Patent Publication Number: US-2009225296-A1

Title: Projection objective of a microlithographic projection exposure apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 11/464,934, filed Aug. 16, 2006, which is a continuation of International Patent Application PCT/EP2005/000947, filed on Feb. 1, 2005 and claims priority of German Patent Application 10 2004 008 285.5 filed Feb. 20, 2004. The full disclosure of these earlier applications is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a projection objective for microlithographic projection exposure apparatus, such as those used for the production of large-scale integrated electrical circuits and other microstructured components. The invention relates in particular to projection objectives in which at least one optical element can be deformed with the aid of an actuator. The invention furthermore relates to a method for correcting optical properties of such a projection objective. 
     2. Description of the Prior Art 
     The purpose of projection objectives in microlithographic projection exposure apparatus is to project a reduced image of structures contained in a mask onto a photosensitive layer, which is applied on a support. Very stringent requirements are placed on the imaging properties of the projection objectives owing to the small size of the structures to be image. 
     U.S. Pat. No. 6,388,823 assigned to the Applicant discloses a projection objective in which forces can be exerted on an optical element with the aid of actuators, which are distributed around the circumference of the optical element. These forces lead to bending of the optical element which, for example, may be a lens or a mirror. The bending does not affect the thickness profile of the optical element, or at least does not substantially affect it. In particular, astigmatic imaging errors can be corrected with such a bendable optical element. 
     It has however been found that the bending is not only accompanied by the desired corrective effect; rather, undesired impairment of the imaging properties of the projection objective can also occur. This applies in particular to optical elements which are isostatically mounted. Isostatic mounting means that the optical element is held merely at three mounting points offset by 120° along the circumference. 
     Such undesired impairment of the imaging properties, however, may also be caused by optical elements which are not bent but otherwise deformed in order to correct imaging errors, as is known per se in the prior art. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a projection objective of a microlithographic projection exposure apparatus, in which such undesired impairment of the imaging due to deliberate deformations of optical elements is substantially avoided. 
     It is a further object of the invention to provide an improved method for correcting optical properties of a projection objective. 
     The first object is achieved by a projection objective comprising a plurality of optical elements and at least one actuator, by which a selected optical element of the projection objective can be deformed. According to the invention, the projection objective has at least one manipulator by which the spatial position of one of the optical elements can be modified preferably automatically as a function of a force exerted by at least one actuator. 
     With respect to the method, the object is achieved in such a projection objective in that the spatial position of one of the optical elements is modified as a function of a force exerted by the at least one actuator. 
     The invention is based on the discovery that, particularly when the relevant optical element is mounted only at relatively few points as is the case for instance with isostatic mounting, the actuators induce forces which lead not only to a deformation but also to a change in the spatial position of the optical element. The type of position change depends in particular on the points at which the optical element is held and where the forces generated by the actuators engage on the optical element. With isostatic mounting of the optical element in which three mounting points are provided offset by 120° along the circumference, for example, tilting of the optical element takes place about a transverse axis of the optical element when it is bent in a saddle fashion to correct astigmatism. With a different constellation of mounting points of the optical element and actuators engaging thereon, the position change may also consist in a translational displacement of the optical element in a plane perpendicular to the symmetry axis. 
     However, undesired position changes due to deformations can take place even when the optical element is mounted at very many points. The reasons for this are in general fabrication tolerances and mounting-induced asymmetries. 
     With the aid of the at least one manipulator according to the invention, it is possible to compensate for the aforementioned position changes of the optical element which lead to impairment of the imaging properties. A single manipulator is not sufficient for this in most cases, so that a plurality of manipulators are to be provided and their effects on the optical element should be appropriately tuned. 
     The at least one manipulator preferably acts on the selected optical element for which the undesired position change has occurred owing to the deformation. Nevertheless, it is also possible for an optical element other than the deformed optical element to be tilted, displaced or otherwise have its position changed with the aid of the at least one manipulator, so as to compensate for the effects which have been caused by the position change of the selected optical element owing to the deformation. 
