Patent Publication Number: US-5835275-A

Title: Catadioptric system for photolithography

Description:
FIELD OF THE INVENTION 
     The present invention pertains to catadioptric systems for photolithography and in particular, to high-resolution catadioptric systems for use with short-wavelength illumination. 
     BACKGROUND OF THE INVENTION 
     Catadioptric optical systems such as those disclosed in Japanese patent document 234722 (1992) and U.S. Pat. No. 4,779,966 have been developed to correct field curvature in photolithography systems. In these catadioptric systems, a first imaging system containing a concave mirror forms an intermediate image of a reticle. A plane mirror directs a light flux from the intermediate image to a second imaging system that re-images the intermediate image onto a wafer. These catadioptric systems have only one plane mirror and the reticle and the wafer are not parallel, thereby complicating synchronous scan of the reticle and the wafer. Other catadioptric systems have been developed wherein the second imaging system comprises a second plane mirror, permitting the reticle and the wafer to be parallel. 
     In general, catadioptric optical systems with tilted plane mirrors exhibit large aberrations unless the tilts of the plane mirrors are fixed precisely with respect to other portions of the optical system. In catadioptric systems that produce high-resolution images, the angular alignment tolerances of the plane mirrors must be very small and catadioptric system assembly is difficult. 
     SUMMARY OF THE INVENTION 
     Catadioptric systems according to the present invention have relaxed angular alignment tolerances for the plane mirrors or other reflecting surfaces that permit the reticle and wafer to be in parallel planes. Catadioptric systems according to the present invention preferably comprise, from objectwise to imagewise, a first imaging system comprising a single-pass optical system and a double-pass optical system, and a second imaging system. The double-pass optical system comprises, from objectwise to imagewise, a double-pass lens group and a concave mirror. 
     The single-pass optical system receives a light flux from an object, e.g., a reticle, and transmits the light flux to the double-pass optical system. The double-pass lens group receives the light flux from the single-pass optical system and transmits the light flux to the concave mirror. The concave mirror reflects the light flux back through the double-pass lens group and an intermediate image is formed. A first plane mirror or other reflector placed near the intermediate image reflects the light flux to the second imaging system and a image of the reticle in formed, generally on the surface of a wafer or other substrate. 
     The second imaging system preferably comprises a first lens group and a second lens group. In addition, the catadioptric systems preferably comprise a second plane mirror or other reflector and an aperture. The second plane mirror and the aperture preferably satisfy at least one of the conditions: 
     
         L.sub.1 /L&lt;0.1 
    
     
         L.sub.1 /L.sub.2 &lt;0.2 
    
     wherein L 1  is the axial distance from the second plane mirror to the aperture, L is the axial distance from the reticle, or other object, to the wafer, or other image location, and L 2  is the axial distance from the first plane mirror to the wafer W, or other image location. 
     The catadioptric systems preferably satisfy the condition: 
     
         |L.sub.1 /f.sub.1 |&lt;1.5 
    
     wherein f 1  is the focal length of the second lens group of the second imaging system. 
     It is further preferable that the catadioptric systems satisfy the condition: 
     
         |β.sub.1 |&lt;0.2 
    
     wherein β 1  is the magnification of the second lens group of the second imaging system. 
     The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description of a preferred embodiment which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic optical diagram of a catadioptric system according to Example Embodiment 1. 
     FIGS. 2(a)-2(b) are plots of transverse aberrations exhibited by Example Embodiment 1. FIG. 2(a) depicts transverse aberrations at an image height of Y=18.6 mm. FIG. 2(b) depicts transverse aberrations at an image height of Y=5 mm. 
     FIG. 3 is a schematic optical diagram of a catadioptric system according to Example Embodiment 2. 
     FIGS. 4(a)-4(b) are plots of transverse aberrations exhibited by Example Embodiment 2. FIG. 4(a) depicts transverse aberrations at an image height of Y=18.6 mm. FIG. 4(b) depicts transverse aberrations at an image height of Y=5 mm. 
     FIG. 5 is a schematic elevational view of a projection-exposure apparatus comprising a catadioptric system according to the invention. 
    
