Abstract:
There is provided an illumination system. The illumination system includes a source for light having a wavelength ≦193 nm, a field plane, and a collector having a mirror shell for receiving a part of the light. The mirror shell is arranged so that a real image of the source is formed and comes to lie in a plane that is defocused relative to the field plane by more than 30 mm, so that the field plane is illuminated in a predetermined region, substantially homogeneously.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International Application No. PCT/EP03/00491, filed Jan. 20, 2003, which claims priority of German Patent Application No. 102 08 854.3, filed Mar. 1, 2002. The content of these applications is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an illumination system with a light source and a field plane, comprising at least one collector with at least one mirror shell which is arranged in a rotationally symmetrical way around an axis for receiving a part of the light of a light source, so that an aperture diaphragm plane arranged in the light path after the collector is illuminated in an annular way. 
     2. Description of the Related Art 
     A projection exposure system is known from U.S. Pat. No. 5,737,137 which comprises a laser plasma source as a light source. The illumination system of the projection exposure system according to U.S. Pat. No. 5,737,137 is arranged in such a way that an exit pupil of the illumination system is illuminated in a circular (i.e. annular) fashion. In order to achieve this, the illumination system comprises an arrangement of three mirrors, with two mirrors forming an inverse Cassegrain or Schwarzschild system with a 25% obscuration. The mirrors of the Schwarzschild system are normal-incidence mirrors with a multi-layer system on which the light rays impinge under angles α&lt;30°. 
     The projective objective of the projection exposure system as known from U.S. Pat. No. 5,737,137 comprises two mirrors with an aperture through which the light passes through. 
     Moreover, the system according to U.S. Pat. No. 5,737,137 is a system with critical illumination. 
     The disadvantageous aspect in the system according to U.S. Pat. No. 5,737,137 is that only normal-incidence mirrors are used in the illumination system directly downstream of the source. Although the entry of debris into the projection objective system can be prevented according to U.S. Pat. No. 5,737,137, the normal-incidence mirrors of the illumination system will degrade very rapidly by the debris of the source. As a result of degradation of the multi-layer system, normal-incidence mirrors lose reflectivity to a very high extent over time. 
     A further disadvantage of the system according to U.S. Pat. No. 5,737,137 is that for a homogeneous illumination of the field plane at critical illumination with a small source it is necessary to have a very large lateral magnification. In the case of very large magnifications however, the dimension especially of the first mirror will become very large and thus difficult to produce with an inverse Cassegrain or Schwarzschild system. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide an illumination system which provides an annular illumination in an aperture diaphragm plane or a plane conjugated thereto. The exit pupil of the illumination system which coincides with the entrance pupil of the downstream projection objective is then also illuminated in an annular way. The disadvantages of the illumination system as known from U.S. Pat. No. 5,737,137 are to be avoided. Moreover, a homogeneous illumination in a field plane is to be enabled with the illumination system in accordance with the invention. 
     This is achieved in such a way that an illumination system comprises a collector with at least one mirror shell which is rotationally symmetrical around an axis. 
     In a respective configuration of the mirror shells, such a collector projects a light source situated in front of the collector into a real light source image. The real light source image is also known as secondary light source. 
     In accordance with the invention, a homogeneous illumination of a field plane can be achieved in two ways. According to a first embodiment of the invention the mirror shells are dimensioned in such a way that a strongly enlarged image of the light source is projected with a lateral magnification of β&gt;5 close to the field plane. 
     According to a second embodiment of the invention, the mirror shells are dimensioned in such a way that the field plane is defocused relative to the light source image, leading to a homogeneous illumination in a predetermined region in the field plane. The illumination is no longer critical in this case. The position of the homogeneously illuminated field plane which is defocused relative to the position of the light source image, is defined namely through the point of intersection of a first inner marginal ray of a first light bundle and a second inner marginal ray of a second light bundle or through the point of intersection of a first outer marginal ray of a light bundles and a second outer marginal ray of a light bundles. The first and second light bundles are determined by the extended light source and the aperture ray. A collector shell receives rays in a specific annular aperture from the light source. A first middle aperture ray is determined between the inner aperture ray and the outer