Method for processing reference element of interferometer

Abstract

The invention relates to a method (10) for processing a reference element (14) of an interferometer configured to measure a surface shape of a test object, in which the reference element (14) is transmissive to measurement radiation of the interferometer and comprises a first surface (16) serving as a reference surface for interferometry of the test object. The method (10) comprises the steps of: generating (60) a first interferogram by superimposing a first measurement wave (64) generated by reflection at a reference surface (16) with a second measurement wave (66) generated by interaction with a standardized test object (32); generating (70) a second interferogram by superimposing the first measurement wave (64) generated by the reflection at the reference surface (16) with a third measurement wave (72, 172) passing through a beam path (73, 173) different from the beam path (67) through which the second measurement wave passes; and evaluating (68, 74) the interferogram and processing (78) the first surface (16) and another surface opposite the first surface (16) of the reference element (14) based on the evaluation results. The invention further relates to a corresponding device (12) for processing a reference element (14).

Description

[0001] This application claims the priority of German patent application 10 2022 209 651.7, dated September 14, 2022. The entire disclosure content of this patent application is incorporated into the present application by reference. Technical Field

[0002] The invention relates to a method and a device for processing a reference element of an interferometer, the interferometer being configured to measure the surface shape of a test object, the reference element being transmissive for the measuring radiation of the interferometer and comprising a first surface serving as a reference surface for the interferometric measurement of the test object. Background Art

[0003] Interferometric measuring devices which generate test waves with a wavefront adapted to the target shape of the surface are known and are used to measure very accurately aspherical optical surfaces of test objects (e.g. microlithographic optical elements). For example, computer generated holograms (CGHs) are used as diffractive optical elements for this purpose. The diffractive structures required for generating the test waves can be determined by computer-aided simulation of the interferometer and the target surface of the test object and then prepared as CGHs on a substrate. In the case of superposition of the test wave reflected off the surface with a reference wave, an interference pattern is created and this allows the deviation of the surface from the target shape to be determined with high accuracy.

[0004] The reference wave can also be generated by a diffractive element and have a spherical or plane wavefront. Complex coded CGHs are used for this purpose, for example, and comprise superimposed diffractive structures for simultaneously generating a test wave and a reference wave with different propagation directions in each case. However, isolated structural errors in the diffraction pattern of a complex coded CGH can lead to unwanted reflections in the process, the latter affecting one of the two waves generated at the corresponding position much more than the other. This is due in particular to the very different diffractive structures used to generate the corresponding aspherical or spherical or plane wavefronts. The affected waves have phase perturbations, which can lead to artifacts in the interference pattern and thus to errors in the surface shape determination.

[0005] It is therefore proposed to use a reference wave with a wavefront that is very similar or identical to the test wave and to generate said wave not using a diffractive element but with the aid of a reference element, also referred to as a matrix. The test wave having a wavefront adapted to the target surface and generated by the diffractive element initially passes through an optically transmissive reference element having an interface that is also adapted to the target surface as a reference surface. At the reference surface, part of the test wave is reflected as a reference wave, while the other part continues to the test object and is reflected back from the surface to be measured. The reflected test wave passes through the reference element again and, together with the reference wave, is incident on the detection plane of the detector. From the interference pattern thus generated and captured, the deviation of the surface from the target surface can be determined very accurately. Potentially unwanted reflections of the diffractive element affect the test wave and the reference wave equally in this process and therefore do not lead to errors in the interference pattern.

[0006] However, a problem with such interferometric measurement devices is that the reference surface of the reference element must be dimensionally very stable with respect to temperature fluctuations. Even small deviations from the shape of the target reference surface lead to measurement errors in the determination of the surface shape of the test object. Therefore, low thermal expansion materials are preferably used for the reference element. However, low thermal expansion materials have the disadvantage of being optically very inhomogeneous. The refractive index inhomogeneities can be 1000 times greater than for quartz. Since both the test wave and the reference wave pass through the material, the influences in the interference pattern are eliminated in the case of perfect adjustment. However, as a result of the inhomogeneities, even the smallest errors in the adjustment of the test object relative to the reference element lead to measurement errors, which make it significantly more difficult or impossible to determine the surface shape of the test object down to the sub-nanometer range. Summary of the invention

[0007] Issues of concern

[0008] The problem addressed by the present invention is to provide a method and an apparatus, by which the above-mentioned problems are solved, in particular the measurement accuracy in an interferometer with a reference element is improved.

[0009] Solution according to the invention

[0010] According to the present invention, the aforementioned problem is solved, for example, by a method for processing a reference element of an interferometer configured to measure the surface shape of a test object. The reference element is transmissive to the measuring radiation of the interferometer and includes a first surface used as a reference surface for the interferometric measurement of the test object. The method includes generating a first interference pattern by superimposing a first measuring wave generated by reflection at the reference surface with a second measuring wave generated by interaction with a standardized test object. In addition, a second interference pattern is generated by superimposing the first measuring wave generated by reflection at the reference surface with a third measuring wave, wherein the third measuring wave passes through a beam path different from the beam path through which the second measuring wave passes. In addition, the method includes evaluating the interference pattern and, based on the evaluation result, processing a first surface of the reference element and another surface of the reference element opposite to the first surface.

