METHOD FOR CALIBRATING A MEASURING APPARATUS

Abstract

A method for calibrating a measuring device (10) for interferometrically determining a shape of an optical surface (12) of an object under test (14). The measuring device includes a module plane (32) for arranging a diffractive optical test module (30) which is configured to generate a test wave (34) that is directed at the optical surface and that has a wavefront at least approximately adapted to a target shape (60) of the optical surface. The method includes: arranging a diffractive optical calibration module (44) in the module plane for generating a calibration wave (80), acquiring a calibration interferogram (88) generated using the calibration wave in a detector plane (43) of the measuring device, and determining a position assignment distribution (46) of points (52) in the module plane to corresponding points (54) in the detector plane from the acquired calibration interferogram.

Claims

1. A method for calibrating a measuring device for interferometrically determining a shape of an optical surface of an object under test, wherein the measuring device comprises a module plane for the arrangement of a diffractive optical test module which is configured to generate a test wave that is a directed at the optical surface and that has a wavefront at least approximately adapted to a target shape of the optical surface, said method comprising: arranging a diffractive optical calibration module in the module plane for generating a calibration wave, acquiring a calibration interferogram generated with the calibration wave in a detector plane of the measuring device, and determining a position assignment distribution of points in the module plane to corresponding points in the detector plane from the acquired calibration interferogram.

2. The method as claimed in claim 1, further comprising determining a distortion of an optical unit arranged between the module plane and the detector plane by determining the position assignment distribution.

3. The method as claimed in claim 1, wherein the calibration module has a diffractive structure pattern with a configuration known in advance and the configuration known in advance is used during the determination of the position assignment distribution.

4. The method as claimed in claim 3, wherein the diffractive structure pattern of the calibration module comprises a two-dimensionally modulated optical property.

5. The method as claimed in claim 4, wherein the two-dimensionally modulated optical property brings about a phase modulation in a wave that interacts with the diffractive structure pattern.

6. A method for determining a shape of an optical surface of an object under test, comprising: carrying out the calibration method as claimed in claim 1 for determining the position assignment distribution of points in the module plane to corresponding points in the detector plane, arranging the test module in the module plane for generating the test wave that is directed at the optical surface, and recording a test interferogram generated with the test wave.

7. The method as claimed in claim 6, further comprising determining the shape of the optical surface from the test interferogram using the position assignment distribution.

8. The method as claimed in claim 7, further comprising implementing the determination of the shape of the optical surface in accordance with a position assignment distribution of points on the optical surface of the object under test to points in the module plane.

9. The method as claimed in claim 6, wherein the test wave generated by the test module has a rotationally symmetric wavefront which is adapted to the target shape of the optical surface such that a maximum deviation of the test wave from the target shape is between 100 nm and 10 μm.

10. The method as claimed in claim 6, wherein the test module comprises a diffractive test pattern configured to generate the test wave and comprises adjustment structures for determining the lateral position of the test module in the detector plane.

11. The method as claimed in claim 10, wherein the calibration module comprises diffractive calibration patterns which each have, at least in sections, an identical configuration, and the adjustment structures are formed, at least in part, by the calibration pattern of the test module.

12. The method as claimed in claim 10, wherein the calibration pattern of the test module surrounds a diffractive test pattern configured to generate the test wave.

13. An optical element comprising an optical surface, wherein the optical surface is assigned a non-rotationally symmetric target shape, a maximum deviation of which from a best-adapted rotationally symmetric reference surface is between 100 nm and 100 μm and wherein a maximum deviation of an actual shape of the optical surface from the target shape is no more than 1/1000 of the maximum deviation of the target shape from the rotationally symmetric reference surface.

14. The optical element as claimed in claim 13, wherein the rotationally symmetric reference surface has a deviation of at least 100 μm from any spherical surface.

15. The optical element as claimed in claim 13, wherein the maximum deviation of the actual shape of the optical surface from the target shape is no more than 1/10 000 of the maximum deviation of the target shape from the rotationally symmetric reference surface.

16. The optical element as claimed claim 13, wherein the maximum deviation of the actual shape of the optical surface from the target shape is no more than 0.2 nm.

17. A diffractive optical element comprising a diffractive structure pattern which has a grating-shaped basic structure having a modulation such that a determination of the diffractive structure pattern with an interferogram recorded by a detector is performed with an accuracy that is better than a pitch between two adjacent pixels of the detector, wherein the interferogram is generated by superimposing a wave extending across the diffractive structure pattern with a reference wave.

