Abstract:
In some embodiments, the disclosure provides a system that includes an optical element group including a plurality of optical elements configured to project a pattern of an object in an object plane to an image plane. The system also includes a unit configured to detect an image selected from an image of at least part of the projection the pattern of the object, and an image of a measurement element arranged in the area of the object. The image is created via at least some of the optical elements in the optical element group. The unit is configured to determine an imaging error in the projection of the pattern of the object from the object plane to the image plane. The device is configured to be used in microlithography.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present patent application is a continuation under 35 U.S.C. 120 of international patent application serial number PCT/EP2006/070119, filed Dec. 21, 2006, which under 35 U.S.C. 119(e)(1) claims the benefit of U.S. Provisional Application No. 60/753,718, filed Dec. 23, 2005, and German Application No. 10 2005 062 618.1, filed Dec. 23, 2005. The present application incorporates by reference the entire contents of both international patent application serial number PCT/EP2006/070119 and U.S. Provisional Application No. 60/753,718. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an optical imaging device. The invention may be put to use in the context of microlithography which is used in the manufacture of microelectronic circuits. Accordingly, the invention also relates to a mask for use in this type of optical imaging device. Lastly, the invention relates to a method for determining an imaging error as well as to an imaging process that makes use of said method. 
     Particularly in the realm of microlithography, it is necessary among other things, and besides using components made with the highest precision possible, that the components of the imaging device, i.e. for example the optical elements such as lenses or mirrors, be positioned as precisely as possible in order to achieve a commensurately high image quality. The exacting quality requirements, which lie in the microscopic range in the order of magnitude of a few nanometers or less, are to a large extent a consequence of the continuing need to increase the resolution of the optical systems used in the manufacture of microelectronic circuits, in order to advance the miniaturization of the microelectronic circuits that are to be produced. 
     With the increased resolution which is normally accompanied by a shift to shorter wavelengths of the light being used, not only is a higher accuracy required in the positioning of the optical elements being used, but of course the requirements in regard to minimizing the imaging errors of the entire optical system are also increased. 
     In view of the short operating wavelengths in the UV range which are used in microlithography, for example operating wavelengths around 193 nm, but also in particular in the so-called extreme UV range (EUV) with operating wavelengths around 13 nm, it is often proposed, in order to satisfy the stringent requirements imposed on the positioning of the individual components, that the positions of individual components such as the mask table, the optical elements and the substrate table (for example a wafer table) be determined individually relative to a reference, for example a reference structure, which is often established in a so-called metrology frame, and that these components then be actively positioned relative to each other. 
     This solution has on the one hand the disadvantage that normally no real-time measurement of the position of the projection pattern of the mask on the substrate (in most cases a wafer) takes place, but that the relative positions of the components and the position of the image are only indirectly arrived at from the individual position data of the components relative to the reference. The respective measurement errors will add up cumulatively, possibly leading to a relatively high total measurement error. Furthermore, this involves a large number of elements that have to be actively positioned, all of which have to be set and controlled in their positions with the required angular accuracy in the nanorad (nrad) range and a translatory position accuracy in the picometer (pm) range. This further entails particularly severe requirements on the thermal stability of the reference and the supporting structure for the optical elements. Normally, only a few dozen nanometers per degree Kelvin (nm/K) can be tolerated here for the thermal expansion. 
     On the other hand, there are further a number of solutions known, where the position of the image of the projection pattern of the mask on the substrate is determined in real time. The position of the image of the projection pattern on the substrate can be corrected here with significantly fewer active elements, possibly even with only one active element. This simplifies not only the dynamic control of the rest of the components, the requirements to be imposed on the thermal stability of the reference and the supporting structure for the optical elements are also markedly less severe. 
     The real-time determination of the position of the image of the projection pattern of the mask on the substrate is often performed according to the so-called laser pointer principle. This involves directing a collimated laser beam along a path starting from a light source arranged in the area of the mask and proceeding near the path of the image-producing light through the optical elements participating in the formation of the image all the way to the area of the substrate, and to capture the laser beam with a detector in that place. Even the smallest deviation of the optical elements from their correct position will now cause a deflection of the laser beam from its target position which is registered through the detector and used to make a correction. A method of this kind has been disclosed for example in US 2003/0234993 A1 (Hazelton et al.). 
