Measuring device for interferometric determination of surface shape

Abstract

A measuring device (10) for interferometrically determining the shape of a surface (12) of a test object (14) comprises: a radiation source for providing an input wave (42); a multiple-coded diffractive optical element (60) configured to generate a test wave (66) and at least one calibration wave (70) from the input wave by diffraction, the test wave being directed towards the test object (14) and having a wavefront in the form of a free-form surface, wherein the wavefront of the calibration wave has a non-rotationally symmetric shape (68f), wherein a cross-section through the wavefront of the calibration wave (70) has a curved shape in each case along cross-sectional regions that are mutually laterally aligned, and wherein the curved shapes in the various cross-sectional regions differ in terms of an aperture parameter. Furthermore, the measuring device comprises a detection device (46) for detecting a calibration interferogram formed by superimposing a reference wave (40) and the calibration wave after interaction with a calibration object (74).

Description

[0001] This application claims the priority of German Patent Application 10 2019 214 979.0, filed on September 30, 2019. The entire disclosure of this patent application is incorporated herein by reference. Background Art

[0002] The present invention relates to a measuring device for interferometrically determining the shape of the surface of a test object, to a method for calibrating a diffractive optical element, to a method for interferometrically determining the shape of the surface of a test object, and to an optical element for a projection lens of a microlithography exposure device.

[0003] For highly accurate interferometric measurement of the surface shape of a test object (such as a microlithography optical element), a diffractive optical arrangement is typically used as a so-called null optic. In this case, the wavefront of the test wave is adapted to the desired shape of the surface by a diffractive element, such that the individual rays of the test wave will be incident perpendicularly onto the desired shape at each position and will thus be reflected back on themselves. The deviation from the desired shape can be determined by superimposing a reference wave on the reflected test wave. The diffractive element used can be, for example, a computer-generated hologram (CGH).

[0004] The accuracy of the shape measurement depends on the accuracy of the CGH. The decisive factor here is not necessarily the most precise possible manufacture of the CGH, but rather the most precise measurement of all possible errors in the CGH. The known errors can be calculated when measuring the shape of the test piece. Thus, the CGH forms a reference. While all non-rotationally symmetric errors can be calibrated with high precision for rotationally symmetric aspherical surfaces, in the case of freeform surfaces, i.e., non-spherical surfaces without rotational symmetry, all CGH errors affect the shape measurement. Therefore, the requirements for the accuracy of the CGH measurement are significantly increased. It is important to know with high precision the distortion of the diffractive structure of the CGH, i.e., the lateral position of the diffractive structure relative to its desired position, and the profile shape of the CGH. However, the measurement accuracy of these parameters that can be determined using the measuring devices known in the prior art is insufficient to meet the increasing requirements.

[0005] DE 10 2012 217 800 A1 describes such a measuring arrangement with a complex-coded CGH. The light wave is initially split into a reference wave and a test wave using a Fizeau element. The test wave is then converted by the complex-coded CGH into a test wave with a wavefront adapted to the desired shape of the surface and a calibration wave with a spherical or planar wavefront. For this purpose, the CGH contains appropriately configured diffraction structures. The calibration wave is used to calibrate the CGH. Subsequently, the test object is arranged in the test position and the measurement is carried out using the test wave. The test wave is reflected by the surface of the test object, transformed back by the CGH, and after passing through the channel of the Fizeau element, the test wave is superimposed by the reference wave. The shape of the surface can be determined from the interference pattern captured in a plane. Although the calibration of the CGH improves the accuracy, this is not always sufficient for advanced applications.

[0006] Fundamental problem

[0007] The object of the present invention is to provide a measuring device and a measuring method of the type mentioned in the background section, by means of which the above problems can be solved, in particular it is possible to determine the shape of an optical surface in the form of a free-form surface with increased accuracy. Summary of the invention

[0008] According to the present invention, the above object can be achieved, for example, by a measuring device for interferometrically determining the shape of the surface of a test object, wherein the measuring device comprises: a radiation source for providing an input wave, a multi-coded diffractive optical element configured to generate a test wave and at least one calibration wave from the input wave by diffraction, the test wave being directed towards the test object and having a wavefront in the form of a free-form surface, wherein the wavefront of the calibration wave has a non-rotationally symmetric shape. The cross-section passing through the wavefront of the calibration wave along cross-sectional surfaces aligned transversely to each other has a curved shape in each case. The curved shapes in different cross-sectional surfaces differ in terms of the aperture parameter here. Furthermore, the measuring device comprises a capturing device for capturing a calibration interference pattern formed by superimposing a reference wave and the calibration wave after interaction with a calibration object.

[0009] A free-form surface should be understood to mean a shape with a deviation from any rotationally symmetric aspherical surface greater than 5 μm, in particular greater than 10 μm. Furthermore, the free-form surface deviates from any spherical surface by at least 0.05 mm, in particular at least 0.1 mm, at least 1 mm or at least 5 mm. In particular, the measuring device comprises an evaluation device for determining the shape of the surface of the test object by evaluating the interference pattern captured by superimposing a reference wave and the test wave after interaction with the surface, taking into account the interference pattern formed by superimposing with the calibration wave.

[0010] The design of the calibration wave with a non-rotationally symmetric wavefront according to the present invention allows the difference between the wavefront of the test wave and the wavefront of the calibration wave (hereinafter also referred to as the test wavefront difference) to be minimized.

[0011] The present invention is based on the following finding: with respect to a free-form test wave to be generated, the better the calibration quality of the diffractive optical element, the smaller the deviation of the shape of the calibration wave from the wavefront shape of the test wave. The reason is that the more similar the partial structures of the multi-coded diffractive optical element responsible for generating the calibration wave and the test wave are, the more similar the calibration wave and the test wave are to each other. During the manufacture of the diffractive optical element, the similar partial structures are in turn subject to similar manufacturing errors, and their groove width profiles have relatively small differences.

