Abstract:
The disclosure relates to a microlithography projection exposure system having optical corrective elements configured to modify the imaging characteristics, as well as related systems and component.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 12/171,394, filed Jul. 11, 2008, which is a continuation of international application No. PCT/EP2006/012120, filed Dec. 15, 2006, which claims benefit of U.S. Ser. No. 11/341,894, filed Jan. 30, 2006. The contents of U.S. Application Ser. No. 12/171,394 and international application No. PCT/EP2006/012120 are hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    The disclosure relates to a microlithography projection exposure system having optical corrective elements configured to modify the imaging characteristics, as well as related systems and components. 
       BACKGROUND 
       [0003]      FIG. 1  shows such an example of a projection exposure system  1  that includes an illumination apparatus  3  and an apparatus  4  to accommodate and position a mask provided with a grid-like structure, a so-called reticle  5 , by which the subsequent structures on a wafer  2  are determined. Projection exposure system  1  also includes an apparatus  6  to hold, move and position the wafer  2 , and an imaging apparatus, namely a projection objective  7  having a plurality of optical elements, such as lenses  8  which are mounted in an objective housing  10  of the projection objective  7  via frames  9 . 
         [0004]    Typically, a basic functional principle in this case provides that the structures inserted into the reticle  5  are imaged reduced in size on the wafer  2 . The illumination apparatus  3  provides a projection beam  11  of electromagnetic radiation for imaging the reticle  5  on the wafer  2 , for example from the visible band, the UV or EUV band. A laser or the like can be used as a source of this radiation. The radiation is formed in the illumination apparatus  3  by optical elements such that the projection beam  11  has the desired properties with regard to diameter, polarization, shape of the wavefront and the like when it is incident on the reticle  5 . The optical elements may be refractive, reflective, or different types of components, or combinations thereof. 
         [0005]    Often, after exposure, the wafer  2  is moved on in the direction of the arrow, so that a multiplicity of individual regions, each having the structure prescribed by the reticle  5 , are exposed on the same wafer  2 . Due to the step-like feeding movement of the wafer  2  in the projection exposure system  1 , it is often also referred to as a stepper. Optionally, a scanning image of each area is carried out in many modern machines, and such systems are commonly referred to as scanners. 
         [0006]    An image of the reticle  5  is generated via the projection beam  11  and is transferred to the wafer  2  with a correspondingly reduced size by the projection objective  7 , as already explained above. The projection objective  7  has a multiplicity of individual refractive, diffractive and/or reflective optical elements such as lenses, mirrors, prisms, end plates and the like. 
       SUMMARY 
       [0007]    In some embodiments, the disclosure provides a device and a method by which flexible correction of imaging defects in a projection exposure system is possible with simultaneously minimal mechanical and thermal loads and minimal contamination of the interior of the system. In certain embodiments, the disclosure provides a device that permits improved correction of imaging defects in projection exposure systems. 
         [0008]    In some embodiments, the projection exposure system used in semiconductor lithography has a first and at least one further optical corrective element, with the first optical corrective element being arranged in the region of a pupil plane of the projection exposure system and the further optical corrective element being arranged at a greater distance from the pupil plane than the first corrective element. In some instances, the first corrective element is arranged at a distance from the pupil plane which corresponds to a sub-aperture ratio of greater than 0.75, such as greater than 0.9. The sub-aperture ratio is a measure of the distance of an object to a pupil plane; a sub-aperture ratio of 1 means that an object is located on the pupil plane. The closer the sub-aperture ratio tends to 0, the greater is the distance between the object and the pupil plane. A more detailed description of the definition of the sub-aperture ratio can be found in the U.S. provisional application U.S. No. 60/696118, from the same applicant. There, the sub-aperture ratio is described as the ratio of the principal beam height to the marginal beam height VM on the optically active surface of an optical element. The further optical corrective element can be arranged at a distance from the pupil plane which corresponds to a sub-aperture ratio of less than 0.75, such as less than 0.5. This arrangement of the two optical corrective elements can provide the advantage of efficient correction of image defects, such as constant image defects over the entire image plane, in the region of the pupil plane. Because the optical elements arranged in the region of the pupil plane can generate constant image defects over the entire image plane, effective correction of such defects is may be possible by this approach. 
