Abstract:
A faceted mirror apparatus includes a support plate having a plurality of stepped reception bores into which are fitted the base portions of respective ones of a plurality of mirror facets having mirrored surfaces. An outer surface of each of the facets bears on an inner wall to provide a stable alignment of the optical axes of the mirrored surfaces such that the apparatus generates a beam of rays from impinging rays.

Description:
[0001]    This application is a continuation of U.S. application Ser. No. 11/695,626 filed Apr. 3, 2007 which is a divisional of U.S. application Ser. No. 10/936,317 filed Sep. 8, 2004 which in turn is a continuation of International Application No. PCT/EP2004/000331 filed on Jan. 17, 2004 which designates the U.S. and claims priority to German Application No. 103 02 664.9 filed Jan. 24, 2003. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to faceted mirrors having a plurality of mirror facets, in particular for an illumination instrument in a projection exposure system for microlithography, and here in particular for use with illumination in the extreme ultraviolet range. 
         [0003]    Faceted mirrors comprise a plurality of mirror facets and are already known in the prior art. 
         [0004]    Prior citation WO 03/067304 is based on manipulators which allow adjustment of the mirror facets. For example, it is known that the mirror facets have a spherical body, a mirror surface being arranged in a recess of the spherical body and the side of the spherical body remote from the mirror surface being mounted in a bearing instrument. At each of the mirror facets, a lever element is arranged on the side of the spherical body remote from the mirror surface. Adjusting means, by which the spherical body can be moved about its mid-point, engage on the lever element in a region remote from the spherical body. 
         [0005]    With a structure of this type, it is not readily possible to achieve the requisite accuracies for relatively small mirror facets when used with radiation in the extreme ultraviolet range. 
         [0006]    Mirror facets whose mirror surfaces are arranged on a support element are furthermore known from prior citation WO 03/067288. The support element has adjusting means by which the angular position of the mirror surface can be adjusted in at least one space direction in a plane at least approximately perpendicular to the optical axis of the mirror surface. 
         [0007]    For further prior art, reference is made to WO 03/050586 A2, DE 100 30 495 A1, EP 0 145 243 A2 and XP-002281181 “Ion Beam and Plasma Jet Etching for Optical Component Fabrication” in Lithographic and Micromachining Techniques for Optical Component Fabrication, Ernst-Bernhard Kley, Hans Peter Herzig, editors, Proceedings of SPIE Vol. 4440 (2001). 
         [0008]    The structure of these mirror facets is relatively elaborate, so that such faceted mirror arrangements entail an increased adjustment outlay and are relatively expensive. 
         [0009]    Faceted mirrors need to withstand heavy thermal loads due to the radiation, especially in the extreme ultraviolet range, and the known arrangements meet the stringent requirements concerning thermal loads only to a small extent. 
         [0010]    In respect of this, it is an object of the invention to optimize a faceted mirror, especially for ultrahigh vacuum requirements, and to provide a structure which makes do with the fewest possible parts and guarantees a reliable, long-term stable and simple structure. 
       SUMMARY OF THE INVENTION 
       [0011]    According to the invention, in a method for the production of a faceted mirror having a plurality of mirror facets which generate a beam of rays, the mirror facets having mirror surfaces each with an optical axis, and they are each fitted into reception bores of a support plate, this object is achieved in that the mirror facets are made in a first method step, after which at least one of the mirror facets is fitted into the associated reception bore of the support plate in a second method step, after which the ACTUAL position of the optical axis of at least one mirror surface of an associated mirror facet fitted into the support plate is determined in a third step and compared with a SET position of a predetermined optical axis, after which, knowing the measured values determined for the at least one mirror facet in the third method step, reprocessing of the mirror facet and/or of the reception bore is carried out in a further method step if there is an angular deviation between the ACTUAL position and the SET position. 
         [0012]    The respectively selected optical axis can be determined via a reference face according to the invention. For example, an arbitrarily picked mirror surface of a mirror facet fitted into the support plate may be used as a reference face. Further mirror facets, and preferably all of them, are then aligned with their mirror surfaces with respect to the mirror surface thus acting as a reference face in order to achieve the required beam of rays, which likewise has a principal optical axis. 