18. The diffractive optical element as claimed in claim 17, wherein the modulation is configured such that the determination of a position of the diffractive structure pattern is performed with an accuracy that is better than one tenth of the pitch between the two adjacent pixels of the detector.

19. The diffractive optical element as claimed in claim 17, wherein the modulation of the diffractive structure pattern is a phase modulation.

20. The diffractive optical element as claimed in claim 17, wherein the modulation is periodic.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The above and further advantageous features of the invention are illustrated in the following detailed description of exemplary embodiments according to the invention with reference to the accompanying schematic drawings. In the drawings:

[0044] FIG. 1 shows an embodiment of a measuring device for interferometrically determining the shape of an optical surface of an object under test, comprising a diffractive optical test module that is arranged in a module plane and is configured to generate a test wave which is radiated onto the optical surface,

[0045] FIG. 2 shows the measuring device according to FIG. 1 when carrying out a calibration method, within the scope of which a calibration module is arranged in the module plane instead of the test module,

[0046] FIG. 3 shows a more detailed illustration of an optical surface of a test object, and of target faces and reference surfaces, which are assigned to the optical surface, and

[0047] FIG. 4 shows an illustration of the data flow according to an embodiment of measuring the optical surface in the form of a nano free-form surface.

DETAILED DESCRIPTION

[0048] 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.

[0049] 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 y-direction extends perpendicularly to the plane of the drawing into this plane, the x-direction extends toward the right, and the z-direction extends upward.

[0050] FIG. 1 depicts an exemplary embodiment of a measuring device 10 for interferometrically determining the shape of an optical surface 12 of an object under test 14. The measuring device 10 can be used, in particular, to determine a deviation of the actual shape of the surface 12 from a target shape. By way of example, a non-spherical surface may be provided as the surface 12 to be measured.

[0051] The measuring device 10 is particularly suitable for measuring a surface 12 of a mirror of a microlithographic projection lens. This surface 12 can be configured for reflecting extreme ultraviolet (EUV) radiation, that is to say radiation with a wavelength of less than 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.8 nm. The non-spherical surface of the mirror can be what is known as a “nano free-form surface”, for example.

[0052] FIG. 3 illustrates the optical surface 12 in the form of such a “nano free-form surface”, shown in FIG. 1, in more detail. A target shape 60 is assigned to the optical surface 12 and has a so-called free-form deviation distribution h(x,y) from a best-fitted rotationally symmetric reference surface 64, the maximum value Δ.sub.1 of this distribution ranging between 100 nm and 100 μm, in particular between 100 nm and 10 μm. Such a deviation 68 of the target shape 60 from the best-fitted rotationally symmetric reference surface 64 with a deviation value of Δ.sub.1 is plotted in FIG. 3 in exemplary fashion. In the present illustration, this represents the greatest deviation of the target shape 60 from the reference surface 64, that is to say the specified maximum deviation is Δ.sub.1. According to one embodiment, the best-fitted rotationally symmetric reference surface 64 can be understood to mean the rotationally symmetric face whose maximum deviation Δ.sub.1 from the target shape 60 is the smallest. Alternatively, the best-fitted rotationally symmetric reference surface 64 can also be determined by minimizing the root mean square—abbreviated RMS—of the deviation Δ.sub.1 (√{square root over (<|Δ.sub.1|.sup.2>)}) or by minimizing the mean deviation <|Δ.sub.1|>.

[0053] The actual shape of the optical surface 12, in turn, typically has deviations from the target shape 60 on account of manufacturing errors. Such a deviation 62 is plotted in FIG. 3 in exemplary fashion. In this case, the deviation 62 represents a maximum deviation, with the value δ, of the actual shape of the optical surface 12 from the target shape 60. What should be noted here is that, for illustrative purposes, the deviation 62 is illustrated in FIG. 3 with a deflection similar to that of the deviation 68, but, in reality, δ is typically substantially smaller than Δ.sub.1. By way of example, δ can be approximately 0.2 nm, in particular approximately 0.05 nm. Therefore, a “nano free-form surface” can also be defined by virtue of its actual form 12 having a maximum deviation from its best-fitted rotationally symmetric reference surface 60 of between 100 nm and 10 μm.

