Interferometric measuring method and measuring device for measuring the shape of a surface

Abstract

Method (80) for interferometric measurement of the shape of a surface (12) of a test object (14), comprising the steps: - respective measurement of the surface shape using an interferometric measuring device (10) in at least two different measuring configurations, - Differentiation (92) of the measurement results (82, 84) and separation (96) of measurement artifacts attributable to the respective measurement configuration (88, 90), - Subtracting (98) the associated measurement artifacts (88, 90) from at least one of the measurement results (82, 84).

Description

Background of the invention

[0001] The invention relates to a method for interferometrically measuring the shape of a surface of a test object. Furthermore, the invention relates to a measuring device with an interferometer for measuring the shape of a surface of a test object.

[0002] Numerous interferometric methods and devices are known for measuring the surface shape of a test object. For very precise determination of a surface shape, as may be required, for example, for optical elements used in microlithography, interferometers with a diffractive optical element are employed to generate a test wave with a wavefront adapted to a target shape of the surface. A computer-generated hologram (CGH) can be used as the diffractive element. For example, EP 2 478 328 B1 describes a Fizeau interferometer with a CGH as the diffractive element. The test wave with the adapted wavefront would strike the target shape perpendicularly at every point and be reflected back by it.Deviations from the target shape result in an interference pattern when the reflected test wave is superimposed with a reference wave, which allows the actual surface shape to be determined with high accuracy.

[0003] The interference patterns acquired through interferometric measurements, also known as interferograms, can exhibit disruptive artifacts when analyzed. Periodic measurement errors, such as fringes, are common. These periodic artifacts can be caused by various factors, including inaccurate positioning of optical elements in the measuring device, periodic polishing errors in an optical element, CGH recording errors, fringing during phase-step analysis, and more. The artifacts may differ depending on the measurement configuration. Furthermore, the artifacts may not be reproducible between measurements.

[0004] Several methods are known for separating these systematic errors or reproducibility errors from the interferometric measurements generated by the surface of the test object. One method involves averaging several measurements with sufficiently distinct systematic errors. This can be achieved by modifying the measuring device, such as changing the position of one or more optical elements. However, a very high number of measurements is required for effective suppression of the measurement error. Averaging multiple measurements with a statistically independent distribution of the systematic error leads to very slow convergence, which decreases monotonically with the number of measurements.

[0005] Furthermore, signal filtering of the measurement data can be performed to eliminate periodic systematic errors. For example, Fourier filtering can be used to suppress affected spatial frequencies of the interferometric measurements. However, this always removes a portion of the actual image signal in the suppressed Fourier region. The restored image does not correspond to the ideal, error-free image. Averaging multiple measurements with a statistically independent distribution of measurement errors, combined with signal filtering, is also possible. However, due to the influence of the filtering on the actual measurement data, a large number of measurements is required to achieve a sufficient reduction in measurement errors.

[0006] Furthermore, appropriate calibration of the measuring device also leads to a reduction in measurement errors. However, this requires good reproducibility of the systematic error between the calibration and test object surfaces across different configurations. The known methods are therefore either very time-consuming or distort the measurement result. Underlying task

[0007] It is an object of the invention to provide a method and a device with which the aforementioned problems are solved, and in particular a reduction of measurement errors in an interferometric measurement of a surface shape of a test object is achieved with less time expenditure and less influence on measured values. Inventive solution

[0008] The aforementioned problem can be solved according to the invention, for example, with a method for interferometrically measuring the shape of a surface of a test object, in which the surface shape is measured using an interferometric measuring device in at least two different measurement configurations. Furthermore, the measurement results are differentiated, measurement artifacts attributable to the respective measurement configuration are separated, and the associated measurement artifacts are subtracted from at least one of the measurement results.

[0009] Test objects include, for example, optical elements whose surface properties need to be determined very precisely. Such optical elements are, for instance, mirrors in a projection exposure system for microlithography using extreme ultraviolet (EUV) radiation. The EUV radiation used in such a projection exposure system has a wavelength of less than 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.8 nm.