[0012] By configuring the diffractive optical element in such a way as to generate a calibration wave having a non-rotationally symmetric wavefront, it becomes possible to keep the test wavefront error as small as possible. This in turn allows the use of calibration data determined by the calibration wave to predict with a high level of accuracy the influence of manufacturing errors in the diffractive optical element on the test wave.

[0013] As described above, the cross-section of the wavefront of the calibration wave passing through the cross-sectional surfaces aligned transversely to each other has a curved shape in each case, where the curved shapes in different cross-sectional surfaces differ in terms of the aperture parameter. In particular, the cross-sectional surfaces are arranged perpendicular to each other.

[0014] According to an embodiment, the curved shapes in two different cross-sectional surfaces differ in terms of their curvature directions.

[0015] According to other embodiments, the curved shape approximates a partial segment of a circular shape in each case and the aperture parameter is the radius of the circle. This is to be understood as meaning that the curved shapes in different cross-sections have a partial segment of a circular shape or approximately have a partial segment of a circular shape in each case, in particular the curved shapes correspond to a partial segment of a circular shape in each case.

[0016] According to other embodiments, the curved shape approximates a partial segment of a parabolic shape in each case and the aperture parameter is the parabola aperture. This means that the curved shapes in different cross-sections correspond to a partial segment of a parabolic shape or approximately correspond to a partial segment of a parabolic shape in each case. The partial segment of the parabolic shape particularly includes the apex region of the parabola, in particular the partial segment has the symmetric apex region of the parabola.

[0017] According to other embodiments, the wavefront of the calibration wave has an astigmatic shape.

[0018] According to other embodiments, the non-rotationally symmetric shape of the calibration wave corresponds to the shape of a section of the surface of a solid of revolution formed by rotating a surface (also hereinafter referred to as a surface of revolution) that is symmetric about a symmetry axis about a rotation axis. A section of the surface of such a solid of revolution is symmetric with respect to a plane and can thus also be described as mirror-symmetric. According to a first variant of the embodiment, the surface of revolution is circular and the rotation axis is truly parallel to the symmetry axis. In this case, the resulting solid of revolution is a torus. According to a second variant of the embodiment, the surface of revolution is non-rotationally symmetric, for example elliptical.

[0019] According to other embodiments, the wavefront of the calibration wave deviates from any rotationally symmetric shape by at least 50 μm, in particular at least 100 μm.

[0020] According to other embodiments, the diffractive optical element is configured to generate at least three calibration waves from the input wave by diffraction in addition to the test wave.

[0021] The above object can also be achieved, for example, by a method for calibrating a diffractive optical element that is configured to generate a test wave with a wavefront in the form of a free-form surface for interferometric determination of the shape of the surface of a test object. The method according to the invention includes providing a diffractive optical element that has multiple coding and radiates in an input wave such that, in addition to the test wave, calibration waves directed at a calibration object are generated by diffraction at the multiple coding, wherein the wavefront of the calibration waves has a non-rotationally symmetric shape. Furthermore, the method according to the invention includes arranging the calibration object at different calibration positions such that the calibration waves are in each case substantially perpendicularly incident on different regions of the surface of the calibration object. Furthermore, the method according to the invention includes capturing and comparing calibration interferograms that are generated by superimposing a reference wave and the calibration waves after interaction with the calibration object at different calibration positions.

[0022] According to the method of the invention, the calibration object is arranged at different calibration positions such that the calibration waves are in each case substantially perpendicularly incident on different regions of the surface of the calibration object. Furthermore, the calibration interferograms generated at different calibration positions are compared with one another. In other words, different sub-apertures of the calibration object are irradiated with calibration waves at each calibration position. By appropriately evaluating the entirety of the various calibration interferograms with an evaluation algorithm, the wavefront error of the calibration waves can be separated from the shape error of the calibration object.

[0023] According to an embodiment, the various calibration positions of the calibration object are set by a combination of shifting and tilting of the calibration object. Furthermore, the backtracking errors occurring at different calibration positions can be taken into account by calculation.

[0024] Furthermore, the present invention provides a method for interferometrically determining the shape of an optical surface of a test object. The method comprises the steps of: providing a multi-coded diffractive optical element configured to generate a test wave and at least one calibration wave from an input wave by diffraction, the test wave being directed towards the test object and having a wavefront in the form of a free-form surface; calibrating the diffractive optical element according to one of the foregoing embodiments or embodiment variants; capturing a measurement interferogram generated by superimposing a reference wave and the test wave after interaction with the surface of the test object; and determining the shape of the surface of the test object by evaluating the measurement interferogram taking into account the calibration interferogram.

[0025] According to an embodiment, for the purpose of testing a plurality of similar but different free-form surfaces, the sum of all test wavefront differences, i.e., the difference between the wavefronts of the test wave and the calibration wave, can be kept small, for example by selecting the achieved test wavefront difference as the average of the test wavefront differences of all tested free-form surfaces.

[0026] Furthermore, the present invention provides a method for interferometrically determining the respective shapes of a plurality of surfaces. Each of these surfaces has the form of a free-form surface, wherein the astigmatic component of the deviation of the respective free-form surface from the best-fit sphere lies between 70% and 90%, in particular between 75% and 85%. The method according to the present invention comprises the steps of: calibrating the shape measurement results determined by interferometrically measuring the surfaces by means of a uniform calibration surface, wherein the astigmatic component of the deviation of the uniform calibration surface from the best-fit sphere corresponds to the average of the astigmatic components of the plurality of surfaces.

[0027] Regarding the definition of the astigmatic component, reference is made in particular to DE 10 2013 226 668 A1. The measured surfaces are similar to each other due to the mentioned astigmatic component, but different from each other. In other words, in order to determine the respective shapes of a plurality of surfaces by using a calibration surface separately, only a single calibration surface is required. The determination of the shape measurement results of the surfaces can in each case be carried out analogously to the above-described interferometric method, wherein the calibration wave is adapted in each case to the mentioned uniform calibration surface.

