WO2024056501A1

WO2024056501A1 - Method for processing a reference element for an interferometer

Info

Publication number: WO2024056501A1
Application number: PCT/EP2023/074540
Authority: WO
Inventors: Jochen Hetzler
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2022-09-14
Filing date: 2023-09-07
Publication date: 2024-03-21
Also published as: DE102022209651A1

Abstract

The invention relates to a method (10) for processing a reference element (14) for an interferometer configured to measure a surface shape of a test object, wherein the reference element (14) is transmissive in relation to a measurement radiation of the interferometer and comprises a first surface (16) that serves as reference surface for the interferometric measurement of the test object. The method (10) comprises the steps of creating (60) a first interferogram by superimposing a first measurement wave (64) created by reflection at the reference surface (16) on a second measurement wave (66) created by interaction with a standard test object (32); creating (70) a second interferogram by superimposing the first measurement wave (64) created by reflection at the reference surface (16) on a third measurement wave (72, 172), the third measurement wave traveling along a beam path (73; 173) that differs from a beam path (67) traveled by the second measurement wave; and evaluating (68, 74) the interferograms and processing (78) the first surface (16) of the reference element (14) and a further surface opposite the first surface (16) on the basis of the evaluation result. The invention also relates to a corresponding device (12) for processing a reference element (14).

Description

Method for processing a reference element for an interferometer

The present application claims the priority of German patent application 10 2022 209 651.7 dated September 14, 2022. The entire disclosure of this patent application is incorporated by reference into the present application.

Background of the invention

The invention relates to a method and a device for processing a reference element for an interferometer configured to measure a surface shape of a test object, wherein the reference element is transparent to a measuring radiation of the interferometer and has a first surface which serves as a reference surface for the interferometric measurement of the test object.

For a very precise measurement of an aspherical optical surface of a test specimen, for example an optical element for microlithography, interferometric measuring devices are known which generate a test wave with a wave front adapted to a target shape of the surface. For this purpose, for example, a computer-generated hologram (CGH) is used as a diffractive optical element. The diffractive structures necessary to generate the test wave can be determined through a computer-aided simulation of the interferometer together with the target surface of the test object and produced on a substrate as CGH. When the test wave reflected from the surface is superimposed on a reference wave, an interferogram is created with which deviations of the surface from the target shape can be determined with high precision.

The reference wave can also be generated by the diffractive element and have a spherical or flat wave front. For example, a complex coded CGH is used, which includes superimposed diffractive structures for the simultaneous generation of the test wave and the reference wave, each with a different direction of propagation. However, selective structural errors in the diffraction pattern of the complex-coded CGH can lead to interference reflections, which affect one of the two waves generated at the respective location much more strongly than the other. This is due in particular to the very different diffractive structures for generating the respective aspherical or spherical or flat wavefront. The affected wave has a phase disorder, which can lead to distortions in the interference pattern and thus to errors in determining the surface shape.

It was therefore proposed to use a reference wave with a wave front that is very similar or identical to the test wave and to generate this not from the diffractive element but by means of a reference element, also known as a matrix. The test wave generated by the diffractive element with a wave front adapted to the target surface first passes through the optically transparent reference element with an interface also adapted to the target surface as a reference surface. A portion of the test wave is reflected at the reference surface as a reference wave, while another portion 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, hits a detection plane of a detector. From the interference pattern generated and recorded in this way, a deviation of the surface from the target surface can be determined very precisely. Any interference reflections from the diffractive element affect the test wave and the reference wave equally and therefore do not lead to errors in the interference pattern.

However, one problem with such an interferometric measuring device is that the reference surface of the reference element is very dimensionally stable must be resistant to temperature fluctuations. Even small shape deviations from the target reference surface lead to measurement errors when determining the surface shape of the test specimen. A material with low thermal expansion is therefore preferably used for the reference element. However, materials with low thermal expansion have the disadvantage that they are optically very inhomogeneous. The refractive index inhomogeneity can be a factor of 1000 greater than with quartz. Since both the test wave and the reference wave pass through the material, the effect is canceled out if the adjustment is perfect in the interferogram. However, even the smallest errors in the adjustment of the test object relative to the reference element lead to measurement errors due to the inhomogeneity, which make it significantly more difficult or impossible to determine the surface shape of the test object down to the sub-nanometer range.

