Abstract:
The invention relates to a device for holding a beam splitter element having an optically active beam splitter layer in an optical imaging device, the beam splitter element being connected to at least one support element that is fastened in the housing of the imaging device. The connection between the beam splitter element and said at least one support element is designed in such a way that the position of the beam splitter layer of the beam splitter element remains nearly constant relative to the housing independently of temperatures and of thermal stresses acting upon the beam splitter element.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a device for holding a beam splitter element having an optically active beam splitter layer in an optical imaging device, the beam splitter element being connected to at least one support element that is fastened in the housing of the imaging device.  
         [0003]     2. Description of the Related Art  
         [0004]     In projection exposure machines for microlithography having catadioptric projection objectives that have a polarizing beam splitter cube, the beam splitter cube deflects the light beam coming from the reticle into a cantilever arm with a concave mirror, while the light beam returning from the cantilever arm is let directly through the beam splitter cube. In this case, temperature changes during the ongoing operation result, in particular, from the absorption of light in the beam splitter layer of the beam splitter cube, but also from that in other layers as well as in the entire volume of the beam splitter cube. It is known that a rise in temperature results in material expansion and associated changes in the geometry of the beam splitter cube depending on the mounting of the latter. In addition, because of the fact that radiation passes through them repeatedly, specific regions of the beam splitter cube are heated more strongly than other regions.  
         [0005]     Known designs use the beam splitter cube with the beam splitter layer vertically, i.e. in the direction of the gravity. Furthermore the volume of the beam splitter cube in such systems is much higher and the power of the laser used is much lower. Since the bearing is applied to a side face of the cube, the place for the bearing can be chosen freely and there is no area, through which radiation passes, that may not be used for the bearing. Most known systems use a beam splitter cube made of quartz comprising much lower thermal expansion than calcium fluoride (CaF 2 ), which is presently used. In the case of mounts, which hold such a beam splitter cube in the region of one or more of its side faces, undesired movements such as displacements or rotations of the beam splitter layer in the interior of the beam splitter cube, can occur. This movement, which stems, from the length expansions of the material of the beam splitter cube and, on the other hand, from deformations of the beam splitter layer owing to thermally induced stresses, worsens the imaging behavior of the catadioptric projection objective of the projection exposure machine. The thermal expansion of the beam splitter cube material can tilt and displace the plane of the beam splitter layer such that the light beam coming from the reticle is no longer reflected exactly into the cantilever arm. In addition, the beam splitter cube and the beam splitter layer can also themselves be deformed such that not only is the beam wrongly deflected, but aberrations occur in the imaging of the projection objective.  
         [0006]     Such undesired changes of the beam splitter layer can also occur, of course, in the event of global warming in the objective.  
       SUMMARY OF THE INVENTION  
       [0007]     It is therefore the object of the present invention to provide a device for holding a beam splitter element having an optically active beam splitter layer of the type mentioned at the beginning and which disposes of the disadvantages of the prior art and permits a precise and stable beam path through the beam splitter element, in particular during operation.  
         [0008]     This object is achieved according to the invention when the connection between the beam splitter element and the at least one support element is designed in such a way that the position of the beam splitter layer of the beam splitter element remains nearly constant relative to the housing independently of temperatures and of thermal stresses acting upon the beam splitter element.  
         [0009]     These measures ensure in a simple and advantageous way that, in the event of changed temperatures in the projection objective or in the beam splitter cube itself during operation of the projection exposure machine, for example, there is no reduction in the imaging quality of the projection objective since the beam splitter layer is constantly kept in its position as no deformations or changes in position occur. A projection light beam is thereby prevented from being wrongly deflected.  
         [0010]     According to the invention, it can further be provided that the connection between the beam splitter element and the at least one support element is designed in such a way that it is arranged at least approximately in the region of the plane in which the beam splitter layer lies.  
         [0011]     It is thereby possible for the beam splitter cube, upon being heated, to expand about the point of intersection of the optical axis of the reticle beam path and the cantilever arm beam path, there being no change in the position of the beam splitter layer, and the point of intersection between the optical axis of the reticle beam path and the cantilever arm beam path remaining stationary.  