[0011] The reference element is configured in particular for use in an interferometer for measuring the surface of a microlithographic optical element, for example for measuring a mirror of a microlithographic projection exposure apparatus using extreme ultraviolet (EUV) radiation. The EUV radiation used has a wavelength of less than 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.8 nm.

[0012] The reference element is transmissive for the measuring radiation and may also be referred to as an optical matrix. In particular, the reference element is configured to absorb only a portion of the intensity of the measuring radiation, for example up to 80% or up to 60% of the intensity of the measuring radiation. This means that the reference element is an element through which the measuring radiation passes, wherein a portion of the intensity of the measuring radiation may be absorbed by the reference element. A portion of the intensity that is not absorbed or that is transmitted through the material of the reference element is reflected from a reference surface or a first surface of the reference element as a first measuring wave. For example, the reflected portion may constitute only about 4% of the non-absorbed intensity. According to one embodiment, the measuring radiation passes through the reference element as a test wave, wherein a portion of the test wave is reflected from the first surface as a first measuring wave, and the unreflected portion of the test wave is incident on a standardized test object downstream of the reference element and is reflected from the standardized test object as a second measuring wave. According to another embodiment, the reference element has a maximum thickness or optical channel length of at least 200 mm.

[0013] The standardized test object may also be referred to as a master test object, and is a test object whose surface corresponds to a target shape with very high accuracy. The target shape may correspond to the target shape of the test object measured using a reference element after processing the reference element. In particular, the master test object is manufactured using a measurement method known from the prior art, which is usually very complex and used to measure the surface shape with high accuracy. For example, these measurement methods may include the use of a multi-encoded diffractive optical element, through which not only a test object wave and a reference wave can be generated, but also a calibration wave can be generated for highly accurate characterization of defects in the diffraction pattern of the diffractive optical element. In this case, the measurement can be performed in a high vacuum or in a vacuum region with a lower pressure. For example, such a multi-encoded diffractive optical element in the form of a quadruple-encoded CGH for creating two calibration waves is described in US 10,337,850 B2. In a similar manner thereto, a five-fold-encoded CGH can also be used to generate three calibration waves.

[0014] For example, the calibration element takes the form of a spherical or plane reflector and can also be referred to as an internal reference. According to one embodiment of the invention, the measuring radiation as a test wave passes through a beam splitter, which allows a portion of the test wave to pass to the reference element and another portion to pass to the calibration element. In the reference element, a portion of the test wave first passes through the reference element and is then reflected from the reference surface as a first measuring wave. Another portion of the test wave is reflected from the calibration element as a third measuring wave.

[0015] The interference pattern is preferably evaluated using known computer-aided calculation methods or simulations. For example, in the process the deviation of the reference surface from the target surface, the influence of optical inhomogeneities in the reference element on its optical properties, further optical properties of the reference element or any combination of these features are determined. In particular, the reference element can be processed using devices and methods for mechanically removing material from optical surfaces, for example, as used in the manufacture of intrinsically corrected aspheres (ICA). Such removal means use, for example, an ion beam in order to incorporate any desired correction profile into the optical element.

[0016] In the interferometric measurement of the test object, the solution according to the invention allows to avoid errors that can be traced back to the fact that the material of the reference element is optically inhomogeneous and therefore has a high refractive index inhomogeneity. Among these errors are retrace errors and errors caused by a displacement of the point of incidence of the beam of the test wave during the interferometric measurement due to the refractive index inhomogeneity, in particular errors caused by a slight tilt of the test object relative to the reference element during the interferometric measurement.

[0017] According to the method of the present invention, a first surface and another surface of a reference element opposite to the first surface are processed based on the evaluation result. The first surface may also be referred to as the front side or reference surface, and the other surface may be referred to as the back side of the reference element. For example, the reference element is configured and arranged so that the measurement radiation enters the reference element as a test wave through the other surface, the measurement radiation is partially reflected off the first surface as a first measurement wave, and the first measurement wave leaves the reference element again through the other surface. Therefore, the optical properties of the reference element relative to the first measurement wave and possibly also relative to another measurement wave from the test object are affected by both the processing of the first surface and the processing of the other surface.

[0018] According to one embodiment, the first and second measuring waves both pass through a reference element, and the third measuring wave is generated by interaction with a calibration element, the third measuring wave not passing through the reference element.

[0019] According to a further embodiment, the third measurement wave is also generated by interaction with a standardized test object, wherein the standardized test object is in a different tilt position when generating the third measurement wave than when generating the second measurement wave.

[0020] According to another embodiment, the first surface of the reference element is processed based on the evaluation result of the first interference pattern. The first interference pattern is created by superimposing a first measurement wave reflected off the first surface or the reference surface with a second measurement wave, which is generated by interaction with the standardized test object (e.g. by reflection off the surface of the standardized test object). Thus, the deviation of the first surface of the reference element from the precisely known surface of the standardized test object can be determined with the aid of the first interference pattern. In particular, the first surface of the reference element is processed based on the evaluation result to adapt to the target surface. For example, the wavefront of the measurement radiation at the position of the surface of the standardized test object or the reference surface of the reference element is used as the target surface.

