Patent Publication Number: US-2022221269-A1

Title: Measuring apparatus for interferometrically determining a surface shape

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation of International Application PCT/EP2020/076557 which has an international filing date of Sep. 23, 2020, and the disclosure of which is incorporated in its entirety into the present Continuation by reference. This Continuation also claims foreign priority under 35 U.S.C. § 119(a)-(d) to and also incorporates by reference, in its entirety, German Patent Application DE 10 2019 214 979.0 filed on Sep. 30, 2019. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a measurement apparatus for interferometrically determining a shape of a surface of a test object, to a method for calibrating a diffractive optical element, to a method for interferometrically determining a shape of a surface of a test object, and to an optical element for a projection lens of a microlithographic exposure apparatus. 
     BACKGROUND 
     For the highly accurate interferometric measurement of a surface shape of a test object, such as a microlithographic optical element, diffractive optical arrangements are often used as what are known as zero optics. In this case, the wavefront of a test wave is adapted by way of a diffractive element to a desired (i.e., predetermined) shape of the surface in such a way that the individual rays of the test wave would be incident on the desired shape in perpendicular fashion at every location and be reflected back on themselves thereby. Deviations from the desired shape can be determined by the superposition of a reference wave on the reflected test wave. The diffractive element used can be a computer-generated hologram (CGH), for example. 
     The accuracy of the shape measurement depends on the accuracy of the CGH. The decisive factor here is not necessarily the most exact possible production of the CGH, but rather a most exact measurement of all possible errors in the CGH. Known errors can be calculated out when measuring the shape of the test piece. The CGH thus forms the reference. While a calibration of all non-rotationally symmetric errors is possible with high accuracy for rotationally symmetric aspheres, in the case of free-form surfaces, that is to say non-spherical surfaces without rotational symmetry, all CGH errors affect the shape measurement. As a result, the requirements regarding the accuracy of the measurement of the CGH increase significantly. It is important to know with high precision the distortion of the diffractive structures of the CGH, that is to say the lateral positions of the diffractive structures in relation to their desired positions, and also the profile shape of the CGH. However, the measurement accuracy with which these parameters can be determined using measurement devices known from the prior art is not sufficient for the constantly increasing requirements. 
     DE 10 2012 217 800 A1 describes such a measurement arrangement having a complex coded CGH. A light wave is initially split into a reference wave and a test wave using a Fizeau element. The test wave is then converted by the complex coded CGH into a test wave having a wavefront that is adapted to the desired shape of the surface and calibration waves having a spherical or plane wavefront. To this end, the CGH contains suitably configured diffractive structures. The calibration waves are used to calibrate the CGH. A test object is subsequently arranged in the test position, and a measurement with the test wave is carried out. The test wave is reflected by the surface of the test object, transformed back by the CGH, and, following the passage through the Fizeau element, it is superimposed by the reference wave. It is possible to determine the shape of the surface from the interferogram captured in a plane. Although the calibration of the CGH improves the accuracy, this is not always sufficient for advanced applications. 
     SUMMARY 
     It is an object of the invention to provide a measurement apparatus and a measurement method of the type mentioned in the introductory part, with which the aforementioned problems are solved and, in particular, a determination of the shape of optical surfaces in the form of free-form surfaces with improved accuracy is made possible. 
     The aforementioned object can be achieved according to the invention, for example, with a measurement apparatus for interferometrically determining a shape of a surface of a test object, wherein the measurement apparatus comprises: a radiation source for providing an input wave, a multiply-encoded diffractive optical element, which is configured to produce by diffraction from the input wave a test wave, which is directed at the test object and has a wavefront in the form of a free-form surface, and at least one calibration wave, wherein the calibration wave has a wavefront with a non-rotationally symmetric shape. Cross sections through the wavefront of the calibration wave along cross-sectional surfaces aligned transversely to one another in each case have a curved shape. The curved shapes in the different cross-sectional surfaces differ here in terms of an opening parameter. Furthermore, the measurement apparatus comprises a capture device for capturing a calibration interferogram formed by the superimposition of a reference wave with the calibration wave after interaction with a calibration object. 
     A free-form surface is to be understood to mean a shape with a deviation from any rotationally symmetric asphere of more than 5 μm, in particular more than 10 μm. Furthermore, the free-form surface deviates from any sphere by at least 0.05 mm, in particular at least 0.1 mm, at least 1 mm or at least 5 mm. In particular, the measurement apparatus comprises an evaluation device for determining the shape of the surface of the test object by evaluating an interferogram captured by the superimposition of the reference wave with the test wave after interaction with the surface, taking into account the interferogram formed by the superimposition with the calibration wave. 
     The design according to the invention of the calibration wave with a non-rotationally symmetric wavefront allows the difference between the wavefront of the test wave and the wavefront of the calibration wave, hereinafter also referred to as the test wavefront difference, to be minimized. 
     The invention makes use of the finding that the quality of the calibration of the diffractive optical element with regard to the free-form test wave to be produced is all the better, the smaller the deviation of the shape of the calibration wave from the wavefront shape of the test wave is. The reason for this is that the partial structures of the multiply-encoded diffractive optical element responsible for producing the calibration wave and the test wave are all the more similar, the more similar the calibration wave and the test wave are to one another. During the manufacture of the diffractive optical element, similar partial structures are in turn subject to similar manufacturing errors and have comparatively small differences in their groove width profiles. 
     By configuring the diffractive optical element such that it produces the calibration wave with a non-rotationally symmetric wavefront, it becomes possible to keep the test wavefront difference as small as possible. This in turn allows effects of manufacturing errors in the diffractive optical element on the test wave to be predicted with a high level of accuracy using calibration data determined with the calibration wave. 
