Abstract:
An optical reduction system for use in the photolithographic manufacture of semiconductor devices having one or more quarter-wave plates operating near the long conjugate end. A quarter-wave plate after the reticle provides linearly polarized light at or near the beamsplitter. A quarter-wave plate before the reticle provides circularly polarized or generally unpolarized light at or near the reticle. Additional quarter-wave plates are used to further reduce transmission loss and asymmetries from feature orientation. The optical reduction system provides a relatively high numerical aperture of 0.7 capable of patterning features smaller than 0.25 microns over a 26 mm×5 mm 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 elements of different refracting power to widen the spectral bandwidth which can be achieved.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to optical systems used in semiconductor manufacturing.  
           [0003]    2. Background Art  
           [0004]    Semiconductor devices are typically manufactured using various photolithographic techniques. The circuitry used in a semiconductor chip is projected from a reticle onto a wafer. This projection 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 0.25 microns. The need to develop an optical system capable of reproducing very fine component features requires the improvement of system performance.  
           [0005]    A conventional optical system is disclosed in U.S. Pat. No. 5,537,260 entitled “Catadioptric Optical Reduction System with High Numerical Aperture” issued Jul. 16, 1996 to Williamson, which is incorporated by reference herein in its entirety. This reference describes an optical reduction system having a numerical aperture of 0.35. Another optical system is described in U.S. Pat. No. 4,953,960 entitled “Optical Reduction System” issuing Sep. 4, 1990 to Williamson, which is incorporated by reference herein in its entirety. This reference describes an optical system operating in the range of 248 nanometers and having a numerical aperture of 0.45.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    While these optical systems perform adequately for their intended purpose, there is an ever increasing need to improve system performance. The present inventor has identified that a need exists for eliminating diffraction induced by bias at the reticle. Further, there is a need for an optical system having low reticle diffraction capable of acceptable system performance over a large spectral waveband.  
           [0007]    Reticle diffraction induced by results from the way linearly polarized light interacts with the features of the reticle. The feature orientation of the reticle is determined by the semiconductor device being projected. Since there is an increasing need to reduce the size of semiconductor devices and feature orientation is dictated by the application of the semiconductor device, the present inventor focused on treating reticle diffraction.  
           [0008]    Linearly polarized light is typically used in certain photolithographic projection optic systems. Diffraction results from the interaction of light and the features on the reticle. Linearly polarized light travels through the reticle differently depending on the orientation of its features. Asymmetries result from this interaction. The asymmetries or print biases are then projected through the optical system onto the wafer. Print bias is significant enough to alter the thickness of the lines projected on the wafer. Variations on the wafer affect the performance of the semiconductor device, and in some cases prevent the device from performing to required specifications.  
           [0009]    The use of circularly polarized light at the reticle can eliminate the asymmetries which result from feature orientation. This circularly polarized light is indistinguishable from unpolarized light in its imaging behavior. The imaging behavior of unpolarized light is such that it diffracts equally regardless of the orientation of the feature through which it is projected. Thus the print biases are reduced throughout the optical system.  
           [0010]    However, other factors, such as transmission loss, prevent the use of circularly polarized light throughout an optical system. Thus, the present invention involves the use of phase shifters, which can take the form of wave plates, retardation plates and the like, to selectively alter the polarization of the light before the reticle and optical system.  
           [0011]    In one embodiment, the present invention is a catadioptric optical reduction system for use in the photolithographic manufacture of semiconductor devices having one or more quarter-wave plates operating near the long conjugate end. A quarter-wave plate after the reticle provides linearly polarized light at or near the beamsplitter. A quarter-wave plate before the reticle provides circularly polarized or generally unpolarized light at or near the reticle. Additional quarter-wave plates are used to further reduce transmission loss and asymmetries from feature orientation. The catadioptric optical reduction system provides a relatively high numerical aperture of 0.7 capable of patterning features smaller than 0.25 microns over a 26 mm×5 mm 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 elements of different refracting power to widen the spectral bandwidth which can be achieved.  
           [0012]    In another embodiment, the present invention is a catadioptric reduction system having, from the object or long conjugate end to the reduced image or short conjugate end, an first quarter-wave plate, a reticle, a second quarter-wave plate, a first lens group, a second lens group, a beamsplitter cube, a concentric concave mirror, and a third lens group. The first quarter-wave plate operates to circularly polarize the radiation passed to the reticle. The second quarter-wave plate operates to linearly polarize the radiation after the reticle before the first lens group. The concave mirror operates 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 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.  
           [0013]    Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES  
       [0014]    The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.  
         [0015]    In the drawings:  
         [0016]    [0016]FIG. 1 is a schematic illustration of a conventional optical projection system.  
         [0017]    [0017]FIG. 2A is an illustration of diffraction at the reticle.  
         [0018]    [0018]FIG. 2B is an illustration of the properties of a quarter-wave plate.  
         [0019]    [0019]FIG. 2C is an illustration of the properties of a half waveplate.  
         [0020]    [0020]FIG. 3 is a schematic illustration of the present invention using more than two quarter-wave plates.  
         [0021]    [0021]FIG. 4 is a schematic illustration of an alternative embodiment.  
         [0022]    [0022]FIG. 5 is a schematic illustration of one embodiment of the present invention using a single refracting material.  
         [0023]    [0023]FIG. 6 is another embodiment of the present invention using two different refracting materials.  