aperture ray of the annular aperture under which a ray from the light source defines a first point of intersection with the collector shell. The middle aperture beam between light source and collector shell, which aperture beam is reflected relative to the axis of rotation of the collector shell on which or close to which the light source is situated, defines a second middle aperture beam and a second point of intersection. A first and second point of intersection lie together with the optical axis and thus in a meridional plane. The first light bundle is the light bundle which is received in the first point of intersection by the collector shell from the extended light source. The second light bundle is the light bundle which is received in a second point of intersection by the collector shell from the extended light source. 
     Preferably, a structural mask is placed in the field plane or a plane conjugated thereto, which mask is projected onto a light-sensitive object by means of a projection objective. 
     The relevant aspect for the collector in accordance with the invention, which comprises mirror shells which are rotationally symmetrical about an axis, is that the received rays are received under an angle of &gt;70° relative to the normal of the surface, i.e. the collector is a so-called grazing-incidence collector. 
     Grazing-incidence collectors have the advantage as compared with normal-incidence collectors that they will degrade only to a low extent by the debris of the source, i.e. they hardly lose reflectivity. Moreover, grazing-incidence mirrors are always arranged in a simpler way because as a rule they only have one optical coating which is preferably made of ruthenium, rhodium or gold. Moreover, reflectivity &gt;80% can be achieved at lower requirements placed on the surface roughness. 
     In an advantageous embodiment of the invention the collector not only comprises a single rotationally-symmetrical mirror shell, but also a plurality of such rotationally-symmetrical mirror shells, with the mirror shells being arranged within each other about a common rotational axis. 
     Preferably, the mirror shells are arranged in such a way that the annular illumination of each mirror shell in the aperture diaphragm plane for adjacent mirror shells are continually adjacent to each other. In a preferred embodiment, the middle aperture ray is then defined by the middle ray between an outer marginal ray between one point of the light source and the outermost shell and an inner marginal ray between the point of the light source and the innermost shell. In a further embodiment, a middle aperture ray is defined for every single collector shell, so that for each collector shell a homogeneous illumination is achieved independently in the same homogeneously illuminated field plane. In this case the images of the light source of the individual collector shells do not lie necessarily in the same plane. 
     Collectors with two reflections are especially preferable. Collectors with two reflections can be configured as nested Wolter systems with first mirror shells which are annular sections of hyperboloids and second mirror shells which are annular sections of ellipsoids. Wolter systems are known from literature, e.g. from Wolter, “Annalen der Physik” 10 (Annals of Physics), p. 94 to 114, 1952. Reference is hereby made to J. Optics, Vol. 15, 270 to 280, 1984, concerning Wolter systems with a real intermediate image of the source which is formed by the combination of a hyperboloid surface with an ellipsoid surface. 
     An especial advantage of Wolter systems is that in a Wolter system with two reflections with incident angles of &gt;70° relative to the normal of the surface it is possible to achieve a maximum collection aperture of NA max =0.985 according to an aperture angle of 80°. In this embodiment, one is still in the highly reflective range of reflection under grazing incidence with a reflectivity of &gt;70%. 
     It is especially preferable when the illumination system between collector and exit pupil comprises at least two optical elements with positive refractive power. In this way it possible to ensure that the field plane which is situated in the light path starting with the light source after the aperture diaphragm plane is projected into a conjugated plane to the field plane which comes to lie before the exit pupil of the illumination system. The aperture diaphragm plane is a plane which is conjugated relative to the exit pupil of the illumination system. In the plane which is conjugated to the field plane, the structured mask to be illuminated (and in particular the reticle) can be arranged in a projection exposure system. 
     In order to have the lowest possible light losses in the system, the two optical elements with positive refractive power are operated under angles of &gt;70° or &lt;20° relative to the normal of the surface. A further advantage is that in this case the illumination of the reticle occurs in a non-polarized manner. 
     An annular illumination in the exit pupil with σ out ≧0.55 is achieved with the illumination system in accordance with the invention. In this respect, σ out  is given by the quotient of the sine of the angle to the optical axis of the outer marginal ray of the annular illumination and the objective aperture. Parallel thereto it is possible to define a σ in-volume  via the inner marginal ray of the annular illumination, for which values of σ in &lt;0.35 can be achieved. 