[0010] The measuring device includes, for example, a Fizeau, Twyman-Green, Michelson, Mach-Zehnder, or point diffraction interferometer, which is configured for interferometric measurement of the surface shape of the test object. Such a Fizeau interferometer is described, for example, in EP 2 478 328 B1. The interferometer provides as a measurement result an image of the interference pattern formed when the test wave reflected from the surface is superimposed on a reference wave. This image is also called an interferogram and typically represents a two-dimensional matrix of image values ​​or pixels. The interferogram contains interference patterns that can be used to determine the shape of the surface. The determined shape of the surface often contains measurement artifacts, such as periodically occurring fringes, which limit the measurement accuracy.

[0011] For the different measurement configurations, a change is preferably made to the arrangement or orientation of the test object, one or more optical elements, or one or more groups of optical elements of the interferometer. A change can be selected where the respective measurement artifacts of the measurement configurations differ more significantly from each other compared to other changes.

[0012] The difference in the specific surface shapes between the two measurement configurations therefore no longer contains any surface-induced structures, but only the measurement artifacts of both configurations. Separating these images yields two images, each containing only the artifact of one measurement configuration. Subtracting the artifact of a measurement configuration from the corresponding original measurement removes the artifact. This results in a clean measurement result without interfering artifacts. DE 11 2021 003 255 T5 describes a method for background subtraction, but without differentiating two measurements and without separating the artifacts attributable to each measurement configuration. US 5,502,566 A describes a method for measuring multiple surfaces using adapted positional tests, but also without separating the artifacts attributable to each measurement configuration.

[0013] According to one embodiment of the invention, the two measurement results are corrected by subtracting the associated measurement artifacts from each result, and the corrected results are then calculated together. In particular, an average is calculated from both corrected results to obtain a final result. For example, an average value is calculated for each pixel of the two corrected results.

[0014] According to a further inventive embodiment, the different measurement configurations are distinguished by different rotational positions or orientations of the test object or by different orientations of a Fizeau surface. In particular, between two measurements, the test object is rotated about an axis of symmetry of the surface of the test object or about an optical axis of the beam path to set different measurement configurations. Preferably, a rotational position is selected in which the measurement artifacts of the respective measurement configuration exhibit greater differences compared to other rotational positions. The rotational position is chosen, for example, such that the stripe-shaped measurement artifacts of the different measurement configurations are as approximately orthogonal to each other as possible, i.e., their inner product is much smaller than the product of their norms.The rotation is performed, for example, using a rotatable mount for the measuring device holding the test object. This embodiment is particularly suitable for measuring a test object with a rotationally symmetrical surface.

[0015] In an embodiment according to the invention, a test wave is generated on the surface of the test object by means of a diffractive optical element of the measuring device, and the different measuring configurations differ with respect to the diffractive optical element.

[0016] In particular, according to one embodiment, the diffractive optical element generates a test wave with a wavefront adapted to a target shape of the test object's surface. If the surface matches the target shape, the test wave is reflected back on itself and transformed back into the original measurement wave by the diffractive optical element. The diffractive optical element comprises, for example, a so-called computer-generated hologram (CGH). The diffractive structures necessary for generating the test wave can be determined by a computer-aided simulation of the measurement result together with the target surface of the test object and produced as a CGH on a substrate.

[0017] According to one embodiment of the invention, the different measurement configurations are distinguished by different rotational positions of the diffractive optical element. In particular, between two measurements, the diffractive optical element is rotated about an optical axis of the beam path to set different measurement configurations. For example, the rotation is carried out using a rotatable mount of the measuring device for the diffractive optical element. Preferably, a rotational position is selected in which the measurement artifacts of the respective measurement configuration differ more significantly from those of other rotational positions. The rotational position is chosen, for example, such that the measurement artifacts of the different measurement configurations are as approximately orthogonal to each other as possible.The different measurement configurations may also involve a different arrangement or orientation of other optical elements or element groups of the measuring device, or be based on other changes in the position or orientation of the diffractive optical element, the measured surface, or the Fizeau surface.

[0018] According to another embodiment of the invention, the different measurement configurations differ by the use of different diffractive optical elements. In other words, a first diffractive optical element is used in the first measurement configuration, and a second diffractive optical element is used in the second measurement configuration. Although the different diffractive optical elements are nominally identical in configuration, they exhibit different manufacturing defects, as certain manufacturing defects occur randomly. The measurement errors are as approximately orthogonal to each other as possible. Thus, the different diffractive optical elements produce different measurement artifacts in the interferometric measurement result.