     As mentioned above, cross sections through the wavefront of the calibration wave along cross-sectional surfaces aligned transversely to one another in each case have a curved shape, wherein the curved shapes in the different cross-sectional surfaces differ in terms of an opening parameter. In particular, the cross-sectional surfaces are aligned perpendicular to one another. 
     According to an embodiment, the curved shapes in the two different cross-sectional surfaces differ in terms of their direction of curvature. 
     According to a further embodiment, the curved shape approximates in each case a segment of a circular shape and the opening parameter is the circle radius. This is to be understood to mean that the curved shape in the different cross sections in each case has a segment of a circular shape or has approximately the segment of the circular shape, in particular the curved shape corresponds in each case to a segment of a circular shape. 
     According to a further embodiment, the curved shape approximates in each case a segment of a parabolic shape and the opening parameter is the parabola opening. This means that the curved shape in the different cross sections in each case corresponds to a segment of a parabolic shape or approximately to the segment of the parabolic shape. The segment of the parabolic shape comprises in particular the vertex region of the parabola, in particular the segment has a symmetric vertex region of the parabola. 
     According to a further embodiment, the wavefront of the calibration wave has an astigmatic shape. 
     According to a further embodiment, the non-rotationally symmetric shape of the calibration wave corresponds to the shape of a section of a surface of a solid of revolution, which is formed by rotation of a surface which is symmetrical to an axis of symmetry, hereinafter also referred to as a surface of revolution, about an axis of rotation. The section of the surface of this solid of revolution has a symmetry to a plane and can therefore also be described as mirror-symmetric. According to a first embodiment variant, the surface of revolution is circular and the axis of rotation is truly parallel to the axis of symmetry. In this case, the solid of revolution that results is a torus. According to a second embodiment variant, the surface of revolution is non-rotationally symmetric, for example elliptical. 
     According to a further embodiment, the wavefront of the calibration wave deviates from any rotationally symmetric shape by at least 50 μm, in particular by at least 100 μm. 
     According to a further embodiment, the diffractive optical element is configured to produce in addition to the test wave at least three calibration waves by diffraction from the input wave. 
     The aforementioned object can also be achieved, for example, by a method for calibrating a diffractive optical element, which is configured to produce a test wave with a wavefront in the form of a free-form surface for interferometrically determining a shape of a surface of a test object. The method according to the invention comprises providing the diffractive optical element with multiple encoding and radiating in an input wave so that a calibration wave that is directed at a calibration object is produced in addition to the test wave by diffraction at the multiple encoding, wherein the calibration wave has a wavefront with a non-rotationally symmetric shape. Furthermore, the method according to the invention comprises arranging the calibration object at different calibration positions such that the calibration wave is in each case substantially perpendicularly incident on different regions of a surface of the calibration object. Furthermore, the method according to the invention comprises capturing and comparing calibration interferograms which are produced by the superposition of a reference wave with the calibration wave after interaction with the calibration object at the different calibration positions. 
     According to the method according to the invention, the calibration object is arranged at different calibration positions such that the calibration wave is substantially perpendicularly incident in each case on different regions of a surface of the calibration object. Furthermore, the calibration interferograms produced at the different calibration positions are compared with one another. In other words, different subapertures of the calibration object are irradiated with the calibration wave in the various calibration positions. By evaluating the totality of the various calibration interferograms using a suitable evaluation algorithm, wavefront errors of the calibration wave can be separated from shape errors of the calibration object. 
     According to an embodiment, the various calibration positions of the calibration object are set by a combination of shifting and tilting the calibration object. Furthermore, retrace errors occurring for the different calibration positions can be taken into account by calculation. 
     Furthermore, the invention provides a method for interferometrically determining a shape of a surface of a test object. This method comprises the steps of: providing a multiply-encoded diffractive optical element, which is configured to produce by diffraction from an input wave a test wave, which is directed at the test object and has a wavefront in the form of a free-form surface, and at least one calibration wave, calibrating the diffractive optical element according to one of the previously described embodiments or embodiment variants, capturing a measurement interferogram produced by the superimposition of the reference wave with the test wave after interaction with the surface of the test object, and ascertaining the shape of the surface of the test object by evaluating the measurement interferogram, taking into account the calibration interferogram. 
     According to an embodiment, for the purpose of testing a plurality of similar yet different free-form surfaces, the totality of all test wavefront differences, i.e. the differences between the wavefronts of the test wave and calibration wave, can be kept small, for example by choosing the realized test wavefront difference as the mean value of the test wavefront differences of all the tested free-form surfaces. 
     Furthermore, the invention provides a method for interferometrically determining a respective shape of a multiplicity of surfaces. The surfaces each have the form of a free-form surface, wherein an astigmatic component of a deviation of the respective free-form surface from a best-fit sphere lies between 70% and 90%, in particular between 75% and 85%. The method according to the invention comprises the step of: calibrating interferometrically determined shape measurement results of the surfaces with a uniform calibration surface, wherein a deviation of the uniform calibration surface from a best-fit sphere has an astigmatic component which corresponds to the mean value of the astigmatic components of the multiplicity of surfaces. 
     With regard to the definition of the astigmatic component, reference is made in particular to DE 10 2013 226 668 A1. The measured surfaces are similar to one another due to the mentioned astigmatic component, but differ from one another. In other words, to determine the respective shape of a multiplicity of surfaces by respectively using a calibration surface, only a single calibration surface is required. The determination of the shape measurement results of the surfaces can in each case be carried out analogously to the interferometric measurement method described above, wherein the calibration wave is adapted in each case to the uniform calibration surface mentioned. 