    
     DETAILED DESCRIPTION 
     For convenience in describing the Example Embodiments, an axis is a line or connected series of line segments through a center of curvature of a reflecting or refracting surface. As is well-known, optical systems with tilted reflecting surfaces have optical axes consisting of connected line segments. Directions along an axis toward an object or an image are called &#34;objectwise&#34; and &#34;imagewise,&#34; respectively. 
     With reference to FIG. 1 and FIG. 3, for example, catadioptric systems according to Example Embodiments 1-2, respectively, project a demagnified image of a reticle R, or other object, onto a wafer W, or other substrate. The catadioptric systems preferably comprise, from objectwise to imagewise, a first imaging system A that forms an intermediate image of the reticle R, a first plane mirror M 1  placed near the intermediate image, and a second imaging system B that re-images the intermediate image onto the wafer W. 
     The first imaging system A preferably comprises, from objectwise to imagewise, a single-pass optical system Al and a double-pass optical system A 2 . The double-pass optical system A 2  preferably comprises, from the single-pass optical system A 1  and along an axis, a double-pass lens group A 2L  and a concave mirror M c . The double-pass lens group A 2L  preferably comprises a concave lens L c  placed axially adjacent the concave mirror M c . 
     The second imaging system B preferably comprises, from objectwise to imagewise, a first lens group B 1  and a second lens group B 2 . The second imaging system further comprises an aperture S. In addition, the second imaging system comprises a second plane mirror M 2  between the first lens group B 1  and the second lens group B 2 . In both Example Embodiments 1-2, the second plane mirror M 2  is objectwise of the aperture S, but the second plane mirror M 2  can be imagewise of the aperture S. 
     The single-pass optical system A 1  receives a light flux from the reticle R and directs the light flux to the double-pass optical system A 2 . The double-pass lens group A 2L  transmits the light flux to the concave mirror M c . The concave mirror M c  receives the light flux and reflects the light flux back through the double-pass lens group A 2L . The light flux then exits the double-pass optical system A 2  and is directed by the first plane mirror M 1  to the second imaging system B. 
     The first lens group B 1  of the second imaging system B receives the light flux reflected by the first plane mirror M 1  and directs the light flux to the second plane mirror M 2 . The second plane mirror M 2  then reflects the light flux to the second lens group B 2  and an image is formed on the wafer W. 
     The catadioptric systems of the preferred embodiments are suitable for photolithography systems in which the reticle R is scanned through an off-axis, slit or arc-shaped illumination region. Patterns from the entire reticle R are projected onto the wafer W by synchronously scanning the wafer W while projecting patterns from the reticle R as the reticle R scans through the illumination region. 
     The second plane mirror M 2  and the aperture S are preferably arranged to satisfy at least one of the following Conditional Expressions 1-2: 
     
         L.sub.1 /L&lt;0.1                                             (1) 
    
     
         L.sub.1 /L.sub.2 &lt;0.2                                      (2) 
    
     wherein L 1  is the axial distance from the second plane mirror M 2  to the aperture S, L is the axial distance from the reticle R, or other object, to the wafer W, or other image location, and L 2  is the axial distance from the first plane mirror M 1  to the wafer W, or other image location. Because the second plane mirror M 2  is near the aperture S, the light flux reflected by the second plane mirror M 2  is substantially collimated. Off-axis aberrations such as coma, astigmatism, and distortion due to tilt of the second plane mirror M 2  are therefore reduced and tilt tolerances for the second plane mirror M 2  are relaxed. If Conditional Expressions 1-2 are not satisfied, then off-axis aberrations are unacceptably large. 
     Also, the catadioptric systems preferably satisfy Conditional Expression 3: 
     
         |L.sub.1 /f.sub.1 |&lt;1.5                  (3) 
    
     wherein f 1  is the focal length of the second lens group B 2  of the second imaging system B. Satisfying Conditional Expression 3 reduces aberrations in the second imaging system B. If Conditional Expression 3 is not satisfied, then f 1  is too small and correction of aberrations in the second imaging system B is difficult. 
     It is further preferable that Conditional Expression 4 be satisfied: 
     
         |L.sub.1 /f.sub.1 |&lt;1.5                  (4) 
    