[0021] According to another embodiment, another surface of the reference element is processed based on the evaluation result of the second interference pattern. The second interference pattern is created by superimposing the first measurement wave reflected from the first surface or the reference surface with the third measurement wave, which is generated as an internal reference by interaction with the calibration element (for example, by reflection from the surface of the calibration element). In contrast to the third measurement wave, the first measurement wave has in this case passed through the reference element. It is therefore possible to determine with the aid of the second interference pattern the optical properties of the reference element that adversely affect the first measurement wave when the first measurement wave passes through the reference element. In particular, another surface of the reference element is processed based on the evaluation result in order to compensate for optical inhomogeneities in the reference element, such as refractive index inhomogeneities.

[0022] In an embodiment according to the invention, the reference element comprises a temperature range of 5°C to 35°C having an absolute value of not more than 200×10 -6 K -1 Low thermal expansion material with an average thermal expansion coefficient of at least -200x10 -6 K -1 And at most +200x10 -6 K -1 In particular, the material has an absolute value of not more than 50×10 -6 K -1 The low thermal expansion material comprises, for example, a silicate glass, such as ULE® glass. ULE® glass stands for "Ultra Low Expansion" glass, a product of Corning Inc., denoted by Corning code 2972. As an alternative or in addition thereto, the silicate glass may consist of Zerodur® glass (a product of Schott). The high thermal stability of the reference surface of the reference element is achieved by the low thermal expansion.

[0023] According to one embodiment of the present invention, the first surface of the reference element has a non-spherical shape. The first surface is also referred to as a reference surface and may correspond in particular to a non-spherical surface of a standardized test object or a non-spherical target surface of a test object. In this article, an aspherical surface is understood to be an aspherical surface or a free-form surface. An aspherical surface should be understood as a rotationally symmetric surface that deviates from any spherical surface by at least 0.05 mm, in particular by at least 0.1 mm, at least 1 mm or at least 5 mm. In this article, such an aspherical surface is also referred to as a rotationally symmetric aspherical surface or simply as an aspherical surface. A free-form surface is understood to be a shape that deviates from any rotationally symmetric aspherical surface by at least 5 μm, in particular by at least 10 μm. In addition, the free-form surface deviates from any sphere by at least 0.05 mm, in particular by at least 0.1 mm, at least 1 mm or at least 5 mm.

[0024] According to another embodiment of the invention, the first measurement wave and the second measurement wave are generated by a test wave, the wavefront of which is adapted to the non-spherical target shape of the surface of the standardized test object. In particular, the wavefront of the test wave at the position of the surface of the standardized test object is only negligibly different from the target shape. For example, the wavefront of the test wave deviates from the target shape by less than 1 mm, or less than 100 μm. Therefore, the test wave is reflected back on itself without substantially any directional change at the standardized test object. The wavefront of the test wave is preferably adapted to the target shape by means of a diffractive optical element. The diffractive optical element is, for example, configured as a computer generated hologram (CGH) having a suitably formed diffraction pattern for generating the above-mentioned wavefront.

[0025] In one inventive embodiment of the present invention, a closing device for a third measurement wave is opened for generating a second interference pattern. In particular, the opened closing device allows the third measurement wave from the calibration element to pass, while when the closing device is closed, the beam path downstream of the calibration element is blocked for the third measurement wave. The closing element can be closed after the second interference pattern has been captured. As an alternative or in addition to this, the closing device can also be used to allow measurement radiation to pass upstream of the calibration element or to block measurement radiation. For example, the closing device is configured as a shutter. In addition to the opening of the closing device, the removal of the standardized test object from the beam path of the measurement radiation can be performed, or the closure of another closing device can be performed to block the second measurement wave. Therefore, when the second interference pattern is generated by superimposing the first and third measurement waves, interference with the second measurement wave is prevented.

[0026] According to one embodiment of the invention, there is a final generation and evaluation of the interference pattern, the result of which is stored as a calibration deviation for the measurement of the test object. In particular, the generation and evaluation of the interference pattern and the processing of the surface of the reference element based on the evaluation result are initially performed iteratively. For example, the calibration deviation takes into account the deviation of the reference surface of the reference element from the target shape during the measurement of the test object, as well as the influence of the optical inhomogeneity of the reference element on the measurement wave when the measurement wave passes through the reference element.

[0027] According to another embodiment, a test object is subsequently arranged instead of the standardized test object and the deviation of the surface of the test object from the target surface is determined by means of the generation and evaluation of an interference pattern. Such a measurement of the test object can be carried out immediately after the processing of the reference element or the storage of the calibration deviation. Preferably, the test object is arranged in the interferometer at the same position and with the same alignment as the standardized test object and an interference pattern is captured, which is formed when the measurement waves generated by reflection at the reference surface are superimposed with the measurement waves generated by the interaction with the test object.

[0028] In addition, the above-mentioned problem can be solved, for example, by a device for processing a reference element of an interferometer, the interferometer being configured to measure the surface shape of a test object, the reference element being transmissive to the measuring radiation of the interferometer and comprising a first surface serving as a reference surface for the interferometric measurement of the test object. The device comprises a standardized test object having a standardized surface and a capture module for capturing a first interference pattern, the first interference pattern being formed by superimposing a first measurement wave generated by reflection at the reference surface with a second measurement wave generated by interaction with the standardized surface. The capture module is also configured to capture a second interference pattern formed by superimposing the first measurement wave generated by reflection at the reference surface with a third measurement wave, wherein the third measurement wave passes through a beam path different from the beam path through which the second measurement wave passes. The device also comprises an evaluation device for evaluating the interference pattern and a processing device for processing at least the first surface of the reference element based on the evaluation result. In particular, the standardized surface corresponds to a target shape with high precision.