[0019] According to one embodiment of the invention, separating measurement artifacts resulting from the respective measurement configuration includes filtering measurement artifacts. This involves filtering out measurement artifacts from one of the two measurements with different measurement configurations, while measurement artifacts from the other measurement are not filtered. In particular, the filtering is performed using an asymmetric bandpass filter. For example, a combination of convolution filters or a combination of non-linear filters, such as median filters, is used as the bandpass filter in the spatial domain. A bandpass filter can also be used to suppress unwanted spatial frequencies in the frequency domain. For this purpose, a Fourier transform of the difference between the measurement results into the frequency domain is first performed, and after removing unwanted frequencies of a measurement artifact, a back-transformation into the spatial domain is carried out.In one embodiment, the difference between the measurement results is transformed into a coordinate system before filtering, in which one or both measurement artifacts sparsely occupy a discrete frequency space. In other words, after the transformation, representations of the measurement artifacts in the discrete frequency space occupy only a few matrix elements or pixels.

[0020] According to another embodiment, separating measurement artifacts resulting from the respective measurement configuration involves adapting the parameters of a model used to describe measurement artifacts to those artifacts. This adaptation can be performed to artifacts from one or both measurements with different measurement configurations. For example, a set of basis vectors is selected, and the coefficients of a linear combination of these basis vectors are adapted to the measurement artifacts. Subsequently, the separation of basis vectors contributing to the artifacts from one of the measurements with the different measurement configuration is carried out. Periodic functions, such as sine and cosine functions, can be used as basis vectors.

[0021] In one embodiment according to the invention, for each pair of measurements with different measurement configurations and measurement ranges on the surface, the measurement results within an overlap region of the measurement ranges are used as measurement results. Thus, only in the overlap region are the measurement results of two measurements with different measurement configurations differentiated, measurement artifacts separated, and measurement artifacts subtracted from the measurement results. In other words, a measurement result cleansed of measurement artifacts is generated only for the overlap region. In particular, for more than two measurements with different measurement configurations and measurement ranges, the measurement results in the respective overlap region are used as measurement results for all possible pairs of measurements. In this way, a cleansed measurement result is obtained for each overlap region.

[0022] According to one embodiment of the invention, measurement results from different overlapping areas are combined into a single overall measurement result after subtracting measurement artifacts. For combining measurement results or interferograms from different overlapping areas, an image stitching method is used, for example. Stitching, particularly in photography, refers to the process of combining individual images into a single large image. In areas with two or more interferograms, the measurement results can be averaged.

[0023] According to a further embodiment, measurement results are extrapolated for areas of the surface outside of overlap regions. In particular, areas of the surface for which no measurement result or only one measurement result is available are extrapolated using the measurement results from adjacent overlap regions. When extrapolating over a region, measurement results within the region or from more distant overlap regions can additionally or alternatively be taken into account.

[0024] The aforementioned task can also be solved, for example, with a measuring device for measuring the shape of a test object's surface, wherein the measuring device comprises an interferometer for measuring the surface shape in at least two different measurement configurations. Furthermore, the measuring device comprises a difference module for subtracting one measurement result from another and subtracting any associated measurement artifact from a measurement result, as well as a separation module for separating measurement artifacts resulting from the respective measurement configuration.

[0025] The interferometer is, for example, a Fizeau, Twyman-Green, Michelson, Mach-Zehnder, or point diffraction interferometer. According to one embodiment, the interferometer comprises a diffractive optical element for generating a test wave directed at the test object. The differential module and the separation module are configured to process evaluated interferograms as measurement results and are each arranged either within an evaluation unit of the measuring device or separately from it within the measuring device.

