Testing Diffractive Optical Elements for Interferometers

Abstract

The invention relates to a diffractive optical element (10) of a test interferometer (100) for measuring the shape of an optical surface (102), comprising: a diffractive shape measurement structure (16) arranged on a use surface (14) of the diffractive optical element and configured to generate a test wave (122) for irradiating the optical surface to be measured when the diffractive optical element is arranged in the test interferometer; and at least one test field (18) configured to measure a plurality of profile properties of the test structure positioned within the test field, the profile properties characterizing a profile of the test structure extending laterally relative to the use surface and including a flank angle of the profile of the test structure, a profile depth of the test structure, and a depth of microgrooves appearing in a base region of a groove-shaped profile of the test structure. The test field is arranged at a position of the use surface other than the diffractive shape measurement structure, so that the test field is surrounded by a plurality of diffractive shape measurement structures.

Description

[0001] This application claims the priority of German patent application 10 2019 212 520.4 filed on August 21, 2019. The entire disclosure content of this patent application is incorporated into the present application by reference. Background Art

[0002] The invention relates to a diffractive optical element for a test interferometer for measuring the shape of an optical surface, to a test interferometer of this type, to a calibration method for calibrating the diffractive optical element, to a measuring method for measuring the shape of an optical surface, and to a method for manufacturing the diffractive optical element.

[0003] Microlithography is used, for example, for the manufacture of microstructured components such as integrated circuits or LCDs. The microlithography process is carried out in an apparatus known as a projection exposure apparatus comprising an illumination device and a projection lens. In projection lenses designed for the EUV range, which operate in a wavelength range below 100 nm, for example at approximately 13.5 nm or approximately 6.8 nm, mirrors are used as optical components for the imaging process. High-accuracy testing of mirror surfaces is carried out in test interferometers, in particular in which diffractive optical elements in the form of computer generated holograms (CGHs) are used to adapt the test waves to the target shape of the mirror surface to be tested.

[0004] In fact, in the context of testing mirror surfaces, it is necessary to reliably distinguish the surface defects to be determined from the manufacturing defects typically present in the CGH. In this regard, DE 10 2018 200 568 A1 describes the characterization of the CGH with respect to said manufacturing defects by means of a so-called diffraction measuring station.

[0005] In addition to the imperfections of the structural pattern of the diffractive structure in the plane of the CGH surface, equally important are the imperfections of the geometry of the diffractive structure, in particular the contour of the diffractive structure, i.e. the contour of the diffractive structure in the cross-sectional image transversely to the CGH surface. The contour of complex diffractive structures on the CGH as well as particularly small manufacturing imperfections (profile deviations) in the plane of the CGH surface can be reconstructed very inaccurately in the conventional manner only from the measurements of the diffraction measuring station.

[0006] Purpose of the Invention

[0007] The object of the present invention is to provide a diffractive optical element, a test interferometer, a calibration method and a measuring method, or a combination of measuring methods of the type mentioned in the background technology, which can solve the above-mentioned problems and through which the geometric properties of the diffractive structure on the diffractive optical element used for the test interferometer can be measured with high accuracy. Summary of the invention

[0008] The above-mentioned object can be achieved according to the present invention, for example, by a diffractive optical element of a test interferometer for measuring the shape of an optical surface, the diffractive optical element comprising: a diffractive shape measurement structure arranged on a use surface of the diffractive optical element and configured to generate a test wave for irradiating the optical surface to be measured when the diffractive optical element is arranged in the test interferometer; and at least one test field configured to measure a plurality of profile properties of the test structure contained in the test field. The profile properties characterize a profile line of the test structure extending laterally relative to the use surface and include a flank angle of the profile line of the test structure, a profile depth of the test structure, and a depth of microgrooves appearing in the bottom region of the profile of the groove shape of the test structure. The test field is arranged at a position of the use surface other than the diffractive shape measurement structure, so that the test field is surrounded by a plurality of diffractive shape measurement structures.

[0009] The expression that the test field is surrounded by a plurality of diffractive shape measurement structures means that the diffractive shape measurement structures are arranged around the test field at an angle of at least 180°, in particular at an angle of 360°. In this case, the diffractive shape measurement structures can surround the test field over the entire area or be arranged in area regions surrounding the test field such that gaps remain between the area regions.

[0010] In particular, a plurality of test fields of this type are arranged in each case at the location of the use surface instead of the diffractive shape measurement structure, so that the respective test field is surrounded by the plurality of diffractive shape measurement structures. The contour properties characterizing the contour lines of the test structure extending transversely to the use surface represent in each case the geometrical properties of the test structure.

[0011] At least one test field is provided at a location of a use surface having the mentioned test structure, so that profile properties of the test structure contained therein can be measured with high measurement accuracy to deduce corresponding profile properties of the diffraction shape measurement structure therefrom, thereby determining the geometrical properties of the diffraction shape measurement structure with high accuracy.

[0012] The indication that the form measurement structures are diffractive should be understood to mean that they have diffractive properties which are related to the wavelength of the test waves used in the test interferometer.

[0013] The profile depth is often also referred to as the etch depth, since the profile shape is usually formed by an etching process. A micro-trench, often also referred to using the English term "μ-trench", forms a substructure compared to a main trench formed by a trench shape profile, ie it is significantly smaller than the main trench.

[0014] According to one embodiment, the use surface is arranged on the top side of the diffractive optical element, the top side comprises a central region which accounts for at least 50% of its area, and the test field is arranged in the central region. The central region of the top side defined in this way is therefore completely surrounded by the edge region.

[0015] According to other embodiments, the test fields are arranged at a plurality of locations on the use surface in each case instead of the diffractive shape measurement structure, so that the respective test field is surrounded by a plurality of diffractive shape measurement structures, in particular at least 10 test fields, at least 50 test fields or at least 100 test fields. The shape measurement structure and the test fields are arranged at a plurality of locations on the use surface in each case, that is, they are arranged in each case distributed on the use surface. The locations where the shape measurement structure is arranged differ from the locations where the test fields are arranged.

[0016] According to other embodiments, at least some of the test fields have a regular arrangement. According to other embodiments, at least some of the test fields have an irregular arrangement.

[0017] According to other embodiments, the test structure and the shape measurement structure each have a structural pattern extending along the use surface and a contour line extending laterally relative to the use surface and characterized by at least one contour property, wherein the structural pattern of the test structure is configured so that the measurement accuracy achievable during the measurement of the contour property of the test structure is improved compared to other measurement accuracies achievable during the measurement of the contour property of the shape measurement structure.

[0018] The contour shape measurement of the form measuring structure is concerned with the measurement of the contour shape of the form measuring structure, which likewise extends transversely to the use surface. In particular, the measurement accuracy that can be achieved in each case during the contour shape measurement of the test structure and the form measuring structure relates to the same measuring method. An increased measurement accuracy is understood to mean a reduction in the maximum deviation of the measured shape from the actual shape.

[0019] According to other embodiments, the test fields each comprise a set of test structures, wherein the structural patterns of the test structures in each set are of the same type, in particular identical. That is, the structural pattern of the test structures in the set of a first test field is also contained in the same type of form or in the same form in the set of another test field. The same type of form should be understood to mean that the basic structural pattern remains unchanged or varies by a maximum of + / - 10%, while another parameter of the test structure can vary to a greater extent. In this regard, for example, the test structures of the individual test fields can remain identical with respect to their periodicity, whereas the area occupancy of the test structures varies from test field to test field, for example by changing the line width.

[0020] According to other embodiments, the structural patterns of different test structures of the corresponding test field are configured to measure different contour properties of the test structure. That is, the structural pattern of the first type of test structure of the corresponding test field is configured to measure a first property of the contour shape of the first type of test structure, and the structural pattern of the second type of test structure of the corresponding test field is configured to measure a second property of the contour shape of the test structure.

[0021] According to a further embodiment, the structure pattern of the test structure is configured for measurement by a diffraction measuring station. In other words, the structure pattern of the test structure is configured such that it is possible to measure at least one profile property of the test structure by a diffraction measuring station; in particular, the structure pattern is optimized for measurement by a diffraction measuring station. In this case, in particular the shape of the test structure can be designed such that at least one profile property to be measured is measurable by a diffraction measuring station with a higher accuracy than by other measurement methods. According to this embodiment, the test structure is a diffractive test structure in relation to the wavelength used by the diffraction measuring station.

[0022] According to other embodiments, the structural pattern of the test structure is configured for measurement by means of a scanning probe microscope. A scanning probe microscope, also referred to as SPM, may be implemented as a scanning force microscope or an atomic force microscope (AFM). In other words, the structural pattern of the test structure is configured such that it is possible to measure at least one property of the contour shape of the test structure by means of a scanning probe microscope; in particular, the structural pattern is optimized for measurement by means of a scanning probe microscope. In this case, in particular, the shape of the test structure may be designed such that at least one property of the contour shape to be measured may be measured by means of a scanning probe microscope with a higher accuracy than by means of other measurement methods.

[0023] According to other embodiments, the structure pattern of the test structure is configured for measurement by a diffraction measurement station and for measurement by a scanning probe microscope.

[0024] According to further embodiments, the test structures of the test field include at least one structure pattern configured for measurement by means of a diffraction measurement station and at least one other structure pattern configured for measurement by means of a scanning probe microscope.

[0025] According to other embodiments, the structural pattern of the test structure has periodically repeated and identically oriented edges, wherein the periodicity of the edges is below the resolution of a diffraction measuring station operating with visible light. According to one embodiment, the resolution of the diffraction measuring station operating with visible light is below 300 μm, in particular below 100 μm, below 50 μm or below 10 μm. In particular, the periodically repeated edges have a spacing between 100 nm and 1 μm.

