Abstract:
A catadioptric optical reduction system for use in the photolithographic manufacture of semiconductors having a concave mirror operating near unit magnification, or close to a concentric condition. A lens group before the mirror provides only enough power to image the entrance pupil at infinity to the aperture stop at or near the concave mirror. A lens group after the mirror provides a larger proportion of reduction from object to image size, as well as projecting the aperture stop to an infinite exit pupil. An aspheric concave mirror is used to further reduce high order aberrations. The catadioptric optical reduction system provides a relatively high numerical aperture of 0.7 capable of patterning features smaller than 0.35 microns over a 26×5 millimeter field. The optical reduction system is thereby well adapted to a step and scan microlithographic exposure tool as used in semiconductor manufacturing. Several other embodiments combine glasses of different refracting power to widen the spectral bandwidth which can be achieved.

Description:
This application is a continuation-in-part, of application Ser. No. 08/009,284, filed Jan. 26, 1993. 
    
    
     BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     This invention relates generally to optical systems used in semiconductor manufacturing, and more particularly to an unobscured catadioptric optical reduction system having a relatively high numerical aperture. 
     DESCRIPTION OF THE RELATED ART 
     Semiconductors are usually manufactured using various photolithographic techniques. The circuitry used in a semiconductor is reproduced from a reticle onto a semiconductor chip. This reproduction is often accomplished with the use of optical systems. The design of these optical systems is often complex, and it is difficult to obtain the desired resolution necessary for reproducing the ever-decreasing size of components being placed on a semiconductor chip. Therefore, there has been much effort expended to develop an optical reduction system capable of reproducing very fine component features, less than 0.35 microns. In addition to the need to develop an optical system capable of reproducing very fine component features, there is also the need to augment system performance by increasing numerical aperture. Increasing numerical aperture provides for a higher resolution optical system. 
     An optical system similar to that of the present invention is disclosed in U.S. Pat. No. 4,953,960 entitled &#34;Optical Reduction System&#34; issuing Sep. 4, 1990 to Williamson, which is herein incorporated by reference. Therein disclosed is an optical reduction system operating in the range of 248 nanometers and having a numerical aperture of 0.45. Another similar optical system is disclosed in U.S. Pat. No. 5,089,913 entitled &#34;High Resolution Reduction Catadioptric Relay Lens&#34; issuing Feb. 18, 1992 to Singh et al, which is herein incorporated by reference. Therein disclosed is an optical system having a restricted spectral waveband at 248 nanometers, and having a numerical aperture of 0.6. 
     While these prior optical systems perform adequately for their intended purpose, there is an ever increasing need to improve system performance by increasing numerical aperture. Therefore, there is a need for an optical system having a relatively high numerical aperture capable of acceptable system performance over a relatively large spectral waveband. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a catadioptric reduction system having, from the object or long conjugate end to the reduced image or short conjugate end, a first lens group, a second lens group, a beamsplitter cube, a substantially or nearly concentric concave mirror, and a third lens group. The concave mirror operates substantially near unit magnification. This reduces the aberrations introduced by the mirror and the diameter of radiation entering the beamsplitter cube. The first and second lens groups before the concave mirror provide only enough power to image the entrance pupil at infinity at the aperture stop at or near the concave mirror. The third lens group after the concave mirror provides a substantial portion of the reduction from object to image of the optical system, as well as projecting the aperture stop to an infinite exit pupil. High-order aberrations are reduced by using an aspheric concave mirror. 
     Accordingly, it is an object of the present invention to provide an optical system having a relatively high numerical aperture. 
     It is another object of the present invention to provide an optical system having a spectral bandwidth substantially wider than could have previously been obtained at such a high numerical aperture. 
     It is an advantage of the present invention that the concave mirror operates substantially closer to unit magnification. 
     It is a feature of the present invention that the lens groups before the concave mirror provide only enough power to image the entrance pupil at infinity at the aperture stop at or near the concave mirror. 
     It is yet another feature of the present invention that the lens group after the concave mirror provides most of the reduction from object to image of the system. 
     These and other objects, advantages, and features will become readily apparent in view of the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of one embodiment of the present invention using a single refracting material. 
     FIG. 2 is another embodiment of the present invention using two different refracting materials. 
     FIG. 3 is yet another embodiment of the present invention using more than two different refracting materials. 