[0026] The features specified for the aforementioned embodiments, exemplary embodiments, or variants, etc., of the method according to the invention can be transferred accordingly to the measuring device according to the invention, and vice versa. These and other features of the embodiments according to the invention are explained in the description of the figures and the claims. The individual features can be implemented either separately or in combination as embodiments of the invention. Furthermore, they can describe advantageous embodiments that are independently patentable and whose protection may be claimed only during or after the filing of the application. Brief description of the drawings

[0027] The foregoing, as well as further advantageous features of the invention, are illustrated in the following detailed description of exemplary embodiments of the invention with reference to the accompanying schematic drawings. These show: Fig. 1 a schematic illustration of an embodiment of the measuring device according to the invention for measuring the shape of a surface of a test object, Fig. 2 a first embodiment of the inventive method for interferometric measurement of the shape of a surface of a test object in a schematic illustration, Fig. 3 a schematic representation of the separation of measurement artifacts from two measurements with different measurement configurations, as well as Fig. 4 A second embodiment of the inventive method for interferometric measurement of the shape of a surface of a test object in a schematic illustration. Detailed description of embodiments according to the invention

[0028] In the exemplary embodiments or variants described below, functionally or structurally similar elements are, as far as possible, provided with the same or similar reference numerals. Therefore, to understand the features of the individual elements of a particular exemplary embodiment, reference should be made to the description of other exemplary embodiments or to the general description of the invention.

[0029] In Fig. Figure 1 schematically illustrates a measuring device 10 for measuring the shape of a surface 12 of a test object 14. The test object 14 is, for example, a mirror of a projection exposure system for microlithography using extreme ultraviolet (EUV) radiation. The EUV radiation has a wavelength of less than 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.8 nm. However, the measuring device 10 is also suitable for very precise measurement of the surface of other optical elements or other objects.

[0030] The measuring device 10 comprises an interferometer 16, which in this embodiment is configured as a Fizeau interferometer. Alternatively, a Twyman-Green, Michelson, Mach-Zehnder, or point diffraction interferometer, or any other suitable interferometer type, can be used.

[0031] The interferometer 16 includes an illumination device 18 for providing sufficiently coherent measurement radiation 20 with a plane wavefront for interferometric measurements. In alternative embodiments, measurement radiation with a differently shaped wavefront, such as a spherical wavefront, can also be provided. For the measurement radiation 20, a radiation source 21 of the illumination device 18, e.g., a helium-neon laser or another suitable laser, generates coherent electromagnetic radiation 22. This is focused by a focusing lens 24 onto an aperture of a diaphragm 26 such that a divergent measurement radiation 28 emerges from the aperture and stray light is suppressed. The divergent measurement radiation 28 passes through a group of lens elements designed as a collimator 30, which focuses the divergent measurement radiation 28 into the measurement radiation 20 with a parallel beam path.transformed into a plane wavefront and suitable diameter for measurement.

[0032] The measurement radiation 20 travels along an optical axis 32, partially passes through a beam splitter 34, and strikes a Fizeau element 36 of the measuring device 10. A portion of the measurement radiation 20 is reflected back by a Fizeau surface 38 of the Fizeau element 36 as a reference wave 40. Another portion of the measurement radiation 20 passes through the Fizeau element 36 and propagates as an input wave 42 with a planar wavefront to a diffractive element 44 with diffractive structures 46 on a diffracting surface 48. The diffractive optical element 44 and the diffractive structures 46 are configured such that the input wave 42 is diffracted to form a test wave 50 with a wavefront adapted to a target shape of the surface 12. The test wave 50 strikes the surface 12 of the test object 14 in autocollimation and is reflected back by it.The reflected test wave 51 contains information about the deviation of the shape of the surface 12 from its nominal shape.

[0033] The diffractive optical element 44 comprises a computer-generated hologram (CGH). The diffractive structures required to generate the test wave 50 can be determined by a computer-aided simulation of the interferometer 10 together with the target surface of the test object 14, e.g., using beam tracing. Subsequently, the calculated diffractive structures are applied as a grating to a substrate as a CGH using a lithographic process or similar method. Alternatively, a conventional hologram can be used. This can be produced, for example, by exposing a photographic plate with a superposition of reference light and light reflected from an object with the target shape.

[0034] The reflected test wave 51 passes through the diffractive optical element 44 and the Fizeau element 36 and is partially directed by the beam splitter 34 towards an interferometer camera 52. The reference wave 40 reflected by the Fizeau element 36 is also partially reflected by the beam splitter 34 towards the interferometer camera 52. An objective 54 with an aperture 55, formed by a group of lens elements, focuses the reflected test wave 51 originating from the surface 12 of the test object 14 onto a detection plane 56 of a detector 58. The reference wave 40 reflected by the Fizeau element 36 is also focused onto the detection plane 56. The superposition of the reflected test wave 51 and the reference wave 40 generates an interference pattern, which is captured by the detector 58 as an image or interferogram in the form of a two-dimensional raster graphic or matrix of image points or pixels.