         [0024]    [0024]FIG. 7 is another embodiment of the present invention using more than two different refracting materials.  
         [0025]    [0025]FIG. 8 is another embodiment of the present invention.  
         [0026]    [0026]FIG. 9 is yet a further embodiment of the present invention. 
     
    
       [0027]    The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    I. Overview  
         [0029]    A. Conventional Optical System  
         [0030]    B. Reticle Diffraction  
         [0031]    C. Polarization and Wave Plates  
         [0032]    II. Terminology  
         [0033]    III. Example Implementations  
         [0034]    A. Optical System With Elimination of Reticle Diffraction Induced Bias  
         [0035]    B. Alternate Embodiment  
         [0036]    C. Further Embodiments  
         [0037]    IV. Alternate Implementation  
         [0038]    I. Overview  
         [0039]    A. Conventional Optical System  
         [0040]    [0040]FIG. 1 illustrates a conventional optical reduction system. From its long conjugate end where the reticle is placed to its short conjugate end where the wafer is placed, it possesses a first optical component group  120 , a beamsplitter cube  150 , a first quarter-wave plate  140 , a concave mirror  130 , a second quarter-wave plate  160 , and a second optical component group  170 . A feature of any optical system is the interdependence of numerical aperture size and spectral radiation requirements. In order to efficiently illuminate the image or wafer plane  180 , linearly polarized light is used. The limitations of linearly polarized light are introduced above and discussed in the following sections.  
         [0041]    B. Reticle Diffraction Induced Bias  
         [0042]    As recognized by the present inventor, the use of linearly polarized light at numerical apertures greater than about 0.5 introduces small, but noticeable, asymmetries in the imaging. These asymmetries in imaging are caused at least in part by diffraction of the linearly polarized light at certain feature orientations. FIG. 2A illustrates the asymmetries or print biases which result from the use of linearly polarized light at the reticle  110 . Simply, reticle  110  is placed in the path of both linearly polarized light  205  and circularly polarized light  210 . The two types of light are separated by separator  215 . After the reticle, the intensity of the light is distributed differently, as shown by distribution curves  220  and  225 . The results are shown on wafer  180 . Here, the projected image  230  resulting from the use of linearly polarized light  205  is not as clear or sharp as projected image  235  which results from the use of circularly polarized light  210 .  
         [0043]    Circularly polarized light  210  is indistinguishable from unpolarized light in its imaging behavior. The imaging behavior of unpolarized light is such that it diffracts equally regardless of the orientation of the feature through which it is projected. When the projection optic cannot accept unpolarized light, but requires linearly polarized light, it is possible to provide circularly polarized light to illuminate the reticle and thereby eliminate the feature orientation bias. Thus, print biases are reduced.  
         [0044]    C. Polarization and Wave Plates  
         [0045]    The properties of wave plates are shown in FIGS. 2B and 2C. FIG. 2B illustrates the properties of a quarter-wave plate. Linearly polarized input  240  enters the wave plate  245  at the input polarization plane  255 . The optic axis  250  and other factors discussed in detail below determine the orientation of the output light. Here, wave plate  245  is designed to produce circularly polarized output  260 .  
         [0046]    Similarly, FIG. 2C illustrates the properties of a half-wave plate. Linearly polarized input  265  enters the wave plate  270  at the input polarization plane  280 . The optic axis  275  and other factors discussed in detail below determine the orientation of the output light. Here, wave plate  270  is designed to produce linearly polarized with plane of polarization retarded output  285 .  
         [0047]    Wave plates (retardation plates or phase shifters) are made from materials which exhibit birefringence. Birefringent materials, such as crystals, are generally anisotropic. This means that the atomic binding forces on the electron clouds are different in different directions and as a result so are the refractive indices.  
         [0048]    In the case of uniaxial birefringent crystals, a single symmetry axis (actually a direction) known as the optic axis (shown in FIGS. 2B and 2C as elements  250  and  275 , respectively) displays two distinct principal indices of refraction: the maximum index n o  (the slow axis) and the minimum index n e  (the fast axis). These two indices correspond to light field oscillations parallel and perpendicular to the optic axis.  
         [0049]    The maximum index results in ordinary rays passing through the material. The minimum index results in extraordinary rays passing through the material. The velocities of the extraordinary and ordinary rays through the birefringent materials vary intensely with their refractive indices. The difference in velocities gives rise to a phase difference when the two beams recombine. In the case of an incident linearly polarized beam this is given by:  
         α   =     2      π                 d          (       n   e     -     n   o       )     λ         ;                         
 
         [0050]    where α is the phase difference; d is the thickness of wave plate; n e , no are the refractive indices of the extraordinary and ordinary rays respectively, and λ is the wavelength. Thus, at any specific wavelength the phase difference is governed by the thickness of the wave plate.  
         [0051]    As discussed above, FIG. 2B illustrates the operation of a quarter-wave plate. The thickness of the quarter-wave plate is such that the phase difference is {fraction (1/4)}-wavelength (zero order) or some multiple of {fraction (1/4)} wavelength (multiple order).  
         [0052]    If the angle between the electric field vector of the incident linearly polarized beam and the retarder principal plane of the quarter-wave plate is 45 degrees, the emergent beam is circularly polarized.  
