Abstract:
In order to provide an antireflection film of a wide wavelength bandwidth excellent in environment resistance performance in the ultraviolet wavelength region, the antireflection film is constructed in four-layered, or five-layered, or six-layered structure, using Al 2 O 3  for high-index layers and AlF 3  or MgF 2  for low-index layers. Vacuum evaporation, sputtering, or CVD is used for formation of the antireflection film.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antireflection film as a coating over a surface of an optical element and, more particularly, to an antireflection film effective against ultraviolet light of the wavelengths not more than 250 nm. 
     2. Related Background Art 
     The antireflection films for the ultraviolet light, using fluoride films, are described in Japanese Patent Laid-Open Application Nos. 61-77001, 7-244205, and 7-244217. 
     SUMMARY OF THE INVENTION 
     The Japanese Patent Laid-Open Application Nos. 7-244205 and 7-244217 disclose NdF 3 , LaF 3 , etc. as high-index fluoride films. These films, however, have a problem that they are inferior in environment resistance (characteristic stability under high temperature and high humidity environments) to oxide films such as Al 2 O 3 . 
     It is thus difficult to apply such films to a projection optical system using a lot of lenses like a refracting projection optical system used in exposure apparatus (so called steppers) for projecting and printing a reticle pattern onto a semiconductor wafer in a semiconductor fabrication step. In the case of the projection optical system using such many lenses, even if change is small in the antireflection characteristics of each surface due to time-dependent change, the total change of the overall projection optical system would result in a large characteristic shift. 
     An object of the present invention is thus to provide an antireflection film of a wide bandwidth excellent in the environment resistance performance (stability of optical characteristics with a lapse of time) in the ultraviolet wavelength region, an optical element with the antireflection film, and a method for producing the antireflection film. 
     The inventors found that use of Al 2 O 3  film as a high-index film permitted production of the densest film among high-index film materials available in the ultraviolet wavelength region and considerably improved the environment resistance performance (the temporal stability of optical characteristics) of the antireflection film. 
     We also clarified that films of AlF 3  including the same metal Al were preferably used as low-index films to be alternately stacked with Al 2 O 3  and that loss was small at the interfaces. It was also clarified that MgF 2  could replace AlF 3 . 
     Good results were obtained when the film-forming methods were vacuum evaporation; and dense films were able to be formed by sputtering, whereby the environment resistance performance was improved. In addition, a uniform film can also be produced on a curved surface of a lens or the like by Chemical Vapor Deposition (CVD). 
     In view of expansion of the antireflection bandwidth and thickness errors due to manufacturing errors, an antireflection film of the present invention for accomplishing the above object is an antireflection film comprising: 
     an alternately multilayered film having a designed center wavelength λ 0  in a range of wavelengths 190 nm to 250 nm and comprising high-index layers of a refractive index n h  and low-index layers of a refractive index n l  in order from the side of a substrate to the side of air, said alternately multilayered film having a four-layered structure, wherein when a refractive index of the substrate is n, the refractive indices satisfy the following conditions; 
     
       
         1.35≦n l ≦1.45 
       
     
     
       
         1.60≦n h ≦1.85 
       
     
     
       
         n l ≦n≦n h ; and 
       
     
     wherein where optical thicknesses (refractive index x geometrical thickness) of the respective layers are represented by d 1 , d 2 , d 3 , and d 4  in order from the first layer to the fourth layer when counted from the substrate side, the first layer and the third layer are the high-index layers, the second layer and the fourth layer are the low-index layers, and the optical thicknesses satisfy the following conditions: 
     
       
         0.38 λ 0 ≦d 1 ≦0.43 λ 0   
       
     
     
       
         0.38 λ 0 ≦d 2 ≦0.43 λ 0   
       
     
     
       
         0.20 λ 0 ≦d 3 ≦0.25 λ 0   
       
     
     
       
         0.20 λ 0 ≦d 4 ≦0.25 λ 0 . 
       