[0029] According to one embodiment of the device, a closing device is arranged in the beam path of the third measurement wave for the purpose of blocking or letting the third measurement wave through. For example, the closing device takes the form of a shutter and preferably lets the third measurement wave through only during the capture of the second interference pattern. Different embodiments include another closing device for letting the second measurement wave through or blocking the second measurement wave. The closing device lets the second measurement wave from the standardized test object pass, for example, only during the capture of the first interference pattern.

[0030] The features indicated in the above-mentioned embodiments, exemplary embodiments or embodiment variants etc. of the method according to the invention can be applied accordingly to the device for processing a reference element according to the invention, and vice versa. These and other features according to the embodiments of the invention will be explained in the description of the drawings and in the claims. The individual features can be implemented as embodiments of the invention individually or in combination. In addition, they can describe advantageous embodiments that can be protected independently and, depending on the circumstances, only claim protection for this embodiment during or after the pendency of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The foregoing and other advantageous features of the invention will be explained in the following detailed description of an embodiment according to an exemplary embodiment of the invention or with reference to the attached schematic drawings, in which:

[0032] Figure 1 An exemplary embodiment of a method for processing a reference element and an exemplary embodiment of a corresponding device are shown in the schematic diagram.

[0033] Figure 2 A further exemplary embodiment of a method for processing a reference element and a further exemplary embodiment of a corresponding device are shown in a schematic diagram.

[0034] Figure 3 Shows that when using Figure 1 a diagrammatic representation of beam paths of a test wave and a reference wave when measuring a surface shape of a test object by a method of processing a reference element, and

[0035] Figure 4 Shows that when using Figure 2 Illustration of beam paths of a test wave and a reference wave when measuring a surface shape of a test object using a reference element processed by a method. DETAILED DESCRIPTION

[0036] In the exemplary embodiments or embodiments or implementation variants described below, elements that are similar to each other in function or structure are provided with the same or similar reference numerals as much as possible. Therefore, in order to understand the features of the individual elements of a specific exemplary embodiment, reference should be made to the description of other exemplary embodiments or the general description of the present invention.

[0037] Figure 1 A first exemplary embodiment of a method 10 and an apparatus 12 for processing a reference element 14 is schematically shown. The reference element 14 is used in an interferometer for measuring a test object and comprises a first surface 16, which for this purpose is also referred to as a reference surface. In this exemplary embodiment, the reference element 14 is configured to measure a surface of a microlithography optical element as a test object, for example for measuring a mirror of a microlithography projection exposure apparatus using extreme ultraviolet (EUV) radiation. In this case, the EUV radiation used for exposure purposes has a wavelength of less than 100 nm, in particular a wavelength of about 13.5 nm or about 6.8 nm.

[0038] The reference surface 16 of the reference element 14 should correspond to the target surface of the test object to the greatest extent possible, i.e. to the non-spherical target surface of the microlithography EUV mirror in this example. For high thermal stability of the reference surface 16, the reference element 14 comprises a low thermal expansion material. For example, the material has an absolute value of not more than 50x10 -6 K -1 The material or reference element 14 is also transmissive for the utilized measuring radiation. That is, the reference element 14 absorbs only approximately 80% of the intensity of the measuring radiation, wherein the reference element 14 can have a maximum thickness or optical path length of 200 mm or more. Therefore, the reference element 14 can also be referred to as an optical matrix. When the reference element 14 is used in an interferometer for measuring a test object, the measuring waves of the interferometer pass through the reference element 14.

[0039] In addition to the reference element 14, the device 12 comprises an illumination module 18 having a radiation source 20 and a waveguide 22 for providing a measurement radiation 24 that is sufficiently coherent for interferometric measurements. For example, a laser, such as a helium-neon laser with a wavelength of approximately 633 nm, is provided as the radiation source 20. However, the measurement radiation 24 may also have a different wavelength in the visible or invisible wavelength range of electromagnetic radiation. In alternative embodiments, an optical device having lens elements, mirror elements, etc. may be provided instead of the waveguide 24.

[0040] Furthermore, the device 12 comprises a beam splitter 26, a deflection mirror 28, a diffractive optical element 30 and a standardized test object 32. The diffractive optical element 30 takes the form of a computer generated hologram (CGH) and comprises a diffractive structure which converts the measuring radiation 24 into a test wave 34 having a wavefront which is adapted to a non-spherical target shape of a surface 36 of the standardized test object 32. At the location of the surface 36, the wavefront of the test wave 34 therefore only differs insignificantly from the target shape. The surface 36 of the standardized test object 32 corresponds very accurately to the target shape of the test object to be measured at a later stage using the reference element 14. Therefore, the surface 36 in this exemplary embodiment corresponds with high accuracy to the non-spherical surface of the EUV mirror to be measured.