Underlying task

It is an object of the invention to provide a method and a device with which the aforementioned problems are solved and, in particular, the measurement accuracy of an interferometer with a reference element is improved.

Solution according to the invention

According to the invention, the aforementioned object can be achieved, for example, with a method for processing a reference element for an interferometer configured to measure a surface shape of a test object. The reference element is transparent to measuring radiation from the interferometer and has a first surface which serves as a reference surface for the interferometric measurement of the test object. The method includes generating a first interferogram by superimposing one by reflection The first measurement wave generated by the reference surface with a second measurement wave generated by interaction with a standard test specimen. Furthermore, a second interferogram is generated by superimposing the first measuring wave generated by reflection on the reference surface with a third measuring wave, the third measuring wave passing through a beam path which differs from a beam path passed through by the second measuring wave. The method further includes evaluating the interferograms and processing the first surface of the reference element as well as a further surface of the reference element opposite the first surface based on the evaluation result.

The reference element is configured in particular for an interferometer for measuring a surface of an optical element in microlithography, for example for measuring a mirror of a projection exposure system for microlithography with 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.

The reference element is transparent to the measuring radiation and can 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 measurement radiation, for example up to 80% or up to 60% of the intensity of the measurement radiation. This means that the reference element is an element that is transmissive for the measurement radiation, with part of the intensity of the measurement radiation being able to be absorbed by the reference element. A part of the intensity that is not absorbed or transmitted through the material of the reference element is reflected as a first measurement wave on the first surface or reference surface of the reference element. The reflected part can, for example, only be approximately 4% of the unabsorbed intensity. According to one embodiment, the measuring radiation passes through the reference element as a test wave, with a portion of the test wave being reflected on the first surface as a first measuring wave, while a portion is not reflected portion of the test wave hits the standard test specimen behind the reference element and is reflected by it as a second measurement wave. According to a further embodiment, the reference element has a maximum thickness or optical passage length of at least 200 mm.

The standard test specimen can also be referred to as a master test specimen and is a test specimen whose surface corresponds to a target shape with very high accuracy. This target shape can correspond to the target shape of test specimens which are measured with the reference element after it has been processed. The master test specimen is produced in particular using measurement methods known from the prior art, which are often very complex, for highly precise measurement of the surface shape. These measurement methods can, for example, include the use of a multiply coded diffractive optical element, with which not only a test object wave and a reference wave, but also calibration waves can be generated for the highly precise characterization of errors in the diffraction pattern of the diffractive optical element. The measurement can be carried out in a high vacuum or in a vacuum area with lower pressure. Such a multiply coded diffractive optical element in the form of a fourfold coded CGH for generating two calibration waves is described, for example, in US 10,337,850 B2. Similarly, a five-fold coded CGH can also be used, with which three calibration waves can be generated.

The calibration element is designed, for example, as a spherical or flat mirror and can also be referred to as an internal reference. According to one embodiment of the invention, the measuring radiation passes through a beam splitter as a test wave, which allows a portion of the test wave to pass to the reference element and another portion to the calibration element. With the reference element, the portion of the test wave first passes through the reference element and is then reflected by the reference surface as the first measurement wave. The other portion of the test wave is reflected by the calibration element as a third measurement wave. The interferograms are preferably evaluated using known computer-aided calculation methods or simulations. For example, a deviation of the reference surface from a 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. The processing of the reference element can be carried out in particular with devices and methods for the mechanical removal of material on optical surfaces, such as those used in the production of intrinsically corrected aspheres (ICA). Such removal devices use, for example, an ion beam to incorporate any correction profiles into an optical element.

