Abstract:
A low-deformation mounting device of a rotationally asymmetric optical element, particularly an optical element in a projection lens used in semiconductor lithography. Said optical element is mounted in a frame and is provided with at least three connection surfaces that are positioned perpendicular to each other. Frame-connecting members are disposed in such a way that at least one but no more than two degrees of translational freedom and at least one but no more than two degrees of rotational freedom are obtained by means of said connecting members.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a device for the low-deformation mounting of a rotationally asymmetric optical element, in particular of an optical element in a projection lens system for semiconductor lithography, which is mounted in a frame.  
         [0003]     2. Description of the Related Art  
         [0004]     In particular in the case of projection lens systems in semiconductor lithography for producing semiconductor elements, it is necessary to know the precise installation position of an optical element or of an optical subassembly in relation to reference surfaces. Furthermore, it is often necessary for an optical element or a subassembly, following removal and reinstallation, to be positioned precisely in relation to the previous position, it also being intended, in particular, for the same, or at least very similar, deformation to occur as in the case of installation first time around, in order that reproducibility is achieved and there are no chances in respect of the imaging quality of the projection lens system.  
         [0005]     A reproducible installation position and at least more or less identical deformation forces are particularly difficult to achieve in the case of rotationally asymmetric optical elements. This applies, in particular, to beam splitter cubes, prisms and double mirrors.  
       SUMMARY OF THE INVENTION  
       [0006]     The object of the present invention is thus to provide a mounting which is intended for a rotationally asymmetric optical element and by means of which reproducibility is achieved, in particular following removal and reinstallation. The intention, in particular, is for external influences during installation and operation not to give rise to any chances in the original deformation of the optical element.  
         [0007]     This object is achieved according to the invention by a device, the optical element being mounted in a frame such that at least three application surfaces are provided on the optical element, in an angle in relation to one another, preferably orthogonally, wherein attachment members for attachment to the frame being arranged such that in each case at least one but not more than two degree/s of translational freedom and degree/s of rotational freedom is/are provided by the attachment members.  
         [0008]     The mounting according to the invention achieves reproducibility for reinstallation of the optical element, the original deformation forces also being reproduced or maintained. This is the case, in particular, when the attachment members are each provided with a connecting member via which the optical element is fixed to the frame.  
         [0009]     One of the essential features of the invention is that two degrees of translational freedom are available at each attachment location during installation. Following installation, that is to say following connection of each attachment location to the optical element via a connecting member, one degree of translational freedom is eliminated. This renders the mounting stiffer overall, as a result of which vibrations can be better avoided.  
         [0010]     The attachment members will be very advantageously designed as solid-state articulations, it being possible for these to have leaf-spring-like, elastic elements.  
         [0011]     Such a mounting is free of play.  
         [0012]     If mounting via at least two leaf-spring-like elastic elements which are arranged perpendicularly to one another is provided, then there are still two degrees of translational freedom following installation, redundancy of the mounting being avoided as a result.  
         [0013]     A very advantageous configuration of the invention is achieved if it is provided that the lines of effect of the possible lateral translational movements intersect one another at a point.  
         [0014]     This arrangement gives rise to temperature compensation since, in the case of different temperature expansions of the optical element and frame, the attachment locations and/or bearing points move such that the optical element does not have to change shape.  
         [0015]     An advantageous configuration of straightforward design is achieved when the attachment members slant in relation to the edges of the optical element, the lines of effect of the possible lateral translational movements intersecting one another at a corner of the optical element. In the case of an optical element forming a cube, this makes it possible to expand or contract, true to form, from the corner of the cube as fixed point.  
         [0016]     Advantageous further configurations and developments of the inventions can be seen from the exemplary embodiments which are described in principle hereinbelow with reference to the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  shows a schematic illustration of a projection exposure installation with a projection lens system;  
         [0018]      FIG. 2  shows a beam splitter cube which is mounted in a frame;  
         [0019]      FIG. 3  shows an enlarged illustration of a bearing device for the beam splitter cube;  
         [0020]      FIG. 4  shows an enlarged illustration of a bearing device for the beam splitter cube in a second embodiment;  
         [0021]      FIG. 5  shows an enlarged illustration of a bearing device for the beam splitter cube in a third embodiment;  
         [0022]      FIG. 6  shows a beam splitter cube in a configuration with obliquely positioned mounting devices;  
         [0023]      FIG. 7  shows an adjustable contact-pressure device with a spring; and  
         [0024]      FIG. 8  shows an adjustable contact-pressure device with a magnet. 
     
    
     DETAILED DESCRIPTION  
       [0025]      FIG. 1  illustrates., in principle, a projection exposure installation with a projection lens system  1  for microlithography for producing semiconductor elements.  