[0041] The device 12 further comprises a capture module 38 for capturing the interference pattern with a scattered light aperture 40, a collimator 42 and a detector 44 (e.g. a digital camera), a calibration element 46 with a closure device 48 as an internal reference, an evaluation device 50 for evaluating the captured interference pattern and a processing device 52 for the reference element 14. The evaluation device 50 comprises a computer and uses computer-aided calculation methods or simulations to evaluate the interference pattern. The processing device 52 is configured for mechanically removing material from the first surface 16 or the reference surface and other surfaces of the reference element 14. For this purpose, the processing device 52 uses, for example, an ion beam.

[0042] In the following, the apparatus 12 and the functionality of the individual components of the apparatus 12 are described together with the method 10 for processing a reference element 14 .

[0043] Initially, a first interference pattern is generated 60 and captured. For this purpose, the reference element 14 is arranged at the standardized test object 52 in the device 12. In this case, the first surface 16 or reference surface of the reference element 14 points to the surface 36 of the standardized test object 32, while the back side of the reference element opposite to the first surface 36, as another surface 62, points to the diffractive optical element. The precise positioning of the reference element 14 in the device 12 can be achieved by Figure 1 This is achieved by means of an adjustment element not shown in the figure.

[0044] The measuring radiation 24 exiting from the outlet of the waveguide 22 is incident on a beam splitter 26. A portion of the measuring radiation 24 is deflected by the beam splitter 26 in the direction of a calibration element 46 in the form of a spherical mirror and is blocked by a closed closure 48. Another portion of the measuring radiation 24 continues in the direction of an optional deflection mirror 28 and is guided from there towards a diffractive optical element 30. When passing through the diffractive optical element 30, the measuring radiation 24 with a spherical wavefront is transformed into a test wave 34 with a wavefront adapted to an aspherical surface 36 of a standardized test object 32.

[0045] The test wave 34 enters the reference element 14 through the back side 62 of the reference element 14. At the front side or reference surface 16 of the reference element 14, a portion of the test wave 34 is reflected as a first measurement wave 64, while another portion of the test wave 34 continues to the standardized test object 32 and is reflected from its surface 36 back onto itself as a second measurement wave 66. Then, both the first measurement wave 64 and the second measurement wave 66 pass through the reference element 24 and are transformed back during passage through the diffractive optical element 30. Both the measurement waves 64, 66 enter the capture module 38 via the deflection mirror 28 and the beam splitter 26. By superposition of the first measurement wave 64 and the second measurement wave 66, a first interference pattern is created in the capture plane of the detector 44.

[0046] The captured first interference pattern is then evaluated by the evaluation device 50. The evaluation process is Figure 1 6 is indicated in FIG. 6 by arrow 68. Since the first measurement wave 64 has been reflected from the reference surface 16 of the reference element 14 and the second measurement wave 66 has been reflected from the surface 36 of the standardized test object 32, the deviation of the reference surface 14 relative to the surface 36 of the standardized test object can be determined in particular from the first interference pattern. Thus, a correction for adapting the reference surface 14 to the target surface is determined by means of the evaluation device 50 and transmitted to the processing device 52.

[0047] According to the method 10, a second interference pattern is also generated 70 and captured. This can be performed before or after the generation 60 of the first interference pattern or the evaluation 68 of the first interference pattern. Initially, the closing element 48 is opened and the standardized test object 32 is removed from the beam path of the test wave 34. As an alternative to this, another closing element in the beam path of the test wave 34 can be closed upstream of the standardized test object 32. A portion of the measurement radiation 24 from the illumination module 18 is now deflected at the beam splitter 26 in the direction of the calibration element 46 and is reflected from the calibration element back to the beam splitter 26 as a third measurement wave 72. Therefore, the calibration element 46 can also be referred to as an internal reference.

[0048] Another part of the measuring radiation 24 is passed from the beam splitter 26 via the deflection mirror 28 and the diffractive optical element 30 to the reference element 14, as in the generation 60 of the first interference pattern, then enters the reference element and is reflected off the reference surface 16 of the reference element 14 as a first measuring wave 64. Via the diffractive optical element 30 and the deflection mirror 28, the first measuring wave 64 is passed back to the beam splitter 26 and now enters the capture module 38 together with the third measuring wave 72. The second interference pattern generated by the superposition of the first measuring wave 64 and the third measuring wave 72 is captured by the detector 44 and transmitted to the evaluation device 50.

[0049] The evaluation device 50 then evaluates the second interference pattern; this is Figure 1 74 in FIG. As is clear from the above explanation, the second measuring wave 66 passes through a beam path 67 which extends from the surface 36 of the standardized test object 32 via the deflection mirror 28 and in reflection via the beam splitter 26 to the detector 44. The third measuring wave 72 passes through a beam path 73 which extends from the calibration element 46 to the detector 44 and through the beam splitter 26. Therefore, the beam path 67 of the second measuring wave 66 is different from the beam path 73 of the third measuring wave 72.

[0050] Due to the difference between the two beam paths 67 and 73, in particular because the first measuring wave 64 has passed the reference element 14 twice compared to the third measuring wave 72, it is possible to determine in particular optical properties of the reference element 14 from the second interference pattern, such as local refractive index inhomogeneities 76 or other material inhomogeneities. With the aid of the evaluation device 50, a correction of the back side 62 of the reference element 14 for compensating the optical inhomogeneities 76 is thus determined and transmitted to the processing device 52.