     If both the at least one actuator, which can deform the optical element, and the at least one manipulator, which induces the position change, can be actuated fluidically i.e. hydraulically or pneumatically, then it is preferable for the at least one actuator and the at least one manipulator to be connected to a fluidic pressure system so that they can both be fluidically actuated simultaneously. Specifically, fluidic pressure systems provide the opportunity to make forces act simultaneously at different positions inside the system. With an appropriate design of the pressure system, it is then possible to induce an automatic position change with the at least one manipulator so that, when the at least one actuator is actuated, the at least one manipulator without its own additional control or regulation applies the required force which leads to the desired position change of the optical element. 
     One possibility for simultaneous actuation of the at least one actuator and the at least one manipulator consists in designing the pressure system so that changes in the fluid pressure applied to the actuator lead to corresponding changes in the fluid pressure applied to the manipulator. For example, this may be done by connecting the manipulator in parallel with the actuator in the pressure system. 
     In a particularly preferred configuration of the invention a resilient compensating element, whose deformations caused by pressure fluctuations can be transmitted via a transmission element to the at least one manipulator, is integrated into a pressure line leading to the at least one actuator. The manipulator will in this way be actuated not by changes in the fluid pressure in the pressure line, but mechanically via the transmission element which is coupled to the compensating element. The purpose of the resilient compensating element, which may for example be a kind of bellows, is to decouple the actuator from supply devices of the pressure system and the housing of the projection objective so that vibrations cannot be transmitted onto the selected optical element. Since pressure changes in such a compensating element lead to deformations and therefore to movements, a mechanical force which can be used for the position change of the one optical element with the aid of the at least one manipulator can be derived from the compensating element or the pressure line connected to it. 
     If such transmission of forces from the compensating element to the at least one manipulator is not provided, then it may be expedient to improve the decoupling of the pressure line from the optical element by integrating a plurality of resilient compensating elements in a pressure line, which are arranged relative to one another so that the deformation forces formed therein in the event of a pressure change mutually compensate least substantially. Deformation forces which act in the event of a pressure change in a compensating element cannot therefore be undesirably transmitted to the optical element. This force-free connection of the optical element to a supply device of the pressure system may also advantageously be used independently of the manipulators according to the invention. 
     Such an arrangement can be produced most simply if two compensating elements, connected together to a pressure feed line and together to a pressure discharge line, are arranged lying diametrically opposite each other. Merely one additional compensating element is required in such an arrangement. 
     The at least one manipulator itself may engage directly on the optical element or, alternatively, it may engage thereon via a mounting which contains the selected optical element. If the at least one manipulator engages on the mounting, then when the at least one manipulator is actuated this leads to a relative movement between the mounting and the optical element contained in it, on the one hand, and the housing of the objective on the other hand. 
     If the mounting is in turn supported in a frame, which is fixed relative to the housing of the objective, then the at least one manipulator may also engage on such a frame. In this way, the mounting of the optical element in the mounting is not affected by the at least one manipulator. 
     Instead of a fluidic pressure system, the at least one actuator and/or the at least one manipulator may of course also be actuated with the aid of other controlling elements. For example, precisely adjustable forces may be exerted with the aid of piezoelectric elements. In this case, it is generally necessary to provide a control or regulating device for driving the at least one actuator and/or the at least one manipulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing in which: 
         FIG. 1  shows a catadioptric projection objective of a microlithographic exposure apparatus having a mirror unit in a meridian section; 
         FIG. 2  shows a side view of the mirror unit for the projection objective shown in  FIG. 1  according to a first exemplary embodiment in an enlarged, highly schematised representation; 
         FIG. 3  shows a section along the line III-III of the mirror unit shown in  FIG. 2 ; 
         FIG. 4  shows the mirror unit shown in  FIG. 2 , but with tilting due to bending generated by actuators; 
         FIG. 5  shows the mirror unit shown in  FIG. 4 , in which the tilting is rectified by manipulators; 
         FIG. 6  shows a side view of the mirror unit for the projection objective shown in  FIG. 1  according to a second exemplary embodiment in an enlarged, highly schematised representation; 
         FIG. 7  shows a side view of the mirror unit for the projection objective shown in  FIG. 1  according to a third exemplary embodiment in an enlarged, highly schematised representation 
         FIG. 8  shows the mirror unit shown in  FIGS. 1 to 5  with a hydraulic system; 
         FIG. 9  shows the mirror unit shown in  FIGS. 1 to 5  with another hydraulic system; 
         FIG. 10  shows the mirror unit shown in  FIGS. 1 to 5  with a further hydraulic system in the undeformed state; 
         FIG. 11  shows the mirror unit shown in  FIG. 10  in the deformed state with tilting correction; 
         FIG. 12  shows a compensating unit for force-free connection of a mirror unit to a housing via a pressure line. 
