Interferometric measuring device

Abstract

A measuring device (10; 110; 210; 310; 410; 510; 610; 710) for the interferometric determination of a property (50; 52) of a shape (50) of a test surface (12) of a test object (14) comprises an irradiation device (22) for generating an input wave (24), a splitting module (18; 118; 318; 418; 518) configured to generate two plane waves (32, 34) with parallel propagation directions, offset from each other transversely to the propagation directions, a wavefront matching module (20; 720) for generating two measurement waves (44, 46) by matching the respective wavefronts of the offset plane waves to a target shape of the optical test surface, and a detector (56) for detecting at least one interferogram (64). which is generated by superimposing the measuring waves (44r, 46r) after their interaction with the test surface.

Description

Background of the invention

[0001] The invention relates to a measuring device and a method for the interferometric determination of a property of the shape of a test surface of a test object. For example, an optical element for microlithography can be measured as the test object. The determined shape property can, in particular, be the shape of the test surface itself. Due to the need for ever smaller structures, increasingly higher demands are being 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.

[0002] For the high-precision interferometric measurement of optical surfaces down to the sub-nanometer range, interferometric measuring devices and methods are known 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 perpendicularly at every point and be reflected back by it. Deviations from the target shape can then be determined using the interferogram formed by superimposing the reflected test wave and the reference wave.

[0003] For example, WO 2016 / 188620 A2 describes a measuring device with a complexly coded computer-generated hologram (CGH) as a diffractive optical element. The CGH generates a test wave from an input wave, directed at the test surface to be measured, with a wavefront at least partially adapted to a target shape of the test surface, and a reference wave. The reference wave is reflected back to the CGH by a reflective optical element. Reference is also made to DE 10 2011 004 376 B3.

[0004] With such previously known measuring devices, the problem arises that measurement errors can occur, for example, due to air schlieren or lattice structure defects in the CGH. To minimize measurement errors caused by air schlieren, the measuring device can be operated in a high vacuum. Calibration procedures can be used to avoid measurement errors caused by lattice structure defects. However, all these measures increase the effort required for the interferometric measurement and / or the complexity of the measuring device. Underlying task

[0005] It is an object of the invention to provide a measuring device and a method with which the aforementioned problems can be solved, and in particular a property of a shape of a test surface can be determined with very high accuracy under reasonable effort. Inventive solution

[0006] The aforementioned problem can be solved according to the invention, for example, with a measuring device for the interferometric determination of a property of the shape of a test surface of a test object. The measuring device comprises an irradiation unit for generating an input wave and a splitting module configured to generate two plane waves with parallel propagation directions from the input wave, which are offset from each other transversely to the propagation directions. Furthermore, the measuring device comprises a wavefront matching module for generating two measurement waves by adapting the respective wavefronts of the offset plane waves to a target shape of the optical test surface, and a detector for acquiring at least one interferogram, which is generated by superimposing the measurement waves after their interaction with the test surface.

[0007] The property of the test surface shape that can be determined by the measuring device can, for example, be a derivative of the test surface shape, the configuration of the test surface shape itself, or the shape of the test surface itself, as explained in more detail below. The plane waves, offset from one another and with parallel propagation directions, can also be referred to as sheared plane waves. The displacement occurs in a lateral direction, i.e., perpendicular to the propagation direction of the input wave. The extent of the displacement is at least one wavelength of the input wave, advantageously at least a multiple of the wavelength of the input wave, i.e., at a wavelength in the visible range, at least approximately 0.5 µm, and in particular at least approximately 10 µm. The wavefront matching module can be configured as a diffractive optical element, such as a CGH, or as a lens, in particular as an aspherical lens.

[0008] By generating the two measuring waves using sheared plane waves, these waves, which function as a test wave and a reference wave, travel along virtually the same beam path through the cavity between the wavefront matching module and the test surface. This means that any air turbulence in the cavity affects the two measuring waves essentially in the same way, resulting in no or only minimal alteration of the interferogram. Therefore, the inventive configuration allows the measuring device to be operated at normal pressure or at only slightly reduced ambient pressure, such as in rough or fine vacuum. Operation in high vacuum can be avoided in any case, thus reducing the effort required to operate the measuring device. According to one embodiment, a specially designed flowbox can also ensure a laminar, homogeneous, uniform, and steady flow through the cavity suitable for normal pressure.Undesirable effects caused by such a reproducible laminar flow can possibly be almost completely eliminated by calibration against a so-called golden mirror.

[0009] Furthermore, errors or deviations of the wavefront matching module in the inventive configuration of the measuring device have an almost identical effect on the sheared plane waves. Therefore, in the case of the wavefront matching module being configured as a diffractive optical element, potential grating structure errors, or in the case of the wavefront matching module being configured as an aspherical lens, potential lens errors, have an almost identical effect on the two measuring waves. Consequently, complex calibration procedures for the wavefront matching module can be largely dispensed with.

[0010] The generation of parallel plane waves according to the invention by the splitting module has the advantage that the two measuring waves are, apart from a lateral offset, exactly identical copies of each other; that is, both measuring waves possess exactly one wavefront adapted to the surface to be measured. The difference between the two measuring waves lies solely in the fact that, when the measuring waves are decomposed into local waves and the origin is identified in the light source, the local waves with the same origin in the light source or with identical coherence are laterally offset in the two measuring waves.

[0011] The two measurement waves thus travel back on themselves with perfect autocollimation, ensuring that they meet precisely at the detector, even if the (ideal or nominal) shape of the test surface deviates significantly from the shape of a flat plate. This reliably avoids measurement artifacts and enables high measurement accuracy for test objects with any surface shape, especially those that deviate considerably from a flat surface. The methodology itself guarantees that, even in the idealized case, the two measurement waves will travel back on themselves exactly and interfere perfectly at the detector, as the waves arrive at identical or nearly identical points.

