Abstract:
A catadioptric objective comprises a plurality of lenses and at least two deflecting mirrors that have reflecting surfaces that are at a specific angle, in particular of 90°, to one another. The two deflecting mirrors are arranged with their reflecting surfaces on a common base member whose position in the objective can be set.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a catadioptric objective, specifically it relates to a projection exposure objective for semiconductor microlithography, comprising a plurality of lenses and comprising at least two deflecting mirrors that have reflecting surfaces that are at a specific angle, in particular of 90°, to one another. 
   2. Description of the Related Art 
   In the case of catadioptric objectives for microlithography, for example in the case of wavelengths of 157 nm, two deflecting mirrors are used that must be at a very precisely defined angle of 90° to one another. This angular position must also be retained exactly. Moreover, the natural frequency of the mirrors must be very high so that no aberrations occur. For this reason, they may also not vibrate relative to one another. 
   WO 95/30918 discloses binoculars, mirrors being held against reference surfaces and being adjusted in a specific angular position relative to one another. (FIGS. 16 to 20). 
   SUMMARY OF THE INVENTION 
   It is the object of the present invention to create a catadioptric objective, the deflecting mirrors being adjusted and held exactly relative to one another, and the aim being to provide a high stiffness simultaneously. 
   This objective is achieved according to the invention by means of a catadioptric objective, wherein the two deflecting mirrors are arranged with their reflecting surfaces on a common base member whose position in the objective can be set. 
   A very stiff configuration can be achieved by means of the combination according to the invention of the two deflecting mirrors on a common base member. Moreover, the mirrors cannot vibrate in phase opposition. Again, a configuration that saves a great deal of space is achieved by the common arrangement on the base member. 
   A further very substantial advantage consists in that during adjustment of the two deflecting mirrors, during settings or adjustments, their position relative to one another is basically maintained exactly, and thus a prescribed angle is always observed between the two reflecting surfaces. 
   Owing to the fact that the adjustment of the two deflecting mirrors in the objective is performed jointly via the base member, the manipulation or installation is facilitated in practice. This is the case, in particular, whenever for this purpose the base member is provided with a reference surface for the adjustment. 
   Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description of exemplary embodiments of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a first configuration of two deflecting mirrors with a common base member, 
       FIG. 2  shows a configuration of two deflecting mirrors with a common base member and a circumferential bearing groove, 
       FIG. 3  shows the embodiment according to  FIG. 1  with a cover plate for coating the reflecting surfaces, 
       FIG. 4  shows a detailed enlargement in accordance with IV according to  FIG. 3 , 
       FIG. 5  shows a second embodiment of two deflecting mirrors with a common base member, 
       FIG. 6  shows a section along the lines VI—VI according to  FIG. 5 , 
       FIG. 7  shows an embodiment with soldered deflecting mirrors, 
       FIG. 8  shows an embodiment in which the two deflecting mirrors are connected to the base member via adjustable holders, and 
       FIG. 9  shows the principle of a design of a catadioptric objective with the deflecting mirrors according to the invention. 
       FIG. 10  shows an illustration of the principle of the catadioptric objective of the subject invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a first embodiment, two deflecting mirrors  2  and  3  with reflecting surfaces  2   a  and  3   a  being arranged on a base member  1 . The base member has the form of a prism with a right angle at which the two deflecting mirrors  2  and  3  abut one another. The reflecting surfaces  2   a  and  3   a  are formed by an appropriate coating. A material of low thermal expansion and which can be highly polished is preferably used for this purpose as substrate. The base member  1  has a rear surface  4  for mounting a manipulator  26  for the purpose of initiating an adjusting movement or for connection with a mount of a catadioptric objective illustrated in FIG.  10 . Furthermore, a reference surface  5  is provided on the base member for the adjustment of the base member  1 , and thus of the two deflecting mirrors  2  and  3  on the base member  1 . 
   In order to reduce the mass, the base member  1  is provided with a milled-out portion  6  (illustrated only by way of example). 
