Patent Number: 
Section: description

The illumination systems pursuant to the invention described below illuminate a segment of a ring field as shown in FIG. 1. An arc shaped field 11 in a reticle plane is imaged into a wafer plane by a projection objective. According to FIG. 1, the width of the arc shaped field 11 is xcex94r and the mean radius is R0. The arc shaped field extends over an angular range of 2xc2x7xcex10 and an arc of xe2x80x832xc2x7S0. The angle xcex10 is defined from the y-axis to the field edge, the arc length S0 is defined from the center of the field to the field edge along the arc at the mean radius R0. The scanning energy SE(x) at x is found to be the line integral over the intensity E(x,y) along the scan direction, which is the y-direction in this embodiment:       SE    ⁡          (      x      )        =            ∫              x        =        const                    xe2x80x83              ⁢                  E        ⁡                  (                      x            ,            y                    )                    ⁢              xe2x80x83            ⁢              ⅆ        y             in which E(x,y) is the intensity distribution in the x-y plane. Each point on the reticle or wafer contains the scanning energy SE(x) corresponding to its x coordinate. If uniform exposure is desired, it is advantageous for the scanning energy to be largely independent of x. In photolithography, it is desirable to have a uniform scanning energy distribution in the wafer plane. The resist on the wafer is very sensitive to the level of light striking the wafer plane. Preferably, each point on the wafer receives the same quantity of light or the same quantity of scanning energy. As described below, the scanning energy can be controlled by the design of the field lens group. By way of example, an EUV illumination system is shown in FIG. 2. In this embodiment a Laser-Produced-Plasma source 200 is used to generate the photons at xcex=13 nm. The light of the source is collected with an ellipsoidal mirror 21 and directed to a first mirror 22 comprising several rectangular mirror elements. The single mirror elements are called field facets, because they are imaged in an image plane 26 of the illumination system. In this embodiment the field facets are plane mirror elements in which each field facet is tilted by a different amount. The ellipsoidal mirror 21 images the light source 200 in an aperture stop plane 23. Due to the tilted field facets, the image of the light source is divided into several secondary light sources 201 so that the number of secondary light sources 201 depends on the number of tilted field facets. The secondary light sources 201 are imaged in an exit pupil 27 of the illumination system using a field mirror 24 and a field mirror 25. The location of the exit pupil 27 depends on the design of the projection objective, which is not shown in FIG. 2. In this embodiment, the field mirrors 24 and 25 are grazing incidence mirrors with a toroidal shape. The imaging of the field facets in the image plane 26 is influenced by the field mirrors 24 and 25. They introduce distortion to shape the arc shaped images of the rectangular field facets and to control the illumination intensity in the plane of the image plane 26, where a reticle is typically located. This will be further explained below. The tilt angles of the field facets are chosen to overlay the arc shaped images of the field facets at least partly in the image plane 26. The embodiment of FIG. 2 is only an example. The source is not limited to Laser-Produced-Plasma sources. Lasers for wavelength xe2x89xa6193 nm, Pinch Plasma sources, synchrotrons, wigglers or undulators for wavelength between 10-20 nm are also possible light sources. The collector unit is adapted for the angular and spatial characteristic of the different light sources. The illumination system does not need to be purely reflective. Catadioptric or dioptric components are also possible. FIG. 3 shows, in a schematic three-dimensional view, the imaging of one field facet 31 to an image plane 35. The beam path of this central field facet 31 located on the optical axis is representative of all other field facets. An incoming beam 300 is focused to a secondary light source 301 using the field facet 31. The field facet 31 is in this case a concave mirror element. The secondary light source 301 is spot-like if a point source is used. The beam diverges after the secondary light source 301. Without a field mirror 33 and a field mirror 34, the image of the rectangular field facet 31 would be rectangular. The imaging of the field facet 31 is distorted to produce an arc shaped field 302. The distortion is provided by the field mirrors 33 and 34. Two mirrors are necessary to produce the proper orientation of the arc. A reflected beam 303 is focused at the exit pupil of the illumination system using the field mirrors 33 and 34. The exit pupil is not shown in FIG. 3. The field mirrors 33 and 34 image the secondary light source 301 into the exit pupil. For real sources the secondary light source 301 is extended. To get a sharp image of the field facet 31 it is advantageous to image the field facet 31 into the image plane 35 using another mirror 32. The mirror 32 located at the secondary light source 301 is called a pupil facet and has a concave surface. Each secondary light source has such a pupil facet. FIG. 3 shows a light path for one pair of field facet 31 and pupil facet 32. In a case of a plurality of field facets 31, there is a corresponding number of pupil facets 32, which are located at the plane of the secondary light sources. In such a case, the plurality of mirror elements 32 forms another faceted mirror. The term xe2x80x9centenduexe2x80x9d refers to a phase-space volume of a light source. Pupil facets are necessary only for extended light sources, which have a high entendue value. In the case of a point source, the secondary light source is also a point, and a pupil facet would have no influence on the imaging. In FIG. 2 the source diameter of the Laser-Produced-Plasma source 200 is only 50 xcexcm, so the pupil facets are not required. For higher source diameters the mirror with the pupil facets is added at the aperture stop plane 23. To eliminate vignetting, the tilt angle of the mirror 22 with the field facets is increased. The field mirrors 24, 25, 33, 34 shown in FIGS. 2 and 3 form the arc shaped field 302, image the plane of the aperture stop 23 in the exit pupil plane 27 of the illumination system, and control the illumination distribution in the arc shaped field 302. As will be described in the following paragraphs, the imaging of the central field facet 31 shown in FIG. 3 is used to optimize the design of the field mirrors 33 and 34. The form of the images of other field facets is determined by a field lens group nearly in the same way as for the central