Abstract:
An optical imaging device, in particular for microlithography, including an imaging unit adapted to image an object point on an image point and a measurement device. The imaging unit has a first optical element group having at least one first optical element. The imaging device is adapted to participate in the imaging of the object point on the image point, and the measurement unit is adapted to determine at least one image defect occurring on the image point when the object point is imaged. The measuring device includes at least one measurement light source, one second optical element group and at least one detection unit. The measurement light source transmits at least one measurement light bundle. The second optical element group includes at least one optical reference element and one second optical element, the elements adapted to direct the at least one measurement light bundle to the at least one detection unit, to produce at least one detection signal. The second optical element has a defined spatial relationship with the first optical element. The optical reference element has an at least partially reflecting first optical surface and the second optical element has an at least partially reflecting second optical surface. The measurement device is adapted to determine the at least one image defect using the at least one detection signal. The first optical surface and the second optical surface are positioned relative to one another such that a multiple reflection of the at least one measurement light bundle occurs between them.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation of International Application PCT/EP2009/058224, with an international filing date of Jun. 30, 2009, which was published under PCT Article 21(2) in German, and the complete disclosure of which, including amendments, is incorporated into the present application by reference. The present application claims the priority of German Patent Application 10 2008 030 664.9 and of U.S. Provisional Patent Application 61/133,616, both filed on Jul. 1, 2008, and the contents of both of which are also incorporated into the present application by reference. 
    
    
     FIELD OF AND BACKGROUND OF THE INVENTION 
     The present invention relates to optical imaging devices. The invention can be used in connection with microlithography, e.g. as employed for the production of microelectronic circuits. It therefore furthermore relates to an imaging method. 
     Particularly in the field of microlithography, besides using components made with the greatest possible precision, it is necessary inter alia to position the components of the imaging device, for example the optical elements such as lenses or mirrors, as exactly as possible in order to achieve a correspondingly high image quality. The high accuracy requirements, which are in the microscopic range of the order of a few nanometers or less, are not least a consequence of the constant need to increase the resolution of the optical systems used for the production of microelectronic circuits, in order to push forward miniaturisation of the microelectronic circuits to be produced. 
     With increased resolution, and the generally concomitant reduction of the wavelength of the light being used, not only do the requirements for the positioning accuracy of the optical elements used increase. Also, the requirements with respect to minimising the imaging errors of the overall optical arrangement increase. 
     In order to comply with the stringent requirements for positioning the components involved with the short wavelengths in the ultraviolet (UV) range used in microlithography, for example with wavelengths in the region of 193 nm, but also particularly in the so-called extreme UV range (EUV) with working wavelengths in the range of from 5 nm to 20 nm (usually in the region of 13 nm), it is often proposed to capture the positions of the individual components such as the mask stage, the optical elements and the substrate stage (for example a wafer stage), respectively and individually with respect to a reference (for example a reference structure, which is often formed by a so-called metrology frame) and then to position these components actively with respect to one another. Such a procedure is known for example from U.S. Pat. No. 7,221,460 B2 (Ohtsuka), the entire disclosure of which is incorporated herein by reference. 
     This solution has on the one hand the disadvantage that, generally, no real time measurement of the position of the image of the projection pattern of the mask on the substrate (usually a wafer) is carried out, but instead the relative position of the components and the position of the image are merely deduced indirectly from the individual position data of the components. In this case, the respective measurement errors add up, so that a comparatively high overall measurement error can sometimes occur. Furthermore, this entails a large number of elements to be positioned accurately, all of which have to be positioned and measured with respect to their position with the corresponding angular accuracy in the nanorad range (nrad) or less and a translation accuracy in the picometer range (pm). This also leads to particularly stringent requirements for the thermal stability of the reference and the support structure for the optical elements. Here, only a few dozens of nanometers per kelvin (nm/K) are generally permissible in respect of the thermal expansion. 
     On the other hand, a range of solutions is also known in which the quality of the imaging of the object points of an object plane (projection pattern of the mask) onto the image points in an image plane (in which the substrate is to be arranged), particularly the position of the image of the projection pattern in the image plane, is determined in real time. In this case, the imaging quality, particularly the position of the image of the projection pattern on the image plane, can in principle be corrected with far fewer active elements, and sometimes even with just one active element. Not only does this simplify the dynamic driving of the other components, but also much less stringent requirements need to be placed on the thermal stability of the reference and the support structure for the optical elements. 
     For example, it is known to determine the aberrations of the projection beam path directly, i.e. with wavefront sensors arranged in the image plane at the position of the substrate. To this end, however, the exposure operation of the substrate needs to be interrupted. This is a practicable solution for projection devices which are sufficiently stable as a function of time, that is to say the monitoring and correction of imaging errors can be carried out at sufficiently large time intervals (from a few hours to days or even weeks). Measurement and correction at shorter time intervals, as would be necessary for perturbations acting within a short time (for example thermal perturbations), is either not possible since the measurement would in principle take too long, or is undesirable since the exposure process ought not to be interrupted or the effects on the productivity of the imaging device are unacceptable. 
     Real time determination of the position of the image of the projection pattern of the mask on the substrate is often carried out according to the so-called laser pointing principle. In this case, a measurement light bundle in the form of a collimated light beam is guided from a light source arranged in the region of the mask in the vicinity of the path of the useful light (i.e. at the image field edge) via the optical elements involved in the imaging as far as into the region of the substrate, and captured there by a detector. Even very small deviations of the optical elements from their setpoint positions then generate a deviation of the light beam from its setpoint position, which is captured by the detector and used for correction. Such a method is known for example from US 2003/0234993 A1 (Hazelton et al.), the entire disclosure of which is also incorporated herein by reference. 
     Here, owing to the guiding of the laser beam via the optical elements involved in the imaging, it is sometimes not only possible to determine deviations with respect to the correct position of the image of the projection pattern of the mask on the substrate, but moreover other errors (for example distortions etc.) in the imaging can be captured. All these position errors and other errors are combined under the term imaging error in the present description. 
     These variants for determination of the imaging error, however, often require intervention in the projection beam path (for example by introducing beam splitters) and sometimes entail an undesired loss of radiation power etc. 
     As already mentioned, the problems explained above in relation to the angular accuracy and the translation accuracy for the positioning and orientation of the components involved in the imaging have particularly great repercussions in imaging devices which operate in the EUV range (for example in the 13.5 nm wavelength range), as is known for example from U.S. Pat. No. 7,226,177 B2 (Sasaki et al.), the entire disclosure of which is also incorporated herein by reference. 
     These objectives currently operate exclusively with reflective optical elements (that is to say mirrors or the like), which guide the EUV light by reflection, with, mostly, four, six or eight mirrors mostly being used. During operation of the imaging device, these mirrors generally change their position and/or orientation (owing to mechanical and/or thermal perturbations), since the support structure cannot be absolutely stable (mechanically and/or thermally). 
     The beam path in such an EUV objective may be a plurality of meters long. In the systems known from U.S. Pat. No. 7,226,177 B2 (Sasaki et al.), for instance, the individual spacings of the optical surfaces lead to an overall beam path length of about 2.5 m from the first mirror (M 1 ) to the plane of the substrate to be exposed. In relation to this, a permissible image displacement with structures to be exposed in the region of about 25 nm must also lie only in the nm range. An admissible image displacement of 1 nm in this case entails a maximum permissible mirror tilt of only 0.2 nrad for the first mirror M 1 . For the subsequent mirrors (M 2  to M 6 ) in the beam path, these angular tolerances increase stepwise since their distances from the plane of the substrate become smaller and smaller. 
     A correspondingly precise measurement (and subsequent correction) of the alignment of such mirrors, however, is not possible with the previously known devices. There are furthermore combinations of mirror tilts whose effects in relation to the overall image displacement cancel out, and therefore do not cause any image displacement. Such mirror tilt combinations nevertheless lead to deviations of the wavefront, that is to say aberrations, which cannot, however, be identified with the previous methods. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical imaging device and/or an imaging method which do not have the disadvantages mentioned above, or at least have them to a lesser extent, and particularly which straightforwardly allow maximally direct determination and, if appropriate, correction of imaging errors in real time. 
     It is furthermore an object of the invention to provide an optical imaging device and/or an imaging method which allow maximally direct determination and, if appropriate, correction of imaging errors with few elements. 
     It is furthermore an object of the invention to provide an optical imaging device and/or an imaging method which allow maximally precise determination and, if appropriate, correction of imaging errors due to a change in the orientation of the optical elements. 