[0012] In contrast, the irradiation of waves, especially plane waves or expanding waves, with non-parallel propagation directions, onto the wavefront matching module and / or the use of measurement waves with non-parallel propagation directions would not lead to an exact autocollimation of the measurement waves, which in turn would result in increased measurement artifacts.

[0013] Furthermore, qualification of the wavefront adaptation module, for example with regard to material inhomogeneities of the wavefront adaptation module, is possible against a plane mirror, i.e. instead of the test object, a plane mirror is arranged during qualification, also called calibration.

[0014] According to various embodiments, the splitting module can be configured to generate more than two plane waves from the input wave, for example, at least three or at least five plane waves, with parallel propagation directions that are offset from each other. In this case, the wavefront adaptation module can be configured accordingly to generate more than two measurement waves, for example, at least three or at least five measurement waves, by adapting the respective wavefronts of the offset plane waves to a target shape of the optical test surface.According to various further embodiments, the measuring device can also comprise more than one splitting module, for example at least two, at least three, or at least five splitting modules, each configured to generate at least two plane waves with parallel propagation directions from the input wave, which are offset from each other transversely to the propagation directions. According to various further embodiments, the measuring device can also comprise more than one wavefront matching module, for example at least two, at least three, or at least five wavefront matching modules, for generating at least two, at least three, or at least five measuring waves by adapting the respective wavefront of the offset plane waves to a target shape of the optical test surface.

[0015] According to one embodiment, the measuring device is configured such that, after interacting with the test surface, the measurement waves pass through the splitting module in the opposite direction to the radiation of the input wave before reaching the detector, thereby offsetting the measurement waves towards each other. "Alignment towards each other" here means that the measurement waves are closer together after the offset than before. In particular, the offset occurs such that the measurement waves are precisely aligned.

[0016] According to a further embodiment, the measuring device comprises an evaluation unit which is configured to determine at least one derivative of the shape of the test surface using the at least one interferogram acquired.

[0017] According to another embodiment, the measuring device is configured to vary the splitting direction of the two plane shafts. This can be achieved, for example, by rotating the splitting module. Advantageously, the derivatives of the test surface in different directions are then determined.

[0018] According to another embodiment, the measuring device is configured to vary the lateral offset of the two plane waves and thus the magnitude of the derivative differential or the spatial resolution. Both the directions of the derivatives and the magnitude of the derivative differentials, and therefore the spatial resolution, can be changed, while the condition of autocollimation of the measuring waves at the mirror is always met.

[0019] According to another embodiment, the evaluation device is configured to determine derivatives of the shape of the test surface at several locations on the test surface and to determine the shape of the test surface by integrating the determined derivatives. Advantageously, derivatives in different directions at the individual locations on the test surface are used for integration. The splitting direction is understood to be the direction in which the propagation directions of the two measurement waves are offset from each other.

[0020] Additionally, measurements of varying magnitudes in the splitting differences can be utilized by different beam offsets and further incorporated into the calculation. Besides considering derivatives in different directions, it is also advantageous to include derivative directions rotated by 180°, and thus derivative differences with the opposite sign, so that both left- and right-hand derivatives are captured in a given direction at the surface.

[0021] According to a further embodiment, the splitting module comprises at least one beam direction splitting element, according to different embodiments at least two, at least three or at least five beam direction splitting elements, and at least one direction alignment element, according to different embodiments at least two, at least three or at least five direction alignment elements, wherein the at least one beam direction splitting element is configured to generate at least two intermediate waves, according to different embodiments at least three or at least five intermediate waves, with different propagation directions from the input wave, and the direction alignment element is configured to generate the at least two plane waves offset from each other by aligning the propagation directions of the at least two intermediate waves.

[0022] According to one embodiment, the distance between the beam direction splitting element and the direction alignment element is on the order of the distance between the wavefront matching module and the test surface. The same applies to the distance between the direction alignment element and the wavefront matching module. The spatial resolution can be adjusted by selecting the distance between the beam direction splitting element and the direction alignment element. Varying this distance can, on the one hand, vary the offset of the measurement waves, thus affecting the magnitude of the derivative differentials and consequently the spatial resolution. On the other hand, this distance variation can simultaneously be used for phase shifting, although the distance variation will typically be smaller than when adjusting the spatial resolution.

[0023] According to another embodiment, the beam direction splitting element and / or the direction alignment element is configured as a diffractive optical element, in particular as a CGH. According to one embodiment variant, the CGHs are tilted relative to each other to avoid reflections.

[0024] According to another embodiment, the beam splitting element and / or the direction alignment element is each configured to diffract incoming radiation only into the zeroth and first diffraction order in magnitude, for example into the 0th and +1st diffraction order.

[0025] According to another embodiment, the beam splitting element and / or the direction alignment element are each configured to diffract incoming radiation only into the +1st and -1st diffraction orders.

[0026] According to another embodiment, the beam direction splitting element and the direction alignment element are diffractive optical elements configured inversely to each other.

[0027] According to another embodiment, the beam direction splitting element and / or the direction alignment element is configured as a shear prism. For example, a Nomarski prism, a Wollaston prism, or a Rochon prism are suitable for this purpose.

[0028] According to a further embodiment, the measuring device also includes a variable phase delay element (also called a phase retarder) for phase shifting. This element can, for example, contain a liquid crystal. The phase delay element causes different phase retarding for wavefronts of different polarizations; thus, depending on the arrangement of the phase delay element in the beam path, it already causes different phase retarding for the two intermediate waves of the two measuring waves.