         [0012]     It is advantageous when the connection between the beam splitter element, designed as beam splitter cube, and the at least one support element is designed as a bearing, the bearing arrangement being determined statically by at least three bearings in such a way that no constraining forces act on the beam splitter cube in the event of a temperature change—in particular a global one—and of the associated volumetric change.  
         [0013]     These measures advantageously provide between the beam splitter cube and the support element a connection that in the event of a temperature change in the beam splitter cube permits a volumetric change in conjunction with retention of the configuration, that is to say the edge length ratio of the beam splitter cube. The function of the beam splitter cube is maintained not only when the point of intersection of the optical axes of the reticle and cantilever arm beam paths remains stationary, but also when is imaged layer into itself again into itself again in the event of a deformation of the beam splitter cube, for example owing to heating of the beam splitter layer. Moreover, the beam splitter cube is mounted in a statically determined fashion.  
         [0014]     In another design refinement of the invention, it can be provided that the connection between the beam splitter element, designed as beam splitter cube, and the at least one support element is designed as a bearing, the bearing arrangement being overdetermined statically by a number of bearings in such a way that no constraining forces act on the beam splitter cube in the event of a temperature change and of the associated volumetric change.  
         [0015]     A statically overdetermined bearing arrangement of the beam splitter cube is likewise advantageously possible owing to these measures. However, it is here a precondition for a constant configuration in conjunction with a volumetric change of the beam splitter cube that the beam splitter cube is exposed only to a global temperature change without other influences such as mounting tolerances etc.  
         [0016]     Advantageous refinements and developments of the invention emerge from the further subclaims and from the exemplary embodiments described below in principle with the aid of the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  shows a schematic representation with the mode of operation of a projection objective for microlithography and having a beam splitter cube;  
         [0018]      FIG. 2  shows a sectional view of the beam splitter cube mounted in a device according to the invention;  
         [0019]      FIG. 3  shows a schematic perspective view of an inventive mount of the beam splitter cube;  
         [0020]      FIG. 4   a  shows a perspective detailed view of inventive mounting regions of the beam splitter cube;  
         [0021]      FIG. 4   b  shows A perspective view of a mounting part corresponding to the mounting region illustrated in  FIG. 4   a;    
         [0022]      FIG. 5  shows a schematic sectional view of the beam splitter cube having an inventive mount in the optically unused regions;  
         [0023]      FIG. 6  shows a side view of a beam splitter cube having a first embodiment of an inventive statically determined bearing arrangement;  
         [0024]      FIG. 7  shows a plan view of the beam splitter cube from  FIG. 6 ;  
         [0025]      FIG. 8  shows a plan view of the beam splitter cube having a second inventive embodiment of a statically determined bearing arrangement;  
         [0026]      FIG. 9  shows a side view of a beam splitter cube having a third inventive embodiment of a statically determined bearing arrangement;  
         [0027]      FIG. 10  shows a plan view of a beam splitter cube having a fourth inventive statically overdetermined bearing arrangement;  
         [0028]      FIG. 11  shows a plan view of a beam splitter cube having a fifth inventive statically overdetermined bearing arrangement;  
         [0029]      FIG. 12  shows a schematic view from below of the beam splitter cube from  FIG. 3 , with decoupling elements;  
         [0030]      FIG. 13  shows a perspective view of a beam splitter cube in a further embodiment of an inventive mount, with flexible elements;  
         [0031]      FIG. 14  shows a schematic plan view of the beam splitter cube in the mount from  FIG. 13 ;  
         [0032]      FIG. 15  shows a side view of the mount according to  FIG. 13 , with a beam trap;  
         [0033]      FIG. 16  shows a side view of a connection of the beam trap according to  FIG. 15 , for heat department;  
         [0034]      FIG. 17   a  shows a perspective view of a first flexible element of the mount from  FIG. 14 ;  
         [0035]      FIG. 17   b  shows a perspective view of a second flexible element of the mount from  FIG. 14 ; and  
         [0036]      FIG. 17   c  shows a perspective view of a third flexible element of the mount from  FIG. 14 . 