[0054] The rotationally symmetric reference surface 64 can be specified by a minimum value Δ.sub.2 for its maximum deviation 72 from a best-fitted spherical surface 70. The best-fitted spherical surface should be understood to mean the sphere whose maximum deviation from the rotationally symmetric reference surface 60 is the smallest. The specification of the maximum deviation by the minimum value Δ.sub.2 can therefore also be related to the maximum deviation 72 from any spherical surface, that is to say by virtue of specifying that the rotationally symmetric reference surface 60 has a deviation of at least Δ.sub.2 in relation to any spherical surface. According to one embodiment, Δ.sub.2 may also be approximately 0 μm; in this case, the rotationally symmetric reference surface 64 is a sphere.

[0055] The measuring device 10 illustrated in FIG. 1 comprises a light source 16 for providing sufficiently coherent measurement radiation as an input wave 18 in the form of an expanded wave. In this case, the light source 16 may have, for example, a laser with a beam-expanding optical unit. By way of example, a frequency doubling Nd:Yag laser with a wavelength of approximately 532 nm or a helium-neon laser with a wavelength of approximately 633 nm can be provided to this end. However, the measurement radiation may also have a different wavelength in the visible or non-visible wavelength range of electromagnetic radiation. The light source 16 constitutes merely one example of a light source that may be used for the measuring arrangement 10.

[0056] The measurement radiation provided by the light sources 16 leaves the light source 16 as an input wave 18 with a spherical wavefront and propagates divergently. In this case, the input wave 18 initially passes through a beam splitter 20 and subsequently strikes a collimator 22 for converting the wavefront of the input wave 18 into a planar wavefront.

[0057] Next, a reference element 24 in the form of a Fizeau element with a Fizeau face 26 for splitting off a reference wave 28 in reflection is situated in the beam path of the input wave 18. The reference wave 28 returns in the direction of the collimator 22 and has a plane wavefront 29, as illustrated in FIG. 1.

[0058] The portion of the input wave 18 passing through the reference element 24 strikes a diffractive optical test module 30 in the form of a computer-generated hologram (CGH) with diffractive structures 57. The diffractive structures 57 comprise a diffractive test pattern 58 and a diffractive adjustment pattern 59 arranged in the surround of the test pattern 58. The diffractive adjustment pattern 59 can surround the test pattern 58 and have a ring-shaped form in this case, as shown in FIG. 1. Alternatively, it may only partly surround the test pattern 58 and, for instance, have the form of a ring section. As shown in FIG. 1, the diffractive test pattern 58 may be circular or else have any other form. In general, the adjustment pattern 59 can be configured such that it has a cutout, at least in a central region, for the diffractive test pattern 58 that generates the test wave 34. Alternatively, the cutout may also be arranged in an off-centered region. The diffractive optical test module 30 is arranged in a module plane 32, specifically it is arranged such that the diffractive structures 57 of the CGH are located in the module plane 32.

[0059] The diffractive test pattern 58 of the diffractive optical test module 30 is used to approximate the wavefront of the input wave 18 to the target shape 60 of the optical surface 12 to be measured that is illustrated in FIG. 3 and to consequently generate a test wave 34. In the aforementioned case where the optical surface 12 is configured as a so-called “nano free-form surface”, the wavefront of the test wave 34 is adapted to the rotationally symmetric reference surface 64, from which the target shape 60 of the optical surface 12 deviates between 100 nm and 10 μm in accordance with the aforementioned embodiment.

[0060] The diffractive adjustment pattern 59 is configured to reflect the radiation of the input wave 18 incident thereon in Littrow reflection, that is to say back onto itself. The wave that is generated in this case is referred to as an adjustment wave 74. By way of example, the diffractive adjustment pattern 59 can be configured as a two-dimensionally cosine-modulated phase grating, as illustrated in a detailed view 85 in FIG. 1.

[0061] The test wave 34 is reflected at the optical surface 12 of the test object 14 and returns to the diffractive optical test module 30 as a returning test wave 34r. In this case, the returning test wave 34r has a wavefront 35a, impressed in which are both the deviation 62 of the optical surface 12 from the target shape 60 and the deviation 68 of the target shape 60 from the reference surface 64. After passing through the diffractive optical test module 30 again, the returning test wave 34r once again has a plane wavefront, in which, however, the deviations 62 and 68 are likewise impressed.

[0062] After passage through the reference element 24, the test wave 34r returns together with the reference wave 28 to the beam splitter 20. The beam splitter 20 guides the combination of the returning measurement wave 34r and the reference wave 28 out of the beam path of the input wave 18. Further, the measuring device 10 contains an acquisition apparatus 36 with a stop 38, an eyepiece 40 and an interferometer camera 42 for acquiring a test interferogram 41 generated by superimposing the reference wave 28 with the test wave 34r in a detector plane 43 or acquisition plane of the interferometer camera 42.