[0029] According to another embodiment, the direction alignment element is configured as a shear prism, which is arranged in reverse orientation to a shear prism of the same type serving as the beam direction splitting element.

[0030] According to another embodiment, the beam-direction splitting element is configured to split the second intermediate wave from the input wave, wherein the first intermediate wave is a portion of the input wave that passes through the beam-direction splitting element without deflection. The beam-direction splitting element can, for example, be configured as a Rochon prism or as a CGH.

[0031] According to another embodiment, the propagation directions of the intermediate waves are oriented symmetrically to the propagation direction of the input wave.

[0032] According to another embodiment, the measuring device is configured to change the distance between the beam direction splitting element and the direction alignment element. Changing this distance can serve to alter the relative phase between the two offset plane waves. This is known as phase shifting. With diffractive optical elements, particularly CGHs, phase shifting is also possible by laterally displacing the diffractive optical element. Therefore, if both the beam direction splitting element and the direction alignment element are implemented as diffractive optical elements, phase shifting can also be achieved by laterally displacing either the beam direction splitting element or the direction alignment element.Changing the distance between the beam direction splitting element and the direction alignment element can also serve the purpose of varying the size of the derivative differentials and thus the spatial resolution.

[0033] Designing the beam direction splitting element and / or the direction matching element in the form of a CGH has the advantage that additional diffractive auxiliary structures can be present on the CGH (e.g. Cateye or Littrow structures), which are advantageous for adjusting the two elements of the splitting module to each other or relative to the interferometer (entire optics in front of the splitting module, i.e. illumination and imaging unit) as well as for adjusting the wavefront matching module against the splitting module or the entire interferometer.

[0034] According to another embodiment, the splitting module comprises a shear plate. This is preferably parallel-sided, but can also be wedge-shaped. In any case, the shear plate is configured such that a first output wave is generated from the light of the input wave, which exits the shear plate at the opposite side from the input without further reflection. The second output wave is generated from the light of the input beam, which is reflected back and forth between the output and input sides of the shear plate before exiting. The phase shifting can be achieved, for example, by tilting the shear plate. Tilting the shear plate causes the path length difference of the two beams within the plate to change.

[0035] According to another embodiment, the measuring device is configured to measure a mirror for EUV microlithography, in particular a mirror of an illumination system or a projection lens of an EUV projection exposure system, as the test object.

[0036] The aforementioned task can also be solved, for example, using a method for the interferometric determination of a property of the shape of a test surface of a test object. The method involves generating two plane waves with two parallel propagation directions, offset perpendicular to the propagation directions, by splitting an input wave. Furthermore, two measurement waves are generated from the plane waves by adapting the respective wavefronts of the offset plane waves to a target shape of the optical test surface. At least one interferogram is then generated by superimposing the measurement waves after their interaction with the test surface and recording the interferogram.

[0037] The features specified for the aforementioned embodiments, exemplary embodiments, or variants, etc., of the measuring device according to the invention can be transferred accordingly to the method according to the invention, and vice versa. These and other features of the embodiments according to the invention are explained in the description of the figures and the claims. The individual features can be implemented either separately or in combination as embodiments of the invention. Furthermore, they can describe advantageous embodiments that are independently patentable and whose protection may be claimed only during or after the filing of the application. Brief description of the drawings

[0038] The foregoing, as well as further advantageous features of the invention, are illustrated in the following detailed description of exemplary embodiments or embodiments or variants of the invention with reference to the accompanying schematic drawings. These show: Fig. 1 An embodiment of a measuring device for the interferometric determination of the shape of a test surface of a test object, comprising a splitting module consisting of two diffractive optical elements for generating two parallel plane waves and a wavefront matching module for generating two measuring waves from the plane waves. Fig. 2 an embodiment of an arrangement for qualifying or calibrating the wavefront matching module according to Fig. 1, Fig. 3 an embodiment of an arrangement for qualifying or calibrating the interferometer including the splitting module of the measuring device according to Fig. 1, Fig. 4 Another embodiment of a measuring device for the interferometric determination of the shape of a test surface of a test object with opposite Fig. 1 differently designed diffractive optical elements of the splitting module, Fig. 5 Another embodiment of a measuring device for the interferometric determination of the shape of a test surface of a test object, in which the diffractive optical elements of the splitting module are arranged at an angle, Fig. 6 Another embodiment of a measuring device for the interferometric determination of the shape of a test surface of a test object, in which one of the two diffractive optical elements of the splitting module is formed by a shear prism, Fig. 7 Another embodiment of a measuring device for the interferometric determination of the shape of a test surface of a test object, in which both diffractive optical elements of the splitting module are formed by shear prisms, Fig. 8 Another embodiment of a measuring device for the interferometric determination of the shape of a test surface of a test object, in which one of the two diffractive optical elements of the splitting module is configured as a Rochon prism, Fig. 9 a further embodiment of a measuring device for the interferometric determination of the shape of a test surface of a test object, in which the splitting modulus is formed by a shear plate, as well as Fig. 10 Another embodiment of a measuring device for the interferometric determination of the shape of a test surface of a test object, in which the wavefront matching module is formed by a lens. Detailed description of embodiments according to the invention

[0039] 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.

[0040] 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 perpendicularly out of the plane of the drawing, the x-direction to the right and the z-direction downwards.