     
    
     DETAILED DESCRIPTION  
       [0037]      FIG. 1  is a schematic representation of a projection exposure machine having a projection objective  1  for microlithography, for the purpose of producing semiconductor components.  
         [0038]     It has an illumination system  2  with a laser (not illustrated) as light source. Located in the object plane of the projection exposure machine is a reticle  3  whose structure is to be imaged at an appropriately reduced scale onto a wafer  4  that is arranged below the projection objective  1  and is located in the image plane.  
         [0039]     The projection objective  1  is provided with a first vertical objective part  1   a  and a second objective part  1   b , which is at least approximately horizontal or inclined at up to 30° to the horizontal. Located in the objective part  1   b  are a number of lenses  5  and a concave mirror  6 , which are arranged in an objective housing  7  of the objective part  1   b . A beam splitter cube  10  is provided for deflecting the projection beam (see arrow) from the vertical objective part  1   a  with a vertical optical axis  8  in the objective part  1   b  with an at least approximately horizontal optical axis  9 . The optical axis  9  can provided. The optical axis  9  can also be inclined at up to 30° to the horizontal, if appropriate.  
         [0040]     After reflection of the beams at the concave mirror  6  and a subsequent transit through the beam splitter cube  10 , these strike a deflecting mirror  11 . The horizontal beam path along the optical axis  9  is deflected, in turn, at the deflecting mirror  11  onto a vertical optical axis  12 . A third vertical objective part  1   c  with a further lens group  13  is located below the deflecting mirror  11 . In addition, three λ/4 plates  14 ,  15  and  16  are also located in the beam path. The λ/4 plate  14  is located in the projection objective  1  between the reticle  3  and the beam splitter cube  10  downstream of a lens or lens group  17 . The λ/4 plate  15  is located in the beam path of the horizontal objective part  1   b , and the λ/4 plate  16  is located in the third objective part  1   c . The three λ/4 plates serve the purpose of completely rotating the polarization once and thereby, inter alia, minimizes beam losses.  
         [0041]     The light absorption in the beam splitter layer  18 , in particular, but also that in other layers as well as in the entire volume of the beam splitter cube  10  leads to temperature changes in the projection objective  1  with the beam splitter cube  10 , which has a beam splitter layer  18 . A rise in temperature, for example, results in material expansion and associated changes of the geometry as a function of a holding device  19  of the beam splitter cube  10 , which is connected to the objective housing  7 .  
         [0042]     So that the light beam is deflected from the optical axis  8  exactly onto the optical axis  9 , the plane of the beam splitter layer  18  must run exactly through a point of intersection  20  of the optical axes  8 ,  9 . In addition, the normal to the plane of the beam splitter layer  18  must be inclined to the optical axis  8  and to the optical axis  9  at half the angle which by beam splitter layer  18  at half the angle which is enclosed by the optical axis  8 ,  9 .  
         [0043]     A portion of the light that passes through the beam splitter cube  10  is absorbed by the latter and, as mentioned above, results in heating of the beam splitter cube  10 . The beam splitter layer  18  can be tilted and displaced by the thermal expansion of the material of the cube, as a result of which the light beam, coming from the reticle, of the optical axis  8  is no longer reflected exactly onto the optical axis  9 . In addition, the beam splitter cube  10  and its beam splitter layer  18  can themselves be deformed such that not only is the beam wrongly deflected, but aberrations also occur in the imaging of the projection objective  1 .  
         [0044]     Some of these negative influences can be balanced out by displacing or tilting or manipulating other optical elements of the projection objective  1  or the beam splitter cube  10  itself. In the case of such a correction of the beam splitter layer  18  in the beam splitter cube  10 , the position of the beam splitter layer  18  is determined during operation and actively corrected, for example by moving special manipulators. In this case, the position can be determined by a separate measuring system for the beam splitter layer. For this purpose, one or more beams are directed in a defined fashion in an unused objective region onto the beam splitter layer, and the deflected beam or beams is/are detected. As an alternative, the position of the image itself can be ascertained by means of alignment markings and serve as basis for the correction.  