     FIG. 4 is another embodiment of the present invention. 
     FIG. 5 is yet a further embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates one embodiment of the optical reduction system of the present invention. From its long conjugant end, it comprises an object or reticle plane 10, a first lens group LG1, a folding mirror 20, a second lens group LG2, a beamsplitter cube 30, a first quarter-waveplate 32, a concave mirror 34, a second quarter-waveplate 38, and a third lens group LG3. The image is formed at image or wafer plane 50. The first lens group LG1 comprises a shell 12, a spaced doublet including positive lens 14 and negative lens 16, and positive lens 18. The shell 12 is an almost zero power lens. The second lens group LG2 comprises a positive lens 22, a spaced doublet including a negative lens 24 and a positive lens 26, and negative lens 28. The third lens group LG3 comprises two positive lenses 40 and 42, which are strongly positive, shell 44, and two positive lenses 46 and 48, which are weakly positive. The folding mirror 20 is not essential to the operation of the present invention. However, the folding mirror permits the object and image planes to be parallel which is convenient for the intended application of the optical system of the present invention, which is the manufacture of semiconductor devices using photolithography with a step and scan system. 
     Radiation enters the system at the reticle or long conjugate end and passes through the first lens group LG1, is reflected by the folding mirror 20, and passes through the second lens group LG2. The radiation enters the beamsplitter cube 30 and is reflected from surface 36 passing through quarter-waveplate 32 and reflected by concave mirror 34. The radiation then passes back through the quarter-waveplate 32, the beamsplitter cube 30, the quarter-waveplate 38, lens group LG3, and is focused at the image or wafer plane 50. 
     The lens groups before the mirror, LG1 and LG2, provide only enough power to image the entrance pupil at infinity to the aperture stop 31 at or near the concave mirror 34. The combined power of lens groups LG1 and LG2 is slightly negative. The shell 12 and air spaced doublet 14 and 16 assist in aberration corrections including astigmatism, field curvature, and distortion. The lens group LG3, after the concave mirror 34, provides most of the reduction from object to image size, as well as projecting the aperture stop to an infinite exit pupil. The two strongly positive lenses 40 and 42 provide a high numerical aperture at the image and exit pupils and infinity. The shell 44 has almost no power. The two weakly positive lenses 46 and 48 help correct high order aberrations. The concave mirror 34 may provide a reduction ratio of between 1.6 and 2.7 times that of the total system. 
     The negative lens 24 in the second lens group LG2 provides a strongly diverging beam directed at the beamsplitter cube 30 and concave mirror 34. The strongly positive lens 22 provides lateral color correction. The air space doublet comprising lenses 24 and 26 helps to correct spherical aberrations and coma. Concave mirror 34 is preferably aspheric, therefore helping further reduce high order aberrations. 
     The transmission losses introduced by the beamsplitter cube 30 are minimized by illuminating the object or reticle with plane polarized light and including a true quarter-waveplate 32 between the beamsplitter cube 30 and the concave mirror 34. By true quarter-waveplate is meant a thickness of birefringent material which introduces a quarter of a wave phase retardance between the S and P polarization states. This is in contrast to an integral number of half plus quarter waves or two thicknesses of material whose phase retardance differs by a quarter wave. The deleterious effects of large angle of incidence variations are thereby minimized at the high numerical aperture by the use of such true zero order waveplates, and by restricting the field size in the plane of incidence. Additionally, by increasing the numerical aperture in lens group LG3, after the concave mirror 34 and beamsplitter cube 30, the greatest angular range is not seen in these elements. 
     However, the use of plane polarized light at numerical apertures greater than about 0.5 introduces small but noticeable asymmetries in the imaging. In the present invention, this can effectively be removed by introducing a second quarter-waveplate 38 after the final passage through the beamsplitter cube 30, thereby converting the plane polarized light into circularly polarized light. This circularly polarized light is indistinguishable from unpolarized light in its imaging behavior. 
