Abstract:
The invention relates to an optical imaging device, in particular an objective  1  for microlithography in the field of EUVL for producing semiconductor components, having a beam path  2 , a plurality of optical elements  3  and a diaphragm device  7  with an adjustable diaphragm opening shape. The diaphragm device has a diaphragm store  7   a,    7   b  with a plurality of different diaphragm openings  6  with fixed shapes in each case, which can be introduced into the beam path  2.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an optical imaging device, in particular an objective for microlithography in the field of EUVL for producing semiconductor elements, having a beam path, a plurality of optical elements and a diaphragm device with an adjustable diaphragm opening shape. 
     2. Description of the Related Art 
     It is generally known to use various diaphragms as system diaphragms in optical imaging devices. The diameter of the light beam bundle in the beam path of the optical imaging device can be varied by means of these diaphragms, of which the opening diameter can be varied, in particular. 
     So-called iris diaphragms, which have at least four—but mostly more—thin blades which are generally in the shape of a sickle and are supported at one end rotatably in a fixed mount are particularly widespread. In this arrangement, the other end is provided as guiding device with a pin which is inserted in a groove or slot of a rotatable ring such that the rotation of the rotatable ring moves the blades in such a way that the remaining opening diameter for the diaphragm can be varied. 
     DE 101 11 299 A1 discloses such an iris diaphragm, in particular for an exposure objective in semiconductor lithography, having a plurality of blades which are guided with the aid of guide elements, and can be moved by at least one drive device for the purpose of adjusting the diaphragm opening. The guide elements are designed such that the blades can be moved in an at least approximately linear fashion in a radial direction in relation to the optical axis of the iris diaphragm. 
     DE 199 55 984 A1 discloses a further diaphragm for stopping down an optical imaging device. 
     Known diaphragms, in particular the iris diaphragms which can be adjusted continuously via blades, are less suitable for use in stopping down an optical system used in microlithography, chiefly in the field of EUVL, since more stringent demands are placed here on the installation space available, which these cannot satisfy because of their construction. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the present invention to create an optical imaging device of the type mentioned at the beginning which can be stopped down with the aid of a diaphragm which requires only a small installation space. 
     This object is achieved according to the invention by virtue of the fact that the diaphragm device has a diaphragm store with a plurality of different diaphragm openings with fixed shapes in each case, which can be introduced into the beam path. 
     The measures according to the invention create in a simple and advantageous way an optical imaging device having a diaphragm mechanism in the case of which the shapes of the diaphragm openings are permanently determined and can be stored in a very small space. There are no restrictions on the geometry of the shapes of the diaphragm openings, and so both circular and elliptical or other geometries can be used for the diaphragm openings. By contrast with the known blade-type iris diaphragms, the masses to be moved are comparatively small, and so changing the diaphragms in the optical imaging device can be undertaken very quickly. The most varied types of diaphragms can be brought into use by means of the existing diaphragm store with diaphragm openings. 
     It is very advantageous when the diaphragm store is designed as a revolving disc diaphragm stack, in particular arranged outside the optical imaging device, with a plurality of revolving disc diaphragms which are provided with diaphragm openings and are, in particular, accommodated in separate plug-in units. 
     These measures yield a further space-saving design of the diaphragm device, in particular outside the optical imaging device, as a result of which comparatively many different revolving disc diaphragms can be stored in the revolving disc diaphragm stack. By contrast with an inner arrangement, the arrangement of the diaphragm device outside the optical imaging device additionally minimizes contamination of the optical imaging device by the diaphragm device. Moreover, the diaphragm device can be dynamically decoupled from the optical imaging device such that no disturbing vibrations are introduced by the diaphragm device to the optical elements arranged in the optical imaging device. 
     Moreover, it can be provided in one structural configuration of the invention that a sheet-metal strip which is wound onto two rollers and held tensioned is provided as a diaphragm store, the sheet-metal strip having a plurality of, in particular, various diaphragm openings of fixed shapes, and it being possible by rotating the rollers to adjust the diaphragm setting by varying the diaphragm openings. 
     This results in a very highly dynamic adjustment of the various diaphragms, which can be stored in a very small space. The masses to be moved are comparatively small and there are no restrictions on the geometry of the diaphragm openings. Changing diaphragms can be undertaken speedily. 
