MEASURING METHOD FOR THE INTERFEROMETRIC DETERMINATION OF A SURFACE SHAPE

Description

[0001] The present application claims priority over German patent application 10 2019 204 096.9 dated March 26, 2019. Background of the invention

[0002] The invention relates to a measuring method and a measuring arrangement for the interferometric determination of the shape of an optical surface of a test object. Furthermore, the invention relates to a method for designing phase gratings. For example, an optical element for microlithography is measured as the test object. Due to the need for ever smaller structures, increasingly higher demands are placed on the optical properties of the optical elements used in microlithography. The optical surface shape of these optical elements must therefore be determined with the highest possible accuracy.

[0003] For the high-precision interferometric measurement of optical surfaces down to the sub-nanometer range, interferometric measuring arrangements and methods are known, for example, from WO 2016 / 188620 A2, in which a diffractive optical element generates a test wave and a reference wave from an input wave. The wavefront of the test wave can be adapted by the diffractive optical element to a target surface of the test object in such a way that it would impinge on the target shape at every point perpendicular to it and be reflected back by it. Deviations from the target shape of the test object can then be determined using the interferogram formed by superimposing the reflected test wave and the reference wave.

[0004] One problem with these known measuring setups and methods is that the diffractive optical element causes measurement errors, thereby reducing the measurement accuracy in determining the surface shape. Underlying task

[0005] It is an object of the invention to provide a measuring method and a measuring arrangement of the type mentioned above, which solves the aforementioned problems and in particular improves the measuring accuracy in determining the surface shape. Inventive solution

[0006] The aforementioned problem is solved according to the invention with the following measurement method for the interferometric determination of a shape of a surface of a test object according to claim 6.

[0007] The measurement method comprises arranging a first diffractive optical element in the beam path of an input wave to generate a first test wave with a wavefront at least partially adapted to a target shape of the optical surface, acquiring a first interferogram generated by the first test wave after interaction with the surface of the test object, and arranging a further diffractive optical element in place of the first diffractive optical element in the beam path of the input wave to generate a further test wave with a wavefront at least partially adapted to the target shape of the optical surface, wherein the first diffractive optical element and the further diffractive optical element differ in the configuration of their respective diffraction structures, and wherein the first test wave and the second test wave are configured such that they correspond to the same points on the surface of the test object.

[0008] Furthermore, the measurement procedure includes capturing an additional interferogram generated by the second test wave after interaction with the surface of the test object, as well as determining the shape of the test object's surface by computing the two interferograms. In other words, the two interferograms are not generated simultaneously, but sequentially. The first diffractive optical element and the second diffractive optical element differ in their diffraction properties; that is, they are not configured identically.

[0009] The solution according to the invention is based on the understanding that interference radiation can arise within the measuring arrangement, which distorts the interferogram at one or more interference points. Such interference radiation can be generated at specific locations of the diffractive optical element, which are also referred to in this text as interference source points. Interference radiation generated at a diffractive optical element is also referred to below as an interference wave. Interference waves can be generated whose respective direction of propagation coincides with the direction of propagation of the test wave and whose respective wavefront differs from the wavefront of the test wave. Furthermore, interference waves can be generated at the diffractive optical element whose direction of propagation differs from the direction of propagation of the test wave, but which nevertheless, if necessary,Further deflection at the diffractive optical element can generate interference points in the interferogram. Furthermore, interference waves can also be generated by direct reflection at the diffractive optical element. Additionally, interference radiation can arise independently of the diffractive optical element due to reflection paths within the measurement setup used for the measurement procedure. Such reflection paths can, for example, be caused by double reflections at lenses.

[0010] By arranging a further diffractive optical element in the beam path of the input wave according to the invention and capturing a further interferogram generated thereby, it becomes possible to at least largely eliminate the measurement errors caused by the interfering radiation and thus improve the measurement accuracy in determining the surface shape.

[0011] According to a further embodiment, the two diffractive optical elements differ at least in that the line densities of respective diffraction patterns corresponding to the same location on the test object differ by at least 10 lines per millimeter between the two diffractive optical elements. In other words, the line density of respective diffraction patterns corresponding to the same location on the test object of the first diffractive optical element is at least 10 lines per millimeter higher or lower than the line density of the second diffractive optical element. According to a further embodiment, the two diffractive optical elements differ at least in that the line densities of respective diffraction patterns corresponding to the same location on the test object differ by at least 100 lines per millimeter between the two diffractive optical elements.

[0012] According to a further embodiment, the measurement method is carried out using a measurement setup comprising a radiation source for providing the input wave, a first holding device for holding one of the two diffractive optical elements, and a second holding device for holding the test object. After the first interferogram has been acquired, the configuration of the measurement setup is changed, and the next interferogram is acquired in the changed configuration. That is, the two interferograms are acquired in different configurations of the measurement setup.

[0013] According to a further embodiment, changing the configuration of the measuring arrangement by manipulating at least one of the holding devices results in a changed relative position between the relevant diffractive optical element and the test object. The manipulation includes a corresponding adjustment of at least one of the holding devices; this adjustment can be performed manually or automatically.

[0014] According to a further embodiment, the change in relative position comprises a change in the relative tilt angle between the relevant diffractive optical element and the test object. According to a further embodiment, the change in relative tilt angle comprises a tilting of the relevant diffractive optical element, in particular about a tilting axis arranged transversely to the direction of propagation. According to a further embodiment, the change in relative tilt angle comprises a tilting of the test object. When calculating the two interferograms, according to one embodiment, the deformation of the test object's surface caused by the tilting of the test object and the associated change in the beam path are modeled in the measurement setup. This can be done using finite element modeling.

[0015] According to a further embodiment, the change in relative position comprises a translational movement between the relevant diffractive optical element and the test object. In particular, the translational movement comprises a displacement of the test object, i.e., the second holding device is manipulated such that the test object performs a translational movement.