         [0053]    Additionally, when a quarter-wave plate is double passed, e.g., when the light passes through it twice because it is reflected off a mirror, it acts as a half-wave plate.  
         [0054]    By quarter-wave plate is meant a thickness of birefringent material which introduces a quarter of a wavelength of the incident light. 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 zero order wave plates, and by restricting the field size in the plane of incidence.  
         [0055]    Similarly, FIG. 2C illustrates the operation of a half-wave plate. The thickness of a half-wave plate is such that the phase difference is {fraction (1/2)}-wavelength (zero order) or some odd multiple of {fraction (1/2)}-wavelength (multiple order). A linearly polarized beam incident on a half-wave plate emerges as a linearly polarized beam but rotated such that its angle to the optical axis is twice that of the incident beam.  
         [0056]    II. Terminology  
         [0057]    To more clearly delineate the present invention, an effort is made throughout the specification to adhere to the following term definitions as consistently as possible.  
         [0058]    The term “circuitry” refers to the features designed for use in a semiconductor device.  
         [0059]    The term “feature orientation” refers to the patterns printed on a reticle to projection.  
         [0060]    The term “long conjugate end” refers to the plane at the object or reticle end of the optical system.  
         [0061]    The term “print bias” refers to the variations in the lines on the wafer produced by asymmetries in the optical system. Asymmetries are produced by diffraction at various stages of the system and the reticle.  
         [0062]    The term “semiconductor” refers to a solid state substance that can be electrically altered.  
         [0063]    The term “semiconductor chip” refers to semiconductor device possessing any number of transistors or other components.  
         [0064]    The term “semiconductor device” refers to electronic equipment possessing semiconductor chips or other elements.  
         [0065]    The term “short conjugate end” refers to the plane at the image or wafer end of the optical system.  
         [0066]    The term “wave plate” refers to retardation plates or phase shifters made from materials which exhibit birefringence.  
         [0067]    III. Example Implementations  
         [0068]    A. Optical System With Elimination of Reticle Diffraction Induced Bias  
         [0069]    The present invention uses circularly polarized light to eliminate the reticle diffraction induced biases of conventional systems. FIG. 3 illustrates an embodiment of the present invention that eliminates such asymmetries or print biases. A first quarter-wave plate  305  is introduced before the object or reticle plane  110 . First quarter-wave plate  305  converts the linearly polarized light into circularly polarized light, as illustrated in FIG. 2B. As discussed above, circularly polarized light is indistinguishable from unpolarized light in its imaging behavior. The imaging behavior of unpolarized light is such that it diffracts equally regardless of the orientation of the feature through which it is projected. Thus the print biases which result from reticle diffraction are reduced.  
         [0070]    In order to minimize transmission loss through the rest of the optical system, second quarter-wave plate  315  is inserted to linearly polarize the radiation before the optical component group  320 .  
         [0071]    With respect to quarter-wave plates  305 ,  315 ,  340  and  360 , one orientation is to have the first quarter-wave plate  305  oriented with its fast axis parallel to that of the input light. The second quarter-wave plate  315  and fourth quarter-wave plate  360  have their fast axes in a parallel orientation but perpendicular to the fast axis of third quarter-wave plate  340 .  
         [0072]    B. Alternate Embodiment  
         [0073]    It is also apparent to one skilled in the relevant art that second quarter-wave plate  315  could be inserted into the system anywhere before the beamsplitter  350 . This aspect is shown in FIG. 4 where second quarter-wave plate  425  serves the same function. The transmission loss caused by the use of circularly polarized light within optical component group  320  influences the placement of second quarter-wave plate  425 .  
         [0074]    Specifically, the use of unpolarized or circularly polarized light at the beamsplitter would cause a transmission loss of 50%. If a non-polarized beamsplitter were to be used, 75% of the light would be lost. Therefore, while alternate embodiments are possible, they may not be feasibly implemented.  
         [0075]    With respect to quarter-wave plates  405 ,  425 ,  440  and  460 , one orientation is to have the first quarter-wave plate  405  oriented with its fast axis parallel to that of the input light. The second quarter-wave plate  425  and fourth quarter-wave plate  460  have their fast axes in a parallel orientation but perpendicular to the fast axis of third quarter-wave plate  440 .  
         [0076]    C. Further Embodiments  
         [0077]    The present invention can be implemented in various projection optic systems. For example, the present invention can be implemented in catadioptric systems as described in detail herein, as well as refractive and reflective systems. On skilled in the relevant art, based at least on the teachings provided herein, would recognize that the embodiments of the present invention are applicable to other reduction systems. More detailed embodiments of the present invention as provided below.  
         [0078]    [0078]FIG. 5 illustrates one embodiment of the optical reduction system of the present invention. From its long conjugate end, it comprises an first quarter-wave plate  508 , an object or reticle plane  110 , a second quarter-wave plate  511 , a first lens group LG 1 , a folding mirror  520 , a second lens group LG 2 , a beamsplitter cube  530 , a third quarter-wave plate  532 , a concave mirror  534 , a second quarter-wave plate  538 , and a third lens group LG 3 . The image is formed at image or wafer plane  180 . The first lens group LG 1  comprises a shell  512 , a spaced doublet including positive lens  514  and negative lens  516 , and positive lens  518 . The shell  512  is an almost zero power or zero power lens. The second lens group LG 2  comprises a positive lens  522 , a spaced doublet including a negative lens  524  and a positive lens  526 , and negative lens  528 . The third lens group LG 3  comprises two positive lenses  540  and  542 , which are strongly positive, shell  544 , and two positive lenses  546  and  548 , which are weakly positive. The first quarter-wave plate  508  passes circularly polarized light incident upon the object or reticle plane  110 . The folding mirror  520  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 one 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.  