The solution according to the invention makes it possible to avoid errors in the interferometric measurement of the test specimen that are due to the fact that the material of the reference element is optically inhomogeneous and therefore has a high refractive index inhomogeneity. These include, among other things, retrace errors and errors that are caused by the impact points of the beams of a test wave being shifted during the interferometric measurement due to the refractive index inhomogeneities, in particular caused by slight tilting of the test specimen relative to the reference element during the interferometric measurement.

According to the method according to the invention, both the first surface and a further surface of the reference element opposite the first surface are processed on the basis of the evaluation result. The first surface can also be referred to as the front or reference surface and the further surface as the back of the reference element. For example, the reference element is configured and arranged in such a way that measurement radiation enters the reference element as a test wave through the further surface, is partially reflected on the first surface as a first measurement wave, and the first measuring wave emerges from the reference element through the further surface. Thus, both machining of the first surface and machining of the further surface influence the optical properties of the reference element with respect to the first measuring wave and possibly also a further measuring wave coming from a test object.

According to one embodiment, both the first measurement wave and the second measurement wave pass through the reference element and the third measurement wave is generated by interaction with a calibration element, wherein the third measurement wave does not pass through the reference element.

According to a further embodiment, the third measurement wave is also generated by interaction with the standard test specimen, the standard test specimen having a different tilting position when the third measurement wave is generated than when the second measurement wave is generated.

According to a further embodiment, the first surface of the reference element is processed based on an evaluation result of the first interferogram. The first interferogram is created by superimposing the first measurement wave reflected from the first surface or reference surface with the second measurement wave, which is generated by interaction with the standard test object, for example by reflection on the surface of the standard test object. Thus, with the aid of the first interferogram, a deviation of the first surface of the reference element from the precisely known surface of the standard test specimen can be determined. In particular, based on the evaluation result, the first surface of the reference element is processed to adapt it to a target surface. For example, the surface of the standard test specimen or the wavefront of the measurement radiation at the location of the reference surface of the reference element is used as the target surface.

According to a further embodiment, the further surface of the reference element is based on an evaluation result of the second Interferogram processed. The second interferogram is created by superimposing the first measurement wave reflected from the first surface or reference surface with the third measurement wave, which is generated as an internal reference by interaction with the calibration element, for example by reflection on the surface of the calibration element. In contrast to the third measurement wave, the first measurement wave passed through the reference element. With the help of the second interferogram, optical properties of the reference element can be determined, which affect the first measurement wave as it passes through the reference element. In particular, based on the evaluation result, the further surface of the reference element is processed to compensate for optical inhomogeneities in the reference element, such as refractive index inhomogeneities.

In an embodiment according to the invention, the reference element has a material of low thermal expansion with an average coefficient of thermal expansion in the temperature range from 5°C to 35°C of at most 200x1 O' ⁶ K' ¹ . This means that the coefficient of thermal expansion in the temperature range mentioned is at least -200x10' ⁶ K' ¹ and at most +200x1 O' ⁶ K' ¹ . In particular, the material has an average coefficient of thermal expansion of at most 50x1 O' ⁶ K' ¹ in the temperature range mentioned. For example, the low thermal expansion material includes a silicate glass, such as ULE® glass. ULE® glass stands for “Ultra Low Expansion” glass and is a product from Corning, Inc. marked with the Corning Code 2972. Alternatively or additionally, the silicate glass can consist of Zerodur® glass, a product from Schott. With the low thermal expansion, a high thermal stability of the reference surface of the reference element is achieved.

According to an embodiment of the 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 can in particular be a non- correspond to the spherical surface of the standard test specimen or a non-spherical target surface of a test specimen. A non-spherical surface here means an aspherical surface or a free-form surface. An aspherical surface is to be understood as meaning a rotationally symmetrical surface that 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. Such an aspherical surface is also referred to in this text as a rotationally symmetrical asphere or simply as an asphere. A free-form surface is to be understood as meaning a shape with a deviation from any rotationally symmetrical asphere of at least 5 pm, in particular at least 10 pm. Furthermore, 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.