     In order to guide the light below the reticle arranged in the field plane, at least one deflection or deviation mirror is provided advantageously as a third optical element in the light path between the first and second optical element. This mirror can preferably be a planar mirror. 
     In order to enable the operation of the reticle in reflection, the reticle is not arranged precisely perpendicular to the optical axis. Instead, the reticle is positioned inclined to an axis being perpendicular to the optical axis, with the angle of inclination being approximately 3° with respect to the conjugated field plane. 
     In addition to the illumination system, the invention also provides a projection exposure system which is characterized in that the illumination system comprises at least one collector which comprises a mirror shell which is arranged in a rotationally symmetrical way around an axis and reflects light in grazing incidence, i.e. with angles &gt;70° relative to the normal of the surface, as well as a projection objective which can be arranged as a Schwarzschild system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is to be described below by way of example by reference to the embodiments shown in the drawings, wherein: 
         FIG. 1  shows the general configuration of an illumination system in accordance with the invention; 
         FIG. 2  shows the establishment of the homogeneous illumination in the field plane in the case of defocused arrangement in relation to the light source; 
         FIG. 3  show the establishment of the homogeneous illumination in the field plane in the case of defocused arrangement in relation to the light source with different defocusing for different collector shells; 
         FIG. 4  shows the energy distribution in the field plane in a system according to  FIG. 2 ; 
         FIG. 5  shows an illumination system with three optical elements; 
         FIG. 6  shows the inclined reticle; 
         FIG. 7  shows a folded system with a total of four optical elements; 
         FIG. 8  shows an explanatory diagram for the coordinates of the nested collector. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an illumination system in accordance with the invention in an exemplary refractive illustration. The illumination system in accordance with the invention comprises a light source  1 . The light source  1  is preferably a plasma source such as a dense-plasma-focus source or a hollow-cathode source. The light of the light source  1  is received by the collector  3 . The schematically illustrated collector  3  comprises in the illustrated embodiment a total of three mirror shells  5 . 1 ,  5 . 2  and  5 . 3  which are arranged within each other and which receive the light of the light source  1  under grazing incidence and project an image  7  of the light source. Nested, reflective collectors such as collector  3  necessary comprise a central obscuration, below a certain aperture angle NA min  the radiation of the source can no longer be received. This radiation is therefore blocked off with a diaphragm so that it cannot reach the subsequent illumination system. The diaphragm is designated in  FIG. 1  with reference numeral  9 . 
     The first aperture diaphragm plane  20  which is situated in the light path downstream of the collector  3  is situated conjugated to the exit pupil  10  of the illumination system. The collector shells  5 . 1 ,  5 . 2 ,  5 . 3  of the collector  3  are chosen in such a way that in conjunction with the downstream optical elements in the exit pupil  10  of the illumination system, a predetermined aperture  12  is illuminated in an annular fashion. The predetermined aperture  12  in the exit pupil  10  is delimited by the aperture diaphragms  16  in the first aperture diaphragm plane. Since a further plane  18  which is conjugated to the aperture diaphragm plane  20  is formed in the system, an alternative or second aperture diaphragm  14  can be provided. The second aperture diaphragm  14  in plane  18  has the advantage that it can be chosen in this position having a smaller dimension. 
     As a result of the non-received aperture of the source radiation below the minimal aperture angle NA min , the desired annular illumination is obtained in a mandatory manner in the aperture diaphragm plane  20  and in the plane conjugated thereto in which the exit pupil  10  comes to lie. 
     In order to obtain the predetermined homogeneous illumination in the field plane, the light source  1  is not projected exactly into the field plane, but is slightly defocused. In the present case the defocusing between the image  7  of the light source and that of the field plane  30  is approximately 60 mm. A field diaphragm  34  can be arranged in the field plane  30 . The structured mask, the so-called reticle is arranged in the field plane  32  which is conjugated to the field plane  30 . The reticle is projected onto a light-sensitive object by means of a projection objective which is shown in  FIG. 5  by way of an example. The field diaphragm  34  delimits the ray bundle to the desired field size of 3 mm×3 mm for example on the structure-bearing mask, blocks the further radiation and thus advantageously reduces the thermal load on the subsequent optical elements. 