         [0079]    Radiation enters the system at the long conjugate end and passes through the first lens group LG 1 , is reflected by the folding mirror  520 , and passes through the second lens group LG 2 . The radiation enters the beamsplitter cube  530  and is reflected from surface  536  passing through quarter-wave plate  532  and reflected by concave mirror  534 . The radiation then passes back through the quarter-wave plate  532 , the beamsplitter cube  530 , the quarter-wave plate  538 , lens group LG 3 , and is focused at the image or wafer plane  180 .  
         [0080]    Lens groups upstream of the mirror, LG 1  and LG 2 , provide only enough power to image the entrance pupil at infinity to the aperture stop  531  at or near the concave mirror  534 . The combined power of lens groups LG 1  and LG 2  is slightly negative. The shell  512  and air spaced doublet  514  and  516  assist in aberration corrections including astigmatism, field curvature, and distortion. The lens group LG 3 , after the concave mirror  534 , 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  540  and  542  provide a high numerical aperture at the image and exit pupils and infinity. The shell  544  has almost no power. The two weakly positive lenses  546  and  548  help correct high order aberrations. The concave mirror  534  may provide a reduction ratio of between 1.6 and 2.7 times that of the total system.  
         [0081]    The negative lens  524  in the second lens group LG 2  provides a strongly diverging beam directed at the beamsplitter cube  530  and concave mirror  534 . The strongly positive lens  522  provides lateral color correction. The air space doublet comprising lenses  524  and  526  helps to correct spherical aberrations and coma. Concave mirror  534  is preferably aspheric, therefore helping further reduce high order aberrations.  
         [0082]    The transmission losses introduced by the beamsplitter cube  530  are minimized by illuminating the object or reticle with linearly polarized light and including a quarter-wave plate  532  between the beamsplitter cube  530  and the concave mirror  534 . Additionally, by increasing the numerical aperture in lens group LG 3 , after the concave mirror  534  and beamsplitter cube  530 , the greatest angular range is not seen in these elements.  
         [0083]    However, the use of linearly 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 another quarter-wave plate  538  after the final passage through the beamsplitter cube  530 , thereby converting the linearly polarized light into circularly polarized light. This circularly polarized light is basically indistinguishable from unpolarized light in its imaging behavior.  
         [0084]    The optical system illustrated in FIG. 5 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 plane  110 . 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 LG 1  the numerical aperture is reduced to 0.12, a consequence of the positive power needed in lens group LG 1  to image the entrance pupil at infinity to the aperture stop of the system close to the concave mirror  534 . The numerical aperture after leaving the second lens group LG 2  and entering the beamsplitter is 0.19. Therefore, the emerging numerical aperture from the second lens group LG 2 , which is 0.19, is larger than the entering or object space numerical aperture of lens group LG 1 , which is 0.175. In other words, the second lens group LG 2  has an emerging numerical aperture greater than the entering numerical aperture of the first lens group LG 1 . 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 LG 2 . 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  534  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 LG 3  effectively doubles the numerical aperture to its final value of 0.7 at the wafer or image plane  180 .  
         [0085]    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 LG 2  and the strongly positive third lens group LG 3 . The use of the beamsplitter cube  530  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. 5 according to the present invention is given in Table 1 below.  
                                                                                                                                                                                                                                                                                                                                           TABLE 1                               Radius of Curvature       Aperture       Element   (mm)   Thickness   Diameter (mm)            Number   Front   Back   (mm)   Front   Back   Glass                    508   Infinite   Infinite   4.500   123.0000   123.0000   Silica       Space           0.7500            110   Infinite   63.3853                Space           0.7500                   511   Infinite   Infinite   4.500   123.0000   123.0000   Silica       Space           0.7500       512   −158.7745   −177.8880   15.0000   124.0478   131.7725   Silica       Space           36.1130       514   −556.6911   −202.0072   22.2126   148.3881   152.5669   Silica       Space           38.7188       516   −183.7199   −558.8803   15.0000   156.5546   166.5750   Silica       Space           10.0674       518   427.2527   −612.2450   28.8010   177.4010   179.0292   Silica       Space           132.3320            520   Infinite   −74.0000   184.6402   Reflection            522   −240.4810   2050.9592   −33.3135   188.4055   185.3395   Silica       Space           −29.3434       524   421.7829   −145.6176   −12.0000   175.5823   169.0234   Silica       Space           −4.2326       526   −150.4759   472.0653   −46.5091   171.4244   169.9587   Silica       Space           −2.0000       528   −1472.2790   −138.2223   −15.0000   165.3586   154.8084   Silica       Space           −27.2060       530   Infinite   Infinite   −91.8186   155.6662   253.0917   Silica            536   Infinite       253.0917   Reflection            530   Infinite   Infinite   91.8186   253.0917   253.0917   Silica       Space           2.0000       532   Infinite   Infinite   6.0000   185.8693   186.8401   Silica       Space           17.9918            Stop       188.0655                534   Aspheric   −17.9918   188.0655   Reflection            532   Infinite   Infinite   −6.0000   183.5471   180.1419   Silica       Space           −2.0000       530   Infinite   Infinite   −91.8186   178.3346   149.2832   Silica       530   Infinite   Infinite   −70.000   149.2832   128.8604   Silica       Space           −2.0000       538   Infinite   Infinite   −4.500   127.9681   126.6552   Silica       Space           −0.7500       540   −175.1330   1737.4442   −17.7754   121.4715   118.2689   Silica       Space           −0.7500       542   −108.8178   −580.1370   −18.2407   104.5228   97.7967   Silica       Space           −0.7500       544   −202.2637   −86.6025   −31.1216   91.7061   57.4968   Silica       Space           −2.3507       546   −122.1235   −488.7122   −17.9476   56.4818   41.1675   Silica       Space           −0.2000       548   −160.8506   −360.1907   −6.1500   39.4528   33.5764   Silica       Space           −4.000            180   Infinite       26.5019                      
 