     With this arrangement it is possible, due to the fact that the laser beam is directed through the optical elements participating in the formation of the image, to determine not only possible deviations from the correct position of the image of the projection pattern of the mask on the substrate, but to also register other errors in the image (for example distortions, etc.). All of these position errors and other errors are herein collectively referred to as imaging errors. 
     The foregoing solution again has the disadvantage that the positioning of the light source and the detector are subject to correspondingly stringent requirements in regard to angular accuracy in the nanorad range (nrad) and translatory accuracy in the picometer range (pm) as well as in regard to thermal stability. Most of all, the light source normally has to be supported by an elaborate thermally stable supporting structure which is normally arranged far from the reference (for example in a so-called metrology frame). In addition, expensive measures are again necessary to determine the positions of these components relative to a reference, with the result also entering into the calculation of the correction. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the present invention to provide an optical imaging device, a mask for the latter, a method for the determination of imaging errors, and an imaging process, in which the aforementioned disadvantages are absent or at least exist only to a smaller extent, and whereby in particular imaging errors can be corrected and, if applicable, a correction of imaging errors can be determined in the most direct way possible, in a simple manner, and in real time. 
     The present invention has the further objective to provide an optical imaging device, a mask for the latter, a method for the determination of imaging errors, and an imaging process, whereby imaging errors can be determined in the most direct way possible and, if applicable, can be corrected with a small number of elements. 
     The present invention is based on the observation that such a direct determination and, if applicable, a correction of imaging errors can be carried out in a simple way if a determination of the position of the mask is made from a location near the substrate through at least a large part of the first optical element which participate in the projection of the image of the projection pattern of the mask on the substrate, or through second optical elements which are functionally connected to the first optical elements. To accomplish this according to the invention, a measurement image is generated of a measurement element that is located in the area of the mask or in the area of the projection pattern by projecting said measurement element through at least a major part of the first optical elements which participate in the formation of the image of the projection pattern of the mask on the substrate, or through second optical elements which are functionally connected to the first optical elements, and by evaluating said measurement image in order to determine the imaging errors. 
     Using a measurement element has the advantage that it can be brought into an appropriately defined and stable position relative to the mask or its projection pattern in a straightforward way which requires only a small effort and expense, unlike the laser pointer methods of the prior art, where the active light source has to be appropriately defined and stably positioned. With the inventive concept, it is possible in particular to place a passive measurement element in a simple manner directly at the mask or in the immediate proximity of the projection pattern. However, in addition or as an alternative to using such a measurement element, it is also possible to use at least a part of the projection pattern for the measurement image. 
     This possibility of capturing the measurement image, and thus among other things the mask position in the immediate proximity of the substrate has a number of advantages. For example, any errors (position errors, dynamic errors, thermal errors, etc.) which originate from the optical elements, the mask device, the substrate device, or a reference, are removed. The measurement image which is captured in this way in most cases represents with good approximation the actual image of the projection pattern on the substrate, so that an appropriate correction can be made on this basis. If necessary, the difference between the measurement image capturing position and the substrate position can be determined and taken into account in a simple manner through a reference measurement of the measurement image capturing position in relation to the substrate position. Furthermore and besides, it is also possible with this arrangement to correct any enlargement error of the optical element group as well as any scale error of further reference measurement systems automatically, possibly in a single step. 
     A correction of the imaging errors that have been determined in this manner can be made simply by way of at least one of the components that participate in the projection. Thus, the correction can possibly be effected by using only one of the actively controllable components that participate in the projection of the image. Preferably, this is a component which is controllable within a large bandwidth. It is also possible here that the actively controllable component is constituted by the substrate device itself. However, one of the optical elements can function as the actively controllable component. In some cases it is also possible to envision an appropriate automatic regulation of the correction process which adapts the correction of the imaging errors to changing boundary conditions. 
     The present invention therefore has as one of its subjects an optical imaging device, in particular for use in microlithography, with a mask device serving to receive a mask containing a projection pattern, with an optical element group, a substrate device serving to receive a substrate and a measuring device. The optical element group includes a plurality of optical elements and is designed to project an image of the projection pattern onto the substrate. The measuring device is designed to determine at least one imaging error which occurs in the projection of the projection pattern on the substrate. The measuring device further includes a detecting device, wherein the detecting device serves to capture or detect a measurement image of at least a part of the projection pattern and/or of at least one measurement element that is located in the area of the mask device, which measurement image is generated through at least a major part of the optical elements of the optical element group, and wherein the measuring device is designed for the determination of the imaging error through the use of the measurement image. 