[0041] In Fig. Figure 1 schematically illustrates an embodiment 10 of a measuring device for the interferometric determination of a property of a shape z(x,y) (reference numeral 50) of a test surface 12 of a test object 14. The determined property can be one or more derivatives dz / dx or dz / dy (reference numeral 52) of the shape z(x,y) of the test surface 12, or the shape 50 of the test surface 12 itself. The shape of the test surface 12, or in particular a desired shape of the test surface 12, can be non-spherical, and in particular can be designed as an aspherical or freeform surface. An aspherical surface is understood to be a rotationally symmetric surface that deviates from any arbitrary sphere by at least 0.05 mm, in particular by at least 0.1 mm, by at least 1 mm, or by at least 5 mm. In this text, such an aspherical surface is also referred to as a rotationally symmetric asphere or simply as an asphere.A freeform surface is defined as a shape with a deviation from any rotationally symmetric asphere of at least 5 µm, in particular at least 10 µm. Furthermore, the freeform surface deviates from any sphere by at least 0.05 mm, in particular by at least 0.1 mm, by at least 1 mm, or by at least 5 mm.

[0042] The measuring device 10 is particularly suitable for the high-precision measurement of the optical surface of an aspherical mirror or a freeform mirror for microlithography using exposure radiation in the extreme ultraviolet (EUV) spectral range. The EUV wavelength range extends to wavelengths below 100 nm and specifically includes wavelengths of approximately 13.5 nm or 6.8 nm. For error reduction or correction of such optical surfaces, shape determination down to the sub-nanometer range is necessary. The measuring device 10 is also suitable for measuring the surface of many other objects.

[0043] The measuring device 10 comprises a beam generation and evaluation unit 16, a splitting module 18, and a wavefront matching module configured as a diffractive optical element 20. The beam generation and evaluation unit 16 comprises an irradiation unit 22, which serves to generate an input wave 24 and includes a radiation source 26 and a collimator lens 28.

[0044] The radiation source 26 generates a measurement radiation 30 in the form of an expanding wave that is sufficiently coherent for interference. According to one embodiment, the radiation source 26 is a helium-neon laser with a wavelength of approximately 633 nm. The measurement radiation 30 can also have a different wavelength in the visible or non-visible wavelength range of electromagnetic radiation. The measurement radiation 30 generated by the radiation source 26 first passes through a beam splitter 31 and then through the collimator lens 28.

[0045] The input wave 24 generated by the irradiation device 22 is a plane wave, i.e., it has a plane wavefront 25. The input wave 24 impinges on the splitting module 18, which is configured to split the input wave 24 into two plane waves 32 and 34, i.e., waves with plane wavefronts 33 and 35 respectively, with parallel directions of propagation. In other words, the beam paths of the plane waves 32 and 34 are offset from each other in a splitting direction, perpendicular to their direction of propagation. In the exemplary embodiment according to Fig. 1. The splitting occurs by separating the plane wave 34 from the input wave 24, while the plane wave 32 is formed from the remaining radiation component of the input wave 24. The in Fig. The splitting direction shown in Figure 1 is the x-direction.

[0046] In the exemplary embodiment according to Fig. The splitting module 18 comprises a beam direction splitting element in the form of a diffractive optical element 36 configured as a CGH, and a direction alignment element, also configured as a CGH diffractive optical element 38. The beam direction splitting element is configured to generate two intermediate waves 40 and 42 with different propagation directions from the input wave 24. The intermediate waves 40 and 42 have plane wavefronts 41 and 43 that are tilted relative to each other. In the exemplary embodiment, the generation of the intermediate waves 40 and 42 is carried out according to Fig. 1. The intermediate wave 42 is produced by splitting off the input wave 24 with a slightly tilted propagation direction, while the intermediate wave 40 is formed from the remaining radiation component of the input wave 24. For this purpose, the diffractive optical element 36 is configured to generate a 0th and a 1st diffraction order of the input wave 24 at the transmission point, with the 0th diffraction order functioning as the intermediate wave 40 and the 1st diffraction order as the intermediate wave 42.

[0047] The direction alignment element 38 is configured to align the propagation directions of the two intermediate waves 40 and 42, thereby generating the two plane waves 32 and 34 that are offset from each other. In the exemplary embodiment according to Fig. 1. This is achieved by slightly deflecting the intermediate wave 42 such that it, now in the form of the plane wave 32, has the same direction of propagation as the intermediate wave 40. For this purpose, the diffractive optical element 38 is configured to generate a 0th and a 1st diffraction order of the intermediate waves 40 and 42 respectively upon transmission, with the 0th diffraction order of the intermediate wave 40 functioning as the plane wave 32 and the 1st diffraction order of the intermediate wave 42 functioning as the plane wave 34. In other words, the two diffractive optical elements 36 and 38 are configured inversely to each other.

[0048] The wavefront matching module, in the form of the diffractive optical element 20 (e.g., a CGH), is configured to adapt the respective wavefronts of the plane waves 32 and 34 to a target shape of the test surface 12 by first-order diffraction, thereby generating, for example, two measurement waves 44 and 46, each with non-spherical wavefronts 45 and 47. By adapting the wavefronts 45 and 47 to the target shape, the measurement waves 44 and 46 would strike the test surface 12 perpendicularly at every point and be reflected back precisely on themselves if the test surface exactly matched the target shape. Deviations from the target shape, when the reflected measurement waves 44r and 46r (i.e., the measurement waves 44 and 46) interact with the test surface 12, result in an interference pattern that allows for a highly accurate determination of the actual surface shape.

[0049] The reflected measurement waves 44r and 46r pass through the diffractive optical element 20, which functions as a wavefront matching module, and are thereby converted back into parallel waves that are offset from each other. These waves differ from the plane waves 32 and 34 only by deviations caused by the deviation of the test surface 12 from its target shape. The reflected measurement waves 44r and 46r then propagate in the beam paths of the intermediate waves 40 and 42 and the input wave 24, but in the opposite direction to the radiation of the input wave 24. As they pass through the splitting module 18 in the reverse direction, the measurement waves 44r and 46r are offset towards each other, such that they coincide exactly.