[0016] According to a further embodiment, the measuring arrangement comprises a reference element for reflecting a reference wave coupled from the input wave, wherein the reference element is tilted when the configuration of the measuring arrangement is changed. The reference element can be a mirror for back-reflecting a reference wave coupled from the input wave at the corresponding diffractive optical element. Furthermore, the reference element can also be a so-called Fizeau element, which is configured to couple the reference wave from the input wave by reflection. The respective reference wave is superimposed with the corresponding test wave to generate the corresponding interferogram. The tilting occurs, in particular, about a tilting axis arranged transversely to the beam path of the input wave.In the case where the reference wave is coupled out of the input wave at the diffractive optical element, the tilting in particular continues to take place about a tilting axis which is arranged transversely to the plane spanned by the propagation direction of the input wave and the propagation direction of the reference wave after its coupling out at the diffractive optical element.

[0017] According to a further embodiment, the measuring arrangement comprises a deflecting mirror for deflecting the input wave before it strikes the relevant diffractive optical element, wherein the deflecting mirror is tilted when the configuration of the measuring arrangement is changed. In the case where the reference wave is coupled out from the input wave at the diffractive optical element, the tilting occurs in particular about a tilting axis that is arranged transversely to the plane spanned by the propagation direction of the input wave and the propagation direction of the reference wave after its coupling out at the diffractive optical element.

[0018] According to another embodiment, changing the configuration of the measuring arrangement changes the wavelength of the input wave.

[0019] According to a further embodiment, the diffractive optical elements each have at least two superimposed diffractive structural patterns. One of the diffractive structural patterns can be configured to generate the test wave and the other to generate the reference wave. Such a diffractive optical element with at least two superimposed diffractive structural patterns can comprise a complexly coded phase grating.

[0020] The aforementioned problem is further solved according to the invention with a measuring arrangement for the interferometric determination of the shape of a surface of a test object according to claim 18. The measuring arrangement comprises a radiation source for providing an input wave, a first holding device for arranging a first diffractive optical element in the beam path of the input wave for generating a first test wave whose wavefront is at least partially adapted to a target shape of the surface, and alternatively for arranging a further diffractive optical element in the beam path of the input wave for generating a further test wave whose wavefront is also at least partially adapted to the target shape of the surface, wherein the first test wave and the second test wave are configured such that they correspond to the same points on the surface of the test object.a second holding device for holding the test object in the beam path of the respective test wave, and an evaluation device configured to determine the shape of the surface of the test object by calculating a first interferogram, which is generated when the first diffractive optical element is placed in the beam path of the input wave by means of the further test wave, with a further interferogram, which is generated when the first diffractive optical element is placed in the beam path of the input wave by means of the further test wave.

[0021] As described above, the second interferogram is generated when the additional diffractive optical element is positioned in the beam path of the input wave. This second diffractive optical element is placed in the beam path of the input wave in place of the first diffractive optical element. The two interferograms are generated by the test wave produced by the respective diffractive optical element after interaction with the surface of the test object. Each test wave has a wavefront that is at least partially adapted to a target shape of the optical surface.

[0022] The features specified for the aforementioned embodiments, exemplary embodiments, or variants of the measuring method according to the invention can be transferred accordingly to the measuring arrangement according to the invention. These and other features of the embodiments according to the invention are explained in the description of the figures and the claims.

[0023] Furthermore, the invention provides a method for designing phase gratings, which comprises generating a plurality of different phase grating designs, each configured to generate a test wave from an input wave directed onto the phase grating, based on predefined boundary conditions. The design method according to the invention further comprises determining the positions of disturbance points for each of the generated designs, which are contained in an interferogram corresponding to the respective design, wherein the respective interferogram can be generated in a measuring device using the test wave associated with the respective design.Furthermore, the design method according to the invention comprises identifying interference points with the same position in the interferograms of at least two of the generated designs, as well as selecting a combination of at least two of the generated designs, taking into account the number of interference points with the same position compared to other combinations. Interference points occur due to interference radiation, which at the corresponding interference point exhibits a wavefront that differs from the test wave. The positions of interference points in the corresponding interferogram are determined based on a simulation and / or experimentally.

[0024] In particular, the combination of at least two of the generated designs is selected whose number of identically positioned disturbance points is lower than the number of identically positioned disturbance points found in other combinations. For example, the combination of at least two of the generated designs can be selected such that the number of identically positioned disturbance points is minimized. In other words, in this embodiment, the number of identically positioned disturbance points in the selected combination is lower than in all other combinations.

[0025] According to one embodiment, the combination of at least two of the generated designs is selected such that the number of disturbance points in the same position falls below a predetermined threshold.

[0026] According to a further embodiment of the design method, at least one of the interference points of one of the generated designs is produced by an interference wave. This interference wave is generated from the input wave at a point on the phase grating associated with the interference point, adjacent to the test wave. The propagation direction of this interference wave coincides with that of the test wave, and its wavefront differs from that of the test wave. In particular, all interference points of the generated designs are produced by such an interference wave. The point on the phase grating associated with the interference point of the interferogram is also referred to in this text as the interference output point.

[0027] The statement that the propagation direction of the interfering wave coincides with the propagation direction of the test wave means that the propagation directions differ so slightly that, when using a diffractive element manufactured using the design in question for interferometric surface shape determination, the interfering wave hits the corresponding point in the interferogram generated by the test wave and thus distorts a measurement at that point that is based solely on this interferogram.

[0028] According to one embodiment of the design method, the designs relate to complexly coded phase lattices and each has at least two superimposed diffractive structural patterns. The different designs are understood to mean that the designs each differ from one another in the configuration of at least one of the structural patterns.