     A further subject of the present invention is a mask for an optical imaging device according to the invention, which mask includes the projection pattern and the measurement element. 
     A further subject addressed by the invention is an imaging method, in particular for microlithography, wherein a projection pattern is projected onto a substrate by means of the optical elements of an optical element group, and wherein the imaging method includes the determination of at least one imaging error which occurs in producing the image of the projection pattern on the substrate. A measurement image is registered which is generated through at least a major part of the optical elements of the optical element group by projecting at least a part of the projection pattern and/or at least one measurement element which is arranged in the area of the projection pattern. The imaging errors are then determined through the use of the measurement image. 
     Finally, a further subject addressed by the present invention is a method for the determination of an imaging error which occurs in the projection of an image of a projection pattern onto a substrate by means of the optical elements of an optical element group, in particular for use in microlithography. A measurement image is captured which is generated through at least a major part of the optical elements of the optical element group by projecting at least a part of the projection pattern and/or at least one measurement element which is arranged in the area of the projection pattern. The imaging errors are again determined through the use of the measurement image. 
     Further preferred embodiments of the invention are presented in the subordinate claims and in the following description of examples of preferred embodiments wherein the attach drawings are being referred to. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a schematic illustration of a preferred embodiment of the optical imaging device according to the invention with a preferred embodiment of the mask according to the invention; 
         FIG. 2  represents a schematic view of the mask of  FIG. 1 ; 
         FIG. 3  represents a flowchart of a preferred embodiment of the imaging method according to the invention, using the imaging device of  FIG. 1 , wherein a preferred embodiment of the method according to the invention is used to detect imaging errors; and 
         FIG. 4  represents a flowchart of a further preferred embodiment of the imaging method according to the invention, wherein a further preferred embodiment of the method according to the invention is used to detect imaging errors. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, a preferred embodiment of the optical imaging device for microlithography applications is described, making reference to  FIGS. 1 to 3 . 
       FIG. 1  shows a schematic representation of a preferred embodiment of the optical imaging device according to the invention in the form of a microlithography device  101  which operates with light of a first wavelength in the extreme UV range (EUV). The microlithography device  101  includes an optical projection system  102  with an illumination system  103 , a mask  104 , and an optical device in the form of an objective  105  with an optical axis  105 . 1 . The illumination system  103  illuminates by way of a suitable light-conducting system (not shown in the drawing) the reflective mask  104 . 1  of a mask device  104  with a projection light bundle (not shown in the drawing). On the mask  104 . 1  which is arranged on a mask table  104 . 2 , there is a projection pattern  104 . 3  which is projected with the projection light bundle by way of the optical elements in the objective  105  onto a substrate  106 . 1 , for example a wafer on a substrate device  106 . 
     The objective  105  includes an optical element group  105 . 2  which is constituted by a number of first optical elements  107 ,  108 ,  109 ,  110 ,  111 ,  112 , which are supported in the housing  105 . 3  of the objective  105 . As the operating wavelength is in the EUV range (approx. 13 nm), the optical elements  107 ,  108 ,  109 ,  110 ,  111 ,  112  are reflective optical elements, i.e. mirrors or similar elements. 
     There is further a measuring device  113 , which includes a detection unit in the form of an encoder  113 . 1  attached to the housing  105 . 3  of the objective  105 , several measurement elements  113 . 2  arranged in the area of the mask device  104 , and a guiding device  113 . 3  attached to the housing  105 . 3  of the objective  105 . The encoder  113 . 1  registers a measurement image of the measurement elements  113 . 2  which, as indicated in  FIG. 1  by the ray path  113 . 4  of a measurement light bundle, is directed through all optical elements  107 ,  108 ,  109 ,  110 ,  111 ,  112  and the guiding device  113 . 3  to the encoder  113 . 1 . The detection occurs here for a second wavelength of the measurement light bundle which is different from the first wavelength of the projection light bundle which serves to project the projection pattern  104 . 3  onto the wafer, wherein said second wavelength is optimized for the maximum sensitivity of the encoder  113 . 1 . However, it should be understood that in other variants of the invention, the detection of the measurement image can also be performed with the wavelength used for projecting the projection pattern onto the wafer. In particular, the measurement light bundle can in this case be branched off from the projection light bundle through suitable means. 