     
    
    
     DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS 
       FIG. 1  shows a simplified representation of a meridian section through a projection objective, denoted overall by  10 , of a microlithographic projection exposure apparatus. The projection objective, which is also described in PCT/EP03/04015 assigned to the Applicant, is used to project a reduced image of structures contained in a reticle  11  onto a photosensitive layer  12 , which consists of a photoresist and is applied on a substrate  13 . The reticle  11  is arranged in an object plane OP and the photosensitive layer  12  is arranged in an image plane IP of the projection objective  10 . 
     After passing through the reticle  11 , projection light  14  indicated by dashes in  FIG. 1  which is generated by an illuminating device of the projection exposure apparatus and has a wavelength of 157 nm in the exemplary embodiment represented, travels through a plane-parallel plate  15  and a lens L 1  into a beam splitter cube  16 . There, the projection light  14  is reflected by a polarisation-selective beam splitter layer  17  contained in it and sent through a lens L 2 , a quarter-wave plate  18  and two further lenses L 3  and L 4  onto a mirror unit  19  having a spherical mirror  20  and a mounting, which will be explained in more detail below. 
     After reflection by the spherical mirror  20 , the projection light  14  passes back through the lenses L 4  and L 3 , the quarter-wave plate  18  and the lens L 2 , and strikes the polarisation-selective beam splitter layer  17 . There, however, the projection light  14  is not reflected but transmitted since the polarisation of the projection light  14  has been rotated through 90° by the double transit through the quarter-wave plate  18 . From the beam splitter cube  16 , the projection light  14  travels via a plane mirror  21  into a purely dioptric parts  23  of the projection objective  10 , in which refractive optical elements (not denoted in detail) are arranged along an optical axis indicated by  25 . 
       FIGS. 2 and 3  give a highly schematised representation of the mirror unit  19  for the projection objective  10  according to a first exemplary embodiment, respectively in a side view and a horizontal section along the line III-III. The mirror  20  of the mirror unit  19  is contained in an isostatic mounting, which is formed by three mounting units  22   a ,  22   b  and  22   c . The mounting units  22   a ,  22   b  and  22   c  are distributed at intervals of 120° over the circumference of the mirror  20 , as can be seen in  FIG. 3 . Owing to the isostatic mounting, the mirror  20  is contained stiffly but nevertheless almost force-free in the mounting. Possible designs for the mounting units  22   a ,  22   b ,  22   c  are known in the prior art, see for example EP 1 245 982 A2, which corresponds to US Pat. Appl. 2002/0176094 A1. 
     The mounting units  22   a ,  22   b ,  22   c  are fastened on a frame  24  which, for example, may have a circularly round base shape. For the isostatic mounting described here, it is nevertheless also conceivable to use a triangular base shape as likewise known from the aforementioned EP 1 245 982 A2. 
     The frame  24  is fixed relative to a housing  28  of the projection objective  10  via three manipulators  26   a ,  26   b  and  26   c , which are arranged at the vertices of an imaginary equilateral triangle. The manipulators  26   a  to  26   c  in the exemplary embodiment represented respectively comprise two hydraulically telescopable cylinders, so that the length of the manipulators  26   a  to  26   c  can be mechanically varied. With appropriate driving of the manipulators  26   a  to  26   c , the frame  24  can be tilted relative to the housing  28  about any axis perpendicular to the optical axis  25  of the projection objective  10 . 