         [0013]    Instead of a mirror surface as the reference face, a reference face may also be provided on the support plate, with respect to which further mirror surfaces, and preferably all of them, are aligned with their respective optical axes in order to generate the intended beam of rays. A plurality of reference faces may furthermore be used for alignment of the mirror facets. This is particularly advantageous when the mirror facets are aligned in groups. Method steps two, three and the further method step may be carried out either individually, successively in groups or even all together for the mirror facets to be fitted into the reception bores of the support plate. 
         [0014]    Very advantageous reprocessing can be carried out by so-called ion beam figuring (IBF). This is controlled, highly accurate surface smoothing by ion beams in order to reduce the roughness and for extremely exact surfaces. Any angular deviation from the intended angle can thereby be reduced further. 
         [0015]    As an alternative, it is also possible to carry out the reprocessing by evaporation of metal interlayers, for example gold layers. The metal layers are then correspondingly evaporated onto the mirror facet and/or support plate, so that any angular deviations are reduced further. 
         [0016]    In another method according to the invention for the production of a faceted mirror having a plurality of mirror facets which generate a beam of rays, the mirror facets having mirror surfaces each with an optical axis, and they are each fitted into reception bores of a support plate, the mirror facets may be made in a first method step, after which at least one of the mirror facets is measured by a measuring device in a second method step, after which, knowing the measurement result, the associated reception bore is formed in the support plate, after which the at least one mirror facet is fitted into the associated reception bore of the support plate in a third method step, after which the ACTUAL position of the optical axis of the at least one mirror surface of the mirror facet fitted into the support plate is determined in a fourth step and compared with a SET position of a predetermined optical axis, after which, knowing the measured values determined for the at least one mirror facet in the fourth method step, reprocessing of the mirror facet and/or of the reception bore is carried out in a further method step if there is an angular deviation between the ACTUAL position and the SET position. 
         [0017]    In this method according to the invention, knowing these values, the associated reception bore is formed in the support plate by using the measured values obtained in the second method step for the at least one measured mirror facet with respect to its optical axis, after which the fine tuning and optional reprocessing in order to achieve a best-possible fabrication accuracy are carried out in the further method step. 
         [0018]    In this exemplary method as well, the mirror facets may be measured individually or successively in groups or even all together. The same applies to the reception bores determined with the aid of the measured values being formed in the support plate. 
         [0019]    In order to achieve the requisite accuracy, the fabrication is thus performed in steps. The individual facets are first made to within the best-possible angular error, for example 200″. This deviation is then determined by a measuring device, for example an angle measuring instrument, preferably with an autocollimation telescope with a positioning stage. The reception bores for the mirror facets are then formed in the support plate with the best-possible fabrication accuracy, which is from 20″ to 100″, preferably from 30″ to 50″. It is then possible to establish directly which of the mirror facets will be fitted with which of the previously measured angular errors into which of the reception bores. If need be, the reception bores may therefore be corrected directly according to the measured angular errors of the mirror facets. The residual error after the mirror facets have been fitted into the respective reception bores is in turn determined via an angle measuring instrument. Owing to the fabrication accuracies to be achieved, this error lies in a range that makes it possible to correct the residual error via a precision surface processing method, for example the IBF method or the evaporation of metal interlayers. 
         [0020]    It is thus possible to achieve the requisite quality concerning alignment of the individual mirror facets. The mirror facets are subsequently fixed in the support plate so that a stable alignment is guaranteed even over a long period of time. Furthermore, the direct contact between the mirror facet and the support plate leads to a structure which ideally dissipates the heat absorbed by the mirror facets. This allows a simpler, less expensive, very stable, less shock-sensitive, material-reducing, adhesive-free and thermally unproblematic structure of a faceted mirror in the ultrahigh vacuum range, especially for use in EUV lithography. 
         [0021]    Compared to solutions with individual manipulators for the mirror facets, the method for the production of faceted mirrors not only has advantages concerning a low assembly and adjustment outlay and inexpensive production, but this method also makes it possible to produce substantially smaller faceted mirrors and to arrange a very large number of them in a faceted mirror, a diameter range of from 3 mm to 50 mm being readily achievable for the mirror facets. 