         [0045]     However, it is more advantageous to counteract the aberration wherever it occurs, specifically at the beam splitter cube  10  or the holding device  19  thereof.  
         [0046]     Inventive mounts, holders and bearing arrangements of the beam splitter cube  10  that minimize decentering/tilting of the beam splitter layer  18  and deformation of the beam splitter cube  10  in the event of heating of the beam splitter cube  10  are illustrated in FIGS.  2  to  16  and described below with the aid of these figures.  
         [0047]     As can be seen from  FIG. 2 , the beam splitter cube  10  is held by a holder  21  in the plane of the beam splitter layer  18 . Since, as indicated in  FIG. 3 , the beam splitter cube  10  is assembled form two prisms  22   a ,  22   b  assembled in the region of its beam splitter layer  18 , it is thereby possible to produce a supporting location  23  by virtue of the fact that the prism  22   a  projects laterally beyond the prism  22   b , that is to say has extensions  24 . In this arrangement, the prism  22   a  lies on the holder  21 , and a prism  22   b  hangs from the prism  22   a  via wringing in the beam splitter layer  18 . The beam splitter cube  10  is advantageously held on the optically unused sides by the holder  21 . Of course, in another exemplary embodiment it would also be possible for the prism  22   b  to be held by the holder  21  and the prism  22   a  could be fitted to the prism  22   b  by wringing such that only pressure loading by the weight of the upper prism  22   a  takes place instead of severe tensile loading of the weight of the lower prism  22   b.    
         [0048]     In a further exemplary embodiment, it would also, of course, be possible to connect the two prisms  22   a ,  22   b  by means of elements fitted laterally thereon, and to mount these on the holder  21  at the level of the beam splitter layer  18  or of the horizontal optical axis  9  of the beam splitter cube  10 .  
         [0049]     In the present exemplary embodiment, the holder  21  consists of ZERODUR, and in another exemplary embodiment this could also be formed from another preferably thermally stable material such as, for example, quartz or INVAR or from the same material as the housing  7  of the projection objective  1 . The poorest possible thermal conduction of the holder  21  is advantageous here in order to be able to dissipate the heat in a targeted fashion via a cooling device.  
         [0050]     As indicated in  FIG. 12 , it is also possible to provide decoupling elements between the holder  21  and the projecting extensions  24  so as to compensate the different expansions of the prisms  22   a ,  22   b  and the holder  21 . The decoupling elements  400  are designed in such a way that they secure the beam splitter cube  10  in its position without preventing an affine enlargement of the beam splitter cube  10 . In a further exemplary embodiment, this function of the decoupling elements  400  could also be integrated directly in the holder  21 , for example by suitable precuts in the holder  21  in the region of the supporting locations  23 . Below the beam splitter cube  10 , the holder  21  offers sufficient space for a cooling plate  25  comprising a surface that absorbs the 157 nm radiation, and heat dissipating means. The cooling plate  25  is designed as a blackened metal sheet. It is provided with conducting copper strips (not illustrated in more detail) for the purpose of dissipating heat.  
         [0051]     The holder  21  with the beam splitter cube  10  and the cooling plate  25  is held in a statically determined further holder which simultaneously permits reproducible installation and removal. An upper part  26   a  of the reproducible holder is permanently connected to the holder  21  or fabricated together with the latter from one part. A lower part  26   b  of the reproducible holder has a spherical surface at the bottom, the center  27  of the sphere being at the center of the beam splitter cube  10  such that the position of the center  27  of the beam splitter cube  10  does not vary during possible adjustment. A vary. A tilt-adjustable mount  28  supports this complete arrangement and has a contact surface  29   a  with the spherical surface, and a contact surface  29   b  with the objective housing  7  of the projection objective  1 . The connection of the mount  28  with the objective housing  7  of the projection lens  1  is produced by a screwed connection. The contact surface  29   a  on the mount  28  has the same radius as the lower part  26   b , supported thereon, for wringing after adjustment. In another exemplary embodiment, it could also have the form of a roof edge so as to produce a defined annular support for cementing the two parts to one another (not illustrated).  