[0051] Finally, a processing 78 of the reference surface 14 or the first surface and a processing 80 of the back side 62 or the further surface of the reference element 14 are performed by means of the processing device 52. In this case, the reference surface 14 is processed 78 based on a correction determined by means of the first interference pattern and used to adapt the target surface or surface 36 of the standardized test object. The back side 62 is processed 80 according to a correction determined by means of the second interference pattern and used to compensate for the inhomogeneities 76 in the material of the reference element 14. This post-processing of the optical surface is also referred to as ICA (intrinsically corrected asphere).

[0052] According to an exemplary embodiment, the generation 60, 70 and evaluation 68, 74 of the two interference patterns and the processing 78, 80 of the surface 16, 62 of the reference element 14 may be performed in succession for a plurality of iterations. In addition, the final processing 78, 80 of the surface 16, 62 may be followed by an updated generation 60, 70 and evaluation 68, 74 of the interference patterns in order to obtain calibration deviations for subsequent measurements of the test object using the reference element 14. For example, the calibration deviations may be taken into account when evaluating the measurements of the test object.

[0053] In the case of a correction, the reference element 14 can now be used, despite the material inhomogeneities 76, as an optical matrix for very accurate measurements of test objects in an interferometer, such as an EUV mirror. The device 12 can also be used with the corrected reference element 14 as such an interferometer. To this end, the closing element 48 must be closed and the standardized test object 32 replaced by the test object 82, as in Figure 3 In a manner similar to the first interference pattern, the deviation of the surface 84 of the test object 82 from the reference surface 16 of the reference element 14 can be determined very precisely from the interference pattern now captured.

[0054] Figure 3 The surface measurement of the test object 82 is shown based on Figure 1 1 , which shows only the reference element 14 and the test object 82 , which has been arranged to replace the standardized test object 32 . Figure 3 For reference Figure 1 The effects of the processing of the reference element 14 on the beam path associated with the measurement of the surface 84 of the test object 82 are described. Figure 1 The processing of back side 62 and reference surface 16 of reference element 14 carried out on the basis of the measuring method described in , is also referred to as a first exemplary embodiment of the processing of reference element 14 .

[0055] When measuring surface 84, a portion of the radiated test wave 34 is reflected from reference surface 16 of reference element 14 as reference wave 86. The portion of the test wave 34 radiated by reference element 14 is reflected off surface 84 of the test object as test object wave 88. Figure 3 The light beams 34k, 86k and 88k shown using solid lines in the region of the reference element 14 and in the intermediate space between the reference element 14 and the test object 82 show the corresponding (corrected) beam profile of the reference element 14 after processing the reference element 14 in the first exemplary embodiment.

[0056] The intermediate space between the reference element 14 and the test object 82 is also referred to as the interferometer cavity 90. The interferometer cavity 90 is understood to mean the region of the interferometer in which the test object wave 88 and the reference wave 86 do not travel along the same beam path.

[0057] Dashed beam 34i represents the path of test wave 34 for the case where the optical material of reference element 14 is completely homogeneous (i.e., has no inhomogeneities), and therefore no corrections are made to back side 62 of reference element 14. In this ideal case, if deviations of test object 82 from its target shape are not taken into account, beam 34i of test wave 34 is perpendicularly incident on reference surface 16 and surface 84 of test object 82. The reference wave and test object wave generated in this case will return along the beam path of dashed beam 34i.

[0058] Dashed beams 34r, 86r, and 88r represent examples of real beam paths in the absence of treatment of reference element 14. The illustrated situation relates to beam paths that occur in the presence of inhomogeneities 76 in the refractive index of the optical material of reference element 14 for incident test wave 34, reference wave 86, and test object wave 88, without correction of back side 62 of reference element 14. Beams 34r of incoming test wave 34 are each bent to the right in the region of inhomogeneities 76 and are therefore no longer perpendicularly incident on reference surface 16. As a result, beam 86r of reference wave 86 does not return along beam path 34r of incoming test wave 34, but rather at an angle thereto.

[0059] Furthermore, light beam 34r of test wave 34 passing through reference surface 16 is likewise not incident perpendicularly on surface 84 of test object 82. Therefore, light beam 88r of test object wave 88 also returns at an angle to light beam path 34r of incident test wave 34. Light beams 86r and 88r are parallel, but offset from each other in the present case. This leads to errors in the measurement result of the surface shape of test object 82.

[0060] In particular, the advantageous common path principle is destroyed by beam offsets. Errors outside interferometer cavity 90 (e.g., in beam splitter 26 or in collimator 42 of capture module 38) do not lead to measurement errors in an interferometer based on the common path principle because test object wave 88 and reference wave 86 are equally affected. However, since the common path principle is destroyed in the beam path depicted by the dashed beam, errors outside interferometer cavity 90 lead to errors in the measurement of the surface shape of test object 82.

[0061] In the above-described first embodiment of the processing of reference element 14, back side 62 of reference element 14 is corrected in its shape so that light beam 34k of test wave 34 takes the route shown by the solid line in reference element 14. The route is configured in such a way that light beam 34k of test wave 34 is each incident perpendicularly on reference surface 16 of reference element 14 and on surface 84 of test object 82. This has the result that light beams 86k and 88k of reference wave 86 and test object wave 88 each return into the beam path of the incident test wave and thus overlap, thereby avoiding the above-described errors in the measurement result of the surface shape of test object 82.