[0029] According to a further embodiment of the design method, the interference wave, due to an interaction of the input wave with the diffractive structural patterns that differs from that during the generation of the test wave, exhibits a wavefront that is distinct from that of the test wave. In particular, the interference wave exhibits a phase shift relative to the test wave. That is, the interaction of the input wave with the at least two superimposed diffractive structural patterns differs during the generation of the interference wave compared to the generation of the test wave. According to one embodiment, the diffractive structural patterns contribute with a different weighting during the generation of the interference wave compared to the generation of the test wave, and this weighting can also be zero.

[0030] For example, in the case of a design with two overlapping structural patterns, a test wave can be generated using the +1st diffraction order at the first structural pattern. A disturbance wave with the same propagation direction can then be created, for instance, by superimposing a wave generated using the +2nd diffraction order at the first structural pattern with a wave generated using the -1st diffraction order at the second structural pattern.

[0031] According to another example, in the case of a design with three overlapping structural patterns, a test wave can also be generated using the +1st diffraction order at the first structural pattern. A disturbance wave with the same propagation direction can then be formed, for example, by superimposing the wave generated using the +1st diffraction order at the first structural pattern with a wave generated using the -1st diffraction order at the second structural pattern and a wave generated using the +1st diffraction order at the third structural pattern.

[0032] According to one embodiment, the interference wave has an intensity that is at least 10⁻⁶, in particular at least 10⁻⁵, at least 10⁻⁴, at least 10⁻³ or at least 10⁻² of the intensity of the test wave.

[0033] According to a further embodiment of the design method, a defect budget characterizing wavefront errors of the test shaft caused by known manufacturing inaccuracies is calculated for each of the generated designs. When selecting the combination of at least two of the generated designs, the calculated defect budgets of the generated designs are also taken into account. The known manufacturing inaccuracies relate to the manufacturing inaccuracies of a phase grating manufactured based on the respective design. In particular, a balance is struck between the goal of minimizing the number of positionally identical disturbance points and the goal of keeping the defect budgets of the selected designs as small as possible.

[0034] This can be done, for example, by means of an optimization calculation based on a suitable objective function.

[0035] According to one embodiment of the measurement method according to the invention, the diffractive optical elements comprise phase gratings, the designs of which are determined by means of the design method in one of the embodiments described above. In particular, the measurement method comprises the steps of the design method for determining a combination of at least two phase grating designs, wherein these designs are used to manufacture the first diffractive optical element and the further diffractive optical element. Brief description of the drawings

[0036] The foregoing, as well as further advantageous features of the invention, are illustrated in the following detailed description of exemplary embodiments of the invention with reference to the accompanying schematic drawings. These show: Fig. 1 a first embodiment of a measuring arrangement according to the invention for the interferometric determination of the shape of an optical surface of a test object with a first embodiment of a diffractive optical element for generating a test wave and a reference wave, Fig. 2 Disturbance point distributions of using the measurement setup according to Fig. 1 recorded interferograms, Fig. 3 another embodiment of a diffractive optical element for use in the measuring arrangement according to Fig. 1 with additional calibration mirrors, Fig. 4 a further embodiment of a measuring arrangement according to the invention for the interferometric determination of the shape of an optical surface of a test object with a diffractive optical element for generating a test wave, as well as Fig. 5 a flowchart illustrating a method for designing phase gratings for diffractive optical elements for use in the measurement setup according to Fig. 1 or Fig. 4 . Detailed description of embodiments according to the invention

[0037] In the exemplary embodiments or variants described below, functionally or structurally similar elements are, as far as possible, provided with the same or similar reference numerals. Therefore, to understand the features of the individual elements of a particular exemplary embodiment, reference should be made to the description of other exemplary embodiments or to the general description of the invention.

[0038] To facilitate description, a Cartesian xyz coordinate system is shown in the drawing, from which the respective positional relationships of the components depicted in the figures can be derived. Fig. 1 The y-direction runs perpendicular to the plane of the drawing, into it, the x-direction to the right, and the z-direction upwards.

[0039] In Fig. 1 An embodiment of a measuring arrangement 10 for the interferometric determination of the shape of an optical surface 12 of a test object 14 is illustrated. The measuring arrangement 10 can be used, in particular, to determine a deviation of the actual shape of the surface 12 from a target shape. The test object 14 can, for example, be a mirror of a projection lens for EUV microlithography with a non-spherical surface for reflecting EUV radiation with a wavelength of less than 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.8 nm. The non-spherical surface of the mirror can have a freeform surface with a deviation from any rotationally symmetric asphere of more than 5 µm and a deviation from any sphere of at least 1 mm.

[0040] The measuring arrangement 10 includes a radiation source 16 for providing a sufficiently coherent measuring radiation as an input wave 18. In this embodiment, the radiation source 16 comprises a waveguide 20 with an exit surface 22. The waveguide 22 is connected to a Fig. 1 A radiation generation module (not shown), e.g., in the form of a laser, is connected. For example, a helium-neon laser with a wavelength of approximately 633 nm can be provided. However, the illumination radiation can also have a different wavelength in the visible or invisible wavelength range of electromagnetic radiation. In one embodiment, the wavelength of the radiation source 16 is variable, which in Fig. 1 This is illustrated by a wavelength adjustment control 24. The radiation source 16 with the waveguide 20 is merely an example of a radiation source that can be used for the measuring arrangement. In alternative embodiments, an optical arrangement with lens elements, mirror elements, or the like can be provided instead of the waveguide 20 to supply a suitable input wave 18.

[0041] The measuring arrangement 10 further comprises a deflecting mirror 26 and a first holding device 28. The deflecting mirror 26 serves to deflect the input wave 18 onto a diffractive optical element held by the first holding device 28. This is shown in the illustration in Fig. 1 A first diffractive optical element designated by reference numeral 30. This can be removed from the holding device 28 and a second diffractive optical element 32 or another diffractive optical element can be arranged in the holding device 28 instead.