     As mentioned above, a deviation of the uniform calibration surface from a best-fit sphere has an astigmatic component that corresponds to the mean value of the astigmatic components of the multiplicity of surfaces. That means that the astigmatic component assigned to the uniform calibration surface deviates from the mean value mentioned by at most 10%, in particular at most 5%. 
     Furthermore provided, according to the invention, is an optical element for a projection lens of a microlithographic exposure apparatus with an optical surface having a desired shape in the form of a free-form surface and a deviation of the actual shape of the optical surface from the desired shape with a root mean square of at most 100 pm, wherein the desired shape has a maximum deviation from its best-fit sphere in the range of 0.1 mm and 20 mm. In particular, the root mean square of the deviation of the actual shape from the desired shape is at most 20 pm, in particular at most 10 μm. The root mean square is also known by the abbreviation RMS. A microlithographic exposure apparatus comprises a radiation source, an illumination system for irradiating a mask, and a projection lens for imaging mask structures onto a substrate. Furthermore, the desired shape deviates from any rotationally symmetric asphere by at least 5 in particular by at least 10 μm. 
     At the point of maximum deviation from the best-fit sphere, the desired shape has a deviation value which is at least 0.1 mm and at most 20 mm. In other words, the maximum deviation of the desired shape from any sphere is at least 0.1 mm, but the deviation from the best-fit sphere is at most 20 mm. According to an embodiment, the maximum deviation of the desired shape from its best-fit sphere is at least 1 mm, in particular at least 5 mm. According to a further embodiment, the maximum deviation is at most 8 mm. 
     According to an embodiment variant, the best-fit sphere can be understood to mean the sphere in which the maximum deviation from the desired shape is smallest. Alternatively, the best-fit rotationally symmetric reference surface can also be determined by minimizing the root mean square of the deviation or by minimizing the mean deviation. A free-form surface is understood to mean a non-spherical surface without rotational symmetry. 
     According to a further embodiment, the optical element is configured as a mirror element for a microlithographic exposure apparatus in the EUV wavelength range. The EUV wavelength range (extreme ultraviolet wavelength range) is understood to mean the wavelength range below 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.8 nm. 
     The features specified with respect to the aforementioned embodiments, exemplary embodiments or embodiment variants, etc., of the measurement apparatus according to the invention can be correspondingly applied to the calibration method according to the invention or the method according to the invention for interferometrically determining shapes, and vice versa. These and other features of the embodiments according to the invention will be explained in the description of the figures and in the claims. The individual features can be implemented, either separately or in combination, as embodiments of the invention. Furthermore, they can describe advantageous embodiments which are independently protectable and protection for which is claimed if appropriate only during or after pendency of the application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantageous features of the invention will be illustrated in the following detailed description of exemplary embodiments according to the invention with reference to the accompanying schematic drawings. In the drawings: 
         FIG. 1  shows an embodiment of a measurement apparatus for interferometrically determining a shape of an optical surface of a test object with a diffractive optical element for producing a test wave adapted to a desired shape of the surface, 
         FIG. 2  shows the diffractive optical element according to  FIG. 1  with a multiply-encoded diffractive structure pattern for producing calibration waves in addition to the test wave, 
         FIG. 3A  shows the diffractive optical element according to  FIG. 2  during the measurement of a first calibration object with a first calibration wave, 
         FIG. 3B  shows the diffractive optical element according to  FIG. 2  during the measurement of a second calibration object with a second calibration wave, 
         FIG. 3C  shows the diffractive optical element according to  FIG. 2  during the measurement of a third calibration object with a third calibration wave, 
         FIG. 3D  shows the diffractive optical element according to  FIG. 2  during the measurement of the optical surface of the test object, 
         FIG. 3E  shows an illustration of the first calibration wave according to  FIG. 3A  and of the test wave according to  FIG. 3C  when a disadvantageous special case arises, 
         FIG. 4  shows an embodiment of a ring torus, on which a first surface section is marked, 
         FIG. 5  is a sectional view along the line V-V in  FIG. 4 , 
         FIG. 6  is a sectional view along the line VI-VI in  FIG. 4 , 
         FIG. 7  shows the ring torus according to  FIG. 4 , on which a further surface section is marked, 
         FIG. 8  is a sectional view along the line VIII-VIII in  FIG. 7 , 
         FIG. 9  is a sectional view along the line IX-IX in  FIG. 7 , 
         FIG. 10  shows the ring torus according to  FIG. 4  in a cutaway view, 
         FIG. 11  shows an embodiment of an elliptical spindle torus, on which a surface section is marked, 
         FIG. 12  is a sectional view along the line XII-XII in  FIG. 11 , 
         FIG. 13  is a sectional view along the line XIII-XIII in  FIG. 11 , 
         FIG. 14A  shows the diffractive optical element according to  FIG. 1  and a calibration object which is arranged in a first calibration position, 
         FIG. 14B  shows the diffractive optical element according to  FIG. 1  and a calibration object which is arranged in a second calibration position, 
         FIG. 15  shows a top view of the calibration object with an illustration of surfaces irradiated in different calibration positions, 
         FIG. 16A  shows an exemplary desired shape of the optical surface of the test object according to  FIG. 1  in a first sectional plane, an exemplary non-rotationally symmetric shape of a calibration wave, and a circular shape best fitted with respect to the desired shape, 
         FIG. 16B  shows an exemplary desired shape of the optical surface of the test object according to  FIG. 1  in a second sectional plane, an exemplary non-rotationally symmetric shape of a calibration wave, and a circular shape best fitted with respect to the desired shape, and 
         FIG. 17  shows an exemplary embodiment of the optical surface of an optical element of a projection lens of a microlithographic exposure apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     In the exemplary embodiments or embodiments or embodiment variants described below, elements which are functionally or structurally similar to one another are provided with the same or similar reference signs as far as possible. Therefore, for understanding the features of the individual elements of a specific exemplary embodiment, reference should be made to the description of other exemplary embodiments or the general description of the invention. 