         [0052]     The extensions  24  fitted along the beam splitter layer  18  of the beam splitter cube  10  are illustrated schematically in  FIG. 4   a . The extensions  24  are fitted on the two prisms  22   a  and  22   b  in  FIG. 4   a . The prisms  22   a ,  22   b  are to be conceived in  FIG. 4   a  as continuing prismatically upward and downward, respectively. It is very advantageous that the extensions  24  are arranged in different subregions of the respective prisms  22   a  and  22   b . The result of this is the production of cutouts  30   a  and  30   b  that are diametrically opposite one another and into which a mount part  300  illustrated in  FIG. 4   b  can be fitted.  
         [0053]     As indicated in  FIG. 5 , the beam splitter cube  10  has an optically used oval region  31   a  and an optically unused region  31   b . Accordingly, the beam splitter cube  10  can be provided easily and advantageously in the optically unused region  31   b , for example in the corner regions there, with cutouts  32  for holding fastening elements  33  for the purpose of fastening on a mount that is not illustrated. It is rational in this case for the mounting likewise to be done on the faces of the beam splitter cube  10  averted from the light.  
         [0054]     The inventors have found that the function of the beam splitter cube  10  is ensured when the beam splitter layer  18  is, at it were, imaged into itself again in the event of a change of shape of the beam splitter cube  10  owing to heating. In this case, the beam splitter cube  10  is permitted to experience only a volumetric change and not a change in configuration, that is to say not a change in the edge length ratios. Proposed for this purpose are suitable mounting arrangements of the beam splitter cube in a mount in accordance with FIGS.  6  to  11 , which is connected to the objective housing  7  and not illustrated in more detail. The basic idea in this case is that no constraining forces that change configuration are allowed to act on the beam splitter cube  10  when the latter is heating up.  
         [0055]     Both statically determined and statically overdetermined solutions are conceivable for bearing purposes, the statically overdetermined bearing arrangements preceding only from a purely global temperature change in the beam splitter cube  10 , that is to say from a temperature change that affects all the parts. The volumetric expansion of the beam splitter cube  10  is indicated schematically in FIGS.  6  to  11  by dotted lines and bearing points.  
         [0056]      FIG. 6  shows schematically the bearing arrangement for a beam splitter cube  10  with a describing coordinate system  34  at the point of intersection  20  of the optical axes  8  and  9 . The x-axis points into the plane of the page, the y-axis points in the direction of the optical axis  9 , and the z-axis points in the direction of the optical axis  8  or in the direction of the weight force.  
         [0057]     As may be seen from  FIG. 6 , the bearing arrangement is marked on the underside of the cube by three bearings  40   a ,  41   a ,  42   a  (the bearing  42   a  is covered here by the bearing  41   a  and visible in  FIG. 7 ), which can in each case support a force in the z-direction. The transmissibility of forces in the plane defined by the x- and y-axes is distributed over the bearings  40   a ,  41   a ,  42   a  such that given a global  41   a ,  42   a  distributed, that given a global temperature change they permit a volumetric change of the beam splitter cube  10  without causing a change in configuration and without changing the position of the beam splitter plane  18 .  
         [0058]     The beam splitter cube  10  is mounted such that the beam splitter layer  18  is imaged into itself again given a global temperature change, and in the process the point of intersection  20  of the optical axes  8  and  9  comes to lie again on the beam splitter layer  18 . Since the bearing arrangement is statically determined, no constraining forces act on the beam splitter cube  10  in the event of a temperature change.  
         [0059]     At the edge where the beam splitter layer  18  intersects the underside of the cube, the beam splitter cube  10  is secured by the bearing  40   a , designed as a fixed bearing, and it is supported on the oppositely situated lower edge by the bearing  41   a , designed as movable bearing, with a translational capacity, and by the bearing  42   a , designed as movable bearing, the two translational capacities (the movable bearing  41   a  covers the movable bearing  42   a  in  FIG. 6 ).  
