Patent Number: 
Section: description

A schematic drawing showing the principal components for deriving the formulas for defocusing by changing the refractive power of the first raster elements is shown in FIG. 1. As is shown in FIG. 1, the first raster elements have a positive optical power, thus providing a convergent light bundle with a focal point in the plane of the images of the light sources. FIG. 1 is shown in refractive representation. A person skilled in the art can transfer the parameters to the reflective optics necessary for exposure systems with wavelengths xe2x89xa6193 nm, particularly for EUV lithograpy, without inventive activity. The principle components of the system shown in FIG. 1 include a light source 100, a first raster element, e.g., field raster element 1, and a second raster element, e.g., pupil raster element 3. If light source 100 has a diameter X, and if pupil raster element 3 has a diameter D and is filled with illumination to a ratio xcexa, then the distance between field raster element 1 and pupil raster element 3 must be selected suitably as L=sxe2x80x2+xcex94z. The following is valid:                     β        =                                            s              xe2x80x2                        s                    =                                                    X                xe2x80x2                            X                        =                                          sin                ⁢                                  xe2x80x83                                ⁢                σ                                            sin                ⁢                                  xe2x80x83                                ⁢                                  σ                  xe2x80x2                                                                                        (        1        )             Thus the size of field raster element 1 is determined by the magnification of the imaging system arranged in the light path after field raster element 1. For example, assuming field raster element 1 is of a size FXxc3x97FY=2.8 mmxc3x9746 mm, it follows for an angle of aperture "sgr", as shown in FIG. 1:                               tan          ⁢                      xe2x80x83                    ⁢                      σ            x                          =                                            FX                              2                ⁢                s                                      ⁢            and            ⁢                          xe2x80x83                        ⁢            tan            ⁢                          xe2x80x83                        ⁢                          σ              y                                =                      FY                          2              ⁢              s                                                          (        2        )             xcex2: magnification of the field raster elements FX: width of the field raster elements FY: length of the field raster elements s: distance between the object, e.g. the light source and the field raster elements, i.e. object intersection distance sxe2x80x2: distance between the field raster elements and the image of the object, e.g. the image of the light source, i.e. image intersection distance "sgr": angle of aperture For the further derivation, only the y-component is necessary, since the y-component has a large divergence angle. In addition, tan "sgr" can be set approximately equal to sin "sgr". The refractive power of field raster element 1 can be determined as follows, based on the definition of xcex2. The known imaging equation of the first order reads:                               p          1                =                                            -                              1                f                                      ⁢                          x              0                                +                                    1              β                        ⁢                          p              0                                                          (        3        )             wherein: p1: optical direction cosine in the image space of the field raster element p2: optical direction cosine in the object space of the field raster element f: focal length of the field raster element xcex2: magnification of the field raster element. The object-side optical direction cosine p0 of the upper aperture beam of the upper light source edge X/2 is given by:                               p          0                =                                            sin              ⁢                              xe2x80x83                            ⁢              α                        ≈                          tan              ⁢                              xe2x80x83                            ⁢              α                                =                      -                                          (                                  FY                  -                  X                                )                                            2                ⁢                s                                                                        (        4        )             and the image-side optical direction cosine p1 of the assigned lower edge beam of the image of the upper edge of the light source is given by:                               p          1                =                              sin            ⁢                          xe2x80x83                        ⁢                          α              xe2x80x2                                =                                    tan              ⁢                              xe2x80x83                            ⁢                              α                xe2x80x2                                      =                          -                                                (                                      FY                    +                                          κ                      ·                      D                                                        )                                                  2                  ⁢                  L                                                                                        (        5        )             If s, L, D and X are given, then the refractive power of the field raster element is determined as follows. With the magnification as a function of focal length and distance to the vertex of the surface:                               β          =                      f                          f              +              s                                      ,                            (        6        )                                          p          1                =                                            -                              1                f                                      ⁢                          x              0                                +                                                    f                +                s                            f                        ⁢                                          p                0                            .                                                          (        7        )             Solving for f and inserting Eq. (4) and (5), with x0=X/2, the following results:                     f        =                                            FY              ·              s              ·              L                                                                        (                                      FY                    +                                          κ                      ⁢                                              xe2x80x83                                            ⁢                      D                                                        )                                ⁢                s                            -                                                (                                      FY                    -                    X                                    )                                ⁢                L                                              .                                    (        8        )             With the values: s=1200 mm for the object intersection distance; L=900 mm for the distance between the pupil raster element and the field raster element; D=10 mm for the diameter of the pupil raster element; X=3 mm for the diameter of the light source; and xcexa=0.8 for the filling ratio. For s, L, D, X, xcexa it follows that: f=480.092 mm for the focal length of the first raster element; Rxcx9cxe2x88x922f=xe2x88x92960.184 mm for the radius of the first raster element; and xcex94z=99.744 mm for the displacement of the pupil raster element beyond the focal point (Xxe2x80x2) for defocusing of the pupil raster element. A member of the convergent light bundle is characterized by a numerical aperture NA and has a diameter Xxe2x80x2 at a focal point. A member of the pupil raster element has a diameter D and is characterized by a defocusing xcex94z, wherein the defocusing xcex94z is given by:                                                         0.1              ⁢              D                        -                          X              xe2x80x2                                            2            ·            NA                           less than                   Δ          ⁢                      xe2x80x83                    ⁢          z                 less than                               D            -                          X              xe2x80x2                                            2            ·            NA                                              (        9        )             The illumination system is characterized by a distance s from the light source to the field raster elements, and a distance L from the field raster elements to sthe pupil raster elements. The light source has a diameter X, and a member of the pupil raster elements has a diameter D. A member of the field raster elements is characterized by an extension FY that describes a longer of two extensions of the member of field raster elements, and a focal distance f. The focal distance f lies within the following boundaries:                                           FY            ·            s            ·            L                                                              (                                  FY                  +                                      0.5                    ⁢                    D                                                  )                            ·              s                        -                                          (                                  FY                  -                  X                                )                            ·              L                                       greater than         f         greater than                               FY            ·            s            ·            L                                                              (                                  FY                  +                  D                                )                            ·              s                        -                                          (                                  FY                  -                  X                                )                            ·              L                                                          (        10        )             so that defocusing occurs. FIG. 2 shows the illumination of a second optical element 104 (see FIG. 4) of a double faceted illumination system with a plurality of pupil raster elements for a source assumed to be spherical, without defocusing, having a diameter of 3 mm. A double faceted illumination system with a second optical element is shown, for example, in FIG. 4 or 5. The focal distance of the field raster elements is f=514.286 mm, and R=xe2x88x921028.571 mm. The collected aperture amounts to 0.09. The etendue is therefore: E=xcfx80/4xc2x7X2xc2x7xcfx80NA2≈0.2.xe2x80x83xe2x80x83(11) The etendue of the light source thus