     In order to facilitate the description, a Cartesian xyz-coordinate system is indicated in the drawing, from which system the respective positional relationship of the components illustrated in the figures is evident. In  FIG. 1 , the x-direction extends perpendicular to the plane of the drawing into said plane, the y-direction extends upward, and the z-direction extends to the right. 
       FIG. 1  illustrates an interferometric measurement apparatus  10  in an exemplary embodiment according to the invention. The measurement apparatus  10  is suitable for interferometrically determining a shape of a surface  12  of a test object  14  in the form of an optical element. This is accomplished by ascertaining a deviation of the actual shape of the surface  12  from a predetermined, i.e., desired shape. 
     The test object  14  can be designed, for example, in the form of an optical lens element or a mirror. In the case shown, the test object  14  is a concave mirror for EUV lithography, i.e. a mirror that is designed for use in a microlithographic projection exposure apparatus with an exposure wavelength in the extreme ultraviolet (EUV) wavelength range, in particular for use in the projection lens of the projection exposure apparatus. The EUV wavelength range extends to wavelengths below 100 nm and relates in particular to wavelengths of approximately 13.5 nm and/or approximately 6.8 nm. 
     The optical test object  14  is mounted in the measurement apparatus  10  with a holder (not shown in the drawing). The measurement apparatus  10  is configured to measure optical surfaces  12  whose desired shape is a free-form surface. In this text, a free-form surface is to be understood to mean a shape with a deviation from any rotationally symmetric asphere of more than 5 μm, in particular more than 10 μm; furthermore, the free-form surface deviates from any sphere by at least 0.1 mm, in particular at least 1 mm or at least 5 mm. 
     The interferometric measurement apparatus  10  comprises an interferometer  16 , which in turn comprises a light source  18 , a beam splitter  34 , and a capture device  46  in the form of an interferometer camera. The light source  18  produces illumination radiation  20  and for this purpose comprises, for example, a laser  22 , such as a helium-neon laser for producing a laser beam  24 . The illumination radiation  20  has sufficient coherent light to carry out an interferometric measurement. In the case of a helium-neon laser, the wavelength of the illumination radiation  20  is approximately 633 nm. However, the wavelength of the illumination radiation  20  may also have other wavelengths in the visible and non-visible wavelength ranges of electromagnetic radiation. 
     The laser beam  24  is focused by a focusing lens element  26  onto a stop  28  such that a divergent beam  30  of coherent light emanates from the aperture. The wavefront of the divergent beam  30  is substantially spherical. The divergent beam  30  is collimated by a lens group  32 , as a result of which the illumination radiation  20  is produced with a wavefront that is substantially plane in the present case. The illumination radiation  20  propagates along an optical axis  56  of the interferometer  16  and passes through the beam splitter  34 . 
     The illumination radiation  20  is then incident on a Fizeau element  36  with a Fizeau surface  38 . A part of the light of the illumination radiation  20  is reflected as a reference wave  40  at the Fizeau surface  38 . The light of the illumination radiation  20  passing through the Fizeau element  36  propagates further along the optical axis  56  with a plane wavefront  44  as an incoming measurement wave, referred to below as input wave  42 , and is incident on a multiply-encoded diffractive optical element  60 . In other embodiments of the measurement apparatus  10 , the wavefront of the input wave  42  may also be spherical. 
     The diffractive optical element  60  comprises a substrate  62  which is transmissive to the wavelength of the illumination radiation  20  and a diffractive structure pattern  64 , arranged on the substrate  52 , in the form of a computer-generated hologram (CGH). 
     In a first embodiment, the structure pattern  64  is configured in such a way that the input wave  42  is split by diffraction at the structure pattern  64  into a test wave  66  in the form of a free-form surface and at least one calibration wave  68  (see  FIG. 2 ) having a non-rotationally symmetric shape. In general, a non-rotationally symmetric shape is to be understood to mean a shape which deviates from any rotationally symmetric shape by at least 50 μm. According to an embodiment variant, the non-rotationally symmetric shape deviates from any rotationally symmetric asphere by more than 5 μm; furthermore, the non-rotationally symmetric shape can have the above-defined shape of a free-form surface. Various embodiments of the non-rotationally symmetric shape will be described below, which is characterized by further parameters in addition to the general feature of the deviation of at least 50 μm from any rotationally symmetric shape. 
     As an alternative to the interferometric measurement apparatus  10  illustrated in  FIG. 1  with a Fizeau element  36  for producing the reference wave  40 , the reference wave can also be produced at the diffractive optical element  60 , as shown for example in FIG. 1 of DE 10 2015 209 490 A1, and be reflected by a reference mirror. 
     In the embodiment illustrated in  FIG. 2 , two further calibration waves  70  and  72  are produced at the structural pattern  64  in addition to the calibration wave  68 . The calibration waves  70  and  72  may each have a plane or a spherical wavefront. According to an embodiment, one or both of the further calibration waves  70  and  72  each has, like the calibration wave  68 , a non-rotationally symmetric shape, but one that is of a different type than the calibration wave  68 . 
     The test wave  66  is also shown in  FIG. 1  and is used to measure the actual shape of the optical surface  12  of the test object  14 . For this purpose, the test wave  66  has a wavefront that is adapted to the desired shape of the optical surface  12 . As mentioned above, the test wave  66  has the shape of a free-form surface. 