         [0060]     The beam splitter cube  10  shown in  FIG. 6  is illustrated in  FIG. 7  from above, the displacement directions of the movable bearings  41   a  and  42   a  being illustrated by arrows. In the case of the movable bearing  41   a  with only one translational capacity, translational capacity is directed to the bearing  40   a , while the movable bearing  42   a  the two translational capacities can be displaced in any direction of the plane of the page.  
         [0061]     As shown in  FIG. 8 , each bearing  40   b ,  41   b ,  42   b  has exactly one translational capacity in the plane defined by the x- and y-axes. Here, all translational capacities point to a point on the cut edge of the underside of the cube with the beam splitter layer  18 . This ensures that the beam splitter layer ensures that the beam splitter layer  18  is imaged into itself again given a global temperature change.  FIG. 8  illustrates the directions of the translational capacities of the three bearings  40   b ,  41   b ,  42   b  in the plane defined by the x- and y-axes.  
         [0062]     Further capacities exist when three bearings  40   c ,  41   c  and  42   c  are translationally fixed only in the z-direction, but are completely freely movable in the plane defined by the x- and y-axes.  
         [0063]     Further bearings  43   c ,  44   c ,  45   c  are required in order to support the beam splitter cube  10  in a statically defined fashion in this case, each bearing being translationally fixed in one direction.  
         [0064]      FIG. 9  shows an arrangement with the bearings  40   c ,  41   c ,  42   c ,  43   c ,  44   c ,  45   c  from the side where the beam splitter layer  18  remains in its position in the event of a temperature change. In this illustration, the bearing  42   c  is covered by the bearing  41   c , and the bearing  44   c  is covered by the bearing  43   c.    
         [0065]     Of course, it is also possible to have the most varied statically determined bearing arrangements in which one or more bearings are fixed in two translational directions, while the remaining bearings are fixed only in terms of one translational direction, and in which the beam splitter layer  18  is not displaced given a temperature change.  
         [0066]     Considering a deformation of the beam splitter cube  10  given only a global temperature change and not owing to other influences such as assembly tolerances etc, there are also statically overdetermined bearing possibilities that permit a volumetric change of the beam splitter cube  10  without displacement of the beam splitter layer  18 . In principle, the beam splitter cube  10  can then be mounted at any desired number of points, one variant consisting in that each bearing each bearing is movable in at least one translational direction, and all the free translational directions intersect a point in the plane of the beam splitter layer  18  such that the plane of the beam splitter layer  18  is not displaced given a global temperature change.  
         [0067]     Such a bearing arrangement with four bearings  40   d ,  41   d ,  42   d  and  43   d  is illustrated from above in  FIG. 10 .  
         [0068]     In a further variant, illustrated in  FIG. 11 , the bearing  40   e  is secured in all degrees of translational freedom, the other bearings  41   e  and  42   e  requiring to be movable at least in translation, and all the free translational directions requiring to intersect in the fixed bearing  40   e . The fixed bearing  40   e  acts on the beam splitter cube  10  at the beam splitter plane  18  such that there is no change in the position of the beam splitter layer  18  given a global temperature change. As may be seen from  FIG. 11 , this variant with the fixed bearing  40   e  and the bearings  41   e  and  42   e  is illustrated from above.  
         [0069]      FIGS. 13 and 14  illustrate a further embodiment of an inventive mount of the beam splitter cube  10  on the support element  19 . For this purpose, the beam splitter cube  10  is supported by two front or first flexible elements or leaf spring elements  50   a ,  50   b , and a further rear flexible element, or leaf spring element  51 . The front leaf spring elements  50   a ,  50   b  are flexible in the Y-direction and stiff in the X- and in the Z-direction. The design of the rear flexible element  51  is symmetrical about the X-axis, flexible in the X-direction, and stiff in the Y- as well as in the Z-directions. Thermal expansions of the beam splitter cube  10  are therefore permitted in the X- and Y-directions.  