[0028] As described above, the astigmatic component of the deviation of the uniform calibration surface from the best-fit sphere corresponds to the average of the astigmatic components of the plurality of surfaces. This means that the deviation of the astigmatic component assigned to the uniform calibration surface from the mentioned average is at most 10%, in particular at most 5%.

[0029] Furthermore, according to the present invention, there is provided an optical element for a projection lens of a microlithography exposure apparatus, the optical surface of which has a desired shape in the form of a freeform surface, and the deviation of the actual shape of the optical surface from the desired shape has a root mean square of at most 100 pm, wherein the maximum deviation of the desired shape from its best-fit spherical surface is in the range of 0.1 mm and 20 mm. In particular, the root mean square of the deviation of the actual shape from the desired shape is at most 20 pm, especially at most 10 pm. The root mean square is also known by the abbreviation RMS. The microlithography exposure apparatus includes a radiation source, an illumination system for a radiation mask, and a projection lens for imaging a mask structure onto a substrate. Furthermore, the desired shape deviates from any rotationally symmetric aspherical surface by at least 5 μm, especially at least 10 μm.

[0030] At the point of maximum deviation from the best-fit spherical surface, the deviation value of the desired shape is at least 0.1 mm and at most 20 mm. In other words, the maximum deviation of the desired shape from any spherical surface is at least 0.1 mm, but the deviation from the best-fit spherical surface is at most 20 mm. According to an embodiment, the maximum deviation of the desired shape from its best-fit spherical surface is at least 1 mm, especially at least 5 mm. According to another embodiment, the maximum deviation is at most 8 mm.

[0031] According to a variant of the embodiment, the best-fit spherical surface can be understood to mean the spherical surface with the minimum maximum deviation from the desired shape. Alternatively, the best-fit rotationally symmetric reference surface can also be determined by minimizing the root mean square of the deviation or by minimizing the average deviation. The freeform surface is understood to mean an aspherical surface without rotational symmetry.

[0032] According to other embodiments, the optical element is configured as a mirror element for a microlithography exposure apparatus in the EUV wavelength range. The EUV wavelength range (extreme ultraviolet wavelength range) is understood to mean a wavelength range below 100 nm, especially approximately 13.5 nm or approximately 6.8 nm.

[0033] Regarding the specified features of the above embodiments, exemplary embodiments, and variant embodiments, etc. of the measuring device according to the present invention, they can be correspondingly applied to the calibration method according to the present invention or the method for interferometric shape determination according to the present invention, and vice versa. These and other features of the embodiments according to the present invention will be explained in the description of the drawings. The individual features can be implemented separately or in combination as embodiments of the present invention. Furthermore, during or after the pendency of this application, they can describe independent protectable advantageous embodiments and the protection claimed for them only under reasonable circumstances. Description of the Drawings

[0034] The above and other advantageous features of the present invention are illustrated in the following detailed description of exemplary embodiments of the present invention with reference to the accompanying schematic drawings. In the drawings:

[0035] Figure 1 An embodiment of a measuring device for interferometric determination of the shape of an optical surface of a test object is shown, which uses a diffractive optical element for generating a test wave adapted to a desired shape of the surface.

[0036] Figure 2 Shows according to Figure 1 A diffractive optical element having a multiple-coded diffractive structure pattern for generating a calibration wave in addition to the test wave.

[0037] Figure 3a Shows the diffractive optical element according to Figure 2 During the measurement of a first calibration object by a first calibration wave.

[0038] Figure 3b Shows the diffractive optical element according to Figure 2 During the measurement of a second calibration object by a second calibration wave.

[0039] Figure 3c Shows the diffractive optical element according to Figure 2 During the measurement of a third calibration object by a third calibration wave.

[0040] Figure 3d Shows the diffractive optical element according to Figure 2 During the measurement of the optical surface of the test object.

[0041] Figure 3e Shows a diagram of a first calibration wave according to Figure 3a And a test wave according to Figure 3c When an adverse special situation occurs.

[0042] Figure 4 An embodiment of an annular torus is shown, on which a first surface section is marked.

[0043] Figure 5 Is a cross-sectional view along Figure 4 The center line V-V.

[0044] Figure 6 Is a cross-sectional view along Figure 4 The center line VI-VI.

[0045] Figure 7 Shows an annular torus according to Figure 4 On which other surface sections are marked.

[0046] Figure 8 Is a cross-sectional view along Figure 7Cross-sectional view of the median line VIII-VIII

[0047] Figure 9 is along Figure 7 Cross-sectional view of the median line IX-IX

[0048] Figure 10 Shows in a sectional view the toroidal torus according to Figure 4

[0049] Figure 11 Shows an embodiment of an elliptical spindle torus, on which surface sections are marked

[0050] Figure 12 is along Figure 11 Cross-sectional view of the median line XII-XII

[0051] Figure 13 is along Figure 11 Cross-sectional view of the median line XIII-XIII

[0052] Figure 14a Shows the diffractive optical element according to Figure 1 and the calibration object arranged at the first calibration position

[0053] Figure 14b Shows the diffractive optical element according to Figure 1 and the calibration object arranged at the second calibration position

[0054] Figure 15 Shows a top view of the calibration object, in which the surfaces irradiated at different calibration positions are shown

[0055] Figure 16a Shows the exemplary desired shape of the optical surface of the test object according to Figure 1 in the first section, the exemplary non-rotationally symmetric shape of the calibration wave, and the circular shape that best fits the desired shape

[0056] Figure 16b Shows the exemplary desired shape of the optical surface of the test object according to Figure 1 in the second section, the exemplary non-rotationally symmetric shape of the calibration wave, and the circular shape that best fits the desired shape, and

[0057] Figure 17 Shows an exemplary embodiment of the optical surface of the optical element of the projection lens of a microlithographic exposure apparatus Detailed Description

[0058] ​In the exemplary embodiments, embodiments or embodiment variants described below, elements that are functionally or structurally similar 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 particular exemplary embodiment, reference should be made to the description of other exemplary embodiments of the present invention or the general description of the invention.