     The guiding device  113 . 3  in the present example is a direction-changing mirror which is attached to the housing  105 . 3  of the objective  105  in the vicinity of the wafer  106 . 1 . This arrangement allows the encoder  113 . 1  to be arranged in the housing  105 . 3  of the objective  105  in a location that is advantageous in regard to the installation of the encoder, its accessibility and/or the design freedom of other components. It should be understood however, that other variants of the invention can also have a differently configured guiding device. In particular, it is also possible that the substrate, if it is given the required reflective properties, would function at least as a part of the guiding device, as indicated in  FIG. 1  by the dotted ray path  113 . 5 . In this case, it may be possible to omit a guiding device which would create an obscuration for the projection of the projection pattern on the wafer. 
     The measuring device  113  further includes a position-determining device in the form of a reference-measuring device  113 . 6  which is arranged on the housing  105 . 3  of the objective  105  in the immediate proximity of the encoder  113 . 1 . This reference-measuring device  113 . 6  serves in a conventional manner to determine the relative position of the wafer  106 . 1  in reference to the encoder  113 . 1 . 
     In other words, the housing  105 . 3  of the objective  105  thus represents a reference element, in relation to which the aforementioned measurements are performed. However, it should be understood, that in other variants of the invention other components of the imaging device could serve as reference element. For example, the supporting structure  114  for the housing  105 . 3  of the objective  105 , which is often referred to as metrology frame, can function as reference element. Likewise, one of the optical elements  107 ,  108 ,  109 ,  110 ,  111 ,  112  can also constitute the reference element. Particularly suitable for this purpose are heavy mirrors or similar elements which are arranged in the proximity of the substrate. Finally, the substrate device itself, for example the substrate table, can constitute the reference element, for example by arranging the encoder on the substrate table. 
     It should further be understood that the measurement image of the measurement elements in other variants of the invention could also be directed through only a major part of the optical elements of the element group, so that a few of the optical elements, for example at the end of the light path, could be bypassed. This is possible especially in a case where the bypassed optical element is sufficiently stable in regard to its position and thermal properties and/or if this optical element itself constitutes the reference element. 
     From the measurement image and from the position of the wafer  106 . 1  in relation to the encoder  113 . 1  as determined with the reference-measuring device  113 . 6 , it is possible to determine in real time the one or more imaging errors in the projection of the projection pattern  104 . 3  onto the wafer  106 . 1 . Thus, based on an observed deviation of the measurement image in the encoder  113 . 1  from a target condition, one can draw conclusions in regard to a deviation of the image of the projection pattern  104 . 3  on the wafer  106 . 1  from the target position and/or from the target geometry. 
     Thus, based on position deviations that are found by way of the measurement image, it is possible to draw conclusions about the positions of the components which participate in the projection of the projection pattern  104 . 3  onto the wafer  106 . 1 , while deviations of the geometry which are found through the measurement image allow conclusions to be drawn about further imaging errors in the projection of the projection pattern  104 . 3  onto the wafer  106 . 1 . 
     As can be seen in  FIG. 2 , the measurement elements  113 . 2  in the present example are two measurement patterns in the form of two-dimensional grids which are arranged on the mask  104 . 1  immediately beside the projection pattern  104 . 3  and which extend along the scanning direction  104 . 4  of the mask device  104 . The two-dimensional grids  113 . 2  can have been put on the mask  104 . 1  for example through a suitable simple photographic exposure process or the like. This can possibly be accomplished at the same time and/or with the same process by which the projection pattern  104 . 3  is produced. 
     With such two-dimensional grids  113 . 2 , it is particularly simple to register the position deviations described above, because with the grid geometry being known, the encoder  113 . 1  (possibly after an appropriate calibration) only needs to register and count the pulses resulting from the displacement of the measurement image of the grid  113 . 2  in order to establish the position deviation. 
     Furthermore, the encoder  113 . 1  can also register deviations in the geometry of the measurement image of the grid  113 . 2 , for example a distortion of the grid  113 . 2 , and draw conclusions from those deviations in regard to further imaging errors in the projection of the projection pattern  104 . 3  onto the wafer  106 . 1 . 