[0026] The periodically repeated edges may be, for example, the edges of periodically arranged parallel straight lines, each point of which points in the same direction. That is, the lines are parallel to each other, and according to one embodiment, the periodicity of the lines is between 100 nm and 2 μm, in particular between 300 nm and 1000 nm, for example approximately 500 nm. In this case, the ratio of the line width to the gap between the lines may vary between 1:1 and 1:10. In the case of a width to gap ratio of 1:1, the periodicity of the line pattern is twice the amplitude of the corresponding line width. According to various embodiments, the line width may be at most 1000 nm, at most 500 nm, at most 300 nm, at most 200 nm, or at most 50 nm.

[0027] According to one embodiment, the test fields each contain a plurality of structural patterns that differ in different combinations of the above parameters. For example, the various structural patterns have different edge periodicities; in this regard, for example, the edge periodicity of one structural pattern can be 100 nm, the second 500 nm and the third 1 μm, etc. Furthermore, the test fields can contain structural patterns with mutually deviating width-to-space ratios, etc.

[0028] Furthermore, the periodically repeated edge can also be an edge of periodically arranged parallel lines with regular interruptions, which point in the same direction. In this case, the interruptions can be alternately offset row by row in the row direction, thereby obtaining a brick pattern.

[0029] According to other embodiments, the edge periodicity is smaller than 4 μm, in particular smaller than 1 μm or smaller than 400 nm.

[0030] According to other embodiments, the test fields each include further test structures having likewise periodically repeated and identically oriented edges, wherein the edges of the further test structures are oriented transversely, in particular perpendicularly, relative to the edges of the first test structures.

[0031] According to other embodiments, the test structure has a periodically arranged two-dimensional structure. The latter can be, for example, a rectangular structure, in particular the brick pattern mentioned above, or a structure with line segments arranged perpendicular to each other, for example a structure in the shape of the letter "F".

[0032] According to other embodiments, the test field comprises an unstructured test field section or the test fields each comprise an unstructured test field section, wherein the diffractive optical element has an antireflection coating at least in the region of the unstructured test field section. In particular, the antireflection coating is arranged on the back side of the diffractive optical element, i.e. on the side of the diffractive optical element opposite to the used side of the diffractive optical element having the diffractive shape measurement structure.

[0033] According to other embodiments, the test field has or the test field has in each case a test field section with a shape measurement structure for monitoring. This test field section is also referred to as a background window. It is used for monitoring the measurement to determine whether the area in which the relevant test field is arranged on the diffractive optical element represents the position of the use surface, which contains the shape measurement structure used during the measurement with the test interferometer. In order to carry out the reference measurement, for example, the shape measurement structure can first be measured by a selected measurement method (such as, for example, by measuring in a diffraction measurement station or by a scanning probe microscope), which shape measurement structure is provided for measurement with the test interferometer in the closest possible vicinity of the reference test field section. Afterwards, the above-mentioned shape measurement structure in the reference test field section of the test field is measured by the same measurement method. By comparing the measurement results, it can be ensured that the measurement results in the test field represent the shape measurement structure provided for measurement with the test interferometer.

[0034] According to other embodiments, the test field comprises or the test field comprises in each case a test field section serving as a reference, wherein reference structures are resolvable by optical microscopy. These reference structures may comprise markings which can be used to verify the correct alignment of the test field with respect to rotation and with respect to translation. Furthermore, the reference structures may each comprise an identification number for unambiguously assigning a measurement established with respect to a specific test field in order to design the properties of the test structure on which the measurement is based.

[0035] Furthermore, the invention provides a test interferometer for measuring the shape of an optical surface, comprising a diffractive optical element in one of the above-described embodiments or embodiment variants. Furthermore, the test interferometer comprises: an interferometry module for generating an interference pattern by superimposing a reference wave with the test wave generated by the diffractive shape measurement structure after the test wave interacts with the optical surface; and an evaluation unit for determining the shape of the optical surface by evaluating the interference pattern taking into account at least one predetermined calibration value of the diffractive optical element.

[0036] In addition, the present invention provides a calibration method for calibrating a diffractive optical element, the diffractive optical element comprising a diffractive shape measurement structure arranged on a use surface of the diffractive optical element, and each diffractive shape measurement structure is configured to generate a test wave when the diffractive optical element is arranged in a test interferometer configured to measure the shape of the optical surface, and the test wave is used to illuminate the optical surface to be measured. The calibration method according to the present invention includes arranging the diffractive optical element in a measuring device and measuring a test field arranged at multiple positions on the use surface and each having a test structure, the test structure having a contour line extending laterally relative to the use surface, for determining multiple profile properties of the test structure related to the contour line. The profile properties include the wing angle of the contour line of the test structure, the profile depth of the test structure, and the depth of the micro-grooves appearing in the bottom area of ​​the groove shape profile of the test structure. In addition, the calibration method according to the present invention includes determining a calibration value for calibrating the shape measurement structure during the measurement of the optical surface from the determined profile properties.

[0037] The calibration values ​​can then be taken into account during the measurement of the shape of the optical surface in a test interferometer, wherein the test waves used in this case are generated by a shape measuring structure. The calibration values ​​are determined in particular by simulation calculations using Maxwell's equations. The profile depth is often also referred to as the etching depth, since the profile shape is usually formed by an etching process. A micro-groove, which is also often referred to using the English term "μ-trench", constitutes a substructure compared to a main groove formed by a groove-shaped profile, that is, it is significantly smaller than the main groove.

[0038] According to various embodiments, the diffractive optical element calibrated in the calibration method is configured according to one of the above-described embodiments or embodiment variants.

[0039] According to other embodiments of the calibration method, the measuring device includes a diffraction measuring station, during the measurement of the test field, a plurality of test waves, each differing in at least one optical parameter, are radiated onto the test field, and at least one of the profile properties is determined by evaluating the intensity distribution of the test waves recorded by the diffraction measuring station after interaction with the test field.

[0040] According to other embodiments, the different test waves differ in their wavelength and / or their polarization. According to one embodiment, the wavelength is between 300 nm and 800 nm. According to various embodiments, test waves with at least 4, for example 7 to 12 wavelengths, and 2 to 4 different polarization directions are used. With, for example, 7 different wavelengths and 4 different polarization directions (e.g. 0°, 45°, 90° and 135°), 28 test waves, each with different optical parameters, are radiated onto the test field.

[0041] According to other embodiments, the measuring device comprises a scanning probe microscope. According to other embodiments, the profile property further comprises a structure width of the test structure.

[0042] According to other embodiments, in the calibration method, at least one profile property of the test structure is determined, i.e. a shape property of a structure pattern extending along the use surface and formed by the test structure, and further calibration values ​​for calibrating the shape measurement structure are determined therefrom. In other words, the profile property describes the deviation of the plan view profile from the desired target profile, in particular the anisotropic deviation therefrom. According to one embodiment, the profile property comprises a structure width of the test structure.

[0043] According to further embodiments, in the calibration method, furthermore, the transmission or reflection properties of unstructured areas of the diffractive optical element (effect of the antireflection layer) are determined by a test field.

[0044] According to other embodiments, the test field is measured by a plurality of different measurement methods and the profile properties are determined by calculation of the measurement results determined by the different measurement methods. The different measurement methods may include the above-mentioned method for determining at least one of the profile properties by means of an intensity distribution recorded by a diffraction measuring station and a method for determining at least one of the profile properties by means of a scanning probe microscope. Other measurement methods that can be used here include transmission electron measurements (TEM), measurements using near-field scanning microscopes, such as so-called TSOM (through-focus scanning optical microscope), x-ray measurements (XRT), and scatterometry methods performed independently of the diffraction measuring station, such as goniometrics, ellipsometry, reflectometry, etc.

[0045] The calculation of the measurement results determined by different measurement methods can be realized, for example, by the Bayesian method described in the publication "Improving optical measurement uncertainty with combined multitool metrology using a Bayesian approach" published in Applied Optics, Vol. 51, No. 25, pp. 6196-6206, September 2012, which is realized by back and forth iteration and / or by parameter separation. In addition, a common comprehensive model can be used for different measurement methods.

[0046] In addition, the present invention provides a measuring method for measuring the shape of an optical surface, which comprises the following steps: determining a calibration value of a diffractive optical element by a calibration method in one of the above-mentioned embodiments or embodiment variations, generating a test wave by a shape measurement structure of the diffractive optical element, recording an interference pattern generated by superimposing a reference wave with the test wave after interacting with the optical surface, and determining the shape of the optical surface by evaluating the recorded interference pattern taking into account the calibration value.

[0047] In addition, the present invention provides a method for manufacturing a diffractive optical element for a test interferometer for measuring the shape of an optical surface. The method according to the present invention comprises the following steps: manufacturing a diffractive optical element having a diffractive shape measuring structure, the diffractive shape measuring structure being arranged on a use surface of the corresponding diffractive optical element in each case and being configured to generate a test wave for irradiating the optical surface to be measured when the corresponding diffractive optical element is arranged in the test interferometer, wherein at least one test field is arranged at a position on the use surface of each diffractive optical element instead of the diffractive shape measuring structure, so that at least one test field is surrounded by a plurality of diffractive shape measuring structures. In addition, the method according to the present invention comprises measuring the test field of the diffractive optical element in order to monitor the stability of the manufacturing process.