[0059] For the sake 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 obvious. Figure 1 In this coordinate system, the x-direction extends perpendicular to the plane of the drawing into the plane, the y-direction extends upwards, and the z-direction extends to the right.

[0060] Figure 1 An interferometric device 10 according to an exemplary embodiment of the present invention is illustrated. The measuring device 10 is adapted for interferometrically determining the shape of the surface 12 of a test object 14 in the form of an optical element. This is achieved by determining the deviation of the actual shape of the surface 12 from the desired shape.

[0061] The test object 14 can, for example, be designed in the form of an optical lens element or a mirror. In the case shown, the test object 14 is a concave mirror for EUV lithography, i.e., a mirror designed to be used in a microlithographic projection exposure apparatus using exposure wavelengths in the EUV wavelength range (especially in the projection lens of a projection exposure apparatus). The EUV wavelength range extends to wavelengths below 100 nm, especially to wavelengths of approximately 13.5 nm and / or approximately 6.8 nm.

[0062] The optical test object 14 is mounted in the measuring device 10 by means of a holding device (not shown in the figures). The measuring device 10 is configured to measure an optical surface 12 whose desired shape is a free-form surface. In the present context, a free-form surface should be understood to mean a shape that deviates from any rotationally symmetric aspherical surface by more than 5 μm, especially by more than 10 μm; furthermore, the free-form surface deviates from any spherical surface by at least 0.1 mm, especially by at least 1 mm or at least 5 mm.

[0063] The interferometric device 10 includes an interferometer 16, which in turn includes a light source 18, a beam splitter 34, and a capture device 46 in the form of an interferometer camera. The light source 18 generates illumination radiation 20 and for this purpose includes, for example, a laser 22, such as a helium-neon laser for generating a laser beam 24. The illumination radiation 20 has sufficient coherent light for performing interferometry. In the case of a helium-neon laser, the wavelength of the illumination radiation 20 is approximately 633 nm. However, the wavelength of the illumination radiation 20 can also have other wavelengths in the visible and invisible wavelength ranges of electromagnetic radiation.

[0064] The laser beam 24 is focused by the focusing lens element 26 onto the aperture stop 28 such that a divergent beam 30 of coherent light exits from the aperture. The wavefront of the divergent beam 30 is substantially spherical. The divergent beam 30 is collimated by the lens group 32, and the resulting illumination radiation 20 has a wavefront that is substantially planar in the present case. The illumination radiation 20 propagates along the optical axis 56 of the interferometer 16 and passes through the beam splitter 34.

[0065] The illumination radiation 20 then impinges on a Fizeau element 36 having a Fizeau surface 38. A portion of the light of the illumination radiation 20 is reflected at the Fizeau surface 38 as a reference wave 40. The light of the illumination radiation 20 that passes through the Fizeau element 36 further propagates along the optical axis 56 with a planar wavefront 44 as an incident measurement wave (hereinafter referred to as the input wave 42), and impinges on the multi-coded diffractive optical element 60. In other embodiments of the measuring device 10, the wavefront of the input wave 42 can also be spherical.

[0066] The diffractive optical element 60 includes a substrate 62 that is transmissive to the wavelength of the illumination radiation 20 and a diffractive structure pattern 64 arranged on the substrate 52 in the form of a computer-generated hologram (CGH).

[0067] In a first embodiment, the structure pattern 64 is configured such that the input wave 42 is diffracted at the structure pattern 64 into a test wave 66 in the form of a freeform surface and at least one calibration wave 68 having a non-rotationally symmetric shape (see Figure 2 ). Generally, a non-rotationally symmetric shape should be understood to mean a shape that deviates from any rotationally symmetric shape by at least 50 μm. According to an embodiment variant, the non-rotationally symmetric shape deviates from any rotationally symmetric aspherical surface by more than 5 μm; furthermore, the non-rotationally symmetric shape can have the shape of a freeform surface as defined above. Various embodiments of the non-rotationally symmetric shape will be described below, which are characterized by other parameters in addition to the general feature of deviating from any rotationally symmetric shape by at least 50 μm.

[0068] As an Figure 1 alternative to the interferometric device 10 having the Fizeau element 36 for generating the reference wave 40 as shown in Figure 1 , the reference wave can also be generated at the diffractive optical element 60, for example as shown in DE 10 2015209 490A1,

[0069] and reflected by a reference mirror. Figure 2 In the embodiment shown in , in addition to the calibration wave 68, two other calibration waves 70 and 72 are also generated at the structure pattern 64. The calibration waves 70 and 72 can each have a planar or spherical wavefront. According to an embodiment, one or both of the other calibration waves 70 and 72 each have a non-rotationally symmetric shape similar to that of the calibration wave 68, but having a different type of shape from the calibration wave 68.

[0070] The test wave 66 is also shown in Figure 1 and is used to measure the actual shape of the optical surface 12 of the test object 14. For this purpose, the wavefront of the test wave 66 is adapted to the desired shape of the optical surface 12. As described above, the test wave 66 has the shape of a freeform surface.

[0071] Before measuring the test object 14 (as Figure 1 shown, where the test object 14 is arranged in the beam path of the test wave 66), the measuring device 10 first operates in a calibration mode. In this mode, initially, a first calibration object 74 instead of the test object 14 is arranged on the output wave side relative to the diffractive optical element 60, precisely in the beam path of the first calibration wave 68, as Figure 3a schematically illustrated in. Except for error deviations, the shape of the calibration wave 68 corresponds to the shape of the calibration surface 76 of the calibration object 74. In other words, the shape of the calibration surface 76 of the calibration object 74 is adapted to the above non-rotationally symmetric desired shape of the calibration wave 68. This non-rotationally symmetric desired shape thus serves as the desired shape for both the calibration wave 68 and the calibration object 74.

