Abstract:
The invention concerns a microlithographic reduction projection catadioptric objective having an even number greater than two of curved mirrors, being devoid of planar folding mirrors and featuring an unobscured aperture. The objective has a plurality of optical elements, and no more than two optical elements deviate substantially from disk form. The objective has an object side and an image side, and has in sequence from the object side to the image side a catadioptric group providing a real intermediate image, a catoptric or catadioptric group providing a virtual image, and a dioptric group providing a real image.

Description:
Cross-References to Related Applications—Not applicable.  
       Statement Regarding Federally Sponsored Research or Development—Not applicable.  
       Reference to a Microfiche Appendix—Not applicable.  
       BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The invention concerns a microlithographic reduction projection catadioptric objective comprising an even number greater than two of curved mirrors, being devoid of planar folding mirrors and featuring an unobscured aperture.  
           [0003]    2. Background Art  
           [0004]    Such objectives are known from European Patent document EP 0 779 528 A (FIG. 3) as variants of pure catoptric objectives, with six mirrors and three lenses. All optical surfaces are symmetric to a common axis and an object plane and an image plane are situated on this axis upstream and downstream of the objective. However, all but one of the mirrors need to be cut off sections of bodies of revolution, so that mounting and adjustment face difficulties. The lenses serve only as correcting elements of minor effect. The most imageward mirror is concave.  
           [0005]    U.S. Pat. No. 4,701,035 (FIG. 12) shows a similar objective. This one, however, has nine mirrors, two lenses and two intermediate images. The object plane and image plane are situated within the envelope of the objective.  
           [0006]    In both cases the image field is an off-axis ring sector.  
           [0007]    A fully axially symmetric catadioptric objective is known from German Patent document DE 196 39 586 A (corresponding to U.S. patent application Ser. No. 09/263,788), e.g., with two opposing concave mirrors, an image field centered at the axis, and a central obscuration of the aperture.  
           [0008]    Another type of catadioptric objective suitable for microlithographic reduction projection has only one concave mirror, but at least one folding mirror, and is known from U.S. Pat. No. 5,052,763 and European Patent document EP 0 869 383A inter alia and is referenced here as “h-design”.  
           [0009]    U.S. Pat. No. 5,323,263 discloses a microlithographic reduction projection catadioptric objective with multiple folding mirrors, where an intermediate image is arranged subsequent to a first concave mirror and a singly passed lens group.  
           [0010]    U.S. Pat. No. 5,575,207 and U.S. Pat. No. 4,685,777 show very similar multiply folded catadioptric objectives.  
         SUMMARY OF THE INVENTION  
         [0011]    It is an object of the invention to provide a generic objective of good capabilities of chromatic correction for typical bandwidths of excimer laser light sources, which allows for a high imageside numerical aperture, and which reduces complexity of mounting and adjusting.  
           [0012]    The solution to this problem is found in the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention is described in detail with respect to the drawings, wherein:  
         [0014]    [0014]FIG. 1 shows a front end of an objective;  
         [0015]    [0015]FIG. 2 shows the lens plan of a version of the objective; and  
         [0016]    [0016]FIG. 3 shows the lens plan of another version of the objective.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0017]    An important concept of the present invention is to replace the front end of an “h-design” objective with a different front end that provides a single axis system.  
         [0018]    In the simplest version of this new front end, set up to be part of a −0.25 reduction. 0,75 image side NA system with a 7 mm×26 mm rectangular image field size, the optical elements are shown in the lens section of FIG. 1. This catadioptric partial system provides a virtual image on the right hand side, which has enough axial chromatic aberration to compensate for a conventional focusing lens group that forms a 0.75 NA image. A real pupil or aperture plane is formed on the right hand end of the system. The system shown has enough Petzval sum so that the focusing lens group can be made up of mostly positive power lenses.  
         [0019]    There is only one field lens LI in this system, which is close to the object plane (Ob) end of the system. That location is an advantage with respect to lens heating. There are no aspherics in this front end, and none are needed. The mirrors M 1  to M 4  are all spherical and coaxial to the common optical axis. It is possible to correct this front end system for spherical aberration of the pupil, but that requires a somewhat larger concave mirror than shown here. Spherical aberration can as well be corrected in the focusing lens group and therefore the size of the concave mirror M 3  is minimized. Decreased size of mirror M 3  simplifies the mechanical construction of the system. In the example of FIG. 1, the concave mirror M 3  has an illuminated area that is about 165 mm wide in the plane of the drawing and about 500 mm in the orthogonal direction, for a 7 mm×26 mm image field size.  