     wherein β 1  is the magnification of the second lens group B 2  of the second imaging system B. Satisfying Conditional Expression 4 provides that the light flux at the second plane mirror M 2  is substantially collimated, i.e. an angular diameter of the light flux is less than the object-side numerical aperture. If Conditional Expression 4 is not satisfied, then the light flux has a large angular diameter and the second plane mirror M 2  tends to reflect the light flux non-uniformly. 
     Tables 1-2 contain specifications for the catadioptric systems of Example Embodiments 1-2, respectively. In Tables 1-2, the first column lists surfaces numbered in order from objectwise to imagewise along an axis along which the light flux from the reticle R propagates; the second column, labeled &#34;r, &#34; lists corresponding curvature radii; the third column, labeled &#34;d, &#34; lists axial separations between adjacent, imagewise surfaces; the fourth column, labeled &#34;Material, &#34; lists lens materials; and the fifth column, labeled &#34;Group, &#34; lists the group number of the surfaces. In the fifth column, a &#34;*&#34; indicates a surface of the double-pass optical system A2 encountered by a light flux after reflection from the concave mirror C m . Refractive indices of fused silica (SiO 2 ) and fluorite (CaF 2 ) at a wavelength of 193 nm are n=1.56019 and n=1.50138, respectively. 
     Table 3 contains values of the Conditional Expressions 1-4 and values of L 1 , L, L 2 , and f1 for Example Embodiments 1-2. 
     With reference to FIG. 2 and FIG. 4, it is apparent that the catadioptric systems are well-corrected for aberrations over a wavelength range of 193.4 nm±0.1 nm at image heights of Y=18.6 mm and Y=5 mm. 
     
                       TABLE 1                                                     
______________________________________                                    
(Example Embodiment 1)                                                    
Surf.  r            d                                                     
No.    (mm)         (mm)    Material  Group                               
______________________________________                                    
0      --           49.998            R                                   
1      369.115      18.000  SiO.sub.2 A.sub.1                             
2      245.893      0.500                                                 
3      227.674      33.705  CaF.sub.2 A.sub.1                             
4      -373.082     18.803                                                
5      -324.258     20.532  SiO.sub.2 A.sub.1                             
6      332.817      1.674                                                 
7      340.581      20.389  SiO.sub.2 A.sub.1                             
8      604.750      27.395                                                
9      ∞      35.900  SiO.sub.2 A.sub.1                             
10     ∞      16.943                                                
11     391.176      30.000  CaF.sub.2 A.sub.2                             
12     -982.727     6.592                                                 
13     -417.793     20.000  SiO.sub.2 A.sub.2                             
14     -1216.731    261.353                                               
15     478.547      40.000  CaF.sub.2 A.sub.2                             
16     -908.632     11.323                                                
17     325.213      20.000  SiO.sub.2 A.sub.2                             
18     208.331      48.917                                                
19     -196.257     20.000  SiO.sub.2 A.sub.2                             
20     1370.871     0.500                                                 
21     430.209      42.793  CaF.sub.2 A.sub.2                             
22     -366.694     61.625                                                
23     247.465      25.000  SiO.sub.2 A.sub.2                             
24     286.274      68.753                                                
25     508.228      40.000  SiO.sub.2 A.sub.2                             
26     -930.828     27.931                                                
27     -313.824     25.000  SiO.sub.2 A.sub.2                             
28     -1017.267    19.454                                                
29     -276.064     25.000  SiO.sub.2 A.sub.2                             
30     1335.454     32.821                                                
31     -360.416     32.821            A.sub.2 (M.sub.c)                   
32     1335.454     25.000  SiO.sub.2 A.sub.2*                            
33     -276.064     19.454                                                
34     -1017.267    25.000  SiO.sub.2 A.sub.2*                            
35     -313.824     27.931                                                
36     -930.828     40.000  SiO.sub.2 A.sub.2*                            
37     508.228      68.753                                                
38     286.274      25.000  SiO.sub.2 A.sub.2*                            
39     247.465      61.625                                                
40     -366.694     42.793  CaF.sub.2 A.sub.2*                            
41     430.209      0.500                                                 
42     1370.871     20.000  SiO.sub.2 A.sub.2*                            
43     -196.257     48.917                                                
44     208.331      20.000  SiO.sub.2 A.sub.2*                            
45     325.213      11.323                                                
46     -908.632     40.000  CaF.sub.2 A.sub.2*                            
47     478.547      261.353                                               
48     -1216.731    20.000  SiO.sub.2 A.sub.2*                            
49     -417.793     6.592                                                 
50     -982.727     30.000  CaF.sub.2 A.sub.2*                            
51     391.176      1.943                                                 
52     ∞      236.637           M.sub.1                             
53     471.443      36.090  CaF.sub.2 B.sub.1                             
54     -1089.261    3.979                                                 
55     306.858      20.000  SiO.sub.2 B.sub.1                             
56     247.195      312.806                                               
57     812.165      25.000  SiO.sub.2 B.sub.1                             
58     2628.418     145.000                                               
59     ∞      145.508           M.sub.2                             
60     -1094.809    30.000  SiO.sub.2 B.sub.2                             
61     1598.936     30.114                                                
62     --           81.437            S                                   
63     -266.544     45.218  CaF.sub.2 B.sub.2                             
64     2115.935     0.550                                                 
65     -213.134     30.096  SiO.sub.2 B.sub.2                             
66     -642.205     15.142                                                
67     1328.716     30.000  SiO.sub.2 B.sub.2                             
68     -654.044     1.236                                                 
69     -210.004     45.167  SiO.sub.2 B.sub.2                             
70     -304.557     19.703                                                
71     -166.497     45.000  SiO.sub.2 B.sub.2                             
72     -72.336      6.218                                                 
73     -71.786      66.262  SiO.sub.2 B.sub.2                             
74     2042.086     17.000                                                
75     --                             W                                   
______________________________________                                    
 