     
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  shows a structure of an EUV illumination instrument with a light source, an illumination instrument and a projection objective; 
           [0023]      FIG. 2  shows a longitudinal section of a cylindrically designed mirror facet, the mirror facet being mounted in a support plate; 
           [0024]      FIG. 3  shows a brief representation relating to a production method for a faceted mirror; 
           [0025]      FIGS. 4   a  and  4   b  show excerpts of a longitudinal section corresponding to  FIG. 2  with examples of reprocessing faces; 
           [0026]      FIG. 5  shows a longitudinal section of a conically designed mirror facet, the mirror facet being mounted in a support plate; and 
           [0027]      FIG. 6  shows a plan view of a faceted mirror, which contains both embodiments of the mirror facets represented in  FIG. 2  and  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION  
       [0028]      FIG. 1  shows a summary representation of an EUV projection exposure system with a complete EUV illumination instrument  1  with a light source la and a projection objective  2  (only briefly represented). The illumination instrument  1  also contains a plane mirror  3 , a first faceted mirror  4  as a first optical element with a multiplicity of faceted mirrors, a subsequently arranged second faceted mirror  5  as a second optical element with a multiplicity of faceted mirrors, and two imaging mirrors  6   a  and  6   b . The imaging mirrors  6   a  and  6   b  are used to image the mirror facets of the faceted mirror  5  into an entry pupil of the projection objective  2 . A reticle  7  can be displaced as a scanning system in the y direction. A reticle plane  8  also simultaneously constitutes the object plane. 
         [0029]    In order to bring different light channels into the optical path of the illumination instrument for the setting adjustment, for example, there are a greater number of mirror facets on the second faceted mirror  5  than correspond to the number of mirror facets on the first faceted mirror  4 . The mirror facets are not represented in  FIG. 1  for the sake of clarity. As the object to be exposed, there is a wafer  11  on a support unit  9 . 
         [0030]      FIG. 2  shows an individual mirror facet  12  with a mirror surface  15 , which is formed by a cylindrical mirror facet head  13  and a cylindrical mirror facet base  14 . In the exemplary embodiment, the cylindrical mirror facet head  13  has a diameter of for example 20 mm, and the cylindrical mirror facet base  14  has a diameter of about 8 mm. The mirror facet  12  has an overall length of 60 mm, for example. For reasons of processing and thermal loading, silicon is selected as the material of the mirror facet  12 . The mirror facet  12  may of course be made of a stainless steel alloy or other materials which meet the requirements for polishability, mechanical, thermal and long-term stability and ultrahigh vacuum compatibility (UHV compatibility). What is important when selecting the materials for the mirror facets  12 , in particular, is that the materials being used should have a high thermal conductivity so that the heat generated because of the beam power can be dissipated. 
         [0031]    The mirror surface  15  of the mirror facet head  13  may be spherical or concave with a radius of, for example, 2000 mm. The mirror surfaces  15  may also be designed to be plane, spherical, aspherical, convex or concave. A marking  23  (see  FIG. 6 ) is also applied to the mirror surface  15  for correct azimuthal orientation of the mirror facet  12 . The marking  23  needs to be aligned with a corresponding marking on a plane support plate  16 , which has reception bores  22  for the mirror facets  12 . The support plate  16  is used to receive all the mirror facets  12 , which together form the faceted mirror  4  or  5 . The mirror facets  12  are in this case received individually in a particular reception bore  22 . The longitudinal axes of the individual reception bores  22  are generally at different angles with respect to the optical axis, so that an intended beam of rays (see reference numeral “ 27 ” in  FIG. 1 ) is obtained overall from the multiplicity of mirror facets  12  fitted into the support plate  16 . 
         [0032]    The support plate  16  may also be designed to be aspherical, if the mirror facets  12  are not intended to be arranged in a plane. For example, the support plate  16  is formed by steel with a thickness of, for example, 50 mm. The support plate  16  may also be made of other materials, for example silicon, so that good heat dissipation is achieved. 
         [0033]    The cylindrical mirror facet base  14  is provided with a screw thread  17  in order to hold the mirror facet  12  in its position with a defined force, for example produced by a threaded nut  18  and a spring  19 , after it has been reprocessed. The spring  19  may be formed as a cylindrical spring or as a spring washer made of stainless steel. This is particularly advantageous when materials with different thermal expansion coefficients are being used for the support plate  16  and for the mirror facet  12 , as in the present exemplary embodiment. 
         [0034]    Owing to the different longitudinal axes of the reception bores  22 , the mirror facets  12  are arranged at different inclination angles on the support plate  16 , so that the impinging rays are reflected in a predetermined direction in order to generate the beam of rays  27 . The optical axis  20  of each mirror facet  12  therefore needs to lie in a particular set direction. For this reason, the mirror surface  15  must be aligned very accurately. 
