero at a maximally exposed location of the lens with increased H 2  content throughout a projected useful life of the lens with increased H 2  content. 
     
     
         21 . The method according to  claim 18 , wherein determining the lens parameter comprises determining a permissible H 2  variation over an optically used area of the lens with increased H 2  content in operation. 
     
     
         22 . The method according to  claim 21 , wherein the permissible H 2  variation over the optically used area of the lens with increased H 2  content is determined based on at least one of:
 a homogeneity of the refractive index of the lens with increased H 2  content, and a homogeneity of compaction and transient absorption for the lens with increased H 2  content.   
     
     
         23 . The method according to  claim 21 , wherein determining the lens parameter comprises factoring in an H 2  loss arising during the manufacture of the lens with increased H 2  content. 
     
     
         24 . The method according to  claim 1 , wherein the second H 2  content provides a predetermined average H 2  concentration over an optically useful area of the lens. This is better than 70, but I would be even happier if we could get a claim for the average concentration in the fluence hot spots. A hot spot volume would be defined by the fluence being 80 to 100% of the maximum fluence. 
     
     
         25 . The method according to  claim 24 , wherein the first H 2  content is less than 2·10 15  molecules/cm 3  and the second H 2  content is greater than 1·10 16  molecules/cm 3 . 
     
     
         26 . The method according to  claim 24 , wherein the second H 2  content exhibits a predetermined variation selected in accordance with differing radiation profiles corresponding to differing radiation intensity distributions to which the lens is exposed in operation. 
     
     
         27 . The method according to  claim 1 , wherein the second H 2  content is greater than a minimum content prescribed for a portion of the lens that receives a highest relative irradiation fluence relative to other portions of the lens. 
     
     
         28 . The method according to  claim 27 , wherein the first H 2  content is less than 2·10 15  molecules/cm 3  and the second H 2  content is greater than 1·10 16  molecules/cm 3 . 
     
     
         29 . The method according to  claim 1 , further comprising, during the treating of the precursor product in the H 2 -containing atmosphere, performing a contamination test using a witness sample of synthetic quartz, wherein performing the contamination test comprises at least one of mass spectroscopy, fluorescence spectroscopy, chemical analysis, and a UV radiation transmissivity measurement. 
     
     
         30 . The method according to  claim 1 , wherein the working comprises at least one of: grinding, lapping, asphericizing and polishing the treated precursor product to a specified surface shape. 
     
     
         31 . The method according to  claim 1 , wherein the working comprises removing a surface layer of between 0.05 mm and 2 mm thickness from at least the part of the one base surface. 
     
     
         32 . The method according to  claim 31 , wherein, prior to the removing, the surface layer has a non-constant thickness, which, at minimum, is 0.05 mm to 2 mm. 
     
     
         33 . The method according to  claim 31 , wherein the part of the one base surface corresponds to an optically used portion of the lens. 
     
     
         34 . The method according to  claim 1 , further comprising finishing the final lens shape to produce at least one final lens surface. 
     
     
         35 . The method according to  claim 34 , wherein the finishing comprises at least one of: mechanically polishing the precursor product to a specified surface roughness, fine removal of material by ion beam figuring or magneto-rheological finishing, and applying an anti-reflection coating. 
     
     
         36 . The method according to  claim 29 , wherein the working comprises removing a surface layer of between 0.05 mm and 2 mm thickness from at least the part of the one base surface, and wherein at least one result of the contamination test determines the surface layer thickness to be removed from at least the part of the one base surface.