[0072] The calibration wave 68 impinges on the calibration surface 76 of the calibration object 74 and is thereby reflected back on itself. The reflected calibration wave 68 passes through the diffractive optical element 60 again and, after being reflected at the beam splitter 34, is directed onto the capture surface 50 of the camera chip 52 of the capture device 46 by the lens system 48 of the capture device 46 (see Figure 1 , where the test wave 66 is replaced by the calibration wave 68).

[0073] Due to the superposition with the reference wave 40, a calibration interference pattern is generated on the capture surface 50, and the deviation of the calibration wave 68 from its desired wavefront is determined from this pattern by the evaluation device 54. However, this occurs under the assumption that any deviation of the calibration object 74 from the desired shape can be neglected. The actual wavefront of the calibration wave 68 is thus determined with a high level of accuracy by the calibration object. The deviation of the calibration wave 68 from its desired wavefront is stored as a calibration deviation K1 in the evaluation device 54.

[0074] According to Figure 14a , 14b and the embodiment variant for determining the calibration deviation K1 shown in 15, the calibration object 74 is measured at a plurality of different calibration positions. For this purpose, the calibration object 74 is preferably configured such that the diameter of the calibration surface 76 is at least 5%, especially at least 10% larger than the diameter of the calibration wave 68, i.e., when the calibration wave 68 is radiated centrally, the corresponding non-radiated peripheral region remains on the calibration surface 76.

[0075] To set different calibration positions, the calibrator 74 is displaced by a positioning device (not shown in the figure) by a combination of tilting and in particular shifting. Figure 14a A first calibration position of the calibrator 74 is shown, in which the calibration wave 68 impinges substantially centrally on the calibration surface 76. In this case, the individual rays of the calibration wave 68 impinge substantially perpendicularly on the calibration surface 76.

[0076] Figure 14b A second calibration position is shown, which is set starting from the first calibration position by tilting the calibrator 74 downwards (i.e., tilting with respect to the x-axis), with the result that the area of the calibration surface irradiated by the calibration wave 68 is shifted upwards, for example, by at least 5% or at least 10% of the diameter of the calibration wave 68. Here, the individual rays of the calibration wave 68 also impinge substantially perpendicularly on the calibration surface. To ensure this, in addition to the above-mentioned tilting, an appropriate shift of the calibrator 74 in the yz plane also takes place.

[0077] As will be explained in more detail below, for example, the cross-sectional shape of the calibration surface 76 and the cross-sectional shape of the wavefront of the calibration wave 70 can be circular or parabolic. By an appropriate combination of tilting and shifting, even in Figure 14b the second calibration position shown, in both the case of a circular and a parabolic cross-sectional shape, substantially perpendicular radiation can be produced by the individual rays of the calibration wave 68. In the case of a circular cross-sectional shape, the combination of tilting and shifting is also referred to as "spherization".

[0078] Figure 15 The corresponding surfaces irradiated by the calibration wave 68 in different calibration positions are illustrated in a plan view of the calibration surface 76. The central calibration position described with reference to Figure 14a is denoted by P1, and the downward-tilted calibration position described with reference to Figure 14b is denoted by P2. By appropriately tilting the calibrator 74 upwards, the calibration position denoted by P3 can be set to be shifted simultaneously in the yz plane. In addition, the calibration positions denoted by P4 and P5 can be set by tilting the calibrator 74 to the left or right (i.e., tilting with respect to the y-axis) and shifting the calibrator 74 simultaneously in the xy plane.

[0079] According to an embodiment, for some or all of these other calibration positions P2 to P5 and possibly for other calibration positions similar to the calibration position P1, corresponding calibration interferograms are recorded. Since the evaluation device 54 evaluates all the recorded calibration interferograms by means of a suitable evaluation algorithm, the wavefront error of the calibration wave 68 can now be separated from the shape error of the calibration object 74. In other words, according to this embodiment, the actual deviation of the calibration object 74 from the desired shape can be taken into account, and thus the actual wavefront of the calibration wave 68 and the calibration deviation K1 can be determined with further improved accuracy. In order to further improve the accuracy of the calibration deviation K1, the retrograde error occurring for the various calibration positions P1 to P5 can be taken into account by calculation, that is, the error accumulated due to lens errors in the optical unit of the interferometric device 10, which depends on the beam path of the test wave 66 through the optical unit.

[0080] According to an embodiment, during the evaluation of different calibration interferograms, the wavefront segments of the calibration wave 68, which exist at different calibration positions P1 to P5 and illuminate corresponding sub-apertures of the calibration surface 76, are combined by a known stitching method.

[0081] In addition to the calibration deviation K1 determined by the calibration wave 68, according to Figure 2 the illustrated embodiment, other calibration waves 70 and 72 can be used to determine other calibration deviations K2 and K3. The propagation directions of the calibration waves 70 and 72 are different from each other and from the propagation direction of the calibration wave 68. The determination of the calibration deviations K2 and K3 is similar to the determination of the calibration deviation K1, which is determined by arranging the Figure 3b illustrated calibration object 78 having a calibration surface 80 in the beam path of the calibration wave 70 and arranging the Figure 3c illustrated calibration object 82 having a calibration surface 84 in the beam path of the calibration wave 72.

[0082] By evaluating the determined calibration deviations K1 to K3, the x and y coordinates of the distortion of the phase function of the diffraction structure pattern 64 on the diffractive optical element 60 that generates the calibration waves 68, 70, and 72 can now be determined. In addition, the shape and / or profile deviation of the substrate surface of the diffractive optical element 60 having the diffraction structure pattern 64 can be determined from the calibration deviations K1 to K3. From the distortion coordinates and the shape and / or profile deviations thus obtained, the distortion in the x and y coordinates and the shape and / or profile deviation of the entire diffraction structure pattern 64 are then inferred. These deviation data are stored in the evaluation device 54 and are used to correct the test wave 66 during the subsequent measurement of the surface shape of the test object 14.