     An actuator unit denoted overall by  30 , by which the mirror  20  can be bent, is arranged on the lower side of the mirror  20 . To this end, the actuator unit  30  comprises a plurality of individual actuators (not shown because they are known per se) which are arranged around the optical axis  25 . The actuator unit  30  may be designed so that the individual actuators act on the mirror  20  from the circumference. Possible embodiments of an actuator unit  30  having a plurality of individual actuators can be found in DE 198 59 634 A1, which corresponds to U.S. Pat. No. 6,307,688, and DE 198 27 603 A1, which corresponds to U.S. Pat. No. 6,388,823. 
     In the exemplary embodiment represented, it is assumed that a second-order astigmatism can be corrected with the aid of the actuator unit  30 . To this end, the individual actuators contained in the actuator unit  30  should engage on the mirror  20  so that the mirror  20  is bent in a saddle fashion owing to the bending moments thereby generated. This saddle-like bending is indicated in  FIG. 3  by circled capital letters H and T. Circles containing a letter H indicate positions on the mirror  20  which lie uppermost, in so far as they are furthest away from the frame  24 . Circles containing a letter T indicate positions which lie lowermost and are thus at the smallest distance from the frame  24 . 
     In order to generate such saddle-like bending, the individual actuators of the actuator unit  30  may be arranged and actuated so that compressive forces in the direction of the optical axis  25  are generated at two points lying opposite each other at an angle of 180°, whereas oppositely directed forces act on the mirror  20  at two points respectively offset by angles of 90° thereto. 
     As can be seen clearly from  FIG. 3 , such forces in the isostatic mounting as shown lead to tilting of the mirror  20  about a horizontal axis  34 , which extends through the lower-lying points T. The direction of the tilting is indicated by an arrow  36  in  FIG. 3 . The reason for the tilting is the fact that the two-fold symmetry of the forces cannot be made to match the three-fold symmetry of the mounting in the mounting units  22   a ,  22   b ,  22   c . With mounting at four suitably selected points, this tilting would not occur. Mounting at four points, however, is known to be disadvantageous in many respects compared to isostatic mounting. 
     The tilting about the axis  34  is absorbed by the mounting units  22   a  to  22   c  which, to this end, may have resilient properties as described in EP 1 245 982 A2, which corresponds to US Pat. Appl. 2002/0176094 A1. 
       FIG. 4  shows the mirror  20  in a side view corresponding to  FIG. 2 , although the mirror  20  is tilted about the axis  34  in the aforementioned way owing to the saddle-like bending (which cannot be seen in  FIG. 4 ). The symmetry axis  32  of the mirror  20 , which normally coincides with the optical axis  25  of the projection objective  20 , is offset by the tilt angle relative to the original setpoint position owing to the tilting. This tilt angle is represented very exaggeratedly for the sake of clarity. 
     The three manipulators  26   a  to  26   c  are now driven so that the tilted symmetry axis  32  of the mirror  20  is returned to its setpoint position which is vertical in  FIG. 4 . To this end, the manipulators  26   a  to  26   c  which are arranged outside the tilting axis are adjusted so that the entire frame  24 , including the mounting units  22   a  to  22   c  fastened therein and the mirror  20  contained therein, is tilted back by the required compensating angle. In magnitude, this compensating angle coincides with the tilt angle by which the mirror  20  is tilted relative to the frame  24  during the bending. Owing to the compensatory tilting generated by the three manipulators  26   a  to  26   c , as can be seen clearly in  FIG. 5 , the symmetry axis  32  of the mirror  20  even after it is bent lies in the setpoint position pointing vertically upwards in  FIG. 5 . The tilting of the mirror  20  in the mounting units  22   a  to  22   c , as generated by the bending, does not therefore impair the imaging properties of the mirror  20 . 