     It should be understood in this context that it would also be possible in other variants of the invention to use at least two one-dimensional grids of different orientation instead of the two-dimensional grid. As should further be understood, it would also be possible in other variants of the invention to use instead of the grids  113 . 2  any other measurement elements and/or instead of the encoders  113 . 1  any other detection units which allow the detection of the position deviation and/or of further imaging errors based on the measurement image. 
     It should further be understood that the one or more measurement elements in other variants of the invention need not be arranged directly at the mask. Rather, it is for example also possible to arrange the one or more measurement elements separately from the mask device in the proximity of the mask or of the projection pattern, if a defined spatial relationship to the mask or to the projection pattern is assured. 
     The use of passive measurement elements  113 . 2  arranged directly at the mask  104 . 1  has the advantage that without special measures, they have a defined and stable position relative to the mask  104 . 1  and to the projection pattern  104 . 3  of the mask. Unlike the laser pointer methods known from the prior art where the active light source has to be appropriately defined and held in a stable position, the measures required for this purpose under the inventive concept are only of a minor scope. 
     The concept of capturing the measurement image and thus among other things the position of the mask  104 . 1  directly at a location in the immediate proximity of the wafer  106 . 1  has a number of advantages. For example, any measurement errors (position errors, dynamic errors, thermal errors, etc.) which originate from the optical elements  107 ,  108 ,  109 ,  110 ,  111 ,  112 , from the mask device  104 , the substrate device  106  and the supporting structure  114  are removed. The measurement image which is registered in this way in most cases represents with good approximation the actual image of the projection pattern  104 . 3  on the wafer  106 . 1 , so that an appropriate correction can be made on this basis, as will be explained in detail in the following. The deviation of the measurement image detecting position, i.e. the position of the encoder  113 . 1 , from the substrate position, i.e. the position of the wafer  106 . 1 , is determined and taken into account through the reference measurement performed by the reference-measuring device  113 . 5 . Furthermore and besides, it is also possible with this arrangement to correct any enlargement error of the optical element group as well as any scale error of further reference measurement systems automatically, possibly in a single step. 
     The correction of the imaging errors determined by the encoder  113 . 1  is effected by way of a correction device in the form of a control device  115  that is connected to the measuring device  113 . The control device  115  as a central control device is connected to the active components that participate in the projection of the projection pattern  104 . 3  onto the wafer  106 . 1 , as illustrated in  FIG. 1  by the control lines  115 . 1 , which are in part only symbolically indicated. Of course, the connections to the active components do not necessarily have to be permanent hard-wired connections. Rather, these communications can also at least in some sections be wireless connections which may exist only temporarily. 
     The control device  115  processes the imaging errors determined by the encoder  113 . 1  and, based on said imaging errors, calculates commands for the actuator elements of one of the active components participating in the projection of the projection pattern  104 . 3  onto the wafer  106 . 1 . This active component is in the present example the last optical element  112  in the light path of the projection of the projection pattern  104 . 3  onto the wafer  106 . 1 . 
     However, it should be clear that in other variants of the invention, one or more other components that participate in the projection of the projection pattern  104 . 3  onto the wafer  106 . 1  can be configured as active components and can be used for the correction of the imaging errors that were determined. Preferably such a component is controllable within a large bandwidth. It is also possible that the controllable component is constituted by the substrate device itself. 
     It should further be understood that the control commands for the actuator elements of the active components that participate in the projection of the projection patterns  104 . 3  onto the wafer  106 . 1  can also be determined in other ways. For example, these commands can be taken directly from pertinent look-up tables and/or other models that are stored in memory. 
       FIG. 3  shows a flowchart diagram of a preferred embodiment of the imaging method according to the invention, consisting of a microlithography method which is performed with the microlithography device  101  of  FIGS. 1 and 2 , which operates according to the so-called scanner principle and wherein a preferred embodiment of the method for determining the imaging error is being used. 
     First, the process sequence of the microlithography method is started in a step  116 . 1 . Next, the microlithography device  101  of  FIG. 1  is made available for the process in a step  116 . 2 . 
     In an imaging phase  116 . 3 , a determination of the imaging error occurs in a step  116 . 4  in parallel with the exposure of the wafer  106 . 1 . As described above in the context of  FIGS. 1 and 2 , the measurement image of the two-dimensional grid  113 . 2  is measured by the encoder  113 . 1  and the reference measurement is performed by the reference-measuring device  113 . 6 , and the results are processed by a processing unit of the measuring device  113  which is for example integrated in the encoder  113 . 1 . 