field facet 31. Thus, the design of the field lens group, which in turn controls the scanning energy, can be optimized through the imaging of the central field facet 31. This facet can be considered as a homogeneously radiating surface. In the real system with all field facets homogeneity results from the superimposition of the images of all field facets. When optimizing the design of the field lens group, the goals include controlling the scanning energy, producing an arc shaped field, and imaging of the plane with secondary light sources to an exit pupil of the illumination system. The given components include a first mirror with field facets 31, a second mirror with pupil facets 32, a field lens group including mirror 33 and mirror 34, image plane 35 and an exit pupil plane (not shown in FIG. 3). The field lens group, in this case the shapes of mirror 33 and mirror 34, will be designed. Without the field lens group, the shape of the illuminated field in image plane 35 would be rectangular, the illuminated field would not be distorted, and there would be no pupil imaging. As a first step, the complexity of the process of designing the field lens group is reduced by considering the imaging of only the central field facet 31, rather than considering all of the facets. Facet 31 is imaged to image plane 35 using pupil facet 32. The design of the field lens group requires (1) controlling the scanning energy by introducing distortion perpendicular to the scanning direction, (2) producing an arc shaped field, and (3) imaging the secondary light sources 301 to the exit pupil of the illumination system. The field lens group only influences the field facet imaging by distorting this imaging. The main component of the field facet imaging is due to the pupil facet 32 (or to a camera obscura). As a second step, a simulation is constructed for all the field facets, the pupil facets and the field lens group designed in the first step. Normally, the field lens group influences the imaging of the other field facets in a manner similar to that of the imaging of the central field facet. If the imaging is not similar, the design of the field lens group must be corrected. Such corrections are typically small. A superimposition of the images of all field facets results in intensity homogeneity in the image plane. This is similar to the principle of a fly-eye integrator. Since the central field facet is representative of all field facets, design complexity is reduced by considering only the central field facet. To simulate the intensity distribution in the image plane only with a central light channel defined by field facet 31 and pupil facet 32, the central field facet 31 is regarded as a homogeneous radiating surface. FIG. 4 shows, schematically, an imaging of a rectangular field 41 on an arc shaped field 42 at an image plane of an illumination system. The rectangular field 41 can be a homogeneously radiating real or virtual surface in a plane conjugated to a reticle plane. FIG. 4 shows the correlation between rectangular field 41 and arc shaped field 42, and it also shows the orientation and definition of the coordinate system. The description of the scanning energy control, as set forth in the following pages, is independent of the design layout of the field facets or pupil facets. Accordingly, only a homogeneously radiating rectangular field is being considered. In FIG. 3, the rectangular field is given by central field facet 31. A length xw at the rectangular field 41 is imaged on an arc length s at the arc shaped field 42, and a length yw is imaged on a radial length r. The origin of the coordinate systems is the center of the field for the rectangular field 41 and the optical axis for the arc shaped field 42. When the field lens group consists of mirror(s) or lens(es) with anamorphotic power, for example toroidal mirrors or lenses, the image formation can be divided into two components xcex2s and xcex2rad: xcex2s:xwxe2x86x92s xcex2rad:ywxe2x86x92r wherein xcex2rad: radial imaging of yw on r xcex2s: azimuthal imaging of xw on s (xw, yw): horizontal and vertical coordinates of a field point on the rectangular field 41. (s,r): radial and azimuthal coordinates of a field point on the arc shaped field 42. Assuming a homogeneous intensity distribution Ew (x,y)=Ew0 in the x-y plane of the rectangular field, the intensity distribution Er (s,r) in the plane of the arc shaped field 42 is obtained by the influence of the field lens group. The index w below stands for the plane of the rectangular field, the index r below stands for the plane of the arc shaped field. If the azimuthal image formation xcex2s is free of distortion, the intensity distribution in the plane of the reticle is also homogeneous Er (x,y)=Er0. Since the scan path increases towards the edge of the field, the scanning energy SE(xr) in the plane of the reticle is a function of xr       SE    ⁡          (              x        r            )        =            E      r      0        ⁢                  ∫                  Scan          ⁢                      xe2x80x83                    ⁢          path          ⁢                      xe2x80x83                    ⁢          at          ⁢                      xe2x80x83                    ⁢                      x            r                              ⁢              ⅆ        y             The following equation applies:       SE    ⁡          (              x        r            )        =                    E        r        0            ⁢                        ∫                      Scan            ⁢                          xe2x80x83                        ⁢            path            ⁢                          xe2x80x83                        ⁢            at            ⁢                          xe2x80x83                        ⁢                          x              r                                      ⁢                  ⅆ          y                      =                  E        r        0            ·              (                                                                              (                                                            R                      0                                        +                                                                  Δ                        ⁢                                                  xe2x80x83                                                ⁢                        r                                            2                                                        )                                2                            -                              x                r                2                                              -                                                                      (                                                            R                      0                                        -                                                                  Δ                        ⁢                                                  xe2x80x83                                                ⁢                        r                                            2                                                        )                                2                            -                              x                r                2                                                    )             For xcex94r less than R0 and xr less than R0, this equation can be expanded in a Taylor series, which is discontinued after the first order:       SE    ⁡          (              x        r            )        =                    E        r        0            ⁢                        ∫                      Scan            ⁢                          xe2x80x83                        ⁢            path            ⁢                          xe2x80x83                        ⁢            at            ⁢                          xe2x80x83                        ⁢                          x              r                                      ⁢                  ⅆ          y                      =                  E        r        0            ·              1                              1            -                                          (                                                      x                    r                                                        R                    0                                                  )                            2                                           The following parameters can be assumed for the arc shaped field 42 by way of example: R0=100.0 mm xcex94r=6.0 mm; xe2x88x923.0 mmxe2x89xa6rxe2x89xa63.0 mm xcex10=30xc2x0 With homogeneous intensity distribution Er0 the scanning energy SE(xr) rises at the edge of the field xr=50.0 mm, to SE (xr=50.0 mm)=1.15xc2x7SE (xr=0.0)=SEmax. The uniformity error produced is thus       Uniformity    ⁢          xe2x80x83        [    %    ]    =            100      ⁢              %        ·                                            SE              max                        -                          SE              min                                                          SE              max                        +                          SE              min                                            =          7.2      ⁢              %        .             The maximum scanning energy SEmax is obtained at the field edge (xr=50.0 mm), the minimum scanning energy SEmin is obtained at the center of the field (xr=0.0). With R0=200.0 mm xcex94r=6.0 mm; xe2x88x923.0 mmxe2x89xa6rxe2x89xa63.0 mm xcex10=14.5xc2x0 we obtain SE (xr=50.0)=1.03xc2x7SE (xr=0.0). The uniformity error produced is thus       Uniformity    ⁢          xe2x80x83        [    %    ]    =            100      ⁢              %        ·                                            SE              max                        -                          SE              min                                                          SE              max                        +                          SE              min                                            =          1.6      ⁢              %        .             The rise of the scanning energy toward the edge of the field is considerably smaller for larger radius R0 of the arc shaped field 42 and smaller arc angles xcex10. The uniformity can be substantially improved pursuant to the invention if the field lens group is designed so that the image formation in the plane of the reticle is distorted azimuthally, i.e., a location-dependent magnification is introduced.             β      S        ⁡          (              x        W            )        =      s          x      W       It is generally true that the intensity of irradiation E is defined as the quotient of the radiation flux d"PHgr" divided by the area element dA struck by the radiation flux, thus:   E  =            d      ⁢              xe2x80x83            ⁢      Φ              d      ⁢              xe2x80x83            ⁢      A       The area element for the case of the arc shaped field is given by A=dsxc2x7dr ds: arc increment. dr: radial increment. If the azimuthal image formation is distorted, the distorted intensity Erv in the plane of the reticle behaves as the reciprocal of the quotient of the distorted arc increment dsv divided by the undistorted arc increment dsv=0:             E      r      V              E      r              V        =        0              =                    dr        ·                  ds                      V            =            0                                      dr        ·                  ds          V                      =          1                        ds          V                          ds                      V            =            0                               Since with undistorted image formation the arc increment dsv=0 is proportional to the x-increment dxw at the rectangular field 41 dsv=0xe2x88x9ddxw, it follows that       E    r    V    ∝      1                  ds        V                    dx        w             The intensity Erv(xr) in the plane of the reticle can be controlled by varying the quotient             ds      V              dx      w        . The relationship between scanning energy SE(xr) and azimuthal imaging magnification xcex2s is to be derived as follows:       SE    ⁡          (              x        r            )        =            ∫              Scan        ⁢                  xe2x80x83                ⁢        path        ⁢                  xe2x80x83                ⁢        at        ⁢                  xe2x80x83                ⁢                  x          r                            xe2x80x83              ⁢                  E        ⁡                  (                                    x              r                        ,                          y              r                                )                    ⁢              xe2x80x83            ⁢              ⅆ        y             The intensity E (xr,yr) can be written as the product of the functions g(r) and f(s). The function g(r) is only dependent on the radial direction r, the function f(s) is only dependent on the azimuthal extent s: E (xr,yr)=g (r)xc2x7f (s). For xcex94r less than R and xcex94r less than xr, g(r) should be independent of the x-position xr in the plane of the reticle and f(s) should be independent of the y-position yr in the plane of the reticle. Since s and xr, from       sin    ⁡          (              s                  R          0                    )        =            x      r              R      0       are directly coupled to one another, SE(xr) can also be written as a function of s:       SE    ⁡          (      s      )        =            ∫              Scan        ⁢                  xe2x80x83                ⁢        path        ⁢                  xe2x80x83                ⁢        at        ⁢                  xe2x80x83                ⁢                  s          (                      x            r                    )                      ⁢                            f          ⁡                      (            s            )                          ·                  g          ⁡                      (            r            )                              ⁢              ⅆ        y             Since f(s) is independent of yr, it follows that:       SE    ⁡          (      s      )        =            f      ⁡              (        s        )              ·                  ∫                  Scan          ⁢                      xe2x80x83                    ⁢          path          ⁢                      xe2x80x83                    ⁢          at          ⁢                      xe2x80x83                    ⁢          s                    ⁢                        g          ⁡                      (            r            )                          ⁢                  ⅆ          y                     and since       dr          dy      r        =      cos    ⁡          (              s                  R          0                    )       then:       SE    ⁡          (      s      )        =            f      ⁡              (        s        )              ·          1              cos        ⁡                  (                      s                          R              0                                )                      ·                  ∫                              -            Δ                    ⁢                      xe2x80x83                    ⁢          r                                      +            Δ                    ⁢                      xe2x80x83                    ⁢          r                    ⁢                        g          ⁡                      (            r            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          r                     The derivation of the distorted intensity ErV has shown the following proportionality for the function f(s):       f    ⁡          (      s      )        ∝      1                  ⅆ        s                    ⅆ                  x          w                     Since       ∫                  -        Δ            ⁢              xe2x80x83            ⁢      r                      +        Δ            ⁢              xe2x80x83            ⁢      r        ⁢            g      ⁡              (        r        )              ⁢          xe2x80x83        ⁢          ⅆ      r       is independent of s, it follow that:       SE    ⁡          (      s      )        ∝      1                            ⅆ          s                          ⅆ                      x            w                              ·              cos        ⁡                  (                      s                          R              0                                )                     Considering the coupling of s and xr, it follows that       SE    ⁡          (              x        r            )        ∝      1                  ⅆ                  x          r                            ⅆ                  x          w                     From the quotient       ⅆ          x      r            ⅆ          x      w       the scanning energy can thus be set directly, with xr being the x-component of a field point on the arc shaped field 42 and xw being the x-component of a field point on the rectangular field 41. From a given curve of scanning energy SE(xr) or SE(s) in the plane of the reticle, the azimuthal imaging magnification xcex2s can be calculated with these formulas.             SE      ⁡              (        s        )              =          c      ·              1                                            ⅆ              s                                      ⅆ                              x                w                                              ·                      cos            ⁡                          (                              s                                  R                  0                                            )                                                              ⅆ        s                    ⅆ                  x          w                      =          c      ·              1                              SE            ⁡                          (              s              )                                ·                      cos            ⁡                          (                              s                                  R                  0                                            )                                                      x      w        =                  c        xe2x80x2            ·                        ∫          0          s                ⁢                                            SE              ⁡                              (                                  s                  xe2x80x2                                )                                      ·                          cos              ⁡                              (                                                      s                    xe2x80x2                                                        R                    0                                                  )                                              ⁢                      xe2x80x83                    ⁢                      ⅆ                          s              xe2x80x2                                           The constant cxe2x80x2 is obtained from the boundary condition that the edge of the rectangular field 41 at xwMax has to be imaged on the edge of the arc shaped field at Smax=S0. s(xw), and therefore the imaging magnification xcex2s (xw), is consequently known as a function of xw:       β    s    =                    β        s            ⁡              (                  x          w                )              =                  s        ⁡                  (                      x            w                    )                            x        w             The aforementioned equation for the azimuthal magnification xcex2s is to be solved by way of example for constant scanning energy SE(xr) in the plane of the reticle. For constant scanning energy SE0 in the plane of the reticle, the azimuthal imaging magnification is derived as follows:             x      w        =                            c          xe2x80x2                ·                              ∫            0            s                    ⁢                                                    SE                0                            ·                              cos                ⁡                                  (                                                            