     
     Another antireflection film of the present invention is an antireflection film comprising: 
     an alternately multilayered film having a designed center wavelength As in a range of wavelengths 190 nm to 250 nm and comprising high-index layers of a refractive index n h  and low-index layers of a refractive index n l  in order from the side of a substrate to the side of air, said alternately multilayered film having a five-layered structure, wherein when a refractive index of the substrate is n, the refractive indices satisfy the following conditions; 
     
       
         1.35≦n l ≦1.45 
       
     
     
       
         1.60≦n h ≦1.85 
       
     
     
       
         n l ≦n≦n h ; and 
       
     
     wherein where optical thicknesses (refractive index x geometrical thickness) of the respective layers are represented by d 1 , d 2 , d 3 , d 4 , and d 5  in order from the first layer to the fifth layer when counted from the substrate side, the first layer, the third layer, and the fifth layer are the low-index layers, the second layer and the fourth layer are the high-index layers, and the optical thicknesses satisfy the following conditions: 
     
       
         0.42 λ 0 ≦d 1 ≦0.47 λ 0   
       
     
     
       
         0.37 λ 0 ≦d 2 ≦0.42 λ 0   
       
     
     
       
         0.37 λ 0 ≦d 3 ≦0.42 λ 0   
       
     
     
       
         0.20 λ 0 ≦d 4 ≦0.25 λ 0   
       
     
     
       
         0.20 λ 0 ≦d 5 ≦0.25 λ 0   
       
     
     Still another antireflection film of the present invention is an antireflection film comprising: 
     an alternately multilayered film having a designed center wavelength λ 0  in a range of wavelengths 190 nm to 250 nm and comprising high-index layers of a refractive index n h  and low-index layers of a refractive index n l  in order from the side of a substrate to the side of air, said alternately multilayered film having a six-layered structure, wherein when a refractive index of the substrate is n, the refractive indices satisfy the following conditions; 
     
       
         1.35≦n l ≦1.45 
       
     
     
       
         1.60≦n h ≦1.85 
       
     
     
       
         n l ≦n≦n h ; and 
       
     
     wherein where optical thicknesses (refractive index x geometrical thickness) of the respective layers are represented by d 1 , d 2 , d 3 , d 4 , d 5 , and d 6  in order from the first layer to the sixth layer when counted from the substrate side, the first layer, the third layer, and the fifth layer are the high-index layers, the second layer, the fourth layer, and the sixth layer are the low-index layers, and the optical thicknesses satisfy the following conditions: 
     
       
         0.40 λ 0 ≦d 1 ≦0.45 λ 0   
       
     
     
       
         0.37 λ 0 ≦d 2 ≦0.42 λ 0   
       
     
     
       
         0.45 λ 0 ≦d 3 ≦0.50 λ 0   
       
     
     
       
         0.04 λ 0 ≦d 4 ≦0.09 λ 0   
       
     
     
       
         0.28 λ 0 ≦d 5 ≦0.33 λ 0   
       
     
     
       
         0.20 λ 0 ≦d 6 ≦0.25 λ 0   
       
     
     The high-index layers are Al 2 O 3  as described above and the low-index layers are preferably AlF 3  or may be MgF 2 . 
     The present invention has the following effects. 
     (1) The present invention was able to provide the antireflection films of the wide bandwidth in the ultraviolet region by the above-stated film structures, using Al 2 O 3  for the high-index layers and AlF 3  or MgF 2  for the low-index layers. 
     (2) The present invention was able to provide the antireflection films in the ultraviolet wavelength region, excellent in the environment resistance, by using Al 2 O 3  for the high-index layers. 
     (3) The present invention was able to provide the antireflection films for ultraviolet light using the Al 2 O 3  layers, which were superior in the environment resistance to those constructed only of fluoride layers. 
     (4) The present invention was able to provide the antireflection films for ultraviolet light of the wide bandwidth in the four-layered, five-layered, and six-layered structures by optimization of the film configuration, and the production method thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram to show reflectance characteristics of the antireflection film in Example 1; 
     FIG. 2 is a diagram to show reflectance characteristics of the antireflection film in Example 2; 
     FIG. 3 is a diagram to show reflectance characteristics of the antireflection film in Example 3; 
     FIG. 4 is a diagram to show reflectance characteristics of the antireflection film in Example 4; 
     FIG. 5 is a diagram to show reflectance characteristics of the antireflection film in Example 5; and 
     FIG. 6 is a diagram to show reflectance characteristics of the antireflection film in Example 6. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention will be described in detail by examples, but it should be noted that the present invention is by no means intended to be limited to these examples and that the present invention involves all modifications and changes within the scope of the present invention. 
     EXAMPLE 1 
     Using the high-index films of Al 2 O 3  and the low-index films of AlF 3  or MgF 2 , the antireflection film was produced in the four-layered structure as shown in Table 1 against the ultraviolet light with the designed center wavelength λ 0 =193 nm. The antireflection film was produced in the thicknesses of Table 1, using a synthetic quarts substrate. Reflection characteristics of the antireflection film thus produced were measured and the results thereof are illustrated in FIG.  1 . It was verified that the wavelength bandwidth of the reflectances not more than 0.2% was as wide as about 40 nm. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Optical 
               