[0042] The corresponding diffractive optical element held by the holding device 28, as shown in the illustration according to Fig. 1 The diffractive optical element 30 serves to generate a test wave 34 and a reference wave 36 from the input wave 18. Furthermore, the measuring arrangement 10 includes a reference element 38 designed as a reflective optical element for reflecting the reference wave 36 into a reflected reference wave 36r. The reference element 38 is attached to a third holding device 78.

[0043] The diffractive optical element 30 is designed as a complex-encoded CGH and contains diffraction structures 40, which, according to the in Fig. 1 In the illustrated embodiment, two diffractive structural patterns are arranged superimposed in one plane. The diffractive optical element 30 according to Fig. 1 It is therefore also referred to as a doubly complex encoded computer-generated hologram (CGH). Alternatively, the diffraction patterns could also consist of more than two diffractive structure patterns arranged superimposed in one plane, e.g., five superimposed diffractive structure patterns as shown below with reference to Fig. 3 explained in more detail below. The diffractive optical element 130 according to Fig. 3 It is therefore also referred to as five-fold complex encoded CGH.

[0044] The two diffractive structural patterns of the diffractive optical element 30 according to Fig. 1 These can be formed, for example, by a first structural pattern in the form of a basic grating and a second diffractive structural pattern in the form of a superlattice. One of the diffractive structural patterns is configured to generate the test wave 34, which is directed towards the test object 14 held by a second holding device 42 and is configured with a wavefront that is at least partially adapted to a target shape of the optical surface 12. The test wave 34 is reflected at the optical surface 12 of the test object 14 and travels back to the diffractive optical element 30 as a reflected test wave 34r. Due to the wavefront being adapted to the target shape of the optical surface 12, the test wave 34 strikes the optical surface 12 at a substantially perpendicular angle at every point and is reflected back on itself.

[0045] The other diffractive structure pattern generates the reference wave 36, which is directed towards the reference element 38 and has a plane wavefront. In alternative embodiments, a singly coded CGH with a diffractive structure or another optical grating can be used instead of the complexly coded CGH. The test wave 34 can, for example, be generated in a first diffraction order and the reference wave 36 in zeroth or any other diffraction order at the diffractive structure. The reference element 38 is designed as a plane mirror for the back reflection of the reference wave 36 with a plane wavefront. In another embodiment, the reference wave 36 can have a spherical wavefront and the reference element 38 can be designed as a spherical mirror.

[0046] The reflected test wave 34r from surface 12 passes through the diffractive optical element 30 again and is diffracted once more. This results in a reverse transformation of the reflected test wave 34r into an approximately spherical wave, whose wavefront exhibits deviations from a spherical wavefront due to deviations of the surface 12 of the test object from the target shape. The reflected reference wave 36r, reflected from the reference element 38, also passes through the diffractive optical element 30 again and is diffracted once more. This results in a reverse transformation of the reflected reference wave 36r into a spherical wave. In an alternative embodiment with a collimator in the beam path of the input wave 18 to generate an input wave 18 with a planar wavefront, no adaptation of the wavefront of the reference wave 36r by the diffractive optical element 30 is necessary.

[0047] The diffractive optical element 30 thus also serves to superimpose the reflected test wave 34r with the reflected reference wave 36r. Furthermore, the measuring arrangement 10 includes a detection device 44 with a beam splitter 46 for expelling the combination of the reflected test wave 34r and the reflected reference wave 36r from the beam path of the input wave 18 and an interferometer camera 48 for capturing an interferogram generated by superimposing the test wave 34r with the reference wave 36r.

[0048] The reflected test wave 34r and the reflected reference wave 36r converge as beams on the beam splitter 46 and are reflected by it towards the interferometer camera 48. Both convergent beams pass through an eyepiece 50 of the interferometer camera 48 and finally strike a two-dimensionally resolving detector 52 of the interferometer camera 48. In the present embodiment, in which the radiation source 16 is provided with a wave adjustment control 24, the eyepiece 50 is configured as a mirror. In alternative embodiments, where no wavelength adjustment is provided, the eyepiece can also be configured as a lens. The detector 48 can, for example, be designed as a CCD sensor and detects an interferogram generated by the interfering waves. At the focus of the convergent beams of waves 34r and 36r, a Fig. 1 The aperture (not shown) may be arranged as a spatial filter to reduce scattered radiation.

[0049] Furthermore, the measuring arrangement 10 comprises an evaluation unit 54 for determining the actual shape of the optical surface 12 of the test object 14 from at least two interferograms recorded by the detector 48. First, a first interferogram, which is generated when the first diffractive optical element 30 is positioned on the detector 48 at the first holding device 28, is combined with at least one further interferogram, which is generated when the second diffractive optical element 32 and, if applicable, further diffractive optical elements are positioned on the detector 48 at the first holding device 28 as described in more detail below. The evaluation unit 54 then determines the actual shape of the optical surface 12 from the combined interferograms.Alternatively or additionally, the measuring device 10 can include a data storage device or an interface to a network to enable the determination of the surface shape by means of the stored or network-transmitted interferograms by an external evaluation unit.

[0050] The interferograms recorded by detector 48 typically exhibit interference point distributions. Fig. 1 An example of such a disturbance point distribution 56-1 for the first interferogram recorded using the first diffractive optical element 30 is shown. The black, dot-like regions indicate disturbance points 58 in the interferogram. These disturbance points 58 occur due to interference radiation, which exhibits a wavefront at the corresponding disturbance point 58 that differs from the test wave 34.

[0051] The interference radiation responsible for the interference points 58 can be generated at the diffractive optical element 30, in which case the interference radiation is referred to in this text as interference wave 60.