s                      xe2x80x2                                                              R                      0                                                        )                                                      ⁢                          xe2x80x83                        ⁢                          ⅆ                              s                xe2x80x2                                                        =                        c          xe2x80x3                ·                              ∫            0            s                    ⁢                                    cos              ⁡                              (                                                      s                    xe2x80x2                                                        R                    0                                                  )                                      ⁢                          ⅆ                              s                xe2x80x2                                                                    x      w        =                            c          xe2x80x3                ·                              [                          sin              ⁡                              (                                                      s                    xe2x80x2                                                        R                    0                                                  )                                      ]                    0          s                    =                        c          xe2x80x3                ·                  sin          ⁡                      (                          s                              R                0                                      )                                          s      ⁡              (                  x          w                )              =                            R          0                ·        a            ⁢              xe2x80x83            ⁢              sin        ⁡                  (                                    x              w                                      c              xe2x80x3                                )                     and thus             β      s        ⁡          (              x        w            )        =            R      0        ·                  a        ⁢                  xe2x80x83                ⁢                  sin          ⁡                      (                                          x                w                                            c                xe2x80x3                                      )                                      x        w             An illumination system will be considered below with: Rectangular field 41 in a plane conjugated to the plane of the reticle: xe2x88x928.75 mmxe2x89xa6xwxe2x89xa68.75 mm xe2x88x920.5 mmxe2x89xa6ywxe2x89xa60.5 mm Arc shaped field 42 in the plane of the reticle: xe2x88x9252.5 mmxe2x89xa6sxe2x89xa652.5 mm xe2x88x923.0 mmxe2x89xa6rxe2x89xa63.0 mm With the boundary condition s(xw=xe2x88x928.75)=52.5 mm the constant cxe2x80x3 is obtained as follows: cxe2x80x3=954.983, and thus       β    s    =            R      0        ·                  a        ⁢                  xe2x80x83                ⁢                  sin          ⁡                      (                                          x                w                            954.983                        )                                      x        w             If the design of the field lens group generates this curve of the azimuthal imaging magnification, then a constant scanning energy is obtained in the plane of the reticle for the system defined above by way of example. With variation of the azimuthal magnification xcex2s, it is necessary for use in lithographic systems to consider that the field lens group, in addition to field formation, also determines the imaging of the secondary light sources, or the aperture stop plane, into the entrance pupil of the projection objective. This as well as the geometric boundary conditions does not permit an arbitrarily large distortion correction. The previously described uniformity correction is not restricted to the illumination system with a faceted mirror described by way of example, but can be used in general. By distorting the image formation in the reticle plane perpendicular to the scanning direction the intensity distribution, and thus the scanning energy distribution, can be controlled. Typically, the illumination system contains a real or virtual plane with secondary light sources. This is always the case, in particular, with Kxc3x6hler illumination systems. The aforementioned real or virtual plane is imaged in the entrance pupil of the objective using the field lens group, with the arc shaped field being produced in the pupil plane of this image formation. The pupil plane of the pupil imaging is, in this case, the plane of the reticle. Some examples of embodiment of illumination systems will be described below, where the distribution of scanning energy is controlled by the design of the field lens group. The general layout of the illumination systems is shown in FIG. 2. The optical data of the illumination system are summarized in table 1. The illumination system of FIG. 2 and Table 1 is optimized for a Laser-Produced-Plasma source 200 at xcex=13 nm with a source diameter of 50 xcexcm. The solid angle xcexa9 of the collected radiation is xcexa9≈2xcfx80. The mirror 22 with field facets has a diameter of 70.0 mm, and the plane field facets have a rectangular shape with x-y dimensions of 17.5 mmxc3x971.0 mm. The mirror 22 consists of 220 field facets. Each facet is tilted relative to the local x- and y-axis to overlay the images of the field facets at least partly in the image plane 26. The field facets at the edge of mirror 22 have the largest tilt angles in the order of 6xc2x0. The mirror 22 is tilted by the angle xcex1x=7.3xc2x0 to bend the optical axis by 14.6xc2x0. The aperture stop plane 23 in this example is not accessible. The first and second field mirrors 24 and 25 are grazing incidence mirrors. Each of them bends the optical axis by 160xc2x0. The field mirror 24 is a concave mirror, and the mirror 25 is a convex mirror. They are optimized to control the illumination intensity, the field shaping and the pupil imaging. In the following embodiments only these two mirrors will be replaced. Their position and tilt angle will always be the same. It will be shown, that by modifying the surface shape, it is possible to change the intensity distribution while keeping the pupil imaging and the field shaping in tolerance. The arc shaped field in the plane of the reticle 26 can be described by R0=100.0 mm xcex94r=6.0 mm; xe2x88x923.0 mmxe2x89xa6rxe2x89xa63.0 mm xcex10=30xc2x0 The reticle 26 is tilted by xcex1x=2.97xc2x0 