[0035] Furthermore, the measuring device 10 includes a holder 60 for rotatably positioning and fixing the diffractive optical element 44. The holder 60 allows, in particular, the diffractive optical element 44 to be rotated about the optical axis 32 and subsequently fixed in a selected rotational position. Similarly, a test object holder 62 for rotatably positioning and fixing the test object 14 allows the test object 14 to be rotated about the optical axis 32 or an axis of symmetry of the surface of the test object 14 and subsequently fixed in a selected rotational position. By changing the rotational position of the diffractive optical element 44 or the test object 14, different measurement configurations of the measuring device 10 can be set.

[0036] Furthermore, the measuring device 10 includes an evaluation unit 64 for determining a deviation of the surface shape 12 of the test object 14 from the target shape based on interference patterns in the acquired interferogram. Since the determined target shape is superimposed by interfering measurement artifacts such as periodically occurring fringes, the measuring device 10 includes a difference module 66 and a separation module 68 for removing the measurement artifacts. The difference module 66 is configured to subtract one measurement result from another measurement result and to subtract an associated measurement artifact from a measurement result. The separation module 68 is designed to separate measurement artifacts resulting from the respective measurement configuration.

[0037] The following describes the further functioning of the measuring device 10 together with various embodiments of a method for interferometric measurement of the shape of a surface 12 of a test object 14.

[0038] In Fig. Figure 2 schematically illustrates a first method 80 for the interferometric measurement of the shape of a surface 12 of a test object 14. To determine the surface shape, two measurements are performed with different measurement configurations. The interferograms evaluated by the measuring device 10 are shown as the first measurement result 82 and the second measurement result 84 in Figure 2. Fig. Figure 2 shows that both measurement results 82 and 84 contain structures 86 caused by the shape of the surface 12 of the test object 14. These structures 86 are superimposed in the first measurement result 82 by first stripe-shaped measurement artifacts 88 and in the second measurement result by second stripe-shaped measurement artifacts 90.

[0039] In this embodiment, the different measurement configurations are determined by different rotational positions of the diffractive optical element 44 or the test object 14. Alternatively, the diffractive optical element 44 can be replaced by another diffractive element that is nominally identical in configuration but has different random manufacturing defects. The different measurement configurations can also involve a different arrangement or orientation of other optical elements or element groups of the measuring device 10, or be based on other changes in the position or orientation of the diffractive optical element 44 or the test object 14.

[0040] First, two different measurement configurations are selected that generate measurement artifacts 88, 90 that are as different as possible, while the structures 86 of the surface 12 are essentially the same. In this embodiment, the measurement configurations are determined, for example, by selecting two different rotational positions of the diffractive optical element 44. The selection is made such that the stripe-shaped measurement artifacts 88, 90 of the different measurement configurations exhibit the largest possible angle to each other.

[0041] Then, using the holder 60 of the diffractive optical element, the first of the two rotational positions is set and the first measurement is performed. Subsequently, the second rotational position is set using the holder 60 and the second measurement is performed. The procedure is analogous for measurement configurations with different rotational positions of the test object 14 or for other defined measurement configurations. First, the first measurement configuration is set and a first measurement is performed, and then the second measurement configuration is set and a second measurement is performed. The rotational position of the test object 14 is set using the test object holder 62.

[0042] The difference module now calculates the difference between the two measurement results 82 and 84. This is done in Fig. 2 schematically represented. The difference 94 contains a superposition of the measurement artifacts 88, 90 from both measurements and essentially no longer any structures 86 caused by the surface 12. Subsequently, a separation 96 of the measurement artifacts 88, 90 in the difference 94 is carried out using the separation module 68. This is described further below with reference to Fig. 3. Filtering the difference 94 in more detail or adjusting the parameters of a model to the difference 94.

[0043] Subtracting the now separately obtained measurement artifacts 88 from the first measurement from the first measurement result 82 using the difference module 66 yields a corrected first measurement result 100 without interfering measurement artifacts 88. Similarly, subtracting the measurement artifacts 90 from the second measurement yields a corrected second measurement result 102. Averaging 104 of the two corrected measurement results 100 and 102 finally leads to an averaged measurement result 106. The evaluation unit 64 uses the averaged measurement result 106 to determine a deviation of the surface shape 12 of the test object 14 from a target shape. With a known deviation from a target shape, the shape of the surface 12 is also known.