[0050] The wave generated in the 0th diffraction order of the measurement wave 44r as it passes through the diffractive optical element 38 is referred to as measurement wave 44r in the further course of the beam, while the 1st diffraction order of the measurement wave 44r represents an unused excess wave 72. Similarly, the wave generated in the 1st diffraction order of the measurement wave 46r as it passes through the diffractive optical element 38 is referred to as measurement wave 46r in the further course of the beam, while the 0th diffraction order of the measurement wave 46r also represents an unused excess wave 72. Similarly, unused excess waves 72 are generated at the diffractive optical element 36.

[0051] Such excess waves can also arise on the outward path when the input wave 24 passes through the diffractive optical element 36 and the diffractive optical element 38. According to one embodiment, the diffractive optical elements 36 and 38 can be configured as blaze gratings, so that the undesired diffraction orders are strongly suppressed in their efficiencies.

[0052] Thus, after passing through the splitting module 18 in reverse, the reflected measurement waves 44r and 46r travel in the same beam path, i.e., they are no longer offset from each other, and exhibit wavefronts 60 and 62 that differ only by the deviations from plane wavefronts caused by the deviation of the test surface 12 from its target shape. Since the measurement waves 44 and 46r strike the test surface 12 slightly offset from each other, these deviations of the test surface 12 manifest themselves as phase deviations of the wavefronts 60 and 62.

[0053] The reflected measurement waves 44r and 46r are deflected in the beam generation and evaluation unit 16, after passing through the collimator lens 28, by the beam splitter 31 towards a camera 58. Both reflected measurement waves 44r and 46r pass through an eyepiece 54 and strike a detector 56 of the camera 58. The detector 56 includes, for example, a CCD sensor and detects several interferograms 64 generated by the superposition of the reflected measurement waves 44r and 46r. The aforementioned excess waves 72 have such a beam path that their radiation contributes nothing or only negligibly to the interferograms 64, since these excess waves no longer strike the camera 58 at the same locations as the desired waves, and therefore the condition for constructive interference is no longer met.The local excess waves and the local useful waves to the same source point arrive at different locations on the camera, or - put another way - the excess waves and the useful waves are no longer completely coherent with each other at a point on the camera 58, since they originate from different source locations.

[0054] The recorded interferograms 64 are generated by changing the relative phase between the two offset plane waves 32 and 34 or the two measuring waves 44 and 46, a process known as phase shifting. This is done in the embodiment according to Fig. 1. by changing the distance between the diffractive optical element 38, which serves as a direction-aligning element, and the diffractive optical element 36, which serves as a beam-direction splitting element. The change in distance is effected by axially displacing the diffractive optical element 38. A displacement device, illustrated by a double arrow 39, is provided for this purpose. Alternatively, the diffractive optical element 36 can also be axially displaced. According to other embodiments, the phase shift can also be achieved by varying the wavelength λ of the measurement radiation 30 or by other means. The phase shift can also be achieved by a lateral displacement of the beam-direction splitting element designed as a diffractive optical element 36 or of the direction-aligning element designed as a diffractive optical element 38.

[0055] Furthermore, the measuring device 10 is configured to vary the splitting direction of the two plane shafts 32 and 34. For this purpose, in the embodiment according to Fig. 1. The splitting module 18 is rotatably mounted axially, i.e., with respect to the direction of incidence of the input wave 24. Thus, according to one embodiment, after the acquisition of the interferograms 64 (hereinafter also referred to as the first set of interferograms) in the Fig. In the position shown in Figure 1, where the splitting direction is oriented in the x-direction, the splitting module 18 is rotated by 90°. A rotating device 65, illustrated by a double arrow, is provided for this purpose. This orients the splitting direction in the y-direction. In this arrangement, a second set of interferograms 64 is determined with appropriate phase shifting.

[0056] Both sets of recorded interferograms 64 are transmitted to an evaluation unit 66, which comprises a first evaluation module 68 and a second evaluation module 70. In the first evaluation module 68, derivatives dz / dx and dz / dy (reference symbols 52) of the form 50 of the test surface 12 are determined from the interferograms 64. The first set of interferograms 64, which were determined for splitting in the x-direction, serves to determine the derivatives dz / dx, and the second set of interferograms 64, which were determined for splitting in the y-direction, serves to determine the derivatives dz / dy.

[0057] The derivatives 52 are determined at the various coordinate points (x,y) of the test surface 12, i.e., at several locations on the test surface 12. In the second evaluation module 70, the shape 50 of the test surface 12 is determined by integrating the determined derivatives dz / dx and dz / dy. The measuring device 10 is operated in air at normal pressure. This is possible because the interfering waves, namely the measuring waves 44r and 46r, follow almost the same beam path, and therefore air turbulence or air pressure fluctuations have hardly any influence on the measurement accuracy of the measuring device 10. Thus, it is unnecessary to arrange the measuring device 10 in a vacuum, especially in a high vacuum.

[0058] The size of the beam splitting of the intermediate waves 40 and 42, and thus the spatial resolution of the shape measurement, can be varied by changing the distance between the diffractive optical elements 36 and 38.

[0059] Another advantage of the measuring device 10 according to Fig. The advantage lies in the fact that the test surface 12 also serves as a reference surface, thus eliminating the need for an additional optical element as a reference mirror. This eliminates the effort required for the manufacture and testing of such a reference mirror. Furthermore, potential errors affecting the measurement are eliminated. In other test optics with a reference or Fizeau element, the test and reference waves take significantly different light paths and therefore experience large differences in errors. Moreover, using the test surface 12 as a reference surface enables a similarly high contrast regardless of the reflectivity of the test mirror.