in respect to the optical axis. The position of the exit pupil 27 of the illumination system is defined by the given design of the projection objective. A notable feature of the present invention is the asphericity of the mirror surfaces that provide a favorable uniformity of scanning energy on the one hand, and on the other hand a favorable telecentricity. While the asphericity of the mirror surfaces will be varied, the tilt angles and spacing of the mirrors are to be kept constant. The following examples are presented and compared with reference to the following parameters:       Uniformity    ⁢          xe2x80x83        [    %    ]    =      100    ⁢          %      ·                                    SE            max                    -                      SE            min                                                SE            max                    +                      SE            min                               SEmam: maximum scanning energy in the illuminated field. SEmin: minimum scanning energy in the illuminated field. maximum telecentricity error xcex94imax over the illuminated field in the reticle plane xcex94imax=[iactxe2x88x92iref]max in [mrad] iact: angle of a centroid ray with respect to the plane of the reticle at a field point. iref: angle of a chief ray of the projection objective with respect to the plane of the reticle at the same field point. The maximum telecentricity error xcex94imax will be calculated for each field point in the illuminated field. The direction of the centroid ray is influenced by the source characteristics and the design of the illumination system. The direction of the chief ray of the projection objective in the plane of the reticle depends only on the design of the projection objective. Typically the chief rays hit the wafer plane telecentrically. To get the telecentricity error in the wafer plane the telecentricity error in the reticle plane has to be divided by the magnification of the projection objective. Typically the projection objective is a reduction objective with a magnification of xcex2=xe2x88x920.25, and therefore the telecentricity error in the wafer plane is four times the telecentricity error in the reticle plane. geometric parameters of the first field mirror: Rx, Ry Kx, Ky  geometric parameters of the second field mirror: Rx, Ry Kx, Ky  Both field mirrors are toroidal mirrors with surface parameters defined in the x- and y-direction. R describes the Radius, K the conical constant. It is also possible to vary higher aspherical constants, but in the examples shown below only the radii and conical constants will be varied. 1st Example of Embodiment: For field mirrors with purely spherical x and y cross sections, the following characteristics are obtained: Uniformity=10.7% xcex94imax=0.24 mrad Field mirror 1: Rx=xe2x88x92290.18, Ry=xe2x88x928391.89, Kx=0.0, Ky=0.0 Field mirror 2: Rx=xe2x88x921494.60, Ry=xe2x88x9224635.09, Kx=0.0, Ky=0.0 The curve of the scanning energy over the x direction in the plane of the reticle is plotted in FIG. 5 as a solid line 51. Because the system is symmetric to the y-axis, only the positive part of the curve is shown. The scanning energy is normalized at the center of the field at 100%. The scanning energy rises toward the edge of the field to 124%. The calculation takes into consideration only the imaging of one representative field facet, in this case the central field facet, which is assumed to be a homogenous radiating surface. However, this relationship is also maintained for the entire system, as shown by the result for all of the field facets in FIG. 6. The curves of FIG. 6 are the result of a simulation with a Laser-produced-Plasma source 200 and the whole illumination system according to FIG. 2. The solid line 61 represents the scanning energy for toroidal field mirrors of the 1st embodiment without conic constants. A comparison of the solid lines or the dashed lines of FIG. 5 and FIG. 6 shows similar characteristics, that is they are almost identical. The curves in FIG. 5 were calculated (1) by considering only a homogeneously radiating rectangular field, i.e., the central field facet, and (2) the Taylor series was discontinued after the first series. However, the curves in FIG. 6 are a result of a simulation with the real illumination system. It is apparent from a comparison of the curves of FIG. 5 and FIG. 6 that the theoretical model can be used to predict scanning energy distribution, including that of a multifaceted system, and that the following approximations are possible: Reduction of the problem to the imaging of a rectangular field, in this case the central field facet. xcex94r less than R: Discontinuation of the Taylor series after the first order. Systems comprising toroidal field mirrors in which the conic constants can be varied and in which the field mirrors are post-optimized, with their tilt angle and their position being retained, will be presented below. 2nd Example of Embodiment: Uniformity=2.7% xcex94imax=1.77 mrad Field mirror 1: Rx=xe2x88x92275.24, Ry=xe2x88x927347.29, Kx=xe2x88x923.813, Ky=xe2x88x92385.81 Field mirror 2: Rx=1067.99, Ry=14032.72, Kx=667.20, Ky=xe2x88x9225452.70 The dashed curve 52 in FIG. 5 shows the curve of scanning energy expected from the design for the central field facet; the curve scanning energy obtained with the entire system of all of the field facets is shown as dashed curve 62 in FIG. 6. The improvement of the scanning uniformity is obvious using the conical constants in the design of the field mirrors. The necessary surface corrections on the two field mirrors 24 and 25 of FIG. 2 are shown in the illustrations of FIG. 7 and FIG. 8 as contour plots. The mirrors are bounded according to the illuminated regions on the mirrors. The bounding lines are shown as reference 71 in FIG. 7 and reference 81 in FIG. 8. The contour plots show the sagitta differences of the surfaces of the first and second embodiment in millimeters. For the first field mirror 24 the maximum sagitta difference is on the order of magnitude of 0.4 mm in FIG. 7. There is also a sign reversal of the sagitta differences. For the second field mirror 25 the maximum sagitta difference is on the order of magnitude of 0.1 mm in FIG. 8. The second embodiment was optimized to get the best improvement of the scanning uniformity accepting an arising telecentricity error. The telecentricity violation of 1.77 mrad in the reticle plane of the second embodiment is problematic for a lithographic system. The following examples demonstrate embodiments in which the maximum telecentricity violation in the plane of the reticle is less or equal 1.0 mrad. The design shown in the example of embodiment 1 is the starting point for the design of the field mirrors in the following examples. In each example, different sets of surface parameters have been optimized. 