         [0035]    After the mirror facets  12  have been made, for example, the relation between its mirror surface  15  and a mirror back face  15 ′ remote from the mirror surface  15  is measured for each mirror facet  12  by a measuring device  28  (not shown, see  FIG. 3 ). Also, the relation between the lateral face  21  of the mirror facet head  13  and the mirror surface  15  may advantageously be measured so that an error to be corrected can be determined. This means that a set angle direction and an actual angle direction are determined, the difference between them is taken and the angular deviation then needs to be eliminated. 
         [0036]    This may be done in a plurality of steps. The first step deals with the reception bores  22 . Since the mirror surfaces  15  can only be made with an accuracy of about 200″ with respect to the support plate  16 , the reception bores  22  are respectively processed at the place of the position-determined lateral faces  21  of the mirror facet  12  until the two relational faces are tilted by the predetermined angular correction (in most cases 200″). The finely processed lateral face  21  with a measured error can then additionally achieve the requisite accuracy of about 10″ by evaporating a wedge-shaped metal interlayer. 
         [0037]    In addition or as an alternative, if the necessary accuracy has not yet been achieved, it is possible for the mirror surface  15  to be reprocessed by ion beam figuring (IBF). 
         [0038]    Optionally, the necessary accuracy may also be achieved only with ion beam figuring, ablation of the face to be treated being carried out in the final processing by ions. The ablation of typically 1 to 2 μm necessary for the IBF processing can be carried out in this processing step without impairing the surface roughness. A final accuracy of about 1″ can thus be achieved. 
         [0039]    In the exemplary embodiment according to  FIG. 2 , the reception bore  22  is respectively designed as a stepped bore with a step ledge  22   a  for matching to the cylindrical mirror facet head  13  and the cylindrical mirror facet base  14 . Instead of relational faces exactly assigned to each other between the lateral face  21  and the inner wall of the reception bore  22 , a mirror back face  15 ′, that is to say a face on the side of the mirror facet head  13  remote from the mirror surface  15 , may respectively be provided as an alternative in this region as an angle-defining guide face in conjunction with the step ledge  22   a . Instead of reprocessing the lateral face  21  or the reception bore  22  in the region of the lateral face  21 , in this case either the mirror back face  15 ′ or the step ledge  22   a  is reprocessed. The reprocessing may again be carried out, for example, by ion beam figuring or by application of a metal interlayer on one of the two parts. Since the reprocessing involves angular corrections, during the reprocessing either wedge-shaped IBF ablation or wedge-shaped application of a metal interlayer is carried out in respect of the face to be reprocessed, in order to achieve an angular correction. In other words: On the face to be processed, a layer is applied or ablated which has a different thickness or is ablated obliquely to its surface, in order to generate the intended angular correction. 
         [0040]    The alignment of a mirror facet  12  and the reprocessing method will briefly be described below with reference to  FIGS. 3 ,  4   a  and  4   b . In order to be able to determine the exact place and the scope of the reprocessing, one or more mirror facets  12  are fitted, for example provisionally, into the associated reception bores  22  after having been measured by the measuring device  28 , and the reflectivity is measured (see  FIG. 3  top). In order to use the multiplicity of mirror facets  12  fitted into the support plate  16  to generate the beam of rays  27 , which is for example guided to the faceted mirror  5 , the mirror facets  12  respectively need to be aligned with their optical axis  20  in a predetermined way. To that end, a reference face should be provided. This may be done, for example, by a mirror surface  15  of a mirror facet  12 , as represented for example in  FIG. 3  left bottom. In relation to the optical axis  20  of this mirror facet  12 , all the other optical axes of the remaining mirror facets  12  are then measured with regard to their respective position and in each case correspondingly aligned so that the mirror facets  12  together lead to the intended beam of rays  27 . To that end, as required, processing faces of the mirror facets  12 , or of the bores  22  in which they are received, will be reprocessed accordingly. 
         [0041]    Examples of possible reprocessing faces are represented in  FIGS. 4   a  and  4   b . For clearer representation, the angular changes indicated in  FIGS. 4   a  and  4   b  and the wedge-shaped ablations or applications are indicated greatly enlarged. In practice, the corrections generally take place in the range of up to 100″. 