     Advantageous refinements and developments of the invention arise from the further subclaims. Various embodiments of the invention are explained in principle below with the aid of the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  shows a detail of a projection objective for microlithography in the field of EUVL, with a typical beam path; 
         FIG. 1   b  shows a view from above of a revolving disc diaphragm suitable for the projection objective in accordance with  FIG. 1   a;    
         FIG. 2  shows an illustration of a revolving disc diaphragm stack with a plurality of revolving disc diaphragms which can be introduced into the beam path of the projection objective in accordance with  FIG. 1   a;    
         FIGS. 3   a  to  3   c  show side views of three embodiments of a revolving disc diaphragm; 
         FIGS. 4   a  to  4   c  show illustrations of three embodiments of a diaphragm device with a revolving disc diaphragm stack; 
         FIG. 5  shows a view of a diaphragm device with a lifting device, a holding device and with spring elements as stop for a revolving disc diaphragm; 
         FIGS. 6   a  to  6   c  show illustrations of three embodiments of electromagnetic holding devices for positioning the revolving disc diaphragm; 
         FIGS. 7   a  and  7   b  show illustrations of two embodiments of a contamination monitoring means for a mirror; 
         FIG. 8  shows a side view of an inventive external diaphragm device with a lifting device; 
         FIG. 9  shows a side view of a further embodiment of a diaphragm device with a lifting device; 
         FIGS. 10   a  to  10   c  show perspective views of three embodiments of a lifting device; 
         FIG. 11  shows a perspective view of a robot gripper arm for unloading a revolving disc diaphragm stack; 
         FIG. 12  shows a perspective view of a further embodiment of the diaphragm device with two rollers on which a sheet-metal strip is wound; 
         FIG. 13  shows a side view of the diaphragm device from  FIG. 12 ; and 
         FIG. 14  shows the principle of the design of an EUV projection exposure machine with a light source, an illuminating system and a projection objective. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1   a  shows a detail of a projection objective  1  for use in the field of EUVL, with its typical beam path  2  between mirrors  3  arranged on a housing  1   a , illustrated by dashes, of the projection objective  1 , and an object plane  4  (explained in more detail in  FIG. 14 ). Arranged in the beam path  2  is a diaphragm  5  with a diaphragm opening  6  which serves to stop down the light beam of the projection objective  1 . 
     As may be seen, stringent requirements are placed on the nature and the installation space of the diaphragm  5  here. This is required principally on a side  5 ′ of the diaphragm  5  that is emphasized by a circle. Consequently, the diaphragm opening  6  should be decentral as illustrated in  FIG. 1   b . This requisite arrangement of the diaphragm opening  6  on the diaphragm  5 , as well as the small installation space in the projection objective  1  complicate the use of conventional, continuously adjustable iris diaphragms (for example, by means of blades) in the case of such a projection objective  1 , in particular in the case of operating wavelengths in the field of EUVL. 
       FIG. 2  shows the detail of the projection objective  1  in a design with a diaphragm device  7  with a revolving disc diaphragm stack  7   a ,  7   b , which has individual diaphragms  5 , designed as revolving disc diaphragms, with fixed geometries (see  FIG. 1   b ) stacked vertically one above another. The diaphragm openings  6  can also have elliptical or other shapes instead of the circular shape illustrated. The revolving disc diaphragms  5  are preferably brought into the beam path  2  of the projection objective  1  to an operating position  9  (indicated by dots) provided therefor via directions indicated by arrows  8 . As may be seen from  FIG. 1   b , the revolving disc diaphragms  5  are shaped in such a way that they have a thin rim on the side of the neighbouring light beam, and a broad rim over the remainder of the circumference. 
     As may be seen in  FIGS. 3   a  to  3   c , the optimum physical spacing of revolving disc diaphragms  5   a  to  5   c  is different for different sizes of diaphragm in relation to the mirrors  3  arranged upstream thereof in the beam direction. In order to be able to ensure this when mounting the revolving disc diaphragms  5   a  to  5   c  at a uniform height h with reference to the mirrors  3 , the latter are provided with different heights with reference to the ranges  10  of their mountings. 
     As illustrated in  FIG. 4   a , the revolving disc diaphragm stack  7   a  has a plurality of revolving disc diaphragms  5  which are accommodated in separate plug-in units  11 . Each plug-in unit  11  can be rotated out (indicated by the arrow  12  in  FIG. 4   a ) individually by means of an articulated element (not illustrated) common to all the plug-in units  11 , such that in each case one revolving disc diaphragm  5  can be rotated out in order subsequently to be lifted (indicated in  FIG. 4   a  by the dotted arrow  8 ) into the beam path  2  of the projection objective  1  to its operating position  9 , as explained at a later point in time. The swivelling movement of the plug-in units  11  can be accomplished by means of a gearwheel drive which is fitted on a lifting mechanism or a module housing and can be arranged in such a way that it moves the gearwheel teeth as the plug-in unit  11  passes. Alternatively, in other exemplary embodiments it would also be possible to provide other drive mechanisms, in particular friction wheels, magnetic clutches or special electric motors with rotors which are installed in the plug-in units  11 . 