     The present invention is based on the cognition that such a direct determination and if appropriate correction of imaging errors of an imaging unit is readily possible in real time in parallel with the actual imaging process (for example the exposure of a substrate or the like) if the determination of the imaging error is carried out by means of a separate measuring device having measurement light source, capturing unit and separate measurement optics, the optical elements of which guide a measurement light bundle from the measurement light source onto the capturing unit and are respectively in a relationship, defined at all times, with one of the optical elements of the imaging unit. The relationship, defined at all times, between the respective optical elements of the measurement optics and one of the optical elements of the imaging unit ensures that a state change (change in position and/or orientation) of one of the optical elements of the imaging unit leads to a defined change in the aberrations of the measurement optics and therefore in the capturing signal of the capturing unit, which can in turn be assigned to a corresponding imaging error of the imaging unit. 
     The optical elements of the measurement optics, which are at least in part arranged separately, make it possible to configure the measurement optics so that only (if at all) a small impairment of the imaging actually carried out by the imaging unit takes place owing to the measurement carried out in parallel with the actual imaging. 
     Unlike in the previously known solutions, the measurement optics can furthermore be configured advantageously so that the changes in the capturing signal can be assigned as unequivocally as possible to the individual state changes of the optical elements of the imaging unit. The sensitivity of the measurement optics in all degrees of freedom is known beforehand, for example having been calculated and/or determined experimentally (for example a change in the Z7/Z8 coma centring and the Z5/Z6 astigmatism centring corresponds to the ratio 4:1 for a translation of a lens etc.). 
     In the event of poor conditioning of individual measurement optical elements (for example low sensitivity, uniqueness, separability or orthogonality of the equation system of the individual sensitivities etc.) it is readily possible to improve the conditioning by adding extra measuring systems. This may, for example, be done by linking an extra measuring system of the same type to a different location on the imaging unit (for example rotated through 90° or 180° with respect to an optical axis of the imaging unit). Likewise, controlled optimisation or adaptation of the conditioning of two or more different measurement optical elements may be carried out. The measurement optics may (individually or in any desired combination) consist of refractive, diffractive and reflective optical elements of any configuration (spherical, aspherical, cylindrical, plane optical surfaces or freeform surfaces). 
     The present invention therefore relates to an optical imaging device, in particular for microlithography, having an imaging unit for imaging an object point on an image point and a measuring device, wherein the imaging unit comprises a first optical element group having at least one first optical element, which is adapted to participate in the imaging of the object point on the image point, and the measuring device is adapted to determine at least one imaging error which occurs when imaging the object point on the image point. The measuring device comprises at least one measurement light source, a second optical element group and at least one capturing unit, the measurement light source emitting at least one measurement light bundle. The second optical element group comprises a plurality of second optical elements, which are adapted to guide the at least one measurement light bundle onto the at least one capturing unit in order to generate at least one capturing signal. Each second optical element is in a defined spatial relationship with one of the first optical elements, at least one second optical element being different from the first optical elements. The measuring device is adapted to determine the at least one imaging error by using the at least one capturing signal. 
     The present invention also relates to an imaging method, in particular for microlithography, in which an object point is imaged on an image point by an imaging unit, the imaging unit comprising a first optical element group having a plurality of first optical elements which participate in the imaging of the object point on the image point, and a measuring device determining at least one imaging error which occurs when imaging the object point on the image point. The measuring device comprises at least one measurement light source, a second element group and at least one capturing unit, the measurement light source emitting at least one measurement light bundle. The at least one measurement light bundle is guided by means of a plurality of second optical elements of the second optical element group onto the at least one capturing unit in order to generate at least one capturing signal. Each second optical element is in a defined spatial relationship with the first optical element, at least one second optical element being different from the first optical elements. The measuring device determines the at least one imaging error by using the at least one capturing signal. 
     The present invention is furthermore based on the cognition that precise determination and if appropriate correction of imaging errors of an imaging unit caused by a change in the orientation (tilting) of optical elements of the imaging unit is readily possible in real time in parallel with the actual imaging process (for example the exposure of a substrate or the like) if the determination of the imaging error is carried out by means of a separate measuring device having measurement light source, capturing unit and measurement optics, the optical elements of which guide a measurement light bundle from the measurement light source onto the capturing unit. One of the optical elements of the measurement optics has at all times a defined relationship with one of the optical elements of the imaging unit, and is assigned to a reference element of the measurement optics so that multiple reflection of the measurement light bundle takes place between the reference element and the assigned element of the measurement optics. 
     The effect achieved by this direct multiple reflection of the measurement light bundle (and thus multiple passage through the optical cavity formed by the reference element and the optical element of the measurement optics) between these two optical elements is that an angular deviation or tilts between these two elements of the measurement optics is incorporated into the measurement wavefront while being multiplied by a factor depending on the number of reflections. Finally, the multiple reflection thus achieves optical amplification which results in an increase of the angular resolution of the measuring device. With a correspondingly large number of reflections in the optical cavity, it is readily possible to achieve angular resolutions in the sub-nrad range. 
     With a usual interferometric measurement, for example, reliable detection of wavefront angular tilts of λ/1000 over a predetermined measurement aperture is possible. If this measurement aperture has a diameter of 100 mm, for example, then this corresponds to an angular resolution of 6 nrad when the measurement wavelength λ is the wavelength of a conventional helium-neon laser. As already explained in the introduction, however, the requirements for the angular resolution of the measuring device are at least a factor of 10 greater. Yet with the invention, this problem can be resolved simply by a correspondingly large number of reflections inside the optical cavity. 
     The relationship, defined at all times, between the optical element of the measurement optics and one of the optical elements of the imaging unit furthermore ensures that a state change (change in position and/or orientation) of one of the optical elements of the imaging unit leads to a defined change in the aberrations of the measurement optics and therefore in the capturing signal of the capturing unit, which can in turn be assigned to a corresponding imaging error of the imaging unit. 
     The present invention therefore also relates to an optical imaging device, in particular for microlithography, having an imaging unit for imaging an object point on an image point and a measuring device, wherein the imaging unit comprises a first optical element group having at least one first optical element, which is adapted to participate in the imaging of the object point on the image point, and the measuring device is adapted to determine at least one imaging error which occurs when imaging the object point on the image point. The measuring device comprises at least one measurement light source, a second optical element group and at least one capturing unit, the measurement light source emitting at least one measurement light bundle. The second optical element group comprises at least one optical reference element and one second optical element, which are adapted to guide the at least one measurement light bundle onto the at least one capturing unit in order to generate at least one capturing signal. The second optical element has a defined spatial relationship with the first optical element. The optical reference element has an at least partially reflective first optical surface, while the second optical element has an at least partially reflective second optical surface. The measuring device is adapted to determine the at least one imaging error by using the at least one capturing signal. The first optical surface and the second optical surface are assigned to one another so that multiple reflection of the at least one measurement light bundle takes place between them. 
     The present invention furthermore relates to an imaging method, in particular for microlithography, in which an object point is imaged on an image point by an imaging unit, a first optical element group of the imaging unit having at least one first optical element participating in the imaging of the object point on the image point, and a measuring device determining at least one imaging error which occurs when imaging the object point on the image point. The measuring device comprises at least one measurement light source, a second element group and at least one capturing unit, the measurement light source emitting at least one measurement light bundle. In order to generate at least one capturing signal, at least one optical reference element and a second optical element of the second optical element group guide the at least one measurement light bundle onto the at least one capturing unit, the second optical element having a defined spatial relationship with the first optical element. The optical reference element has an at least partially reflective first optical surface, while the second optical element has an at least partially reflective second optical surface. The measuring device determines the at least one imaging error by using the at least one capturing signal. The first optical surface and the second optical surface are assigned to one another so that multiple reflection of the at least one measurement light bundle takes place between them. 
     The present invention furthermore relates to an optical imaging device, in particular for microlithography, having an imaging unit for imaging an object point on an image point and a measuring device, the measuring device being adapted to determine at least one imaging error which occurs when imaging the object point on the image point. The measuring device comprises at least one measurement light source, an optical element group having at least one optical element and at least one capturing unit. The measurement light source emits at least one measurement light bundle, in particular a plurality of measurement light bundles, while the optical element group is adapted to guide the at least one measurement light bundle onto the at least one capturing unit in order to generate at least one capturing signal. In this case, thermal shielding is provided for the at least one optical element. 
     The present invention furthermore relates to an optical imaging device having an imaging unit for imaging an object point on an image point and a measuring device, the imaging unit having an optical element group with at least one optical element and the measuring device being adapted to determine state changes of the at least one optical element. The measuring device is adapted to determine the state changes of the at least one optical element during transport of the optical imaging device and register them in a log. 
     The present invention also relates to an optical imaging device having an imaging unit for imaging an object point on an image point, the imaging unit having an optical element group with at least one optical element. The imaging unit is adapted to image the object point on the image point by using light with a wavelength in the EUV range, particularly in the range of from 5 nm to 20 nm, and comprises a support structure having at least one structural element, by means of which the at least one optical element is supported. The at least one structural element comprises a material or a material combination having a coefficient of thermal expansion of more than 0.6.10 −6  K −1 , in particular more than 1.2·10 −6  K −1 . 