     Dependent on the imaging errors in the projection of the projection pattern  104 . 3  onto the wafer  106 . 1  which are determined in the foregoing step  116 . 4  the correction of the imaging error as described above in the context of  FIGS. 1 and 2  is performed in step  116 . 5  by the control device  115  by sending appropriate commands to the actuating elements of the optical element  112 . 
     As mentioned, the determination and correction of the imaging error is performed in parallel with the exposure of the wafer  106 . 1 . At least as long as no imaging errors are found which would make it necessary to stop the exposure of the wafer  106 . 1 , the exposure occurs thus simultaneously with, and independently of, the determination and correction of the imaging error. 
     A further step  116 . 6  consists of a test as to whether a further correction cycle needs to be performed. If this is not the case, the process sequence is terminated in step  116 . 7 . Otherwise, the process loops back to step  116 . 4 . 
       FIG. 4  shows a flowchart diagram of a further preferred embodiment of the imaging method according to the invention, consisting of a microlithography process, which is performed with the microlithography device of  FIGS. 1 and 2  which in this case operates according to the so-called stepper principle and wherein a preferred embodiment of the method for the determination of the imaging error is being used. 
     First, the process sequence of the microlithography method is started in a step  216 . 1 . Next, the microlithography device  101  of  FIG. 1  is made available for the process in a step  216 . 2 . 
     In an imaging phase  216 . 3 , a determination of the imaging error occurs in a step  216 . 4 . As described above in the context of  FIGS. 1 and 2 , the measurement image of the two-dimensional grid  113 . 2  is measured by the encoder  113 . 1  and the reference measurement is performed by the reference-measuring device  113 . 6 , and the results are processed by a processing unit of the measuring device  113  which is for example integrated in the encoder  113 . 1 . 
     Dependent on the imaging errors in the projection of the projection pattern  104 . 3  onto the wafer  106 . 1  which are determined in the foregoing step  216 . 4 , the correction of the imaging error as described above in the context of  FIGS. 1 and 2  is performed in step  216 . 5  by the control device  115  by sending appropriate commands to the actuating elements of the optical element  112 . 
     In a further step  216 . 6  the wafer  106 . 1  is exposed. The next-following step  216 . 7  consists of a test as to whether a further exposure cycle needs to be performed. If this is not the case, the process sequence is terminated in step  216 . 8 . Otherwise, the process loops back to repeat the phase  216 . 3 . 
     In further variants of the invention, the measurement image is not—or not exclusively—produced by way of the first optical elements  107 ,  108 ,  109 ,  110 ,  111 ,  112 , but at least in part through a group of second optical elements, one or more of which are operatively connected to one of the first optical elements  107 ,  108 ,  109 ,  110 ,  111 ,  112 . Such a second optical element can for this purpose be, e.g., rigidly connected to the respective first optical element  107 ,  108 ,  109 ,  110 ,  111 ,  112 , so that a correlation is known or can be sufficiently well determined or estimated between the status changes (for example position changes) of the respective first optical element  107 ,  108 ,  109 ,  110 ,  111 ,  112  and the second optical element that is connected to it. 
     The second optical elements can be constituted—possibly depending on the wavelength of the measurement light bundle—by refractive, reflective, or diffractive optical elements used either exclusively or in any combination. 
     It is further considered to be clear that the concept of a measurement image is not limited to a projection of separate measurement elements. Rather, it can also be envisioned that the measurement image consists either entirely or in part of an image generated by means of the measurement light bundle of at least a part of the projection pattern  104 . 3 . The projection pattern  104 . 3  normally also has a sufficiently known and measurable structure, so that from its image which is projected by means of the measurement light bundle the imaging error can be determined with sufficient accuracy. 
     The present invention has been described hereinabove through examples in which the optical element group consists exclusively of reflective optical elements. However, it should be noted at this point that the invention can of course also find application for optical element groups which alone or in any combination include refractive, reflective or diffractive optical elements, in particular in a case of a projection with different first wavelengths. 
     It further needs to be noted that the present invention has been described hereinabove through an example taken from the realm of microlithography. It should, however, be understood that the present invention can likewise be used for any other applications or imaging processes.