         [0020]    The greatest distance of any ray from the common optical axis is 370 mm in this example. This is substantially less than for many designs of the “h-design” type, where the concave mirror thickness and mount thickness must be added in to the sideways ray path distance after the fold mirror, from the axis to the concave mirror. The package envelope of this new design is more attractive.  
         [0021]    More axial chromatic aberration and Petzval curvature can be introduced by the front end (FE) than in the example of FIG. 1, by increasing the power of the negative lens L 2  near the concave mirror M 1 . A strong lens L 2  however, tends to introduce too much overcorrected spherical aberration and makes the intermediate image aberrations too large. Thus, a better version of the design has two concave lenses near the concave mirror.  
         [0022]    The field lens LI near the object plane Ob can also be split into two weaker lenses, to help control pupil aberration. Finally, the convex mirror M 2  that is near the reticle (Ob) can be split off from the field lens LI surface and made to be a separate optical element. This more complicated design is capable of better performance.  
         [0023]    It is possible to make this system meet all of the first-order specifications of a typical microlithographic objective as well as correct for Petzval curvature, and axial and lateral color correction, with only positive lenses in the telecentric focusing group (TFG). An example is shown in FIG. 2, without any other kind of aberration correction. The lens heating is substantially uniform, as the beam diameter is large on all the lenses L 21  to L 29 .  
         [0024]    [0024]FIG. 3 shows a further embodiment example. The front end FE&#39; features a field lens group split into 3 lenses L 31  to L 33 , which helps achieve a good quality telecentricity. Also, the focussing lens group (FLG&#39;) now has more lenses L 36  to L 44 . This focussing lens group FLG&#39; has a few aspherics. There are also some aspherics in the catadioptric front end FE&#39; of the design that simplify correction, though they are not compulsory. The large mirror M 33  is still a sphere, as this simplifies production.  
         [0025]    Preferred locations of the aspheric surface are near an aperture or pupil plane, namely on mirror M 31  or on lenses L 34 , L 35 , where the marginal ray height exceeds 80% of the height of the neighboring aperture, and on the other hand on some distant locations with marginal ray height less than 80% of the height of the next aperture. Examples of the latter are surfaces of the field lens group or of the last two lenses next to the image plane Im.  
         [0026]    The polychromatic r.m.s. wavefront error value in this design now varies from 0.05 to 0.13 waves over a 26×7 mm field at 0.75 NA in a 4× design.  
         [0027]    The catadioptric front end FE&#39; is now somewhat more complicated than in FIGS. 1 and 2. The design is both side telecentric and corrected for pupil aberration and distortion. The working distance is 34 mm on the reticle end (Ob) and 12 mm on the wafer end (Im). The system length is about 1200 mm.  
         [0028]    The focusing lens group FLG&#39; is almost all positive lenses (except L 41 ), with no strong curves. The very large amount of aberration at the intermediate image is because the two concave lenses L 31 , L 35  next to the concave mirror M 31  do not have the optimum bending under this aspect.  
         [0029]    Table I provides lens data for this embodiment.  
         [0030]    Mechanical construction of the lens barrel for this type of objective is very advantageous when compared with catadioptric systems with folding of the optical axis (as “h-design” etc.). Here, only the mirrors M 32  and M 33  cannot be full disks. Mirror M 33 , however, can be extended to a full annular body that can be mounted in a rotationally symmetric structure. The barrel must be cut between the lenses L 33  and L 36  at a lower side of the drawing of FIG. 3 to provide passage to the light beam, but generally can be cylindrical. Only mirror M 33  must be positioned outside this cylindrical barrel, but at a very moderate distance.  
         [0031]    With “h-designs”, a similar effect needs additional folding. Folding mirrors are generally not desirable, as they cause intensity losses and quality degradation of the light beam, and production costs and adjustment work without benefit to image quality.  
         [0032]    It is possible to produce mirror M 33  as an annular blank, and it can be mounted as this annular part in a cylindrical barrel that is extended in diameter in this area.  
