Abstract:
An illumination system for a microlithography projection illumination equipment, in which a secondary light source is imaged on a reticle . The distortion of the image can be set by at least one variable optical path between optical elements, and the uniformity of the illumination is changed because of the changed distortion, in particular, in that the uniformity is increased toward the edge.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not Applicable. 
     Statement Regarding Federally sponsored Research or Development 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an illumination system for a microlithographic projection equipment, a REMA objective, and a process for the operation of a REMA objective. 
     2. Discussion of Relevant Prior Act 
     An illumination system for a microlithographic projection illumination equipment is known from U.S. Pat. No. 4,851,882, with a simple REMA objective which has a field diaphragm, namely the reticle mask (REMA), on which the reticle with the structured lithographic mask is imaged. A zoom system is provided, mounted in front, in order to insure optimum illumination of the whole REMA diaphragm surface with little loss of light, in the case of reticles with different mask dimensions and correspondingly different aperture measurements of the REMA diaphragm. There are no specific embodiment examples. 
     Highly developed REMA objectives are described in German Patent Application DE-A 195 48 805 and in the German Patent Application DE 196 53 983.8 of Carl Zeiss. They are suitable for cooperating with zoom-axicon illumination objectives according to European Patent EP-A 0 687 956 of Carl Zeiss, and for an arrangement of the REMA diaphragm at the exit of a glass rod according to U.S. Pat. No. 5,646,715. The cited applications of Carl Zeiss are incorporated by reference into the present application. These all have in common, with the present application, the co-inventor Wangler. 
     European Patent 0 500 393 B1 describes a microlithographic projection illumination system with variable quadrupole illumination, in which there is provided, between a honeycomb condenser and the reticle mask, an optics with the possibility of adjustment in order to adjust the uniformity of illumination. Since the skilled person is given in this patent only functional optical groups in the nature of block diagrams, but not a specific embodiment example, this disclosure has more of a functional character. 
     U.S. Pat. No. 5,311,362 describes a microlithographic projection illumination system with variable numerical aperture of the illumination system, in which the numerical aperture of the projection objective is also variable and an optical path length between lenses of the projection objective can be varied in dependence on the two numerical apertures, whereby aberrations, primarily the vertical spherical aberration, are corrected. 
     In general, various embodiments of projection objectives with adjusting means for the correction of variable imaging errors are known. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention has as its object to improve an illumination system such that optimum uniformity, i.e. regularity of the illumination over the whole surface of the wafer, is attained. This is to hold for the most diverse disturbing influences which can also arise with variable aperture of the illumination as regards numerical aperture and shape of aperture such a circle, ring, or quadrupole. 
     This object is attained by means of an illumination system in which a secondary light source is imaged on a reticle and distortion of the image of the secondary light source is adjusted by adjusting at least one variable optical path between optical elements or by a REMA objective. This object is also achieved by a REMA objective comprising optical elements, in which the optical path between two optical elements is adjustable. The object of the invention is also achieved by an operating process comprising changing at least one optical path between two optical elements and the REMA objective in dependence on the variable aperture of the illumination system. 
     For the first time, the distortion of the illumination is thus adjustably provided, and hence control and regulation of the uniformity of the illumination is derived. 
     Likewise for the first time, adjustment is provided in a REMA objective. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in more detail with reference to preferred embodiments and to the accompanying drawings, in which: 
     FIG. 1 shows a projection illumination equipment with displaceable lens groups in the REMA objective; 
     FIG. 2 shows the lens section of a REMA objective with displaceable lens groups. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic overview of the optical portion of a whole projection illumination equipment (wafer scanner), in which the REMA objective  123  according to the invention with three lens groups  100 ,  200 ,  300  including the displaceable lens group  301  is integrated. 
     An excimer laser  50  with a wavelength of 248 nm serves as the light source. A device  60  serves for beam formation and coherence reduction. A zoom axicon objective  70  facilitates the setting of various kinds of illumination according to requirements. For this purpose, an adjustable zoom  71  and an adjustable axicon pair  72  are provided. The whole arrangement is known as in German patent application DE 196 53 983 of Carl Zeiss. 
     The light is coupled into a glass rod  80 , which serves for mixing and homogenizing. 
     A reticle masking system  90  directly adjoins the glass rod  30 , and lies in the object plane of the REMA objective  123 . This objective consists of a first lens group  100 , a pupil plane (diaphragm plane)  14 , a second lens group  200 , a deflecting mirror  240 , a third lens group  300 , and an image plane  19 . Suitable REMA objectives are known from German Patents DE-A 195 48 805 and DE 196 53 983. 
     The reticle  330  is arranged in the image plane  19  of the REMA objective  123 , and is precisely positioned by the change and adjusting unit  331 . 
     There follows a projection objective  400 , which in this embodiment is the catadioptric objective according to WO 95/32446 with a pupil plane  410 . A wafer  500 , with associated adjusting and movement unit  501 , is arranged in the image plane of the projection objective  400 . 
     The following is based on the arrangement being constructed as a scanner, i.e., the reticle  33  and wafer  500  are synchronously moved linearly in the speed ratio of the imaging scale factor of the projection objective  400 , and that the illumination system 50-123 produces a narrow slit which is oriented transversely of the direction of motion. 
     The lens group  301 , adjustable according to the invention, is provided in the third lens group  300  of the REMA objective  123 , and affects the optical path length by changing an air space. A control unit  600  is connected to the adjustable members  71 ,  72 ,  301 , and controls these in mutual dependence. 
     The REMA objective shown in the lens section of FIG. 2 is a modification of the objective according to FIG. 1 of German Patent DE 196 53 983. The measurements according to Table 1 are taken from Table 1 of the cited document for the surfaces  1  through  19 . 
     However, the air space between surfaces  16  and  17  is axially adjustable with an actuator  161 , so that it can be increased (or else reduced) by several millimeters. 
     The lenses  17 ,  18  and the gray filter  21  are collectively displaced. The distortion of the REMA objective is thereby increased (reduced), and the intensity distribution in the image plane  19  (at the reticle  33 ) is increased (reduced) toward the edge. The same effect occurs at the wafer. The conjugate points of the objective  123  remain unchanged. 
     In the embodiment shown, and in many similar embodiments, there thereby results a worsening of the edge steepness when the REMA system  90  is imaged on the reticle  33 . However, this can be compensated, in that either the objective distance is slightly increased by the displacement of the REMA system  9  with an actuator  91 , or a further air space of the REMA objective  123  is slightly changed. In the embodiment, this is the air space between the surfaces  3  and  4 , and is axially changed with the actuator  31 . 
     It is provided in the example that the air space 16/17 is increased by 2.2. mm, which effects an increase of intensity at the edge of the image field of 0.5%. Associated with this is a broadening of the edge by 0.1 mm and a telecentric impairment by 0.1 mrad. 
     By shortening the object width ½ by 0.02 mm, the broadening is reduced to only 0.01 mm, with the same telecentric quality. Alternatively or supplementary thereto, the air space ¾ can also be made smaller. 
     Table 1 gives the design data of this REMA objective for the basic position of the displaceable elements. 
     The amount of light transported remains constant while the air spaces are varied. If it is considered as an alternative solution that a gray filter is used with a darkening that increases toward the middle, in each case a portion of the light is lost by absorption. 
     The proposed solution is applied when the intensity distribution in the reticle plane  19  or  33  is to be varied within narrow limits (delta I=±0.5% to 2.0%) without losses, continuously and controllably from the exterior. The uniformity at the wafer is then also correspondingly controllable. 
     The uniformity correction can then be coupled to the control  600 , which e.g. also controls the zoom axicon function. 
     If the variation path of the air space is further increased, then the worsening of the telecentric effect is no longer tolerable in comparison with the increase in uniformity. 
     When the deviations of uniformity vary from small apertures to large ring apertures, it is appropriate to combine the displacement of the lens group  301 , as a means of correction, with a gray value graduated filter ( 21  in FIG.  2 ), which has a radial graduation of transmission corresponding to the average value. For this purpose, the distribution of illumination over the wafer  500  can be evaluated taking account of the whole projection equipment in the specific example. 
     The displacement according to the invention then still corrects by the average value only the variable components corresponding to the illumination aperture. 
     