[0048] According to one embodiment, one or more test fields have the same design on different diffractive optical elements. In particular, the respective test fields are configured to measure at least one geometric property of a test structure contained in the test field. The diffractive optical element manufactured in the manufacturing method can in particular be present in the above-mentioned embodiments or embodiment variants of the diffractive optical element according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The above and other advantageous features of the present invention are illustrated in the following detailed description of exemplary embodiments according to the present invention with reference to the attached schematic drawings. In the drawings:

[0050] Figure 1 An embodiment of a diffractive optical element is shown in plan view, comprising a diffractive shape measuring structure arranged on a use surface and a test field distributed on the use surface,

[0051] Figure 2 The plan view shows the Figure 1 Details of an example of a structural pattern of a diffraction shape measurement structure,

[0052] Figure 3 Shown through Figure 2 The diffraction shape in the cross section of the measurement structure along line III-III′,

[0053] Figure 4 The plan view shows the Figure 1An embodiment of a test field of the invention, the test field having a plurality of test field sections,

[0054] Figure 5a The plan view shows the Figure 4 an embodiment of one of the test field segments in the form of a so-called horizontal line test field segment,

[0055] Figure 5b The plan view shows the Figure 4 an embodiment of one of the test field segments in the form of a so-called vertical line test field segment,

[0056] Figure 5c The plan view shows the Figure 4 an embodiment of one of the test field segments in the form of a so-called brick-pattern test field segment,

[0057] Figure 5d The plan view shows the Figure 4 an embodiment of one of the test field segments in the form of a first so-called F-pattern test field segment,

[0058] Figure 5e The plan view shows the Figure 4 an embodiment of one of the test field segments in the form of a second so-called F-pattern test field segment,

[0059] Figure 5f Shown according to Figure 4 A cross-sectional view of a diffractive optical element in the region of an unstructured test field section,

[0060] Figure 6 shows the use of Figure 1 Example of a test interferometer for measuring the shape of an optical surface of a diffractive optical element,

[0061] Figure 7 shows the Figure 1 Embodiment of a diffraction measuring station for measuring profile properties of a test structure in a test field of a diffractive optical element,

[0062] Figure 8 shows the Figure 1 Example of a scanning probe microscope for measuring profile properties of a test structure in a test field of a diffractive optical element,

[0063] Figure 9a The plan view shows the Figure 1 Further details of an embodiment of a structural pattern of a diffraction shape measurement structure,

[0064] Figure 9b Shown through the Figure 9aThe diffraction shape measures the cross section of the structure along line BB′,

[0065] Fig.10a The plan view shows the Figure 4 an embodiment of a structural pattern of a test structure in one of the test field sections,

[0066] Fig.10b Shown through the Fig.10a The cross section of the test structure along line BB′,

[0067] Fig.11 A diagram showing the relationship between the depth and width of a trench of a test structure,

[0068] Fig.12a The plan view shows the Figure 4 Embodiments of other structural patterns of the test structures in one of the test field sections,

[0069] Figure 12b Shown through the Fig.12a The cross section of the test structure along line BB′,

[0070] Fig.13a The diffractive optical element is shown relative to the incident light according to Figure 7 Example of the transmission behavior of the test radiation in a diffraction measuring station,

[0071] Fig.13b Shown in accordance with Fig.13a The transmission behavior is corrected before and after by Figure 7 The distribution of the flank angles of the test structure determined by the diffraction measuring station,

[0072] Fig.14a shows a diagram of a rectangular structure having a first type of outer shape deviation from its target shape,

[0073] Fig.14b shows a diagram of a rectangular structure having a second type of outer shape deviation from its target shape,

[0074] Fig.14c a diagram showing a rectangular structure having a third type of profile deviation from its target shape, and

[0075] Fig.15 A flow chart is shown for illustrating an exemplary embodiment of calculating measurement results determined by different measurement methods. DETAILED DESCRIPTION

[0076] In the exemplary embodiments or embodiments or embodiment variants described below, elements similar to each other in function or structure are provided with the same or similar reference numerals as much as possible. Therefore, in order to understand the features of the individual elements of a specific exemplary embodiment, reference should be made to the description of other exemplary embodiments of the present invention or the general description of the invention.

[0077] For ease of description, a Cartesian xyz coordinate system is indicated in the drawings, from which the corresponding positional relationships of the components shown in the drawings are apparent. Figure 1 , the z direction is perpendicular to the plane of the drawing and extends away from the plane, the x direction extends to the right and the y direction extends upward.

[0078] Figure 1 An embodiment of a diffractive optical element 10 in the form of a computer generated hologram (CGH) is shown. The diffractive optical element 10 is used to measure the shape of an optical surface 102 of a test object 104 by means of a test interferometer 100, as described below with reference to Figure 6 1. The diffractive optical element 10 is explained in more detail. To this end, the diffractive optical element 10 includes a diffractive shape measuring structure 16 in the form of a CGH structure arranged on a use surface 14 of the diffractive optical element 10. The use surface 14 extends over a large part of a surface 12 of the diffractive optical element 10. In the present embodiment, the diffractive optical element 10 is configured as a disk, and the surface 12 corresponds to the top side of the disk. In the present embodiment, the use surface 14 is implemented in the form of an ellipse, with the minor semi-axis oriented in the x-direction, so that neither the left nor the right edge segments of the surface 12 belong to the use surface 14.

[0079] In the present embodiment, the use surface 14 is completely covered by the diffraction shape measurement structure 16 except for the area provided for the test fields 18. The test fields 18, which are also referred to as markers, are arranged at a plurality of locations on the use surface 14, wherein some of the test fields 18 have a regular arrangement. However, so-called forbidden areas 22 and preferred areas 20 are also defined on the use surface 14. Although no test fields 14 are arranged in the forbidden areas 22, the test fields 14 are particularly preferably or arranged in a particularly high density in the preferred areas 20. Figure 4 5 explain the exact structure of the test field 14 in more detail.

[0080] Figure 6An embodiment of the above-described test interferometer 100 for measuring the shape of an optical surface 102 of a test object 104 according to the invention is shown. In the case shown, the optical surface 102 is the surface of a reflector. Alternatively, for example, the surface of a lens element can also be checked. The test interferometer 100 is implemented as a high-coherence interferometer in the form of a Fizeau interferometer. The test interferometer 100 includes a test radiation source 106 for generating a test radiation 108, for example in the visible wavelength range. The test radiation source 106 can include, for example, a laser, such as a helium-neon laser. The test radiation 108 propagates along the optical axis 110 of the test interferometer 100 and first passes through a beam splitter 112.

[0081] The test radiation 108 then impinges on a focusing lens element in order to convert the test radiation 108 into a plane wave which then impinges on a reference element 116 in the form of a Fizeau element having a Fizeau surface 118. A portion of the test radiation 108 is reflected at the Fizeau surface 118 as a reference wave 120. In the present example, that portion of the test radiation 108 which passes through the Fizeau surface 118 has a plane wavefront and is referred to below as incident test radiation 108i. The incident test radiation 108i then passes through the diffractive optical element 10. In this case, the wavefront of the test radiation 108i is adapted to the target shape of the surface 102 of the test object 104 by means of a diffractive shape measurement structure 16 arranged on the diffractive optical element. The wave which occurs in this case is referred to herein as a test wave 122.

[0082] The test wave 122 with the adapted wavefront is then reflected at the surface to be measured 102. The reflected test wave 122r returns in the beam path of the incident test radiation 108i in the opposite direction, passing through the diffractive optical element 10 and the reference element 116 in the process and is then guided together with the reference wave 120 by the beam splitter 112 via the aperture 124 and the eyepiece 126 onto the surface of the detector camera 128. In this context, the arrangement comprising the reference element 116, the focusing lens element 114, the beam splitter 112, the aperture 124 and the eyepiece 126 is also referred to as the interferometry module 127.

[0083] As a result of the superposition of the two radiation parts, an intensity distribution in the form of an interference pattern appears on the detector camera 128. In an embodiment according to the invention, the reference element 116 is connected to a displacement unit, for example in the form of a piezoelectric element. The displacement unit allows the reference element 116 to be displaced in the direction of the optical axis 110 by a fraction of the wavelength of the test radiation 108. The phase of the reference wave 120 can be changed by such a displacement. This has the result that the intensity distribution generated on the detector camera 128 is changed. The intensity distributions that occur for different positions of the reference element 116 are recorded by the detector camera 128 and evaluated in the evaluation unit 130.

[0084] In this case, the evaluation unit 130 takes into account the previously determined calibration values ​​86 of the diffractive optical element 10. The calibration values ​​86 relate to the profile properties 36 of the diffractive shape measurement structure 16, which characterize the contour lines 26 of the shape measurement structure 16 extending transversely to the use surface 14. The process for determining the calibration values ​​130 is explained in detail below. The result of the evaluation of the recorded intensity distribution is the deviation of the actual shape of the optical surface 102 from its target shape, from which the actual shape of the optical surface 102 is then derived.

[0085] Figure 2 Detail of an embodiment of a structure pattern 24 of a diffractive shape measuring structure 16 is shown, which structure pattern 24 extends along the use surface 14. In other words, the structure pattern 24 is understood to mean a pattern discernible in a plan view of the diffractive shape measuring structure 16. Figure 2 In the embodiment shown, the structure pattern 24 of the diffractive shape measurement structure 16 is essentially formed by a pattern of horizontal lines, the respective shapes of which are distorted in an irregular manner.

[0086] According to one embodiment, the structure pattern 24 of the diffractive shape measurement structure 16 may be configured as a separately encoded CGH pattern for use in Figure 6 A test wave 122 is generated from an incident test radiation 108i in a test interferometer 100 configured as a Fizeau interferometer. According to other embodiments, the structural pattern 24 can also be configured as a multiply encoded CGH pattern. In the case of such a multiply encoded CGH pattern, the structural pattern 24 contains a superposition of multiple CGH patterns, so that the incident test radiation 108i is simultaneously converted into multiple exit waves by first-order diffraction in each case. In addition to the test wave 122, the exit waves here can also include a calibration wave and, optionally, considering the type of interferometer used with a reference reflector arranged downstream of the diffractive optical element 10, a reference wave. According to a variant embodiment, the structural pattern 24 is configured as a five-times encoded CHG structural pattern for generating a test wave 122, a reference wave and three calibration waves.