     FIG. 2  shows an essentially identical configuration, a different geometry with attachments  7  on the rear surface  4  being provided on the rear surface. 
   In order to induce as few and uniform stresses as possible during vacuum deposition of the reflecting surfaces  2   a  and  3   a  or of the reflecting layer, it is favorable to apply the two reflecting surfaces  2   a  and  3   a  in one operation. It is possible for this purpose to provide a circumferential shoulder  8  in accordance with  FIG. 1 , or else a circumferential bearing groove  9  in accordance with FIG.  2 . 
   If different layers are to be applied to the two reflecting surfaces  2   a  and  3   a , the surface respectively not to be coated must be protected. This can be performed by means of a cover plate  10 , as illustrated in FIG.  3  and in an enlarged detail in FIG.  4 . The cover plate  10  should be provided in this case, on its side facing the reflecting surface  2   a  or  3   a  to be covered, with a set-back surface  11 , in order to protect the scratch-sensitive surface, in particular the extremely sensitive optical surface, on the reflecting surface  2   a  or  3   a . The cover plate  10  therefore bears on the reflecting surface  2   a  or  3   a  only in a region not used optically, as represented by “12”. 
   It is advantageous in this case if the cover provided by the cover plate  10  goes beyond the edge at which the two reflecting surfaces  2   a  and  3   a  abut one another, since this prevents inadequate deposition of the cover. This is visible from the enlarged illustration in  FIG. 4 , the cover plate  10  being provided for this purpose with an extension  13  embracing the common edge. 
     FIGS. 5 and 6  show an exemplary embodiment, the entire unit being assembled from a plurality of parts. In this case, the two reflecting surfaces  2   a  and  3   a  of the deflecting mirrors  2  and  3  are supported by an angle  14  and are stiffened by a plurality of stiffening ribs  15  and a back plate  16 . The individual components can be connected by soldering, sintering or else in any other way. A base member  1  in the form of a frame is created with the aid of this configuration. The stiffening ribs  15 , which can be provided with cutouts or bores  6  of different size in order to yield corresponding weight reductions extend, arranged at a spacing from one another, between the angle  14  and the back plate  16 . The optically polishable layer can be applied after grinding of the optically used surfaces. 
     FIGS. 7 and 8  show a largely similar configuration to that in  FIGS. 5 and 6 . What is different is that the angle  14  is formed in this case from two individual mirror plates  14   a  and  14   b . The two mirror plates  14   a  and  14   b  are soldered to one another at their common edge, or connected to one another in a different way. A connection to the back plate  16  can be performed in a similar way at their rear ends. Stiffening ribs  15  that are provided with a multiplicity of bores  6  for reducing weight are provided, in turn for stiffening purposes. 
     FIG. 9  shows an embodiment similar to the embodiment according to  FIGS. 7 and 8 . The reflecting layers  2   a  and  3   a  are applied to the mirror plates  14   a  and  14   b . The two mirror plates  14   a  and  14   b  are mounted on the base member  1  by means of adjustable holders  17 . The angular position of the two mirror plates  14   a  and  14   b  can be set relative to one another by means of the adjustable holders. 
     FIG. 10  shows an illustration of the principle of a catadioptric objective  25  (indicated only by dashes) with a beam path illustrated only in principle. An illuminated region  18 , for example a reticle, is imaged on a wafer  19 . In this process, the light is reflected downstream of a first lens group  20  (only one being illustrated for simplification) with a vertical optical axis on the top reflecting surface  2   a  of the two deflecting mirrors  2  and  3  into a lens group  21  with a virtually horizontal axis  22 , and turned back from there at an arcuate mirror  23 . The light is then sent at the lower reflecting surface  3   a  into a lower lens group  24  with, once again, a vertical optical axis. The base member  1 , on which the two deflecting mirrors  2  and  3  are arranged, is connected (in a way not shown in more detail) to the mount of the objective  25 , it also being possible to provide corresponding adjusting elements and manipulator  26 .