     Other preferred configurations of the invention are set forth in the dependent claims and in the following description of preferred exemplary embodiments, which refer to the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a highly schematised representation of a preferred embodiment of the optical imaging device according to the invention, with which a preferred embodiment of the imaging method according to the invention can be carried out; 
         FIG. 2  is a more concrete schematic representation of the imaging device of  FIG. 1 ; 
         FIG. 3  is a schematic representation of a part of the imaging device of  FIG. 1 ; 
         FIG. 4  is a flow chart of a preferred embodiment of the imaging method according to the invention, which can be carried out with the imaging device of  FIG. 1 ; 
         FIG. 5  is a schematic representation of a part of another preferred embodiment of the imaging device according to the invention; 
         FIG. 6  is a highly schematised representation of another preferred embodiment of the optical imaging device according to the invention; 
         FIG. 7  is a highly schematised representation of another preferred embodiment of the optical imaging device according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     First Embodiment 
     A preferred embodiment of the optical imaging device  101  according to the invention for microlithography will be described below with reference to  FIGS. 1 to 4 . To simplify the representation in the figures an xyz coordinate system, to which reference is made, is introduced in the figures. 
       FIG. 1  shows a highly schematised representation of the optical imaging device according to the invention in the form of a microlithography device  101 , which operates with light having a first wavelength in the UV range. The microlithography device  101  comprises an imaging unit in the form of an optical projection system  102  having an illumination system  103 , a mask device  104  and an optical device in the form of an objective  105  with an optical axis  105 . 1 . The illumination system  103  illuminates the mask  104 . 1  of the mask device  104  with a projection light bundle  101 . 1  (not further represented in this section). 
     On the mask  104 . 1 , which is arranged in a mask stage  104 . 2 , there is a projection pattern  104 . 3  in an object plane (or more generally on an arbitrarily shaped object surface) having individual object points which are projected by the projection light bundle  101 . 1  via optical elements arranged in the objective  105  onto image points in an image plane (or more generally on an arbitrarily shaped image surface) on a substrate  106 . 1 , for example a so-called wafer, of a substrate device  106 . 
     To this end, the objective  105  comprises a first optical element group  105 . 2 , which is formed by a plurality of first optical elements  107 ,  108 ,  109 ,  110 ,  111 ,  112  that are mounted in the housing  105 . 3  of the objective  105 , which is in turn supported on a base structure  101 . 2 . The first optical elements  107  to  112  project the projection light bundle  101 . 1  onto the substrate  106 . 1 , and thus image an object point of the projection pattern onto an image point on the substrate  106 . 1 . 
     A measuring device  113  is furthermore provided, with which (as will be explained in more detail below) an imaging error of the projection system  102  during imaging of the projection pattern of the mask  104 . 1  on the substrate  106 . 1  is determined. To this end, the measuring device  113  comprises a first measurement unit  113 . 1  having a measurement light source  114 , a second optical element group  115  and a first capturing unit  116 . The second optical element group  115  in turn comprises a plurality of second optical elements  117 ,  118 ,  119 ,  120 ,  121 ,  122 , each of which is in a spatial relationship, defined at all times, with one of the first optical elements  107  to  112 , as will be explained in more detail below. 
     In the example shown, the measurement light source  114  and the capturing unit  116  are rigidly connected to a reference structure  123 , which is supported on the base structure  101 . 2 . The reference structure  123 , the measurement light source  114  and the capturing unit  116  are components which have been sufficiently stabilised thermally and mechanically, so that there is an accurately defined spatial relationship between the measurement light source  114  and the capturing unit  116  at all times. It is, however, to be understood that in other variants of the invention the spatial relationship between the measurement light source and the capturing unit may also be captured by means of a corresponding measurement technique (continuously or intermittently) and used in the course of determining the imaging error of the projection system  102 . 
     In order to determine the imaging error of the projection system  102 , the measurement light source  114  emits a measurement light bundle  114 . 1 , which is guided or projected onto the capturing unit  116  via the second optical elements  117  to  122 . As a function of the position and shape of the incident wavefront of the measurement light bundle  114 . 1 , the capturing unit  116  generates a first capturing signal S 1  which is delivered to a processing unit  124  of the measuring device  123 . The processing unit  124  then uses this first capturing signal S 1  in order to determine the imaging error of the projection system  102 . 
     In preferred variants of the invention, in order to determine the imaging error of the projection system  102 , the measurement light source  114  emits at least one further measurement light bundle in addition to the measurement light bundle  114 . 1 , as is indicated in  FIG. 1  by the measurement light bundle  114 . 2 . The use of a plurality of measurement light bundles is known for example from WO 01/63233 A1 (Wegmann), the entire disclosure of which is incorporated herein by reference. This further measurement light bundle  114 . 2  is furthermore guided or projected onto the capturing unit  116  by means of the second optical elements  117  to  122 . As a function of the position and shape of the incident wavefront of the further measurement light bundle  114 . 2 , the capturing unit  116  generates a defined further first capturing signal S 1  which is delivered to a processing unit  124  of the measuring device  113 . The processing unit  124  then uses this further first capturing signal S 1  in order to determine the imaging error of the projection system  102 . 
     In other words, a plurality of measurement channels can be formed in the first measurement unit  113 . 1 , by means of which the relevant imaging error or errors of the projection system  102  can be determined. For each measurement channel, a separate light source which generates the corresponding measurement light bundle  114 . 1  or  114 . 2  may be provided in the measurement light source  114 . Likewise, the capturing unit  116  may have a plurality of separate sensors (for example one sensor per measurement channel). It is, however, also possible to provide a common sensor for a plurality of or even all of the measurement channels. 
     The measurements in the individual measurement channels of the first measurement unit  113 . 1  may be carried out in an arbitrarily predeterminable time sequence. The measurements in the individual measurement channels are preferably carried out chronologically synchronously (that is to say essentially simultaneously), since particularly reliable inferences about the respective imaging errors are thereby possible using the known correlation of the measurements from the individual measurement channels. 
     The second optical elements  117  to  122 , as already explained above, are connected to the first optical elements  107  to  112  so that all times there is an accurately defined spatial relationship between the respective first optical element  107  to  112  and the second optical element  117  to  122  assigned to it. In the example shown in  FIG. 1 , for this purpose the respective second optical element  117  to  121  is connected directly and rigidly to the first optical element  107  to  111  assigned to it. 
     To this end, the respective optical element  117  to  121  may be fastened directly on a holding structure (for example a frame or a holding ring etc.) of the assigned first optical element  107  to  111  in order to ensure the spatial relationship, accurately defined at all times, between the second optical element  117  to  122  and the relevant first optical element  107  to  112 . Likewise, it is possible for the second optical element  117  to  121  to be fastened (by a sufficiently stable suitable connection technique) directly on the assigned first optical element  107  to  111 . 
     On the other hand, the second optical element  122  is connected to the associated first optical element  112  by means of a gearing device  125  supported on the reference structure  123 . Depending on the configuration of the gearing device  125 , a defined movement transmission takes place between a first movement of the first optical element  112  and the second movement, resulting therefrom, of the second optical element  122 . 
     The effect which can then be achieved with the gearing device  125  (as represented in  FIG. 1 ) is that a first movement of the first optical element  112  in a first degree of freedom causes a second movement of the second optical element in a second degree of freedom, which is different from the first degree of freedom. In this case, the type (translation or rotation) of the first and second degrees of freedom may optionally also differ. For example, a translation of the first optical element  112  along the z axis generates inter alia a rotation of the second optical element  122  about an axis parallel to the y axis. 
     As an alternative or in addition, an effect achievable using a corresponding configuration of the gearing device  125  is moreover that the movements of the first optical element  112  and of the second optical element  122  take place in the same degree of freedom. In the case of the gearing device  125  of  FIG. 1 , for example, a translation of the first optical element  112  along the x or y axis generates an identical translation of the second optical element  122  along the x or y axis, respectively. Likewise, a rotation of the first optical element  112  about an axis parallel to the y or z axis results in a rotation of the second optical element  122  about an axis parallel to the y or z axis, respectively. 
     The gearing device  125  may in principle be configured in any desired suitable way. For instance, it may be configured in one part or a plurality of parts. A suitable number of lever elements and articulations connecting them may be provided. Preferably, the respective articulations are configured as solid-state articulations (flexural articulations, leaf spring elements etc.) in order to minimise the effects of manufacturing inaccuracies (for example undesired play) and readily ensure the spatial relationship, defined at all times, between the first and second optical elements. 