According to a further embodiment of the invention, the first measurement wave and the second measurement wave are generated from a test wave whose wave front is adapted to a non-spherical target shape of the surface of the standard test specimen. In particular, the wave front of the test wave at the location of the surface of the standard test specimen only deviates insignificantly 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 pm. This means that the test wave in the standard test specimen is essentially reflected back into itself without changing direction. The wavefront of the test wave is adapted to the desired shape preferably by means of a diffractive optical element. The diffractive optical element is configured, for example, as a computer-generated hologram (CGH) with a correspondingly designed diffraction pattern to generate the wavefront described above.

In an embodiment of the invention according to the invention, a closure device for the third measuring shaft is opened in order to generate the second interferogram. In particular, the opened closure device leaves the The third measuring wave coming from the calibration element passes through, while when the shutter device is closed, the beam path after the calibration element for the third measuring wave is blocked. After the second interferogram has been detected, the closure element can be closed. Alternatively or additionally, the closure device can also be used to pass through or block the measuring radiation in front of the calibration element. The closure device is configured, for example, as a shutter. In addition to opening the closure device, the standard test specimen can be removed from the beam path of the measuring radiation or a further closure device can be closed to block the second measurement wave. Thus, when generating the second interferogram, interference with the second measurement wave is prevented by superimposing the first and third measurement waves.

According to one embodiment of the invention, interferograms are finally generated and evaluated, with the result being saved as a calibration deviation for a measurement of a test object. In particular, the generation and evaluation of the interferograms and the processing of surfaces of the reference element are initially carried out iteratively based on the evaluation result. When measuring a test object, the calibration deviation takes into account, for example, a deviation of the reference surface of the reference element from a target shape together with the influence of optical inhomogeneities of the reference element on measuring shafts as they pass through the reference element.

According to a further embodiment, a test object is then arranged instead of the standard test object and a deviation of a surface of the test object from a target surface is determined by generating and evaluating an interferogram. Such measurement of test items can take place immediately after the reference element has been processed or the calibration deviations have been saved. Preferably, the test specimen is arranged at the same position and orientation as the standard test specimen in the interferometer and an interferogram is recorded, which occurs when superimposed on a by reflection The measuring wave generated by the reference surface is formed with a measuring wave generated by interaction with the test object.

The aforementioned task can further be solved, for example, with a device for processing a reference element for an interferometer configured to measure a surface shape of a test object, the reference element being transparent to a measuring radiation of the interferometer and having a first surface which serves as a reference surface for the interferometric Measurement of the test specimen is used. The device comprises a standard test specimen with a standard surface and a detection module for detecting a first interferogram, which is formed by superimposing a first measurement wave generated by reflection on the reference surface with a second measurement wave generated by interaction with the standard surface. The detection module is further configured to detect a second interferogram, which is formed by superimposing the first measurement wave generated by reflection on the reference surface with a third measurement wave, the third measurement wave passing through a beam path which differs from a beam path passed through by the second measuring wave. The device further contains an evaluation device for evaluating the interferograms and a processing device for processing at least the first surface of the reference element based on the evaluation result. In particular, the standard surface corresponds to a target shape with a high level of accuracy.

According to one embodiment of the device, a closure device is arranged in the beam path of the third measuring wave for blocking or allowing the third measuring wave to pass through. The closure device is designed, for example, as a shutter and preferably only allows the third measurement wave to pass while the second interferogram is being detected. Another embodiment includes a further closure device for allowing or blocking the second measuring shaft. This closure device releases the second measuring shaft For example, the standard test specimen can only pass during the acquisition of the first interferogram.

The features specified with regard to the above-listed embodiments, exemplary embodiments or embodiment variants, etc. of the method according to the invention can be transferred accordingly to the device according to the invention for processing a reference element 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 protectable and whose protection may only be claimed during or after the application is pending.