     Before the measurement of the test object  14 , for which the latter is arranged, as illustrated in  FIG. 1 , in the beam path of the test wave  66 , the measurement apparatus  10  is first operated in a calibration mode. In this mode, rather than the test object  14 , a first calibration object  74  is initially arranged on the output wave side with respect to the diffractive optical element  60 , to be precise in the beam path of the first calibration wave  68 , as is illustrated schematically in  FIG. 3A . With the exception of error deviations, the shape of the calibration wave  68  corresponds to the shape of a calibration surface  76  of the calibration object  74 , in other words the shape of the calibration surface  76  of the calibration object  74  is adapted to the abovementioned non-rotationally symmetric desired shape of the calibration wave  68 . This non-rotationally symmetric desired shape thus serves as a desired shape both for the calibration wave  68  and for the calibration object  74 . 
     The calibration wave  68  is incident on the calibration surface  76  of the calibration object  74  and is thereby reflected back on itself. The reflected calibration wave  68  again passes through the diffractive optical element  60  and, after being reflected at the beam splitter  34 , is directed by a lens system  48  of the capture device  46  onto a capture surface  50  of a camera chip  52  of the capture device  46  (cf.  FIG. 1  with the test wave  66  replaced by the calibration wave  68 ). 
     A calibration interference pattern is produced on the capture surface  50  owing to the superposition with the reference wave  40 , from which pattern the deviation of the calibration wave  68  from its desired wavefront is determined by with an evaluation device  54 . However, this takes place on the assumption that any deviations of the calibration object  74  from the desired shape are negligible. The actual wavefront of the calibration wave  68  is thus determined with a high level of accuracy using the calibration object. The deviations of the calibration wave  68  from its desired wavefront are stored as calibration deviations K 1  in the evaluation device  54 . 
     According to an embodiment variant of the determination of the calibration deviations K 1  illustrated in  FIGS. 14A, 14B and 15 , the calibration object  74  is measured at a plurality of different calibration positions. For this purpose, the calibration object  74  is preferably configured in such a way that the diameter of the calibration surface  76  is larger than the diameter of the calibration wave  68  by at least 5%, in particular at least 10%, i.e. when the calibration wave  68  is irradiated centrally, a corresponding non-irradiated peripheral region remains on the calibration surface  76 . 
     To set the different calibration positions, the calibration object  74  is displaced with a combination of tilting and, in particular, shifting, with a positioning device (not shown in the drawing).  FIG. 14A  shows a first calibration position of the calibration object  74 , in which the calibration wave  68  is incident on the calibration surface  76  substantially centrally. The individual rays of the calibration wave  68  are incident on the calibration surface  76  substantially perpendicularly in this case. 
       FIG. 14B  shows a second calibration position which, starting from the first calibration position, is set by tilting the calibration object  74  downward (i.e. tilting with respect to the x-axis), with the result that the region of the calibration surface irradiated by the calibration wave  68  is, for example, shifted upward by at least 5% or at least 10% of the diameter of the calibration wave  68 . Here, the individual rays of the calibration wave  68  are also incident on the calibration surface substantially perpendicularly. In order to ensure this, a suitable shift of the calibration object  74  in the yz-plane takes place in addition to the abovementioned tilting. 
     As explained in more detail below, the cross-sectional shape of the calibration surface  76  and thus also of the wavefront of the calibration wave  70  can be circular or parabolic, for example. With a suitable combination of tilting and shifting, a substantially perpendicular irradiation by the individual rays of the calibration wave  68  can be brought about, both in the case of a circular and a parabolic cross-sectional shape, even in the second calibration position illustrated in  FIG. 14B . In the case of a circular cross-sectional shape, the combination of tilting and shifting is also referred to as “spherization.” 
       FIG. 15  illustrates the respective surface irradiated by the calibration wave  68  in different calibration positions in a plan view of the calibration surface  76 . The central calibration position described with reference to  FIG. 14A  is denoted by P 1 , and the downwardly tilted calibration position described with reference to  FIG. 14B  is denoted by P 2 . By appropriately tilting the calibration object  74  upward, the calibration position denoted by P 3  can be set with a simultaneous shift in the yz-plane. Furthermore, the calibration positions denoted by P 4  and P 5  can be set by tilting the calibration object to the left or to the right (i.e. tilting with respect to the y-axis) while simultaneously shifting the calibration object  74  in the xy-plane. 
     According to an embodiment, corresponding calibration interferograms are recorded for some or all of these further calibration positions P 2  to P 5 , and possibly for further calibration positions, analogously to the calibration position P 1 . Owing to an evaluation of all the recorded calibration interferograms by the evaluation device  54  using a suitable evaluation algorithm, wavefront errors of the calibration wave  68  can now be separated from shape errors of the calibration object  74 . In other words, according to this embodiment, the actual deviations of the calibration object  74  from the desired shape can be taken into account and the actual wavefront of the calibration wave  68  and the calibration deviations K 1  can thus be determined with a further improved accuracy. To further improve the accuracy of the calibration deviations K 1 , the retrace errors occurring for the various calibration positions P 1  to P 5 , i.e. the errors accumulated due to the lens errors in the optical unit of the interferometric measurement apparatus  10  in dependence on the beam path of the test wave  66  through the optical unit, can be taken into account by calculation. 
     According to an embodiment, during the evaluation of the different calibration interferograms, the wavefront sections of the calibration wave  68  that are present in the different calibration positions P 1  to P 5  and illuminate respective subapertures of the calibration surface  76  are combined by stitching methods, as they are known. 