       
         
               
             
               
               
               
               
               
             
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Scale: 4.444:1  Wavelength: 248.33 nm 
               
             
          
           
               
                   
                   
                 Radius 
                 Thickness 
                 Material 
               
               
                   
                   
               
               
                   
                  1 
                   
                 55.240 
               
               
                   
                  2 
                 −38.258 
                 46.424 
                 Quartz 
               
               
                   
                  3 
                 −66.551 
                 0.633 
               
               
                   
                  4 
                 881.696 
                 45.341 
                 Quartz 
               
               
                   
                  5 
                 −190.791 
                 0.924 
               
               
                   
                  6 
                 374.111 
                 47.958 
                 Quartz 
               
               
                   
                  7 
                 −287.518 
                 222.221 
               
               
                   
                  8 
                 diaphragm 
                 17.900 
               
               
                   
                  9 
                 ∞ 
                 79.903 
               
               
                   
                 10 
                 164.908 
                 52.530 
                 Quartz 
               
               
                   
                 11 
                 −1246.141 
                 27.586 
               
               
                   
                 12 
                 280.226 
                 19.580 
                 Quartz 
               
               
                   
                 13 
                 114.495 
                 133.941 
               
               
                   
                 14 
                 ∞ 
                 365.253 
               
               
                   
                 15 
                 −216.480 
                 12.551 
                 Quartz 
               
               
                   
                 16 
                 −113.446 
                 1.399 
               
               
                   
                 17 
                 −329.0.56 
                 10.797 
                 Quartz 
               
               
                   
                 18 
                 −552.687 
                 60.000 
               
               
                   
                 19 
                 ∞ 
                 0.000 
               
               
                   
                   
               
             
          
           
               
                 Surface 
                 Aspheric Constants 
               
               
                   
               
             
          
           
               
                  7 
                 K = −0.00640071 
                 C1 = 0.347156E−07 
                 C2 = 
               
               
                   
                   
                   
                 0.802432- 
               
               
                   
                   
                   
                 E−13 
               
               
                   
                 C3 = −0.769512E−17 
                 C4 = 
               
               
                   
                   
                 0.157667E−21 
               
               
                 11 
                 K = +0.00104108 
                 C1 = 0.431697E−07 
                 C2 = 
               
               
                   
                   
                   
                 −0.564977- 
               
               
                   
                   
                   
                 E−13 
               
               
                   
                 C3 = −0.125201E−16 
                 C4 = 
               
               
                   
                   
                 0.486357E−21 
               
               
                 17 
                 K = +0/00121471 
                 C1 = −0.991033E−07 
                 C2 = 
               
               
                   
                   
                   
                 −0.130790- 
               
               
                   
                   
                   
                 E−11 
               
               
                   
                 C3 = −0.414621E−14 
                 C4 = 0.200482E−17 
                 C5 = 
               
               
                   
                   
                   
                 −0.392671- 
               
               
                   
                   
                   
                 E−21