[0063] An evaluation apparatus 56 of the measuring device 10 determines the actual shape of the optical surface 12 of the test object 14 from one or more test interferograms 41 acquired by the interferometer camera 42. To this end, the evaluation apparatus 56 has a suitable data processing unit and uses corresponding calculation methods known to a person skilled in the art. According to the invention, the evaluation apparatus 56 uses a position assignment distribution T.sub.2 (cf. reference sign 46) of points 52 in the module plane 32 to corresponding points 54 in the detector plane 43, which position assignment distribution is determined using a calibration module 44 and is explained in more detail below with reference to FIG. 2, and, in particular, a further position assignment distribution T.sub.1 (cf. reference sign 48) of points 50 on the surface 12 of the object under test 14 to corresponding points 52 in the module plane 32 when evaluating the interferograms. An exemplary embodiment of the evaluation taking place in the evaluation apparatus 56 with regard to one or more of the test interferograms 41 for determining the actual shape of the optical surface 12 is described below with reference to the fourth process phase P4 in FIG. 4.

[0064] Alternatively or additionally, the measuring device 10 can contain a data memory or an interface to a network to make possible a determination of the surface shape using the test interferogram or interferograms 41 that is or are stored or transmitted via the network by way of an external evaluation unit.

[0065] By combining the position assignment distributions T.sub.1 and T.sub.2, the respective positions of the points 50, which are distributed like a grid over the surface 12 of the object under test 14, are assigned to the positions on the detector plane 43 on which the points 50 are imaged during the measurement operation of the measuring device 10 illustrated in FIG. 1. The corresponding positions in the detector plane 43 are characterized by the aforementioned points 54. Here, the position assignment distribution T.sub.1 specifies the assignment of the points 50 on the surface 12 of the object under test 14 to the corresponding points 52 in the module plane 32 while the position assignment distribution T.sub.2 in turn specifies the assignment of the points 52 to the corresponding points 54 in the detector plane 43.

[0066] According to an embodiment, the position assignment distribution T.sub.1 is determined directly from the design data of the diffractive optical test module 30 using an optics design model. Here, the accuracy of the production of the diffractive test pattern 58 and the adjustment of the object under test 14 in relation to the test module 30 being sufficiently accurate is initially ensured. The position assignment distribution T.sub.1 can be calculated externally and, as illustrated in FIG. 1, be transmitted to the evaluation apparatus 56. Alternatively, the position assignment distribution T.sub.1 can also be calculated directly from the design data of the test pattern 58 by the evaluation apparatus 56.

[0067] The position assignment distribution T.sub.2 reflects lateral imaging aberrations which are generated by the adjustment of the light source and the optical unit arranged between the module plane 32 and the detector plane 43, that is to say by the collimator 22, the beam splitter 20 and the eyepiece 40 in the present case. These lateral imaging aberrations may comprise imaging scale, orthogonality and further orders of lateral imaging aberrations which are referred to as distortion. For illustrative purposes, the arrangement of the points 54 in the detector plane 43, shown in FIG. 1, has a distortion in relation to the points 52 that are arranged in a regular grid in the module plane 32. As already mentioned above, the position assignment distribution T.sub.2 is determined by measurement using the calibration module 44.

[0068] In the edge region of the test interferogram 41 corresponding to the ring-shaped adjustment pattern 59, the reference wave 28 is superimposed with the adjustment wave 74. The information contained in this region of the test interferogram 41 can firstly be used to adjust the test module 30 before the acquisition of the test interferogram or interferograms 41, used to determine the shape of the surface 12 of the object under test 14, in the module plane 32 in accordance with the positioning of the calibration module 44 present when determining the position assignment distribution T.sub.2. Secondly, the information contained in the edge region of the test interferogram 41 can also be used to adapt by calculation the position assignment distribution T.sub.2 determined using the calibration module 44 to the positioning of the test module 30 present when acquiring the test interferogram or interferograms 41 for determining the shape of the surface 12 of the object under test 14.

[0069] To determine the position assignment distribution T.sub.2, the calibration module 44 is arranged in the module plane 32 of the measuring device 10 according to FIG. 1 instead of the test module 30, and so the arrangement illustrated in FIG. 2 arises. Like the optical test module 30, the calibration module 44 is configured as a computer-generated holograms (CGH) and comprises diffractive structures 82 in the form of a diffractive structure pattern, which is also referred to as diffractive calibration pattern 84 below. The diffractive calibration pattern 84 in the present embodiment is embodied in the form of a Littrow grating with a two-dimensional phase modulation flush over the entire beam cross section of the input wave 18. FIG. 2 shows, in a detailed view 85 of the calibration pattern, an embodiment of the phase modulation in the form of a two-dimensional cosine grating.