Brief description of the drawings

The above and further advantageous features of the invention are illustrated in the following detailed description of exemplary embodiments or embodiments according to the invention with reference to the attached schematic drawing. It shows:

Fig. 1 shows an exemplary embodiment of the method for processing a reference element together with an exemplary embodiment of a corresponding device in a schematic illustration.

2 shows a further exemplary embodiment of the method for processing a reference element together with a further exemplary embodiment of a corresponding device in a schematic illustration, 3 shows an illustration of beam paths of a test wave and a reference wave when measuring a surface shape of a test object using the reference element processed using the method according to FIG. 1, and

4 shows an illustration of beam paths of a test wave and a reference wave when measuring a surface shape of a test object using the reference element processed using the method according to FIG. 2.

Detailed description of exemplary embodiments of the invention

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

Fig. 1 shows schematically a first exemplary embodiment of a method 10 and a device 12 for processing a reference element 14. The reference element 14 is used in an interferometer for measuring a test object and has a first surface 16 for this purpose, which is also referred to as a reference surface. In this exemplary embodiment, the reference element 14 is configured for measuring a surface of an optical element for microlithography as a test specimen, for example for measuring a mirror of a projection exposure system for microlithography with extreme ultraviolet (EUV) radiation. The EUV radiation used for exposure has a wavelength of less than 100 nm, in particular a wavelength of approximately 13.5 nm or approximately 6.8 nm. The reference surface 16 of the reference element 14 should largely correspond to a target surface of the test object, i.e. in this example a non-spherical target surface of an EUV mirror for microlithography. For high thermal stability of the reference surface 16, the reference element 14 contains a material with low thermal expansion. For example, the material has an average thermal expansion coefficient of at most 50x1 O' ⁶ K' ¹ in the temperature range from 5°C to 35°C. Furthermore, the material or the reference element 14 is transparent to the measurement radiation used. This means that the reference element 14 absorbs approximately only 80% of the intensity of the measurement radiation, whereby the reference element 14 can have a maximum thickness or optical passage length of 200 mm or more. The reference element 14 can therefore also be referred to as an optical matrix. When using the reference element 14 in an interferometer for measuring test objects, measuring waves of the interferometer pass through the reference element 14.

In addition to the reference element 14, the device 12 contains an illumination module 18 with a radiation source 20 and a waveguide 22 for providing measuring radiation 24 that is sufficiently coherent for interferometric measurements. The radiation source 20 is, for example, a laser, for example a helium-neon laser with a wavelength of approximately 633 nm, provided. However, the measuring radiation 24 can also have a different wavelength in the visible or non-visible wavelength range of electromagnetic radiation. In alternative embodiments, an optical arrangement with lens elements, mirror elements or the like can be provided instead of the waveguide 24.

Furthermore, the device 12 contains a beam splitter 26, a deflection mirror 28, a diffractive optical element 30 and a standard test specimen 32. The diffractive optical element 30 is designed as a computer-generated hologram (CGH) and includes diffractive structures which transmit the measuring radiation 24 into a test wave 34 one to a non-spherical target shape of a surface 36 the wavefront adapted to the standard test specimen 32 is transformed. At the location of the surface 36, the wave front of the test wave 34 therefore only deviates insignificantly from the desired shape. The surface 36 of the standard test specimen 32 corresponds with high accuracy to the target shape of the test specimens to be measured later with the reference element 14. Thus, the surface 36 in this exemplary embodiment corresponds with high accuracy to the non-spherical surface of an EUV mirror to be measured.

The device 12 further contains a detection module 38 with a lens hood 40, a collimator 42 and a detector 44, e.g. a digital camera, for detecting interferograms, a calibration element 46 with a shutter device 48 as an internal reference, an evaluation device 50 for evaluating detected interferograms 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 interferograms. The processing device 52 is configured for mechanical removal of material from the first surface 16 or reference surface and other surfaces of the reference element 14. For this purpose, the processing device 52 uses, for example, an ion beam.

The functionality of the device 12 and individual components of the device 12 together with the method 10 for processing the reference element 14 will be described below.