     In the example shown in  FIG. 1 , the second optical elements are furthermore arranged so that the optical axis  115  defined by them extends in the direction of the optical axis  105 . 1 . Accordingly, with rigid coupling, a translation of the first optical elements  107  to  111  in the direction of the z axis is converted into a translation of the second optical elements  117  to  121  in the direction of the z axis. It is, however, to be understood that in other variants of the invention the directions of respective optical axes may also differ at least for individual optical elements. 
     The effect achieved by the mechanical coupling defined at all times between the respective first optical elements  107  to  112  and the associated second optical elements  117  to  122  is that a state change (here a change in position and/or orientation) of the respective first optical element  107  to  112  necessarily entails a corresponding state change (position and/or orientation) of the assigned second optical element  117  to  122 . 
     The state changes of the second optical elements  117  to  122  in turn entail a change in the imaging properties or aberrations of the second optical element group  115 , which lead to a modification of the geometry and/or position of the wavefront of the measurement light bundle  114 . 1  incident on the capturing unit  116 . This in turn entails a change in the first capturing signal S 1 , which the capturing unit  116  delivers. A suitable measurement technique of the capturing unit  116  can thus capture the change in the imaging properties of the second optical element group, for example the modification of the incident wavefront, with one or more wavefront sensors at one or more field points. 
     Various known arrangements and principles may be employed for the wavefront measurement. These include inter alia interferometers (Michelson, Twyman, Green, shearing, point diffraction interferometers etc.). These also include wavefront sensors which are based on segmenting the pupil (i.e. dividing it into sub-pupils) and determining the positions or position changes of the focal positions of the bundles from the sub-pupils. Examples of this are sensors of the Hartmann, Shack-Hartmann type etc. 
     Another type sensor which may be used operates according to a measurement method which is referred to as phase retrieval (from a point image) (the so-called “phase retrieval measurement technique” or “aerial image measurement technique”). In this case, the corresponding phase distribution in the pupil is calculated from the three-dimensional intensity distribution in the region of the respective point image of the measurement image. In order to acquire the three-dimensional intensity distribution, a two-dimensional sensor (for example a camera which acquires a plane intensity distribution) is moved through the focal region or the beam waist in the third dimension (transversely to the measurement plane of the sensor). The capturing of the intensity distribution thus takes place stepwise. The wavefront in the exit pupil can then be calculated back from this image stack obtained in this way (using so-called “phase retrieval algorithms”). 
     It is furthermore to be understood that the state change to be detected of the second optical elements  117  to  122  may possibly also impact on simpler measurement quantities of the measuring device  113 , for example on a lateral image offset or a focal change. In this case, measurement techniques other than a wavefront measurement technique may be provided, for example focal sensors (change in the focal position along the optical axis) or moiré techniques as sensors for the offset of the image position or the change of scale and/or distortion of the measurement optics. Naturally, combinations of wavefront sensors and position sensors are also possible. 
     Using a previously determined model of the measuring device  113 , stored in the processing unit  124 , the state changes (position and/or orientation) of the second optical elements  117  to  122  can be deduced with the aid of the measurement data of the wavefront sensors. The model of the measuring device  113  may have been determined beforehand theoretically (for example by corresponding simulation calculations) and/or experimentally (for example via corresponding calibration) for the measuring device  113 . The model of the measuring device  113  gives the relationship between the measurement data of the wavefront sensors and the respective state changes of the second optical elements  117  to  122 . 
     Owing to the above-described defined coupling between the first optical elements  107  to  112  and the second optical elements  117  to  122 , the processing unit  124  in turn determines the corresponding state changes of the first optical elements  107  to  112  from the state changes of the second optical elements  117  to  122 . 
     From these state changes of the first optical elements  107  to  112 , using the previously determined model of the first optical element group  105 . 2  stored in the processing unit  124 , the processing unit  124  determines the current imaging properties or current imaging errors of the first optical element group  105 . 2 . The model of the first optical element group  105 . 2  may have been determined beforehand theoretically (for example by corresponding simulation calculations) and/or experimentally (for example by means of corresponding calibration) for the first optical element group  105 . 2 . The model of the first optical element group  105 . 2  gives the relationship between the state changes of the first optical elements  107  to  112  and the respective imaging errors. 
     The processing unit  124  then forwards the respective currently determined imaging errors to a control device  126  of a correction device  127 . From the currently determined imaging errors of the first optical element group  105 . 2 , the control device  126  determines control signals for actuator devices  128  and  129 . The actuator devices  128  and  129  are connected to individual first optical elements  109  and  112 , respectively, and are used in a well-known way to modify the position and/or orientation and/or geometry of the respective first optical element  109  or  112  in order to reduce the currently determined imaging error. 
     The coupling with a spatial relationship defined at all times, between the first optical elements  107  to  112  and the second optical elements  117  to  122  of the measuring device  113  affords the advantage that the determination and correction of the imaging error can take place in real time during the actual exposure of the substrate  106 . 1 , without thereby interfering with the process of exposing the substrate. 
     In the present example, merely two actuator devices  128  and  129  are provided. It is, however, to be understood that in other variants of the invention any other desired number of actuator devices may be provided. In particular, each first optical element  107  to  112  may be provided with a corresponding actuator device (as indicated in  FIG. 1  by the corresponding dashed outlines). 
     The measuring device  113  is preferably configured so that the changes in the imaging properties of the second optical element group  115 , and therefore the changes in the measurement data of the wavefront sensors, can be assigned as unequivocally as possible to particular state changes of the individual second optical elements  117  to  122 . In this case, particularly by using one or more gearing devices  125 , it is possible to convert the movement of a first optical element into a movement of the associated second optical element, which increases the uniqueness of the assignment of the changes in the measurement data to the state changes of the individual second optical elements  117  to  122 . In particular, the change achievable by such a gearing device in the type of degree of freedom (translation or rotation) may be of considerable advantage here. 
     The sensitivity of the measuring device  113  for all degrees of freedom of the second optical elements  117  to  122  relevant to the present case is known from the configuration of the measuring device  113  (for example, the change in the Z7/Z8 coma centring and the Z5/Z6 astigmatism centring corresponds to the ratio 4:1 for a translation of optical elements) and correspondingly saved in the stored model of the measuring device  113 . 
     In the event of poor conditioning of the second optical element group  115  (for example low sensitivity, uniqueness, separability or orthogonality of the equation system of the sensitivity to state changes of the individual second optical elements) the conditioning can be improved by the addition of extra optical element groups. To this end, for example, an identical (to the second optical element group  115 ) third optical element group may be linked to a different position (for example rotated through 90° or 180° with respect to the z axis) on the first optical element group in an identical or similar fashion. It is likewise possible to achieve optimisation of the conditioning of the measuring device  113  by controlled optimisation of the configuration and coupling of two or a plurality of mutually differing optical element groups to the first optical element group  107  to  112 , as will further be described below in connection with  FIGS. 2 and 3 . 
     The measuring device  113  may in particular be configured so that its sensitivity corresponds to the sensitivity of the projection system  102 , or at least so that there is a simple (for example proportional) relationship between these sensitivities. In this context, by suitable selection of the movement transmission, the gearing device  125  makes it possible to adapt the sensitivity of the measuring device  113  to the measurement task in question. 
     Such adaptation of the sensitivity of the measuring device  113  by means of one or more gearing devices may, for example, be advantageous when an imaging error of the projection system  102  is more sensitive to a state change of one or more of the first optical elements  107  to  112  than the measuring device  113  is to a state change of the assigned second optical element  117  to  122 . The sensitivity of the measuring device  113  can then be suitably increased in a straightforward way by the movement transmission. In particular, the effect achievable by the movement transmission is that the sensitivity of the measuring device  113  to a state change of one of the second optical elements  117  to  122  reaches or even exceeds the sensitivity of an imaging error of the projection system  102  to a state change of the associated first optical element  107  to  112 . 
     Furthermore, in this context it may also be advantageous to convert the translation of a first optical element  107  to  112  into a rotation of the assigned second optical element  117  to  122  with such a gearing device. Lastly, such gearing devices may be necessary in order to accommodate other constraints (and available installation space, distances to be covered etc. 
     Since it is possible that the kinematics and sensitivity of the measuring device  113  cannot be theoretically modelled or practically implemented well enough, or variations have to be taken into account, calibration of the measuring device  113  may as mentioned be necessary in order to compile or adapt the model. For such calibration, the objective  105  may initially be optimised with a system interferometer, i.e. all the components may for example be brought into their setpoint state (setpoint position, setpoint orientation and setpoint geometry) using corresponding manipulators. Here, the setpoint state refers to a state in which the aberrations cause the minimal achievable deviation from an ideal state. The ideal state depends on various parameters of the imaging, for example the shape and the dimension of the structures to be imaged, the illumination setting, the size of the imaging field used etc. 