[0044] Fig. Figure 3 illustrates the separation of measurement artifacts 88, 90 from two measurements with different measurement configurations according to Fig. 2. By filtering 110 of the difference 94 between the two measurement results 82, 84. In this process, the measurement artifacts of one measurement are filtered out. What remains are the measurement artifacts of the other measurement.

[0045] First, for both measurement artifacts 88 and 90, a transformation 112 of the difference 94 is performed into a coordinate system in which the measurement artifacts 88 and 90 of a measurement sparsely occupy a discrete frequency space in the transformed difference 114; that is, the representation of the measurement artifacts occupies only a few matrix elements in the discrete frequency space. Then, an asymmetric bandpass filter 116 is applied. For example, a combination of convolution filters or a combination of non-linear filters, such as median filters, is used as the bandpass filter in the spatial domain.

[0046] Alternatively, unwanted spatial frequencies can be suppressed in the frequency domain. This involves first performing a Fourier transform of the transformed difference 114 into the frequency domain, and after removing unwanted frequencies of a measurement artifact, performing a reverse transformation into the spatial domain.

[0047] The filtered difference 118 now contains only measurement artifacts 88 and 90 from one measurement. Finally, a back-transformation 120 of the filtered difference 118 into the original coordinate system is performed. The measurement artifacts 88 from the first measurement and the measurement artifacts 90 from the second measurement are now separate.

[0048] In an alternative embodiment, the measurement artifacts 88 and 90 are separated by adjusting the parameters of a model to the difference 94. For example, a set of basis vectors is selected, and the coefficients of a linear combination of these basis vectors are adjusted to the difference 94. Subsequently, a separation is performed between basis vectors contributing to measurement artifacts 88 of the first measurement and those contributing to measurement artifacts 90 of the second measurement. Periodic functions such as sine and cosine functions can be used as basis vectors.

[0049] In Fig. Figure 4 schematically illustrates a second embodiment of method 80 for the interferometric measurement of the surface shape of a test object 14. In this embodiment, more than two measurements with different measurement configurations and measurement ranges are performed on the surface 12. Fig. Figure 4 shows exemplary measurement results from three measurements with different measurement configurations and ranges. However, the procedure can also be performed with more than three measurements.

[0050] First, the procedure is carried out analogously to the procedure according to Fig. 2. A selection of different measurement configurations is available for each measurement. The respective measurement area on surface 12 can be taken into account. The appropriate measurement configuration is then set before each measurement, and the measurement is performed. The difference is then calculated for all possible pairs of measurement results with different measurement configurations (dashed box 130). In this example, a first difference 138 is calculated from a first measurement result 132 and a second measurement result 134; a second difference 140 is calculated from the first measurement result 132 and a third measurement result 136; and a third difference 142 is calculated from the second measurement result 134 and the third measurement result 136. Only an overlap area of ​​both measurement results is used in each case.

[0051] Differences 138, 140, and 142 contain only the superimposed measurement artifacts of both measurement results in the respective overlap area. These are processed according to the procedure described below. Fig. 2 separated, subtracted from the respective measurement result in the overlap area, and the measurement results of each pair of measurements, thus adjusted, are averaged for an overlap area, dashed box 144.

[0052] Subsequently, overlapping areas 146 are combined with adjusted and averaged measurement results 148, 150, 152 to form an overall measurement result 154 within a total measurement area. A stitching method is used for this purpose, e.g., a technique known from photography for combining several individual images into one. In areas with multiple averaged measurement results 148, 150, 152, further averaging of measurement results can be performed. Furthermore, for areas of the surface 12 outside of overlapping areas, measurement results can be extrapolated using the measurement results 148, 150, 152 from adjacent overlapping areas.