[0087] Figure 3 Shows Figure 2 1 through the diffractive shape measuring structure 16 along the line III-III′ and thus the contour line 26 of the relevant part of the shape measuring structure 16, which extends transversely relative to the use surface 14. The contour line 26 essentially shows the contour of the groove 28 extending in the x-direction. The groove 28 has side walls, also called flank regions 30, and a bottom region 32. The width d of the groove 28 at the level of the surface 12 is denoted by the reference numeral 37. The height difference between the surface 12 of the diffractive optical element 10 and the bottom region 32 is referred to as the profile depth d. p(reference numeral 36-1) or the etching depth for the illustrated case, in which the trench 28 is produced by etching techniques in the material 19 of the diffractive optical element 10. The material 19 may be quartz glass, for example.

[0088] Due to the etching process, the flank regions of the groove 28 do not extend completely vertically, but are each inclined at a flank angle relative to the vertical to the surface 12; in this case, the flank angle of the left flank region 30 is denoted by α1 (reference numeral 36-2) and the flank angle of the right flank region is denoted by α2 (reference numeral 36-3). In addition, the etching process used has the undesirable side effect that microgrooves 34 are formed in each case at the transition between the flank regions and the bottom region 32.

[0089] Figure 3 The depth d of the micro grooves M The profile depth 36-1, wing angles 36-2 and 36-3 of the groove 28, the depth 36-4 of the microgrooves 34, and optional other parameters are referred to as profile properties 36 of the contour 26 of the shape measurement structure 16. If a target profile is assumed for the groove 28 with vertical wing areas, a flat bottom area without microgrooves 34, and a predefined profile depth, the profile properties 36 are parameters that can be used to characterize the manufacturing deviation of the actual contour 26 of the shape measurement structure 16 from its target profile. However, the manufacturing deviation may be different depending on the target profile. Figure 6 This causes undesirable aberrations in the wavefront of the test wave 122 in the test interferometer 100 .

[0090] The lack of correction for these aberrations in the evaluation of the intensity distribution recorded by the detector camera 128 then leads to defects in the shape of the optical surface 102 determined by the test interferometer 100. However, in order to be able to correct for manufacturing deviations in the shape measurement structure 16, the above reference is provided on the use surface 14 of the diffractive optical element 10. Figure 1 The test field 18 mentioned is in this case based on a test structure 38 contained therein, so that the profile properties 36 of the shape measuring structure 16 can be estimated as accurately as possible and provided to the evaluation unit 130 of the test interferometer 100 for defect correction when determining the shape of the optical surface 102 .

[0091] Figure 4An embodiment of one of the test fields 18 is illustrated in plan view. The test field is configured as a matrix consisting of test field segments 40 arranged in five rows and five columns. Other embodiments of the test field 18 may also include a matrix consisting of more or fewer rows and columns. In the embodiment shown, the test field segments 40 have a rectangular shape, in particular a square shape, and are represented according to the scheme 40-CR, where "C" represents the relevant column in the matrix and "R" represents the relevant row in the matrix. In this regard, for example, Figure 4 The test field section denoted by "H1" in FIG. 1 is denoted by reference numeral 40-21. According to one embodiment, the side length of the test field 18 is in the range of 0.1 mm to 3 mm, in particular in the range of 0.5 mm to 1.5 mm.

[0092] Figure 5a The plan view shows Figure 4 , a test field segment 40-21 identified by "H1". The test field segment, like the test field segments identified by "H3", "H4", "H5", "H6", "H7" and "H8", is also configured as a so-called horizontal line test field segment and for this purpose comprises a test structure 38, whose structural pattern 39 comprises parallel straight lines 42 which are periodically arranged horizontally (i.e. in the x-direction) in a plan view. The lines 42 are separated by corresponding gaps 44. The lines 42 also have an upper edge 46-1 and a lower edge 46-2. The structural pattern 39 of the horizontal line test field segment 40-21 having an upper edge 46-1 and a lower edge 46-2 comprises in each case edges which are periodically repeated and oriented in the same manner.

[0093] The period of edge 46-1 or, respectively, 46-2 is given by Figure 5a The period p in the diffraction measurement station 60 is indicated, and according to one embodiment, the period is lower than the resolution of the diffraction measurement station 60 operating with visible light, as referred to below. Figure 7 The diffraction measuring station is explained in more detail. According to one embodiment, such a diffraction measuring station has a resolution below 300 μm, in particular below 100 μm, below 50 μm or below 10 μm.

[0094] According to various embodiment variants, the period of the edge 46-1 or respectively 46-2 may lie, for example, between 100 nm and 1 μm, in particular between 300 nm and 800 nm, for example approximately 500 nm. In this case, the ratio of the respective width of the line 42 to the respective width of the gap 44, the so-called width / gap ratio, may lie between 1:1 (e.g. Figure 5a The ratio varies between 1:10 and 1:10. Figure 4 The test field sections identified by "H2", "H3", "H4", "H5", "H6", "H7" and "H8" include Figure 5aPeriodically arranged parallel straight lines 42 of the type shown may have different width / gap ratios and / or different periods p; in particular, they may comprise lines 42 having different widths and having the same period.

[0095] If we observe along line III-III′ in the cross section, Figure 5a If a test structure 38 is formed, an outline 58 of the relevant part of the test structure 38 appears, which corresponds in structure to the following. Figure 3 The contour line 58 of the relevant part of the shape measurement structure 16 is shown. That is, according to Figure 5a , the profile of the sequence of voids 44, lines 42 and further voids 44 along the line III-III′ is likewise the profile of a groove 28 extending along the x-direction. The groove 28 also has flank regions 30, a bottom region 32 and microgrooves 34. Therefore, corresponding profile properties 36 can also be determined for the test structure 38, in particular the profile depth 36-1 of the groove 28, the flank angles 36-2 and 36-3 and the depth 36-4 of the microgrooves 34, and the corresponding profile properties 36 are used as an approximate estimate of the corresponding profile properties 36 of the shape measurement structure 16 arranged in the environment of the relevant test field 18.

[0096] Figure 5b The plan view shows Figure 4 The test field segment 40-11 is identified by "V1" in FIG. The test field segment, like the test field segments identified by "V2", "V3", "V4", "V5", "V6" and "V7", is also configured as a so-called vertical line test field segment. The vertical line test field segment includes a test structure 38, whose corresponding structure pattern 39 includes parallel straight lines 42 arranged periodically vertically (i.e. in the y direction) in a plan view. Here, according to Figure 5b The structural pattern 39 is based on Figure 5a The result of rotating the structural pattern 39 by 90°. For the right edge 46-3 and the left edge 46-4 of the line 42, the above given Figure 5a The same explanations as for the edges 46-1 and 46-2 of the horizontal line test field segment 40-21 apply to their periods. Figure 5b The test structure 38 is observed along the line III-III′ extending in the x-direction in the cross section, then Figure 3 The contour line 58 shown in the yz cross section in FIG. 5 occurs analogously for the xz cross section. Figure 4 The test field sections identified by "V2", "V3", "V4", "V5", "V6" and "V7" in FIG. Figure 5b The type shown has periodically arranged parallel straight lines 42 with different width / gap ratios and different periods p.

[0097] Figure 5cThe plan view shows Figure 4 4 . The test field segment 40-22 is identified by "B1" in FIG. 4 . This test field segment, like the test field segments identified by "B2", "B3" and "B4", is also configured as a so-called brick pattern test field segment. For this purpose, the test field segment 40-22 comprises rows of periodically arranged two-dimensional structures in the form of rectangular structures 50 or brick-shaped structures - arranged in the horizontal direction. The rows are repeated in the y direction, interrupted by linear gaps 44, wherein consecutive rows are offset in each case in the x direction, thus producing a brick pattern. In an alternative representation, according to Figure 5c The structural pattern also corresponds to Figure 5a The difference is that regular breaks 48 are provided in the lines 42. Figure 4 The test field sections identified by "B2", "B3" and "B4" in each include Figure 5a , but differs, inter alia, in the orientation of lines 42, the period of lines 42, the line / space ratio, the period of breaks 48, and / or the offset pattern of the brick rows.

[0098] Figure 5d and 5e The plan view shows Figure 4 4 and 5. The test field segments 40-33 and 40-43 are respectively identified by "F1" and "F2". These test field segments and the test field segment identified by "F3" are configured as so-called F-pattern test field segments and, for this purpose, include a two-dimensional structure in the form of the letter "F" arranged periodically. The structural patterns 39 in the various F-pattern test field segments differ in period, spacing and size of the letter "F" (e.g. Figure 5d and 5e The structural pattern 39 in the figure may be different) and / or the orientation of the letter "F" may be different.

[0099] The structural patterns 39 of the test structures 38 in the individual test field sections 40 of the above-mentioned test field 18 are selected in a targeted manner so that some or all of the above-mentioned profile properties 36 can be measured with a particularly high measurement accuracy by means of a measuring device provided therefor. In any case, the structural patterns 39 of the test structures 38 are configured in such a way that some or all of the profile properties 36 are measurable by means of a measuring device provided therefor with an increased measurement accuracy compared to the measurement accuracy achievable during the measurement of the corresponding profile properties 36 of the shape measurement structure 16. In particular, the above-mentioned diffraction measuring station 60 and the following with reference to Figure 8 A scanning probe microscope 84 , explained in more detail, may be envisaged as a measuring device for measuring the profile property 36 .

[0100] Figure 4 The brick pattern test field segments identified by "B1" to "B4" in Figure 5c) and the F pattern test field segments identified by "F1" to "F3" (see Figure 5d and 5e ), in addition to being applicable to measuring profile properties indicative of a profile line 58 extending transversely relative to the use surface 14, is also applicable to measuring one or more profile properties of the test structure 38. The measured profile properties of the test structure 38 can be applied to the associated shape measurement structure 16. This is similar to how the profile properties 36 are applied, as described below.