[0070] Expressed differently, the calibration pattern 84 comprises a diffractive structure pattern which has a grating-shaped basic structure, wherein the grating-shaped basic structure has a phase modulation in turn. The phase modulation is configured such that this facilitates a determination of the position of the diffractive structure pattern in the module plane 32 on the basis of an interferogram, also referred to as calibration interferogram 88 below, recorded by the interferometer camera 42, which is also referred to as a detector, with an accuracy that is better than a distance between adjacent pixels of the detector. Hence, the accuracy of determining the position is better than the pixel resolution of the detector. According to further embodiments, the accuracy is better than one fifth, better than one tenth or better than one twentieth of the distance between two adjacent pixels of the detector.

[0071] By modulating the diffractive structure pattern of the calibration pattern 84, a variation, known in advance, in the wavefront of the wave extending across the diffractive structure pattern, that is to say of the calibration wave 80, is generated, which variation is generated at the calibration pattern 84 by the reflection of the input wave18 back onto itself while having a two-dimensional phase signature 86 known in advance impressed thereon. As illustrated in FIG. 2, the impressed two-dimensional phase signature 86 may have a checkerboard-like structure, for example. However, unlike what the expression could be construed to imply, such a checkerboard-like structure does not have a modulation with constant regions and discontinuities therebetween. Rather, the curve of the modulation is continuous with finite gradients. Thus, in particular, the structure can be approximated to a twice cosine-modulated phase grating. Hence, a corresponding two-dimensional structure is generated in the calibration interferogram recorded by the interferometer camera 42. During the evaluation of the interference pattern, this two-dimensional structure can be fitted by way of suitable algorithms. On account of the prior knowledge of the two-dimensional structure, the position of the diffractive structure pattern can be determined with the aforementioned resolution, which exceeds the pixel resolution of the detector.

[0072] Expressed differently, the diffractive calibration pattern 84 is configured as a phase grating for reflecting the input wave 18 back onto itself while impressing a two-dimensional phase signature 86. Alternatively, the diffractive calibration pattern 84 can also be configured as a modulated intensity grating. What is known as a calibration wave 80 is generated by the Littrow reflection of the input wave at the diffractive calibration pattern 84.

[0073] By way of example, if a very precise electron beam writer is used to produce a calibration pattern 84, the lateral positioning of the structure elements, which bring about the modulation, on the calibration module 44 can be implemented with great accuracy. The accuracy when carrying out a correction of scale and orthogonality, which may be determined by way of a separate measurement of the placement using equipment that is conventional in lithography mask production, may be better than 100 nm. Hence, the calibration pattern 84 establishes a precise scale in the module plane 32. This scale is now measured with great precision by evaluating one or more calibration interferograms 88 that were generated by the superimposition of the reference wave 28 generated by reflection at the Fizeau face 26 with the calibration wave 80 in the detector plane 43. The evaluation is implemented in an evaluation apparatus 90 by measurement of the position of the peaks and valleys of the corresponding calibration interferogram 88 and by appropriate assignment to the corresponding structure elements of the calibration pattern 84. The result of this evaluation then is the position assignment distribution T.sub.2.

[0074] The position assignment distribution T.sub.2 preferably has an accuracy that is better than the resolution of the interferometer camera. Expressed differently, in relation to the pixel resolution of the interferometer camera 42, the position assignment distribution T.sub.2 facilitates a sub-pixel accurate position assignment of coordinates of the test module 30 in camera coordinates.

[0075] According to one embodiment, the ring-shaped adjustment pattern 59 of the diffractive optical test module 30 is configured analogously to the calibration pattern 85, that is to say the adjustment pattern 59 likewise has a phase modulation for a highly accurate determination of position. According to one embodiment variant, the adjustment pattern 59 is structurally identical to the calibration pattern 85 of the calibration module 44, that is to say both patterns for example comprise identical two-dimensionally cosine-modulated phase gratings. Preferably, the edge region of the calibration pattern 85 of the calibration module 44 therefore identically corresponds to the ring-shaped adjustment pattern 59 of the test module 30. This allows the test module 30 to be placed exactly at the identical lateral position in the module plane 32. The lateral assignment between positions in the detector plane 43 and in the module plane 32 from the measurement with the calibration module 44 is accordingly also valid during the measurement with the test module 30.