     After the setpoint state has been achieved, the capturing units of the measuring device  113  (i.e. for example its sensors, measurement transducers etc.) are “zeroed” (i.e. their intrinsic error is set to “zero”). This state is then also the setpoint state for the regulation of the correction of the imaging errors during operation of the microlithography device  101 . The sensitivities of the optical elements of the measuring device  113  to position changes of the components of the projection device  102  (in particular of the first optical elements  107  to  112 ) are then determined by controlled (continuous or stepwise) adjustment of the actuators of the projection device  102 , in order to determine a sensitivity curve. The sensitivity curves determined in this way are then saved in a model, or in a sensitivity matrix, so that the actuator movements necessary for the correction of the imaging errors can be determined and instigated during control operation from the measurement data of the measuring device  113 . 
     In the present example, the static reference structure  123  is used as a reference for the measuring device  113 . It is, however, to be understood that in other variants of the invention a relative (optionally mobile) reference may be expedient. This may in particular be advantageous if the desired measurement quantity comprises a relative position, for example the position of the mask device  104  relative to the substrate device  106 . In this case, a particular reference region of the objective  105  may be expedient as the reference of the measuring device. 
     The measurement light source and/or the capturing unit may also be connected not to the reference structure  123  but to the mask device  104  or substrate device  106 , as indicated in  FIG. 1  by the dashed outlines  130  and  131 . Here, the connection may be carried out similarly to the manner described above for the connection between the first and second optical elements (directly rigidly or via a gearing device), so in this regard reference is made to the comments above. 
     The measurement light source and the capturing unit may furthermore be arranged next to one another, and optionally even integrated in a single unit, in order to establish a defined spatial relationship between them in a straightforward way. In the embodiment of  FIG. 1 , a reflective element, for example a mirror or the like, would then merely need to be arranged for example at the position of the capturing unit  116 , as indicated in  FIG. 1  by the dashed outline  132 . 
     In order to be able to capture state changes of the mask device  104  or the substrate device  106  as well, in other variants of the invention, they may also be coupled to a corresponding second optical element in a similar way as the first optical elements  107  to  112 . 
     The measuring device may (as mentioned above) be equipped with a plurality of measurement channels or measurement principles, in order very rapidly to detect deviations in all six degrees of freedom and make them available for rapid regulation. For example, moiré channels may be provided for translations in the direction of the x and y axes and a rotation about the z axis, while rapid focal detection may be provided for translation in the direction of the z axis and a rotation about the x and y axes. 
     As already mentioned,  FIG. 1  shows a highly schematised representation of the imaging device  101 , in which the first and second optical elements are represented schematically by lenses (for the sake of simpler representation). It is, however, to be understood that the first optical elements  107  to  112  and the second optical elements  117  to  122  may be formed individually or in any desired combination by refractive, diffractive and reflective optical elements. In particular, these optical elements may have any desired suitable geometry. For instance, elements with spherical, aspherical, cylindrical, plane optical surfaces or freeform surfaces may be used as optical surfaces. 
     In the case of the second optical elements  117  to  122 , the geometry of the optical elements may be adapted to the measurement result to be achieved. Thus, the effect achievable by a different configuration of the optical surfaces of the second optical elements  117  to  122  along different degrees of freedom (for example a different curvature along different degrees of freedom) is that the movements along these different degrees of freedom result in different changes in the capturing signals of the capturing unit  116 , which allow correspondingly unequivocal inference of the associated state changes of the respective second optical elements  117  to  122 . 
     For example, the geometry of the second optical element (as for example in the case of the optical element indicated by the dashed contour  132  in  FIG. 1 ) may contribute to simple discrimination of state changes. In a variant with two or more measurement channels (for example with the two measurement light sub-bundles  114 . 1  and  114 . 2 ) and two such (convex or concave) mirrors  132 ,  133  correspondingly arranged next to one another, a translation of the mirror  132 ,  133  along the x axis or the y axis causes equally large tilting of the returning waves in both measurement channels, and a translation along the z axis likewise causes equally large defocusing. A rotation about the z axis, however, causes unequal tilting of the waves while a rotation about the x axis causes unequal defocusing of the waves. 
     In the present example, each of the first optical elements  107  to  112  is coupled to precisely one second optical element  117  to  122 . It is, however, to be understood that in other variants of the invention only some of the first optical elements  107  to  112  may respectively be coupled to one or more of the second optical elements. In particular, a state capturing of sufficiently (thermally and mechanically) stable first optical elements may possibly be dispensed with. 
     As already mentioned above, in the event of insufficient conditioning of the second optical element group  115 , the conditioning may be improved by adding extra optical element groups. In the microlithography device  101 , this is done by a third optical element group  134  of the measuring device  113 , as will be explained below with reference to  FIGS. 2 and 3 . 
       FIG. 2  shows a (compared with  FIG. 1 ) more concrete configuration of the microlithography device  101 , which operates with a projection light bundle  101 . 1  in the EUV range with a wavelength of 13.5 nm. For this reason, the first optical elements  107  to  112  in this embodiment are formed exclusively as reflective optical elements in the form of mirrors.  FIG. 3  shows a schematic detail of the microlithography device  101  of  FIG. 2 . 
     As already mentioned in the introduction, reliable detection of wavefront tilts of λ/1000 over a predetermined measurement aperture is for example possible with a usual interferometric measurement. If this measurement aperture has a diameter of 100 mm, for example, then this corresponds to an angular resolution of 6 nrad when the measurement wavelength λ is the wavelength of a conventional helium-neon laser. 
     The image position during a current process of exposing the substrate  106 . 1  must remain within the scope of specified limits. As mentioned, the beam path in an EUV objective  105  may be a several meters long. With structures to be exposed of the order of 25 nm, however, the image displacements must only lie in the nanometer range. A length of about 2.5 m for the beam path of the projection light bundle  101 . 1  (between the mirror  107  and the substrate  106 . 1 ) and an admissible image displacement of 1 mm entail a maximum admissible mirror tilt of only 0.2 nrad for the mirror  107 . Although these angular tolerances increase stepwise for the subsequent mirrors  108  to  112  in the beam path, since their distance from the image plane (i.e. from the substrate  106 . 1 ) decreases continuously, here again angular tolerances in the sub-nrad range should ideally be complied with as before. 
     In order to achieve this, according to an aspect of the present invention a time-resolved measurement of the orientation of the mirrors  107  to  112  in the sub-nrad range by the measuring device  113  is provided, with the aid of which image position changes can be identified in real time in parallel with the exposure of the substrate  106 . 1  and corrected by the correction device  127 . 
     This will be described below by way of example with the aid of the mirrors  107 ,  108  and  112 . Since tilts (as changes in orientation) of all the mirrors  107  to  112  can contribute to the image displacement, albeit to a different extent, it is preferable for the tilts of all the mirrors to be observed and corrected in this manner described below. 
     For this purpose, the measuring device  113  comprises an interferometric second measurement unit  113 . 2  having a second measurement light source  134 , a third optical element group  135  and a second capturing unit  136 . Besides a row of reflective third optical elements in the form of measurement mirrors  137 ,  138  and  139 , the third optical element group  135  has a partially reflective optical reference element  140 . The third optical elements  137 ,  138  and  139  in the present example are respectively coupled to one of the mirrors  107 ,  108  and  112 , in such a way that there is a spatial relationship, defined at all times, between the mirror  107 ,  108  and  112  and the associated, respective third optical element  137 ,  138  and  139 . To this end, the coupling between the third optical elements  137 ,  138  and  139  and the first optical elements  107 ,  108  and  112  may be configured in a similar way to the coupling described above of the first optical elements  107  to  112  and the second optical elements  117  to  122  (i.e. directly rigidly and/or through gearing devices). In the present example, the third optical elements  137 ,  138  and  139  are connected rigidly to the respective mirror  107 ,  108  and  112 . 
     The resolution in the sub-nrad range when measuring the orientation of the mirrors  107  to  112  is achieved by forming an optical cavity in the manner of a Fizeau cavity between the measurement mirrors  137 ,  139  and the reference element  140 , and respectively between the measurement mirror  137  and the measurement mirror  138 . This cavity is passed through a plurality of times because the optical elements  137  to  140 , which form the respective optical cavity  141 ,  142  and  143 , are arranged so that a measurement light bundle input into the cavity (at the optical elements  137  to  140  forming the respective cavity) experiences a plurality of M reflections before it leaves the cavity again and strikes the second capturing unit  136 . Owing to these M reflections, angular deviations between the two optical elements  137  to  140  forming the cavity are multiplied, so that an angular resolution in the sub-nrad range can readily be achieved even with the aforementioned simple interferometric measurement. 