         [0009]    In some embodiments, an optical corrective element is a plane-parallel plate. Optionally, one, several, or even all optical corrective elements can be plane-parallel plates. Plane-parallel plates as corrective elements can provide the advantage that they are easy to manufacture and replace in the projection exposure system, and that they can be measured in a simple manner via interferometric methods. Furthermore, their corrective action is comparatively robust against eccentricities—particularly when used in the vicinity of a pupil plane. 
         [0010]    In certain embodiments, an optical corrective element can be a screen, such as a vapor-deposited screen of the first order or a variable screen. 
         [0011]    In some embodiments, an optical corrective element is an interference filter or an intensity filter, such as a neutral filter. Here, neutral filters have the property that they allow, in a simple manner, compensation for local deviations in the transmission of the objective, such as in the radial direction. 
         [0012]    One advantageous use of screens is that the zero-order diffraction of the diffraction image generated by the reticle can be efficiently masked or attenuated by a screen arranged on a pupil plane or in the vicinity of a pupil plane, leading to an improvement in the contrast and hence an improvement to the image on the wafer. However, the diffraction occurring at the reticle depends strongly on the type of structures to be exposed and the illumination settings. This makes it desirable to flexibly match the used shape and position of the screen to the respectively given conditions. By way of example, this can be achieved by providing a replacement device which permits a rapid replacement of the optical corrective element as soon as the optical conditions change, for example when a new reticle is used. Here, the use of a replacement device has the particular advantage that, it is not necessary to completely open the objective housing to replace the optical corrective element, as a result of which the risk of contamination of the interior of the objective housing is reduced. Of course, the use of the replacement device is not limited to the rapid replacement of screens; the further optical corrective elements specified above can advantageously also be rapidly replaced by the replacement device. 
         [0013]    The arrangement of the further optical corrective element at a greater distance from the pupil plane than the first optical corrective element means that this corrective element will be closer to a field plane of the projection exposure system than the first optical corrective element. Here, a field plane or an image plane is understood to be a plane in which an image or intermediate image of the object plane is generated. Typically, the optical elements used, for example lenses, are particularly exposed to inhomogeneous loads, which lead to imaging defects. The density of the lens material can locally change or increase in the strongly illuminated areas, so that the imaging properties of the lens change and imaging defects result. Effective correction of such defects can be achieved using the solution according to the disclosure by virtue of the fact that the further optical corrective element is arranged in the region of those optical elements of the projection exposure system which are in the vicinity of the field, since in this manner the defects caused by the effects described above can be rectified in the vicinity of the location of their creation. 
         [0014]    One advantageous procedure for replacing the optical corrective element is to firstly record the application parameters of the projection exposure system, and to predict degradation phenomena on the basis of the recording. Subsequently, at least one matched corrective element can be produced in advance, significantly before the planned point in time of a replacement, and then be replaced at a defined point in time. This procedure can be further improved by additional measurement of the application parameters of the projection exposure system, or prediction of the expected degradation appearances on the basis of drift measurements and/or known illumination parameters. This method has the advantage that the times to replace optical corrective elements can be reduced effectively. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The disclosure is provided in connection with  FIGS. 1 to 7 , in which: 
           [0016]      FIG. 1  shows an exemplary micro lithography projection exposure apparatus; 
           [0017]      FIG. 2  shows an exemplary arrangement of the two optical corrective elements in the projection objective of a projection exposure system; 
           [0018]      FIG. 3  shows an exemplary replacement device for replacing one of the optical corrective elements; 
           [0019]      FIG. 4  shows a replacement device is in the form of a rotating disk; 
           [0020]      FIG. 5  shows the replacement device as a rotating disk; 
           [0021]      FIG. 5   a  shows the replacement device in conjunction with two mutually opposite magazines; 