     It is ensured by the defocusing of the image  7  of the light source relative to the field plane that the deviations in homogeneity of the illumination in the field to be illuminated of approximately 3 mm×3 mm is less than ±5% in the field plane  32 . 
     In the arrangement as shown in  FIG. 1 , the illumination system comprises a condenser lens system with two optical elements  40 ,  42  with positive optical refractive power in order to exchange the sequence of the aperture diaphragm plane  20  which coincides with the end of the nested collector  3  and the field plane  30  which is situated close to the intermediate image  7  of the source. The optical elements  40 ,  42  are shown schematically as refractive components. A possibility to derive reflective components is obvious to the person skilled in the art. The field plane in which the reticle is arranged is the field plane  32  which is conjugated to the field plane  30  and which is formed according to the system from the first optical element  40  and the second optical element  42  and is situated in front of the exit pupil  10 . An image  44  of the light source image  7 , namely a so-called tertiary light source, is formed by the two optical elements  40 ,  42 . 
     In  FIG. 1 , the illumination system further preferably comprises a spectral filter  50  in order to filter the light of the light source  1  and to provide after the filter  50  radiation in the wavelength that is solely required, e.g. a wavelength of 13.5 nm. The spectral filter can be a zirconium foil or a grating spectral filter. 
       FIG. 2  shows again how a homogeneous illumination can be achieved in the field planes  30 . 1 ,  30 . 2  according to the invention by an enlarged source and slight defocusing of the field plane  30 . 1  or the alternative field plane  30 . 2  to the image of the light source  7 . The primary light source  1  is in the present case an extended light source. A first point on the mirror shell  5 . 2  of the nested collector which is given by the first point of intersection S 1  of a first middle aperture ray  102 . 1  with the collector shell  5 . 2  of the collector in the meridional plane receives a first ray bundle  102 . 2  of the light source  1 . The extension of the ray bundle is given by the extension of the light source  1 . An image  7  of the light source  1  is formed by the reflection under grazing incidence, i.e. under an angle of &gt;70° relative to the normal of the surface of the mirror shell  5 . 2 . The size of the formed light source image is defined by the reflected marginal rays  104 . 1  and  105 . 1  and the distance to the first point of intersection S 1 . A plane is understood in this application as being a meridional plane which comprises the axis of rotation HA. 
     In the meridional plane mirrored relative to the optical axis HA there is a second middle aperture ray  102 . 3  and a second point of intersection S 2  with the collector shell  5 . 2  opposite of the first middle aperture ray  102 . 1  and the first point of intersection S 1 . A second luminous beam  102 . 4  of the light source  1  is received in the second point of intersection S 2  of the collector with the mirror shell  5 . 2 . The second outer marginal ray  104 . 2  and the second inner marginal ray  105 . 2  of the second ray bundle  102 . 4  are determined again by the maximum extension of the light source  1 . The inner marginal rays  105 . 1 ,  105 . 2  of the first ray bundle  102 . 2  and the second ray bundle  102 . 4  intersect in a point of intersection  107 . 2  behind the image  7  of the light source  1  in a plane  30 . 2 . The outer marginal rays  104 . 1 ,  104 . 2  of the first ray bundle  102 . 2  and the second ray bundle  102 . 4  intersect before the image  7  of the light source  1  in a point of intersection  107 . 1  in a plane  30 . 1 . 
     A substantially homogeneous illumination (as shown in  FIG. 4 ) is obtained along the X-direction in or close to the planes  30 . 1 ,  30 . 2 . One of the planes  30 . 1 ,  30 . 2  can then be chosen as the field plane of the system. 
     Whereas it was shown in  FIG. 2  for a single mirror shell of a nested collector that a homogeneous illumination can be achieved in planes  30 . 1 ,  30 . 2  in a defocused manner relative to the secondary light source  7 , it is also possible in systems with more than one shell to achieve a homogeneous illumination in the planes  30 . 1 ,  30 . 2 . There is no change in the derivation with respect to the embodiment described in  FIG. 1  with the exception of the choice of the middle aperture ray. The middle aperture ray  102 . 1  is defined in the embodiment shown in  FIG. 2  as the middle ray between an outer marginal ray  109  between a point of the light source and the outermost shell  5 . 1  and an inner marginal ray  111  between the point of the light source and the innermost shell  5 . 3 . 