         [0042]    According to  FIG. 4   a , there is an angular deviation ε as the correction angle between the actual position or the actual value of the optical axis  20 ′ and the set position or the set value of the optical axis  20 . In order to correct the angular error ε, according to  FIG. 4   a , the mirror surface  15  is therefore ablated accordingly in a wedge shape with the correction angle ε. 
         [0043]    As an alternative to this, of course, it is also possible to ablate the mirror facet head  13  with the correction angle ε on its back face  15 ′ remote from the mirror surface  15 . Since the mirror facet head  13  bears with its back face  15 ′ on the step ledge  22   a  of the stepped bore  22 , the position of the longitudinal axis of the mirror facet  12  is in this way corrected accordingly in the reception bore  22 . Of course, a correspondingly large play between the diameter of the stepped bore  22  and the diameter of the cylindrical mirror facet base  14  therefore needs to be provided in the region of the mirror facet base  14 , so that there is enough lateral free space for alignment of the mirror facet  12 . 
         [0044]      FIG. 4   b  represents another alternative for correction of the deviation of the actual position of the optical axis  20 ″ from the set position of the optical axis  20 . Instead of the back face  15 ′, as can be seen, the step ledge  22   a  is reprocessed here in a wedge shape with the correction angle ε by corresponding ablation. Here again, the position of the longitudinal axis of the mirror facet  12  is in this way corrected accordingly so that the angular deviation is minimized as far as possible—within predetermined tolerance limits. The optimum case would, of course, be when the angular deviation of the correction angle ε becomes 0. 
         [0045]    When all the mirror facets have been aligned with their respective optical axes in relation to the reference face, so that they generate the intended beam of rays  27 , then it may sometimes still be necessary merely to correct the position of the support plate  16  overall, if the position of the principal axis of the beam of rays still needs to be aligned more accurately with the subsequent faceted mirror. 
         [0046]    Instead of the reference face of the mirror surface  15  of the mirror facet  12  according to the representation in  FIG. 3  left bottom, of course, any selected measuring face of the support plate  16  may be used as a reference face, relative to which the individual optical axes  20  of the mirror facets  12  will then be aligned. In  FIG. 4   b , the measuring face is merely represented briefly by the reference numeral  28 . The represented measuring or plane face  28  may, for example, be scanned with a coordinate measuring machine and recorded by measuring techniques, after which a relationship with the individual optical axes  20  of the mirror facets  12  is established. 
         [0047]    For alignment of the individual optical axes with the reference face, owing to the spatial dimension of the beam of rays, two tilt axes accordingly need to be taken into account when processing the respective reprocessing face. In the example represented in  FIGS. 4   a  and  4   b  with the correction angle ε, the second correction angle is in a plane lying at 90° to the correction angle ε. 
         [0048]    Instead of the ablation method as represented in  FIGS. 4   a  and  4   b , of course, it is also possible to carry out an application method, for example by evaporating metal interlayers which are likewise to be applied in a wedge shape according to the correction angle ε. This means, for example, that instead of the wedge-shaped ablation of the step ledge  22   a  represented by dashes in  FIG. 4   b , a metal layer is accordingly raised by evaporation in a wedge shape as represented by dots and dashes. In the same way, of course, a wedge-shaped metal interlayer may also be applied to the mirror surface  15  instead of ablation of the mirror surface  15 . The same applies to the back face  15 ′. 
         [0049]    When a metal interlayer is to be evaporated, for example, gold may be used for this since it is very highly suitable with respect to a good thermal junction as well as processing and softness, together with adaptation to the shape in question. Other metals may naturally also be used, for example noble metals, gallium, platinum, silver or indium. It is advantageous to pick a metal which can be deposited very easily but nevertheless produces a good thermal contact. 
         [0050]    Besides the evaporation of metal interlayers or IBF processing, another option is to rotate the tilted mirror facets  12  and  12 ′ about their longitudinal axes. The effect of rotation is that the correction may then only need to be carried out in one angular direction, so that it is possible to simplify the other process steps. If a higher accuracy can be achieved only in one direction, but not in another direction, then the effect of rotating the mirror facets  12  may be that the accuracy is equally high in all directions. If still necessary, the requisite accuracy may be achieved after this by either ion beam figuring or evaporation of metal interlayers. 