           [0022]      FIG. 6  shows a carriage as a replacement device combined with a magazine in the form of a rotating disk; 
           [0023]      FIG. 7  shows the optical corrective elements are arranged together on a holding frame; and 
           [0024]      FIG. 8  shows a concept for mounting an optical corrective element. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 2  shows an exemplary arrangement of the two optical corrective elements  13 ,  14  in the projection objective  7  of a projection exposure system. The projection objective  7  has a plurality of lenses  8  mounted in frames  9 ; furthermore, the location of a pupil plane  12  of the projection objective  7  is indicated by a dashed line. Here, in the region of the pupil plane  12 , the first optical corrective element  13  is connected to the holding elements  16  via manipulators  15 ; a loose arrangement of the optical corrective elements  13  and  14  in the projection exposure system is also conceivable. The manipulators  15  allow variation of the tilting of the optical corrective element  13  or else variation of the distance of the optical corrective element  13  from the pupil plane  12 ; in this case, they can be in the form of piezo-manipulators. Here, the optical corrective element  13  can be fixed to the manipulators  15  by, for example, spring elements, pneumatic elements, magnetic elements, reduced pressure elements or else interlocking elements. The distance at which the first corrective element  13  is arranged from the pupil plane  12  corresponds to a sub-aperture ratio of &gt;0.75. The second optical corrective element  14  is arranged at a distance from the pupil plane  12  and hence from the first optical corrective element  13 ; in this case, the distance of the second optical corrective element  14  from the pupil plane  12  corresponds to a sub-aperture ratio of &lt;0.75. By way of example, the optical corrective elements  13  and  14  here can be plane-parallel plates, screens such as a vapor-deposited screen of the first order, or else variable screens. Furthermore, interference filters or else intensity filters, such as neutral filters, can be used as optical corrective elements  13  or  14 . In this case, the disclosure is not limited to the optical corrective elements  13  and  14  being adjacent, as illustrated in  FIG. 2 ; in fact, it is also feasible for further optical elements to be arranged in the region between the optical corrective elements  13  and  14 . 
         [0026]      FIG. 3  shows an exemplary replacement device  17  for replacing one of the optical corrective elements  13  or  14 . In the example shown in  FIG. 3 , the replacement device  17  is in the form of a carriage. In this case, the replacement device  17  in the form of a carriage is an arrangement of fixed guide rails  19  connected to moveable guide rails  18  by adapter frame  29 , ensuring linear guidance of the optical corrective element  13  into the beam path of the projection objective  7  (not illustrated in  FIG. 3 ). In this case, the moveable guide rails  18 , the adapter frame  29  or else the fixed guide rails  19  can be equipped with sensor units  20  for determining the position of the optical corrective element  13 . The drive of the replacement device  17 , not illustrated in  FIG. 3 , should in this case be selected such that the introduction of vibration or else of heat into the projection objective  7  is kept as low as possible; this can be achieved by the use of linear motors, pneumatic elements or else moving coils for a drive. 
         [0027]    In the example shown in  FIG. 3 , the optical corrective element  13  is an intensity filter for the central shadow. Via the replacement device  17 , the optical corrective element  13  is inserted in the region of the holding element  16  by a linear motion. Here, the final position of the optical corrective element  13  with respect to the other components of the projection objective  7  is determined during the insertion of the optical corrective element  13  into the adapter frame  29 . In this case, the holding element  16  is connected to the objective housing  10  of the projection objective  7  (not illustrated in  FIG. 3 ). 
         [0028]    This measure means that the vast majority of the components of the replacement device  17  have no contact with the interior of the objective. 
         [0029]    This can result in various advantages, such as:
       the components for the positioning of the external optical corrective element  13  can avoid particle created by friction from being deposited on the surfaces of the optical elements arranged in the projection objective  7 , and causing scattered light;   in the case of defect, the optical corrective element  13  can be completely replaced with little effort and without replacing the entire projection objective  7 ;   upgrades/redesigns can be undertaken even in the case of objectives which are already in use without the need to replace the objective; in this case, an optical corrective element  13  which is completely different to the originally used element can also be installed; and/or   the service can be sped up with a reduced downtime of the projection exposure system.       