     The optical system illustrated in FIG. 1 is designed to operate at a reduction ratio of 4 to 1. Therefore, the numerical aperture in the image space is reduced from 0.7 by a factor of 4 to 0.175 at the object or reticle. In other words, the object space numerical aperture is 0.175 and the image space numerical aperture is 0.7. Upon leaving the first lens group LG1 the numerical aperture is reduced to 0.12, a consequence of the positive power needed in lens group LG1 to image the entrance pupil at infinity to the aperture stop of the system close to the concave mirror 34. The numerical aperture after leaving the second lens group LG2 and entering the beamsplitter is 0.19. Therefore, the emerging numerical aperture from the second lens group LG2, which is 0.19, is larger than the entering or object space numerical aperture of lens group LG1, which is 0.175. In other words, the second lens group LG2 has an emerging numerical aperture greater than the entering numerical aperture of the first lens group LG1. This is very similar to the object space numerical aperture, which is 0.175, due to the overall negative power of the second lens group LG2. This is contrary to prior art systems where the numerical aperture entering a beamsplitter cube is typically close to zero or almost collimated. The concave mirror 34 being almost concentric, the numerical aperture of the radiation reflected from it is increased only slightly from 0.19 to 0.35. The third lens group LG3 effectively doubles the numerical aperture to its final value of 0.7 at the wafer or image plane 50. 
     The present invention achieves its relatively high numerical aperture without obstruction by the edges of the beamsplitter cube by means of the negative second group LG2 and the strongly positive third lens group LG3. The use of the beamsplitter cube 30 rather than a plate beamsplitter is important in the present invention because at numerical apertures greater than about 0.45 a beamsplitter cube will provide better performance. There is a reduction of the numerical aperture within the cube by the refractive index of the glass, as well as the absence of aberrations that would be introduced by a tilted plate beamsplitter in the non-collimated beam entering the beamsplitter. The construction data for the lens system illustrated in FIG. 1 according to the present invention is given in Table 1 below. 
     
                                           TABLE 1__________________________________________________________________________Radius of         Aperture DiameterElementCurvature (mm)            Thickness                  (mm)NumberFront Back  (mm)  Front Back  Glass__________________________________________________________________________10   Infinite    63.385312   -158.7745      -177.8880            15.0000                  124.0478                        131.7725                              SilicaSpace            36.113014   -556.6911      -202.0072            22.2126                  148.3881                        152.5669                              SilicaSpace            38.718816   -183.7199      -558.8803            15.0000                  156.5546                        166.5750                              SilicaSpace            10.067418   427.2527      -612.2450            28.8010                  177.4010                        179.0292                              SilicaSpace            132.332020   Infinite    -74.0000                  184.6402    Reflection22   -240.4810      2050.9592            -33.3135                  188.4055                        185.3395                              SilicaSpace            -29.343424   421.7829      -145.6176            -12.0000                  175.5823                        169.0234                              SilicaSpace            -4.232626   -150.4759      472.0653            -46.5091                  171.4244                        169.9587                              SilicaSpace            -2.000028   -1472.2790      -138.2223            -15.0000                  165.3586                        154.8084                              SilicaSpace            -27.206030   Infinite      Infinite            -91.8186                  155.6662                        253.0917                              Silica36   Infinite           253.0917   Reflection30   Infinite      Infinite            91.8186                  253.0917                        253.0917                              SilicaSpace            2.000032   Infinite      Infinite            6.0000                  185.8693                        186.8401                              SilicaSpace            17.9918Stop                   188.065534   Aspheric    -17.9918                  188.0655    Reflection32   Infinite      Infinite            -6.0000                  183.5471                        180.1419                              SilicaSpace            -2.000030   Infinite      Infinite            -91.8186                  178.3346                        149.2832                              Silica30   Infinite      Infinite            -70.000                  149.2832                        128.8604                              SilicaSpace            -2.000038   Infinite      Infinite            -4.5000                  127.9681                        126.6552                              SilicaSpace            -0.750040   -175.1330      1737.4442            -17.7754                  121.4715                        118.2689                              SilicaSpace            -0.750042   -108.8178      -580.1370            -18.2407                  104.5228                        97.7967                              SilicaSpace            -0.750044   -202.2637      -86.6025            -31.1216                  91.7061                        57.4968                              SilicaSpace            -2.350746   -122.1235      -488.7122            -17.9476                  56.4818                        41.1675                              SilicaSpace            -0.200048   -160.8506      -360.1907            -6.1500                  39.4528                        33.5764                              SilicaSpace            -4.00050   Infinite           26.5019__________________________________________________________________________ 
    
     Concave mirror 34 has an aspheric reflective surface according to the following equation. ##EQU1## wherein the constants are as follows, CURV=-0.00289051 
     K=0.000000 
     A=6.08975×10 -11   
     B=2.64378×10 14   
     C=9.82237×10 -19   
     D=7.98056×10 -23   
     E=-5.96805×10 -27   
     F=4.85179×10 -31   
     The lens according to the construction in Table 1 is optimized for radiation centered on 248.4 nanometers. The single refracting material of fused silica and the large portion of refracting power restricts the spectral bandwidth of the embodiment illustrated in FIG. 1 to about 10 picometers or 0.01  nanometers. This spectral bandwidth is more than adequate for a line narrowed krypton fluoride excimer laser light source. The embodiment illustrated in FIG. 1 can be optimized for any wavelength for which fused silica transmits adequately. 