[0101] The contour property is understood to mean a shape property of the structure pattern of the test structure 38 extending along the use surface 14, as described below with reference to Figures 14a to 14c The figures each show a diagram according to Figure 5c The different actual shapes 50a of the rectangular structure 50 deviate from the target shape 50s of the rectangular structure 50 in different ways. These deviations are classified as contour properties. Fig.14a In , the actual shape 50a differs from the target shape 50s in the isotropic displacement of the rectangular edges and the rounding of the corners. Fig.14b In , the edge offset depends on its immediate surroundings. Also, the rounded corners are more pronounced. Fig.14c A particularly clear example of anisotropic edge shift is shown. The top and bottom edges are further away from the position than the left and right edges. Figures 14a to 14c The rounding and edge displacement visible in φ 1 are typically due to diffusion of the photolithographic parts and the so-called proximity effect during the manufacturing process of the diffractive optical element 10 .

[0102] according to Figure 5a or corresponding Figure 5b The embodiment of the structure pattern 39 with lines and spaces having a period of approximately 500 nm and a width / gap ratio of 1:1 is particularly suitable for checking the flank shape of the test structure 38 by means of the diffraction measuring station 60. The intensity values ​​determined by means of the diffraction measuring station 60 for this embodiment of the structure pattern 39 show a high correlation with the flank shape of the test structure 38. Thus, in particular the flank angles 36-2 and 36-3 of the contour line 58 can be derived with high accuracy from the correlated intensity values.

[0103] Using more complex two-dimensional structures, such as Figure 5c The brick pattern test field section or according to Figure 5d or Figure 5e The two-dimensional structure in the F-pattern test field section of the test structure 38 is used in particular to provide a geometric shape in the structure pattern 39 of the test structure 38 which, in addition to the straight line pattern, approximates other geometric shapes contained in the structure pattern 24 of the shape measurement structure 16. In this regard, for example, from Figure 5c The brick pattern test field section has a rectangular structure of 50 and according to Figure 5d and5f The F structures 52 are suitable for emulating transitions or island structures between structural elements oriented perpendicularly to one another in the structural pattern 24 of the form measuring structure.

[0104] Figure 5f Shown according to Figure 4 The diffractive optical element 10 in the region of the test field segment 40-34 is configured as an unstructured test field segment. The diffractive shape measurement structure 16 is arranged on the so-called used side 55 of the disk-shaped diffractive optical element 10. In the present embodiment, an antireflection coating 56 is applied on the back side 57 of the diffractive optical element 10, which is opposite to the used side 55. The coating is adapted to the wavelength of the test radiation 108i incident on the diffractive optical element 10 in the test interferometer 100, i.e. the antireflection coating 56 is configured such that there is almost no intensity loss when the test radiation 108 enters the diffractive optical element 10 on its back side.

[0105] However, if the test radiation 64 having a different wavelength passes through the following reference Figure 7 The effect of the antireflection coating 56 changes when the diffractive optical element 10 is incident on the diffractive optical element 10 during measurement of the test field 18 by a diffraction measuring station 60 explained in more detail. A portion of the test radiation 64 is reflected at the back side 57 of the diffractive optical element 10 (reflected test radiation 64r). The intensity of the transmitted test radiation 64t is reduced accordingly.

[0106] In this case, depending on the wavelength of the test radiation 64, the antireflection coating 56 may also have the effect of reducing the reflection (destructive interference) or even enhancing the reflection (constructive interference) by comparison with the reflection from the back side 57 without the antireflection coating 56. The unstructured test field segment 40-34 is then used to determine the influence of the antireflection coating 56 on the intensity of the transmitted test radiation 64t in the diffraction measurement station 60.

[0107] For this purpose, in the diffraction measuring station 60, measurements of the unstructured test field segments 40-34 and corresponding measurements without the arrangement of the diffractive optical element 10 are carried out for different wavelengths of the test radiation 64. Thus, by applying the Fresnel formula, the corresponding influence of the antireflection coating 56 on the measurements of the other test field segments 40 of the test field 18 is determined and correspondingly taken into account in the evaluation of these measurements.

[0108] Fig.13a The diffractive optical element 10 (see Figure 1 ) is an example of the transmission behavior of a test radiation 64 with a specific wavelength used by a diffraction measuring station 60 at a test field 18 of a ) arranged along a line extending in the x direction. In this case, the corresponding transmittance value T is determined based on the corresponding test field segment 40-34 of the relevant test field 18. From Fig.13a It can be clearly seen that the transmittance for a given wavelength decreases towards the edge of the diffractive optical element 10. To illustrate the correction principle, Fig.13b Shown based on Fig.13a The transmission behavior is corrected before and after the wing angle α (according to Figure 3 36-2 or 36-3), which is determined by evaluating the measurement performed by the diffraction measuring station 60 at the wavelength.

[0109] also, Figure 4 The test field 18 shown includes a monitoring test field segment 40-32 identified by the letter "K". The monitoring test field segment contains a so-called monitoring structure corresponding to the shape measurement structure 16 arranged in the use surface 14. In other words, a segment of the pattern of the use surface 14 with the shape measurement structure 16 is arranged in the monitoring test field segment 40-32. The monitoring test field segment 40-32 (also referred to as a background window) is also used in the diffraction measurement station 60 or the scanning probe microscope 84 to monitor the measurement to determine whether the area where the relevant test field 18 is arranged on the diffractive optical element 10 represents an area of ​​the use surface 14 adjacent to the relevant test field 18, so that the profile property 36 determined by the measurement of the test field 18 at the test structure 38 can be applied to the relevant shape measurement structure 16.

[0110] also, Figure 4 The test field 18 shown comprises a so-called reference test field section 40-14. The test field section comprises reference structures 54 which can be resolved by an optical microscope. These reference structures 54 may comprise markings which can be used to verify the correct alignment of the test field with respect to rotation and with respect to translation. In addition, the reference structures 54 may each comprise an identification number for unambiguously assigning a measurement established with respect to a specific test field 18 in order to design the properties of the test structure 38 on which the measurement is based. The information from the reference test field section 40-14 is used in particular during the measurement of the diffractive optical element 10 by means of a scanning probe microscope 84 which is described in more detail below.

[0111] Figure 7An embodiment of the above-mentioned diffraction measuring station 60 for measuring the profile property 36 of a test structure 38 in a test field 18 of a diffractive optical element 10 is shown. The diffraction measuring station 60 comprises a tunable test radiation source 62 for generating the test radiation 64 already mentioned above, which is monochromatic, the wavelength of which is tunable in a wavelength range between approximately 300 nm and 800 nm. Furthermore, the diffraction measuring station 60 comprises a first focusing lens element 66 for focusing the test radiation 64 provided by the test radiation source 62, a polarizer 68 arranged in the vicinity of the focus of the first focusing lens element 66 and for polarizing the test radiation 64, and a second focusing lens element 70 for radiating the test radiation 64 in the form of a plane test wave 72 onto the entire area of ​​the diffractive optical element 10 to be measured with respect to the profile property 36.

[0112] After passing through the diffractive optical element 10, the test wave 72 passes through two Fourier optical units 74 and 80 (the second Fourier optical unit 80 is at Figure 7 The optical element 10 is represented by two lens elements in the figure) and is directed to a surface measurement detector 82, which can be implemented as a CCD sensor, for example. The aperture stop 76 arranged between the two Fourier optical units 74 and 80 is used to eliminate radiation of diffraction orders higher than the zeroth order diffraction. In the present embodiment, the detector 82 is used to generate spatially resolved information about the brightness or intensity distribution provided by the diffractive optical element 10 in the zeroth order diffraction.

[0113] In accordance with Figure 7 In the embodiment of the present invention, the resolution of the diffraction measuring station 60 (in which only the intensity distribution generated by the zeroth order diffraction is evaluated) lies between 10 μm and 300 μm and is therefore much higher than that contained in the embodiment according to Figures 5a to 5e The periods in the structure pattern 39 of . These structure patterns cannot therefore be resolved during the measurement by the diffraction measuring station 60 . Instead, grayscale or intensity values ​​appear for the individual test field segments 40 .

[0114] Through the Figure 7 During the measurement of the profile property 36 of the test structure 38 by the diffraction measuring station 60, the following procedure is adopted: through the corresponding operation of the tunable test radiation source 62 and the arrangement of different variations of the polarizer 68, the test waves 72 with various combinations of different wavelengths and different polarization settings are successively radiated onto the diffractive optical element 10. Depending on the polarization property of the polarizer 68, a suitable analyzer 68 is arranged.

[0115] According to one embodiment, 7 to 12 different wavelengths are combined with 2 to 4 different polarization settings. The wavelength used is preferably between 300nm and 800nm; in this regard, for example, wavelengths of 300nm, 350nm, 400nm, 450nm, 500nm, 550nm, 600nm, 650nm, 700nm, 750nm and 800nm ​​can be used. For example, polarization directions 0°, 45°, 90° and 135° can be used as polarization settings. In the example mentioned, test waves 72 with 11x4, i.e. 44, different combinations of wavelength and polarization arrangements are radiated onto the diffractive optical element.

[0116] For each of the different combinations of wavelength and polarization settings, the detector 82 of the diffraction measuring station 60 records an associated intensity distribution containing intensity values ​​assigned to the individual test field segments 40 on the surface 12 of the diffractive optical element 10 .

[0117] In the evaluation unit 83, an evaluation algorithm is used to take into account the Figures 5a to 5e All intensity values ​​40 of all recorded intensity distributions assigned to the individual test field segments 40 are processed based on the design information of the various structure patterns 24 of the diffractive optical element 10. This processing results in profile properties 36 of the contour lines 58 of the test structures 38, in particular the corresponding profile depths 36-1, the corresponding flank angles 36-2 and 36-3 of the contour lines 58 of the test structures 38 contained in the various test field segments 40 and the corresponding depths 36-4 of the microgrooves 34. Other profile properties 36 of the contour lines 58 which can be determined as a result of the evaluation of the intensity distributions include, for example, so-called "notches" or so-called "grooves", in which the lower corners in the contour lines 58 are indented or rounded. Furthermore, material properties of the diffractive optical element 10, such as changes in the refractive index, surface roughness or changes in the above-mentioned profile properties of the test structures 38 can also be determined by the evaluation. In particular, the structure width of the test structure can also be determined.