[0052] A first category of interference waves 60 is in Fig. 1 This is illustrated by two interference output points 57-1 and 57-2. Interference output points 57-1 and 57-2 are positions on the diffractive optical element 30 at which, in addition to the test wave 34 directed towards the optical surface 12, an interference wave 60 (in the example interference waves 60-1 and 60-2) is generated from the input wave 18, the propagation direction of which corresponds to the propagation direction of the test wave 34 and the wavefront of which differs from the wavefront of the test wave 34.

[0053] At the interference points 58-1 and 58-2, i.e., the points in the interferogram recorded by detector 48 that correspond to the interference output points 57-1 and 57-2, instead of the desired superposition of only test wave 34r and reference wave 36r, a superposition of three waves occurs: test wave 34r, reference wave 36r, and the respective interference wave 60-1 or 60-2. Due to the contribution of interference wave 60 to the intensity measured at the corresponding point in the interferogram, the measurement at this point is distorted, i.e., the measurement at this point is unusable. Therefore, the interference points can also be referred to as "blind spots." Depending on the accuracy requirement, a measurement is considered unusable as soon as the intensity of the corresponding disturbance wave 60 is at least 10 -6< , in particular at least 10 -5< , at least 10 -4< , at least 10 -3< or at least 10 -2< of the intensity of the test wave 34r at the mentioned location.

[0054] According to one embodiment, one or more of the interference waves 60 can arise due to an interaction of the input wave 18 with the diffractive structural patterns of the diffraction structures 40 at the interference points 58, which is of a different nature than the generation of the test wave 34. Referring to the doubly complex encoded CGH according to Fig. 1 At one or more disturbance points 58, where, as at all other locations of the diffraction structures 40, the test wave 34 is generated by diffraction at the first structure pattern (no diffraction or 0th diffraction order at the second structure pattern), a disturbance wave 60 can be generated by diffraction of the input wave 18 at the first structure pattern in the +2nd diffraction order and simultaneous diffraction at the second structure pattern in the -1st diffraction order, provided the structure patterns are suitably configured. As mentioned above, such a disturbance wave has the same propagation direction but a different wavefront than the test wave 34.

[0055] A second category of interference waves 60, i.e. interference radiation arising at the diffractive optical element 30, is in Fig. 1 The interference wave 60-3, originating from an interference source point 57-3, is shown as an example. In contrast to the interference waves 60-1 and 60-2, the interference wave has a propagation direction that differs from the propagation direction of the test wave 34 in the corresponding radiation path. Therefore, after reflection at the optical surface 12 of the test object 14, the interference wave 60-3 travels back to the diffractive optical element 30 along a tilted path and thus encounters the diffractive optical element 30 at a point 61-3 that differs from the interference source point 57-3. At this point, which is also called the interference wave deflection point 61-3, the interference wave 60-3 is usually deflected differently than is the case for the returning test wave 34r at this point, in such a way that the interference wave 60-3 remains within the beam path leading via the deflecting mirror 26, the beam splitter 46 and the eyepiece 50 to the detector 52.In the exemplary illustration according to . Fig. 1 The interference wave 60-3 hits the detector 52 at the location of the interference point 58-3.

[0056] According to a third category, interference radiation in the form of interference waves can arise at the diffractive optical element 30 through direct reflection of the input wave 18 at the diffractive optical element 30. According to a further embodiment, interference radiation generating interferogram points can also be formed independently of the diffractive optical element due to reflection paths within the measurement setup 10. Such reflection paths are particularly relevant in measurement setups that include one or more lenses in addition to mirrors, where double reflections can occur.

[0057] In the Fig. 3 In the illustrated embodiment, where the diffractive optical element 130 is configured as a five-fold complex-coded CGH, a multitude of combinations of diffraction orders are conceivable for generating interference waves 60. According to one example, interference waves 60 can be generated by diffracting the input wave 18 at the first structure pattern in +1st diffraction order and simultaneously diffracting at the third structure pattern in -1st diffraction order as well as at the fourth structure pattern in +1st diffraction order (no diffraction at the fourth and fifth structure patterns, i.e., in short: [1, 0, -1, 1, 0]).

[0058] In general, interference radiation, as above with reference to Fig. 1 described, also in the form of interference waves with a propagation direction that differs from the propagation direction of the test wave 34, in the form of interference waves reflected directly at the diffractive optical element or in the form of interference radiation that arises due to reflection paths within the measuring arrangement.

[0059] As mentioned above, at least two interferograms are combined to determine the actual shape of the optical surface 12 of the test object 14. For this purpose, after the recording of the first interferogram generated by the first diffractive optical element 30 as described above, the second diffractive optical element 32 is first positioned in the beam path of the input wave 18 in place of the first diffractive optical element 30. In other words, the first diffractive optical element 30 is removed from the holding device 28 and the second diffractive optical element 32 is positioned on the holding device 28.

[0060] The second diffractive optical element 32 is very similar to the first diffractive optical element 30, but differs to some extent in the configuration of its diffraction structures 40. The diffraction structures 40 of the second diffractive optical element 32 also comprise two diffractive structure patterns, the first of which is configured to generate the test wave 34 with a wavefront at least partially adapted to the desired shape of the optical surface 12, and the second diffractive structure pattern for generating the reference wave 36. The test wave 34 and reference wave 36 generated by the second diffractive optical element 32 differ from the corresponding waves 34 and 36 generated by the first diffractive optical element 30 only slightly in their propagation directions. Here, the waves that propagate to the same points on the optical surface 12 of the test object 14 and 36, respectively, are compared.belonging to the reference element 38. In other words, the propagation directions of the test waves 34 and / or the reference waves 36 differ slightly from one another; that is, the corresponding radiation angle of the test wave 34, the reference wave 36, or both waves 34 and 36 from the corresponding diffractive optical element 30 or 32 differs slightly. According to one embodiment, the difference in the propagation directions is at least 0.3°, in particular at least 3°. Alternatively or in addition to the change in the radiation angle with respect to the test wave 34 and the reference wave 36, the change caused by the second diffractive optical element 32 can also involve a rotation of the test wave 34 about its propagation direction.