         [0026]     It has an illuminating system  2  with a laser (not illustrated) as light source. Located in the object plane of the projection exposure installation is a reticle  3 , of which the structure is to be replicated, on a correspondingly reduced scale, on a wafer  4  which is arranged beneath the projection lens system  1  and is located in the image plane.  
         [0027]     The projection lens system  1  is provided with a first vertical lens-system part  1   a  and a second horizontal lens-system part  1   b . Located in the lens-system part  1   b  are a plurality of lenses  5  and a concave mirror  6 , which are arranged in a lens-system housing  7  of the lens-system part  1   b . A beam splitter cube  10  is provided in order to deflect the projection beam (see arrow) from the vertical lens-system part  1   a , with a vertical optical axis  8 , into the horizontal lens-system part  1   b , with a horizontal optical axis  9 .  
         [0028]     Following reflection of the beams on the concave mirror  6  and subsequent passage through the beam splitter cube  10 , these beams impinge on a deflecting mirror  11 . The horizontal beam path  9  is deflected at the deflecting mirror  11 , in turn, into a vertical optical axis  12 . A third vertical lens-system part  1   c  with a further lens group  13  is locates beneath 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 lens system  1 , between the reticle  3  and the beam splitter cube  10 , behind a lens or lens group  17 . The π/4 plate  15  is located in the beam path of the horizontal lens-system part  1   b , and the λ/4 plate  16  is located in the third lens-system part  1   c . The three λ/4 plates serve to provide one full rotation of the polarization, as a result of which, inter alia, beam losses are minimized.  
         [0029]     FIGS.  2  to  8  use an enlarged illustration to describe in more detail the beam splitter cube  10  wish its bearing devices which is illustrated in  FIG. 1 . The beam splitter cube  10  is mounted in a frame  21  which is fixed, in a manner which is not illustrated specifically, to the projection lens system  1 .  
         [0030]     The beam splitter cube  10  is mounted in the frame  21  via three attachment members  22 , which act on three attachment surfaces  23  of the beam splitter cube  10  which are located perpendicularly to one another or orthogonally in relation to one another. As can be seen from  FIG. 2 , the attachment members  22  are designed as solid-state articulations and are integral with the frame  21 . As can also be seen from  FIG. 2 , the attachment surfaces  23  constitute three sides of the beam splitter cube  10 . According to  FIG. 3 , each attachment member  22  has a central part  2 ′ with a threaded bore  25  and two laterally adjoining leaf-spring-like elastic elements  26 . While in each case one end of the elastic elements  26  is connected to the central part  25 , the other ends of the elements  26  are each connected to the frame  21 . The central parts  24  of the attachment members  22  butt, by way of bearing surfaces  27 , against the attachment surfaces  23  of the beam splitter cube  10 .  
         [0031]     The bore  25  of the central part  24  contains a screw  28 , which is screwed into a threaded bore  29  of the beam splitter cube  10  in each case (see the partially broken-away illustration of the beam splitter cube in  FIG. 2 ).  
         [0032]     In the case of the bearing device according to  FIGS. 2 and 3 , there are two degrees of rotational freedom and two degrees of translational freedom per attachment member  22  during installation.  
         [0033]     As can be seen from  FIG. 3  in conjunction with  FIG. 2 , vertical rotation  30  about the longitudinal axis of the screw and axial rotation  31  about the longitudinal axes of the leaf-spring-like elastic elements  26  are possible in each case. In addition, on account of the elasticity of the elements  26 , a lateral translational movement  32  in the direction transverse to the longitudinal axis of the leaf-spring-like elements  26  is possible. Furthermore, prior to the beam splatter cube  10  being connected to the frame  21  via the three screws  28 , a lateral translational movement in the longitudinal direction of the elastic elements  26  is also possible in each case, as is indicated by the dashed arrow  33  in  FIG. 3 . The possibility of lateral translational movement in arrow direction  33 , however, is eliminated on account of the statics of the elastic elements  26  in this direction once the screws  28  have been screwed into the threaded bore  29  of the beam splitter element  10 . As a result, once the beam splitter cube  10  has been connected to the frame  21  via the three screws  28 , only a lateral translational movement  32  in the direction transverse to the longitudinal axes of the elements  26  and to the two rotational movements  30  and  31  is possible in each case.  
         [0034]     Redundancy, which gives rise to deformation at the beam splitter cube  10 , is present in the case of this type of mounting.  
         [0035]      FIGS. 4 and 5  illustrate mounting devices for the beam splitter cube  10  which still have two degrees of translational freedom present following installation.  
         [0036]     Since essentially the same parts are used, and the same construction is present, in the case of the mounting devices illustrated in  FIGS. 4 and 5 , the same parts have also been provided with the same designations.  