         [0086]    Concave mirror  534  has an aspheric reflective surface according to the following equation:  
         Z   =           (   CURV   )          Y   2         1   +       1   -       (     1   +   k     )            (   CURV   )     2          Y   2               +       (   A   )          Y   4       +       (   B   )          Y   6       +       (   D   )          Y   10       +       (   E   )          Y   12       +       (   F   )          Y   14           ;                         
 
         [0087]    wherein the constants are as follows:  
         [0088]    CURV=−0.00289051  
         [0089]    K=0.000000  
         [0090]    A=6.08975×10 −11    
         [0091]    B=2.64378×10 14    
         [0092]    C=9.82237×10 −19    
         [0093]    D=7.98056×10 −2    
         [0094]    E=−5.96805×10 −27    
         [0095]    F=4.85179×10 −3    
         [0096]    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. 5 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. 5 can be optimized for any wavelength for which fused silica transmits adequately.  
         [0097]    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. 6. From its long conjugate end, it comprises a first quarter-wave plate  608 , an object or reticle plane  110 , a second quarter-wave plate  611 , a lens group LG 4 , a folding mirror  622 , a lens group LG 5 , a beamsplitter cube  632  having surface  638 , a third quarter-wave plate  634 , a concave mirror  636 , a fourth quarter-wave plate  640 , and lens group LG 6 . The image is formed at image or wafer plane  180 . The lens group LG 4  comprises a spaced doublet including negative lens  612  and positive lens  614 , a weak positive lens  616 , positive lens  618 , and shell  620 . The lens group LG 5  comprises a positive lens  624 , a negative lens  626 , a positive lens  628 , and a negative lens  630 . The lens group LG 6  comprises two positive lenses  642 , cemented doublet including positive lens  644  and negative lens  646 , positive lens  648 , and cemented doublet including shell  650  and positive lens  652 .  
         [0098]    This second embodiment uses calcium fluoride in one of the individual positive lenses of the lens group LG 4 , negative lenses of the lens group LG 5 , and two of the positive lenses of the lens group LG 6 . The construction data of the second embodiment illustrated in FIG. 6 of the present invention is given in Table 2 below.  
                                                                                                                                                                                                                                                                                                                     TABLE 2                               Radius of Curvature               Element   (mm)   Thickness   Aperture Diameter (mm)            Number   Front   Back   (mm)   Front   Back   Glass                    608   Infinite   Infinite   4.5000   123.0000   123.0000   Silica       Space           0.5000            110   Infinite   60.4852                Space           0.5000                   611   Infinite   Infinite   4.5000   123.0000   123.0000   Silica       612   −205.5158   539.1791   15.2158   124.0926   137.3346   Silica       Space           8.8054       614   2080.9700   −210.6539   32.4984   142.6149   151.7878   Silica       Space           1.2676       616   310.4463   700.3748   40.7304   162.4908   165.2126   CaFl       Space           0.5000       618   634.1820   −798.8523   27.5892   165.4595   166.4747   Silica       Space           0.5000       620   1480.0597   1312.1247   25.4322   168.7516   164.7651   Silica       Space           136.2343            622   Infinite   −74.0000   161.9590   Reflection            624   −761.9176   1088.9351   −19.2150   160.3165   159.2384   Silica       Space           −19.9465       626   648.8361   −202.5872   −12.0000   155.1711   153.0635   CaFl       Space           −7.6304       628   −400.4276   458.5060   −25.8769   153.0635   153.8055   Silica       Space           −2.0000       630   −818.0922   −168.5034   −27.5927   152.6663   147.5200   CaFl       Space           −20.5014       632   Infinite   Infinite   −91.7553   148.6158   252.7349   Silica            638   Infinite       252.7349   Reflection            632   Infinite   Infinite   91.7553   252.7349   252.7349   Silica       Space           2.0000       634   Infinite   Infinite   6.0000   185.8070   187.0026   Silica       Space           18.1636       Stop           188.5681            636   Aspheric   −18.1636   188.5681   Reflection            634   Infinite   Infinite   −6.0000   184.2566   181.1084   Silica       Space           −2.0000       632   Infinite   Infinite   −91.7553   179.3838   151.7747   Silica       632   Infinite   Infinite   −70.0000   151.7747   133.3985   Silica       Space           −2.0000       640   Infinite   Infinite   −4.5000   132.5690   131.3876   Silica       Space           −0.5000       642   −112.0665    −597.6805   −21.4866   123.4895   119.2442   Silica       Space           −0.5000       644   −116.3137   282.3140   −24.0940   107.8451   101.2412   CaFl       646   282.3140   −66.5293   −13.7306   101.2412   72.6862   Silica       Space           −2.6346       648   −77.2627   −374.4800   −17.9594   72.0749   62.7659   Silica       Space           −0.5452       650   −130.1381   −57.1295   −20.8147   58.9696   37.4889   Silica       652   −57.1295   −7305.8777   −6.1425   37.4889   34.3156   CaFl       Space           −4.0000            180   Infinite       26.4992                  
 
         [0099]    wherein the constants for the aspheric mirror  634  used in the equation after Table 1 are as follows:  
         [0100]    CURV=−0.00286744  
         [0101]    K=0.000000  
         [0102]    A=−1.92013×10 −9    
         [0103]    B=−3.50840×10 −4    
         [0104]    C=2.95934×10 −19    
         [0105]    D=−1.10495×10 −22    
         [0106]    E=9.03439×10 −27    
         [0107]    F=−1.39494×10 −3    
         [0108]    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.  