[0118] According to one embodiment, in this case, the following are processed: all vertical line test field segments 40-11, 40-12, 40-13, 40-15, 40-25, 40-35, 40-45, all horizontal line test field segments 40-21, 40-31, 40-41, 40-51, 40-52, 40-53, 40-54, 40-55, all brick pattern test field segments 40-22, 40-42, 40-23, 40-24, all F pattern test field segments 40-33, 40-43, 40-44, as well as the monitoring test field segment 40-32 and the unstructured test field segment 40-34.

[0119] From the determined profile properties 36, the evaluation unit 83 determines, by means of a rigorous computation-based evaluation algorithm, the calibration values ​​86 already mentioned above for the shape measurement structures 16 contained on the use surface 14 of the diffractive optical element 10. In this case, according to an embodiment variant, in the calibration values ​​86, the profile properties 36 determined for the individual test fields 18 distributed on the use surface 14 are respectively assigned to the shape measurement structures 16 arranged in the region of the use surface 14 adjacent to the respective test fields 18. Alternatively, the local assignment of the profile properties assigned to the shape measurement structures 16 can also be determined by interpolation of the profile properties determined at the individual test fields 18.

[0120] According to one embodiment, when determining the calibration value 86 of the shape measurement structure 16 , the evaluation unit 83 uses the depth d p The relationship between the width b—determined on the basis of the test field 18—(see Figure 3 ). Although this relationship can be based on any function, in principle, Fig.11 As shown, usually, the groove depth d p The larger , the greater the width b of the trench 28. This effect occurs in particular in etching processes on a length scale of a few hundred nm.

[0121] As already mentioned above, the effect cannot be directly detected due to the resolution of the diffraction measuring station 60 being higher than 10 μm. As already mentioned above, the horizontal line test field segments 40-21, 40-31, 40-41, 40-51, 40-52, 40-53, 40-54, 40-55 and the vertical line test field segments 40-11, 40-12, 40-13, 40-15, 40-25, 40-35, 40-45 comprise regularly arranged lines 42 with different widths. The evaluation of the intensity values ​​of the individual test field segments recorded by the diffraction measuring station 60 makes it possible to determine the depth d of the groove 28, allocating it from the profile width b known in the design. p The relationship between and width b.

[0122] A similar procedure can be used in the case of two-dimensional structures in the brick pattern test field segments 40-22, 40-42, 40-23, 40-24 and the F pattern test field segments 40-33, 40-43, 40-44, the "groove width" then varying in both dimensions. This relationship is assumed to represent a general structure and is taken into account when determining the calibration value 86 of the shape measurement structure 16 in the evaluation unit 83. Similar to the profile depth d p As a function of the lateral structure definition, there may also be changes in the structure width, such as the groove width b, and other profile parameters, such as the depth d of the microgrooves 34. p (See Figure 3). According to one embodiment, these variations are similarly detected and processed. For two-dimensional structures, in particular, the profile variations can be detected as a function of the surrounding trench width - proximity effects of the photolithography process generated by the CGH. Typical examples are rounding of inner and outer corners (F structures) and rectangular non-isotropic shape defects.

[0123] Figure 8 A scanning probe microscope 84 (also referred to as SPM) is described, which has been mentioned above as an alternative measuring device for measuring the profile property 36 of the test structure 38. In the example shown, the scanning probe microscope 84 is a scanning force microscope or atomic force microscope (also referred to as AFM). A measuring head 89 is incorporated into the scanning probe microscope 84.

[0124] The measuring head 89 is fixed to the frame of the scanning probe microscope 84 by the holding unit 87 ( Figure 8 The frame allows the diffractive optical element 10 to be positioned at any position in any orientation. The holding unit 87 can be rotated about its longitudinal axis extending in the horizontal direction. The piezoelectric actuator 88 is attached to the holding unit 87 of the measuring head 89 and enables the free end of the piezoelectric actuator 88 to move in three spatial directions ( Figure 8 A bending beam, hereinafter referred to as cantilever 90 , as is conventional in the art, is fixed to the free end of the piezoelectric actuator 88 .

[0125] The cantilever 90 has a retaining plate ( Figure 8 ). The opposite end of the cantilever 90 relative to the holding plate carries a measuring probe 92. In the embodiment shown, the measuring probe 92 is embodied such that it is pyramidal or conical at its free end. Furthermore, the measuring probe 92 can also be embodied such that it is cylindrical or inverted conical or hammer-shaped (also called "concave"), for example.

[0126] The cantilever 90 and the measuring probe 92 can be implemented in one piece. As an example, the cantilever 90 and the measuring probe 92 can be manufactured from a metal such as tungsten, cobalt, iridium, a metal alloy or from a semiconductor such as silicon or silicon nitride. It is also possible to manufacture the cantilever 90 and the measuring probe 92 as two separate components and subsequently connect them to each other. This can be achieved, for example, by adhesive bonding. In particular, the measuring probe 92 can also be manufactured in two separate steps.

[0127] The diffractive optical element 10 to be measured is fixed on a sample stage 94. This can be achieved, for example, by placing the diffractive optical element 10 on a supporting point of the sample stage 94 in a vacuum or high vacuum environment.

[0128] like Figure 8As indicated by the arrows, the sample stage 94 can be moved in three spatial directions by the positioning system 96 relative to the measuring head 96 of the scanning probe microscope 84. In addition, the sample stage 94 can be rotated around the normal of the diffractive optical element 10 ( Figure 8 (not shown). Figure 8 In the example of , the positioning system 96 is implemented in the form of multiple micromanipulators. In addition or alternatively, the positioning system 96 can be equipped with a stepper motor and / or a linear drive for moving the diffractive optical element 10. An alternative embodiment of the positioning system 96 can be a piezoelectric actuator. The positioning system 96 is controlled by a signal from a control unit. In an alternative embodiment, the control unit does not move the sample stage 94, but moves the holding unit of the measuring head 89 of the scanning probe microscope 84. In addition, the control unit can perform a rough positioning of the diffractive optical element 10 used as a sample in height (z direction), and the piezoelectric actuator 88 of the measuring head 89 can perform a precise height setting of the scanning probe microscope 84.

[0129] Alternatively or additionally, in other embodiments, the relative movement between the sample and the measurement probe 92 may be divided between the positioning system 96 and the piezoelectric actuator 88. As an example, the positioning system 96 effects the movement of the sample in the sample plane (xy plane), and the piezoelectric actuator 88 enables the measurement probe 92 to move in the normal direction of the sample.

[0130] The scanning probe microscope 92 can be operated in one-dimensional or two-dimensional measurement mode. In the one-dimensional measurement mode, the measurement probe 92 scans the sample in a linear manner in a predefined measurement direction, achieving a high spatial resolution in the scanning direction due to the selected line spacing and a relatively low spatial resolution transversely to the scanning direction (typically, the spatial resolution is reduced by a factor of 100). In the two-dimensional measurement mode, the line spacing is reduced, so that a high spatial resolution is also achieved transversely to the scanning direction, for example, the spatial resolution is only 10 times lower than in the scanning direction. However, the two-dimensional measurement mode is significantly more time-consuming than the one-dimensional measurement mode and is therefore avoided as much as possible.

[0131] In the one-dimensional measurement mode, scanning is preferably performed perpendicular to the plane defined by the axis of symmetry of the cantilever 90 and the measurement probe 92, resulting in reduced measurement artifacts. In the two-dimensional measurement mode, the measurement probe is preferably moved rapidly perpendicular to the axis of symmetry, while slower movements are performed perpendicular to the axis of symmetry.

[0132] Figure 9a An embodiment of a structure pattern 24 of a diffractive shape measurement structure 16 extending along the use surface 14 is shown - different from Figure 2The details shown in FIG. 8 are other details. In this case, the groove-shaped structure has a very irregular shape in cross section, with the flanking regions having different orientations. If the structure pattern shown is then measured by the above-mentioned scanning probe microscope 84 in a one-dimensional measurement mode along the scanning line BB′ in the horizontal scanning direction (x direction), then Figure 9b The contour line 26 shown in FIG.

[0133] However, the exact structure of the contour line 26 is extremely dependent on the exact position of the scan line in the y-direction. Furthermore, the flank angles of the measured contour line 26 are corrupted by the pyramidal or cone-like shape of the measuring probe 92. However, the extent of the corruption depends on the orientation of the flank region 30 in the xy plane, i.e., the flank orientation along the scan line needs to be known very accurately in order to accurately calculate the influence of the shape of the measuring probe 92. Due to these measurement uncertainties, the measurement results determined during the measurement of the structure pattern 24 of the real shape measurement structure 16 in the one-dimensional measurement mode of the scanning probe microscope 84 are usually too inaccurate for the purpose of correcting the surface measurement in the test interferometer 100. In contrast, the measurement in the two-dimensional measurement mode is often too complex.

[0134] According to one embodiment of the invention, then, the structure pattern 39 of the test structure 38 arranged in the test field 18 is measured instead of the shape measurement structure 16. Figure 4 , all vertical line test field segments 40-11, 40-12, 40-13, 40-15, 40-25, 40-35, 40-45, all horizontal line test field segments 40-21, 40-31, 40-41, 40-51, 40-52, 40-53, 40-54, 40-55 and all brick pattern test field segments 40-22, 40-42, 40-23, 40-24 are applicable here. Fig.10b Shows Fig.10a The contour line 58 of the structural pattern 39 in the form of a vertical straight line along the scanning line BB' is shown. In this case, since the orientation of the flank area varies independently of the position of the scanning line in the y direction and does not vary along the scanning line, the influence of the shape of the measuring probe 92 can be calculated very accurately from the measured contour line 58.