[0053] The foregoing description of exemplary embodiments, embodiments, or variants is to be understood as illustrative. The disclosure thereby enables the person skilled in the art to understand the present invention and its associated advantages, and also encompasses, in the understanding of the person skilled in the art, obvious modifications and alterations of the described structures and methods. Therefore, all such modifications and alterations, insofar as they fall within the scope of the invention as defined in the appended claims, as well as equivalents, are to be covered by the protection of the claims. Reference symbol list 10 Measuring device 12 surface 14 Test object 16 Interferometer 18 Lighting equipment 20 Measurement radiation 21 Radiation source 22 electromagnetic radiation 24 Focusing lens 26 aperture 28 divergent measurement radiation 30 Collimator 32 optical axis 34 beam splitters 36 Fizeau element 38 Fizeau surface 40 Reference wave 42 Input shaft 44 diffractive optical element 46 diffractive structures 48 flexural surface 50 test wave 51 reflected test wave 52 Interferometer camera 54 lens 55 aperture 56 Recording level 58 Detector 60 Mounting bracket diffractive optical element 62 Test object holder 64 Evaluation unit 66 Differential module 68 Separation module 80 procedures 82 first measurement result 84 second measurement result 86 Surface Structures 88 first measurement artifacts 90 second measurement artifacts 92 Differentiation 96 Separating measurement artifacts 98 Subtracting measurement artifacts 100 adjusted first measurement result 102 adjusted second measurement result 104 Means of measurement results 106 average adjusted measurement result 110 Filtering measurement artifacts 112 Coordinate transformation 114 transformed difference 116 Applying a filter 118 filtered difference 120 Inverse transformation 130 Differentiation of Overlap Areas 132 first measurement result 134 second measurement result 136 third measurement result 138 first difference 140 second difference 142 third difference 144 Separating, peeling and mediating 146 Merging overlapping areas 148 first averaged measurement result 150 second averaged measurement result 152 third averaged measurement result 154 Total measurement result

Claims

[1] Method (80) for interferometric measurement of the shape of a surface (12) of a test object (14), comprising the steps: - respective measurement of the surface shape using an interferometric measuring device (10) in at least two different measuring configurations, - Differentiation (92) of the measurement results (82, 84) and separation (96) of measurement artifacts attributable to the respective measurement configuration (88, 90), - Subtracting (98) the associated measurement artifacts (88, 90) from at least one of the measurement results (82, 84). [2] Method according to claim 1, wherein the two measurement results (82, 84) are cleaned up by subtracting (98) the respective measurement artifacts (88, 90) from the respective measurement result (82, 84) and the cleaned measurement results (100, 102) are calculated together (104). [3] Method according to claim 1 or 2, wherein the different measurement configurations differ by different rotational positions or orientations of the test object (14) or by different orientations of a Fizeau surface (38). [4] Method according to one of the preceding claims, wherein a test wave (50) is generated on the surface (12) of the test object (14) by means of a diffractive optical element (44) of the measuring device (10) and the different measuring configurations differ with respect to the diffractive optical element (44). [5] Method according to claim 4, wherein the different measurement configurations differ by different rotational positions of the diffractive optical element (44). [6] Method according to claim 4, wherein the different measurement configurations differ by the use of different diffractive optical elements. [7] Method according to one of the preceding claims, wherein the separation (96) of measurement artifacts (88, 90) resulting from the respective measurement configuration comprises filtering (110) of measurement artifacts (88, 99). [8] Method according to one of the preceding claims, wherein the separation (96) of measurement artifacts attributable to the respective measurement configuration comprises adapting parameters of a model for describing measurement artifacts to the measurement artifacts. [9] Method according to one of the preceding claims, wherein for each of two measurements with different measurement configurations and measurement ranges on the surface (12) the measurement results within an overlap area of ​​the measurement ranges are used as measurement results. [10] Method according to claim 9, wherein measurement results (148, 150, 152) from different overlap areas are combined (146) after subtracting measurement artifacts to form an overall measurement result (154). [11] Method according to claim 9 or 10, wherein measurement results are extrapolated for areas of the surface (12) outside of overlap areas. [12] Measuring device (10) for measuring the shape of a surface (12) of a test object (14), comprising: - an interferometer (16) for measuring the surface shape in at least two different measurement configurations, - a difference module (66) for subtracting one measurement result (82) from another measurement result (84) and subtracting an associated measurement artifact (88, 90) from a measurement result (82, 84), as well as - a separator module (68) for separating measurement artifacts (88, 90) resulting from the respective measurement configuration.

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