[0076] FIG. 4 illustrates the data flow according to an embodiment according to the invention of measuring the optical surface 12 in the form of a nano free-form surface. In a first process phase P1, the position assignment distribution T.sub.1 in respect of the assignment of the points 50 on the surface 12 of the object under test 14 to the corresponding points 52 in the module plane 32 is determined using an optics design model from the design data of the diffractive test pattern 58 of the test module 30. Furthermore, the free-form deviation distribution h(x,y) which is denoted by reference sign 92 and which—as already mentioned above—specifies the deviation of the target shape 60 from the rotationally symmetric reference surface 64 is determined from the specifications underlying the design of the diffractive test pattern 58. In a second process phase P2, when the calibration module 44 is arranged in the module plane 32 according to FIG. 2, the measuring device 10 is used to determine the position assignment distribution T.sub.2 in relation to the assignment of the points 52 in the module plane 32 to the corresponding points 54 on the interferometer camera 42 that is arranged in the detector plane 43.

[0077] In a third process phase P3, the surface 12 of the object under test 14 is measured with the measuring device 10 while the test module 30 is arranged in the module plane 32 in accordance with FIG. 1. The measurement data 92 in camera coordinates taken in the process from the test interferogram or interferograms 41 recorded by the interferometer camera 42 are subsequently evaluated in the evaluation apparatus 56 in a fourth process phase P4. In this case, the measurement data 93 are calculated back into the module plane 32 using the inverse position assignment distribution (T.sub.1).sup.−1 and into the coordinate system of the surface 12 using the inverse position assignment distribution (T.sub.2).sup.−1. The back-calculation of the measurement data 93 into the module plane 32 and into the coordinate system of the surface 12 can be implemented sequentially. Alternatively, it is also possible to construct a model of the overall distortion which also contains the design distortion T.sub.1 and to determine remaining model parameters by way of the distortion measurement, the model parameters taking account of components which may occur both before and after the design distortion T.sub.1 within the scope of the overall distortion.

[0078] Subsequently, the free-form deviation distribution h(x,y) is subtracted and hence the shape deviation 94 of the optical surface 12 from the target shape 60 is determined. The shape deviation 94 represents processing data in device-under-test coordinates, which can now be transmitted to a processing machine 96 for post-processing of the optical surface 12. In turn, the actual shape of the optical surface 12 can be determined by adding the shape deviation 94 to the target shape 60.

[0079] 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

[0080] 10 Measuring device [0081] 12 Optical surface [0082] 14 Object under test [0083] 16 Light source [0084] 18 Input wave [0085] 20 Beam splitter [0086] 22 Collimator [0087] 24 Reference element [0088] 26 Fizeau surface [0089] 28 Reference wave [0090] 29 Wavefront of the reference wave [0091] 30 Diffractive optical test module [0092] 32 Module plane [0093] 34 Test wave [0094] 34r Returning test wave [0095] 35a Wavefront of the returning test wave [0096] 35b Wavefront of the returning test wave [0097] 36 Acquisition device [0098] 38 Stop [0099] 40 Eyepiece [0100] 41 Test interferogram [0101] 42 Interferometer camera [0102] 43 Detector plane [0103] 44 Calibration module [0104] 46 Position assignment distribution [0105] 48 Further position assignment distribution [0106] 50 Points on the surface of the object under test [0107] 52 Points on the module plane [0108] 54 Points in the detector plane [0109] 56 Evaluation apparatus [0110] 57 Diffractive structures [0111] 58 Diffractive test pattern [0112] 59 Adjustment pattern [0113] 60 Target shape [0114] 62 Deviation from the target shape [0115] 64 Rotationally symmetric reference surface [0116] 66 Axis of symmetry [0117] 68 Deviation from the reference surface [0118] 70 Best-fitted spherical surface [0119] 72 Maximum deviation from the spherical surface [0120] 74 Adjustment wave [0121] 80 Calibration wave [0122] 82 Diffractive structures [0123] 84 Diffractive calibration pattern [0124] 85 Detailed view of the calibration pattern [0125] 86 Phase signature [0126] 88 Calibration interferogram [0127] 90 Evaluation apparatus [0128] 92 Free-form deviation distribution h(x,y) [0129] 93 Measurement data in camera coordinates [0130] 94 Shape deviation [0131] 96 Processing machine