[0061] Due to the altered emission characteristics of the second diffractive optical element 32, the configuration of the measuring arrangement 10 is adapted to the changed beam path before the corresponding interferogram is acquired. This can be achieved by one or more of the configuration changes of the measuring arrangement 10 described below. For this purpose, the first holding device 28 is configured to change the tilt position of the diffractive optical element 30 or 32 about two tilt axes 62 and 64 that are orthogonal to each other and oriented transversely to the direction of incidence of the input wave 18.

[0062] Furthermore, the second holding device 42 is configured to move the test object 14 in all six rigid body degrees of freedom, i.e., to change the tilting position of the test object 14 about three mutually orthogonal tilting axes 66, 68, and 70, and to move the test object 14 along three mutually orthogonal translation directions 72, 74, and 76. Furthermore, the third holding device 78 is configured to change the tilting position of the reference element 38 about two mutually orthogonal tilting axes 80 and 82 oriented transversely to the direction of incidence of the reference wave 36. Furthermore, the deflecting mirror 26 is arranged to be tiltable about at least one tilting axis 84 oriented perpendicular to the direction of incidence of the input wave 18.

[0063] As mentioned above, the radiation source 16 is equipped with a wave control 24. By changing the wavelength of the input wave 18, the emission angles of waves 34 and 36 from the diffractive optical elements 30 and 32, respectively, can be changed, thus compensating for any deviation between the diffractive optical elements 30 and 32 with respect to these emission angles. A change in wavelength therefore also falls under the previously mentioned possible configuration changes of the measuring arrangement 10.

[0064] The configuration changes achievable by means of the tilting and translational degrees of freedom described above include the adjustment of a changed relative position, in particular by changing the relative tilt position and / or by performing a translational movement, between the diffractive optical element 30 or 32 and the test object 14. Furthermore, these configuration changes can include a tilting of the reference element 38 and / or the deflecting mirror 26.

[0065] In Fig. 2 The top left corner already contains... Fig. 1 The illustrated example of a disturbance point distribution 56-1 of an interferogram recorded by the detector 48 using the first diffractive optical element 30 is shown. As already indicated above, the disturbance points contained therein are not points in the mathematical sense, but rather point-like regions. The area cumulatively occupied by the disturbance points 58 in the disturbance point distribution 56-1 is, in this example, 3.37% of the total area of ​​the interferogram, i.e., the disturbance point fraction is 3.37%.

[0066] In addition to the disturbance point distribution 56-1, in Fig. 2 An example of an interference point distribution 56-2 of a second interferogram recorded as described above using the second diffractive optical element 32 with a corresponding change in the configuration of the measuring arrangement 10 is shown. The interference point fraction of the second interferogram is 3.26% in the present example. However, the interference points 58 in the second interferogram are distributed differently than in the first interferogram, so that the combination of the two interferograms results in an interference point distribution 56-4 with an interference point fraction, i.e., a fraction of "blind spots", of only 0.10%.

[0067] Furthermore, in Fig. 2 This case illustrates how a third interferogram is recorded using a third diffractive optical element, which differs to some extent from the first two diffractive optical elements 30 and 32 according to the criteria explained above, with a corresponding change in the configuration of the measurement setup 10. In this example, the interference distribution 56-3 of the third interferogram shows an interference fraction of 3.46%. When all three interferograms are combined, the resulting interference distribution 56-5 shows an interference fraction of less than 0.01%.

[0068] By combining several interferograms as described above to determine the actual shape of the optical surface 12 of the test object 14, the error influence on the result of the shape determination attributable to the disturbance points 58 is reduced in accordance with the reduction of the disturbance point component in the combined interferograms explained above. The accuracy of the shape determination is thus improved accordingly.

[0069] As mentioned above, illustrates Fig. 3 a diffractive optical element 130 in the form of a five-fold complex coded CGH, which is in the measuring arrangement 10 according to Fig. 1 instead of the first diffractive optical element 30, or in a modified form according to the criteria explained above, instead of a further diffractive optical element, such as the second diffractive optical element 32. When using the diffractive optical element 130 in the measuring arrangement 10 according to Fig. 1 Three additional calibration mirrors 102, 104 and 106 are provided. For the calibration of the diffractive optical element 130 or the measuring arrangement 10, the calibration mirrors 102, 104 and 106 are successively positioned and measured instead of the test object 14.

[0070] The diffraction structures 140 of the diffractive optical element 130 form five diffractive structure patterns arranged superimposed in a plane. In addition to the test wave 34 directed at the surface 12 of the test object 14 to be measured and the reference wave 36 directed at the reference element 38, the diffraction structures 140 generate three calibration waves 108, 110, and 112. The calibration waves 108, 110, and 112 are each directed at one of the calibration mirrors 102, 104, and 106, respectively, and in the present embodiment are all designed as spherical waves whose wavefront is adapted to the shape of the respective calibration mirror. In other embodiments, plane waves can also be used as calibration waves in addition to or instead of spherical waves.

[0071] Using the detector 52, calibration interference patterns generated by superimposing the calibration waves 108, 110, and 112 after their respective reflections at the corresponding calibration mirrors 102, 104, and 106 with the reflected reference wave 36r are recorded. By evaluating the calibration interference patterns, fit and / or placement errors of the diffraction structures 140 of the diffractive optical element 130 can be determined and taken into account when determining the shape of the optical surface 12 by evaluating the interferograms generated by superimposing the reflected test wave 34r with the reflected reference wave 36r.