[0060] Another advantage of the measuring device 10 according to Fig. 1. The advantage is that the contrast is almost independent of the reflectivity of a mirror being measured, in particular almost independent of whether the mirror is coated or not.

[0061] Another advantage of the measuring device 10 according to Fig. The advantage is that, in addition to a coherent light source, an incoherent light source can also be used, with the benefit that disturbing reflections in the interferogram are strongly suppressed.

[0062] Furthermore, it should be mentioned that in the case of a wavefront matching module 20 designed as a diffractive optical element or CGH, this is a diffractive optical element with only one encoded functionality and that the test and reference waves see this module almost identically, so that this diffractive optical element normally has fewer manufacturing defects and one can do without a special additional external qualification as well as a special calibration measurement of the module to be carried out in the test setup with additional mirrors.

[0063] Fig. Figure 2 illustrates the qualification or calibration of the diffractive optical element 20, which functions as a wavefront matching module. For this purpose, a planar calibration mirror 74 is positioned in place of the test object 14. The interferograms resulting from the superposition of the plane waves 32 and 34, referred to here as calibration waves 76 and 78, which pass through the diffractive optical element 20 in the 0th diffraction order, are evaluated on the detector 56. From this, material inhomogeneities in the substrate of the diffractive optical element 20 can be determined, for example.

[0064] Fig. Figure 3 illustrates a qualification or calibration of the interferometer, including the splitting module 18. For this purpose, the plane calibration mirror 74 is positioned in place of the diffractive optical element 20. The interferograms resulting from the superposition of the plane waves 32 and 34 reflected from the calibration mirror 74 onto the detector 16 are evaluated. From this, for example, material inhomogeneities in the substrates of the diffractive optical elements 36 and 37 or other deviations in the beam path of the interferometer can be determined. Taking into account the qualification according to Fig. 3 can be achieved through qualification according to Fig. 2. The error influence of the wavefront matching module can be separated from almost all other error contributions in the interferogram.

[0065] In Fig. Figure 4 shows a further embodiment 110 of a measuring device for the interferometric determination of a property of the shape of a test surface 12 of a test object 14. The measuring device 110 differs from the measuring device 10 according to Fig. 1 only in the configuration of the splitting module 18, which is in Fig. 4, designated with reference numeral 118, and in the phase-shifting method. The beam generation and evaluation unit 16 is in Fig. 4 is shown only schematically.

[0066] The splitting module 118 also comprises diffractive optical elements configured as beam direction splitting elements or direction alignment elements, which are designated by reference numerals 136 and 138. The diffractive optical elements 136 and 138 according to Fig. The elements 4 differ from the diffractive optical elements 36 and 38 in that they are configured to operate in the first diffraction order instead of the 0th / 1st diffraction order. This means that intermediate waves 140 and 142 are generated at the diffractive optical element 136 in the -1st and +1st diffraction orders of the input wave 24 during transmission. Consequently, the propagation directions of the intermediate waves 140 and 142 are symmetrically oriented to the propagation direction of the input wave 24. Furthermore, the first diffraction order of the intermediate wave 140 then serves as plane wave 32, and the first diffraction order of the intermediate wave 142 serves as plane wave 34.

[0067] In Fig. Figure 5 shows a further embodiment 210 of a measuring device for the interferometric determination of a property of the shape of a test surface 12 of a test object 14. The measuring device 210 differs from the measuring device 110 according to Fig. 4 merely insofar as the diffractive optical elements 136 and 138 are tilted relative to the perpendicular arrangement to the beam direction of the input wave 24, advantageously by at least 1°, in particular by at least 10°. The tilting allows the phase shifting, as in the embodiment according to Fig. 1. This is achieved by moving the diffractive optical element 138, which serves as a direction adjustment element. For this purpose, a moving device, illustrated by a double arrow 39, is provided in the measuring device 210.

[0068] Another important advantage of this tilted version of the splitting module is the suppression of the reflected light for reflection at the unstructured surfaces of the beam direction splitting element and the direction alignment element in the non-tilted case, where the reflected light travels back in the direction of the input wave 24.

[0069] In Fig. Figure 6 shows a further embodiment 310 of a measuring device for the interferometric determination of a property of the shape of a test surface 12 of a test object 14. The measuring device 310 differs from the measuring device 210 according to Fig. 5 differs only in that the element serving as the beam splitting element is formed by a shear prism 336 instead of the diffractive optical element 136. The shear prism 336 is configured to split the input wave 24, which in this embodiment comprises unpolarized or diagonally polarized light, into two beams tilted relative to each other with differently polarized light, which then function as the intermediate waves 140 and 142.

[0070] Exemplary embodiments of the shear prism 336 can be designed as a Nomarski prism 336a or as a Wollaston prism 336b. As in Fig. As illustrated in Figure 6, in both the Nomarski prism 336a and the Wollaston prism 336b, the unpolarized or diagonally polarized input wave 24 is separated into two linearly polarized beams perpendicular to each other. These beams are deflected in different directions relative to the input wave 24 and serve as intermediate waves 140 and 142. The intermediate waves 140 can be oriented symmetrically to the propagation direction of the input wave 24.

[0071] The phase shift can be performed in the measuring device 310, as in the measuring device 210 according to Fig. 5, by axially shifting the diffractive optical element 138, which serves as a direction-alignment element. Furthermore, the phase shift can also be achieved by laterally shifting the shear prism 336.

[0072] Alternatively, a variable phase retardation element 339 can be used for phase shifting. This element can be arranged between the diffractive optical element 118 of the splitting module 318 and the diffractive optical element 20, which serves as a wavefront matching module 20. The variable phase retardation element 339, also called a phase retarder, can, for example, contain a liquid crystal and causes different phase retardation for wavefronts of different polarization. Thus, it causes different phase retardation for the two plane waves 32 and 34, or for the two reflected measurement waves 44r and 46r.

