Patent Publication Number: US-2013250428-A1

Title: Magnifying imaging optical unit and metrology system comprising such an imaging optical unit

Description:
The contents of German patent application DE 10 2011 003 302.5 are incorporated by reference. 
     The invention relates to a magnifying imaging optical unit, and to a metrology system comprising such an imaging optical unit. 
     A magnifying imaging optical unit of the type mentioned in the introduction is known for the simulation and analysis of effects of properties of masks for microlithography from DE 102 20 815 A1. Further imaging optical units are known from U.S. Pat. No. 6,894,834 B2, WO 2006/0069725 A1, U.S. Pat. No. 5,071,240, U.S. Pat. No. 7,623,620, US 2008/0175349 A1 and WO 2010/148293 A2. 
     It is an object of the present invention to develop an imaging optical unit of the type mentioned in the introduction in such a way as to take account of increased requirements made of the compactness and the transmission of the imaging optical unit, particularly for a given imaging scale. 
     The object is achieved according to a first aspect according to the invention by means of an imaging optical unit comprising the features specified in claim  1 , and is achieved according to a further aspect according to the invention by means of an imaging optical unit comprising the features specified in claim  5 . 
     It has been recognized according to the invention that when the two imaging partial rays between the second and third mirrors and between the third and fourth mirrors in the imaging beam path pass through the mirror body of the first mirror, compact designs of the imaging optical unit can be realised in which the last mirror in the imaging beam path can nevertheless occupy a position at a large distance from the image field. 
     In an alternative embodiment, only an imaging partial ray between a second mirror in the imaging beam path and a third mirror in the imaging beam path may pass through at least one passage opening in a mirror body of the first mirror in the imaging beam path. The passage opening may be a through-hole or may be an edge side recess in the first mirror M 1 . 
     Systems having a large image-side vertex focal length or a large image-side back focal distance and a correspondingly large imaging scale can thus be realised. The design comprising at most four mirrors ensures low reflection losses, particularly when the imaging optical unit is used with EUV radiation in the wavelength range of between 5 nm and 30 nm. The angle of incidence on the mirrors of the imaging optical unit can also be kept small, which is advantageous for the design of the mirrors with optimized reflectivity. 
     The second imaging partial ray may run between a third mirror and a fourth mirror in the imaging beam path. 
     The imaging optical unit may have exactly three mirrors. In that case, the second imaging partial ray may run between the third mirror in the imaging beam path and the image field. The imaging optical unit may be a catoptric optical device. 
     In so far as, according to claim  2 , the first and second imaging partial rays pass through the same passage opening in the mirror body of the first mirror, the first mirror can be manufactured with relatively little outlay. Separate passage openings in the first mirror for the imaging partial rays that pass through the latter are also possible, which can lead to a low loss of reflection area on the first mirror on account of the passage openings and thus to low reflection losses at the first mirror. 
     Designing the optical unit according to claim  3  allows an even more compact design. Shading the passage opening in the mirror body of the first mirror according to claim  4  reduces or avoids an additional obscuration by the at least one passage opening. In so far as a plurality of passage openings are provided in the first mirror, the imaging optical unit can be designed such that at least one of the passage openings is shaded by one of the mirrors at least in sections in the imaging beam path. 
     A ratio T/β between the structural length T and the imaging scale β of the imaging optical unit according to the further aspect likewise ensures a compact embodiment of the imaging optical unit. The structural length can be 1439 mm, can be 1300 mm, can be 1227 mm, can be 1093 mm, can be 1010 mm, can be at most 1000 mm, can be 900 mm, can be 878 mm, can be at most 800 mm, can be 741 mm and can be 700 mm. The ratio T/β of the structural length and the imaging scale can be less than 1.6, can be 1.502, can be 1.44, can be less than 1.2, can be 1.17, can be less than 1.1, can be less than 1.0, can be 0.98, can be 0.94, can be less than 0.9 and can be 0.87. Other ratios T/β may be realized, depending on the respective embodiment. The imaging scale can be greater than 500, can be greater than 700, can be 711, can be 750, can be greater than 800 and can be 850. An object-side chief ray angle α of at least 6° enables a reflective object to be imaged without components of the imaging optical unit and components of an illumination optical unit disturbing one another. Alternatively, an object-side chief ray angle α between a normal to the object plane and a chief ray of a central object field point can be less than 1°. These alternative chief ray angles for the further aspect of the invention can be optimized for dark field illumination and/or bright field illumination. Depending on the chief ray angle, the examination of a reflective reticle or else of a transmissive reticle, for example of a phase shift mask, is possible. 
     An object-side numerical aperture according to claim  6  allows a large imaging scale. In addition, depending on the design of an illumination optical unit, for illuminating an object, this allows different illumination geometries, for example dark field or bright field illumination. 
     An object field according to claim  7  is suited to the surfaces to be examined particularly when checking lithography masks in projection exposure, particularly in EUV projection exposure. The object field can be rectangular. The object field can have a size of 100 μm×300 μm, 100 μm×400 μm or 100 μm×200 μm. 
     An RMS (root mean square) wavefront aberration according to claim  8  and/or a distortion according to claim  9  result in aberration correction that suffices for object examination particularly with a CCD array. The wavefront aberration (RMS) can be 465 mλ, can be at most 250 mλ, can be 216 mλ, can be at most 31 mλ, can be at most 30 mλ, at most 25 mλ, can be 22 mλ, can be at most 20 mλ, can be at most 10 mλ, can be 6 mλ and can even be just 2 mλ. The maximum distortion can be 63.8 μm, can be at most 50 μm, can be at most 25 μm, can be at most 15 μm, can be 12.3 μm, can be at most 1500 nm, can be 1000 nm, can be 500 nm, can be 400 nm, can be 300 nm, can be 150 nm and can even be just 40 nm. 
     Other object-side numerical apertures, other object field sizes and other RMS wavefront abberations may be realized, depending on the respective embodiment. 
     Chief ray angles in the alternatives according to claim  10  for the first aspect can be optimized for a dark field illumination and/or bright field illumination. Depending on the chief ray angle, the examination of a reflective reticle or else a transmissive reticle, for example of a phase shift mask, is possible. 
     Configurations of the imaging optical unit according to the alternative embodiments in claims  11  and  12  can be prescribed in a manner optimized in respect of structural space depending on the configuration of an illumination optical unit for illuminating the object field. These configurations of the imaging optical unit give rise to corresponding free spaces in which components of the illumination optical unit can be accommodated. 
     An aperture stop according to claim  13  defines the imaging beam path. The aperture stop can be configured in a manner capable of being decentred for variation of a chief ray angle. In addition, the aperture stop can be configured with an adaptable diameter for variation of the object-side numerical aperture. Three imaging partial rays, four imaging partial rays or even five imaging partial rays or partial beams can pass through the aperture. 
     At least two intermediate image planes according to claim  14  increase the degrees of freedom when designing the optical design. This can be used, in particular, in order that the imaging light partial ray between the last mirror and the image field at the level of the first mirror can also be configured compactly such that a passage opening in the first mirror can be provided for this imaging light partial ray as well. A configuration of the imaging optical unit with exactly one intermediate image or completely without an intermediate image is also possible. 
     The advantages of a metrology system according to claim  15  correspond to those which have already been explained above with reference to the imaging optical unit. A CCD sensor, in particular a TDI CCD sensor, can be provided as detection device. 
     The features of the imaging optical units explained above can also be present in combination with one another and may constitute independently relevant aspects of the invention not in detail referred to above. 
    
    
     
       Exemplary embodiments of the invention are explained in greater detail below with reference to the drawing, in which: 
         FIG. 1  schematically shows a metrology system for examining objects, wherein a reflective reticle for EUV projection lithography serves as an object to be examined; 
         FIG. 2  shows, in an illustration similar to  FIG. 1 , a further embodiment of a metrology system, wherein a transmissive reticle for EUV projection lithography, e.g. a phase shift mask, serves as an object to be examined; 
         FIG. 3  shows a meridional section through an embodiment of a magnifying imaging optical unit for use in a metrology system according to  FIG. 1  or  2 , wherein the imaging optical unit serves for simulation and for analysis of effects and of properties of lithography masks, that is to say reticles, on optical imaging within a projection optical unit of a projection exposure apparatus for EUV projection lithography or else for the large-area detection of mask defects; 
         FIG. 4  shows, in a diagram, the dependence of a chief ray distortion CRD on a field height y of an object field of the imaging optical unit according to  FIG. 3 , wherein the field height y runs in a meridional plane that coincides with the plane of the drawing of  FIG. 3  and perpendicularly to an optical axis of the imaging optical unit, wherein a scanning direction for moving a mask to be examined runs along the y-direction; 
         FIG. 5  shows, in an illustration similar to  FIG. 3 , a further embodiment of the imaging optical unit; 
         FIG. 6  shows, in an illustration similar to  FIG. 4 , the dependence of the chief ray distortion CRD against the field height y for the imaging optical unit according to  FIG. 5 ; 
         FIG. 7  shows, in an illustration similar to  FIG. 3 , a further embodiment of the imaging optical unit; 
         FIG. 8  shows, in an illustration similar to  FIG. 4 , the dependence of the chief ray distortion CRD against the field height y for the imaging optical unit according to  FIG. 7 ; 
         FIG. 9  shows, in an illustration similar to  FIG. 3 , a further embodiment of the imaging optical unit; 
         FIG. 10  shows, in an illustration similar to  FIG. 4 , the dependence of the chief ray distortion CRD against the field height y for the imaging optical unit according to  FIG. 9 ; 
         FIG. 11  shows, in an illustration similar to  FIG. 3 , a further embodiment of the imaging optical unit; 
         FIG. 12  shows, in an illustration similar to  FIG. 4 , the dependence of the chief ray distortion CRD against the field height y for the imaging optical unit according to  FIG. 11 ; 
         FIG. 13  shows, in an illustration similar to  FIG. 3 , a further embodiment of the imaging optical unit; 
         FIG. 14  shows, in an illustration similar to  FIG. 4 , the dependence of the chief ray distortion CRD against the field height y for the imaging optical unit according to  FIG. 13 ; 
         FIG. 15  shows, in an illustration similar to  FIG. 3 , a further embodiment of the imaging optical unit; 
         FIG. 16  shows, in an illustration similar to  FIG. 4 , the dependence of the chief ray distortion CRD against the field height y for the imaging optical unit according to  FIG. 15 ; 
         FIG. 17  shows, in an illustration similar to  FIG. 3 , a further embodiment of the imaging optical unit; 
         FIG. 18  shows, in an illustration similar to  FIG. 4 , the dependence of the chief ray distortion CRD against the field height y for the imaging optical unit according to  FIG. 17 ; 
         FIG. 19  shows, in an illustration similar to  FIG. 3 , a further embodiment of the imaging optical unit; 
         FIG. 20  shows, in an illustration similar to  FIG. 4 , the dependence of the chief ray distortion CRD against the field height y for the imaging optical unit according to  FIG. 19 ; and 
         FIGS. 21 to 31  show, in an illustration similar to  FIG. 3 , further embodiments of the imaging optical unit. 
     