               
                   
                 Material 
                 thickness 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Entrance medium 
                 air 
                   
               
               
                   
                 Fourth layer 
                 AlF 3  or MgF 2   
                 0.22922 λ 0   
               
               
                   
                 Third layer 
                 Al 2 O 3   
                 0.22353 λ 0   
               
               
                   
                 Second layer 
                 AlF 3  or MgF 2   
                 0.41996 λ 0   
               
               
                   
                 First layer 
                 Al 2 O 3   
                 0.41223 λ 0   
               
               
                   
                 Exit medium 
                 synthetic quartz 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 2 
     The antireflection film of the five-layered structure against the ultraviolet light with the designed center wavelength λ 0 =193 nm is shown in Table 2. The antireflection film was produced in the thicknesses of Table 2, using a fluorite substrate. The reflection characteristics of the antireflection film thus produced were measured and the results thereof are shown in FIG.  2 . It was verified that the film configuration whose wavelength bandwidth of the reflectances not more than 0.2% was as wide as 44 nm was able to be obtained even in the vacuum ultraviolet wavelength region of the wavelengths not more than 200 nm. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Optical 
               
               
                   
                 Material 
                 thickness 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Entrance medium 
                 air 
                   
               
               
                   
                 Fifth layer 
                 AlF 3  or MgF 2   
                 0.22515 λ 0   
               
               
                   
                 Fourth layer 
                 Al 2 O 3   
                 0.22707 λ 0   
               
               
                   
                 Third layer 
                 AlF 3  or MgF 2   
                 0.39863 λ 0   
               
               
                   
                 Second layer 
                 Al 2 O 3   
                 0.39159 λ 0   
               
               
                   
                 First layer 
                 AlF 3  or MgF 2   
                 0.44195 λ 0   
               
               
                   
                 Exit medium 
                 fluorite 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 3 
     The antireflection film of the six-layered structure against the ultraviolet light with the designed center wavelength λ 0 =193 nm is shown in Table 3. The antireflection film was produced in the thicknesses of Table 3, using a fluorite substrate. The reflection characteristics of the antireflection film thus produced were measured. The results of the measurement of optical characteristics of reflectance are shown in FIG.  3 . It was verified that the film configuration whose wavelength bandwidth of the reflectances not more than 0.2% was as wide as 46 nm was able to be obtained even in the vacuum ultraviolet wavelength region of the wavelengths not more than 200 nm. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Optical 
               
               
                   
                 Material 
                 thickness 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Entrance medium 
                 air 
                   
               
               
                   
                 Sixth layer 
                 AlF 3  or MgF 2   
                 0.23101 λ 0   
               
               
                   
                 Fifth layer 
                 Al 2 O 3   
                 0.30711 λ 0   
               
               
                   
                 Fourth layer 
                 AlF 3  or MgF 2   
                 0.06564 λ 0   
               
               
                   
                 Third layer 
                 Al 2 O 3   
                 0.49072 λ 0   
               
               
                   
                 Second layer 
                 AlF 3  or MgF 2   
                 0.39775 λ 0   
               
               
                   
                 First layer 
                 Al 2 O 3   
                 0.42817 λ 0   
               
               
                   
                 Exit medium 
                 fluorite 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 4 
     Using the high-index films of Al 2 O 3  and the low-index films of AlF 3  or MgF 2 , the antireflection film was produced in the four-layered structure as shown in Table 4 against the ultraviolet light with the designed center wavelength λ 0 =248 nm. The antireflection film was produced in the thicknesses of Table 4, using a synthetic quarts substrate. The reflection characteristics of the antireflection film thus produced were measured. The results of the measurement of the optical characteristics of reflectance are shown in FIG.  4 . It was verified that the wavelength bandwidth of the reflectances not more than 0.5% was as wide as about 70 nm. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Optical 
               
               
                   
                 Material 
                 thickness 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Entrance medium 
                 air 
                   
               
               
                   
                 Fourth layer 
                 AlF 3  or MgF 2   
                 0.22449 λ 0   
               
               
                   
                 Third layer 
                 Al 2 O 3   
                 0.22232 λ 0   
               
               
                   
                 Second layer 
                 AlF 3  or MgF 2   
                 0.41101 λ 0   
               
               
                   