     For this purpose, the second measurement light source  134  emits inter alia a measurement light bundle  134 . 1  which travels to the reference element  140  via a beam splitter  144 . The incident measurement light bundle  134 . 1  passes through the partially reflective reference element  140  and on the exit side it is partially reflected on a surface region  140 . 1  of the optical reference surface so that a returning reference wave is formed. The transmitted fraction passes through the optical cavity  141  formed between the reference element  140  and the first measurement mirror  137 , and is reflected on a first surface region  137 . 1  of the measurement mirror  137 . 
     The surface normals of the reference surface  140 . 1  and of the first surface region  137 . 1  (in a setpoint state) are arranged mutually inclined so that the measurement light bundle  134 . 1  (in this setpoint state) is not folded back on itself. From the first surface region  137 . 1 , the measurement light bundle  134 . 1  is reflected back inside the cavity  141  to the reference body  140 , strikes a preferably totally reflective surface region  140 . 2  and is reflected again there. The measurement light bundle  134 . 1  passes through the cavity  141  again and strikes a surface region  137 . 2  of the first measurement mirror  137  perpendicularly. In this way, the measurement light bundle  134 . 1  is folded back on itself so that it returns along essentially the same path in order to leave the cavity  141  again in the partially reflective region of the surface region  140 . 1  of the reference element  140 . The measurement light bundle  134 . 1  is then guided using the beam splitter  144  onto the capturing unit  136 , where it leads to the generation of a second capturing signal S 2  which is representative of the angular deviations between the first measurement mirror  137  and the reference element  140  and therefore (owing to the coupling which is defined at all times) of a current angular deviation of the first optical element  107  (from a setpoint state). 
     The capturing signal S 2  is forwarded to the processing unit  124 , which then uses the capturing signal S 2  in the manner described above in order to determine the imaging errors of the projection device  102  and correct them with the correction device  127 . 
     Depending on the application, the M=5 reflections inside the cavity  141  as represented in  FIG. 3  may still not be sufficient in order to achieve the angular resolutions required above in the sub-nrad range. It is, however, to be understood that any desired number of reflections inside the cavity  141  can be achieved through simple selection of the alignment of the optical surfaces  137 . 1 ,  140 . 2  and  137 . 21  forming the cavity  141 . At least 21 reflections are preferably provided when passing through the cavity. 
     If the angular deviation between the first measurement mirror  137  and the reference element  140  is so small that the reflected rays of the measurement light bundle  134 . 1  overlap with the incoming measurement rays on the reference element  140 , they will be partially transmitted and travel back to the capturing unit  136  where they can be captured by an aperture device at the intermediate focus of the capturing unit  136 . 
     For the relative shift of the phases of the measurement wave and the reference wave, with a view to a phase-shifting interference evaluation, the reference element  140  may be displaceable along the direction  145  by suitable actuators. As an alternative, a phase shift using a wavelength shift is also possible. If this is intended to be avoided, the widely known so-called DMI method may furthermore be used, as is known from U.S. Pat. No. 5,361,312 (Küchel), the entire disclosure of which is incorporated herein by reference. The cavity  141  can thus be formed so that a carrier frequency appears in the interferogram of the capturing unit  136  and is subsequently eliminated computationally (in the capturing unit  136  or the processing unit  124 ). 
     In a similar way, using a further measurement light beam  134 . 3  and the cavity  143  formed between the reference element  140  and the third measurement mirror  139 , angular deviations between the third measurement mirror  139  and the reference element  140  are determined and used in order to determine and correct the imaging error of the projection device  102 . 
     The second measurement light source  134  furthermore emits a measurement light bundle  134 . 2  that enters the optical cavity  142  which is formed between the first measurement mirror  137  and the second measurement mirror  138 . The measurement light bundle  134 . 2  initially strikes a diffractive element in the form of a grating  137 . 3  in so-called Littrow arrangement. By means of the grating  137 . 3  on the first measurement mirror  137 , on the one hand a returning reference wave is generated. The specular reflection of the measurement light bundle  134 . 2  at the grating (zero-th diffraction order) passes through the cavity  142  a plurality of times in a similar way to that described above (in connection with the measurement light bundle  134 . 1  and the cavity  141 ), until it strikes the surface region  137 . 4  of the first measurement mirror  137  perpendicularly and is reflected back on itself. The measurement light bundle  134 . 2  then travels back along essentially the same path before leaving the cavity  142  again through the grating  137 . 3 . Here, angular deviations (from a setpoint state) between the first measurement mirror  137  and the second measurement mirror  138  are again added using the number of reflections inside the cavity  142 . 
     The measurement light bundle  134 . 2  is then guided by the beam splitter  144  onto the capturing unit  136 , where it leads to the generation of a third capturing signal S 3  which is representative of the angular deviations between the first measurement mirror  137  and the second measurement mirror  138  and therefore (owing to the coupling which is defined at all times) of a current angular deviation of the first optical elements  107  and  108  (from a setpoint state). 
     For phase shifting, one of the two measurement mirrors  137  and  138  may again be mounted displaceably. If this is intended to be avoided (with a view to the defined coupling with the relevant first optical element  107 ,  108 ), here again either so-called wavelength tuning may be used or the aforementioned DMI method may be used. 
     As already mentioned, the reference element  140  and the measurement mirrors  137  to  139  preferably have locally varying reflectivities. The surface region  140 . 1  of the reference element  140 , which generates the reference wave by partial reflection, may for example have a reflectivity of about 40% while the other surface regions  140 . 2  and  140 . 3  have a high reflectivity of for example almost 100%. The surface regions  137 . 1  and  137 . 2  of the first measurement mirror  137  may likewise have a high reflectivity. In this case, the reference wave and the measurement wave have comparable intensities, so that a high contrast of the interference pattern in the region of the capturing unit  136  is advantageously ensured. 
     As can be seen in  FIG. 3 , determination of the lateral displacement (deviation in the x direction and y direction) of the objective  105  with respect to the mask stage  104 . 2  is furthermore provided in the present example, since this displacement affects the lateral image position in the image plane (i.e. on the substrate  106 . 1 ). To this end a mask reference body  104 . 4  is arranged on a non-actuated, i.e. stationary, region of the mask stage. A position measurement unit  146  (formed in a well-known way) linked to the reference structure  123  captures the spatial relationship between the reference body  104 . 4  and the reference element  140  and forwards corresponding information into the processing unit  124  for further processing in the determination of the imaging error. The resolution of the position measurement unit  146  is of the order of the admissible image displacements, i.e. in the nanometer range in the present example. 
     It is to be understood that by corresponding extensions of the second measurement unit  113 . 2 , or by further measurement units, it is also possible to capture the angular deviations of the three other first optical elements  109  to  111  (absolutely or relative to one another). In particular, according to an aspect of the present invention, it is possible to determine all angular deviations relative to a single reference body, namely the reference element  140 , and additionally to capture the position of the entire objective  105  relative to the mask unit  102  in real time in parallel with the exposure of the substrate  106 . 1 . All these quantities are advantageously related to a single common reference body  140 . 
     The reference element  140  is therefore preferably made of a material or a material combination having a low coefficient of thermal expansion (CTE). Materials such as Zerodur, ULE, quartz glass or the like may in particular be used for the partially transmissive part of the reference element  140 . The reference element is preferably configured so that it has an aspect ratio of less than 7, in particular an aspect ratio of less than 3, in order to avoid bending. 
     The reference element  140  is preferably provided with a thermal heat shield in the form of a radiation protection shield  147 , in order to thermally stabilise it. The radiation protection shield  147  may for example be a metal foil or a metal shell with openings for the respective measurement light bundle. Optionally, the radiation protection shield  147  may be cooled actively and/or passively. In particular, temperature regulation may also be provided in order to keep the protective shield  147  at a constant temperature. 
     In this context, it should be mentioned that the provision of such a thermal protection shield for optical elements of the measuring device constitutes a separately protectable inventive concept which is independent from the rest of the configuration of the measuring device. 
     Another advantage of the measuring device  113  according to the invention is that changes in the angular deviations of the measurement mirrors  137  to  139  result in a change in the interference fringe spacing in the interferogram of the capturing unit  136 , and after phase-shifting evaluation also result in a change of the wavefront tilt coefficient. Whereas translational changes of the measurement mirrors  137  to  139  along the respective measurement light bundle  134 . 1  to  134 . 3  merely result in a change of the interference fringe positions in the interferogram of the capturing unit  136 , after the phase-shifting evaluation they thus result in a change of the offset. In this way, in addition to capturing the changes of the angular deviations is also possible to capture translational changes of the measurement mirrors  137  to  139  along the respective measurement light bundle  134 . 1  to  134 . 3 . 