 
         [0034]      FIG. 4  shows the replacement device  17  is in the form of a rotating disk. In this case, the replacement device  17  in the form of a rotating disk has four accommodation units  22  for accommodating optical corrective elements  13 . In the present example, three of the four accommodation units  22  of the replacement device  17  in the form of a rotating disk are provided with optical corrective elements  13 , in this case intensity filters; the fourth accommodation unit  22  remains empty, as a result of which exposure without an intensity filter, for example, becomes possible, or it is possible to provide the empty accommodation unit  22  for replacement of the optical corrective element  13 . The particular advantage of using a rotating disk as a replacement device  17  that this makes it possible to keep the horizontal forces acting on the objective housing  10 , and hence the projection objective  7 , to a minimum, since only torques and no linear forces occur as the acceleration torques during rapid braking or acceleration of the replacement device  17 . In this case, as illustrated in  FIG. 4 , the replacement device  17  in the form of a rotating disk can be partly located outside the objective housing  10 , making it easier to replace the optical corrective elements  13 . This advantage is however offset by the disadvantage that, if part of the replacement device  17  in the form of a rotating disk is arranged outside the objective housing  10 , increased complexity is desired to avoid the influx of dirt into the interior of the objective housing  10 . This problem can be resolved by arranging the replacement device  17  in the form of a rotating disk completely in the interior of the objective housing  10 ; of course, this results in certain limitations with regard to the maximum number of optical corrective elements  13  available for rapid replacement. It is furthermore feasible to arrange the drive of the replacement device in the form of a rotating disk not illustrated in  FIG. 4  both within and outside of the objective housing  10 . 
         [0035]    The replacement device  17  is  FIG. 5  is a rotating disk.  FIG. 5  shows a replacement device  17  in the form of a linear carriage. Here, the accommodation units  22  of the replacement device  17  in the form of a linear carriage are arranged linearly along the profile of the carriage. In this case, the replacement device  17  in the form of a linear carriage can run horizontally through the entire objective housing  10 . It is common to both solutions illustrated in  FIGS. 4 and 5  that the replacement device  17  itself has a plurality of accommodation units  22  and thus has a dual functionality as replacement device  17  on the one hand, and magazine on the other. It is particularly advantageous in the case of this solution that a separate magazine can be dispensed with, as a result of which a significant amount of installation space can be saved. 
         [0036]      FIG. 5   a  provides a high degree of flexibility and rapid replacement in particular. The replacement device  17  is in the form of a linear carriage with two accommodation units  22  for optical corrective elements  13 . In contrast to  FIG. 5 , two stack magazines  23   a  and  23   b  are in this case arranged on opposite sides of the objective housing  10 . Here, the replacement device  17  can be moved horizontally in a linear movement from one magazine  23   a,b  to the other through the entire objective housing  10 . Using this, the removal of one optical corrective element from the beam path of the projection exposure system and the insertion of an optical corrective element can be carried out within the same movement of the replacement device, without changing the direction of the movement. As a result, the number of acceleration and deceleration processes for the replacement of an optical corrective element is minimized, this allowing quicker replacement. Optical corrective elements  13  can be removed from or inserted into the replacement device  17  from both the magazine  23   a  and magazine  23   b.  During the operation of the projection exposure system with an optical corrective element  13  in one of the magazines  23   a  or  23   b,  this arrangement allows the insertion of the fitting optical corrective element  13  for the subsequently provided operational parameters of the system into the accommodation unit  22  of the replacement device  17 . This procedure allows a no-longer required corrective element  13  out of the beam path of the system and, during the course of the same movement, insertion of the new corrective element  13  for the parameters of the system into the beam path within a single linear movement, with a practically unlimited number of different corrective elements. This allows changes to the parameters of the system within a time of &lt;30 ms, such as &lt;10 ms. Via the mentioned measure, the level of utilization of the system can be significantly increased and thus the productivity can be improved. 
         [0037]    Of course, the idea illustrated by  FIG. 5   a  can also be transferred to the other embodiments. 