[0073] In Fig. Figure 7 shows a further embodiment 410 of a measuring device for the interferometric determination of a property of the shape of a test surface 12 of a test object 14. The measuring device 410 differs from the measuring device 310 according to Fig. 6 merely in that the direction alignment element is also formed by a shear prism 438. In the resulting splitting module 418, the function of the shear prism 438 corresponds to that of the shear prism 336 in reverse. That is, the shear prism 438 is arranged in the opposite orientation to the shear prism 336 of the same type. According to an embodiment not shown in the drawing, the splitting module 18 can also have a diffractive optical element as the beam direction splitting element and a shear prism as the direction alignment element.

[0074] In Fig. Figure 8 shows a further embodiment 510 of a measuring device for the interferometric determination of a property of the shape of a test surface 12 of a test object 14. The measuring device 510 differs from the measuring device 310 according to Fig. 6 merely insofar as the shear prism 336 serving as a beam direction splitting element is configured not as a Nomarski prism or as a Wollaston prism, but as a Rochon prism 336c. Unlike in the embodiment according to Fig. 6, in which the intermediate waves 140 and 142 formed by the shear prism 1 336 are oriented symmetrically to the propagation direction of the input wave 24, exhibits in the measuring device 510 according to Fig. 8 due to the use of the Rochon prism 336c, the intermediate wave 40 has the same propagation direction as the input wave 24, while the intermediate wave 42 is tilted relative to the input wave 24.

[0075] The beam guidance in the resulting splitting module 518 thus essentially corresponds to that also in embodiment 10 according to Fig. 1. The present beam path. The diffractive optical element 38 can thus be used as a direction-aligning element according to Fig. 1 or a similarly configured diffractive optical element can be used. Alternatively, a reverse-oriented Rochon prism can be used as a direction-aligning element instead of the diffractive optical element 38. According to a further embodiment, not shown in the drawing, the diffractive optical element 36 is combined with a Rochon prism serving as a direction-aligning element.

[0076] As in Fig. As illustrated in Figure 8, in the Rochon prism 336c, an unpolarized or diagonally polarized input wave 24 is converted into an ordinary beam (intermediate wave 40) that passes through the prism without deflection and an extraordinary beam (intermediate wave 42) that is deflected. The two beams are polarized at right angles to each other. The size of the beam splitting, and thus the spatial resolution of the shape measurement, can be varied by moving the two sub-elements of the Rochon prism or by changing the distance between the Rochon prism, i.e., the shear prism 336, and the diffractive optical element 38.

[0077] In Fig. Figure 9 shows a further embodiment 610 of a measuring device for the interferometric determination of a property of the shape of a test surface 12 of a test object 14. The measuring device 610 differs from the measuring device 10 according to Fig. 1 only insofar as the splitting modulus is not formed by the two diffractive optical elements 36 and 38, but by a shear plate 618.

[0078] The shear plate 618 is parallel-sided, i.e., it has the shape of a cuboid with parallel inlet and outlet surfaces 619, 620 for the measurement radiation 30. The shear plate is arranged at an angle to the input wave 24, i.e., the normal of the inlet and outlet surfaces 619, 620 is tilted relative to the propagation direction of the input wave 24, advantageously by at least 1°, and in particular by at least 10° or at least 20°. A portion of the radiation from the input wave 24 enters the shear plate 618 at surface 619 and is split at the opposite surface 620.

[0079] Part of the radiation striking surface 620 immediately exits the shear plate 618 and forms the first plane wave 32. The remaining part of the radiation is reflected at surface 620. Due to the tilt of the shear plate 618, the reflected beam is tilted relative to the beam striking surface 620. Part of the reflected radiation is reflected again at surface 619 and then exits surface 620 as a second plane wave 34. This second plane wave 34 is offset relative to the first plane wave 32.

[0080] Phase shifting can be achieved by tilting the shear plate 618. For this purpose, the measuring device 610 has a tilting device 639. Other methods of phase shifting, such as the variation of the wavelength of the measuring radiation 30 described above, can also be used here.

[0081] In Fig. Figure 10 shows a further embodiment 710 of a measuring device for the interferometric determination of a property of a shape of a test surface 12 of a test object 14. The measuring device 610 differs from the measuring devices 10, 110, 210, 310, 410, 510 or 610 according to Fig. 1, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8 or 9 merely in that the wavefront matching module is not configured as a diffractive optical element, but as lens 720, in particular as an aspherical lens.