     A wider spectral bandwidth can be achieved by the use of two optical materials with different dispersions. A second embodiment of the present invention is illustrated in FIG. 2. From its long conjugant end, it comprises an object or reticle plane 10, a lens group LG4, a folding mirror 122, a lens group LG5, a beamsplitter cube 132 having surface 138, a first quarter-waveplate 134, a concave mirror 136, a second quarter-waveplate 140, and lens group LG6. The image is formed at image or wafer plane 50. The lens group LG4 comprises a spaced doublet including negative lens 112 and positive lens 114, a weak positive lens 116, positive lens 118, and shell 120. The lens group LG5 comprises a positive lens 124, a negative lens 126, a positive lens 128, and a negative lens 130. The lens group LG6 comprises two positive lenses 142, `cemented` doublet including positive lens 144 and negative lens 146, positive lens 148, and `cemented` doublet including shell 150 and positive lens 152. 
     This second embodiment uses calcium fluoride in one of the individual positive lenses of the lens group LG4, negative lenses of the lens group LG5, and two of the positive lenses of the lens group LG6. The construction data of the second embodiment illustrated in FIG. 2 of the present invention is given in Table 2 below. 
     
                                           TABLE 2__________________________________________________________________________Radius of         Aperture DiameterElementCurvature (mm)            Thickness                  (mm)NumberFront Back  (mm)  Front Back  Glass__________________________________________________________________________10   Infinite    60.4852112  -205.5158      539.1791            15.2158                  124.0926                        137.3346                              SilicaSpace            8.8054114  2080.9700      -210.6539            32.4984                  142.6149                        151.7878                              SilicaSpace            1.2676116  310.4463      700.3748            40.7304                  162.4908                        165.2126 CaFlSpace            0.5000118  634.1820      -798.8523            27.5892                  165.4595                        166.4747                              SilicaSpace            0.5000120  1480.0597      1312.1247            25.4322                  168.7516                        164.7651                              SilicaSpace            136.2343122  Infinite    -74.0000                  161.9590    Reflection124  -761.9176      1088.9351            -19.2150                  160.3165                        159.2384                              SilicaSpace            -19.9465126  648.8361      -202.5872            -12.0000                  155.1711                        153.0635                              CaFlSpace            -7.6304128  -400.4276      458.5060            -25.8769                  153.0635                        153.8055                              SilicaSpace            -2.0000130  -818.0922      -168.5034            -27.5927                  152.6663                        147.5200                              CaFlSpace            -20.5014132  Infinite      Infinite            -91.7553                  148.6158                        252.7349                              Silica138  Infinite          252.7349    Reflection132  Infinite      Infinite            91.7553                  252.7349                        252.7349                              SilicaSpace            2.0000134  Infinite      Infinite            6.0000                  185.8070                        187.0026                              SilicaSpace            18.1636Stop                   188.5681136  Aspheric    -18.1636                  188.5681    Reflection134  Infinite      Infinite            -6.0000                  184.2566                        181.1084                              SilicaSpace            -2.0000132  Infinite      Infinite            -91.7553                  179.3838                        151.7747                              Silica132  Infinite      Infinite            -70.0000                  151.7747                        133.3985                              SilicaSpace            -2.0000140  Infinite      Infinite            -4.5000                  132.5690                        131.3876                              SilicaSpace            -0.5000142  -112.0665      -597.6805            -21.4866                  123.4895                        119.2442                              SilicaSpace            -0.5000144  -116.3137      282.3140            -24.0940                  107.8451                        101.2412                              CaFl146  282.3140      -66.5293            -13.7306                  101.2412                        72.6862                              SilicaSpace            -2.6346148  -77.2627      -374.4800            -17.9594                  72.0749                        62.7659                              SilicaSpace            -0.5452150  -130.1381      -57.1295            -20.8147                  58.9696                        37.4889                              Silica152  -57.1295      -7305.8777            -6.1425                  37.4889                        34.3156                              CaFlSpace            -4.0000ImageInfinite           26.4992__________________________________________________________________________ 
    
     wherein the constants for the aspheric mirror 134 used in the equation after Table 1 are as follows, 
     CURV=-0.00286744 
     K=0.000000 
     A=-1.92013×10 -09   
     B=-3.50840×10 -14   
     C=2.95934×10 -19   
     D=-1.10495×10 -22   
     E=9.03439×10 -27   
     F=-1.39494×10 -31   
     This second embodiment is optimized for radiation centered on 193.3 nanometers and has a spectral bandwidth of about 200 picometers or 0.2 nanometers. A slightly line narrowed argon fluoride excimer laser is an adequate light source. Additionally, the design can be optimized for any wavelength for which both refractive materials transmit adequately. The bandwidth will generally increase for longer wavelengths, as the material dispersions decrease. For example, around 248.4 nanometers such a two-material design will operate over at least a 400 picometers, 0.4 nanometers bandwidth. 