[0083] For this purpose, as Figure 1 and Figure 3dAs shown, the test object 14 is arranged in the beam path of the test wave 66 such that it impinges on the optical surface 12 in a self-collimating manner and is reflected there. The reflected wave then returns to the interferometer 16 through the diffractive optical element 60 as the returned test wave 66. The returned test wave 66 captures interference with the reference wave 40 on the surface 50, thereby generating a test interferogram. The test interferogram is evaluated by the evaluation device 54 and therefrom the deviation of the actual shape of the optical surface 12 from its desired shape is determined. All deviation data previously determined during the calibration surface measurement are taken into account in the evaluation.

[0084] Figure 3e The figure illustrates an unfavorable special case of the diffraction direction matching between the calibration wave and the test wave at the so-called pole 86, but this can be avoided due to the degrees of freedom enabled by the configuration according to the invention of the calibration wave having a non-rotationally symmetric shape. This unfavorable special case is Figure 3e shown in by simultaneously illustrating the individual rays 66-1 to 66-6 of the test wave 66 adapted to the shape of the surface 12 of the test object 14 and the individual rays 68a-1 to 68a-6 of the calibration wave 68a adapted to the calibration surface 76a of an unfavorable embodiment of the first calibration object 74a.

[0085] As Figure 3e shown, in the unfavorable embodiment shown, the individual ray 66-4 (of the test wave 66) and the individual ray 68a-4 (of the calibration wave 68) travel along the same path, and these individual rays 66-4 and 68a-4 emanate from a point of the diffraction structure pattern 64a of the associated diffractive optical element 60a called the pole 86. At the pole 86, the difference between the two associated phase functions of the associated diffraction structure pattern 64a is zero, which results in a relatively large groove spacing in the diffraction structure pattern 64a. These in turn have an adverse effect on the diffraction intensity. In short, this situation results in a gap in the diffraction structure pattern 64a. As described above, such poles can be avoided due to the degrees of freedom in the configuration of the calibration wave 68, which degrees of freedom are available for the calibration wave 68 by using a non-rotationally symmetric shape, in particular an astigmatic shape.

[0086] According to an embodiment, for the interferometric measurement of the respective shapes of a plurality of optical surfaces 12, which optical surfaces 12 are similar in that they have a desired shape, each of which has the form of a freeform surface having an astigmatic component of the deviation of the respective freeform surface from the best-fit sphere between 70% and 90%, a uniform calibration surface can be used to calibrate the shape measurement results. In this case, the uniform calibration surface can be configured such that the deviation of the calibration surface from the best-fit sphere has an astigmatic component corresponding to the average value of the astigmatic components of the plurality of optical surfaces.

[0087] The following will refer to Figures 4 to 13 FIGS. illustrate various embodiments of the above-described non-rotationally symmetric shape of the calibration wave 68 and the associated calibrator 74. As described above, the non-rotationally symmetric shape is characterized in that it deviates from any rotationally symmetric shape by at least 50 μm. The embodiments of the non-rotationally symmetric shape described below can be used for other calibration waves generated at the diffraction structure pattern 64 of the diffractive optical element 60, such as calibration waves 70 and 72.

[0088] In Figures 4 to 13 In all of the embodiments shown, the non-rotationally symmetric shape of the calibration wave 68, hereinafter referred to as 68f, corresponds to Figure 10 the shape of a section of the surface 89 of the rotational solid 88 shown. The rotational solid 88 is formed by rotating a rotational surface 90 that is symmetric about a symmetry axis 92 about a rotation axis 94. In the embodiments shown, the symmetry axis 92 is arranged parallel to the rotation axis 94. In alternative embodiments, the symmetry axis 92 can also be aligned non-parallel to the rotation axis 94. The rotational surface 90 (which can also be described as "mirror symmetric" due to its axial symmetry) can, for example, be configured to be rotationally symmetric and thus circular (see rotational surface 90a), or non-rotationally symmetric, such as elliptical (see rotational surface 90b with a semi-major axis a perpendicular to the rotation axis 94 and rotational surface 90c with a semi-minor axis a perpendicular to the rotation axis 94).

[0089] The distance between the rotation axis 94 and the symmetry axis 92 is the radius R of the rotational solid 88. The radius R can be greater than, equal to, or less than the radius r of the rotational surface 90a or the semi-axis of the rotational surface 90b or 90c perpendicular to the symmetry axis 92. In the case of the circular rotational surface 90a, Figure 10 the toroidal torus shown in has R>r for the rotational solid 88, the so-called angular torus has R=r, and the known spindle torus has R<r. In the case of the circular rotational surface 90a, according to the present invention, R≠0 applies.

[0090] In the case of the elliptical rotational surfaces 90b and 90c, similar solid results are produced, which are herein referred to as an "elliptical toroidal torus" with R>a, an "elliptical angular torus" with R=a, and an "elliptical spindle torus" with R<a. In the case of the elliptical rotational surfaces 90b and 90c, the case of R=0 is also allowed. Figure 11 An example of an "elliptical spindle torus" with R=0 is shown.

[0091] In Figure 4 and 7 In, from Figure 10Two different sections of the surface 89 of the rotary body 88 in the form of an annular torus shown are each selected as an example to be the non-rotationally symmetric shape 68f of the calibration wave 68. This shape 68f is in each case mirror-symmetric with respect to the mirror plane extending through the Figure 4 V-V line specified in Figure 7 or the VIII-VIII line specified in Figure 4 . In the embodiment according to

[0092] Figure 5 , the selected section is located on the outer side of the annular torus and thus has a convex shape. Figure 6 shows a cross-sectional view along the V-V line and Figure 4 shows a cross-sectional view along the VI-VI line, each in the image of the non-rotationally symmetric shape 68f shown in the Figure 5 and 6 right-hand part of 1 or r 2 differ in terms of the opening parameter in the form of their respective radii r

[0093] . In the embodiment according to Figure 7 , the selected section is located on the inner side of the annular torus and thus has a saddle shape. Figure 8 shows a cross-sectional view along the VII-VII line and Figure 9 shows a cross-sectional view along the IX-IX line, both in the image of the non-rotationally symmetric shape 68f shown in the Figure 7 right-hand part of

[0094] Similar to Figure 5 and 6 , Figure 8 and 9 show cross-sections of the wavefront of the calibration wave 68 according to Figure 7 through cross-sectional surfaces arranged perpendicular to each other, precisely once along the yz plane and once along the xz plane. Here, the wavefront also has a circular shape and thus has a curved shape in each of these two cross-sectional surfaces. The circular shapes have different curvature directions and thus different opening parameters, in particular radii with different signs (r 1 >0 or r 2 <0), and the absolute values of the two radii are also different (|r 1 |<|r 2 |).