[0135] As mentioned above Fig.11 The depth d of the groove 28 is pThe relationship to the width b can be determined with particularly high accuracy by corresponding measurements of the relevant profiles in the horizontal line test field segments 40-21, 40-31, 40-41, 40-51, 40-52, 40-53, 40-54, 40-55 and the vertical line test field segments 40-11, 40-12, 40-13, 40-15, 40-25, 40-35, 40-45 by the scanning probe microscope 84. The same applies to the two-dimensional structures in the brick pattern test field segments 40-22, 40-42, 40-23, 40-24 and the F pattern test field segments 40-33, 40-43, 40-44. During the measurement, the reference test field segment 40-14 is preferably used for the correct alignment of the corresponding test field 18.

[0136] Fig.12a The test field section 40 is shown in detail in another embodiment, which essentially has Figure 5c The inverted structure of the brick pattern test field segment 40-22 shown in FIG. and rotated 90°. Figure 5c In contrast to the structural pattern of (wherein the rectangular structure 50 is formed by the grooves), according to Fig.12a The rectangular structures 50 in the structural pattern are surrounded by trenches 28 .

[0137] Figure 12b The scanning probe microscope 84 is shown along Fig.12a BB′ in FIG. 5 . The contour line clearly reveals that the maximum groove depth is in the intersection region between the vertical and horizontal groove sections, while the groove depth has a minimum value 98 at half the distance between the intersection regions, because there the small groove width is dominant. The groove depth characteristics shown can be measured very accurately using a scanning probe microscope 84, and the correspondence with the structure pattern design can be used when determining the calibration value 86 of the shape measurement structure 16.

[0138] The contour properties 36 of the contour line 58 of the test structure 38 are determined in the evaluation unit 97 of the scanning probe microscope 84 by measuring from the vertical line test field segments 40-11, 40-12, 40-13, 40-15, 40-25, 40-35, 40-45, the horizontal line test field segments 40-21, 40-31, 40-41, 40-51, 40-52, 40-53, 40-54, 40-55, the brick pattern test field segments 40-22, 40-42, 40-23, 40-24 and the F pattern test field segments 40-33, 40-43, 40-44, taking into account the monitoring test field segment 40-32 and optionally taking into account the above-mentioned relationship between the design dimensions of the corresponding structural pattern and the groove depth. As measured by diffraction measurement station 60 , profile properties 36 may include profile depth 36 - 1 , corresponding wing angles 36 - 2 and 36 - 3 , and corresponding depth 36 - 4 of microgrooves in profile lines 58 of test structures 38 included in various test field segments 40 .

[0139] Other profile properties 36 of the contour line 58 that can be determined by means of the scanning probe microscope 84 include, for example, the “notches” or “grooves” that have already been explained with respect to the diffraction measuring station 60. Furthermore, by means of the scanning probe microscope 84, material properties of the diffractive optical element 10 can also be determined with regard to the surface roughness or topographic properties of the test structure 38. In particular, the structure width of the test structure can also be determined.

[0140] Similar to the way the evaluation unit 83 of the diffraction measuring station 60 operates, Figure 8 The evaluation unit 83 depicted in FIG. 8 determines calibration values ​​86 for the form measurement structure 16 contained on the use surface 14 of the diffractive optical element 10 as a function of the determined profile property 36 .

[0141] According to an embodiment variant according to the invention, in each case, as described above, at least some of the following are measured both by the diffraction measurement station 60 and by the scanning probe microscope 84: the vertical line test field segments 40-11, 40-12, 40-13, 40-15, 40-25, 40-35, 40-45, the horizontal line test field segments 40-21, 40-31, 40-41, 40-51, 40-52, 40-53, 40-54, 40-55, the brick pattern test field segments 40-22, 40-42, 40-23, 40-24 and the F pattern test field segments 40-33, 40-43, 40-44. Based on the measurement results, optionally taking into account the above-mentioned relationship between the design dimensions of the monitoring test field segment 40-32, the unstructured test field segment 40-34 and the corresponding structure pattern and the trench depth, the profile property 36 of the contour line 58 of the test structure 38 is determined. In other words, the test field is measured by a plurality of different measurement methods, in the present case by a measurement method based on a diffraction measurement station 60 and a scanning probe microscope 84, and the profile property 36 is determined by calculating the measurement results determined by the different measurement methods.

[0142] Other measurement methods that can be used here include transmission electron measurements (TEM), measurements using near-field scanning microscopes, such as the so-called TSOM (through-focus scanning optical microscope), x-ray measurements (XRT), and scatterometry methods performed independently of the diffraction measurement station, such as goniometery, ellipsometry, reflectometry, etc.

[0143] The calculation of the measurement results determined by different measurement methods can be realized, for example, by the Bayesian method described in the publication "Improving optical measurement uncertainty with combined multitool metrology using a Bayesian approach" published in Applied Optics, Vol. 51, No. 25, pp. 6196-6206, September 2012, which is realized by back and forth iteration and / or by parameter separation. In addition, a common comprehensive model can be used for different measurement methods.

[0144] In accordance with Fig.15 An exemplary embodiment of calculating the measurement results determined by the diffraction measuring station 60, wherein the measurement results are determined by alternative measurement methods, is shown in the flow chart of . According to this exemplary embodiment, three different data sets are calculated by nonlinear fitting using the least squares method.

[0145] The first data set consists of various parameters {Pi} i The diffraction efficiency is determined by theoretical prediction or calculation. i} i is a set of parameters describing the geometrical surface of the diffractive optical element 10. These parameters relate in particular to geometrical properties, such as profile properties and / or profile properties, of the test structure 38 contained in the test field 18. In this regard, for example, P1 = etching depth, P2 = flank angle and P3 = web width / profile variation.

[0146] The second data set includes weights w(x, y) determined by measuring diffraction efficiency (referred to above as intensity values) by the diffraction measurement station 60, data conditioning, and corresponding estimated weights w(x, y). The third data set includes weights w(x, y) determined by determining measured values ​​using an alternate measurement method (e.g., a measurement method performed by a scanning probe microscope 84), data conditioning, and corresponding estimated weights w(x, y).

[0147] Data conditioning is usually necessary because the measuring device typically does not measure the relevant parameter directly, but only measures data related to it. As an example, the scanning probe microscope 84 measures the relative height change, and the absolute etch depth can then be derived via an external calibration sample. In a similar manner, the diffraction measurement station 60 performs two measurements with and without the diffractive optical element 10 in the beam path; the resulting diffraction efficiency is the quotient of these two measurements.

[0148] The estimation of the weights of the measurement information is performed in order to correctly take into account their different measurement accuracy in the case of mixed use of a plurality of measurement devices present here. As an example, a scanning probe microscope 84 can be implemented in particular, since it can predict the etching depth particularly accurately, whereas microgrooves can only be measured very roughly by a scanning probe microscope. According to one embodiment variant, the weights are selected relative to the inverse of the measurement error, i.e. Where x, y indicate the position on the diffractive optical element 10 and ΔP i is the measurement error of the ith parameter.

[0149] The result of calculating the three data sets by nonlinear fitting using the least squares method is the parameter {P i} i The use of least squares fitting (also called "nonlinear least squares fitting") is a form of nonlinear regression in which the weighted squares of the differences in the individual measurement channels are minimized:

[0150]

[0151] In this case, w(x, y; λ, p) represents the weight at the position (x, y) of the diffractive optical element 10 for measurement at wavelength λ and polarization p. m (x, y; λ, p), similarly represents the measured intensity of the zeroth order diffraction with respect to x, y, λ, p. R (x, y; λ, p; {P i (x, y)} i ) represents the manufacturing parameter P of the diffractive optical element 10 at this position. i The second part shows that the geometric parameter P can be directly accessed using a second measurement device (such as a scanning probe microscope 84, for example). i . w(x0, y0; x, y) describes the parameters P i The possibility of such a measurement does not take place at the position (x, y) where the reconstruction is carried out, but at a position (x0, y0) of the test field 18 which is slightly further away. i m represents the measured value of the parameter, and P i ——Similar to I R What happens - is the changing parameters of the fit.

[0152] Since the number of measurement channels in the diffraction measurement station is limited and certain residual errors may also occur in the measurement data, when using a detailed CGH manufacturing defect model in practice, it may occur that a variety of combinations of CGH manufacturing defects can reasonably explain the measured diffraction efficiency. This corresponds to a plurality of local minima in the evaluation function during the reconstruction of the profile parameters. It is not possible to state which local minimum is the physical minimum (the profile parameter that most likely describes the actually manufactured CGH), not even by comparing their "depths", i.e. the "depth" of the evaluation function values. By defining the profile parameters, a second measuring device (e.g. a scanning probe microscope, in particular an AFM) can then decide which local minimum is the physical minimum.

[0153] Furthermore, it may be difficult or even impossible to identify specific profile parameters in the zeroth order of a diffraction measurement. An example of this is the profile wing angle. In particular, the case of asymmetric wing angles, such as a left wing of 85° and a right wing of 95° (overhang) are undetectable in the zeroth order of diffraction for symmetry reasons with respect to sign (i.e. whether the left or right wing is overhanging). However, the sign is crucial in the phase used by the form measurement structure (i.e. for the diffraction order). The sign can be correctly defined here by measurements in the marking by means of a scanning probe microscope, in particular by means of an AFM. The jointly considered horizontal and vertical line structure also helps to detect pronounced tilts along the CGH radius.

[0154] The exemplary embodiments, embodiments or embodiment variations described above should be considered as examples. The present disclosure thus achieved first enables those skilled in the art to understand the present invention and the advantages associated therewith, and secondly covers obvious changes and modifications in the structures and methods within the understanding of those skilled in the art. Therefore, all these changes and modifications, as long as they fall within the scope of the present invention as defined by the attached claims, and equivalents are intended to be covered by the protection of the claims.