    
     
                       TABLE 2                                                     
______________________________________                                    
(Example Embodiment 2)                                                    
Surf.  r            d                                                     
No.    (mm)         (mm)    Material Group                                
______________________________________                                    
0      --           60.000           R                                    
1      -210.000     18.000  SiO.sub.2                                     
                                     A.sub.1                              
2      -233.058     1.734                                                 
3      301.818      32.109  CaF.sub.2                                     
                                     A.sub.1                              
4      -415.393     19.449                                                
5      154862.242   15.248  SiO.sub.2                                     
                                     A.sub.1                              
6      -528.109     5.460                                                 
7      -316.309     18.000  SiO.sub.2                                     
                                     A.sub.1                              
8      275.570      74.064                                                
9      342.313      48.000  CaF.sub.2                                     
                                     A.sub.2                              
10     -248.024     1.806                                                 
11     -250.000     20.000  SiO.sub.2                                     
                                     A.sub.2                              
12     3438.110     286.849                                               
13     390.013      40.000  CaF.sub.2                                     
                                     A.sub.2                              
14     -2017.162    22.849                                                
15     421.041      20.000  SiO.sub.2                                     
                                     A.sub.2                              
16     230.317      47.916                                                
17     -222.542     20.000  SiO.sub.2                                     
                                     A.sub.2                              
18     988.626      7.270                                                 
19     11949.023    27.617  CaF.sub.2                                     
                                     A.sub.2                              
20     -328.913     0.500                                                 
21     365.306      42.285  SiO.sub.2                                     
                                     A.sub.2                              
22     -1713.365    160.144                                               
23     -283.704     30.000  SiO.sub.2                                     
                                     A.sub.2                              
24     1076.349     30.701                                                
25     -353.136     30.701           A.sub.2 (M.sub.c)                    
26     1076.349     30.000  SiO.sub.2                                     
                                     A.sub.2*                             
27     -283.704     160.144                                               
28     -1713.365    42.285  SiO.sub.2                                     
                                     A.sub.2*                             
29     365.306      0.500                                                 
30     -328.913     27.617  CaF.sub.2                                     
                                     A.sub.2*                             
31     -11949.023   7.270                                                 
32     988.626      20.000  SiO.sub.2                                     
                                     A.sub.2*                             
33     -222.542     47.916                                                
34     230.317      20.000  SiO.sub.2                                     
                                     A.sub.2*                             
35     421.041      22.849                                                
36     -2017.162    40.000  CaF.sub.2                                     
                                     A.sub.2*                             
37     390.013      286.849                                               
38     3438.110     20.000  SiO.sub.2                                     
                                     A.sub.2*                             
39     -250.000     1.806                                                 
40     -248.024     48.000  CaF.sub.2                                     
                                     A.sub.2*                             
41     342.313      4.064                                                 
42     ∞      180.000          M.sub.1                              
43     506.214      34.041  CaF.sub.2                                     
                                     B.sub.1                              
44     -256.332     3.017                                                 
45     -450.000     20.000  SiO.sub.2                                     
                                     B.sub.1                              
46     -1453.242    422.966                                               
47     ∞      150.000          M.sub.2                              
48     -285.380     30.000  SiO.sub.2                                     
                                     B.sub.2                              
49     -954.824     50.000                                                
50     --           78.332           S                                    
51     -220.000     45.000  CaF.sub.2                                     
                                     B.sub.2                              
52     -2665.536    6.535                                                 
53     -200.000     27.411  SiO.sub.2                                     
                                     B.sub.2                              
54     -516.467     18.844                                                
55     632.373      30.000  SiO.sub.2                                     
                                     B.sub.2                              
56     -1060.585    19.112                                                
57     -553.788     45.000  SiO.sub.2                                     
                                     B.sub.2                              
58     5823.302     0.500                                                 
59     -153.299     45.000  SiO.sub.2                                     
                                     B.sub.2                              
60     -120.000     1.243                                                 
61     -125.615     66.000  SiO.sub.2                                     
                                     B.sub.2                              
62     3036.218     17.000                                                
63     --                            W                                    
______________________________________                                    
 