     At wavelengths longer than 360 nanometers, a wider range of optical glasses begin to have adequate transmission. A third embodiment illustrated in FIG. 3 takes advantage of this wider selection of glasses and further reduced dispersion. From its long conjugant end, it comprises an object or reticle plane 10, a lens group LG7, a folding mirror 222, a lens group LG8, a beamsplitter cube 232 having a surface 238, a first quarter-waveplate 234, a concave mirror 236, a second quarter-waveplate 240, and lens group LG9. The image is formed at image or wafer plane 50. The lens group LG7 comprises a spaced doublet comprising negative lens 212 and positive lens 214, spaced doublet including positive lens 216 and negative lens 218, and positive lens 220. The lens group LG8 comprises a positive lens 224, a negative lens 226, a positive lens 228, and a negative lens 230. The lens group LG9 comprises a positive lenses 242, cemented doublet including positive lens 244 and negative lens 246, positive lens 248, and cemented doublet including shell 250 and positive lens 252. 
     The construction data of the third embodiment illustrated in FIG. 3 is given in Table 3 below. 
     
                                           TABLE 3__________________________________________________________________________Radius of         Aperture DiameterElementCurvature (mm)            Thickness                  (mm)NumberFront Back  (mm)  Front Back  Glass__________________________________________________________________________10   Infinite    59.2960212  -620.7809      361.8305            20.2974                  125.9406                        134.7227                              PBM2YSpace            2.6174214  515.7935      -455.1015            39.8858                  135.3384                        145.6015                              PBM2YSpace            14.7197216  431.3189      -239.4002            36.9329                  155.6269                        157.3014                              BSL7YSpace            0.5000218  -259.6013      685.3286            26.3534                  156.9363                        162.2451                              PBM2YSpace            1.4303220  361.5709      -1853.2955            23.3934                  168.7516                        165.1801                              BAL15YSpace            131.8538222  Infinite    -77.8469                  169.9390    Reflection224  -429.2950      455.4247            -32.3086                  173.0235                        171.1102                              PBL6YSpace            -27.6206226  401.0363      -180.0031            -12.0000                  159.3555                        154.7155                              BSL7YSpace            -5.6227228  -258.4722      1301.3764            -26.1321                  154.7155                        154.1517                              PBM8YSpace            -2.0000230  -1282.8931      -180.2226            -12.0000                  153.1461                        149.4794                              BSL7YSpace            -19.7282232  Infinite      Infinite            -91.7349                  150.4585                        252.6772                              Silica238   Infinite         252.6772    Reflection232  Infinite      Infinite            91.7349                  252.6772                        252.6772                              SpaceSpace            2.0000234  Infinite      Infinite            6.0000                  185.6435                        186.7758                              SilicaSpace            18.2715Stop                   188.1745236  Aspheric    -18.2715                  188.1745    Reflection234  Infinite      Infinite            -6.0000                  183.6393                        180.1377                              SilicaSpace            -2.0000232  