[0155] Reference numerals list

[0156] 10 Diffractive optical elements

[0157] 12 Surface

[0158] 14 Using Surfaces

[0159] 16 Diffraction shape measurement structure

[0160] 18 Test Site

[0161] 20 Preferred Areas

[0162] 22 Prohibited Areas

[0163] 24 Structural pattern of shape measurement structure

[0164] 26. Shape measurement of the structure's contours

[0165] 28 Grooves

[0166] 29 Materials

[0167] 30 Flanking Area

[0168] 32 Bottom area

[0169] 34 Microgrooves

[0170] 36 Contour properties

[0171] 36-1 Profile Depth

[0172] 36-2 Left wing corner

[0173] 36-3 Right wing corner

[0174] 36-4 Depth of micro grooves

[0175] 37 Groove Width

[0176] 38 Test Structure

[0177] 39 Structural pattern of test structure

[0178] 40 Test field section

[0179] 40-11 Vertical Line Test Field Section

[0180] 40-14 Reference test field section

[0181] 40-21 Horizontal test field section

[0182] 40-22 Brick Pattern Test Field Section

[0183] 40-32 Monitoring test field section

[0184] 40-33F Pattern test field section

[0185] 40-34 Unstructured test field section

[0186] 40-43F Pattern test field section

[0187] 42 Straight Line

[0188] 44 Gap

[0189] 46-1 Upper edge

[0190] 46-2 Lower edge

[0191] 46-3 Right Edge

[0192] 46-4 Left Edge

[0193] 48 Interrupt

[0194] 50 Rectangular structure

[0195] 50a The actual shape of the rectangular structure

[0196] 50s Target shape of rectangular structure

[0197] 52 F structure

[0198] 54 Reference Structure

[0199] 55 Side used

[0200] 56 Anti-reflective coating

[0201] 57 dorsal

[0202] 58 Test structure outline

[0203] 60 Diffraction measurement stations

[0204] 62 Tunable test radiation source

[0205] 64 Test Radiation

[0206] 64r Radiated by reflection test

[0207] 64t irradiation tested by transmission

[0208] 66 First focusing lens element

[0209] 68 Polarizer

[0210] 70 second focusing lens element

[0211] 72 test waves

[0212] 74 first focusing lens element

[0213] 76 Aperture stop

[0214] 78 Analyzer

[0215] 80 Second Fourier Optical Unit

[0216] 82 Detector

[0217] 83 evaluation units

[0218] 84 Scanning Probe Microscope

[0219] 86 Calibration value

[0220] 87 Holding unit

[0221] 88 Piezoelectric Actuator

[0222] 89 Measuring head

[0223] 90 Cantilever

[0224] 92 Measuring probe

[0225] 94 Sample stage

[0226] 96 Positioning System

[0227] 97 evaluation units

[0228] 98 Minimum groove depth

[0229] 99u Uncorrected wing angle distribution

[0230] 99K Corrected wing angle distribution

[0231] 100 Test interferometer

[0232] 102 Optical Surfaces

[0233] 104 Test Objects

[0234] 106 Test Radiation Source

[0235] 108 Test Radiation

[0236] 108i Incident Test Radiation

[0237] 110 Optical axis

[0238] 112 Beam Splitter

[0239] 114 focusing lens elements

[0240] 116 Reference Components

[0241] 118 Fizeau surface

[0242] 120 Reference Wave

[0243] 122 Test Wave

[0244] 122R Reflected test wave

[0245] 124 Aperture

[0246] 126 Eyepiece

[0247] 127 Interference Module

[0248] 128 detector camera

[0249] 130 evaluation units

Claims

1. A diffractive optical element for a test interferometer for measuring the shape of an optical surface, comprising: a diffractive shape measuring structure arranged on a use surface of the diffractive optical element and configured to generate a test wave for illuminating an optical surface to be measured when the diffractive optical element is arranged in the test interferometer, and at least one test field configured for measuring a plurality of profile properties of a test structure contained in said test field, wherein the profile properties characterize a profile of the test structure extending laterally relative to the use surface and include a flank angle of the profile of the test structure, a profile depth of the test structure, and a depth of microgrooves occurring in a bottom region of a groove-shaped profile of the test structure, and The test field is arranged at a position of the use surface other than the diffraction shape measurement structure, so that the test field is surrounded by a plurality of the diffraction shape measurement structures.

2. The diffractive optical element according to claim 1, in, At least ten test fields are arranged in each case at a plurality of locations of the usage surface instead of the diffraction shape measurement structure, so that a respective test field is surrounded by a plurality of the diffraction shape measurement structures.

3. The diffractive optical element according to claim 2, in, At least some of the test fields have a regular arrangement.

4. A diffractive optical element according to any one of the preceding claims, in, The test structure and the shape measurement structure each have a structural pattern extending along the usage surface and a contour line extending laterally relative to the usage surface and are characterized by at least one contour property, wherein the structural pattern of the test structure is configured so that a measurement accuracy achievable during measurement of the contour property of the test structure is improved compared to a measurement accuracy achievable during measurement of the contour property of the shape measurement structure.

5. The diffractive optical element according to any one of claims 1 to 3, in, The structure patterns of different test structures of a respective test field are configured for measuring different profile properties of the test structures.

6. The diffractive optical element according to any one of claims 1 to 3, in, The structure pattern of the test structure is configured for measurement by a diffraction measurement station.

7. The diffractive optical element according to any one of claims 1 to 3, in, The structural pattern of the test structure is configured for measurement by a scanning probe microscope.

8. The diffractive optical element according to any one of claims 1 to 3, in, The structural pattern of the test structure is configured for measurement by a diffraction measurement station and for measurement by a scanning probe microscope.

9. The diffractive optical element according to any one of claims 1 to 3, in, The test structure has a structural pattern with periodically repeating and identically oriented edges, wherein the period of the edges is below the resolution of a diffraction measuring station operating with visible light.

10. The diffractive optical element according to claim 9, in, The test fields each comprise a further test structure having likewise periodically repeating and identically oriented edges, wherein the edges of the further test structures are oriented transversely with respect to the edges of the test structure.

11. The diffractive optical element according to any one of claims 1 to 3, in, The test structure has a periodically arranged two-dimensional structure.

12. The diffractive optical element according to any one of claims 1 to 3, in, The test field comprises an unstructured test field section, wherein the diffractive optical element has an antireflection coating at least in the region of the unstructured test field section.

13. A test interferometer for measuring the shape of an optical surface, comprising: - a diffractive optical element according to any one of the preceding claims, and an interferometry module for generating an interference pattern by superimposing a reference wave with the test wave generated by the diffractive shape measurement structure after the test wave has interacted with the optical surface, and - an evaluation unit for determining the shape of the optical surface by evaluating an interference pattern taking into account at least one predetermined calibration value of the diffractive optical element.

14. A calibration method for calibrating a diffractive optical element, the diffractive optical element comprising diffractive shape measurement structures arranged on a use surface of the diffractive optical element, and each diffractive shape measurement structure is configured to generate a test wave when the diffractive optical element is arranged in a test interferometer, the test interferometer being configured to measure the shape of an optical surface, the test wave being used to illuminate the optical surface to be measured, the calibration method comprising the following steps: - arranging the diffractive optical element in a measuring device for measuring the diffractive optical element and measuring, by the measuring device, test fields arranged at a plurality of positions on the use surface and each having a test structure, the test structure having a contour line extending transversely relative to the use surface, for determining a plurality of profile properties of the test structure related to the contour line, wherein the profile properties include a flank angle of the contour line of the test structure, a profile depth of the test structure, and a depth of micro grooves occurring in a bottom region of a groove-shaped contour of the test structure, and - determining a calibration value for calibrating the form measurement structure during the measurement of the optical surface based on the determined profile property, Wherein, the measuring device comprises a diffraction measuring station and / or a scanning probe microscope.

15. The calibration method according to claim 14, in, The measuring device comprises the diffraction measuring station, during the measurement of the test field, a plurality of test waves, each differing in at least one optical parameter, are radiated onto the test field, and at least one of the profile properties is determined by evaluating the intensity distribution of the test waves recorded by the diffraction measuring station after interaction with the test field.

16. The calibration method according to claim 15, in, The different test waves differ in their wavelength and / or their polarization.

17. The calibration method according to any one of claims 14 to 16, in, The test field is measured by means of a plurality of different measuring methods and the profile property is determined by calculation of the measurement results determined by the different measuring methods.

18. A method for measuring the shape of an optical surface, comprising the following steps: - determining a calibration value of a diffractive optical element according to a calibration method according to any one of claims 14 to 17, - generating a test wave by means of a shape measuring structure of the diffractive optical element, - recording an interference pattern generated by superimposing a reference wave with said test wave after interaction with said optical surface, and - Taking into account the calibration values, determining the shape of the optical surface by evaluating the recorded interference pattern.

19. A method for manufacturing a diffractive optical element of a test interferometer for measuring the shape of an optical surface, comprising the following steps: - manufacturing a diffractive optical element with a diffractive shape measuring structure, which is arranged in each case on a use surface of the respective diffractive optical element and is configured to generate a test wave for irradiating the optical surface to be measured when the respective diffractive optical element is arranged in the test interferometer, wherein at least one test field is arranged at at least one position of the use surface of each diffractive optical element instead of the diffractive shape measuring structure, so that the at least one test field is surrounded by a plurality of the diffractive shape measuring structures, wherein the test fields are configured for measuring a plurality of profile properties of the test structures contained in the test fields, wherein the profile properties characterize a profile line of the test structure extending laterally with respect to the use surface and include a flank angle of the profile line of the test structure, a profile depth of the test structure, and a depth of microgrooves occurring in a bottom region of a groove-shaped profile of the test structure, and - Measuring a test field of the diffractive optical element to monitor the stability of the manufacturing process.

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