[0062] The common path principle is thus preserved and errors outside the interferometer cavity 90 do not lead to errors in the measurement of the surface shape of the test object 82. The advantage of the correction implemented in the first embodiment of the processing of the reference element 14 is in particular that the common path principle is observed and thus leads to a significant reduction of error influences caused by interferometer components outside the interferometer cavity 90. These include particles, high-frequency surface defects of the optical unit and also inhomogeneities of the matrix material of the reference element 14. These effects are also referred to as retrace errors.

[0063] The influence of the retrace error on the measurement result increases with the distance between the reference element 14 and the test object 82, i.e. the length of the interferometer cavity 90; in particular, it scales linearly with the distance. Therefore, the correction achieved by means of the first embodiment of the processing of the reference element 14 is particularly advantageous at large distances between the reference element 14 and the test object 82, for example at distances >1 mm, in particular >10 mm.

[0064] Figure 2 A second exemplary embodiment of a method and apparatus 10 for processing a reference element 14 is shown. Figure 1 The method shown differs only in the generation 170 of the second interference pattern shown in the lower part of the figure. This is carried out in a similar manner to the generation 60 of the first interference pattern, with the difference that the standardized test object 32 is tilted here relative to the position assumed in the generation 60 of the first interference pattern.

[0065] In this inclined position 171 a third measurement wave 172 is generated which is superimposed with the first measurement wave 64 generated by reflection from the reference surface 16 to generate a second interference pattern. Back side 62 of reference element 14 is processed based on the correction determined by evaluation 74 of the second interference pattern to compensate for inhomogeneities.

[0066] Figure 4 Similar to Figure 3 The method shows that according to Figure 1 The above-mentioned diagram of the device 12 for measuring the surface of the test object 81 is a partial diagram for illustrating reference Figure 2 The effect of the treatment described with reference to element 14. Figure 2 The processing of back side 62 and reference surface 16 of reference element 14 carried out on the basis of the measuring method described in , is also referred to as a second embodiment of the processing of reference element 14 .

[0067] The beam path of the test wave 34 is shown in FIG. Figure 4 The dashed line 34i in FIG. 1 and the dashed lines for showing the beams 34r, 86r and 88r in the actual situation correspond to Figure 3 . The beam profile that occurs in the second exemplary embodiment after processing back side 62 is shown using a solid line. As a result of correcting back side 62, the beam path of test wave 34 in reference element 14 is adapted so that beam 34k of test wave 34, after passing through reference surface 16, is incident on surface 84 of test object 82 at the same corresponding point as the beam of test wave 34i in the ideal case. In other words, beams 34k and 34i emanating from the same beam of test wave 34 are each incident on the same point on surface 84. However, unlike Figure 3 Compared to the exemplary embodiment shown, the angle of incidence is not the same, which is why in this case a beam offset still occurs between the test object wave 88k and the reference wave 86k.

[0068] However, Figure 4 The second embodiment of the processing of the reference element 14 depicted in illustrative embodiment eliminates the error caused by the inhomogeneities of the optical material of the reference element 14 that the corresponding points of incidence of the individual beams of the test wave 34 on the surface 84 of the test object 82 are no longer exactly known.

[0069] This error causes two subsequent errors: Firstly, the subsequent processing of the surface 84 of the test object 82 takes place at slightly incorrect positions during the manufacturing process. This reduces the convergence of the processing, which leads to a greater manufacturing outlay for the test object. Secondly, in the case that the test object 82 is slightly tilted during the measurement and this tilt should be corrected by calculation, incorrect calculations are performed due to the unknown point of incidence. This reduces the measurement accuracy, which also leads to a greater manufacturing outlay for the test object 82. By processing the reference element 14 in the second embodiment, these error sources can be avoided.

[0070] The greater the tilt of the test object 82 during the measurement, the greater the Figure 4 The correction illustrated in , corrects the larger the error. The correction achieved by the second embodiment of the processing of the reference element 14 is therefore particularly advantageous in the case of large tilts of the test object 82 during the measurement, for example tilts >10 μrad, in particular >100 μrad.

[0071] The above description of exemplary embodiments, embodiments or embodiment variations should be understood as exemplary. The disclosure thus achieved first enables those skilled in the art to understand the present invention and the advantages associated therewith, and secondly includes changes and modifications to the described structures and methods, which are also obvious in the understanding of those skilled in the art. Therefore, all such changes and modifications, as long as they fall within the scope of the present invention as defined in the appended claims, and equivalents are intended to be covered by the protection of the claims.