    
    
       FIG. 1  shows, highly schematically, a metrology system  1  for examining an object  2  in the form of a reticle or a lithography mask for EUV projection lithography. The metrology system  1 , which is also referred to as APMI (Actinic Patterned Mask Inspection), can be used to examine, in particular, defects on the reticle  2  and the effects thereof on imaging in EUV projection lithography. The reticle  2  can be checked, in particular, for patterning errors. The patterning error can subsequently be examined with the aid of an analysis of a so-called aerial image (Aerial Image Metrology System, AIMS). AIMS systems are known from DE 102 20 815 A1. The metrology system  1  is used for examining a reflective reticle  2 . 
     In order to facilitate the representation of positional relationships, a Cartesian xyz coordinate system is used below. The x-axis runs perpendicularly to the plane of the drawing out of the latter in  FIG. 1 . The y-axis runs towards the right in  FIG. 1 . The z-axis runs upwards in  FIG. 1 . 
     The metrology system  1  has an EUV light source  3  for generating illumination and imaging light  4 . The EUV light source can be a plasma source, that is to say an LPP source (laser produced plasma), or a GDP source (gas discharge produced plasma). The EUV light source  3  can also be an EUV laser. The latter can be realised for example by frequency multiplication of longer-wave laser radiation. The EUV light source  3  emits usable illumination and imaging light  4  having a wavelength of 13.5 nm. Other wavelengths in the range of between 5 nm and 100 nm, in particular in the range of between 5 nm and 30 nm, can also be used as illumination and imaging light  4  given a corresponding design of the EUV light source  3 . 
     An illumination optical unit  5  serves for transferring the illumination and imaging light  4  from the EUV light source  3  towards an object field  6 , in which a segment of the reflective reticle  2  is arranged. 
     An imaging optical unit  7  having a high magnification factor, for example of 500, images the object field  6  into an image field  9  via an imaging beam path  8 . A spatially resolving detection device in the form of a CCD sensor  10  detects an intensity distribution of the illumination and imaging light  4  over the image field  9 . A CCD chip of the CCD sensor  10  can be embodied as a time delay and integration CCD chip (time delay and integration charge-coupled device, TDI CCD). A TDI CCD chip can be used, in particular, for examining a reticle  2  moved through the object field  6 . A movement direction of the reticle  2  can run along the y-direction. 
     Illumination and detection of the illumination and imagine light  4  emerging from the object field  6  can take place in various ways. In the case of the metrology system according to  FIG. 1 , illumination is effected with a numerical aperture NA of 0.25, for example. The imaging optical unit  7  can capture this numerical aperture completely or partially, depending on the embodiment. Assuming a perfectly reflective reticle  2 , therefore, the entire illumination and imaging light  4  reflected from the reticle  2  or part of said light can be captured by the imaging optical unit  7 . Such illumination is also known as bright field illumination. Dark field illumination is also possible, in which portions of the illumination and imaging light  4  that are exclusively scattered or diffracted by the reticle  2  are detected by the CCD sensor  10 . 
       FIG. 2  shows a variant of the metrology system  1  that is used for examining a reticle  2  that is at least partly transmissive to the illumination and imaging light  4 , for example for a phase shift mask. Components corresponding to those which have already been explained above with reference to  FIG. 1  bear the same reference numerals and will not be discussed in detail again. 
     In contrast to the embodiment according to  FIG. 1 , in the case of the metrology system  1  according to  FIG. 2 , the imaging optical unit  7  is not arranged in the direction of a reflected beam path of the illumination and imaging light  4 , but rather in the direction of a beam path transmitted through the reticle  2 . In this case, too, bright field or dark field illumination is possible depending on the embodiment of the illumination optical unit  5  and/or the imaging optical unit  7 . 
       FIG. 3  shows an embodiment of the imaging optical unit  7  that can be used in the metrology system  1  in  FIG. 1  or  2 . Components that have already been explained above in connection with the description of the metrology system  1  bear the same reference numerals and will not be discussed in detail again. A Cartesian xyz coordinate system is also used in connection with the description of the imaging optical unit  7  according to  FIG. 3  and with the description of the further embodiments for the imaging optical unit. The x-axis runs perpendicularly to the plane of the drawing into the latter in  FIG. 3 . The y-axis runs upwards in  FIG. 3 . The z-axis runs towards the right in  FIG. 3 . 
     The imaging optical unit  7  according to  FIG. 3  images the object field  6  lying in an object plane  11  into the image field  9  lying in an image plane  12  with a magnification factor of 750. 
       FIG. 3  illustrates, for the visualization of the imaging beam path  8  of the imaging optical unit  7 , the course of chief rays  13  and of coma rays  14 ,  15  which emerge from five object field points lying one above another in the y-direction. The distance between said object field points in the y-direction is so small in the object field  6  that said distance cannot be resolved in the drawing. These five object field points are imaged into five image field points lying one above another in  FIG. 3  in the image field  9 , which are resolved separately in the drawing on account of the high magnification factor. The chief rays  13 , on the one hand, and the coma rays  14 ,  15 , on the other hand, are also designated as imaging rays hereinafter. 
     The object field  6 , on the one hand, and the image field  9 , on the other hand, lie in xy planes spaced apart from one another. The object field  6  has an extent of 40 μm in the y-direction and an extent of 200 μm in the x-direction, that is to say has a field size of 40×200 μm 2 . The object field  6  and the image field  9  are rectangular in each case. 
     The chief rays  13  emerge in the imaging beam path  8  between the object field  6  and the image field  9  from the object field  6  with a chief ray angle α of almost 0° with respect to a normal  16 —running in the z-direction—to a central object field point of the object plane  11 . On account of this practically vanishing chief ray angle α, that is to say on account of the almost perpendicular course of the chief rays  13  on the reticle  2 , the imaging optical unit  7  according to  FIG. 3  can be used for dark field illumination in the metrology system  1  according to  FIG. 2 . The chief ray angle α is less than 1°. Other chief ray angles α, in particular a larger chief ray angle α, are possible. 
     An object-field-side numerical aperture of the imaging optical unit  7  is NAO=0.25. 
     In the image plane  12 , the imaging rays  13  to  15  meet almost perpendicularly to the image plane  12  respectively at one of the five image field points of the image field  9 . The chief rays  13  associated with each of the image field points run parallel to one another. The imaging optical unit  7  according to  FIG. 3  is therefore telecentric on the image side. 
     In the imaging beam path between the object field  6  and the image field  9 , the imaging optical unit  7  has exactly four mirrors, which are designated hereinafter by M 1 , M 2 , M 3  and M 4  in the order in which they are arranged in the imaging beam path. The four mirrors M 1  to M 4  constitute four optical components that are separate from one another. 
     An aperture stop  17  is arranged in the beam path between the object field  6  and the mirror M 1 . The aperture stop  17  is arranged in the region of a first pupil plane of the imaging optical unit  7  according to  FIG. 3  between the object field  6  and the mirror M 1 . A second pupil plane of the imaging optical unit  7  according to  FIG. 3  lies in the imaging beam path  8  between the mirror M 2  and the mirror M 3 . 
     The first mirror M 1  in the beam path between the object field  6  and the image field  9  is embodied aspherically as a concave primary mirror and the further mirrors M 2  to M 4  are embodied spherically. The mirror M 2  is configured in concave fashion, the mirror M 3  is configured in convex fashion and the mirror M 4  is configured in concave fashion. 
       FIG. 3  illustrates the curves of intersection of parent surfaces which are used for the mathematical modelling of the reflection surfaces of the mirrors M 1  to M 4 . Those regions of the reflection surfaces of the mirrors M 1  to M 4  to which the coma rays  14 ,  15  are applied and between the coma rays  14 ,  15  imaging radiation is actually applied are actually physically present in the sectional plane illustrated. 
     An intermediate image  18  lies in the imaging beam path between the mirrors M 1  and M 2 . 
     The imaging optical unit  7  is designed for an operating wavelength of 13.5 nm. 
     The mirrors M 1  to M 4  bear a coating that is highly reflective to the illumination imaging light  4 , which coating can be embodied as a multilayer coating. 
     A first imaging partial ray  19  lies in the imaging beam path  8  between the second mirror M 2  and the third mirror M 3 . A second imaging partial ray  20  lies in the imaging beam path  8  between the third mirror M 3  and the fourth mirror M 4 . The two imaging partial rays  19  and  20  both pass through a passage opening  21  into a mirror body  22  of the first mirror M 1  in the imaging beam path  8 . The mirror body  22  is schematically illustrated only in the vicinity of the passage opening  21  in  FIG. 3 . The two imaging partial rays  19 ,  20  pass through one and the same passage opening  21 . 
     The passage opening  21  is completely shaded by the mirror M 2  in the imaging beam path  8 . This is illustrated in  FIG. 3  by two dashed shadow lines  23  which run in each case from the object field  6  as far as the mirror M 1  and the course of which is defined by the shading edge of the mirror M 2 . 
     An imaging partial ray  24  between the object field  6  and the first mirror M 1  passes through the aperture stop  17 , wherein the aperture stop  17  defines the marginal extent of the imaging partial ray  24 . In addition, a further imaging partial ray  25  of the imaging beam path  8  between the mirror M 1  and the mirror M 2  and also the first imaging partial ray  19  pass through the aperture stop  17 . 
     Optical data of the imaging optical unit  7  according to  FIG. 3  are reproduced below with the aid of two tables. In the column “Radius”, the first table shows the respective radius of curvature of the mirrors M 1  to M 4 . The third column (Thickness) describes the distance in each case to the downstream surface in the z-direction. 
     The second table describes the exact aspherical surface shape of the reflection surfaces of the mirror M 1 , wherein the constants K and A to E should be inserted into the following equation for the sagitta: 
     
       
         
           
             
               z 
                
               
                 ( 
                 h 
                 ) 
               
             
             = 
             
               
                 
                   ch 
                   2 
                 
                 
                   1 
                   + 
                   
                     SQRT 
                      
                     
                       { 
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               1 
                               + 
                               K 
                             
                             ) 
                           
                            
                           
                             c 
                             2 
                           
                            
                           
                             h 
                             2 
                           
                         
                       
                       } 
                     
                   
                 
               
               + 
               
                 Ah 
                 4 
               
               + 
               
                 Bh 
                 6 
               
               + 
               
                 Ch 
                 8 
               
               + 
               
                 Dh 
                 10 
               
               + 
               
                 
                   Eh 
                   12 
                 
                  
                 
                   ( 
                   
                     
                       + 
                       
                         Fh 
                         14 
                       
                     
                     + 
                     
                       Gh 
                       16 
                     
                   
                   ) 
                 
               
             
           
         
       
     
     In this case, h represents the distance from the optical axis, that is to say from the normal  16 , of the imaging optical unit  7 . h 2 =x 2 +y 2  therefore holds true. The reciprocal of “Radius” is inserted into the equation for c. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Surface 
                 Radius 
                 Thickness 
                 Operating mode 
               
               
                   
                   
               
               
                   
                 Object 
                 Infinite 
                 341.321 
                   
               
               
                   
                 Stop 
                 Infinite 
                 458.679 
                   
               
               
                   
                 M1 
                 −661.396 
                 −587.218 
                 REFL 
               
               
                   
                 M2 
                 45.279 
                 606.973 
                 REFL 
               
               
                   