[0082] The foregoing description of exemplary embodiments, embodiments, or variants is to be understood as illustrative. The disclosure thereby enables the person skilled in the art to understand the present invention and its associated advantages, and also encompasses, in the understanding of the person skilled in the art, obvious modifications and alterations of the described structures and methods. Therefore, all such modifications and alterations, insofar as they fall within the scope of the invention as defined in the appended claims, as well as equivalents, are to be covered by the protection of the claims. Reference symbol list 10 Measuring device 12 Test surface 14 Test object 16 Radiation generation and evaluation unit 18 Splitting module 20 diffractive optical element as wavefront matching module 22 Irradiation facility 24 Input wave 25 plane wavefront 26 Radiation source 28 Collimator lens 30 Measurement radiation 31 beam splitters 32 first wave of plans 33 plane wavefront 34 second wave of plans 35 plane wavefront 36 diffractive optical element as beam direction splitting element 38 diffractive optical element as directional alignment element 39 Shifting device 40 first intermediate wave 41 plane wavefront 42 second intermediate wave 43 plane wavefront 44 first measurement wave 44r reflected first measurement wave 45 Wavefront of the first measurement wave 46 second measuring wave 46r reflected second measurement wave 47 Wavefront of the second measurement wave 50 Shape of the test surface 52 derivations of the shape of the test surface 54 eyepiece 56 Detector 58 camera 60 Wavefront of the first reflected measurement wave 62 Wavefront of the second reflected measurement wave 64 Interferogram 65 Rotary device 66 Evaluation unit 68 first evaluation module 70 second evaluation module 72 Surplus wave 74 flat calibration mirrors 76 Calibration wave 78 Calibration wave 110 Measuring device 118 Splitting module 136 diffractive optical element as beam direction splitting element 138 diffractive optical element as directional alignment element 139 Wavelength manipulation device 210 Measuring device 310 Measuring device 318 Splitting module 336 Shear prism 336a Nomarski Prism 336b Wollaston Prism 336c Rochon prism 339 variable phase delay element 410 Measuring device 418 Splitting module 438 Shearing prism 510 Measuring device 518 Splitting module 610 Measuring device 618 shear plate 619 Entrance / exit area 620 Entry / exit area 639 Tilting device 720 lens as wavefront adaptation module QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 2016 / 188620 A2

[0003] DE 10 2011 004 376 B3

[0003]

Claims

[1] Measuring device (10; 110; 210; 310; 410; 510; 610; 710) for the interferometric determination of a property (50; 52) of a shape (50) of a test surface (12) of a test object (14), comprising: - an irradiation device (22) for generating an input wave (24), - a splitting module (18; 118; 318; 418; 518) configured to generate from the input wave two plane waves (32, 34) with parallel propagation directions, which are offset from each other transversely to the propagation directions, - a wavefront adaptation module (20; 720) for generating two measuring waves (44, 46) by adapting the respective wavefront of the mutually offset plane waves to a desired shape of the optical test surface, and - a detector (56) for detecting at least one interferogram (64) which is generated by superposition of the measuring waves (44r, 46r) after their interaction with the test surface. [2] Measuring device according to claim 1, which is configured such that the measuring waves (44r, 46r), after their interaction with the test surface, pass through the splitting module (18) in the opposite direction to the radiation of the input wave (24) before hitting the detector (56), and in the process the measuring waves are offset towards one another. [3] Measuring device according to claim 1 or 2, which comprises an evaluation device (66) which is configured to determine at least one derivative (52) of the shape of the test surface (50) using the at least one detected interferogram (64). [4] Measuring device according to one of the preceding claims, which is configured to vary a splitting direction of the two plane waves (32, 34). [5] Measuring device according to claim 3 or 4, wherein the evaluation device (66) is configured to determine derivatives (52) of the shape of the test surface at a plurality of locations on the test surface and to determine the shape (50) of the test surface by integrating the determined derivatives. [6] Measuring device according to one of the preceding claims, in which the splitting module (18; 118; 318; 418; 518) comprises a beam direction splitting element (36; 136; 336) and a direction adjustment element (38; 138; 438), wherein the beam direction splitting element is configured to generate two intermediate waves (40, 42; 140, 142) with different propagation directions from the input wave (24), and the direction adjustment element is configured to generate the two mutually offset plane waves (32, 34) by adjusting the propagation directions of the two intermediate waves. [7] Measuring device according to claim 6, wherein the beam direction splitting element and / or the direction adjusting element is configured as a diffractive optical element (36; 136, 38; 138). [8] Measuring device according to claim 7, wherein the beam splitting element (36) and / or the direction adjustment element (38) is / are each configured to diffract incoming radiation (24; 40, 42) only into the zeroth and first order of diffraction. [9] Measuring device according to claim 7, wherein the beam splitting element (136) and / or the direction adjustment element (138) are each configured to diffract incoming radiation only into the +1st and -1st diffraction orders. [10] Measuring device according to one of claims 6 to 9, wherein the beam direction splitting element (36; 136) and the direction adjusting element (38; 138) are diffractive optical elements configured inversely to one another. [11] Measuring device according to claim 6, wherein the beam direction splitting element (336) and / or the direction adjusting element (438) is configured as a shear prism. [12] Measuring device according to claim 6 or 11, wherein the direction adjustment element (438) is configured as a shear prism which is arranged in a reverse orientation with respect to a shear prism of the same type serving as the beam direction splitting element (336). [13] Measuring device according to one of claims 6 to 12, wherein the beam direction splitting element (36; 336) is configured to split the second intermediate wave (42) from the input wave (24), wherein the first intermediate wave (40) is a portion of the input wave passing through the beam direction splitting element without deflection. [14] Measuring device according to one of claims 6 to 13, wherein the propagation directions of the intermediate waves (140, 142) are oriented symmetrically to the propagation direction of the input wave (24). [15] Measuring device according to one of claims 6 to 14, which is configured to change a distance between the beam direction splitting element (36) and the direction adjusting element (38). [16] Measuring device according to one of claims 1 to 5, wherein the splitting module (618) comprises a shear plate. [17] Measuring device according to one of the preceding claims, which is configured to measure a mirror for EUV microlithography as a test object (14). [18] Method for the interferometric determination of a property (50; 52) of a shape of a test surface (12) of a test object (14), comprising the steps: - generating two plane waves (32, 34) with two parallel propagation directions, which are offset transversely to the propagation directions, by splitting an input wave, - generating two measuring waves (44, 46) from the plane waves by adapting the respective wavefront of the mutually offset plane waves to a desired shape of the optical test surface, and - generating at least one interferogram (64) by superimposing the measuring waves (44r, 46r) after their interaction with the test surface and detecting the interferogram.

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