     In a preferred embellishment to the present embodiment, the measuring device  113  is configured to be transportable together with the objective  105 , and the measuring device  113  continues to be operated (in a logging step) during transport of the objective  105 . To this end, for example, a mobile electricity supply  113 . 4  (such as a battery or an accumulator etc.) may be provided, which supplies the electrically operated components of the measuring device  113 , in particular the processing unit  124 , with electrical energy. The respective interferograms of the measuring device  113  are captured continuously or at regular time intervals, and optionally stored in a suitable form in a memory  124 . 1  of the processing unit  124 . After transport, the state changes (geometry and/or position and/or orientation) of the first optical elements  107  to  112  can thus be reproduced (in an analysis step) with the aid of the data read out from the memory  124 . 1 , and a corresponding correction can be carried out straightforwardly (in a correction step). 
     In this context, it should be noted that the mobile capturing of state changes of the optical elements of a projection system constitutes a separately protectable inventive concept which is independent from the type of capturing of the state changes or the configuration of the measuring device used. 
       FIG. 4  shows a flow chart of a preferred embodiment of the imaging method according to the invention in the form of a microlithography method, which is carried out with a microlithography device  101  of  FIGS. 1 to 3  operating according to the so-called scanner principle. 
     First, in a step  148 . 1 , the method flow of the microlithography method is started. In a step  148 . 2 , the microlithography device  101  is then made available in the configuration of  FIGS. 1 to 3 . 
     In a capturing and correction sequence  148 . 3  of steps, a determination of the imaging error is initially carried out in a determination step  148 . 4  in parallel with the exposure of the substrate  106 . 1 . To this end, as described in connection with  FIGS. 1 to 3 , the capturing signals S 1  to S 3  are generated by the measurement units  113 . 1  and  113 . 2  of the measuring device  113 , and processed in the processing unit  124 . 
     As a function of the imaging error thereby determined in the imaging of the projection pattern  104 . 3  on the substrate  106 . 1 , the correction of the respective imaging error as described above in connection with  FIGS. 1 to 3  is then carried out by the correction device  127  in the correction step  148 . 5 , by the control device  126  correspondingly driving the actuators  128 ,  129  in the first optical elements  109  and  112 . 
     As mentioned, the determination and correction of the imaging error are carried out in parallel with the exposure of the substrate  106 . 1 . At least so long as no imaging errors are captured, which would necessitate an interruption to the exposure of the substrate  106 . 1 , the exposure thus takes place simultaneously with and/or independently of the determination and correction of the imaging error. 
     In a further step  148 . 6 , a check is then made whether a further correction process should still be carried out. If this is not the case, the method flow ends in step  148 . 7 . Otherwise, it returns to step  148 . 4 . 
     Owing to the highly precise capturing and correction of the imaging errors of the projection system  102  taking place in real time in parallel with the exposure of the substrate  106 . 1 , in one aspect of the present invention it is possible to make the structural components of the microlithography device, which in respect of the imaging errors of the projection system  102  have a substantial influence on the state (geometry and/or position and/or orientation) of the optical elements of the imaging system, i.e. in particular the support structure for the first optical elements  107  to  112 , entirely or partially from thermally more sensitive materials (i.e. for example materials having a comparatively high coefficient of thermal expansion), since thermally induced perturbations that lead to imaging errors can readily be compensated for. 
     In particular, in the case of an EUV system (typically with a working wavelength of from 5 nm to 20 nm), in contrast to the previously known systems it is sometimes even possible to use thermally comparatively sensitive but economical materials such as Invar or the like for these structural components. It is thus possible to use for these structural components materials or material combinations the coefficient of thermal expansion (CTE) of which is close to or more than the coefficient of thermal expansion of Invar, i.e. in particular more than 0.6·10 −6  K −1 , in particular even more than 1.2·10 −6  K −1 . 
     In this context, it should be noted that this capturing and correction of state changes of the optical elements supported by such thermally more sensitive materials of a projection objective operating in the EUV range, in parallel with the exposure of the substrate, constitutes a separately protectable inventive concept which is independent of the type of capturing of the state changes or the configuration of the measuring device used. 
     Second Embodiment 
     Another preferred embodiment of the microlithography device  201  according to the invention will be described below with reference to  FIG. 5 .  FIG. 5  shows a detail of the microlithography device  201 , which corresponds to the detail of  FIG. 3 . 
     Basically, the microlithography device  201  corresponds in structure and functionality to the microlithography device  101  of  FIGS. 1 to 3 , so merely the differences will be discussed here. In particular, components of the same type are provided with references augmented by the value  100 . Unless otherwise mentioned below, reference should be made to the comments above in connection with the first embodiment with respect of the properties of these components. 
     The only material difference of the microlithography device  201  from the microlithography device  101  consists in the configuration of the second measurement unit  213 . 2 . In the microlithography device  201  as well, a time-resolved measurement of the orientation of the mirrors  101  to  112  is carried out in the sub-nrad range, with the aid of which image position changes can be identified in real time in parallel with the exposure of the substrate  106 . 1  and corrected via the correction device  127 . 
     This will be described below by way of example with the aid of the mirrors  107  and  112 . Since tilts (as changes in orientation) of all the mirrors  107  to  112  can contribute to the image displacement, albeit to a different extent, it is preferable for the tilts of all the mirrors to be observed and corrected in this manner described below. 
     In the case of the second measurement unit  213 . 2 , the resolution in the sub-nrad range when measuring the orientation of the mirrors  107  to  112  is again achieved by forming an optical cavity  241  and  243  in the manner of a Fizeau cavity between the measurement mirrors  237  and  239 , respectively, and the reference element  240 . These cavities  241  and  243  are passed through a plurality of times because the optical elements  237 ,  239  and  240 , which form the respective optical cavity  241  or  243 , are arranged so that a measurement light bundle input into the cavity (at the optical elements  237 ,  239  and  240  forming the respective cavity) experiences a plurality of M reflections before it leaves the cavity again and strikes the second capturing unit  236 . Owing to these M reflections, angular deviations between both the optical elements  237 ,  239  and  240  forming the cavity are multiplied, so that an angular resolution in the sub-nrad range can readily be achieved even with the aforementioned simple interferometric measurement. 
     For this purpose, the second measurement light source  234  emits inter alia a measurement light bundle  234 . 1  which travels to the reference element  240  via a beam splitter  244 . The incident measurement light bundle  234 . 1  passes through the partially reflective reference element  240  and, on the exit side, it is partially reflected on a surface region  240 . 1  of the optical reference surface so that a returning reference wave is formed. The transmitted fraction passes through the optical cavity  241  formed between the reference element  240  and the first measurement mirror  237 , and is reflected on a first surface region  237 . 1  of the measurement mirror  237 . 
     Unlike in the embodiment of  FIG. 3 , the surface normals of the reference surface  240 . 1  and of the first surface region  237 . 1  (in a setpoint state) are arranged mutually parallel so that the measurement light bundle  234 . 1  (in this setpoint state) is folded back on itself. From the first surface region  237 . 1 , the measurement light bundle  234 . 1  is reflected inside the cavity  241  back to the reference body  240 , and again strikes the partially reflective surface region  240 . 1  where it is reflected again and folded back on itself. 
     In this variant as well, multiple reflection of the measurement light bundle  234 . 1  thus also takes place in the cavity  241 , and each time the measurement light bundle  234 . 1  is incident on the partially reflective surface region  240 . 1  a part of the measurement light bundle  234 . 1  emerges from the cavity  241  and is guided by the beam splitter  244  onto the capturing unit  236 . 
     Using so-called white light interferometry, these multiple reflections can be selected. To this end, a so-called white light interferometer (more strictly speaking an interferometer with a spatially short-coherence light source) is combined with a retardation section, as is known from U.S. Pat. No. 5,361,312 (Küchel), the entire disclosure of which is incorporated herein by reference. 
     A spatially short-coherence light source  234 . 4  couples quasi-monochromatic light into a retardation section  234 . 5 . Downstream of the retardation section  234 . 5 , the light is launched into a single-mode light guide fibre  234 . 6  and guided to the cavity  241  with a collimator  234 . 7 , as is known from WO 2006/102997 A1 (Altenberger et al.), the entire disclosure of which is incorporated herein by reference. The light guide fibre  234 . 6  eliminates aberrations or tilts of the wavefront downstream of the retardation section  234 . 5  and allows a flexible spatial arrangement of the components. The retardation D in the retardation section  234 . 5  is adjusted so that it corresponds to the optical path length of the part of the multiply reflected measurement light bundle which has experienced the desired number of M reflections in the cavity  241 . Thus, with the distance L 1  (as measured in the direction of the measurement light bundle  234 . 1 ) between the surface region  237 . 1  and the surface region  240 . 1 , the following applies:
 
 D=M·L 1.  (1)
 
     Only multiple reflected rays therefore contribute to the interference, although all the others contribute to a background brightness in the interferogram of the capturing unit  236 . A similar procedure is adopted with the other optical elements  108  to  112 , rendering further discussion apparent to those skilled in the art. 