[0095] In Figure 11 the illustrated embodiment of the solid of revolution 88 in the form of an "elliptic spindle torus" with R = 0, the selected section 68f on the front side of the solid of revolution 88 has a convex shape similar to that of the section according to Figure 5 , but has a parabolic cross-sectional profile. The shape of the section 68f is mirror-symmetrical with respect to the mirror plane extending through the line designated as XII-XII.

[0096] Figure 12 A sectional view along line XII-XII is shown and Figure 13 a sectional view along line XIII-XIII is shown, each in the image of the non-rotationally symmetric shape 68f shown in the right-hand part of Figure 11 . In other words, Figure 12 and 13 show cross-sections of an embodiment of the wavefront of the calibration wave 68 passing through cross-sectional surfaces arranged perpendicular to each other, precisely once along the yz plane and once along the xz plane. In each of these two cross-sectional surfaces, the shape 68f of the wavefront approximates in each case the top region 98 of the parabolic shape 96-1 or 96-2 and thus has a curved shape.

[0097] Figure 12 The illustrated parabolic shape 96-1 can be described as follows: z = -a 1 y 2 + b 1 , and Figure 13 the illustrated parabolic shape 96-2 can be described as follows: z = -a 2 x 2 + b 2 , where in this case: b 1 = 0 and b 2 = 0. The opening parameters a 1 and a 2 of the parabolic shape in the parabolic opening shape are different from each other (a 2 < a 1 ). The wavefront of the calibration wave 68 according to the embodiment of Figure 11 therefore has an astigmatic shape.

[0098] In Figure 16a and 16b , an exemplary desired shape 12a of the optical surface 12 is shown together with the non-rotationally symmetric shape 68f of the calibration wave 68 and the best-fit circular shape 100 in the corresponding sectional views of Figure 12 or Figure 13 , i.e., in two mutually orthogonal cutting planes. For illustrative purposes, as in Figure 12 and 13In , the shape of the respective top regions 98 approximating the parabolic shapes 96-1 and 96-2 is selected as the non-rotationally symmetric shape 68f of the calibration wave 68. In Figure 16a and 16b The best-fit circular shapes 100 drawn therein each represent a cross-sectional view through a spherical surface that best fits the desired shape 12a and thus has the same radius.

[0099] If one now considers Figure 16a and 16b the wobble 102-1 or 102-2, i.e., the maximum deviation, between the desired shape 12a of the optical surface 12 and the shape 68f of the calibration wave 68 in the two cross-sectional views of , it is evident that this wobble is significantly reduced compared to the corresponding wobble 102a-1 or 102a-2 between the desired shape 12a and the best-fit circular shape 100. Due to this reduced wobble, the use of the calibration wave 68 according to the invention having a non-rotationally symmetric shape 68f enables a significant improvement in the achievable calibration accuracy of the diffractive optical element 60 compared to the use of a spherical calibration wave, which is customary in the prior art.

[0100] Figure 17 In , the optical surface 12 of an optical element of the type designated as test object 14 is schematically illustrated in the yz plane. In particular, the optical element 14 is an element of a projection lens of a microlithography exposure apparatus, in particular a mirror element for the EUV wavelength range. Figure 1

[0101] Figure 17 In addition to the optical surface 12 in its actual shape, cross-sectional views of the desired shape 12a of the optical surface and the spherical surface 104 that best fits the desired shape 12a are shown in Figure 17 in a manner Figure 16a similar to . The deviation of the actual shape of the optical surface 12 from the desired shape 12a, described by the two-dimensional deviation D(x,y), is schematically shown in a highly magnified manner, where x and y represent coordinates on the surface 12.

[0102] The root mean square of the deviation D(x,y) determined over the entire optical surface 12 is at most 100 pm, in particular at most 20 pm. In contrast, the desired shape 12a has a maximum deviation Δ compared to the best-fit spherical surface 104, which is in the range of 0.1 mm and 20 mm, i.e., the maximum deviation Δ is at least 0.1 mm and at most 20 mm. In particular, the lower limit value of this range is 1 mm or 5 mm, and the upper limit value can in particular be 8 mm.

[0103] In the cross-section of the optical surface 12 of the optical element 14 in the xz plane, in addition to the deviation described by D(x,y), the optical surface 12 also extends along Figure 16bThe desired shape 12a shown extends. The maximum deviation of the desired shape 12a from the best-fit spherical surface 104 is less than Figure 17 the deviation Δ in the shown section. In this embodiment, according to Figure 17 the maximum deviation in the section of Figure 17 is greater than the maximum deviation in all other possible sections. Therefore, according to

[0104] the maximum deviation Δ is regarded as the maximum deviation Δ from the best-fit spherical surface, as described above. Figure 16a Since the desired shape 12a in the xz plane ( Figure 16b ) is significantly different from the desired shape 12a in the yz plane (

[0105] ), the desired shape 12a deviates significantly from any rotationally symmetric aspherical surface in three-dimensional form. According to this embodiment, the maximum deviation of the desired shape 12a from any rotationally symmetric aspherical surface is at least 5 μm, especially at least 10 μm.