         [0051]      FIG. 5  shows a mirror facet  12 ′ with a conical mirror facet head  13 ′ and a cylindrical mirror facet base  14 . Since the structure of the mirror facet and the reception bore correspond essentially to the exemplary embodiment according to  FIG. 2 , the same reference numerals will also be used for the same parts. The reception bore  22 ′ is accordingly also designed to be conical in the region of the mirror facet head  13 ′ and cylindrical in the region of the mirror facet base  14 . The processing steps for the reception bore  22 ′ are carried out in the same steps as when there is a cylindrical mirror facet head  13 . It should be borne in mind, however, that the lateral faces  21 ′ must have a very high accuracy since the mirror facet head  13 ′ bears directly on the inner wall of the reception bore  22 ′ via the lateral faces  21 ′. The application or evaporation of a wedge-shaped metal interlayer for the reprocessing may be carried out on the mirror surface  15  according to  FIG. 4   a  and/or on the conical lateral face  21 ′ or on the reception bore  22 ′ in the region of the lateral face  21 ′. The same applies to IBF reprocessing. 
         [0052]    The conical guide has an essential advantage. It is not self-locking but self-centering, so as to provide a greater bearing region for the mirror facet head  13 ′ on the inner wall of the reception bore  22 ′ and therefore very good heat dissipation. Very steep angles of the reception bore  22 ′ and of the conical mirror facet head  13 ′ are preferred, in order to obtain a very good position definition. The conical mirror facet head  13 ′ may have a radius of, for example, 2000 mm and a diameter of about 20 mm, in each case measured on the mirror surface  15 . 
         [0053]      FIG. 6  shows a plan view of a faceted mirror  24 . The faceted mirror  24  is, for example, respectively represented with a cylindrical mirror facet  12  and a conical mirror facet  12 ′. For the sake of clarity, only two mirror facets  12  and  12 ′ are actually represented. If need be, of course, there will be substantially more individual mirror facets in a faceted mirror  24 . In practice, moreover, there will generally be either only cylindrical mirror facets  12  or only conical mirror facets  12 ′ in one faceted mirror. 
         [0054]    The marking  23  for the azimuthal alignment can respectively be seen in the—in the drawing—lower region of the mirror facets  12  and  12 ′. One or more marking holes  25  or reference faces  26  on the faceted mirror  24  define the relation with a measuring system, in order to align it according to its position in the projection exposure system. 
         [0055]    A faceted mirror  24  contains, for example, 200 mirror facets, for example only with cylindrical mirror facets  12  or only with conical mirror facets  12 ′. However, it is also possible to provide a mixture of cylindrical and conical mirror facets  12  and  12 ′. 
         [0056]    In a faceted mirror  24  with a support plate  16 , the plate diameter being about 30 cm, there may be angular differences relating to the respective optical axes  20  of the individual mirror facets  12  or  12 ′ of, for example, 6° with respect to a principal direction. The reprocessing according to the invention is carried out so that the respective optical axis of a mirror facet is as exact as possible. The optical axis is in this case given by the optical effect of the mirror surface as a function of its shape. 
         [0057]    For a corresponding angular accuracy of the optical axis, it is therefore necessary for there to be a relation between the mirror surface  15  and the longitudinal axis in the bore  22  of the mirror facet  12 . The measurements and corresponding reprocessing operations described above need to be carried out for this reason. In other words: the longitudinal axis  20  of a mirror facet  12  defines the direction, and the mirror surface  15  with its optical axis defines the optical effect. The two axes need to be correlated so that, in operation, the optical axis subsequently lies accurately in the direction that is necessary in order to produce an intended beam of rays from the multiplicity of individual mirror facets  12  or  12 ′. 
         [0058]    In general, the configuration and alignment of the individual mirror facets  12  will be provided so that the normal of the mirror surface  15  corresponds as accurately as possible to the optical axis  20 . This may not be necessary in principle, however, since the crucial point is that the respective individual optical axes should be aligned so as to obtain an intended beam of rays  27  which has a corresponding accuracy with respect to the principal optical axis of the beam of rays and the shape and size of the beam of rays. 
         [0059]    Since the entire system is operated in a vacuum there should be no blind regions, that is to say no regions which are fully closed, in the region of the reception bores  22  and  22 ′. Optionally, V-shaped groves extending in the longitudinal direction may be formed in the reception bores  22  and  22 ′ in order to evacuate the faces  21  and  21 ′.