     As is shown in  FIG. 3 , it is possible to choose for different collector shells different position of the planes  30 . 1  and  30 . 2  associated to each collector shell. For example, for the outermost shell  5 . 4 , the plane  30 . 1  is situated before the intermediate image  7 . 1  of the outer most shell  5 . 4 , and for the inner shell  5 . 5 , the plane  30 . 2  is situated downstream of the intermediate image  7 . 2  of the inner shell  5 . 5 . Through a suitable choice of the shell parameters it is possible to displace the planes associated with the respective shells in such a way that the planes  30 . 1  and  30 . 2  will coincide. An even better mixing of light and a further improvement in the homogeneity in the field plane  30 . 1  and  30 . 2  to be illuminated is achieved in this manner. The shell parameters can be chosen in such a way that an annular illumination is obtained in the plane  20  in or behind the collector. The middle first aperture ray of the outermost shell  5 . 4  is designated with reference numeral  102 . 1 . 4  and the middle second aperture ray of the outermost shell  5 . 4  is designated with reference numeral  102 . 3 . 4 , the middle first aperture ray of the innermost shell  5 . 5  is designated with  102 . 1 . 5  and the middle second aperture ray of the innermost shell  5 . 5  is designated with  102 . 3 . 5 . 
       FIG. 5  shows a system according to the invention in a reflective illustration. The system shown in  FIG. 5  comprises, in addition to the illumination system, the projection lens system which is situated downstream of the illumination system. The same components as in  FIG. 1  are denoted with reference numbers higher by  1000 . The light of the light source  1001  is received by the nested collector  1003  with two shells  1005 . 1 ,  1005 . 2 . An image of the light source  1001 , the so-called secondary light source  1007 , is provided. Field plane and aperture diaphragm plane  1020  are exchanged in their sequence in the light path with the help of the grazing-incidence mirror  1040  and the normal-incidence mirror  1042  which correspond to the first and second optical element. The conjugated field plane  1032  in which a structured mask is illuminated in a homogeneous way according to the invention is arranged in a defocused manner relative to the image of the secondary light source  1007 , the so-called tertiary light source. This defocusing can clearly be seen in  FIG. 5  because the tertiary light source image  1090  comes to lie already behind the aperture diaphragm  1018 . 
     The exit pupil  1012  of the illumination system coincides with the entrance pupil of the projective objective  1200  which is arranged as a Schwarzschild system. The projection objective  1200  comprises two mirrors  1202 . 1  and  1202 . 2 . The light-sensitive object onto which the mask is projected is situated in the plane  1204 . In order to hold the beam path below the reticle situated in the conjugated field plane  1032 , a third optical element  1206  is provided in the present system which deflects the beam from the first optical element  1040  to the second optical element  1042 . In order to keep the reflection losses at a minimum, the rays impinge on the first optical element  1040  under an angle &gt;70° relative to the normal of the surface and on the normal-incidence mirror  1042  with angles &lt;20° relative to the normal of the surface. A further advantage of this angle of incidence is the substantial independence of reflectivity from the polarization of the incident light, so that there is a non-polarized illumination on the reticle. 
       FIG. 6  shows in more detail again the third optical element  1206  and the second optical element  1042 . The arrangement of the first mirror  1202 . 1  of the objective  1200  which is centered relative to the optical axis HA of the objective  1200  can clearly be seen. Since in EUV lithography it is necessary to operate the reticle in reflection on the one hand and since the optical elements of the present Schwarzschild objective are arranged in a centered manner relative to the optical axis HA, the reticle is inclined by at least the aperture angle (which in the present case is 3.44°) relative to the field plane. The angle of inclination γ of the reticle  1510  relative to the field plane  1032  is thus in the present example γ&gt;3.5°. 
       FIG. 7  shows an alternative system according to Tables 1 and 2 in which the beam path is additionally folded from the light source to the field plane for the purpose of shortening the overall length of the system. In addition, a second planar mirror  2800  is introduced in the beam path so that the illumination system is completely situated below the reticle plane. The same components as in the preceding figures are designated with the same reference numerals increased by  1000  relative to  FIG. 4 . Therefore  2001  designates the light source,  2003  the nested collector consisting of three shells,  2020  the first aperture diaphragm plane,  2030  the conjugated field plane,  2040  the first optical element,  2042  the second optical element,  2200  the projective objective with two mirrors  2202 . 1  and  2202 . 2 , and  2510  the reticle which is inclined relative to the field plane  2032 , and  2204  the plane in which the light-sensitive object is arranged. Preferably, the tilting angle of the first optical element  2040  is chosen in such a way that the beam path between light source  2001  and the first optical element  2040  is not situated in the same plane as the beam path of the subsequent optical elements, as also the reticle  2510  and the light-sensitive object in plane  2204 . In  FIG. 7 , the ray bundle is thus delimited in the field plane  2030  to such an extent that the reticle  2510  is now only illuminated within an area on a surface of 3 mm×3 mm. 
     Table 1: Parameters of the optical elements in the beam path after the collector  2003   
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Parameters of the optical elements in the beam path after the 
               