corresponds approximately to a {fraction (1/25)}th part of the etendue, which could be collected. At the same time, the secondary light sources fill only approximately {fraction (1/25)}th of the pupil raster element plate. FIG. 3 shows the illumination of the pupil raster element plate with defocused secondary light sources for a spherical light source with a diameter of X=3mm and an assumed aperture of NAxcx9c0.09. The etendue remains the same, but a slight deviation from the Kxc3x6hler illumination has been introduced. In the length direction of the field raster elements, the y-direction, the illumination is clearly elongated, so that the thermal load is drastically reduced. Also, in the short direction, the x-direction, an expansion of the light source images results. The expansion is smaller, however, due to the small angle of aperture. Since the field raster elements have a longer expansion FY in y-direction then FX in x-direction, the angle of aperture in y-direction is greater then in the x-direction, see also in the earlier example where FXxc3x97FY=2.8 mmxc3x9746 mm the angle of aperture in y-direction is grater then in x-direction. In a further enhanced form of embodiment, the refractive powers of the field raster elements are adapted to the distances of the corresponding pupil raster elements. As can be seen in FIG. 3, there is more space between the pupil raster elements as compared to the lower half of the second optical component with second raster elements, which take up the respective individual light source images each time, particularly in the outer regions and in the upper half, since the pupil raster elements are arranged on a slightly distorted raster due to the distortion of the imaging optics. Therefore, the sizes of the light source images can be individually influenced by different defocusing, i.e., different focal distances of the individual field raster elements. FIG. 4 is a schematic diagram of an illumination system, in which the invention can be applied. The illumination system comprises a light source or an intermediate image of a light source 100. The light emitted by the light source or the intermediate image of light source 100, from which only three representative rays are depicted, impinges a mirror 102 with a plurality of first raster elements, so-called field raster elements. Mirror 102 is thus also denoted a field raster element mirror. The refractive power of the individual field raster elements is selected according to the invention such that a defocusing is present, so that the second raster elements, the so-called pupil raster elements of the second optical element 104, lie outside the focal point of the beam bundle produced by the first raster elements. The optical elements 106, 108 and 110, i.e., mirrors, arranged in the light path after the second optical element 104 serve essentially for the purpose of forming a field in a reticle plane 114. The reticle is a reflection mask in the reticle plane 114. The reticle can be moved in the depicted direction 116 in an EUV projection system designed as a scanning system. Thus, the illumination system of FIG. 4 includes at least one mirror (106, 108, 110), which is arranged in the light path after the second optical element (104). The at least one mirror (106, 108, 110) images a plane, which is arranged in or in the vicinity of the second optical element, in the exit pupil (112). The exit pupil 112 of the illumination system is illuminated nearly homogeneously by means of the illumination system shown in FIG. 4. The exit pupil 112 coincides with the entrance pupil of a projection objective arranged in the light path after the illumination system. Such a projection objective, for example, with six mirrors, is shown in U.S. patent application Ser. No. 09/503,640, the disclosure content of which is incorporated to the full extent in the present application. The illumination system of the present invention can be employed in an EUV projection exposure apparatus for microlithography. The illumination system would have an exit pupil that partially collects EUV radiation. The illumination system would guide the EUV radiation for illuminating a ring field. The EUV projection apparatus would include (a) a source of the EUV radiation, (b) the illumination system of the present invention, (c) a pattern-bearing mask on a carrier system, where the pattern-bearing mask lies in a plane of the ring field, (d) a projection device with an entrance pupil that coincides with the exit pupil, where the projection objective images an illuminated part of the pattern-bearing mask in an image field, and (e) a light-sensitive substrate on a carrier system in the plane of the image field. Such an EUV projection exposure apparatus can used for producing a microelectronic component, such as a semiconductor chip. FIG. 5 shows an optical part of a projection exposure system proceeding from a physical light source 122 up to an object 124 to be exposed. The same components as in FIG. 4 are denoted with the same reference numbers. The system according to FIG. 5 comprises the physical light source 122, a collector unit 120, which is depicted as a nested collector in the embodiment shown in FIG. 5, the illumination system from FIG. 4, a projection objective 126, for example, with six mirrors 128.1, 128.2, 128.3, 128.4, 128.5, 128.6 according to U.S. patent application Ser. No. 09/503,640 as well as the object 124 to be exposed. The collector unit 120 collects the light of the physical light source 122 and directs the collected light in a light bundle onto the first optical element 102 with raster elements. In FIG. 5 the collector unit 120 is shaped such that a intermediate image of the physical light source 122, the so-called light source 100, is formed. Such a configuration of the collector unit is advantageous, but not necessary. For the first time, an EUV illumination system is indicated by the invention, with which the thermal load on the second faceted mirror elements is reduced. It should be understood that various alternatives and modifications of the present invention can 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.