Infinite      Infinite            -91.7349                  178.3236                        147.9888                              Silica232  Infinite      Infinite            -70.0000                  147.9888                        126.9282                              SilicaSpace            -2.0000240  Infinite      Infinite            -4.5000                  126.0289                        124.6750                              SilicaSpace            -0.5000242  -119.8912      -610.6840            -18.6508                  117.5305                        113.4233                              BSM51YSpace            -0.5000244  -114.1327      384.9135            -21.1139                  102.6172                        96.4137                              BSL7Y246  384.9135      -70.2077            -13.0576                  96.4137                        71.1691                              PBL26YSpace            -2.8552248  -85.7858      -400.3240            -16.9147                  70.5182                        61.2633                              BSM51YSpace            -0.8180250  -151.5235      -54.0114            -19.5810                  57.6234                        37.3909                              BSM51Y252  -54.0114      -2011.1057            -6.3947                  37.3909                        34.2119                              PBL6YSpace            -4.0000ImageInfinite           26.5002__________________________________________________________________________ 
    
     wherein the constants for the aspheric mirror 234 used in the equation after Table 1 as follows, 
     CURV=-0.00291648 
     K=0.000000 
     A=-1.27285×10 -9   
     B=-1.92865×10 -14   
     C=6.21813×10 -19   
     D=-6.80975×10 23   
     E=6.04233×10 -27   
     F=3.64479×10 -32   
     This third embodiment operates over a spectral bandwidth of 8 nanometers centered on 365.5 nanometers. A radiation of this spectral bandwidth can be provided by a filtered mercury arc lamp at the I-line waveband. The optical glasses other than fused silica used in this third embodiment are commonly known as I-line glasses. These optical glasses have the least absorption or solarization effects at the mercury I-line wavelength. These glasses may be found in a commonly available glass catalog provided by O&#39;Hara Corporation, 50 Columbia Road, Branchburg Township, Somerville, N.J. 08876-3519, USA. 
     FIG. 4 illustrates a fourth embodiment of the optical reduction system of the present invention. This embodiment has a numerical aperture of 0.63 and can operate at a spectral bandwidth of 300 picometers, and preferably of 100 picometers, centered on 248.4 nanometers. From the long conjugate end, it includes an object or reticle plane 410, a first lens group LG1, a folding mirror 420, a second lens group LG2, a beamsplitter cube 430, a first quarter-waveplate 432, a concave mirror 434, a second quarter-waveplate 438, and a third lens group LG3. The image is formed at the image or wafer plane 450. 
     The first lens group LG1 comprises a shell 412, a spaced doublet including a positive lens 414 and a negative lens 416, and a positive lens 418. The second lens group LG2 comprises a positive lens 422, a spaced doublet including a negative lens 424 and a positive lens 426, and a negative lens 428. The third lens group LG3 comprises two positive lenses 440 and 442, a shell 444, and two positive lenses 446 and 448. Again, as in the embodiment illustrated in FIG. 1, the folding mirror 420 of FIG. 4 is not essential to the operation of the invention, but nevertheless permits the object 410 and image plane 450 to be parallel to each other which is convenient for the manufacture of semiconductor devices using photolithography. 
     The construction data of the fourth embodiment illustrated in FIG. 4 is given in Table 4 below. 