[0072] Reference numerals list

[0073] 10 Methods for processing reference elements

[0074] 12 Equipment for handling reference elements

[0075] 14 Reference Components

[0076] 16 Reference surface, first surface of the reference element

[0077] 18 lighting modules

[0078] 20 Radiation Sources

[0079] 22 Waveguide

[0080] 24. Measuring Radiation

[0081] 26 beam splitter

[0082] 28 Deflecting mirror

[0083] 30Diffractive optical elements

[0084] 32 Standardized test subjects

[0085] 34 test waves

[0086] 34i ideal test wave beam

[0087] Test wave beam with 34k correction

[0088] 34r Test wave beam under actual conditions

[0089] 36 Standardized test object surfaces

[0090] 38 Capture Module

[0091] 40 scattered light aperture

[0092] 42 Collimator

[0093] 44 detectors

[0094] 46 Calibration elements

[0095] 48 Closure device

[0096] 50 Evaluation Devices

[0097] 52 Processing equipment

[0098] 60 Generate the first interference pattern

[0099] 62 back side, the other surface of the reference element

[0100] 64 First Measurement Wave

[0101] 66 Second measurement wave

[0102] 68 Evaluate the first interferogram

[0103] 70 Generate the second interference pattern

[0104] 72 The third measurement wave

[0105] 74 Evaluate the second interference pattern

[0106] 76 Inhomogeneity

[0107] 78 Reference Surface Treatment

[0108] 80 Back side processing

[0109] 82 test subjects

[0110] 84 Surface

[0111] 86 reference wave

[0112] 86i Ideal reference beam

[0113] Reference wave beam in the case of 86k correction

[0114] 86r Real reference beam

[0115] 88 Test Object Wave

[0116] 88i Ideal test object wave beam

[0117] Beam of the test object wave with 88k correction

[0118] 88r beam of test object wave in real situation

[0119] 90 interferometer cavity

[0120] 170 Generate a second interference pattern

[0121] 171 tilt position

[0122] 172 The third measurement wave

Claims

1. A method (10) for processing a reference element (14) of an interferometer, the interferometer being configured to measure the surface shape of a test object, the reference element (14) being transmissive for the measuring radiation of the interferometer and comprising a first surface (16) serving as a reference surface for the interferometric measurement of the test object, the method comprising the following steps: - generating (60) a first interference pattern by superimposing a first measurement wave (64) generated by reflection at the reference surface (16) with a second measurement wave (66) generated by interaction with a standardized test object (32), - generating (70) a second interference pattern by superimposing the first measurement wave (64) generated by reflection at the reference surface (16) with a third measurement wave (72; 172), wherein the third measurement wave passes through a beam path (73; 173) different from the beam path (67) through which the second measurement wave passes, and - evaluating (68, 74) the interference pattern and, based on the evaluation result, processing (78) the first surface (16) of the reference element (14) and a further surface (62) of the reference element (14) opposite the first surface (16).

2. The method according to claim 1, in, The first measurement wave (64) and the second measurement wave (66) both pass through the reference element (14), and the third measurement wave (72) is generated by interaction with a calibration element (46), wherein the third measurement wave (72) does not pass through the reference element (14).

3. The method according to claim 1, in, The third measurement wave (172) is also generated by interaction with the standardized test object (32), wherein the standardized test object is in a different tilt position when generating the third measurement wave than when generating the second measurement wave (66).

4. The method according to any one of the preceding claims, in, The first surface (16) of the reference element (14) is processed based on an evaluation result of the first interference pattern (78).

5. The method according to any one of the preceding claims, in, The further surface (62) of the reference element (14) is processed based on the evaluation result of the second interference pattern (80).

6. The method according to any one of the preceding claims, in, The reference element (14) comprises a low thermal expansion material having an absolute value of no more than 200 x 10 -6 K -1 The average coefficient of thermal expansion.

7. The method according to any one of the preceding claims, in, The first surface (16) of the reference element (14) has a non-spherical shape.

8. The method according to any one of the preceding claims, in, The first measurement wave (64) and the second measurement wave (66) are generated by a test wave (34), the wavefront of which is adapted to a non-spherical target shape of a surface (36) of the standardized test object (32).

9. The method according to any one of the preceding claims, in, The closure device (48) for the third measuring wave (72) is opened to generate (70) the second interference pattern.

10. The method according to any one of the preceding claims, in, There is final generation (60, 70) and evaluation (68, 74) of the interference pattern, the result being stored as a calibration deviation for the measurement of the test object.

11. The method according to any one of the preceding claims, in, A test object is then arranged instead of the standardized test object (32), and deviations of the surface of the test object from a target surface are determined by generating and evaluating an interference pattern.

12. A device (12) for handling a reference element (14) of an interferometer, the interferometer being configured to measure the surface shape of a test object, the reference element (14) being transmissive for the measuring radiation of the interferometer and comprising a first surface (16) serving as a reference surface for interferometric measurements of the test object, the device comprising: - a standardized test object (32) having a standardized surface (36), - a capture module (38) for capturing a first interference pattern formed by superimposing a first measurement wave (64) generated by reflection at the reference surface (16) with a second measurement wave (66) generated by interaction with the standardization surface (36), the capture module (38) being further configured to capture a second interference pattern formed by superimposing the first measurement wave (64) generated by reflection at the reference surface (16) with a third measurement wave (72), the third measurement wave (72) passing through a beam path (73) different from the beam path (67) through which the second measurement wave passes, and - an evaluation device (50) for evaluating (68, 74) the interference pattern, and - a processing device (52) for processing (78) at least the first surface (16) of the reference element based on the evaluation result.

13. The device according to claim 12, in, The closing device (48) is arranged in the beam path of the third measuring wave (72) for the purpose of blocking or allowing the third measuring wave (72) to pass.

Images