                 First layer 
                 Al 2 O 3   
                 0.41048 λ 0   
               
               
                   
                 Exit medium 
                 synthetic quartz 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 5 
     The antireflection film of the five-layered structure against the ultraviolet light with the designed center wavelength λ 0 =248 nm is shown in Table 5. The antireflection film was produced in the thicknesses of Table 5, using a synthetic quartz substrate. The reflection characteristics of the antireflection film thus produced were measured. The results of the measurement of the optical characteristics of reflectance are shown in FIG.  5 . It was verified that the film configuration whose wavelength bandwidth of the reflectances not more than 0.5% was as wide as 80 nm was able to be obtained even in the ultraviolet wavelength region of from the wavelength 200 nm to the wavelength 300 nm. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                 Optical 
               
               
                   
                 Material 
                 thickness 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Entrance medium 
                 air 
                   
               
               
                   
                 Fifth layer 
                 AlF 3  or MgF 2   
                 0.22401 λ 0   
               
               
                   
                 Fourth layer 
                 Al 2 O 3   
                 0.22444 λ 0   
               
               
                   
                 Third layer 
                 AlF 3  or MgF 2   
                 0.38959 λ 0   
               
               
                   
                 Second layer 
                 Al 2 O 3   
                 0.39518 λ 0   
               
               
                   
                 First layer 
                 AlF 3  or MgF 2   
                 0.45239 λ 0   
               
               
                   
                 Exit medium 
                 synthetic quartz 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 6 
     The antireflection film of the six-layered structure against the ultraviolet light with the designed center wavelength λ 0 =248 nm is shown in Table 6. The antireflection film was produced in the thicknesses of Table 6, using a synthetic quartz substrate. The reflection characteristics of the antireflection film thus produced were measured. The results of the measurement of the optical characteristics of reflectance are shown in FIG.  6 . It was verified that the film configuration whose wavelength bandwidth of the reflectances not more than 0.2% was as wide as 80 nm was able to be obtained even in the ultraviolet wavelength region of from the wavelength 200 nm to the wavelength 300 nm. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                 Optical 
               
               
                   
                 Material 
                 thickness 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Entrance medium 
                 air 
                   
               
               
                   
                 Sixth layer 
                 AlF 3  or MgF 2   
                 0.23036 λ 0   
               
               
                   
                 Fifth layer 
                 Al 2 O 3   
                 0.30467 λ 0   
               
               
                   
                 Fourth layer 
                 AlF 3  or MgF 2   
                 0.06419 λ 0   
               
               
                   
                 Third layer 
                 Al 2 O 3   
                 0.47623 λ 0   
               
               
                   
                 Second layer 
                 AlF 3  or MgF 2   
                 0.40485 λ 0   
               
               
                   
                 First layer 
                 Al 2 O 3   
                 0.43151 λ 0   
               
               
                   
                 Exit medium 
                 synthetic quartz 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 7 
     The antireflection films of Examples 1 to 6 were produced by vacuum evaporation and sputtering. 
     The vacuum evaporation was carried out as follows. The evaporation chamber was evacuated down to the vacuum of 10 −5  Pa or less and thereafter evaporation was carried out for deposition of Al 2 O 3  with an electron gun while introducing oxygen gas at about 20 sccm and for deposition of AlF 3  or MgF 2  by resistance heating under a high vacuum condition. The substrate was heated at the substrate temperature of 200° C. or more. 
     The sputtering was carried out as follows. Using a 99.999% aluminum target, the sputtering was conducted with a process gas mainly containing oxygen gas for deposition of Al 2 O 3  and with mixed gases of Xe and NF 3  for deposition of AlF 3 . The deposition of AlF 3  can also be performed with either one selected from other inert gases such as Ar or He, and fluorine-based gases such as CF 4 , or F 2  gas diluted with an inert gas. Therefore, the antireflection films can be produced by only switching of the gases with the single target. 
     Comparative Example 
     For comparing the antireflection films of Tables 1 to 6 (group A) with antireflection films using only the fluoride films of LaF 3  or NdF 3  instead of Al 2 O 3  (group B), they were kept under circumstances of 60° C. and relative humidity 90% for 1000 hours and they were compared as to appearance and adhesion. From the results of the experiments where the films were kept under the circumstances of 60° C. and relative humidity 90% for 1000 hours, it was verified that all the antireflection films of group B showed clouding or peeling-off in the appearance and were also inferior in the adhesion of tape tests to those of group A.