     
                                           TABLE 4__________________________________________________________________________Radius of             Aperture DiameterElementCurvature (mm)  Thickness                      (mm)NumberFront   Back    (mm)  Front Back  Glass__________________________________________________________________________410  Infinite        63.3853412  -183.5661 CC        -215.7867 CX                17.0000                      122.8436                            130.6579                                  SilicaSpace                46.6205414  -601.1535 CC        -230.9702 CX                21.4839                      149.1476                            153.3103                                  SilicaSpace                68.8075416  -195.1255 CC        -345.4510 CX                15.0000                      161.6789                            170.1025                                  SilicaSpace                3.0000418  435.8058 CX        -1045.1785 CX                24.9351                      177.4520                            178.2672                                  SilicaSpace                130.0000        Decenter (1)420  Infinite        -64.5000                      180.3457    Reflection422  -210.7910 CX        380.1625 CX                -43.1418                      181.6672                            178.0170                                  SilicaSpace                -15.8065424  300.1724 CC        -123.4555 CC                -12.0000                      166.7278                            153.3103                                  SilicaSpace                -3.8871426  -126.8951 CX        972.6391 CX                -41.3263                      154.8530                            151.8327                                  SilicaSpace                -1.5000428  -626.4905 CX        -116.6456 CC                -12.0000                      147.6711                            136.1163                                  SilicaSpace                -31.8384430   Infinite        Infinite                -74.0000                      137.2448                            200.1127                                  Silica        Decenter (2)436  Infinite              200.1127    Reflection430  Infinite        Infinite                74.0000                      200.1127                            200.1127                                  SilicaSpace                2.0000432  Infinite        Infinite                6.0000                      148.6188                            149.0707                                  SilicaSpace                14.4638Stop                       149.6392434  Aspheric        -14.4638                      149.6392    Reflection432  Infinite        Infinite                -6.0000                      144.8563                            141.2737                                  SilicaSpace                -2.0000430  Infinite        Infinite                -74.0000                      139.3606                            117.3979                                  Silica        Decenter (3)430  Infinite        Infinite                -61.0000                      117.3979                            100.5074                                  SilicaSpace                -2.0000438  Infinite        Infinite                -4.5000                      99.6617                            98.4157                                  SilicaSpace                -1.2000440  -157.8776 CX        2282.2178 CX                -13.7501                      94.8267                            91.8775                                  SilicaSpace                -1.2000442  -94.0059 CX        -466.6659 CC                -13.4850                      82.8663                            78.1418                                  SilicaSpace                -1.2000444  -147.2485 CX        -77.8924 CC                -22.2075                      72.7262                            50.6555                                  SilicaSpace                -3.2091446  -159.2880 CX        -519.4850 CC                -13.8321                      49.5648                            39.0473                                  SilicaSpace                -0.2000448  -129.3683 CX        -426.7350 CC                -6.1500                      37.3816                            32.4880                                  SilicaSpaceImage Distance =                -4.0000450  Image   Infinite__________________________________________________________________________ 
    
     The constants for the aspheric mirror 434 used in equation (1) located after Table 1 are as follows: 
     CURV=-0.00332614 
     K=0.000000 
     A=-4.32261E-10 
     B=3.50228E-14 
     C=7.13264E-19 
     D=2.73587E-22 
     This fourth embodiment is optimized for radiation centered on 248.4 nm. The single refracting material of fused silica and the large portion of refracting power restricts the spectral bandwidth of the embodiment depicted in FIG. 4. However, because the fourth embodiment has a maximum numerical aperture of 0.63 rather than of 0.7 as in the first three embodiments, the fourth embodiment provides acceptable imaging over a spectral full-width-half-maximum bandwidth of 300 picometers, or preferably of 100 picometers. Thus, in the former, an unnarrowed, or, in the latter, a narrowed excimer laser can be employed for the illumination source. 
     The fourth embodiment differs from the first three embodiments in that the net power of LG1 and LG2 of the fourth embodiment is weakly positive rather than weakly negative as in the first three embodiments. In addition, this illustrates that the overall focal power of LG1 plus LG2 can be either positive or negative and still permit an infinitely distant entrance pupil to be imaged at or near the concave mirror 434. 
     FIG. 5 illustrates a fifth embodiment of the optical reduction system of the present invention. Preferably, this embodiment has a numerical aperture of 0.60 and operates at a spectral bandwidth of 300 picometers centered on 248.4 nanometers. From the long conjugate end, it includes an object or reticle plane 510, a first lens group LG1, a folding mirror 520, a second lens group LG2, a beamsplitter cube 530, a first quarter-waveplate 532, a concave mirror 534, a second quarter-waveplate 538, and a third lens group LG3. The image is formed at the image or wafer plane. 
     The first lens group LG1 comprises a shell 512, a spaced doublet including a positive lens 514 and a negative lens 516, and a positive lens 518. The second lens group LG2 comprises a positive lens 522, a spaced doublet including a negative lens 524 and a positive lens 526, and a negative lens 528. The third lens group LG3 comprises two positive lenses 540 and 542, a shell 544, and two positive lenses 546 and 548. Again, as in the embodiment illustrated in FIG. 1, the folding mirror 520 of FIG. 5 is not essential to the operation of the invention, but nevertheless permits the object and image planes to be parallel to each other which is convenient for the manufacture of semiconductor devices using photolithography. 
     The construction data of the fifth embodiment illustrated in FIG. 5 is given in Table 5 below. 