     In addition to the calibration deviations K 1  determined with the calibration wave  68 , further calibration deviations K 2  and K 3  can be determined using the further calibration waves  70  and  72  according to the embodiment illustrated in  FIG. 2 . The directions of propagation of the calibration waves  70  and  72  differ from one another and from the direction of propagation of the calibration wave  68 . The calibration deviations K 2  and K 3  are determined analogously to the determination of the calibration deviations K 1  by arranging the calibration object  78  illustrated in  FIG. 3B  with the calibration surface  80  in the beam path of the calibration wave  70  and the calibration object  82  illustrated in  FIG. 3C  with the calibration surface  84  in the beam path of the calibration wave  72 . 
     By evaluating the ascertained calibration deviations K 1  to K 3 , x- and y-coordinates of distortions of the phase functions of the diffractive structure pattern  64  on the diffractive optical element  60  that produce the calibration waves  68 ,  70  and  72  can now be ascertained. Furthermore, shape and/or profile deviations of the substrate surface of the diffractive optical element  60  having the diffractive structure pattern  64  can be determined from the calibration deviations K 1  to K 3 . From the distortion coordinates thus obtained and shape and/or profile deviations, the distortion in x- and y-coordinates and the shape and/or profile deviations of the entire diffractive structure pattern  64  are then inferred. These deviation data are stored in the evaluation device  54  and are used to correct the test wave  66  during the measurement of the surface shape of the test object  14  that now follows. 
     For this purpose, the test object  14 , as shown in  FIG. 1  and  FIG. 3D , is arranged in the beam path of the test wave  66  in such a way that it is incident on the optical surface  12  in autocollimation and is reflected at it. The reflected wave then runs back through the diffractive optical element  60  into the interferometer  16  as a returning test wave  66 . The returning test wave  66  interferes with the reference wave  40  on the capture surface  50 , thereby producing a test interferogram. The test interferogram is evaluated by with the evaluation device  54  and the deviation of the actual shape of the optical surface  12  from its desired shape is ascertained therefrom. All deviation data previously ascertained during the measurement of the calibration surfaces are taken into account in the evaluation. 
       FIG. 3E  illustrates a disadvantageous special case of matching directions of diffraction between the calibration wave and the test wave at what is referred to as a pole  86 , but which can be avoided due to the degrees of freedom made possible by the configuration according to the invention of the calibration wave with a non-rotationally symmetric shape. This disadvantageous special case is shown in  FIG. 3E  by the simultaneous illustration of the individual rays  66 - 1  to  66 - 6  of the test wave  66  adapted to the shape of the surface  12  of the test object  14  and of the individual rays  68   a - 1  to  68   a - 6  of the calibration wave  68   a , which is adapted to the calibration surface  76   a  of a disadvantageous embodiment of the first calibration object  74   a.    
     As can be seen in  FIG. 3E , the individual rays  66 - 4  (of the test wave  66 ) and  68   a - 4  (of the calibration wave  68 ), in the disadvantageous embodiment shown, which emanate from the point of the diffractive structure pattern  64   a  of the associated diffractive optical element  60   a  that is referred to as pole  86 , travel along the same path. At the pole  86 , the difference between the two associated phase functions of the associated diffractive structure pattern  64   a  has a zero, which results in relatively large groove spacings in the diffractive structure pattern  64   a . These in turn can adversely affect the diffraction intensity. Put simply, this case leads to a gap in the diffractive structure pattern  64   a . As already mentioned above, such a pole can be avoided due to the degrees of freedom in the configuration of the calibration wave  68  that are available for the calibration wave  68  through the use of a non-rotationally symmetric shape, in particular an astigmatic shape. 
     According to an embodiment, for the interferometric measurement of a respective shape of a multiplicity of optical surfaces  12 , which are similar in that their desired shapes, each of which has the form of a free-form surface, have a respective astigmatic component of a deviation of the respective free-form surface from a best-fit sphere with a value of between 70% and 90%, a uniform calibration surface can be used to calibrate the shape measurement results. In this case, the uniform calibration surface can be configured in such a way that a deviation of the calibration surface from a best-fit sphere has an astigmatic component which corresponds to the mean value of the astigmatic components of the multiplicity of optical surfaces. 
     Various embodiments of the abovementioned, non-rotationally symmetric shape of the calibration wave  68  and of the associated calibration object  74  will be illustrated below with reference to  FIGS. 4 to 13 . As mentioned above, the non-rotationally symmetric shape is characterized in that it deviates from any rotationally symmetric shape by at least 50 μm. The embodiments of the non-rotationally symmetric shape illustrated below can be used in further calibration waves produced at the diffractive structure pattern  64  of the diffractive optical element  60 , such as the calibration waves  70  and  72 . 
     In all of the embodiments shown in  FIGS. 4 to 13 , the non-rotationally symmetric shape of the calibration wave  68 , hereinafter referred to as  68   f , corresponds to the shape of a section of a surface  89  of a solid of revolution  88  illustrated in  FIG. 10 . This solid of revolution  88  is formed by rotating a surface of revolution  90 , which is symmetrical to an axis of symmetry  92 , about an axis of rotation  94 . In the embodiments shown, the axis of symmetry  92  is arranged parallel to the axis of rotation  94 . In alternative embodiments, the axis of symmetry  92  can also be aligned non-parallel with respect to the axis of rotation  94 . The surface of revolution  90 , which can also be described as “mirror-symmetric” because of its axial symmetry, can, for example, be configured to be rotationally symmetric and thus circular (cf. surface of revolution  90   a ) or non-rotationally symmetric, such as elliptical (cf. surface of revolution  90   b  with the semi-major axis a perpendicular to the axis of rotation  94  and surface of revolution  90   c  with the semi-minor axis a perpendicular to the axis of rotation  94 ). 