     In the present exemplary embodiment, the plug-in units  11  have a uniform overall height. In other exemplary embodiments, however, these can also differ in order to be able to use various sizes of diaphragm (compare  FIGS. 3   a  to  3   c ). 
     After the operating position  9  of the revolving disc diaphragm  5  is reached, the latter is coupled to a holding device or to a stop  13 . The holding device  13  permits a repeatably accurate positioning of the revolving disc diaphragms  5  in the micrometre range. This reduces the accuracy requirements for the separate plug-in units  11 , and also for the overall lifting mechanism (indicated by the arrow  8 ). 
     As may be seen from  FIG. 4   b , instead of lifting the revolving disc diaphragms  5  to the operating position  9  it is also possible in a further embodiment to move a revolving disc diaphragm stack  7   b  vertically (indicated by the arrow  8 ′) until the appropriate revolving disc diaphragm  5  has reached substantially the same height as the holding device  13 , after which the plug-in unit  11  with the appropriate revolving disc diaphragm  5  is rotated out and coupled to the holding device  13  after a possible additional slight vertical movement (arrow  8 ). This embodiment has the advantage that the diaphragm exchange mechanism requires only very little space in front of the mirror  3 , the result being to release this space for additional systems (mirror cleaning systems etc.). An operating range  14  of the vertically displaceable revolving disc diaphragm stack  7   b  is illustrated by dashes or dots and dashes in  FIG. 4   c , as is a free region for additional systems  15 . 
     Especially for the field of EUVL, projection objectives  1  are very sensitive to movements of their individual optical elements, for example mirror  3 , both relative to one another and relative to the structure of their mountings. In order to minimize the transmission of interfering vibrations, the projection objective  1  is isolated from vibrations. Moreover, the individual elements inside the projection objective  1  are connected to one another rigidly (with a high natural frequency) in such a way that they move with one another as a rigid body when excited by any residual vibrations, which are usually of low frequency. 
     It is a complicated undertaking to create an embodiment of the overall diaphragm device  7  with a sufficiently high natural frequency, since relatively large masses have to be moved and the installation space is restricted. Consequently, dynamic movements (vibrations) would be transmitted to the overall projection objective  1  by the diaphragm device  7 . The relative positioning of the diaphragm  5  in relation to the remaining optical elements of the projection objective  1  is less critical in general, however. 
     A possible solution to this problem is for the entire diaphragm device  7  to be mounted on a separate structure dynamically decoupled from the projection objective  1 , but this would make positioning the diaphragm exactly in the projection objective  1  more difficult. 
     A further solution consists in separating the selected revolving disc diaphragm  5  with the holding device  13  from the remainder of the diaphragm device  7  (revolving disc diaphragm stacks  7   a ,  7   b , plug-in units  11 , lifting mechanism, housing, etc.) and arranging them on different structures, the holding device  13  being fastened directly on the optical imaging device or on the projection objective  1 . The remainder of the diaphragm device  7  can be mounted on a separate structure. 
     A further possible solution consists in fastening both the holding device  13  and the lifting mechanism  16  on the projection objective  1 , while the remainder of the diaphragm device  7  is mounted on a separate structure. 
     The holding device  13  ensures that the revolving disc diaphragm  5  is positioned accurately relative to the projection objective  1  and in six degrees of freedom. 
     Furthermore, there is also a need to hold or lock the revolving disc diaphragms  5  in the holding device  13  against the gravity force and other interfering forces. In order to prevent particles from contaminating the mirror surfaces, the revolving disc diaphragm  5  should be locked as gently as possible. 