       FIG. 6  shows a mirror unit  119  suitable for the projection objective  10  according to a second exemplary embodiment. Parts which are the same as in the exemplary embodiment described above are denoted by the same reference numerals, and parts corresponding to one another are denoted by reference numerals increased by 100. 
     In the mirror unit  119 , it is not the frame  24  but only the mirror  20  which is correspondingly tilted in order to compensate for tilting movements due to bending. To this end, a manipulator  126   a  or  126   b  which can adjust the lengths of the three mounting units is respectively integrated into two mounting units  122   a ,  122   b  and a third mounting unit (concealed by the mounting unit  122   b  in  FIG. 6 ). The frame, however, is firmly connected to the housing  28  via connecting elements  38   a ,  38   b . In order to rectify the tilting of the mirror  20 , as represented in  FIG. 4  for the mirror unit  19 , the manipulator  126   a  and the manipulator lying behind it (not visible in  FIG. 6 ) should be retracted and the manipulator  126   b  should be deployed, so that their lengths are respectively reduced and increased. 
       FIG. 7  shows a mirror unit  219  suitable for the projection objective  10  according to a third exemplary embodiment. Parts which are the same as in the mirror unit  19  are denoted by the same reference numerals, and parts corresponding to one another are denoted by reference numerals increased by 200. 
     The mirror unit  219  represented in  FIG. 7  differs from the mirror unit  119  represented in  FIG. 6  merely in that mounting units  222   a ,  222   b  are fastened directly via manipulators  226   a ,  226   b , i.e. without an interposed frame  24 , on the housing  28  of the projection objective  10 . 
       FIG. 8  shows a mirror unit  319  suitable for the projection objective  10  according to a fourth exemplary embodiment. Parts which are the same as in the mirror unit  19  are denoted by the same reference numerals, and parts corresponding to one another are denoted by reference numerals increased by 300. 
     The mirror unit  319  corresponds essentially to the mirror unit  19  as represented in  FIG. 2 . A hydraulic device  40  for supplying a hydraulic liquid to an actuator unit  330  and manipulators  326   a ,  326   b ,  326   c  is also shown. 
     The hydraulic device  40  contains a controller  42  which, via valves (in a manner not represented in detail), controls the pressure of the hydraulic liquid which is used to actuate the individual actuators contained in the actuator unit  30  and the manipulators  26   a ,  26   b ,  26   c . To this end, in the mirror unit  319 , the four individual actuators contained in the actuator unit  30  are connected to the hydraulic device  40  via respectively independent first pressure lines  44   a  to  44   d.    
     The hydraulic device  40  is furthermore connected via second pressure lines  46   a ,  46   b  and  46   c  to the manipulators  326   a ,  326   b ,  326   c . A compensating element  48  in the form of a bellows is integrated into each of the first and second pressure lines  44   a  to  44   d  and  46   a  to  46   c . The purpose of the compensating element is to decouple the hydraulic device  40  from the actuator unit  330  and the manipulators  326   a  to  326   c  so that vibrations, which are caused in the hydraulic device  40  by valves or pumps contained therein or by the projection objective housing  28  connected thereto, cannot be transmitted to the mirror  20  via the first and second pressure lines  44   a  to  44   d  and  46   a  to  46   c.    
     The controller  42  of the hydraulic device  40  is designed so that actuation of the actuator unit  330  is accompanied by synchronous actuation of the manipulators  326   a  to  326   c . To this end, the controller  42  accesses a stored table in which the setpoint excursions of the manipulators  326   a  to  326   c , which are necessary so that the tilting of the mirror  20  due to the bending caused by the actuator unit  330  can be rectified again, are stored for a multiplicity of settings of the individual actuators contained in the actuator unit  330 . 
       FIG. 9  shows a mirror unit  419  suitable for the projection objective  10  according to a fifth exemplary embodiment. Parts which are the same as in the mirror unit  19  are denoted by the same reference numerals, and parts corresponding to one another are denoted by reference numerals increased by 400. 