[0072] In Fig. 4 Figure 1 illustrates a further embodiment of a measuring arrangement 10 for the interferometric determination of the shape of an optical surface 12 of a test object 14. The measuring arrangement 10 according to Fig. 4 differs in this respect from the measurement setup according to Fig. 1 , that instead of the reference element 38 designed as a reflective optical element, a reference element 238 in the form of a Fizeau element is provided, and instead of the diffractive optical element 30 or 32 for generating both the reference wave 36 and the test wave 34, a diffractive optical element 230 or 232 with diffraction structures 240 for generating the test wave 34 is arranged. The Fizeau element is arranged in the beam path of the input wave 18, i.e., upstream of the diffractive optical element 230 or 232, and is configured to reflect a portion of the radiation from the input wave 18 as a reflected reference wave 36r. The measuring arrangement 10 according to Fig. 4 It is thus configured as a Fizeau interferometer.

[0073] The reference element 238 is attached to the third holding device 78 and can thus be tilted about two tilting axes 80 and 82 that are orthogonal to each other and oriented transversely to the direction of incidence of the input wave 18. This tilting must then occur simultaneously with a tilting of the deflecting mirror 26 so that the light still strikes the Fizeau surface perpendicularly. The operation of the measuring arrangement 10 according to Fig. 4 behaves analogously to the operating principle of the measuring arrangement 10 according to Fig. 1 The interferograms recorded by means of the two diffractive optical elements 230 and 232 under configuration changes of the measurement setup 10 are combined to determine the shape of the optical surface 12 of the test object 14. The diffractive optical elements 230 and 232 can, in particular, be configured as complex-coded CGH and, for example, analogous to the one described in Fig. 3 The diffractive optical element 130 shown is designed to generate calibration waves next to the test wave 34.

[0074] In Fig. 5 is a method for designing the phase gratings for the measuring arrangement 10 according to Fig. 1 or Fig. 4 The combination of diffractive optical elements 30 and 32 or 230 and 232, and possibly other diffractive optical elements, used is illustrated by a flowchart. In particular, these are designs of complexly coded phase gratings, each with at least two superimposed diffractive structural patterns. In a first step S1 of the design procedure, a large number of different phase grating designs for a diffractive optical element are generated in a computer based on predefined boundary conditions. The generated phase gratings are each used to generate the test wave 34 according to Fig. 1 configured from the input wave 18 radiated onto the phase grid.

[0075] In a second step S2 of the design procedure, positions of disturbance points 58 are generated for each of the generated designs, which are contained in an interferogram corresponding to the respective design, wherein the respective interferogram can be generated by means of the test wave 34 assigned to the respective design in a measuring arrangement 10.

[0076] In the case of complexly coded phase grids, the disturbance points 58 can be, as above with reference to Fig. 1 or Fig. 3described, correspond to locations where waves diffracted in different diffraction orders at the individual diffractive structural patterns of the phase grating superimpose in such a way that the superimposed wave has the same direction of propagation as the test wave 34. In other words, at the disturbance points 58 corresponding locations 57 of the phase grating, a respective disturbance wave 60 can be generated which, due to an interaction of the input wave 18 with the diffractive structural patterns, which is of a different nature than in the generation of the test wave 34, has a wavefront that differs from the wavefront of the test wave 34.

[0077] In an optional third step S3 of the design procedure, a defect budget is calculated for each of the generated designs, which characterizes wavefront errors of the test wave 34 that are caused by known inaccuracies in the manufacture of a diffractive optical element based on the respective design.

[0078] In a fourth step S4 of the design process, positionally identical disturbance points are identified in the interferograms of at least two of the generated designs. In the subsequent fifth step S5 of the design process, a combination of at least two of the generated designs is selected, taking into account the identified number of positionally identical disturbance points. In particular, the combination of designs is selected whose number of positionally identical disturbance points is lower than the number of positionally identical disturbance points found in other combinations. For example, the combination of designs can be selected such that the number of positionally identical disturbance points is minimized. According to one embodiment, the combination of designs is selected in which the number of positionally identical disturbance points falls below a predetermined threshold.In the event that error budgets have been calculated according to step S3, the calculated error budgets can also be taken into account when selecting the combination of designs.

[0079] The foregoing description of exemplary embodiments, embodiments, or variants is to be understood as illustrative. The disclosure thereby enables the person skilled in the art to understand the present invention and its associated advantages, and also encompasses, in the understanding of the person skilled in the art, obvious modifications and alterations of the described structures and methods. Therefore, all such modifications and alterations, insofar as they fall within the scope of the invention as defined in the appended claims, are intended to be covered. Reference symbol list

[0080] 10 Measuring setup 12 Optical surface 14 Test object 16 Radiation source 18 Input wave 20 Waveguide 22 Output surface 24 Wavelength adjustment control 26 Deflection mirror 28 First holding device 30 First diffractive optical element 32 Second diffractive optical element 34 Test wave 34 Returning test wave 36 Reference wave 36 Returning reference wave 38 Reference element 40 Diffraction structures 42 Second holding device 44 Acquisition device 46 Beam splitter 48 Interferometer camera 50 Eyepiece 52 Detector 54 Evaluation device 56-1 Disturbance point distribution of the first interferogram 56-2 Disturbance point distribution of the second interferogram 57 Disturbance output points 58 Disturbance points 60 Disturbance wave 61-3 Disturbance wave deflection point 62 First tilting axis first holding device 64 second tilting axis of the first holding device 66 first tilting axis of the second holding device 68 second tilting axis of the second holding device 70 third tilting axis of the second holding device 72 first translation device of the secondHolding device 74 Second translation device of the second holding device 76 Third translation device of the second holding device 78 Third holding device 80 First tilting axis of the third holding device 82 Second tilting axis of the third holding device 84 Tilting axis of the deflecting mirror 102 First calibration mirror 104 Second calibration mirror 106 Third calibration mirror 108 First calibration wave 110 Second calibration wave 112 Third calibration wave 130 First diffractive element 140 Diffraction structures 230 First diffractive optical element 232 Second diffractive optical element 238 Reference element 240 Diffraction structures

Claims

1. A method for designing phase gratings, comprising the steps of: - generating a multiplicity of different designs of a phase grating by means of a computing device, each of which is configured to generate a test wave from an input wave radiated onto the phase grating, on the basis of specified boundary conditions, - determining positions of disturbance points for each of the generated designs that are contained in an interferogram corresponding to the respective design, wherein the respective interferogram can be generated in a measurement arrangement by means of the test wave assigned to the respective design, - identifying disturbance points in the same position in the interferograms of in each case at least two of the generated designs, and - selecting a combination of at least two of the generated designs, taking into account the identified number of disturbance points in the same position in comparison with other combinations.