                 M3 
                 37.363 
                 −719.756 
                 REFL 
               
               
                   
                 M4 
                 1492.495 
                 778.296 
                 REFL 
               
               
                   
                 Image 
                 Infinite 
                 0.000 
                   
               
               
                   
                   
               
               
                   
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
                   
               
               
                   
                 M1 
                 0.000000E+00  
                 1.646127E−11 
                 3.681016E−17 
               
               
                   
                   
               
               
                   
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
                   
               
               
                   
                 M1 
                 7.950565E−23  
                 9.621018E−29  
                 1.101070E−33 
               
               
                   
                   
               
            
           
         
       
     
     A structural length T, that is to say, depending on the embodiment of the imaging optical unit, a distance between the object plane  11  and the image plane  12  or the distance between the components of the imaging optical unit  7  that are furthest away from each other in the z-direction, is 878 mm. With respect to this definition of the structural length T, the object field  6  and the image field  9  also are components of the imaging optical unit. A ratio of the structural length T and the imaging scale β is 878 mm/750=1.17 mm. 
     The distance between the last mirror M 4  and the image field  9  is more than 88 percent of the structural length T. 
       FIG. 4  shows in a diagram the dependence of a chief ray distortion CRD in nm on the field height y of the object field  6  of the imaging optical unit  7  according to  FIG. 3 . A distortion profile  26  is approximately parabolic with a minimum of CRD≈−280 nm at a field height y≈23 μm. The highest distortion value CRD≈360 nm is achieved at a field height y=0. At the other field edge, that is to say at the field height y=40 μm, the distortion CRD≈125 nm. Over the entire y-field height of the object field  6 , the distortion CRD in absolute terms is therefore less than 400 nm. Given a pixel size of the CCD sensor  10  of 10 μm×10 μm, the imaging optical unit  7  is therefore corrected well. On account of the rotational symmetry of the imaging optical unit  7  about the optical axis, a corresponding dependence of the distortion CRD on the x-dimension arises. 
     In the case of the imaging optical unit  7 , the etendue (aperture×field size) required for the metrology system  1  can be corrected in a diffraction-limited and distortion-free manner. 
     With reference to  FIGS. 5 and 6 , a description is given below of a further embodiment of an imaging optical unit  27 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiment are explained below. 
     The imaging optical unit  27  has an object-side chief ray angle α between the normal  16  to the object plane  11  and the chief ray  13  of a central object field point of 10°. The imaging optical unit  27  can be used for the bright field illumination of a reflective reticle  2  in the metrology system  1  according to  FIG. 1 . Given an illumination aperture chosen to be appropriately small in the illumination optical unit  5 , which is indicated schematically in  FIG. 5 , a zeroth diffraction order of the illumination imaging light  4  reflected at the reticle  2  is not shaded particularly by the mirror M 2 . 
     The imaging optical unit  27  has a structural length T of 800 mm between the object plane  11  and the image plane  12 . A distance A between the mirror M 4  and the object plane  11  is more than 38 percent of the structural length T. In the case of the imaging optical unit  27 , therefore, enough structural space for the illumination optical unit  5  is present in the vicinity of the object plane  11 . 
     In the case of the imaging optical unit  27 , too, the passage opening  21  lies in the shade of the mirror M 2 . 
     The chief rays  13  of different field points run divergently in the imaging beam path  8  between the last mirror M 4  and the image field  9 . 
     The ratio T/β of the structural length T and the imaging scale β (β=850) is T/β=0.94 in the case of the imaging optical unit  27 . 
     The imaging optical unit  27  has an object-side numerical aperture of 0.24. The object field  6  of the imaging optical unit  27  has a size of 100 μm in the y-direction and 300 μm in the x-direction. 
     An impingement point  28  of the chief ray  13  of the central object field point on the first mirror M 1  in the imaging beam path  8  and an impingement point  29  of the chief ray  13  of the central object field point on the fourth mirror M 4  in the imaging beam path  8  lie on different sides of a plane  30  which is perpendicular to the meridional plane (plane of the drawing in  FIG. 5 ) of the imaging optical unit  27  and in which the normal  16  lies. The plane  30  is therefore defined as that plane which is perpendicular to the meridional plane and contains the normal  16 . The plane  30  lies between the impingement points  28  and  29 . 
       FIG. 6  shows a CRD profile  31  over the field height y of the object field  6  in the case of the imaging optical unit  27 . In the case of a field height y=0, the distortion value CRD≈−40 nm. In the case of a field height y≈20 μm, the distortion value attains a local maximum CRD≈110 nm. In the case of a field height y≈75 μm, the distortion value attains a minimum CRD≈−225 nm. At the field edge y=100 μm, the distortion attains a global maximum CRD≈175 nm. The absolute value of the distortion is therefore less than 250 nm over the entire y-field height. 
     The optical data of the imaging optical unit  27  according to  FIG. 5  are reproduced below with the aid of two tables, which correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Surface 
                 Radius 
                 Thickness 
                 Operating mode 
               
               
                   
                   
               
               
                   
                 Object 
                 Infinite 
                 314.392 
                   
               
               
                   
                 Stop 
                 Infinite 
                 364.472 
                   
               
               
                   
                 M1 
                 −536.900 
                 −469.274 
                 REFL 
               
               
                   
                 M2 
                 48.401 
                 570.410 
                 REFL 
               
               
                   
                 M3 
                 45.000 
                 −470.410 
                 REFL 
               
               
                   
                 M4 
                 −1844.563 
                 490.410 
                 REFL 
               
               
                   
                 Image 
                 Infinite 
                 0.000 
                   
               
               
                   
                   
               
               
                   
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
                   
               
               
                   
                 M1 
                 0.000000E+00 
                  4.357111E−11  
                  1.480406E−16 
               
               
                   
                 M2 
                 0.000000E+00 
                 −1.386259E−07 
                  4.004273E−12 
               
               
                   
                 M4 
                 0.000000E+00 
                  2.326524E−09  
                 −1.752362E−14 
               
               
                   
                   
               
               
                   
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
                   
               
               
                   
                 M1 
                  4.934056E−22 
                 9.065147E−28  
                 1.077925E−32 
               
               
                   
                 M2 
                 −7.308931E−14  
                 1.933971E−16  
                 0.000000E+00 
               
               
                   
                 M4 
                  9.974181E−20 
                 0.000000E+00  
                 0.000000E+00 
               
               
                   
                   
               
            
           
         
       
     
     In the case of the imaging optical unit  27 , therefore, the mirrors M 1 , M 2  and M 4  are embodied as aspherical mirrors. The mirror M 3  is embodied as a spherical mirror. 
     With reference to  FIGS. 7 and 8 , a description is given below of a further embodiment of an imaging optical unit  32 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  32  can be used in the metrology system  1  according to  FIG. 1 , that is to say for examining a reflective reticle  2 . 
     The imaging beam path  8  of the imaging optical unit  32  is similar to that of the imaging optical unit  27 . Between the object field  6  and the mirror M 3 , the imaging beam path  8  of the imaging optical unit  32  can be regarded as mirrored about the plane  30  in comparison with the imaging optical unit  27 . 
     The impingement point  28  of the chief ray  13  of the central object field point on the first mirror M 1  in the imaging beam path  8  and the impingement point  29  of the chief ray of the central object field point on the fourth mirror M 4  in the imaging beam path  8  lie on the same side of the plane  30 . In the case of the imaging optical unit  32 , therefore, the fourth mirror M 4  is not structure-space-limiting for the illumination optical unit  5 , which is indicated schematically in  FIG. 7 . 
     Instead of a single passage opening  21  in the mirror body  22 , two passage openings  21   a ,  21   b  are embodied in the mirror body  22  of the mirror M 1  in the case of the imaging optical unit  32 . Through the passage opening  21   a , the first imaging partial ray  19  between the mirrors M 2  and M 3  passes through the mirror body  22 . Through the further passage opening  21   b , the imaging partial ray  20  between the mirrors M 3  and M 4  passes through the mirror body  22 . 
     The passage opening  21   a  is shaded by the mirror M 2 . 
     The imaging light partial rays  24 ,  25 ,  19  and additionally the second imaging light partial ray  20  pass through the aperture stop  17 . 
     The imaging optical unit  32  has a structural length T of 741 mm. A ratio between the distance A between the mirror M 4  and the object plane  11  and the structural length T is A/T≈0.28. The ratio T/β of the structural length T and the imaging scale β (β=850) is T/β=0.87 in the case of the imaging optical unit  32 . 
     The optical data of the imaging optical unit  32  according to  FIG. 7  are reproduced below with the aid of two tables, which correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Surface 
                 Radius 
                 Thickness 
                 Operating mode 
               
               
                   
                   
               
               
                   
                 Object 
                 Infinite 
                 299.082 
                   
               
               
                   
                 Stop 
                 Infinite 
                 321.628 
                   
               
               
                   
                 M1 
                 −467.134 
                 −400.711 
                 REFL 
               
               
                   
                 M2 
                 49.955 
                 500.811 
                 REFL 
               
               
                   
                 M3 
                 45.000 
                 −501.728 
                 REFL 
               
               
                   
                 M4 
                 −1007.185 
                 521.728 
                 REFL 
               
               
                   
                 Image 
                 Infinite 
                 0.000 
                   
               
               
                   
                   
               
               
                   
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
                   
               
               
                   
                 M1 
                 0.000000E+00  
                  8.920370E−11 
                  3.897637E−16 
               
               
                   
                 M2 
                 0.000000E+00  
                 −2.340808E−07  
                 −8.443464E−11 
               
               
                   
                 M4 
                 0.000000E+00  
                  3.951304E−09 
                 −3.068802E−14 
               
               
                   
                   
               
               
                   
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
                   
               
               
                   
                 M1 
                 1.859259E−21  
                  2.937370E−27 
                 6.606394E−32 
               
               
                   
                 M2 
                 1.060639E−13  
                 −1.686228E−16  
                 0.000000E+00 
               
               
                   