     At this point it should be noted that the imaging method described in connection with  FIGS. 3 and 4  can also be carried out with this alternative embodiment. 
     Third Embodiment 
     Another preferred embodiment of the microlithography device  301  according to the invention will be described below with reference to  FIG. 6 .  FIG. 6  shows a schematic view of the microlithography device  301 , which corresponds to the view of  FIG. 1 . 
     Basically, the microlithography device  201  corresponds in structure and functionality to the microlithography device  101  of  FIGS. 1 to 3 , so merely the differences will be discussed here. In particular, components of the same type are provided with references augmented by the value  200 . Unless otherwise mentioned below, reference should be made to the comments above in connection with the first exemplary embodiment with respect of the properties of these components. 
     The material difference of the microlithography device  301  with respect to the embodiment of  FIG. 1  consists in the configuration of the measuring device  313 . More precisely, the capturing of the state changes of the first optical elements  107  to  112  is carried out with two measurement units  313 . 1  and  313 . 3 . 
     The two measurement units  313 . 1  and  313 . 3  are arranged rotated with respect to the optical axis  105 . 1  of the objective  105  by 180°. The measurement unit  313 . 1  captures (in a similar way to the first measurement unit  113 . 1 ) the state changes of the first optical elements  107 ,  109 ,  110  and  112 , while the measurement unit  313 . 3  (likewise in a similar way to the first measurement unit  113 . 1 ) captures the state changes of the first optical elements  108  and  111 . To this end, the measurement unit  313 . 3  has a further measurement light source  348 , a further optical element group  349  and a further capturing unit  350 . 
     The measurement light source  348  and the capturing unit  350  are again rigidly fastened on a further reference structure  351  which is connected through a rigid connection  352  to the reference structure  123  so that there is a spatial relationship, defined at all times, between the reference structure  123  and the reference structure  351 . Owing to this spatial relationship, defined at all times, between the reference structures  123  and  351  the processing unit  124  can deduce the current state changes of the first optical elements  107  to  112  from the signals of the capturing units  315  and  350  in the manner described above in connection with the first embodiment. 
     It is, however, to be understood that in other variants of the invention the spatial relationship between the reference structure  123  and the reference structure  351  may also be captured using a corresponding measuring device, as indicated in  FIG. 6  by the dotted outline  353 . With the aid of this information together with the capturing signals of the capturing units  315  and  350 , the current state changes of the first optical elements  107  to  112  can likewise be deduced. 
     An interleaved arrangement of the two measurement units  313 . 1  and  313 . 3  is shown in the present example, in which first optical elements whose state change is acquired by one measurement unit are arranged spatially between first optical elements whose state change is acquired by the other measurement unit. It is, however, to be understood that, in other variants of the invention, the capturing of the state changes may be carried out by different measurement units which are not interleaved. For example, the state changes of the first optical elements  107  to  109  could be captured by one measurement unit and the state changes of the first optical elements  110  to  112  could be captured by another measurement unit. Naturally, more than two different measurement units could be provided for this. 
     It is furthermore to be understood that in other variants of the invention the measurement light source and/or the capturing unit of one of the measurement units may be connected not to a reference structure but to one of the first optical elements, the state change of which is captured by another measurement unit. Naturally, in this case the connection may preferably be configured so that, at all times, there is an accurately defined spatial relationship between the relevant components and the first optical element. In  FIG. 6 , this is indicated by the dashed outline  354 . 
     At this point, it should be noted that the imaging method described in connection with  FIGS. 3 and 4  may also be carried out with this alternative embodiment. 
     Fourth Embodiment 
     Another preferred embodiment of the microlithography device  401  according to the invention will be described below with reference to  FIG. 7 .  FIG. 7  shows a schematic view of the microlithography device  401 , which corresponds to the view of  FIG. 1 . 
     Basically, the microlithography device  401  corresponds in structure and functionality to the microlithography device  101  of  FIG. 1 , so merely the differences will be discussed here. In particular, components of the same type are provided with references augmented by the value  300 . Unless otherwise mentioned below, reference should be made to the comments above in connection with the first embodiment with respect of the properties of these components. 
     The material difference of the microlithography device  401  with respect to the embodiment of  FIG. 1  consists in the configuration of the measuring device  413 . More precisely, the capturing of the state changes of the first optical elements  107  to  112  is carried out with a measurement unit  413 . 1 , one of the second optical elements of the second optical element group  415  being formed by one of the first optical elements, namely the first optical element  109 . To this end, the second optical element group  415  comprises guiding devices  455  and  456 , which guide the measurement light bundle  414 . 1  such that it passes through the first optical element  109 . 
     In this variant the measurement light source  414  is furthermore fastened on a reference structure  123 , while the associated capturing unit  416  is arranged on a further reference structure  451 . The measurement light source  414  and the capturing unit  416  are respectively fastened rigidly on the relevant reference structure  123  and  451 , respectively, which are connected to one another through a rigid connection  452  so that there is a spatial relationship, defined accurately at all times, between the reference structure  123  and the reference structure  451 . Owing to this spatial relationship, defined accurately at all times, between the reference structures  123  and  451 , the processing unit  124  can deduce the current state changes of the first optical elements  107  to  112  from the signals of the capturing unit  416  in the manner described above in connection with the first embodiment. 
     It is, however, to be understood that, in other variants of the invention, the spatial relationship between the reference structure  123  and the reference structure  451  may also be captured using a corresponding measuring device, as indicated in  FIG. 7  by the dotted outline  453 . With the aid of this information, together with the capturing signals of the capturing unit  415 , the current state changes of the first optical elements  107  to  112  can likewise be deduced. 
     They are arranged rotated with respect to the optical axis  105 . 1  of the objective  105  by 180°. The measurement unit  413 . 1  (in a similar way to the first measurement unit  113 . 1 ) captures the state changes of the first optical elements  107 ,  109 ,  110  and  112 , while the measurement unit  413 . 3  (likewise in a similar way to the first measurement unit  113 . 1 ) captures the state changes of the first optical elements  108  and  111 . To this end, the measurement unit  413 . 3  has a further measurement light source  448 , a further optical element group  349  and a further capturing unit  450 . 
     The measurement light source  348  and the capturing unit  350  are in turn fastened rigidly on a further reference structure  351 , which is connected to the reference structure  123  through a rigid connection  352  so that there is a spatial relationship, defined accurately at all times, between the reference structure  123  and the reference structure  351 . Owing to this spatial relationship, defined accurately at all times, between the reference structures  123  and  351 , the processing unit  124  can deduce the current state changes of the first optical elements  107  to  112  from the signals of the capturing units  315  and  350  in the manner described above in connection with the first embodiment. 
     It is, however, to be understood that in other variants of the invention the spatial relationship between the reference structure  123  and the reference structure  351  may also be captured with a corresponding measuring device, as indicated in  FIG. 6  by the dotted outline  353 . With the aid of this information, together with the capturing signals of the capturing units  315  and  350 , the current state changes of the first optical elements  107  to  112  can likewise be deduced. 
     In the present example, only one of the second optical elements of the second optical element group  415  is formed by one of the first optical elements  107  to  112 . It is, however, to be understood that, in other variants of the invention, a different number of second optical elements of the second optical element group may also be formed by one of the first optical elements. 
     At this point, it should be noted that the imaging method described in connection with  FIGS. 3 and 4  can also be carried out with this alternative embodiment. 
     The present invention may be used in connection with any production methods for electronic circuits. In this case, any desired working principles may be used, for example the so-called stepper principle. The invention is preferably used in connection with imaging devices operating according to the scanner principle, since the advantages of the present invention in respect of correcting the imaging errors of the imaging device are particularly effective in this case. 
     The present invention has been described above with the aid of examples, in which a multi-channel measurement (with a plurality of measurement light bundles) has only been described in connection with the first measurement unit  113 . 1 . It is, however, to be understood that in other variants of the invention having a plurality of such measurement units, all or at least some of the other measurement units may also carry out such multichannel measurements (with a plurality of measurement light bundles). In other words, a multichannel functionality of the measurements carried out in the scope of the present invention can be achieved on the one hand by using a plurality of measurement units. Likewise, however, it is also possible for the respective measurement unit itself to carry out a multichannel measurement directly. 
     The present invention has been explained above with the aid of examples in which wavelengths in the EUV range are used for the exposure of the substrate. It should however be noted at this point that the invention may naturally also be used in connection with applications in which the exposure of a substrate, or another kind of imaging, is carried out with other wavelengths. 
     It should furthermore be noted that the present invention has been explained above with the aid of an example from the field of microlithography. It is, however, to be understood that the present invention may likewise be used for any other applications or imaging methods. 
     As noted, the description of the 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.