2. The method as claimed in claim 1 or 2, wherein at least one of the disturbance points of one of the generated designs is generated by a disturbance wave that is generated at a point of the phase grating assigned to the disturbance point next to the test wave from the input wave, whose propagation direction corresponds to the propagation direction of the test wave and whose wavefront differs from the wavefront of the test wave.

3. The method as claimed in claim 1 or 2, wherein the designs relate to complex coded phase gratings and each have at least two diffractive structure patterns arranged so as to superpose one another.

4. The method as claimed in claim 2 or 3, wherein, due to an interaction of the input wave with the diffractive structure patterns, which is of a different type than when generating the test wave, the disturbance wave has the wavefront differing from the wavefront of the test wave.

5. The method as claimed in any of the preceding claims, wherein an error budget characterizing wavefront errors of the test wave caused by known manufacturing inaccuracies is calculated for each of the generated designs and the calculated error budgets of the generated designs are also taken into account when selecting the combination of at least two of the generated designs.

6. A measurement method for interferometrically determining a shape of a surface of a test object, comprising the steps of: - arranging a first diffractive optical element in the beam path of an input wave for generating a first test wave having a wavefront that is at least partially adapted to a desired shape of the optical surface, - capturing a first interferogram generated by means of the first test wave after interaction with the surface of the test object, - arranging a further diffractive optical element in place of the first diffractive optical element in the beam path of the input wave for generating a further test wave having a wavefront that is at least partially adapted to the desired shape of the optical surface, wherein the first diffractive optical element and the further diffractive optical element differ in the configuration of respective diffraction structures, and wherein the first test wave and the further test wave are configured such that they are associated to the same points on the surface of the test object, - capturing a further interferogram generated by means of the further test wave after interaction with the surface of the test object, and - determining the shape of the surface of the test object by way of computationally combining the two interferograms.

7. The measurement method as claimed in claim 6, in which the two diffractive optical elements differ at least in that line densities of respective diffraction structures corresponding to the same location on the test object between the two diffractive optical elements differ by at least 10 lines per millimeter.

8. The measurement method as claimed in claim 7, in which the two diffractive optical elements differ at least in that the line densities of respective diffraction structures corresponding to the same location on the test object between the two diffractive optical elements differ by at least 100 lines per millimeter.

9. The measurement method as claimed in any of claims 6 to 8, which is carried out by means of a measurement arrangement, which comprises a radiation source for providing the input wave, a first holding device for respectively holding one of the two diffractive optical elements and a second holding device for holding the test object, wherein a configuration of the measurement arrangement is changed after capturing of the first interferogram and the further interferogram is captured in the changed configuration.

10. The measurement method as claimed in claim 9, in which, during the changing of the configuration of the measurement arrangement, a changed relative position between the relevant diffractive optical element and the test object is set by manipulating at least one of the holding devices.

11. The measurement method as claimed in claim 10, in which the change in the relative position comprises a change in the relative tilt position between the relevant diffractive optical element and the test object.

12. The measurement method as claimed in claim 10 or 11, in which the change in the relative position comprises a translational movement between the relevant diffractive optical element and the test object.

13. The measurement method as claimed in any of claims 9 to 12, in which the measurement arrangement comprises a reference element for reflecting a reference wave decoupled from the input wave and the reference element is tilted when the configuration of the measurement arrangement is changed.

14. The measurement method as claimed in any of claims 9 to 13, in which the measurement arrangement comprises a deflection mirror for deflecting the input wave before it is incident on the relevant diffractive optical element and the deflection mirror is tilted when the configuration of the measurement arrangement is changed.

15. The measurement method as claimed in any of claims 9 to 14, in which a wavelength of the input wave is changed when the configuration of the measurement arrangement is changed.

16. The measurement method as claimed in any of claims 6 to 15, in which the diffractive optical elements each have at least two diffractive structure patterns that are arranged so as to superpose one another.

17. The measurement method as claimed in any of claims 16, wherein the diffractive optical elements comprise phase gratings, the designs of which are determined by means of the method as claimed in claim 1.

18. A measurement arrangement for interferometrically determining a shape of a surface of a test object, having: - a radiation source for providing an input wave, - a first holding device for arranging a first diffractive optical element of the measurement arrangement in the beam path of the input wave for generating a first test wave, the wave front of which being at least partially adapted to a target shape of the surface and, alternatively, for arranging a further diffractive optical element of the measurement arrangement in the beam path of the input wave for generating a further test wave the wave front of which also being at least partially adapted to the target shape of the surface of the test object, wherein the first test wave and the further test wave are configured such that they are associated to the same points on the surface of the test object, - a second holding device for holding the test object in the beam path of the respective test wave, and - an evaluation device, which is configured to determine the shape of the surface of the test object by computationally combining a first interferogram, which is generated by means or the first test wave when the first diffractive optical element is arranged in the beam path of the input wave, with a further interferogram, which is generated by means of the further test wave when the further diffractive optical element is generated in the beam path of the input wave.