                 M4 
                 1.570060E−19  
                  0.000000E+00 
                 0.000000E+00 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 8  shows a profile  33  of the chief ray distortion CRD against the field height y. In principle, the CRD profile  33  of the imaging optical unit  32  according to  FIG. 7  is similar to the CRD profile  31  of the imaging optical unit  27  according to  FIG. 5 . In the case of a field height of y=0, a chief ray distortion CRD of 0 μm is present. In the case of a field height y≈15 μm, a local maximum of the chief ray distortion of CRD≈700 nm is present. In the case of a field height y≈70 μm, a minimum of the chief ray distortion of CRD≈−1400 nm is present. In the case of a field height y≈100 μm, a global maximum of the chief ray distortion CRD≈1400 nm is present. The absolute chief ray distortion is not greater than 1500 nm over the entire y-field height. 
     With reference to  FIGS. 9 and 10 , a description is given below of a further embodiment of an imaging optical unit  34 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  34  has two intermediate images, namely alongside the intermediate image  18  also a further intermediate image  35  in the imaging beam path between the mirrors M 3  and M 4 . 
     A further pupil plane  36  lies between the second intermediate image  35  and the image field  9 , said further pupil plane representing an image of the plane in which the aperture stop  17  is arranged. Adjacent to the pupil plane  36  arranged in the imaging beam path  8  between the mirror M 4  and the image field  9 , an imaging partial ray  37  between the mirror M 4  and the image field  9  has a small diameter in comparison with the transverse dimensions of the image field  9 . The imaging partial ray  37  is the third imaging partial ray that passes through the mirror body  22  of the mirror M 1  of the imaging optical unit  34 , and is therefore also referred to as third imaging partial ray  37 . 
     Similarly to the embodiment of the imaging optical unit  32 , the mirror body  22  of the mirror M 1  has two passage openings  21   a ,  21   b . The first imaging partial ray  19  and the second imaging partial ray  20  pass through the passage opening  21   a . The third imaging partial ray  37  passes through the passage opening  21   b . The passage opening  21   a  is completely shaded by the mirror M 2 . An additional obscuration of the imaging beam path  8  by the passage opening  21   b  is small on account of the small diameter of the passage opening  21   b.    
     The imaging optical unit  39  has a structural length T of 800 mm 
     A ratio between the distance A between the mirror M 4  and the object plane  11  and the structural length T is A/T=0.24. 
     In the case of the imaging optical unit  34 , the chief rays  13  run divergently between the pupil plane  36  and the image field  9 . 
     On account of the imaging beam path being folded at the mirror M 4  back in the direction of the mirror M 1 , this results in an overall very compact imaging optical unit  34  in the y-direction. A distance B between points of the mirrors M 1  to M 4 , of the object field  6  and of the object field  9  which are furthest away from one another in the y-direction and to which imaging radiation is applied is therefore small. The ratio B/T is 0.41 in the case of the imaging optical unit  34 . 
     The optical data of the imaging optical unit  34  according to  FIG. 9  are reproduced below with the aid of two tables, which correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Object 
                 Infinite 
                 376.829 
                   
               
               
                   
                 Stop 
                 Infinite 
                 423.171 
                   
               
               
                   
                 M1 
                 −680.112 
                 −620.000  
                 REFL 
               
               
                   
                 M2 
                 54.939 
                 719.999 
                 REFL 
               
               
                   
                 M3 
                 45.614 
                 −619.999  
                 REFL 
               
               
                   
                 M4 
                 468.493 
                 947.141 
                 REFL 
               
               
                   
                 Image 
                 Infinite 
                 0.000 
                   
               
               
                   
                   
               
               
                   
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
                   
               
               
                   
                 M1 
                 0.000000E+00  
                  1.271439E−11  
                  2.758381E−17 
               
               
                   
                 M2 
                 0.000000E+00  
                  5.407597E−08 
                  7.532271E−11 
               
               
                   
                 M4 
                 0.000000E+00  
                 −5.313806E−10  
                 −1.465797E−15 
               
               
                   
                   
               
               
                   
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
                   
               
               
                   
                 M1 
                  5.668265E−23  
                 7.895876E−29 
                 4.584057E−34 
               
               
                   
                 M2 
                 −1.079043E−14  
                 9.519225E−17 
                 0.000000E+00 
               
               
                   
                 M4 
                 −8.252054E−21 
                 0.000000E+00  
                 0.000000E+00 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 10  shows a chief ray distortion profile or CRD profile  38  over the field height y of the object field  6  of the imaging optical unit  34 . In principle, this CRD profile is similar to that according to  FIGS. 6 and 8 , wherein, in contrast to those profiles, the CRD profile  38  falls to smaller absolute values again at the right-hand field edge in  FIG. 10 . In the case of the field height y≈0, the chief ray distortion CRD z≈−15 nm. In the case of the field height y≈20 μm, the chief ray distortion CRD≈30 nm and has a local maximum there. In the case of the field height y≈55 μm, the CRD profile  38  has a global minimum at CRD y≈−18 nm. In the case of the field height y≈90 μm, the CRD profile has a global maximum at CRD≈40 μm. In absolute terms, the chief ray distortion is always less than 40 nm within the entire y-field height. 
     In the case of the imaging optical unit  34 , the impingement points  28 ,  29  again lie on different sides of the plane  30 . 
     With reference to  FIGS. 11 and 12 , a description is given below of a further embodiment of an imaging optical unit  39 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     In comparison with the imaging optical unit  34 , the imaging optical unit  39  is mirrored by part of its imaging beam path  8  about the plane  30  in a comparable manner to that as explained above in the comparison of the imaging optical units  27  and  32  according to  FIGS. 5 and 7 . In the case of the imaging optical unit  39 , the imaging partial rays  19  and  20  pass through the passage opening  21  of the mirror body  22  of the mirror M 1 . The imaging partial ray  37  runs past the mirror M 1 , that is to say does not pass through the mirror body  22  of the mirror M 1 . 
     All the imaging partial rays  24 ,  25 ,  19 ,  20  and  37  of the imaging beam path  8  pass through the aperture stop  17 . 
     The impingement points  28  and  29  both lie on the same side of the plane  30 . 
     The imaging optical unit  39  has a structural length T of 800 mm and a magnification scale β of 850. The ratio T/β is 0.94 as in the case of the imaging optical unit  27  according to  FIG. 5 . 
     The optical data of the imaging optical unit  39  according to  FIG. 11  are reproduced below with the aid of two tables, which correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Surface 
                 Radius 
                 Thickness  
                 Operating mode 
               
               
                   
                   
               
               
                   
                 Object 
                 Infinite 
                 301.306 
                   
               
               
                   
                 Stop 
                 Infinite 
                 379.389 
                   
               
               
                   
                 M1 
                 −559.837 
                 −500.696 
                 REFL 
               
               
                   
                 M2 
                 48.560 
                 600.000 
                 REFL 
               
               
                   
                 M3 
                 40.000 
                 −680.000 
                 REFL 
               
               
                   
                 M4 
                 409.424 
                 700.000 
                 REFL 
               
               
                   
                 Image 
                 Infinite 
                 0.000 
                   
               
               
                   
                   
               
               
                   
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
                   
               
               
                   
                 M1 
                 0.000000E+00 
                  2.965442E−11 
                  9.292083E−17 
               
               
                   
                 M2 
                 0.000000E+00 
                 −2.649999E−08 
                  3.216689E−11 
               
               
                   
                 M4 
                 0.000000E+00 
                 −1.131277E−09  
                 −3.568456E−15 
               
               
                   
                   
               
               
                   
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
                   
               
               
                   
                 M1 
                  2.937853E−22 
                 5.132600E−28 
                 5.061928E−33 
               
               
                   
                 M2 
                  8.859961E−14 
                 0.000000E+00 
                 0.000000E+00 
               
               
                   
                 M4 
                 −2.085254E−20  
                 0.000000E+00 
                 0.000000E+00 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 12  shows a CRD profile  40  of the imaging optical unit  39  over the field height y of the object field  6 . 
     In the case of the field height y≈0, the distortion CRD≈5 nm. In the case of the field height y≈30 μm, the distortion CRD≈−40 nm and has a local minimum there. In the case of the field height y≈80 μm, the distortion CRD≈150 nm and has a global maximum there. In the case of the field height y≈100 μm, the distortion CRD≈−60 μm. The chief ray distortion CRD is less than 150 nm in absolute terms over the entire y-field height of the object field  6  of the imaging optical unit  39 . 
     With reference to  FIGS. 13 and 14 , a description is given below of a further embodiment of an imaging optical unit  41 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  41  differs from the imaging optical unit  27  according to  FIG. 5  principally in that the mirror M 2  is embodied in convex fashion and the third mirror M 3  is embodied in concave fashion. The intermediate image  18  is arranged between the mirrors M 3  and M 4  in the case of the imaging optical unit  41 . 
     The mirrors M 1  and M 2  are configured in aspherical fashion and the mirrors M 3  and M 4  are configured in spherical fashion. 
     The imaging optical unit  41  has a size of the object field  6  of 100 μm in the y-direction and of 400 μm in the x-direction. The imaging optical unit  41  has a magnification factor (scale) of 850. The imaging optical unit  41  has a structural length T of 800 mm. The ratio T/β is 0.93. The object-side chief ray angle α is 10°. 
     The optical data of the imaging optical unit  41  according to  FIG. 13  are reproduced below with the aid of two tables, which correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Surface 
                 Radius 
                 Thickness 
                 Operating mode 
               
               
                   
                   
               
               
                   
                 Object 
                 Infinite 
                 258.727 
                   
               
               
                   
                 Stop 
                 Infinite 
                 378.264 
                   
               
               
                   
                 M1 
                 −543.947  
                 −456.991 
                 REFL 
               
               
                   
                 M2 
                 −36.455 
                 557.137 
                 REFL 
               
               
                   
                 M3 
                 −40.703 
                 −637.137 
                 REFL 
               
               
                   
                 M4 
                 1563.169 
                 691.213 
                 REFL 
               
               
                   
                 Image 
                 Infinite 
                 0.000 
                   
               
               
                   
                   
               
               
                   
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
                   
               
               
                   
                 M1 
                 0.000000E+00  
                 1.033316E−11  
                  3.279920E−17 
               
               
                   
                 M2 
                 1.308094E−01  
                 0.000000E+00  
                 −5.196086E−10 
               
               
                   
                   
               
               
                   
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
                   
               
               
                   
                 M1 
                 1.148946E−22  
                 1.623072E−28 
                 2.445232E−33 
               
               
                   
                 M2 
                 0.000000E+00  
                 0.000000E+00  
                 0.000000E+00 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 14  shows a CRD profile  42  of the imaging optical unit  41  over the field height y of the object field  6 . 
     In the case of the field height y≈0, the distortion CRD≈170 nm. In the case of the field height y≈65 μm, the distortion CRD≈−250 nm and has a global minimum there. In the case of the field height y≈110 μm, the distortion CRD≈170 nm. The chief ray distortion CRD is less than 260 nm in absolute terms over the entire y-field height of the object field  6  of the imaging optical unit  41 . 
     With reference to  FIGS. 15 and 16 , a description is given below of a further embodiment of an imaging optical unit  43 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     In comparison with the imaging optical unit  41 , the imaging optical unit  43  is mirrored by part of its imaging beam path  8  about the plane  30  in a comparable manner to that as explained above in the comparison of the imaging optical units  27  and  32  according to  FIGS. 5 and 7 . 
     The imaging optical unit  43  has a structural length T of 786 mm and a magnification scale β of 850. The ratio T/β is 0.92. 
     The optical data of the imaging optical unit  43  according to  FIG. 15  are reproduced below with the aid of two tables, which correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Operating mode 
               
               
                   
               
               
                 Object 
                 Infinite 
                 258.747 
                   
               
               
                 Stop 
                 Infinite 
                 377.289 
                   
               
               
                 M1 
                 −542.906 
                 −456.036 
                 REFL 
               
               
                 M2 
                 −36.246 
                 556.120 
                 REFL 
               
               
                 M3 
                 −40.479 
                 −636.120 
                 REFL 
               
               
                 M4 
                 1547.952 
                 685.587 
                 REFL 
               
               
                 Image 
                 Infinite 
                 0.000 
                   
               
               
                   
               
               
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
               
               
                 M1 
                 0.000000E+00 
                 1.049517E−11 
                   3.354943E−17 
               
               
                 M2 
                 1.285065E−01 
                 0.000000E+00 
                 −6.437537E−10 
               
               
                   