         [0033]    It can be seen that concave spherical mirror M 33  is the only mirror extending outside of a cylindrical envelope scribed around all the lenses that has the radius of the lens of greatest radius. This shows again that this type of objective is suitable for mounting in a compact cylindrical barrel of high intrinsic rigidity.  
         [0034]    The lens material in the given examples is calcium fluoride, fluorspar. Other materials standing alone or in combinations, may be used, namely at other wavelengths of excimer lasers. Quartz glass, eventually suitably doped, and fluoride crystals are such suitable materials.  
         [0035]    Four, six and eight or more mirror objective designs known in the field of EUV lithography are generally suitable as starting designs for the front end group of the invention, with the eventual deviation that a virtual image instead of a real image is provided.  
         [0036]    These embodiments are not intended to limit the scope of the invention. For example, in addition to curved mirrors, planar folding mirrors may occasionally be introduced into the system according to the invention.  
         [0037]    All the features of the different claims can be combined in various combinations according to the invention.  
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           TABLE 1                       CODE V&gt; lis       Shafer - design .75NA.4x.75 mm Obj. - hight                                    RDY   THI   RMD   GLA   CCY   THC   GLC                        &gt; OBJ:   INFINITY   34.000000           100   100           1:   147.23281   21.000000       ‘CAF-UV’   100   100       2:   236.79522   1.000000           100   100                 ASP   :                                                    K   :   0.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   0.273300E−07   B   :   0.201130E−11   C   :   −.871260E−16   D   :   0.118100E−19            AC   :   100   BC   :   100   CC   :   100   DC   :   100            3:   145.44401   27.000000       ‘CAF-UV’   100   100           4:   224.64885   51.185724           100   100       5:   −223.00016   25.004072       ‘CAF-UV’   100   100       6:   −184.59445   162.666291           100   100       7:   −97.23630   12.000000       ‘CAF-UV’   100   100       8:   −928.69926   24.980383           100   100       9:   −75.28503   15.000000       ‘CAF-UV’   100   100       10:   −116.14787   3.000000           100   100       11:   −134.28262   −3.000000   REFL       100   100                 ASP   :                                                    K   :   0.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   0.474810E−08   B   :   0.506570E−12   C   :   −.284590E−17   D   :   0.934830E−21            AC   :   100   BC   :   100   CC   :   100   DC   :   100            12:   −116.14787   −15.000000       ‘CAF-UV’   100   100           13:   −75.28503   −24.980383           100   100       14:   −928.69926   −12.000000       ‘CAF-UV’   100   100       15:   −97.23630   −162.666291           100   100       16:   −184.59445   −25.004072       ‘CAF-UV’   100   100       17:   −223.00016   11.195502           100   100       18:   −363.91714   11.195502   REFL       100   100                 ASP   :                                                    K   :   0.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   −.107960E−07   B   :   0.170830E−13   C   :   −.328180E−16   D   :   0.143630E−20            AC   :   100   BC   :   100   CC   :   100   DC   :   100            19:   −223.00016   25.004072       ‘CAF-UV’   100   100           20:   −184.59445   162.666291           100   100       21:   −96.00000   15.000000           100   100                 ASP   :                                                    K   :   -1.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   0.000000E+00   B   :   0.000000E+00   C   :   0.000000E+00   D   :   0.000000E+00            AC   :   100   BC   :   100   CC   :   100   DC   :   100            22:   INFINITY   24.980383       100   100           23:   −247.00000   67.808099       100   100                 ASP   :                                                    K   :   -1.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   0.000000E+00   B   :   0.000000E+00   C   :   0.000000E+00   D   :   0.000000E+00            AC   :   100   BC   :   100   CC   :   100   DC   :   100            24:   −237.00000   266.861281       100   100                     ASP   :                                                    K   :   −1.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   0.000000E+00   B   :   0.000000E+00   C   :   0.000000E+00   D   :   0.000000E+00            AC   :   100   BC   :   100   CC   :   100   DC   :   100            25:   −470.62323   −266.861281   REFL       100   100           26:   −210.84570   266.861281   REFL       100   100                 ASP   :                                                    K   :   0.