         [0038]      FIG. 6  shows a carriage used as the replacement device  17  is combined with a magazine  23  in the form of a rotating disk. In this case, the magazine  23  has four accommodation units  22 , three of which are equipped with optical corrective elements  13 . The fourth accommodation unit  22  is not occupied in the present example; it is available for accommodating an optical corrective element  13  from the interior of the objective housing  10 . The replacement device  17  is, in the form of as a linear carriage which moves in and out of the interior of the objective housing  10  along the guide rails  18  and  19 . This can provide the advantage that the opening, through which the optical corrective elements  13  are inserted into the interior of the objective housing  10 , can be kept small in comparison with the rotating disk solution described in  FIG. 4  and, in this manner, the risk of the introduction of dirt into the interior of the objective housing  10  can be effectively reduced. Moreover, the risk of contamination can be further reduced by providing a part  24 , through which the optical corrective elements  13  pass prior to and after replacement and through which, by way of example, a purge gas is continuously passed, as a result of which dirt possibly penetrating from the outside can be discharged before the optical corrective element  13  reaches the interior of the objective housing  10 . In this case, it is also feasible for the entire replacement device  17  and the magazine  23  to be arranged together in a space through which purge gas passes, so that, during the process of replacing the optical corrective element  13 , there is no contact with the surroundings and, purge gas already flows around the optical corrective elements  13  during their storage in the magazine  23  and the elements are thus protected to the greatest possible extent from contamination. 
         [0039]    In this case, the purge gas can advantageously be discharged from the projection objective in the region of the replacement device  17 ; in other words, the main purge outlet (not illustrated) of the projection objective  7  is located in the region of the replacement device  17 . By this measure, contamination of the interior of the projection objective  7  is avoided particularly effectively. 
         [0040]    In some embodiments, the magazine  23  can also be a stack magazine with optical corrective elements  13  or  14  stacked vertically one above the other. 
         [0041]    It is likewise possible to implement the replacement device  17  in such a manner that a rotating disk provided with a plurality of optical corrective elements  13  is located on a linear carriage and can be inserted completely into the interior of the objective housing  10  or also be removed therefrom. Solutions in which the optical corrective element  13  is replaced by a swinging arm or a double swinging arm are also conceivable. 
         [0042]    Of course it is possible to replacement both the optical corrective elements  13  closer to the pupil plane  12  and the optical corrective elements  14  further away from the pupil plane  12  by the above-described exemplary arrangements. 
         [0043]    A further advantageous implementation of the present disclosure is illustrated in  FIG. 7 . The optical corrective elements  13  or  14  are in this case arranged together on a holding frame  25 . In this case, for example, the first optical corrective element  13  can be permanently integrated in the holding frame in the vicinity of the pupil plane. The holding frame holds the second optical corrective element  14  by the holding elements  16  and the manipulators  15 . In this case, the holding frame  25  can be designed in such a manner that it can easily be replaced as a whole. Furthermore, on account of the described modular design, the second optical corrective element  14  in the holding frame  25  can be replaced without removing it from the objective housing  10 . By way of example, in combination with the magazine  23  in the form of a stack magazine, the second optical corrective element  14  can, in this manner, easily be replaced in a known manner (see the previous figures); furthermore this can provide the advantage that the two optical corrective elements  13  and  14  are arranged within one unit and therefore arranged particularly compactly. 
         [0044]      FIG. 8  shows a bearing concept for mounting an optical corrective element  13  or  14  in the interior of a projection objective  7 , the optical corrective element  13  or  14  being in the form of a fixed screen and being mounted only at two bearing points  27  via the supports  26  and  28  in the interior of the projection objective  7 , not illustrated in  FIG. 8 . In this case, the design of the bearing points  27  can be chosen in such a way that a bearing point made from a hard metal, a ruby prism or hardened steel is used. Here, the first support  26  is implemented as a fixed support; a loose support is used in the present example as a second support  28 , in which the line contact principle is used. 
         [0045]    The described variants and exemplary embodiments of the disclosure should not be considered in isolation; any combinations of the previously illustrated solutions are of course conceivable.