3rd Example of Embodiment: Optimized parameters Rx1st mirror, Ry1st mirror, Kx1st mirror, Ky1st mirror, Rx2nd mirror,Ry2nd mirror, Kx2nd mirror, Ky2nd mirror. Uniformity=4.6% xcex94Imax=1.00 mrad Field mirror 1: Rx=xe2x88x92282.72, Ry=xe2x88x927691.08, Kx=xe2x88x922.754, Ky=xe2x88x92474.838 Field mirror 2: Rx=1253.83, Ry=16826.99, Kx=xe2x88x92572.635, Ky=xe2x88x9232783.857 4th Example of Embodiment: Optimized parameters Rx1st mirror, Kx1st mirror, Ky1st mirror, Rx2nd mirror, Kx2nd mirror, Ky2nd mirror. Uniformity=5.1% xcex94imax=1.00 mrad Field mirror 1: Rx=xe2x88x92285.23, Ry=xe2x88x928391.89, Ky=xe2x88x922.426, Ky=xe2x88x92385.801 Field mirror 2: Rx=1324.42, Ry=24635.09, Kx=xe2x88x92568.266, Ky=xe2x88x9231621.360 5th Example of Embodiment: Optimized parameters Rx1st mirror, Kx1st mirror, Rx2nd mirror, Kx2nd mirror. Uniformity=5.1% xcex94imax=1.00 mrad Field mirror 1: Rx=xe2x88x92280.08, Ry=xe2x88x928391.89, Kx=xe2x88x922.350, Ky=0.0 Field mirror 2: Rx=1181.53, Ry=24635.09, Kx=xe2x88x92475.26, Ky=0.0 6th Example of Embodiment: Optimized parameters Kx1st mirror, Ky1st mirror, Kx2nd mirror, Ky2nd mirror. Uniformity=6.0% xcex94imax=1.00 mrad Field mirror 1: Rx=xe2x88x92290.18, Ry=xe2x88x928391.89, Kx=xe2x88x922.069, Ky=xe2x88x92290.182 Field mirror 2: Rx=1494.60, Ry=24635.09, Kx=xe2x88x92503.171, Ky=xe2x88x921494.602 7th Example of Embodiment: Optimized parameters Kx1st mirror,Kx2nd mirror. Uniformity=7.0% xcex94imax=1.00 mrad Field mirror 1: Rx=xe2x88x92290.18, Ry=xe2x88x928391.89, Kx=xe2x88x921.137, Ky=0.0 Field mirror 2: Rx=1494.60, Ry=24635.09, Kx=xe2x88x92305.384, Ky=0.0 8th Example of Embodiment: Optimized parameters Rx1st mirror, Ry1st mirror, Kx1st mirror, Ky1st mirror. Uniformity=7.8% xcex94imax=1.00 mrad Field mirror 1: Rx=xe2x88x92288.65, Ry=xe2x88x928466.58, Kx=xe2x88x920.566, Ky=139.337 Field mirror 2: Rx=1494.60, Ry=24635.09, Kx=0.0, Ky=0.0 9th Example of Embodiment: Optimized parameters Rx1st mirror, Kx1st mirror, Ky1st mirror. Uniformity=7.8% xcex94imax=1.00 mrad Field mirror 1: Rx=xe2x88x92288.59, Ry=xe2x88x928391.89, Kx=xe2x88x920.580, Ky=111.346 Field mirror 2: Rx=1494.60, Ry=24635.09, Kx=0.0, Ky=0.0 10th Example of Embodiment: Optimized parameters Rx1st mirror, Kx1st mirror. Uniformity=8.1% xcex94imax=1.00 mrad Field mirror 1: Rx=xe2x88x92288.45, Ry=xe2x88x928391.89, Kx=xe2x88x920.574, Ky=0.0 Field mirror 2: Rx=1494.60, Ry=24635.09, Kx=0.0, Ky=0.0 11th Example of Embodiment: Optimized parameters Kx1st mirror, Ky1st mirror. Uniformity=8.5% xcex94imax=1.00 mrad Field mirror 1: Rx=xe2x88x92290.18, Ry=xe2x88x928391.89, Kx=xe2x88x920.304, Ky=xe2x88x92290.182 Field mirror 2: Rx=1494.60, Ry=24635.09, Kx=0.0, Ky=0.0 12th Example of Embodiment: Optimized parameter Kx1st mirror. Uniformity=8.6% xcex94imax=1.00 mrad Field mirror 1: Rx=xe2x88x92290.18, Ry=xe2x88x928391.89, Kx=xe2x88x920.367, Ky=0.0 Field mirror 2: Rx=1494.60, Ry=24635.09, Kx=0.0, Ky=0.0 The results for the various examples of embodiment are summarized in Table 2, with the optimized parameters designated with an xe2x80x9cxxe2x80x9d. Table 2 shows that field mirror 1 and field mirror 2 improve the scanning uniformity to almost the same extent, with the principal fraction of this being carried by the x parameters, which ultimately determine the azimuthal magnification scale xcex2s. While only static correction of uniformity was examined with the exemplary embodiments described so far, in which essentially only the surface was xe2x80x9cwarpedxe2x80x9d, an active variant of the invention will be described below. Actuation in this case can occur by means of mechanical actuators. A possible actuator can be a piezo-element at the rear side of a field mirror to vary the shape of the mirror by changing the voltage to the piezo-element. As stated above, great improvements of uniformity can be produced even when only the x surface parameters are changed. If only the conic constants in the x direction are varied, the sagitta differences have the same algebraic sign over the entire surface, which is advantageous for the surface manipulation. FIG. 9 and FIG. 10 show the sagitta differences between the field mirrors of embodiment #6 and embodiment #1. The conic constants in the x direction were varied here for field mirror 1 and 2. The maximum sagitta differences are 250 xcexcm for the first field mirror 24 and 100 xcexcm for the second field mirror 25. Uniformity is improved from 10.7% to 7.0% with an additional telecentricity violation of 1.0 mrad in the plane of the reticle. This telecentricity violation corresponds to 4.0 mrad in the plane of the wafer, if the projection objective has a magnification of xcex2=xe2x88x920.25. Accordingly the uniformity of scanning energy can be corrected by xc2x13.7% by active manipulation on the mirrors of the field lens group. When only the conic constants in the x direction are varied, the sagitta changes depend almost only on x. The lines with the same sagitta difference are nearly parallel to the y-axis, which is, in this example, the scanning direction. The sagitta distribution pfhref of the reference surfaces (1st embodiment) of the field mirrors can be described by:             pfh      ref        ⁡          (              x        ,        y            )        =                              1                      R            x                          ·                  x          2                    +                        1                      R            y                          ·                  y          2                            1      +                        1          -                                                    (                                  1                                      R                    x                                                  )                            2                        ·                          x              2                                -                                                    (                                  1                                      R                    y                                                  )                            2                        ·                          y              2                                           x and y are the mirror coordinates in the local coordinate system of the mirror surface. Rx and Ry are the radii of the toroidal mirror. The sagitta distribution pfhact of the manipulated surfaces of the field mirrors can be described by:             pfh      ref        ⁡          (              x        ,        y            )        =                              1                      R            x                          ·                  x          2                    +                        1                      R            y                          ·                  y          2                            1      +                        1          -                                    (                              1                +                                  K                  x                                            )                        ·                                          (                                  1                                      R                    x                                                  )                            2                        ·                          x              2                                -                                    (                              1                +                                  K                  y                                            )                        ·                                          (                                  1                                      R                    y                                                  )                            2                        ·                          y              2                                           Kx and Ky are the conical constants. For the sagitta difference xcex94pfh, this gives: xcex94pfh(x,y)=Pfhact(x,y)xe2x88x92pfhref(x,y) In Embodiment #1: Field mirror 1: Rx=xe2x88x92290.18, Ry=xe2x88x928391.89, Kx=0.0, Ky=0.0 Field mirror 2: Rx=xe2x88x921494.60, Ry=xe2x88x9224635.09, Kx=0.0, Ky=0.0 In Embodiment #6: Field mirror 1: Rx=xe2x88x92290.18, Ry=xe2x88x928391.89, Kx=xe2x88x921.137, Ky=0.0 Field mirror 2: Rx=1494.60, Ry=24635.09, Kx=xe2x88x92305.384, Ky=0.0 Preferably, the actuators or mechanical regulators are placed on the mirrors on equipotential lines 92, 102 (sites of equal sagitta difference). In the example of embodiment #6, these rows of identical actuators run almost parallel to the y axis, and therefore, it is unnecessary to control a two-dimensional field of actuators, but it suffices to control only a row of different actuator banks. For example, on the second field mirror an arrangement of actuator rows can be proposed as shown in FIG. 11. The second field mirror is shown in the plan view (x-y-view) at the top and side view (x-z-view) at the bottom of FIG. 11. In the plan view the actuator beams 5xe2x80x2, 4xe2x80x2, 3xe2x80x2, 2xe2x80x2, 1xe2x80x2, 0, 1, 2, 3, 4, 5 are arranged along equipotential lines. Because of the symmetry regarding the y-axis the corresponding actuator beams 5 and 5xe2x80x2, or 4 and 4xe2x80x2, or 3 and 3xe2x80x2, or 2 and 2xe2x80x2, or 1 and 1xe2x80x2 can be activated with the same signal. The actuators in the plan view are represented by lines, and in the side view by arrows. An industrial implementation would be to design the entire row of actuators as actuator beams 5xe2x80x2, 4xe2x80x2, 3xe2x80x2, 2xe2x80x2, 1xe2x80x2, 0, 1, 2, 3, 4, 5. When the beam is actuated, the entire row of actuators is raised or lowered. The distances between the actuator beams can be chosen dependent on the gradient of the sagitta differences. For high values of the gradient a dense arrangement of the actuator beams is necessary, for low values of the gradient the distances can be increased. In the example of FIG. 10 the gradient of the sagitta differences is high at the edges of the illuminated field, so more actuator beams are at the edge of the field than in the center as shown in FIG. 11. An active correction of uniformity can be accomplished as follows using the actuators described above. The curve of scanning energy SEstandard(xr) in the plane of the reticle is established based on the geometric design of the field lens group. Now the scanning energy SEwafer(xwafer) in the plane of the wafer is measured, including all coating, absorption, and vignetting effects. For the lithographic process, SEwafer(xwafer) has to be independent of the x-position xw in the plane of the wafer. If this is not the case, the xw-dependent offset has to be addressed by the illumination system. Since the imaging of the reticle plane to the wafer plane is almost ideal imaging, SEwafer(xwafer) can be converted directly into the plane of the reticle SEwafer(xr) using the given magnification of the projection objective. If the design reference SEstandard(xr) and the measured distribution SEwafer(xr) are normalized at 100% for xr=0.0, then the necessary correction of the surfaces of the field mirrors can be calculated from the difference SEDesakt(xr): SEDesakt(xr)=SEwafer(xr)xe2x88x92SEStandard (xr SEDesakt(xr) determines the azimuthal magnification xcex2s, and from this the necessary corrections for the field lens group. If there is a difference SEDesakt(xr) between the target SEstandard(xr) and actual values SEwafer(xr) due to time-dependent or illumination setting-dependent effects for example, the uniformity of the scanning energy can be corrected by the actuators described above within certain limits. Up to xc2x12.5% uniformity can be corrected with one manipulable field mirror, and up to xc2x15.0% with two manipulable field mirrors. In case of static deviations, e.g., deviations from coating effects, absorption effects, etc., which are known in the design phase, these effects can be taken into consideration in a modified field lens group design, and correction with actuators is then unnecessary. Intensity loss-free control of scanning energy is achieved by the present invention, where the field-dependent scan path, the coating, absorption, and vignetting effects, if known, can be taken into account in the static design of the field lens group. Furthermore, the invention proposes dynamic control with active field mirrors for time-dependent or illumination setting-dependent effects. If a telecentricity error of xc2x14.0 mrad is allowed in the plane of the wafer, the uniformity correction can be up to xc2x15%. In FIG. 12 a projection exposure system comprising an Laser-Produced-Plasma source as light source 120, an illumination system 121 corresponding to the invention, a mask 122, also known as a reticle, a positioning system 123, a projection objective 124 and a wafer 125 to be exposed on a positioning table 126 is shown. The projection objective 124 for EUV lithography is typically a mirror system with an even number of mirrors to have reticle and wafer on different sides of the projection objective 124. Detection units in a reticle plane 128 and in a wafer plane 129 are provided to measure the intensity distribution inside the illuminated field. The measured data are transferred to a computation unit 127. With the measured data the scanning energy and scanning uniformity can be evaluated. If there is a difference between the predetermined and the measured intensity distribution, the surface corrections are computed. The actuator drives 130 at one of the field mirrors are triggered to manipulate the mirror surface. It should be understood that various alternatives and modifications could be devised by those skilled in the art. The present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.