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   −.419940E−08   B   :   −.904030E−13   C   :   −.297400E−17   D   :   −.10634.0E−21            AC   :   100   BC   :   100   CC   :   100   DC   :   100            27:   INFINITY   35.031723           100   100           28:   1621.80000   33.000000       ‘CAF-UV’   100   100                 ASP   :                                                    K   :   0.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   0.155580E−07   B   :   −.854090E−12   C   :   0.123240E−16   D   :   −.559700E−21            AC   :   100   BC   :   100   CC   :   100   DC   :   100            29:   −747.60113   67.859320           100   100           30:   827.21786   27.000000       ‘CAF-UV’   100   100       31:   −1939.50000   20.227637           100   100       32:   197.25357   14.999969       ‘CAF-UV’   100   100       33:   128.31113   39.542169           100   100       34:   −1370.10000   24.000000       ‘CAF-UV   100   100                 ASP   :                                                    K   :   0.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   −.164770E−07   B   :   0.155510E−11   C   :   −.542070E−16   D   :   0.556740E−20            AC   :   100   BC   :   100   CC   :   100   DC   :   100            35:   −253.41246   18.476467           100   100           36:   109.90063   30.001392       ‘CAF-UV’   100   100       STO:   242.23740   22.529315           100   100       38:   −264.99438   46.219742       ‘CAF-UV’   100   100       39:   −372.29467   0.998929           100   100       40:   173.30822   24.000000       ‘CAF-UV’   100   100                 ASP   :                                                    K   :   0.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   0.628520E−07   B   :   −.915530E−11   C   :   −.628040E−15   D   :   −.946620E−19            AC   :   100   BC   :   100   CC   :   100   DC   :   100            41:   1411.60000   4.845900           100   100           42:   110.28842   22.740804       ‘CAF-UV’   100   100       43:   160.79657   13.371732           100   100       44:   69.10873   45.185600       ‘CAF-UV’   100   100       45:   −895.78799   11.999039           100   100                 ASP   :                                                    K   :   0.000000   KC   :   100            IC   :   YES   CUF   :   0.000000   CCF   :   100            A   :   −.113590E−06   B   :   0.281520E−09   C   :   −.171880E−12   D   :   0.507740E−16            AC   :   100   BC   :   100   CC   :   100   DC   :   100            IMG:   INFINITY   0.000000       100   100                SPECIFICATION DATA            NAO   −0.18750                   TEL       DIM   MN       WL   157.63   157.63   157.63       REF   2       WTW   1   1   1            XOB   0.00000   0.00000   0.00000   0.00000   0.00000           0.00000   0.00000       YOB   0.00000   26.51700   40.00000   53.03300   64.95100           70.15600   75.00000       WTF   0.00000   0.00000   1.00000   1.00000   1.00000           1.00000   1.00000       VUX   0.00000   −0.00138   −0.00308   −0.00534   −0.00803           −0.00941   −0.01082       VLX   0.00000   −0.00138   −0.00308   −0.00534   −0.00803           −0.00941   −0.01082       VUY   0.00000   −0.00065   −0.00224   −0.00398   −0.00520           −0.00531   −0.00535       VLY   0.00000   −0.00370   −0.00706   −0.01156   −0.01709           −0.01985   −0.02220            APERTURE DATA/EDGE DEFINITIONS                CA                APERTURE data not specified for surface Obj thru 46            PRIVATE CATALOG                PWL   157.63   157.63   157.63               ‘CAF-UV’   1.558411   1.558410   1.558409            REFRACTIVE INDICES                GLASS CODE       157.63   157.63   157.63           ‘CAF-UV’       1.558409   1.558410   1.558411            No solves defined in system       No pickups defined in system                INFINITE CONJUGATES                EFL   −66053.1391               BFL   −16500.9052           FFL   0.2642E+06           FNO   −0.0000                AT USED CONJUGATES                RED   −0.2500               FNO   0.6667           OBJ DIS   34.0000           TT   1198.5356           IMG DIS   11.9990           OAL   1152.5365                PARAXIAL IMAGE                HT   18.7496               THI   12.0008           ANG   0.0000                ENTRANCE PUPIL                DIA   0.3818E+10               THI   0.1000E+11                EXIT PUPIL                DIA   25217.8299               THI   −16501.3415                CODE V&gt; out t