     The mirror unit  419  differs from the mirror unit  319  shown in  FIG. 8  inter alia in that the controller  42  is replaced by a regulator  42 ′, which is connected via a line  49  (represented by dashes) to position sensors which record the spatial position of the mirror  20 . Furthermore, only one pressure line  50  leads from the hydraulic device  40 ′ to the actuator unit  430 . The latter is connected via a further pressure line  52  in series with manipulators  426   a  to  426   c . Any pressure change in the pressure line  50  therefore leads to a corresponding pressure change in the actuator unit  430  and in the manipulators  426   a  to  426   c . The effect which can be achieved via correspondingly preset valves, which are assigned to the individual actuators of the actuator unit  430  and to the manipulators  426   a  to  426   c , is to set up the desired deformation of the mirror  20  as well as simultaneously tilting of the frame  24  relative to the housing  28  merely by changing the pressure of the hydraulic liquid in the pressure line  50 . Although the actuator unit  430  and the manipulators  426   a  to  426   c  can therefore no longer be controlled independently of one another, the control of the mirror unit  419  is thereby greatly simplified. 
       FIGS. 10 and 11  show a mirror unit  519  suitable for the projection objective  10  according to a sixth exemplary embodiment, respectively with an unbent mirror and a bent mirror. Parts which are the same as in the mirror unit  19  are denoted by the same reference numerals, and parts corresponding to one another are denoted by reference numerals increased by 500. 
     In the mirror unit  519 , only the manipulator  526   b  is actively configured. The other two manipulators  526   a  and  526   c , on the other hand, are passively configured. This means that the manipulators  526   a ,  526   c  can in turn accomplish length changes only passively; an active length change, for example by varying an applied hydraulic pressure, is not possible. 
     The active manipulator  526   b  consists essentially of the upper arm of a lever mechanism  54 , the lower arm  55  of which is coupled to a pressure line  56  which connects a pressure device (not shown) to an actuator unit  530 . 
     When the pressure of the hydraulic liquid in the pressure line  56  increases in order to actuate the individual actuators contained in the actuator unit  530 , this leads to an extension of the resilient compensating element  48  as indicated by a double arrow  58  in  FIG. 11 . Via the lever mechanism  54 , this extension  58  is converted into a tilting movement of the frame  24  in the direction indicated by arrows  60 . The frame  24  is thereby automatically tilted in the desired way each time the mirror  20  bends, without additional control or regulating devices. It is readily possible to adapt the tilt angle by appropriately configuring the lever mechanism  54 . 
     If the tilting movement generated by manipulators is intended to be controllable independently of the actuator unit  30 , as is the case in the mirror unit  319  according to  FIG. 8 , then it is advantageous to replace the compensating elements  48  in the first and second pressure lines  44   a  to  44   d  and  46   a  to  46   c  respectively by a compensating unit  48 ′ as represented in  FIG. 12 . The compensating unit  48 ′ contains a first T-piece  64 , which divides a pressure line  44  into two parallel pressure lines  66 ,  68 . A compensating element  48   a  or  48   b  is respectively integrated into each of the parallel pressure lines  66 ,  68 , and specifically so that the compensating elements  48   a ,  48   b  lie opposite each other. Via a second T-piece  70 , the parallel pressure lines  66 ,  68  are recombined into a pressure line  44 ′ downstream of the compensating elements  48   a ,  48   b.    
     Since the compensating elements  48   a ,  48   b  lie symmetrically opposite each other, length changes of the compensating elements  48   a ,  48   b  as indicated by double arrows  72 ,  74  do not lead to a force in the longitudinal direction of the pressure line  44 ′ leading off from the second T-piece  70 . The take-off through the second T-piece  70 , lying between the two compensating elements  48   a ,  48   b , thus induces a force equilibrium which prevents undesired controlling forces from propagating via the pressure line  44 ′ and an actuator unit and/or a manipulator to the mirror  20 . 
     The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. The applicant seeks, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.