     
                                           TABLE 5__________________________________________________________________________ElementRadius of Curvature (mm)                 Thickness                       Aperture Diameter (mm)NumberFront   Back     (mm)  Front Back   Glass__________________________________________________________________________510  Infinite         62.7514512  -136.1154 CC        -152.5295 CX                 16.8300                       120.7552                             129.4354                                    SilicaSpace                 4.5206514  -270.1396 CC        -191.8742 CX                 20.5341                       132.9152                             139.0377                                    SilicaSpace                 90.8476516  -188.9000 CC        -284.7476 CX                 17.5000                       156.1938                             165.6567                                    SilicaSpace                 2.9700518   433.8174 CX        -841.5599 CX                 25.8293                       173.8279                             174.8334                                    SilicaSpace                 149.4549        Decenter(1)520  Infinite         -61.0000                       177.2183     Reflection522  -190.3251 CX        -8413.4836 CC                 -34.4584                       178.5071                             174.2260                                    SilicaSpace                 -51.5487524   690.5706 CC        -146.4997 CC                 -11.8800                       150.4109                             141.8021                                    SilicaSpace                 -10.6267526  -265.9886 CX         1773.5314 CX                 -24.1851                       142.1592                             141.2400                                    SilicaSpace                 -1.5000528  -244.9899 CX        -142.8558 CC                 -11.8800                       139.3290                             133.8967                                    SilicaSpace                 -21.6411530  Infinite        Infinite -71.2800                       134.3115                             189.7826                                    Silica        Decenter(2)536  Infinite               189.7826     Reflection530  Infinite        Infinite 71.2800                       189.7826                             189.7826                                    SilicaSpace                 1.9800532  Infinite        Infinite 5.9400                       142.3429                             142.6707                                    SilicaSpace                 18.5263Stop                        143.5034534  Aspheric         -18.5263                       143.5034     Reflection532  Infinite        Infinite -5.9400                       134.2788                             130.9398                                    SilicaSpace                 -1.9800530  Infinite        Infinite -71.2800                       130.1221                             111.7247                                    Silica        Decenter(3)530  Infinite        Infinite -60.4000                       111.7247                              96.1353                                    SilicaSpace                 -1.9800538  Infinite        Infinite -4.4550                        95.3562                              94.2064                                    SilicaSpace                 -1.1880540  -127.4561 CX        -1398.8019 CC                 -13.0104                        90.4737                              87.7002                                    SilicaSpace                 -1.1880542   -98.8795 CX        -424.1302 CC                 -12.2874                        80.7016                              76.3270                                    SilicaSpace                 -1.1880544  -132.0104 CX         -70.9574 CC                 -17.8706                        71.0789                              53.4306                                    SilicaSpace                 -3.1246546  -123.1071 CX        -585.4471 CC                 -19.9496                        52.6417                              38.2256                                    SilicaSpace                 -0.1980548  -137.8349 CX        -292.6179 CC                 -6.0885                        36.7251                              31.8484                                    SilicaSpaceImage Distance = -4.0000550  Image   Infinite              26.5000__________________________________________________________________________ 
    
     The constants for the aspheric mirror 534 used in equation (1) located after Table 1 are as follows: 
     CURV=-0.00325995 
     K=0.000000 
     A=-6.91799E-10 
     B=5.26952E-15 
     C=6.10046E-19 
     D=1.59429E-22 
     This fifth embodiment is optimized for radiation centered on 248.4 nm. The single refracting material of fused silica and the large portion of refracting power restricts the spectral bandwidth of the embodiment depicted in FIG. 5. However, because the fifth embodiment has a maximum numerical aperture of 0.6 rather than of 0.7 as in the first three embodiments, the fifth embodiment provides acceptable imaging over a spectral full-width-half-maximum bandwidth of 300 picometers. Thus, an unnarrowed excimer laser can be employed for an illumination source. The fifth embodiment differs from the first three embodiments in that the net power of LG1 and LG2 of the fifth embodiment is weakly positive rather than weakly negative as in the first three embodiments. In addition, this illustrates that the overall focal power of LG1 plus LG2 can be either positive or negative and still permit an infinitely distant entrance pupil to be imaged at or near the concave mirror 534. 
     Although the preferred embodiments have been illustrated and described, it will obvious to those skilled in the art that various modifications may be made without departing from the spirit and scope of thid invention.