               
                 collector 2003 
               
             
          
           
               
                   
                   
                   
                 Angle of incidence 
               
               
                   
                   
                   
                 of the principal ray 
               
               
                   
                   
                   
                 relative to the 
               
               
                   
                   
                 Distance to next 
                 normal of the 
               
               
                 Plane 
                 Radius 
                 mirror 
                 surface 
               
               
                   
               
               
                 2020 
                 Diaphragm plane 
                  1285 mm 
                   0° 
               
               
                 2030 
                 Field plane 
                 209.2 mm 
                   0° 
               
               
                 2040 
                 −330.5 mm 
                 185.8 mm 
                 7.25° 
               
               
                 2018 
                 Diaphragm plane 
                   290 mm 
                   0° 
               
               
                 2800 
                 Planar 
                   70 mm 
                   75° 
               
               
                 2206 
                 Planar 
                   70 mm 
                   72° 
               
               
                 2042 
                 −497.2 mm 
                   100 mm 
                  −16° 
               
               
                 2032 
                 Reticle plane 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Optical data of the collector 2003 
               
             
          
           
               
                 Shell 
                   
                 K 
                 ZS [mm] 
                 ZV [mm] 
                 ZH [mm] 
               
               
                   
               
             
          
           
               
                   
                 Hyperboloid 
                   
               
               
                   
                 R [mm] 
               
             
          
           
               
                 1 
                 3.5076 
                 −1.0399 
                 −1.74 
                 107.56 
                 182.35 
               
               
                 2 
                 5.0414 
                 −1.0571 
                 −2.49 
                 105.05 
                 179.53 
               
               
                 3 
                 7.2534 
                 −1.0814 
                 −3.56 
                 102.83 
                 177.68 
               
             
          
           
               
                   
                 Ellipsoid 
                   
               
               
                   
                 R [mm] 
               
             
          
           
               
                 1 
                 4.6059 
                 −0.9945 
                 −181.56 
                 363.50 
                 454.10 
               
               
                 2 
                 6.4739 
                 −0.9923 
                 −184.74 
                 364.03 
                 457.33 
               
               
                 3 
                 9.0813 
                 −0.9893 
                 −189.80 
                 366.19 
                 463.15 
               
               
                   
               
             
          
         
       
     
     In  FIG. 8  and Table 2, the characteristic data of a Wolter system comprising two segments each, a first hyperboloid segment and a second ellipsoid segment, are shown per mirror shell in an exemplary way for the three mirror shells  5 . 1 ,  5 . 2 ,  5 . 3  of the nested collector of  FIG. 7 . Reference numeral  5 . 1 . 1  designates the first annular segment for the first mirror shell  5 . 1  and  5 . 1 . 2  the second annular segment. ZS designates the z position of the surface apex relative to the position of the light source  1 , ZV and ZH designate the starting and end position of the first segment  5 . 1 . 1  which is a hyperboloid, relating to the position of the surface apex ZS. The reference numerals ZS, ZH and ZV are used in an analogous way for the second segment  5 . 1 . 2  of the mirror shell  5 . 1  which is an ellipsoid. 
     The design data of the collector according to  FIG. 8  are obtained with the radius of curvature R and the conical constant K of the respective mirror segment and the definitions given above as in Table 2. 
     A system is provided for the first time with the present invention in which a homogeneous illumination in a field plane is achieved and the light losses are minimized.