               
               
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
               
               
                 M1 
                 1.086720E−22 
                 2.589792E−28 
                   2.021330E−33 
               
               
                 M2 
                 0.000000E+00 
                 0.000000E+00 
                   0.000000E+00 
               
               
                   
               
            
           
         
       
     
       FIG. 16  shows a CRD profile  44  of the imaging optical unit  43  against the field height y of the object field  6 . This field height profile is similar to the CRD profile  42  according to  FIG. 14 . 
     In the case of the field height y≈0, the distortion CRD≈200 nm. In the case of the field height y≈70 μm, the distortion CRD≈−300 nm and has a global minimum there. In the case of the field height y≈100 μm, the distortion CRD≈250 nm. The chief ray distortion CRD is less than 330 nm in absolute terms over the entire y-field height of the object field  6  of the imaging optical unit  43 . 
     With reference to  FIGS. 17 and 18 , a description is given below of a further embodiment of an imaging optical unit  45 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     In the case of the imaging optical unit  45 , no intermediate image is present between the object field  6  and the image field  9  in the imaging beam path  8 . The mirrors M 2  and M 3  are configured in convex fashion. 
     The imaging optical unit  45  has a structural length T of 1050 mm and a magnification scale β in absolute terms of 850. The ratio T/β is 1.24. 
     The optical data of the imaging optical unit  45  according to  FIG. 17  are reproduced below with the aid of two tables, which correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Operating mode 
               
               
                   
               
               
                 Object 
                 Infinite 
                 256.742 
                   
               
               
                 Stop 
                 Infinite 
                 373.890 
                   
               
               
                 M1 
                 −545.447 
                 −450.631 
                 REFL 
               
               
                 M2 
                 −61.991 
                 820.000 
                 REFL 
               
               
                 M3 
                 59.543 
                 −900.000 
                 REFL 
               
               
                 M4 
                 2477.069 
                 950.000 
                 REFL 
               
               
                 Image 
                 Infinite 
                 0.000 
                   
               
               
                   
               
               
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
               
               
                 M1 
                 0.000000E+00 
                   4.197072E−12 
                   6.316517E−19 
               
               
                 M2 
                 1.125022E−01 
                   0.000000E+00 
                 −1.570881E−10 
               
               
                   
               
               
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
               
               
                 M1 
                 7.807468E−23 
                 −6.468616E−28 
                   3.776136E−33 
               
               
                 M2 
                 0.000000E+00 
                   0.000000E+00 
                   0.000000E+00 
               
               
                   
               
            
           
         
       
     
       FIG. 18  shows a CRD profile  46  of the imaging optical unit  45  against the field height y of the object field  6 . 
     In the case of the field height y≈0, the distortion CRD≈30 μm. Up to the field height y≈10 μm, the distortion remains practically unchanged. In the further profile, the distortion falls to a value CRD≈−62 μm. The chief ray distortion CRD is less than 63 μm in absolute terms over the entire y-field height of the object field  6  of the imaging optical unit  45 . 
     With reference to  FIGS. 19 and 20 , a description is given below of a further embodiment of an imaging optical unit  47 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     In comparison with the imaging optical unit  45  according to  FIG. 17 , the imaging optical unit  47  according to  FIG. 19  is mirrored by part of its imaging beam path  8  about the plane  30  in a comparable manner to that as explained above in the comparison of the imaging optical units  27  and  32  according to  FIGS. 5 and 7 . 
     In the case of the imaging optical unit  47 , the mirrors M 2 , M 3  and M 4  are configured as convex mirrors. 
     The imaging optical unit  47  has a structural length T of 800 mm and a magnification scale β in absolute terms of 850. The ratio T/β is 0.94 as in the case of the imaging optical units  27  and  39 . 
     The optical data of the imaging optical unit  47  according to  FIG. 19  are reproduced below with the aid of two tables, which correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Operating mode 
               
               
                   
               
               
                 Object 
                 Infinite 
                 248.571 
                   
               
               
                 Stop 
                 Infinite 
                 374.783 
                   
               
               
                 M1 
                 −555.686 
                 −443.354 
                 REFL 
               
               
                 M2 
                 −126.546 
                 617.636 
                 REFL 
               
               
                 M3 
                 144.878 
                 −697.636  
                 REFL 
               
               
                 M4 
                 −214.474 
                 700.000 
                 REFL 
               
               
                 Image 
                 Infinite 
                 0.000 
                   
               
               
                   
               
               
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
               
               
                 M1 
                   0.000000E+00 
                 −1.227299E−11 
                 −4.503697E−17 
               
               
                 M2 
                   4.927422E−01 
                   0.000000E+00 
                 −1.277664E−12 
               
               
                 M3 
                   0.000000E+00 
                   6.607818E−08 
                 −2.188416E−12 
               
               
                   
               
               
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
               
               
                 M1 
                 −1.152684E−22 
                 −2.486658E−28 
                 −2.171236E−33 
               
               
                 M2 
                   9.892206E−16  
                   0.000000E+00 
                   0.000000E+00 
               
               
                 M3 
                   0.000000E+00 
                   0.000000E+00 
                   0.000000E+00 
               
               
                   
               
            
           
         
       
     
       FIG. 20  shows a CRD profile  48  of the imaging optical unit  47  against the field height y of the object field  6 . 
     In the case of the field height y≈0, the distortion CRD≈−10 μm. In the case of the field height≈65 μm, the distortion CRD≈12.5 μm and has a global maximum there. In the case of the field height y≈100 μm, the distortion CRD≈−10 μm. The chief ray distortion CRD is less than 12.5 μm over the entire y-field height of the object field  6  of the imaging optical unit  47 . 
     With reference to  FIG. 21 , a description is given below of a further embodiment of an imaging optical unit  49 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     In comparison with the imaging optical unit  7  according to  FIG. 3 , the imaging optical unit  49  according to  FIG. 21  has lower incidence angles of the imaging rays of the imaging beam path  8  on the mirror M 3 . 
     The imaging optical unit  49  has a structural length T of 1088 mm between the object plane  11  and the image plane  12 . A distance A between the mirror M 4  and the object plane is more than 17% of the structural length T. 
     The passage opening  21  lies in the shade of the mirror M 2 . 
     The chief rays  13  of different field points run divergently in the imaging beam path  8  between the last mirror M 4  and the image field  9 . 
     The ratio T/β of the structural length T and the imaging scale β (β=850) is T/β=1.28 in the case of the imaging optical unit  49 . 
     The imaging optical unit  49  has an object-side numerical aperture of 0.25. The object field  6  of the imaging optical unit  49  has a size of 106 μm in the y-direction and 680 μm in the x-direction. 
     The optical data of the imaging optical unit  49  according to  FIG. 21  are reproduced below with the aid of two tables, which correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 328.979 
                   
               
               
                 STOP 
                 INFINITY 
                 446.838 
                   
               
               
                 Mirror 1 
                 −646.249 
                 −608.573 
                 REFL 
               
               
                 Mirror 2 
                 103.429 
                 870.260  
                 REFL 
               
               
                 Mirror 3 
                 96.288 
                 −837.504 
                 REFL 
               
               
                 Mirror 4 
                 −950.126 
                 887.504  
                 REFL 
               
               
                 Image 
                 INFINITY 
                 0.000 
                   
               
               
                   
               
               
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
               
               
                 Mirror 1 
                 0.000000E+00 
                   2.130673E−11  
                   5.114172E−17 
               
               
                 Mirror 2 
                 0.000000E+00 
                 −1.484184E−09 
                   1.588111E−12 
               
               
                 Mirror 3 
                 0.000000E+00 
                   1.168086E−07 
                   3.806841E−11 
               
               
                 Mirror 4 
                 0.000000E+00 
                   2.159545E−09  
                 −9.203407E−15 
               
               
                   
               
               
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
               
               
                 Mirror 1 
                 1.117023E−22  
                   2.162742E−28 
                   8.660117E−34 
               
               
                 Mirror 2 
                 0.000000E+00 
                   0.000000E+00 
                   0.000000E+00 
               
               
                 Mirror 3 
                 0.000000E+00 
                   0.000000E+00 
                   0.000000E+00 
               
               
                 Mirror 4 
                 0.000000E+00 
                   0.000000E+00 
                   0.000000E+00 
               
               
                   
               
            
           
         
       
     
     In the case of the imaging optical unit  49 , therefore, mirrors M 1  to M 4  all are embodied as aspherical mirrors. 
     With reference to  FIG. 22 , a description is given below of a further embodiment of an imaging optical unit  50 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  50  is a variation of the imaging optical unit  49 . 
     The imaging optical unit  50  has a structural length T of 1000 mm between the optic plane  11  and the image plane  12 . 
     In the case of the imaging optical unit  50  the mirror M 2  is displaced along the x-direction such that the mirror M 2  does not obstruct the imaging partial ray  19  between the object field  6  and the mirror M 1 . 
     The ratio T/β of the structural length T and the imaging scale β (β=850) is T/β=1.18 in the case of the imaging optical unit  50 . 
     The imaging optical unit  50  has an object-side numerical aperture of 0.24. The object field  6  of the imaging optical unit  50  has a size of 106 μm in the y-direction and 680 μm in the x-direction. 
     The optical data of the imaging optical unit  50  according to  FIG. 22  are reproduced below with the aid of two tables, which correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 232.242 
                   
               
               
                 STOP 
                 INFINITY 
                 349.832 
                   
               
               
                 Mirror 1 
                 −545.209 
                 −562.074 
                 REFL 
               
               
                 Mirror 2 
                 93.327 
                 822.074 
                 REFL 
               
               
                 Mirror 3 
                 79.639 
                 −742.074 
                 REFL 
               
               
                 Mirror 4 
                 −2702.878 
                 900.000 
                 REFL 
               
               
                 Image 
                 −1669.981 
                 0.000 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Surface 
                 K 
                 A 
                 B 
                 C 
               
               
                   
               
               
                 Mirror 1 
                 0.000000E+00 
                 1.882766E−11 
                 6.580594E−17 
                   2.471942E−22 
               
               
                 Mirror 2 
                 0.000000E+00 
                 6.123538E−08  
                 1.458619E−11 
                   3.110235E−15 
               
               
                 Mirror 3 
                 0.000000E+00 
                 2.192720E−07  
                 1.493226E−10  
                 −2.207925E−13 
               
               
                   
               
            
           
         
       
     
     In the case of the imaging optical unit  50 , therefore, the mirrors M 1  to M 3  are embodied as aspherical mirrors. The mirror M 4  is embodied as a spherical mirror. 
     With reference to  FIG. 23 , a description is given below of a further embodiment of an imaging optical unit  51 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  51  has exactly three mirrors M 1 , M 2  and M 3  in the imaging beam path  8  between the object field  6  and the image field  9 . The image field  9  is not a planar field but is concavely curved. 
     The imaging optical unit  51  has a structural length T of 1010 mm between the object plane  11  and an arrangement plane  52  being parallel to the object plane  11  and representing the position of mirror M 3 . 
     The chief rays  13  of different field points run divergently in the imaging beam path  8  between the last mirror M 3  and the image field  9 . 
     The ratio T/β of the structural length T and the imaging scale β (β=850) is T/β=1.19 in the case of the imaging optical unit  51 . 
     The imaging optical unit  51  has an object-side numerical aperture of 0.24. The object field  6  of the imaging optical unit  51  has a size of 212 μm in the y-direction and 340 μm in the x-direction. 
     The optical data of imaging optical unit  51  according to  FIG. 23  are reproduced below with the aid of two tables which correspond in terms of structures to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 173.326 
                   