         [0109]    At wavelengths longer than 360 nanometers, a wider range of optical glasses begin to have adequate transmission. A third embodiment illustrated in FIG. 7 takes advantage of this wider selection of glasses and further reduced dispersion. From its long conjugate end, it comprises a first quarter-wave plate  708 , an object or reticle plane  110 , a second quarter-wave plate  711 , a lens group LG 7 , a folding mirror  722 , a lens group LG 8 , a beamsplitter cube  732  having a surface  738 , a third quarter-wave plate  734 , a concave mirror  736 , a fourth quarter-wave plate  740 , and lens group LG 9 . The image is formed at image or wafer plane  180 . The lens group LG 7  comprises a spaced doublet comprising negative lens  712  and positive lens  714 , spaced doublet including positive lens  716  and negative lens  718 , and positive lens  720 . The lens group LG 8  comprises a positive lens  724 , a negative lens  726 , a positive lens  728 , and a negative lens  730 . The lens group LG 9  comprises a positive lenses  742 , cemented doublet including positive lens  744  and negative lens  746 , positive lens  748 , and cemented doublet including shell  750  and positive lens  752 .  
         [0110]    The construction data of the third embodiment illustrated in FIG. 7 is given in Table 3 below.  
                                                                                                                                                                                                                                                                                                                                                   TABLE 3                               Radius of Curvature       Aperture       Element   (mm)   Thickness   Diameter (mm)            Number   Front   Back   (mm)   Front   Back   Glass                    708   Infinite   Infinite   4.5000   125.0000   125.0000   Silica       Space           0.5000            110   Infinite   59.2960           Space       0.5000            711   Infinite   Infinite   4.5000   125.0000   125.0000   Silica       712   −620.7809   361.8305   20.2974   125.9406   134.7227   PBM2Y       Space           2.6174       714   515.7935   −455.1015   39.8858   135.3384   145.6015   PBM2Y       Space           14.7197       716   431.3189   −239.4002   36.9329   155.6269   157.3014   BSL7Y       Space           0.5000       718   −259.6013   685.3286   26.3534   156.9363   162.2451   PBM2Y       Space           1.4303       720   361.5709   −1853.2955   23.3934   168.7516   165.1801   BAL15Y       Space           131.8538            722   Infinite   −77.8469   169.9390   Reflection            724   −429.2950   455.4247   −32.3086   173.0235   171.1102   PBL6Y       Space           −27.6206       726   401.0363   −180.0031   −12.0000   159.3555   154.7155   BSL7Y       Space           −5.6227       728   −258.4722   1301.3764   −26.1321   154.7155   154.1517   PBM8Y       Space           −2.0000       730   −1282.8931   −180.2226   −12.0000   153.1461   149.4794   BSL7Y       Space           −19.7282       732   Infinite   Infinite   −91.7349   150.4585   252.6772   Silica            738   Infinite       252.6772   Reflection            732   Infinite   Infinite   91.7349   252.6772   252.6772   Silica       Space           2.0000       734   Infinite   Infinite   6.0000   185.6435   186.7758   Silica       Space           18.2715            Stop               188.1745                736   Aspheric   −18.2715   188.1745   Reflection            734   Infinite   Infinite   −6.0000   183.6393   180.1377   Silica       Space           −2.0000       732   Infinite   Infinite   −91.7349   178.3236   147.9888   Silica       732   Infinite   Infinite   −70.0000   147.9888   126.9282   Silica       Space           −2.000       740   Infinite   Infinite   −4.5000   126.0289   124.6750   Silica       Space           −0.5000       742   −119.8912   −610.6840   −18.6508   117.5305   113.4233   BSM51Y       Space           −0.5000       744   −114.1327   384.9135   −21.1139   102.6172   96.4137   BSL7Y       746   384.9135   −70.2077   −13.0576   96.4137   71.1691   PBL26Y       Space           −2.8552       748   −85.7858   −400.3240   −16.9147   70.5182   61.2633   BSM51Y       Space           −0.8180       750   −151.5235   −54.0114   −19.5810   57.6234   37.3909   BSM51Y       752   −54.0114   −2011.1057   −6.3947   37.3909   34.2119   PBL6Y       Space           −4.0000            180   Infinite       26.5002                      
 
         [0111]    wherein the constants for the aspheric mirror 736 used in the equation after Table 1 as follows:  
         [0112]    CURV=−0.00291648  
         [0113]    K=0.000000  
         [0114]    A=−1.27285×10 −9    
         [0115]    B=−1.92865×10 −14    
         [0116]    C=6.21813×10 −19    
         [0117]    D=−6.80975×10 23    
         [0118]    E=6.04233×10 −27    
         [0119]    F=3.64479×10 −32    
         [0120]    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.  