[0106] List of reference numerals

[0107] 10 Interferometric device

[0108] 12 Optical surface

[0109] 12a Desired surface

[0110] 14 Test object

[0111] 16 Interferometer

[0112] 18 Light source

[0113] 20 Illuminating radiation

[0114] 22 Laser

[0115] 24 Laser beam

[0116] 26 Focusing lens element

[0117] 28 Diaphragm

[0118] 30 Divergent beam

[0119] 32 Lens group

[0120] 34 Beam splitter

[0121] 36 Fizeau element

[0122] 38 Fizeau surface

[0123] 40 Reference wave

[0124] 42 Input wave

[0125] 44 Plane wavefront

[0126] 46 Capture device

[0127] 48 Lens system

[0128] 50 Capture surface

[0129] 52 Camera chip

[0130] 54 Evaluation device

[0131] 56 Optical axis

[0132] 60 Diffractive optical element

[0133] 60a Diffractive optical element in an adverse embodiment

[0134] 62 Substrate

[0135] 64 Diffraction structure pattern

[0136] 64a Diffraction structure pattern in an adverse embodiment

[0137] 66 Test wave

[0138] 66-1 to 66-6 Individual rays of test wave 66

[0139] 68 Calibration wave

[0140] 68a-1 to 68a-6 Individual rays of calibration wave 68a

[0141] 68f Non-rotationally symmetric shape

[0142] 70 Calibration wave

[0143] 72 Calibration wave

[0144] 74 First calibrator

[0145] 74a First calibrator in an adverse embodiment

[0146] 76 Calibration surface

[0147] 76a Calibration surface of calibrator 76

[0148] 78 Second calibrator

[0149] 80 Calibration surface

[0150] 82 Third calibrator

[0151] 84 Calibration surface

[0152] 86 Pole

[0153] 88 Revolving solid

[0154] 89 Surface

[0155] 90, 90a, 90b, 90c Revolving surface

[0156] 92 Axis of symmetry

[0157] 94 Axis of rotation

[0158] 96-1, 96-2 Parabolic shape

[0159] 98 Top region

[0160] 100 Best-fit circular shape

[0161] 102-1, 102-2 Wobble when using non-rotationally symmetric calibration waves

[0162] 102a-1, 102a-2 Wobble when using spherical calibration waves

[0163] 104 Best-fit sphere

Claims

1. A measuring device (10) for interferometrically determining the shape of a surface (12) of a test object (14), comprising: - a radiation source for providing an input wave (42), - a multi-coded diffractive optical element (60) configured to generate a test wave (66) and at least one calibration wave (70) from the input wave by diffraction, the test wave (66) being directed towards the test object (14) and having a wavefront in the form of a free-form surface, wherein the wavefront of the calibration wave has a non-rotationally symmetric shape (68f), wherein the cross-section of the wavefront of the calibration wave (70) passing through cross-sectional surfaces aligned transversely to each other has a curved shape in each case, and wherein the curved shapes in different cross-sectional surfaces differ in terms of an aperture parameter, and a capture device (46) for capturing a calibration interferogram formed by superposing a reference wave (40) and the calibration wave after interaction with a calibration object (74).

2. The measuring device according to claim 1, wherein, the curved shapes in two different cross-sectional surfaces differ in terms of the direction of their curvature.

3. The measuring device according to claim 2, wherein, the curved shape is in each case approximately a partial segment of a circular shape and the aperture parameter is the radius of the circle.

4. The measuring device according to claim 3, wherein, the curved shape is in each case approximately a partial segment of a parabolic shape and the aperture parameter is the parabola aperture.

5. The measuring device according to claim 2 or 4, wherein, the wavefront of the calibration wave has an astigmatic shape.

6. The measuring device according to any one of claims 1 to 4, wherein, the non-rotationally symmetric shape (68f) of the calibration wave (70) corresponds to the shape of a section of the surface of a solid of revolution (88) formed by rotating a surface (90, 90a, 90b, 90c) symmetric about a symmetry axis (92) about a rotation axis (94).

7. The measuring device according to any one of claims 1 to 4, wherein, the wavefront of the calibration wave (70) deviates from any rotationally symmetric shape by at least 50 μm.

8. The measuring device according to any one of claims 1 to 4, wherein, the diffractive optical element (60) is configured to generate at least three calibration waves (68, 70, 72) from the input wave by diffraction in addition to the test wave.

9. A method for calibrating a diffractive optical element (60) configured to generate a test wave (66) having a wavefront in the form of a free-form surface for interferometrically determining the shape of a surface (12) of a test object (14), the method comprising the steps of: - providing the diffractive optical element (60) which has multi-coding and is irradiated with an input wave (42) such that, in addition to the test wave, a calibration wave (70) directed towards a calibration object (74) is generated by diffraction at the multi-coding, wherein the wavefront of the calibration wave has a non-rotationally symmetric shape (68f), - Arrange the calibrator (74) at different calibration positions (P1, P2) such that the calibration wave is incident substantially perpendicularly on different regions of the surface (76) of the calibrator in each case, and Capture and compare calibration interferograms, which are generated by superimposing a reference wave (40) and the calibration wave (70) after interaction with the calibrator at the different calibration positions.

10. The method according to claim 9,[[]]END]] wherein,[[]]END]] the different calibration positions of the calibrator are set by a combination of shifting and tilting of the calibrator (74).

11. A method for interferometric determination of the shape of the surface (12) of a test object (14), comprising the steps of:[[]]END]] - Providing a multi-coded diffractive optical element (60) configured to generate a test wave and at least one calibration wave (70) from an input wave (42) by diffraction, the test wave being directed towards the test object and having a wavefront in the form of a free-form surface,[[]]END]] - Calibrating the diffractive optical element according to claim 9 or 10,[[]]END]] - Capturing a measurement interferogram generated by superimposing the reference wave (40) and the test wave (66) after interaction with the surface of the test object, and - Determining the shape of the surface of the test object by evaluating the measurement interferogram taking into account the calibration interferogram.[[]]END]]

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