               
               
                 STOP 
                 INFINITY 
                 576.674 
                   
               
               
                 Mirror 1 
                 −667.237 
                 −576.674 
                 REFL 
               
               
                 Mirror 2 
                 −50.000 
                 836.674 
                 REFL 
               
               
                 Mirror 3 
                 −55.428 
                 −910.000 
                 REFL 
               
               
                 Image 
                 1118.363 
                 0.000 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Surface 
                 K 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
               
                 Mirror 1 
                 0.000000E+00 
                   1.239362E−12 
                   2.224440E−18 
                   2.381888E−24 
                 2.961774E−29 
               
               
                 Mirror 2 
                 0.000000E+00 
                 −3.373423E−07 
                 −2.670617E−10 
                 −3.891394E−13 
                 1.482808E−15 
               
               
                 Mirror 3 
                 0.000000E+00 
                 −9.836320E−08 
                   0.000000E+00 
                   0.000000E+00 
                 0.000000E+00 
               
               
                 Image 
                 0.000000E+00 
                 −3.940290E−11 
                   0.000000E+00 
                   0.000000E+00 
                 0.000000E+00 
               
               
                   
               
            
           
         
       
     
     In the case of the imaging optical unit  51  all mirrors M 1  to M 3  are embodied as aspherical mirrors. Further, the image field  9  is aspherically curved. 
     With reference to  FIG. 24 , a description is given below of a further embodiment of an imaging optical unit  53 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  53  has exactly three mirrors M 1  to M 3 . 
     Mirror M 2  is convex. 
     The imaging field  9  is concavely curved. 
     The imaging optical unit  53  has an object-side chief ray angle α between the normal  16  to the object plane  11  and the chief ray  13  of a central object field point of 10°. The imaging optical unit  53  can be used for the bright field illumination of a reflective reticle  2  in the metrology system  1  according to  FIG. 1  as is explained above with reference to the imaging optical unit  27  according to  FIGS. 5 and 6 . 
     The imaging optical unit  53  has a structural length T of 1093 mm between the object plane  11  and the arrangement plane  52  of mirror M 3 . 
     The chief rays  13  of different field points run divergently in the imaging beam path  8  between the last mirror M 3  and the image field  9 . 
     The ratio T/β of the structural length T and the imaging scale β (β=850) is T/β=1.29 in the case of the imaging optical unit  53 . 
     The imaging optical unit  53  has an object-side numerical aperture of 0.24. The object field  6  of the imaging optical unit  53  has a size of 212 μm in the y-direction and 340 μm in the x-direction. 
     The impingement point  28  of the chief ray  13  of the central object field point on the first mirror M 1  in the imaging beam path  8  and a central image field point  54  lie on the same side of the plane  30 . 
     The optical data of imaging optical unit  53  according to  FIG. 24  are reproduced below with the aid of two tables which correspond in terms of structures to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 250.000 
                   
               
               
                 STOP 
                 INFINITY 
                 583.122 
                   
               
               
                 Mirror 1 
                 −702.563 
                 −583.122 
                 REFL 
               
               
                 Mirror 2 
                 −50.000 
                 843.212 
                 REFL 
               
               
                 Mirror 3 
                 −50.219 
                 −893.212 
                 REFL 
               
               
                 Image 
                 1814.063 
                 0.000 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Surface 
                 K 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
               
                 Mirror 1 
                 −1.601482E−02 
                 0.000000E+00 
                 −4.392992E−19 
                 −7.984806E−25 
                 −4.607245E−30 
               
               
                 Mirror 2 
                   8.455222E−02  
                 0.000000E+00 
                 −8.959759E−11  
                 −6.520758E−14  
                 −3.194743E−17 
               
               
                 Mirror 3 
                 −5.068107E−01 
                 0.000000E+00 
                 −5.695781E−09  
                   3.720288E−11 
                 −9.829453E−14 
               
               
                 Image 
                   0.000000E+00 
                 4.003240E−09  
                 −5.632790E−14  
                   3.962980E−19 
                 −1.093640E−24 
               
               
                   
               
            
           
         
       
     
     In the case of the imaging optical unit  53 , all mirrors M 1  to M 3  are embodied as aspherical mirrors. In addition, the image field  9  is aspherically curved. 
     With reference to  FIG. 25 , a description is given below of a further embodiment of an imaging optical unit  55 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  55  has exactly three mirrors M 1  to M 3 . The image field  9  is concavely curved. The imaging partial ray  19  between the second mirror M 2  and the third mirror M 3  in the imaging beam path passes through the passage opening  21  in the mirror body  22  of the first mirror M 1 . 
     The imaging optical unit  55  has an object-side chief ray angle α between the normal  16  to the object plane  11  and the chief ray  13  of a central object field point of 10°. The imaging optical unit  55  can be used for the bright field illumination. 
     The imaging optical unit  55  has a structural length T of 1439 mm between the object plane  11  and the arrangement plane  52  of mirror M 3 . 
     The chief rays  13  of different field points run divergently in the imaging beam path  8  between the last mirror M 3  and the image field  9 . 
     The ratio T/β of the structural length T and the imaging scale β (β=711) is T/β=2.02 in the case of the imaging optical unit  55 . 
     The imaging optical unit  55  has an object-side numerical aperture of 0.2. The object field  6  of the imaging optical unit  55  has a size of 306 μm in the y-direction and 408 μm in the x-direction. 
     The impingement point  28  of the chief ray  13  of the central object field point on the first mirror M 1  in the imaging beam path  8  and the central image field point  54  lie on different sides of the plane  30 . 
     The optical data of imaging optical unit  55  according to  FIG. 25  are reproduced below with the aid of two tables which correspond in terms of structures to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 589.163 
                   
               
               
                 STOP 
                 INFINITY 
                 60.837 
                   
               
               
                 Mirror 1 
                 −526.058 
                 −475.342 
                 REFL 
               
               
                 Mirror 2 
                 65.360 
                 1263.987 
                 REFL 
               
               
                 Mirror 3 
                 56.456 
                 −738.645 
                 REFL 
               
               
                 Image 
                 980.894 
                 0.000 
               
               
                   
               
               
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
               
               
                 Mirror 1 
                   0.000000E+00 
                   4.300373E−11 
                   1.548645E−16 
               
               
                 Mirror 2 
                   0.000000E+00 
                 −4.824465E−08 
                   1.001720E−11 
               
               
                 Mirror 3 
                   0.000000E+00 
                   1.064409E−07 
                 −8.351938E−11 
               
               
                 Image 
                   0.000000E+00 
                 −9.399710E−11  
                   1.166900E−15 
               
               
                   
               
               
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
               
               
                 Mirror 1 
                   4.891213E−22 
                   1.852110E−27 
                   7.401320E−33 
               
               
                 Mirror 2 
                 −3.075640E−14 
                   9.015706E−17  
                 −8.848435E−20 
               
               
                 Mirror 3 
                   1.092495E−12  
                 −2.579340E−15  
                   1.506823E−34 
               
               
                 Image 
                 −8.581340E−21 
                   3.578410E−26  
                 −9.483660E−32 
               
               
                   
               
            
           
         
       
     
     In the case of the imaging optical unit  55 , the mirrors M 1  to M 3  are embodied as aspherical mirrors. Further, image field  9  is aspherically curved. 
     With reference to  FIG. 26 , a description is given below of a further embodiment of an imaging optical unit  56 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  56  has exactly three mirrors M 1  to M 3 , none of which is obscured. None of the mirrors M 1  to M 3  therefore has a through-hole for imaging light to pass through. Mirror M 1  may have an edge side recess for passage of the imaging partial ray  19 . 
     The image field  9  is concavely curved. 
     The imaging optical unit  56  has an object-side chief ray angle α between the normal  16  to the object plane  11  and the chief ray  13  of a central object field point of 6°. The imaging optical unit  56  can be used for the bright field illumination. 
     The imaging optical unit  56  has a structural length T of 1300 mm between the object plane  11  and the arrangement plane  52  of mirror M 3 . 
     The chief rays  13  of different field points run divergently in the imaging beam path  8  between the last mirror M 3  and the image field  9 . 
     The ratio T/β of the structural length T and the imaging scale β (β=444) is T/β=2.93. 
     The imaging optical unit  56  has an object-side numerical aperture of 0.125. The object field  6  of the imaging optical unit  56  has a size of 490 μm in the y-direction and 652 μm in the x-direction. 
     An impingement point  28  of the chief ray  13  of the central object field point on the first mirror M 1  in the imaging beam path  8  and the central image field point  54  lie on the same side of the plane  30 . 
     The optical data of imaging optical unit  56  according to  FIG. 26  are reproduced below with the aid of two tables which correspond in terms of structures to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 540.146 
                   
               
               
                 STOP 
                 INFINITY 
                 39.837 
                   
               
               
                 Mirror 1 
                 −441.759 
                 −404.983 
                 REFL 
               
               
                 Mirror 2 
                 92.640 
                 1125.017 
                 REFL 
               
               
                 Mirror 3 
                 75.846 
                 −1100.017 
                 REFL 
               
               
                 Image 
                 1418.455 
                 0.000 
               
               
                   
               
               
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
               
               
                 Mirror 1 
                   0.000000E+00 
                   1.133303E−10 
                   5.556978E−16 
               
               
                 Mirror 2 
                   0.000000E+00 
                 −4.050928E−08 
                 −4.091379E−12 
               
               
                 Mirror 3 
                   0.000000E+00 
                   7.487605E−08 
                 −3.577094E−10 
               
               
                 Image 
                 −1.000000E+01 
                   2.773900E−10 
                 −2.364600E−16 
               
               
                   
               
               
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
               
               
                 Mirror 1 
                   3.170923E−21 
                 −5.865964E−27 
                   2.974805E−31 
               
               
                 Mirror 2 
                   4.020399E−15 
                 −6.638198E−18 
                   4.171862E−21 
               
               
                 Mirror 3 
                   1.316485E−12 
                 −2.503142E−15 
                   1.942998E−18 
               
               
                 Image 
                   9.716070E−22 
                 −3.737610E−27  
                   4.766980E−33 
               
               
                   
               
            
           
         
       
     
     In the case of the imaging optical unit  56 , the mirrors M 1  to M 3  are embodied as aspherical mirrors. Further, the image field  9  is aspherically curved. 
     With reference to  FIG. 27 , a description is given below of a further embodiment of an imaging optical unit  57 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  57  corresponds to the imaging optical unit  55  according to  FIG. 25 . A difference is that mirror M 2  of the imaging optical unit  57  is concave. 
     The imaging optical unit  57  has a structural length T of 1068 mm between the object plane  11  and the arrangement plane  52  of mirror M 3 . 
     The ratio T/β of the structural length T and the imaging scale β (β=711) is T/β=1.50 in the case of the imaging optical unit  57 . 
     The optical data of imaging optical unit  57  according to  FIG. 27  are reproduced below with the aid of two tables which correspond in terms of structures to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 530.284 
                   