         [0121]    [0121]FIG. 8 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 a first quarter-wave plate  808 , an object or reticle plane  110 , a second quarter-wave plate  811 , a first lens group LG 1 , a folding mirror  820 , a second lens group LG 2 , a beamsplitter cube  830 , a first quarter-wave plate  832 , a concave mirror  834 , a second quarter-wave plate  838 , and a third lens group LG 3 . The image is formed at the image or wafer plane  180 .  
         [0122]    The first lens group LG 1  comprises a shell  812 , a spaced doublet including a positive lens  814  and a negative lens  816 , and a positive lens  818 . The second lens group LG 2  comprises a positive lens  822 , a spaced doublet including a negative lens  824  and a positive lens  826 , and a negative lens  828 . The third lens group LG 3  comprises two positive lenses  840  and  842 , a shell  844 , and two positive lenses  846  and  848 . Again, as in the embodiment illustrated in FIG. 5, the folding mirror  820  of FIG. 8 is not essential to the operation of the invention, but nevertheless permits the object  10  and image plane  180  to be parallel to each other which is convenient for the manufacture of semiconductor devices using photolithography.  
         [0123]    The construction data of the fourth embodiment illustrated in FIG. 8 is in Table 4 below.  
                                                                                                                                                                                                                                                                                                                                                             TABLE 4                               Radius of Curvature       Aperture       Element   (mm)   Thickness   Diameter (mm)            Number   Front   Back   (mm)   Front   Back   Glass                    808   Infinite   Infinite   4.5000   122.0000   122.0000   Silica       Space           2.0000            110   Infinite   63.3853                Space           2.0000                   811   Infinite   Infinite   4.5000   122.0000   122.0000   Silica       812   −183.5661   −215.7867CX   17.0000   122.8436   130.6579   Silica       Space           46.6205       814   −601.1535CC   −230.9702CX   21.4839   149.1476   153.3103   Silica       Space           68.8075       816   −195.1255   −345.4510CX   15.0000   161.6789   170.1025   Silica       Space           3.0000       818   435.8058CX   −1045 1785CX      24.9351   177.4250   178.2672   Silica       Space           130.0000               Decenter(1)            820   Infinite   −64.5000   180.3457   Reflection            822   210 7910CX   380 1625CX   −43.1418   181.6672   178.0170   Silica       Space           −15.8065       824   300 1724CC   −123 4555CC     −12.0000   166.7278   152.3101   Silica       Space           −3.8871       826   −126 8915CX     972 6391CX   −41.3263   154.8530   151.8327   Silica       Space           −1.5000       828   −626.4905CX     −116 6456CC     −12.0000   147.6711   136.1163   Silica       Space           −31.8384       830   Infinite   Infinite   −74.0000   137.2448   200.1127   Silica               Decenter(2)|            836   Infinite       200.1128   Reflection            830   Infinite   Infinite   74.0000   200.1127   200.1127   Silica       Space           2.0000       832   Infinite   Infinite   6.0000   148.6188   149.0707   Silica       Space           14.4638            Stop               149.6392                834   Aspheric   −14.4638   149.6392   Reflection            832   Infinite   Infinite   −6.0000   144.8563   141.2737   Silica       Space           −2.0000       830   Infinite   Infinite   −74.000   139.3606   117.3979   Silica               Decenter(3)       830   Infinite   Infinite   −61.000   117.3979   100.5074   Silica       Space           −2.0000       838   Infinite   Infinite   −4.5000   99.6617   98.4157   Silica       Space           −1.2000       840   −157.8776CX   2282.2178CX   −13.7501   94.8267   91.8775   Silica       Space           −1.2000       842    −94 0059CX    −46.6659CC   −13.4850   82.8663   78.1418   Silica       Space           −1.2000       844   −147 2485CX     −77.8924CC   −22.2075   72.7262   50.6555   Silica       Space           −3.2091       846   −159.2880CX   −519 4850CC     −13.8321   49.5648   39.0473   Silica       Space           −0.2000       848   −129 3683CX     −426 7350CC     −6.1500   37.3816   32.4880   Silica            Space   Image Distance =   −4.0000                850   Image   Infinite                  
 
         [0124]    The constants for the aspheric mirror 834 used in the equation located after Table 1 are as follows:  
         [0125]    CURV=−0.00332614  
         [0126]    K=0.000000  
         [0127]    A=−4.32261E−10  
         [0128]    B=3.50228E−14  
         [0129]    C=7.13264E−19  
         [0130]    D=2.73587E−22  
         [0131]    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. 8. 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.  
         [0132]    The fourth embodiment differs from the first three embodiments in that the net power of LG 1  and LG 2  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 LG 1  plus LG 2  can be either positive or negative and still permit an infinitely distant entrance pupil to be imaged at or near the concave mirror  834 .  