     As sketched in  FIG. 5 , the revolving disc diaphragm  5  is conveyed by means of a lifting device  16  from a removal position into its operating position  9 , and held there in the holding device  13 . In the case of the diaphragm device  7  illustrated in  FIG. 5 , use was advantageously made of mainly rotary mechanisms in the diaphragm exchange mechanism since, by contrast with translation mechanisms, fewer particles causing contamination, for example, by friction forces, are produced. As illustrated further in  FIG. 5 , the essentially constant force for holding the revolving disc diaphragm  5  in the holding device  13  is effected in a simple and advantageous way by spring elements  17  of low stiffness. The spring elements  17  should be precompressed in order to avoid a large compression deflection of the spring elements  17  relative to the operating position  9  of the revolving disc diaphragm  5 . An arrow  18  indicates the dynamic decoupling or the vibrational decoupling of the separately-mounted housing  1   a  of the projection objective  1  (indicated by dashes) and of the remainder of the diaphragm mechanism (dashed box  19 ), likewise mounted separately. 
       FIGS. 6   a  to  6   c  illustrate various embodiments of the holding device  13  for fixing and/or positioning the revolving disc diaphragm  5 . 
     As may be seen from  FIG. 6   a , a holding device  13   a  has a permanent magnet  20  and a soft iron core  21  with a coil winding  22 . The revolving disc diaphragms  5  (not illustrated in more detail here) likewise have a soft iron core  21 ′ on the opposite side and are thereby held via magnetic forces. This has the advantage that there are only a few or no open mechanically moveable parts which could lead to further instances of particle contamination. 
     As is illustrated in  FIG. 6   b , a holding device  13   b  is provided on a part  23 , and has a static part  23 ′ and the permanent magnet  20 . The revolving disc diaphragm  5  has the soft iron core  21  by means of which the revolving disc diaphragm  5  is held on the holding device  13   b . In addition, the lifting device  16  (not illustrated in more detail in  FIG. 6   b ) has a switchable electromagnet  20 ′ which is switched in the event of an exchange of diaphragms in such a way that the diaphragm is loosened from the holding device  13   b.    
     Illustrated in  FIG. 6   c  is a third embodiment of a holding device  13   c  which corresponds in essence to the holding device  13   b  from  FIG. 6   b . A soft spring element  24  which engages in a cut-out  25  in the revolving disc diaphragm  5  has been inserted here in addition. 
       FIG. 7   a  shows a holding device  13   d  with a revolving disc diaphragm  5   d . A mirror contamination monitoring means is provided here, in addition. This is effected by fine tungsten lead wires  26  which are guided via the opening in the revolving disc diaphragm  5 . The revolving disc diaphragm  5   d  is fabricated for this purpose from an insulating material such as, for example, a ceramic or similar. The electrical connection with the tungsten lead wires  26  is achieved by three contact points on bearing points  27  of the revolving disc diaphragm  5   d.    
       FIG. 7   b  shows an alternative embodiment of a contamination monitoring means. Here, the tungsten lead wires  26  are integrated in the lifting device  16 . 
     As may be seen from  FIG. 8 , the vertically displaceable revolving disc diaphragm stack  7   b  is arranged outside the projection objective  1  or the housing  1   a  thereof. This protects the projection objective  1  against contamination by the revolving disc diaphragm stack  7   b . The revolving disc diaphragm stack  7   b  is provided with a feeder device  28  which is designed as a moveable robot gripper arm, removes the corresponding revolving disc diaphragm  5  from the revolving disc diaphragm stack  7   b  and inserts it into the beam path  2  of the projection objective  1  through an opening  29  provided for the purpose. An additional lifting device  16 ′ (illustrated in a simplified fashion), likewise arranged outside the projection objective  1 , conveys the revolving disc diaphragm  5  to the holding device  13 , it then being fixed in its operating position  9 . As already described above, the diaphragm exchange mechanisms and the lifting device  16 ′ can be mounted in a dynamically decoupled fashion on different structures. Soft springs  17  of the lifting device  16 ′ ensure a dynamically decoupled connection. The opening  29  in the projection objective  1  or the housing  1   a  is closed during operation. 
     In  FIG. 9 , a lifting device  16 ″ is introduced and mounted inside the housing  1   a  of the projection objective  1 . Surfaces which slide or roll on one another are reduced to an absolute minimum in order to avoid or to minimize particle contamination. This can be implemented by using solid joints and appropriate actuators (voice coil actuator, Lorentz actuator). Surfaces are minimized in order to avoid instances of molecular contamination and, moreover, use is made only of suitable materials with low degassing rates (steels, no plastics or lubricants). Lubrication on bearings can be dispensed with by using solid joints. The mass is to be kept small or the natural frequency of the lifting device  16 ″ is to be kept as high as possible in order not to impair the structure of the projection optics dynamically. 