               
               
                 STOP 
                 INFINITY 
                 49.716 
                   
               
               
                 Mirror 1 
                 −456.922 
                 −405.000 
                 REFL 
               
               
                 Mirror 2 
                 54.461 
                 893.251 
                 REFL 
               
               
                 Mirror 3 
                 47.406 
                 −770.706 
                 REFL 
               
               
                 Image 
                 1027.326 
                 0.000 
               
               
                   
               
               
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
               
               
                 Mirror 1 
                   0.000000E+00 
                   7.603561E−11 
                   3.602510E−16 
               
               
                 Mirror 2 
                   0.000000E+00 
                 −1.304980E−07  
                   6.663337E−12 
               
               
                 Mirror 3 
                   0.000000E+00 
                   1.199487E−07 
                 −1.899920E−09 
               
               
                 Image 
                 −3.180656E+00 
                   0.000000E+00 
                   6.175220E−15 
               
               
                   
               
               
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
               
               
                 Mirror 1 
                   1.503883E−21 
                   7.273048E−27  
                   4.216415E−32 
               
               
                 Mirror 2 
                 −1.221818E−13 
                   4.340964E−16  
                 −5.660438E−19 
               
               
                 Mirror 3 
                   2.997983E−11 
                 −2.140167E−13  
                   5.922798E−16 
               
               
                 Image 
                 −5.465910E−20 
                   2.002020E−25  
                 −1.822510E−31 
               
               
                   
               
            
           
         
       
     
     In the case of the imaging optical unit  57 , the mirrors M 1  to M 3  are embodied as aspherical mirrors. Further, the image field  9  is aspherically curved. 
     With reference to  FIG. 28 , a description is given below of a further embodiment of an imaging optical unit  58 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  58  has exactly four mirrors M 1  to M 4   
     The imaging partial ray  19  between the second mirror M 2  and the third mirror M 3  in the imaging beam path  8  passes the passage opening  21  in the mirror body  22  of the first mirror M 1  of imaging optical unit  58 . 
     The imaging optical unit  58  has an object-side chief ray angle α between the normal  16  to the object plane  11  and the chief ray  13  of a central object field point of 10°. The imaging optical unit  58  can be used for the bright field illumination. 
     The imaging optical unit  58  has a structural length T of 1300 mm between the object plane  11  and the image plane  12 . 
     A distance A between the mirror M 4  and the object plane  11  is more than 38% of the structural length T. In case of the imaging optical unit  58 , enough structural space for the imaging optical unit  5  is present in the vicinity of the object plane  11 . 
     The chief rays  13  of different field points run divergently in the imaging beam path  8  between the last mirror M 3  and the image field  9 . 
     The ratio T/β of the structural length T and the imaging scale β (β=711) is T/β=1.82 in the case of the imaging optical unit  58 . 
     The imaging optical unit  58  has an object-side numerical aperture of 0.2. The object field  6  of the imaging optical unit  58  has a size of 306 μm in the y-direction and 408 μm in the x-direction. 
     An impingement point  28  of the chief ray  13  of the central object field point on the first mirror M 1  in the imaging beam path  8  and an impingement point  29  of the chief ray  13  of central object field point on the fourth mirror M 4  in the imaging beam path  8  lie on different sides of the plane  30 . 
     The optical data of imaging optical unit  58  according to  FIG. 28  are reproduced below with the aid of two tables which correspond in terms of structures to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 296.323 
                   
               
               
                 STOP 
                 INFINITY 
                 341.744 
                   
               
               
                 Mirror 1 
                 −516.195 
                 −463.010 
                 REFL 
               
               
                 Mirror 2 
                 57.307 
                 872.873 
                 REFL 
               
               
                 Mirror 3 
                 50.000 
                 −547.930 
                 REFL 
               
               
                 Mirror 4 
                 1797.024 
                 800.000 
                 REFL 
               
               
                 Image 
                 INFINITY 
                 0.000 
                   
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Surface 
                 K 
                 A 
                 B 
                 C 
               
               
                   
               
               
                 Mirror 1 
                 −6,598742E−02 
                 −1.552137E−11 
                 −5.121132E−17 
                 −2.187397E−22 
               
               
                 Mirror 2 
                   0.000000E+00 
                 −6.282086E−08 
                   2.256927E−11 
                 −1.094029E−13 
               
               
                 Mirror 3 
                   0.000000E+00 
                   2.308663E−07  
                 −4.401882E−09 
                   9.024446E−11 
               
               
                 Mirror 4 
                   0.000000E+00 
                   3.558040E−11  
                 −7.077130E−16 
                   2.458008E−20 
               
               
                   
               
               
                 Surface 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
               
                 Mirror 1 
                   4.055956E−28 
                 −1.134847E−32 
                   2.873614E−38 
                   0.000000E+00 
               
               
                 Mirror 2 
                   5.557270E−16 
                 −1.324566E−18  
                   1.289002E−21 
                   0.000000E+00 
               
               
                 Mirror 3 
                 −1.006407E−12 
                   5.908172E−15 
                 −1.421500E−17 
                   0.000000E+00 
               
               
                 Mirror 4 
                 −5.030491E−25 
                   5.389670E−30  
                 −2.349207E−35 
                   0.000000E+00 
               
               
                   
               
            
           
         
       
     
     In case of the imaging optical unit  58 , all mirrors M 1  to M 4  are embodied as aspherical mirrors. The image field  9  is planar. 
     With reference to  FIG. 29 , a description is given below of a further embodiment of an imaging optical unit  59 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  59  corresponds to the imaging optical unit  58  of  FIG. 28 . 
     A difference is that mirror M 4  of imaging optical unit  59  is spherical. 
     The optical data of imaging optical unit  59  according to  FIG. 29  are reproduced below with the aid of two tables which correspond in terms of structures to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 292.634 
                   
               
               
                 STOP 
                 INFINITY 
                 337.366 
                   
               
               
                 Mirror 1 
                 −508.391 
                 −455.012 
                 REFL 
               
               
                 Mirror 2 
                 56.050 
                 925.011 
                 REFL 
               
               
                 Mirror 3 
                 48.906 
                 −600.000 
                 REFL 
               
               
                 Mirror 4 
                 1554.806 
                 800.000 
                 REFL 
               
               
                 Image 
                 INFINITY 
                 0.000 
                   
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Surface 
                 K 
                 A 
                 B 
                 C 
               
               
                   
               
               
                 Mirror 1 
                   0.000000E+00 
                   4.715547E−11 
                   1.809879E−16 
                   6.262806E−22 
               
               
                 Mirror 2 
                   0.000000E+00 
                 −7.323815E−08 
                   1.341416E−11 
                 −2.837041E−14 
               
               
                 Mirror 3 
                   0.000000E+00 
                   9.968913E−08 
                 −3.661928E−10 
                   6.824245E−12 
               
               
                   
               
               
                 Surface 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
               
                 Mirror 1 
                   2.511566E−27 
                   5.772735E−33 
                   4.915167E−38 
                   0.000000E+00 
               
               
                 Mirror 2 
                   1.263719E−16  
                 −1.541552E−19 
                   1.900983E−23 
                   0.000000E+00 
               
               
                 Mirror 3 
                 −4.729789E−14 
                   1.253633E−16 
                   6.159083E−24 
                   0.000000E+00 
               
               
                   
               
            
           
         
       
     
     In case of the imaging optical unit  59 , the mirrors M 1  to M 3  are embodied as aspherical mirrors. The image field  9  is planar. 
     With reference to  FIG. 30 , a description is given below of a further embodiment of an imaging optical unit  60 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  60  has an object-side chief ray angle α between the normal  16  to the object plane  11  and the chief ray  13  of a central object field point of 10°. The imaging optical unit  60  can be used for the bright field illumination. 
     The imaging optical unit  60  has a structural length T of 1300 mm between the object plane  11  and the image field  9 . The image plane  12  does not run parallel to the object plane  11 . 
     The imaging partial ray  19  between the mirror M 2  and the mirror M 3 , the imaging partial ray  20  between the mirror M 3  and the mirror M 4  and the imaging partial ray  37  between the last mirror M 4  in the imaging beam path  8  of the imaging optical unit  60  all pass mirror M 1  at a small distance. Dependent on the practical design of the mirror M 1 , this mirror M 1  in a first embodiment has a passage opening  21  for passage of the imaging partial ray  19  between the second mirror M 2  and the third mirror M 3  in the imaging beam path and for passage of the imaging partial ray  20  between the third mirror M 3  and the fourth mirror M 4  in the imaging beam path. Such passage may be realized in the mirror M 1  as a through-hole or as an edge side recess. 
     The chief rays  13  of different field points run divergently in the imaging beam path  8  between the last mirror M 4  and the image field  9 . 
     The ratio T/β of the structural length T and the imaging scale β (β=711) is T/β=1.82 in the case of the imaging optical unit  60 . 
     The imaging optical unit  60  has an object-side numerical aperture of 0.2. The object field  6  of the imaging optical unit  60  has a size of 306 μm in the y-direction and 408 μm in the x-direction. 
     An impingement point  28  of the chief ray  13  of the central object field point on the first mirror M 1  in the imaging beam path  8  and an impingement point  29  of the chief ray  13  of the central object field point on the fourth mirror M 4  in the imaging beam path  8  lie on the same side of the plane  30 . 
     Mirror M 3  is planar with very low aspherical contributions. 
     Mirror M 4  has a small diameter as compared to the other mirrors M 1  to M 3 . Mirror M 1  has a large diameter as compared to mirrors M 2  to M 4 . 
     The optical data of the imaging optical unit  60  according to  FIG. 30  are reproduced below with the aid of three tables. The first two tables correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . 
     The third table shows decenter parameters. The parameter YDE is the y-decenter with respect to the local coordinate system of the surface of the respective optical component or field. The parameter ADE gives the tilt angle with respect to the x axis of the local coordinate system of the surface of the respective optical component or field. 
     Decenter type BEN (decenter and bend) corresponds to the fact that a reference axis for description of the following surfaces also is reflected at the surface. Decenter type DAR (decencer and return) corresponds to the fact that only the surface to which this decentered type refers to is decentered. The reference axis for description of the following surfaces remains unchanged. 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 195.298 
                   
               
               
                 STOP 
                 INFINITY 
                 429.199 
                   
               
               
                 Mirror 1 
                 −493.270 
                 −449.497 
                 REFL 
               
               
                 Mirror 2 
                 80.948 
                 549.497 
                 REFL 
               
               
                 Mirror 3 
                 1184860.795 
                 −624.497 
                 REFL 
               
               
                 Mirror 4 
                 −66.100 
                 1216.483 
                 REFL 
               
               
                 Image 
                 INFINITY 
                 0.000 
                   
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Surface 
                 K 
                 A 
                 B 
                 C 
               
               
                   
               
               
                 Mirror 1 
                 −5.816921E−02 
                   0.000000E+00 
                   6.914712E−18 
                 −2.714395E−23 
               
               
                 Mirror 2 
                 −3.078104E−01 
                   0.000000E+00 
                   4.693560E−12  
                 −3.807573E−15 
               