    
     
                       TABLE 3                                                     
______________________________________                                    
Values of Conditional Expressions                                         
Variables and                                                             
             Example    Example                                           
Conditions   Embodiment 1                                                 
                        Embodiment 2                                      
______________________________________                                    
       L.sub.1 (mm)                                                       
                 205.6      230.0                                         
       L (mm)    3287.2     3150.0                                        
       L.sub.2 (mm)                                                       
                 1388.2     1290.0                                        
       f.sub.1 (mm)                                                       
                 192.0      207.5                                         
(1)    L.sub.1 /L                                                         
                 0.06       0.07                                          
(2)    L.sub.1 /L.sub.2                                                   
                 0.15       0.18                                          
(3)    |L.sub.1 /f.sub.1 |                              
                 1.07       1.11                                          
(4)    |β.sub.1 |                                  
                 0.1409     0.1932                                        
______________________________________                                    
 
    
     In Example Embodiment 1, the second mirror M 2  is placed 205.6 mm objectwise of the aperture S; in Example Embodiment 2, the second mirror M 2  is 230 mm objectwise of the aperture S. As a result, coma, astigmatism, and distortion due to tilt of the plane mirror M 2  are small, thereby simplifying the fabrication of a high-resolution catadioptric system. In addition, irregular reflectance of the second mirror M 2  is reduced. 
     In both Example Embodiments 1-2, the second plane mirror M 2  is objectwise of the aperture S. However, the second plane mirror M 2  can be imagewise of the aperture S so long as Conditional Expressions 1-4 are satisfied. 
     A representative projection-exposure (microlithography) apparatus 10 comprising a catadioptric system as described above is schematically illustrated in FIG. 5. The apparatus 10 comprises an illumination optical system 12, a reticle or &#34;mask&#34; 14 defining a pattern for projection, a reticle stage 16 for holding the reticle, a catadioptric system 18 as described above, a photosensitive substrate 20 onto which the catadioptric system projects an image of the pattern defined by the mask 14, a substrate stage 22 for holding the substrate 20, a driver 24 for moving the reticle stage 16 as required during or between exposures, a driver 26 for moving the substrate stage 22 as required during or between exposures, and a controller 28 for controlling operation of the entire apparatus 10. 
     Having illustrated and demonstrated the principles of the invention in example embodiments, it should be apparent to those skilled in the art that the example embodiments can be modified in arrangement and detail without departing from such principles. For example, other reflecting elements such as prisms can be substituted for the first plane mirror M 1  or the second plane mirror M 2  of Example Embodiments 1-2. I claim as the invention all that comes within the scope of these claims.