         [0070]     As may be seen, an additional flexible element or leaf spring element  52  is provided that raises the natural frequency of the arrangement. The leaf spring element  52  is arranged symmetrically in relation to the X-axis and is of stiff design in the X-direction and flexible design in the Z-direction, and prevents a rotation above the Z-axis. The additional leaf spring element  52  is advantageously arranged in the region of the beam splitter cube  10  in which the intensity of the light passing through is low. Provided in each case at the connecting points of the flexible elements  50   a ,  50   b ,  51 ,  52  to the beam splitter cube  10  is a coating that protects adhesive connections  501 ,  511 ,  521  of the flexible elements  50   a ,  50   b ,  51 ,  52  for fastening to the beam splitter cube  10  against irradiation of light (not illustrated). The connecting points of the flexible elements  50   a ,  50   b ,  51 ,  52  are arranged in regions of low irradiation of light.  
         [0071]     Furthermore, as may be seen from  FIG. 13 , a beam or light trap  53  is arranged below the beam splitter cube  10 . The light trap  53  absorbs undesired optical irradiation. In order to ensure good dissipation of heat, the light trap  53  is provided with connecting elements  54  made from copper for the purpose of dissipating heat toward the objective housing  7 . However, in this case the aim is also for a dynamic decoupling from the protective housing  7  to be retained. Provided for this purpose in relation to the objective housing  7  are further connecting elements  55  that are made from ceramic and engage in a comb-like fashion toward a connecting location  56  with alternating grooves and tongues in correspondingly designed grooves and tongues of the connection elements  54 , and permit good dissipation of heat over a small remaining interspace  57  in conjunction with simultaneous dynamic decoupling (indicated schematically in  FIG. 16 ). In addition, helium can be introduced into the remaining interspace  57  and additionally creates good thermal conduction.  
         [0072]     Moreover, in a further exemplary embodiment additional cooling elements (not illustrated) can be provided at outer surfaces of the beam splitter cube  10  in regions that are not used optically.  
         [0073]     After assembling the arrangement, that is to say after assembling the beam splitter cube  10  in the objective  1 , it is possible to compensate the deformations, caused by the weight force, of the beam splitter cube  10  by means of subsequent surface treatment, in particular by ion beam etching of the surface, at least of a further optical element of the imaging device  1 .  
         [0074]     In  FIGS. 17   a - 17   c  the flexible elements  50   a ,  50   b ,  51 ,  52  are shown in detail.  
         [0075]     As may be seen from  FIG. 17   a , the front flexible element  50   a ,  50   b  comprises a top part  502  with the adhesive connection point  501  to the beam splitter cube  10 . The top part  502  is made of a steel material comprising a thermal expansion coefficient which is approximately equal to the thermal expansion coefficient of calcium fluoride (CaF 2 ). Furthermore the front flexible element  50   a ,  50   b  comprises two leaf springs  503  which are soldered or welded to the top part  502  and which are integrated into a base part  504  made of a material comprising a thermal expansion coefficient which is as small as possible (e.g. INVAR). The front flexible element  50   a ,  50   b  is flexible in the Y-direction.  
         [0076]     As may be seen from  FIG. 17   b , the rear flexible element  51  comprises a top part  512  with the adhesive connection point  511  to the beam splitter cube  10 . The top part  512  is made of a steel material comprising a thermal expansion coefficient which is approximately equal to the thermal expansion coefficient of calcium fluoride (CaF 2 ). Furthermore the rear flexible element  51  comprises a leaf spring  513  which is connected to a base part  514  made of a material comprising a thermal expansion coefficient which is as small as possible (e.g. INVAR). The rear flexible element  51  is flexible in the X-direction.  
         [0077]     As may be seen from  FIG. 17   c , the additional flexible element  52  is completely made of a material comprising a thermal expansion coefficient which is as small as possible (e.g. INVAR) and comprises a top part  522  with the adhesive connection point  521  to the beam splitter cube  10 . Furthermore the additional flexible element  52  comprises two leaf springs  523  which are connected to a base part  524 . The additional flexible element  52  is flexible in the Z-direction.