               
                 Mirror 3 
                   0.000000E+00 
                   1.789193E−08 
                 −1.531460E−12 
                   1.582776E−14 
               
               
                 Mirror 4 
                 −3.415509E+00 
                   0.000000E+00 
                 −8.702688E−11 
                   3.212090E−12 
               
               
                   
               
               
                 Surface 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
               
                 Mirror 1 
                   1.391299E−27 
                 −1.360055E−32 
                   6.343599E−38 
                   0.000000E+00 
               
               
                 Mirror 2 
                   9.844683E−18  
                 −1.065946E−20 
                   4.610258E−24  
                   0.000000E+00 
               
               
                 Mirror 3 
                 −6.110633E−17  
                   1.318972E−19  
                 −1.165050E−22  
                   0.000000E+00 
               
               
                 Mirror 4 
                 −5.775782E−14  
                   4.505083E−16  
                 −1.268868E−18  
                   0.000000E+00 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Decenter 
               
               
                   
                 YDE 
                 ADE 
                 type 
               
               
                   
               
               
                 Mirror 3 
                 0.026622 
                 2.337361 
                 BEN 
               
               
                 Mirror 4 
                 −0.029607 
                 0.001951 
                 BEN 
               
               
                 Image 
                 177.886707 
                 0.010589 
                 DAR 
               
               
                   
               
            
           
         
       
     
     In the case of the imaging optical unit  60 , mirrors M 1  to M 4  are embodied as aspherical mirrors. The image field  9  is planar. Mirrors M 3 , M 4  and also the image field are decentered and tilted. 
     With reference to  FIG. 31 , a description is given below of a further embodiment of an imaging optical unit  61 , which can be used instead of the imaging optical unit  7  according to  FIG. 3 . Components and functions corresponding to those which have already been explained in the previous figures bear the same reference numerals and will not be discussed in detail again. The differences relative to the previous exemplary embodiments are explained below. 
     The imaging optical unit  61  corresponds to the imaging optical unit  60  of  FIG. 30 . 
     The imaging optical unit  61  has a structural length T of 700 mm between the object plane  11  and the image field  9 . 
     The ratio T/β of the structural length T and the imaging scale β (β=711) is T/β=0.98 in the case of the imaging optical unit  61 . 
     The imaging optical unit  61  has an object-side numerical aperture of 0.2. The object field  6  of the imaging optical unit  61  has a size of 306 μm in the y-direction and 408 μm in the x-direction. 
     The optical data of the imaging optical unit  61  according to  FIG. 31  are reproduced below with the aid of three tables. The first two tables correspond in terms of structure to the tables of the imaging optical unit  7  according to  FIG. 3 . The third table corresponds in terms of structure to the third table of the imaging optical unit  60  according to  FIG. 30 . 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
               
               
                 Object 
                 INFINITY 
                 194.932 
                   
               
               
                 STOP 
                 INFINITY 
                 355.769 
                   
               
               
                 Mirror 1 
                 −426.179 
                 −375.701 
                 REFL 
               
               
                 Mirror 2 
                 54.782 
                 492.420  
                 REFL 
               
               
                 Mirror 3 
                 79033.237 
                 −557.420 
                 REFL 
               
               
                 Mirror 4 
                 −42.790 
                 607.420 
                 REFL 
               
               
                 Image 
                 INFINITY 
                 0.000 
                   
               
               
                   
               
               
                 Surface 
                 K 
                 A 
                 B 
               
               
                   
               
               
                 Mirror 1 
                 −6.271971E−02 
                   0.000000E+00 
                   9.222134E−18 
               
               
                 Mirror 2 
                 −3.208834E−01 
                   0.000000E+00 
                   2.697892E−11 
               
               
                 Mirror 3 
                   0.000000E+00 
                   3.311824E−08 
                 −7.463884E−13 
               
               
                 Mirror 4 
                 −3.327639E+00 
                   0.000000E+00 
                 −2.733498E−10 
               
               
                   
               
               
                 Surface 
                 C 
                 D 
                 E 
               
               
                   
               
               
                 Mirror 1 
                   2.805504E−23 
                   9.373245E−28 
                 −1.916234E−33 
               
               
                 Mirror 2 
                 −7.789744E−15 
                   5.690974E−17 
                 −6.321964E−20 
               
               
                 Mirror 3 
                   1.269030E−14 
                 −8.891973E−18 
                 −3.128143E−20 
               
               
                 Mirror 4 
                 −2.183069E−12 
                   1.620091E−14 
                 −3.049564E−17 
               
               
                   
               
               
                   
                   
                   
                 Decenter 
               
               
                   
                 YDE 
                 ADE 
                 type 
               
               
                   
               
               
                 Mirror 3 
                 0.064273 
                 3.176114 
                 BEN 
               
               
                 Mirror 4 
                 −0.008075 
                 −0.001267 
                 BEN 
               
               
                 Image 
                 154.764702 
                 0.011999 
                 DAR 
               
               
                   
               
            
           
         
       
     
     In the case of the imaging optical unit  61 , mirrors M 1  to M 4  are embodied as aspherical mirrors. Mirror M 2  again practically is planar, having very low aspherically constributions. The image field  9  is planar. Mirrors M 3 , M 4  and also the image field are decentered and tilted. 
     Some characteristic variables of the imaging optical unit are summarized in the tables below, namely the object-side numerical aperture NAO, the field size, that is to say the size of the object field  6 , the magnification scale β, the structural length T, a wavefront aberration (rms) in units of the used wavelength λ and a maximum distortion, indicated in μm, and also the object-side chief ray angle α of the central object field point. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
             
            
               
                   
                 Imaging 
                 Imaging 
                 Imaging 
                 Imaging 
                 Imaging 
               
               
                   
                 optical unit 
                 optical unit 
                 optical unit 
                 optical unit 
                 optical unit 
               
               
                   
                 7 
                 27 
                 32 
                 34 
                 39 
               
               
                   
               
               
                 NAO 
                  0.25 
                  0.24 
                  0.24 
                  0.24 
                  0.24 
               
               
                 Field size y times x 
                 40 × 200 
                 100 × 300 
                 100 × 400 
                 100 × 300 
                 100 × 200 
               
               
                 [μm × μm] 
                   
                   
                   
                   
                   
               
               
                 Scale β 
                 750   
                 850   
                 850   
                 850   
                 850   
               
               
                 Structural length T [mm] 
                 878   
                 800   
                 741   
                 1227    
                 800   
               
               
                 Wavefront (rms) [λ] 
                   0.031 
                   0.013 
                   0.022 
                   0.002 
                   0.006 
               
               
                 Distortion (max) [μm] 
                  0.4  
                  0.3  
                  1.5  
                  0.04 
                  0.15 
               
               
                 Object-side chief ray 
                      0°   
                      10°   
                      10°   
                      10°   
                      10°   
               
               
                 angle α 
                   
                   
                   
                   
                   
               
               
                 T/β 
                  1.17 
                  0.94 
                  0.87 
                  1.44 
                  0.94 
               
               
                   
               
               
                   
                 Imaging 
                 Imaging 
                 Imaging 
                 Imaging 
                 Imaging 
               
               
                   
                 optical unit 
                 optical unit 
                 optical unit 
                 optical unit 
                 optical unit 
               
               
                   
                 41 
                 43 
                 45 
                 47 
                 49 
               
               
                   
               
               
                 NAO 
                  0.24 
                  0.24 
                  0.24 
                  0.24 
                  0.25 
               
               
                 Field size y times x 
                 100 × 400 
                 100 × 400 
                 100 × 400 
                 100 × 400 
                 106 × 680 
               
               
                 [μm × μm] 
                   
                   
                   
                   
                   
               
               
                 Scale β 
                 850   
                 850   
                 −850     
                 −850     
                 850   
               
               
                 Structural length T [mm] 
                 791   
                 786   
                 1050    
                 800   
                 1088    
               
               
                 Wavefront (rms) [λ] 
                   0.011 
                   0.007 
                   0.465 
                   0.216 
                   0.014 
               
               
                 Distortion (max) [μm] 
                  0.25 
                  0.32 
                 62.8 
                 12.3 
                  7.2  
               
               
                 Object-side chief ray 
                      10°   
                      10°   
                      10°   
                      10°   
                      0°   
               
               
                 angle α 
                   
                   
                   
                   
                   
               
               
                 T/β 
                  0.93 
                  0.92 
                  1.24 
                  0.94 
                  1.28 
               
               
                   
               
               
                   
                 Imaging 
                 Imaging 
                 Imaging 
                 Imaging 
                 Imaging 
               
               
                   
                 optical unit 
                 optical unit 
                 optical unit 
                 optical unit 
                 optical unit 
               
               
                   
                 50 
                 51 
                 53 
                 55 
                 56 
               
               
                   
               
               
                 NAO 
                  0.24 
                  0.24 
                  0.24 
                  0.2  
                   0.125 
               
               
                 Field size y times x 
                 106 × 680 
                 212 × 340 
                 212 × 340 
                 306 × 408 
                 490 × 652 
               
               
                 [μm × μm] 
                   
                   
                   
                   
                   
               
               
                 Scale β 
                 850   
                 850   
                 850   
                 711   
                 444   
               
               
                 Structural length T [mm] 
                 1000    
                 1010    
                 1093    
                 1439    
                 1300    
               
               
                 Wavefront (rms) [λ] 
                   0.008 
                   0.004 
                   0.065 
                   0.0091 
                   0.0108 
               
               
                 Distortion (max) [μm] 
                  0.3  
                  0.1  
                  9.7  
                  0.7  
                  0.4  
               
               
                 Object-side chief ray 
                      0°   
                      0°   
                      0°   
                      10°   
                      6°   
               
               
                 angle α 
                   
                   
                   
                   
                   
               
               
                 T/β 
                  1.18 
                  1.19 
                  1.29 
                  2.02 
                  2.93 
               
               
                   
               
               
                   
                 Imaging 
                 Imaging 
                 Imaging 
                 Imaging 
                 Imaging 
               
               
                   
                 optical unit  
                 optical unit 
                 optical unit 
                 optical unit 
                 optical unit 
               
               
                   
                 57 
                 58 
                 59 
                 60 
                 61 
               
               
                   
               
               
                 NAO 
                  0.2  
                  0.2  
                  0.2  
                  0.2  
                  0.2  
               
               
                 Field size y times x 
                 306 × 408 
                 306 × 408 
                 306 × 408 
                 306 × 408 
                 306 × 408 
               
               
                 [μm × μm] 
                   
                   
                   
                   
                   
               
               
                 Scale β 
                 711   
                 711   
                 711   
                 711   
                 711   
               
               
                 Structural length T [mm] 
                 1068    
                 1300    
                 1300    
                 700   
                 700   
               
               
                 Wavefront (rms) [λ] 
                   0.011 
                   0.011 
                   0.2012 
                   0.022 
                   0.022 
               
               
                 Distortion (max) [μm] 
                  0.7  
                  0.8  
                  1.1  
                  4.2  
                  4.2  
               
               
                 Object-side chief ray 
                      10°   
                      10°   
                      10°   
                      10°   
                      10°   
               
               
                 angle α 
                   
                   
                   
                   
                   
               
               
                 T/β 
                  1.50 
                  1.82 
                  1.82 
                  1.82 
                  0.98