     As may further be seen from  FIG. 9 , the lifting device  16 ″ has the holding device  13  for the revolving disc diaphragm  5 . The revolving disc diaphragm  5  constructed as sheet metal is situated on the feeder device  28 . The feeder device  28  brings the revolving disc diaphragm  5  into the projection optics below the mirror  3 . The revolving disc diaphragm  5  is lifted from the feeder device  28  when the lifting device  16 ″ is raised. The lifting device  16 ″ drives against an inner stop. The revolving disc diaphragm  5  lies on the holding device  13  because of its own weight. Raising upwards can be prevented for example by means of a protective cover (compare  FIG. 10   a ). The revolving disc diaphragm  5  cannot then fall out or collide with the mirror  3 . 
     The following  FIGS. 10   a  to  10   c  show structural configurations  16   a ,  16   b ,  16   c  of the lifting device  16 ″ from  FIG. 9 . They have voice coil actuators (not shown in more detail) for manipulation. Rotary joints are respectively designed as solid joints  30 . 
     As illustrated in  FIG. 10   a , a protective cover  31  prevents the lifting device  16   a , constructed as a rocker, from raising the revolving disc diaphragm  5 . The lifting devices  16   a  to  16   c  have internal end stops which prescribe the respective end positions of the lifting movement. The steering movement of the lifting device  16   a  is indicated by an arrow  32 . 
       FIG. 10   b  shows the lifting device  16   b , which is designed as a set of scales and has a parallelogram guide. It is advantageous in this case that the revolving disc diaphragm  5  can be moved upwards virtually vertically. 
     A pantographic lifting device  16   c  is sketched in  FIG. 10   c.    
       FIG. 11  shows the feeder device  28  designed as a robot gripper arm. The revolving disc diaphragm  5  can be withdrawn from below by the lifting device  16   a ,  16   b ,  16   c  from the receptacle of the feeder device  28 . A locking mechanism  33  fastens the revolving disc diaphragm  5  during transport. In other exemplary embodiments the revolving disc diaphragm  5  can also be configured symmetrically such that fitting may be done from both sides. The feeder device  28  can, in addition, be designed as a double gripper, that is to say with two receptacles for two revolving disc diaphragms  5  (not illustrated). The time for changing diaphragms is thereby substantially shortened. During changing, the feeder device  28  moves with a revolving disc diaphragm  5  into the projection optics of the projection objective  1 . The exchange revolving disc diaphragm  5 , which is already located in the projection optics, is deposited on the second (empty) receptacle. The new revolving disc diaphragm  5  would be taken over by the lifting device  16   a ,  16   b ,  16   c . During a change of diaphragm, the feeder device  28  would therefore have to move one less time into the projection optics. 
     A further embodiment of a diaphragm device  7 ′ for the projection objective  1  is illustrated in  FIG. 12 . The great advantage here is the improved dynamics of the change of diaphragm in conjunction with a small required installation space. As may be seen, an incident light beam  34  is stopped down by a sheet-metal strip  7   c . The latter is provided with openings  35  which, depending on optical requirement exhibit an optimum fixed geometry. The further openings  35  are incised adjacently as diaphragms on the sheet-metal strip  7   c . The sequence of the openings  35  can be varied in order to ensure optimum speed in changing diaphragms, depending on the requirements. 
     The sheet-metal strip  7   c  is wound onto two rollers  36 . These are driven and tensioned such that the sheet-metal strip  7   c  has no “folds”. Two additional tensioning and guiding rollers  37  are fitted in order to avoid diaphragms which shift in the light direction. As a result, the changing diameter of the rollers  36  (including wound-on sheet-metal strip  7   c ) is, in particular, not rendered noticeable by an oblique position of the sheet-metal strip  7   c.    
     The optimum position of the diaphragm openings  35  can be measured, using appropriate sensors (not illustrated) via markings  38  at the edge of the sheet-metal strip  7   c . However, other methods are also conceivable in further exemplary embodiments. 
     A front view of the diaphragm device  7 ′ from  FIG. 12  is illustrated in  FIG. 13 . 
     As may be seen from  FIG. 14 , an EUV projection exposure machine  40  has a light source  41 , an EUV illuminating system  42  for illuminating a field in the object plane  4  in which a pattern-bearing mask is arranged, and the projection objective  1  with the housing  1   a  and the beam path  2  (indicated by dashes) for imaging the pattern-bearing mask in the object plane  4  onto a photosensitive substrate  43 . The diaphragm  5  for stopping down the projection objective  1  is indicated by dots.