     The distance between the axis of rotation  94  and the axis of symmetry  92  is the radius R of the solid of revolution  88 . The radius R can be greater than, equal to or smaller than the radius r of the surface of revolution  90   a  or than the semi-axis of the surface of revolution  90   b  or  90   c  perpendicular to the axis of symmetry  92 . In the case of the circular surface of revolution  90   a , the ring torus shown in  FIG. 10  results for R&gt;r for the solid of revolution  88 , what is known as a horn torus results for R=r, and what is known as a spindle torus results for R&lt;r. In the case of the circular surface of revolution  90   a , R≠0 applies according to the invention. 
     In the case of the elliptical surfaces of revolution  90   b  and  90   c , analogous solids result, which are referred to here as “elliptical ring torus” for R&gt;a, “elliptical horn torus” for R=a, and “elliptical spindle torus” for R&lt;a. In the case of the elliptical surfaces of revolution  90   b  or  90   c , the case R=0 is also permitted. An example of an “elliptical spindle torus” with R=0 is shown in  FIG. 11 . 
     In  FIGS. 4 and 7 , two different sections from the surface  89  of the solid of rotation  88  shown in  FIG. 10  in the form of a ring torus are selected by way of example in each case as the non-rotationally symmetric shape  68   f  of the calibration wave  68 . This shape  68   f  is in each case mirror-symmetrical with respect to a mirror plane extending through the line designated V-V in  FIG. 4  or the line designated VIII-VIII in  FIG. 7 . In the embodiment according to  FIG. 4 , the selected section lies on the outside of the ring torus and thus has a convex shape. 
       FIG. 5  shows a sectional view along the line V-V and  FIG. 6  shows a sectional view along the line VI-VI each in the image of the non-rotationally symmetric shape  68   f  shown in the right-hand portion of  FIG. 4 . In other words,  FIGS. 5 and 6  show cross sections through an embodiment of the wavefront of the calibration wave  68  along cross-sectional surfaces arranged perpendicularly to one another, to be precise once along the yz-plane and once along the xz-plane. In each of these two cross-sectional surfaces, the shape  68   f  of the wavefront has a circular shape and hence a curved shape. The circular shapes have the same direction of curvature and differ in terms of their opening parameters in the form of their respective radius r 1  or r 2 . 
     In the embodiment according to  FIG. 7 , the selected section lies on the inside of the ring torus and thus has a saddle shape.  FIG. 8  illustrates a sectional view along the line VIII-VIII and  FIG. 9  illustrates a sectional view along the line IX-IX in the image of the non-rotationally symmetric shape  68   f  shown in the right-hand portion of  FIG. 7 . 
     Analogously to  FIGS. 5 and 6 ,  FIGS. 8 and 9  show cross sections through an embodiment of the wavefront of the calibration wave  68  according to  FIG. 7  along cross-sectional surfaces arranged perpendicularly to one another, to be precise once along the yz-plane and once along the xz-plane. Here, too, the wavefront has a circular shape and thus a curved shape in each of these two cross-sectional surfaces. The circular shapes have different directions of curvature and thus already different opening parameters, specifically radii with different signs (r 1 &gt;0 or r 2 &lt;0), and furthermore, the absolute values of the two radii also differ (|r 1 |&lt;|r 2 |). 
     In the embodiment of the solid of revolution  88  shown in  FIG. 11  in the form of an “elliptical spindle torus” with R=0, the section  68   f  selected on the front side of the solid of revolution  88  has, analogous to the section according to  FIG. 5 , a convex shape, but with parabolic cross-sectional profiles. The shape of the section  68   f  is mirror-symmetrical with respect to a mirror plane extending through the line designated XII-XII. 
       FIG. 12  shows a sectional view along the line XII-XII and  FIG. 13  shows a sectional view along the line XIII-XIII each in the image of the non-rotationally symmetric shape  68   f  shown in the right-hand portion of  FIG. 11 . In other words,  FIGS. 12 and 13  show cross sections through an embodiment of the wavefront of the calibration wave  68  along cross-sectional surfaces arranged perpendicularly to one another, to be precise once along the yz-plane and once along the xz-plane. In each of these two cross-sectional surfaces, the shape  68   f  of the wavefront is approximated in each case to the vertex region  98  of a parabolic shape  96 - 1  or  96 - 2  and thus has a curved shape. 
     The parabolic shape  96 - 1  shown in  FIG. 12  can be described as follows: z=−a 1 y 2 +b 1 , and the parabolic shape  96 - 2  shown in  FIG. 13  can be described as follows: z=−a 2 x 2 +b 2 , wherein in the present case: b 1 =0 and b 2 =0. The opening parameters of the parabolic shapes in the shape of the parabolic openings a 1  and a 2  differ from each other (a 2 &lt;a 1 ). The wavefront of the calibration wave  68  for the embodiment according to  FIG. 11  thus has an astigmatic shape. 
     In  FIGS. 16A and 16B , an exemplary desired shape  12   a  of the optical surface  12  is shown together with the non-rotationally symmetric shape  68   f  of the calibration wave  68  and a best-fit circular shape  100  in the respective sectional view of  FIG. 12  or  FIG. 13 , i.e. in two mutually orthogonal sectional planes. For illustration purposes, as in  FIGS. 12 and 13 , a shape approximated to the respective vertex region  98  of the parabolic shapes  96 - 1  and  96 - 2  is selected as the non-rotationally symmetric shape  68   f  of the calibration wave  68 . The best-fit circular shapes  100  drawn in  FIGS. 16A and 16B  each represent the sectional view through a sphere that is best fitted to the desired shape  12   a  and thus have the same radius. 