         [0133]    [0133]FIG. 9 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 a first quarter-wave plate  908 , an object or reticle plane  110 , a second quarter-wave plate  911 , a first lens group LG 1 , a folding mirror  920 , a second lens group LG 2 , a beamsplitter cube  930 , a third quarter-wave plate  932 , a concave mirror  934 , a fourth quarter-wave plate  938 , and a third lens group LG 3 . The image is formed at an image or wafer plane  180 .  
         [0134]    The first lens group LG 1  comprises a shell  912 , a spaced doublet including a positive lens  914  and a negative lens  916 , and a positive lens  918 . The second lens group LG 2  comprises a positive lens  922 , a spaced doublet including a negative lens  924  and a positive lens  926 , and a negative lens  928 . The third lens group LG 3  comprises two positive lenses  940  and  942 , a shell  944 , and two positive lenses  946  and  948 . Again, as in the embodiment illustrated in FIG. 5, the folding mirror  920  of FIG. 9 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.  
         [0135]    The construction data of the fifth embodiment illustrated in FIG. 9 is given in Table 5 below.  
                                                                                                                                                                                                                                                                                                                                                                     TABLE 5                               Radius of Curvature       Aperture       Element   (mm)   Thickness   Diameter (mm)            Number   Front   Back   (mm)   Front   Back   Glass                    908   Infinite   Infinite   −4.4550   120.0000   120.0000   Silica       Space           1.1880            910   Infinite   62.7514           Space   1.1880            911   Infinite   Infinite   −4.4550   120.0000   120.0000   Silica       912   −136 1154CC     −152 5295CX     16.8300   120.7552   129.4354   Silica       Space           4.5206       914   −270 1396CC     −191 8742CX     20.5341   132.9152   139.0377   Silica       Space           90.8476       916   −188.9000CC   −284.7476CX   17.5000   156.1938   165.6567   Silica       Space           2.9700       918   433 8174 CX     −841 5599CX     25.8293   173.8279   174.8334   Silica       Space           149.4549               Decenter(1)            920   Infinite   −61.0000   177.2183   Reflection            922   −190.3251CX   −8413 4836CC     −34.4584   178.5071   174.2260   Silica       Space           −51.5487       924   690 5706CC   −146 4997CC     −11.8800   150.4109   141.8021   Silica       Space           −10.6267       526   −265.9886CX   1773.5314CX   −24.1851   142.1851   141.2400   Silica       Space           −1.5000       928   −244 9899CX     −142 8558CC     −11.8800   139.3290   133.8967   Silica       Space           −21.6411       930   Infinite   Infinite   −71.2800   134.3115   189.7826   Silica               Decenter(2)            936   Infinite       189.7826   Reflection            930   Infinite   Infinite   71.2800   189.7826   189.7826   Silica       Space           1.9800       932   Infinite   Infinite   5.9400   142.3429   142.6707   Silica       Space           18.5263            Stop           143.5034                934   Aspheric   −18.5263   143.5034   Reflection            932   Infinite   Infinite   −5.9400   134.2788   130.9398   Silica       Space           −1.9800       930   Infinite   Infinite   −71.2800   130.1221   111.7247   Silica               Decenter (3)       930   Infinite   Infinite   −60.4000   111.7247   96.1353   Silica       Space           −1.9800       938   Infinite   Infinite   −4.4550   95.3562   94.2064   Silica       Space           −1.1880       940   −127 4561CX     −1398 8019CC      −13.0104   90.4737   87.7002   Silica       Space           −1.1880       942    −98 8795 CX   −424 1302CC   −12.2874   80.7016   76.3270   Silica       Space           −1.1880       944   −132.0104CX    −70 9574CC   −17.8706   71.0789   53.4306   Silica       Space           −3.1246       946   −123.1071CX   −585 4471CC   −19.9496   52.6417   38.2256   Silica       Space           −0.1980       948   −137.8349CX   −292 6179CX     −6.0885   36.7251   31.8484   Silica            Space   Image Distance =   −4.0000                950   Image   Infinite       26.5000                      
 
         [0136]    The constants for the aspheric mirror  934  used in the equation located after Table 1 are as follows:  
         [0137]    CURV=−0.00325995  
         [0138]    K=0.000000  
         [0139]    A=−6.91799E−10  
         [0140]    B=5.26952E−15  
         [0141]    C=6.10046E−19  
         [0142]    D=1.59429E−22  
         [0143]    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. 9. 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 LG 1  and LG 2  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 LG 1  plus LG 2  can be either positive or negative and still permit an infinitely distant entrance pupil to be imaged at or near the concave mirror  934 .  
         [0144]    IV. Alternate Implementation  
         [0145]    It is apparent to one skilled in the relevant art that the use of the first quarter-wave plate in any of the above embodiments depends on the initial polarization of the radiation incident on the long conjugate end. Therefore, if the polarization of the light is circular or unpolarized prior to the long conjugate end, then the first quarter-wave plate, used to transform linearly polarized light into circularly polarized light, could be omitted.  
         [0146]    Such an implementation can be shown by omitting first quarter-wave plate  305  from FIG. 3 and/or first quarter-wave plate  405  from FIG. 4. Further implementations of this configuration in the other embodiments described above are obvious to one skilled in the relevant art.  
         [0147]    Conclusion  
         [0148]    While specific embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.