         [0037]     According to  FIG. 4 , the frame  21  is provided with two cut-outs  34  in the form of longitudinal grooves in each case in the region of its mounting devices. The cut-outs  34  extend transversely to the longitudinal axes of the leaf-spring-like elastic elements  26  and are each spaced apart from the outside of the frame  21 , in the region of the connection between the elements  26  and the frame  21 , such that only a narrow crosspiece  35  is present, this crosspiece likewise running transversely to the longitudinal axis or the elements  26 . It is thus the case that the two elongate cut-outs  34 , once the beam splitter cube  10  has been fixed  0  to the frame  21 , still allow a lateral translational movement in arrow direction  33 , since the narrow crosspieces  35  can flex like the leaf-spring-like elastic elements  26 .  
         [0038]      FIG. 5  illustrates a further possibility, in the case of which two lateral translational movements  32  and  33  are likewise possible following installation. In this case, this is made possible by a frame widening  36  which constitutes, at least more or less, a U-profile shape, with two legs  37  which, in relation to their respective longitudinal axes or the stiff surfaces thereof, run perpendicularly to the leaf-spring-like elastic element  26 . As can be seen, this configuration provides just one leaf-spring-like elastic element  26 , which is connected, on one side, to the central part  24  as the actual attachment member and, on the other side, to the transverse part of the U-profile between the two legs  37 .  
         [0039]     In addition to the two degrees of rotational freedom  30  and  31 , the elasticity of the element  26  allows a lateral translational movement  32  and the elasticity of the two leas  37  allows a translational movement  33  which is perpendicular thereto.  
         [0040]      FIG. 6  shows a configuration of a device which is intended for connecting the beam splitter cube  10  to the frame  21  and in which the attachment members  22  slant, or are located at an angle, in relation to the edges of the beam splitter cube  10 . If the angle at which the leaf-spring-like elements  26  are connected to the sides of the frame  21  is 45°, in which case the attachment members  22  run diagonally on the attachment surfaces  23 , then the lines of effect of the possible lateral translational movements intersect one another at a point P, as can be seen from the effect-indicating arrows  38 ,  39  and  40 . Point P is located at a corner of the beam splitter cube  10 . Since the movement directions are directed toward the point P, temperature compensation is achieved; that is to say, on account of different temperature expansions of the beam splitter cube  10  and frame  21 , there is no change in the shape of the beam splitter cube  10 .  
         [0041]      FIG. 7  shows a variant in respect of the screw  28  as connecting member between the frame  21  and the beam splitter cube  20 . In the case of this exemplary embodiment, it is possible to adjust the contact-pressure force. This is achieved by a stressed spring  41  which prevents the attachment surface  23  of the beam splitter cube  10  from lifting off from the bearing surface  27  of the attachment member  22 , to be precise by producing a tensile force between a bolt  42  and a bolt  43 , over which the ends of the spring  41  are wound in each case. The bolt  42  is located on an insert sleeve  44  in the beam splitter cube  10 . The connection between the insert sleeve  44  and the beam splitter cube  10  or between the spring  41  and the bolt  42  will preferably be provided as a quick-acting closure, for example as a bayonet closure, in order thus to allow quick release and reconnection during installation and removal.  
         [0042]     The bolt  43  is connected to a screw  45  which is screwed into the central part  24  of the attachment member  22 .  
         [0043]     The screw  45  can be used to adjust the tensile force of the spring  41  such that, upon removal and reinstallation, the beam splitter cube  10  undergoes the same deformation as before.  
         [0044]      FIG. 8  shows another possible way of regulating the contact-pressure force between the beam splatter cube  10  and the attachment member  22  as part of the frame  21 . In the case of  FIG. 8 , the contact-pressure force is produced by a magnet  46  with a north pole and a south pole, of which the magnetic flux runs via a sleeve  47  in the beam splitter cube  10  and a sleeve  48  in the attachment member  22 . Adjustment of an adjusting screw  49 , which is fixed to the magnet  46 , allows the contact-pressure force between the beam splitter cube  10  and the attachment member  22  to be adjusted, in turn, such that, upon reinstallation, the beam splitter cube  10  assumes the same deformation as during earlier installation.  
         [0045]     The invention has been described, in the case of the exemplary embodiment; with reference to a beam splitter cube. However, it is, of course, also suitable for other types of rotationally asymmetric optical elements, for example a prism or a double mirror.  
         [0046]     Instead of fixing via the three screws  28  or the connecting members illustrated in  FIGS. 7 and 8 , by means of which it is possible to regulate the contact-pressure force, it is, of course, also possible to have other connecting members within the context of the invention.