     If one now considers the swing  102 - 1  or  102 - 2 , i.e. the maximum deviation, between the desired shape  12   a  of the optical surface  12  and the shape  68   f  of the calibration wave  68  in the two sectional views of  FIGS. 16A and 16B , it becomes clear that said swing is significantly reduced compared with the corresponding swing  102   a - 1  or  102   a - 2  between the desired shape  12   a  and the best-fit circular shape  100 . Due to this reduced swing, the use of the calibration wave  68  according to the invention with the non-rotationally symmetric shape  68   f  enables a significant improvement in the achievable calibration accuracy of the diffractive optical element  60  compared with the use of a spherical calibration wave as is customary in the prior art. 
     In  FIG. 17 , the optical surface  12  of an optical element of the type designated as test object  14  in  FIG. 1  is illustrated schematically in the yz-plane. In particular, the optical element  14  is an element of a projection lens of a microlithographic exposure apparatus, in particular a mirror element for the EUV wavelength range. 
     In addition to the optical surface  12  in its actual shape, the desired shape  12   a  of the optical surface and the sectional view of the sphere  104  that is best fitted to the desired shape  12   a  are shown in  FIG. 17  analogously to  FIG. 16A . The deviation of the actual shape of the optical surface  12  from the desired shape  12   a , described by a two-dimensional deviation D(x,y), is schematically illustrated in a greatly enlarged fashion, wherein x and y denote the coordinates on the surface  12 . 
     The root mean square of the deviation D(x,y) ascertained over the entire optical surface  12  is at most 100 μm, in particular at most 20 pm. The desired shape  12   a , by contrast, has a maximum deviation Δ compared with the best-fit sphere  104 , which is in the range from 0.1 mm and 20 mm, i.e. the maximum deviation Δ is at least 0.1 mm and at most 20 mm. In particular, the lower value of the range is 1 mm or 5 mm, and the upper value can be in particular 8 mm. 
     In a cross section of the optical surface  12  of the optical element  14  in the xz-plane, the optical surface  12  extends, aside from the deviation described by D(x, y), along the desired shape  12   a  shown in  FIG. 16B . The maximum deviation of the desired shape  12   a  from the best-fit sphere  104  is smaller in this sectional plane than the deviation Δ in the sectional plane shown in  FIG. 17 . In this embodiment, the maximum deviation in the sectional plane according to  FIG. 17  is greater than in all other possible sectional planes. The maximum deviation Δ according to  FIG. 17  is therefore regarded as the maximum deviation Δ with respect to the best-fit sphere, as already mentioned above. 
     Since the desired shape  12   a  in the xz-plane ( FIG. 16A ) differs significantly from the desired shape  12   a  in the yz-plane ( FIG. 16B ), the desired shape  12   a  deviates significantly in the three-dimensional form from any rotationally symmetric asphere. According to the present embodiment, the maximum deviation of the desired shape  12   a  from any rotationally symmetric asphere is at least 5 in particular at least 10 μm. 
     The above description of exemplary embodiments, embodiments or embodiment variants should be understood to be by way of example. The disclosure effected thereby firstly enables the person skilled in the art to understand the present invention and the advantages associated therewith, and secondly encompasses alterations and modifications of the described structures and methods that are also obvious in the understanding of the person skilled in the art. Therefore, all such alterations and modifications, insofar as they fall within the scope of the invention in accordance with the definition in the accompanying claims, and equivalents are intended to be covered by the protection of the claims. 
     LIST OF REFERENCE SIGNS 
     
         
           10  Interferometric measurement apparatus 
           12  Optical surface 
           12   a  Desired shape 
           14  Test object 
           16  Interferometer 
           18  Light source 
           20  Illumination radiation 
           22  Laser 
           24  Laser beam 
           26  Focusing lens element 
           28  Stop 
           30  Divergent beam 
           32  Lens group 
           34  Beam splitter 
           36  Fizeau element 
           38  Fizeau surface 
           40  Reference wave 
           42  Input wave 
           44  Plane wavefront 
           46  Capture device 
           48  Lens system 
           50  Capture surface 
           52  Camera chip 
           54  Evaluation device 
           56  Optical axis 
           60  Diffractive optical element 
           60   a  Diffractive optical element in a disadvantageous embodiment 
           62  Substrate 
           64  Diffractive structure pattern 
           64   a  Diffractive structure pattern in a disadvantageous embodiment 
           66  Test wave 
           66 - 1  to  66 - 6  Individual rays of the test wave  66   
           68  Calibration wave 
           68   a - 1  to  68   a - 6  Individual rays of the calibration wave  68   a    
           68   f  Non-rotationally symmetric shape 
           70  Calibration wave 
           72  Calibration wave 
           74  First calibration object 
           74   a  First calibration object in a disadvantageous embodiment 
           76  Calibration surface 
           76   a  Calibration surface of the calibration object  76   
           78  Second calibration object 
           80  Calibration surface 
           82  Third calibration object 
           84  Calibration surface 
           86  Pole 
           88  Solid of revolution 
           89  Surface 
           90 ,  90   a ,  90   b ,  90   c  Surface of revolution 
           92  Axis of symmetry 
           94  Axis of rotation 
           96 - 1 ,  96 - 2  Parabolic shape 
           98  Vertex region 
           100  Best-fit circular shape 
           102 - 1 ,  102 - 2  Swing when using a non-rotationally symmetric calibration wave 
           102   a - 1 ,  102   a - 2  Swing when using a spherical calibration wave 
           104  Best-fit sphere