First, a first interferogram is generated 60 and detected. For this purpose, the reference element 14 is arranged in the device 12 at the standard test specimen 52. The first surface 16 or reference surface of the reference element 14 is directed towards the surface 36 of the standard test specimen 32, while the back of the reference element opposite the first surface 36 faces the diffractive optical element as a further surface 62. The exact positioning of the reference element 14 in the device 12 can be done with the help of adjustment elements not shown in FIG. Measuring radiation 24 emerging from an exit opening of the waveguide 22 strikes the beam splitter 26. A portion of the measuring radiation 24 is deflected by the beam splitter 26 in the direction of the calibration element 46 designed as a spherical mirror and blocked by the closed closure device 48. Another portion of the measuring radiation 24 continues in the direction of the optional deflection mirror 28 and is directed from it onto the diffractive optical element 30. When passing through the diffractive optical element 30, the measuring radiation 24 with a spherical wave front is transformed into a test wave 34 with a wave front adapted to the aspherical surface 36 of the standard test specimen 32.

The test shaft 34 enters the reference element 14 through the back 62. At the front 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 standard test specimen 32 and is reflected back into itself from its surface 36 as a second measurement wave 66. Both the first measurement wave 64 and the second measurement wave 66 then pass through the reference element 24 and are transformed back as they pass through the diffractive optical element 30. Both measuring shafts 64, 66 enter the detection module 38 via the deflection mirror 28 and the beam splitter 26. In a detection plane of the detector 44, the first interferogram is created by superimposing the first measuring wave 64 with the second measuring wave 66.

The captured first interferogram is then evaluated by the evaluation device 50. The evaluation process is illustrated in FIG. 1 with an arrow 68. Since the first measurement wave 64 was reflected on the reference surface 16 of the reference element 14 and the second measurement wave 66 on the surface 36 of the standard test object 32, deviations of the reference surface 14 compared to the surface 36 of the standard test object can be determined from the first interferogram. With the help of the evaluation device 50, a correction for the reference surface 14 to adapt to the target surface is determined and transferred to the processing device 52.

According to the method 10, a second interferogram is also generated 70 and detected. This can be carried out before or after the generation 60 of the first interferogram or the evaluation 68 of the first interferogram. First, the closure element 48 is opened and the standard test specimen 32 is removed from the beam path of the test shaft 34. Alternatively, a further closure element can be closed in front of the standard test specimen 32 in the beam path of the test shaft 34. A portion of the measuring radiation 24 coming from the lighting module 18 is now deflected at the beam splitter 26 in the direction of the calibration element 46 and reflected back from it as a third measuring wave 72 to the beam splitter 26. The calibration element 46 can therefore also be referred to as an internal reference.

As in the generation 60 of the first interferogram, a further portion of the measurement radiation 24 runs from the beam splitter 26 via the deflection mirror 28 and the diffractive optical element 30 to the reference element 14, enters it and is reflected on the reference surface 16 of the reference element 14 as the first measurement wave 64 . The first measuring wave 64 runs back to the beam splitter 26 via the diffractive optical element 30 and the deflection mirror 28 and now enters the detection module 38 together with the third measuring wave 72. The second interferogram generated by superimposing the first measuring wave 64 on the third measuring wave 72 is detected by the detector 44 and transferred to the evaluation device 50.

The second interferogram is then evaluated by the evaluation device 50, illustrated in FIG as well as in reflection over the beam splitter 26 up to the detector 44 extends. The third measuring wave 72 passes through a beam path 73, which extends from the calibration element 46 in the passage over the beam splitter 26 to the detector 44. The beam path 67 of the second measuring shaft 66 therefore differs from the beam path 73 of the third measuring shaft 72.

Because of the differences between the two beam paths 67 and 73, in particular since the first measuring wave 64, in contrast to the third measuring wave 72, has passed through the reference element 14 twice, the second interferogram can be used to determine in particular optical properties of the reference element 14, such as local refractive index inhomogeneities 76 or others Material inhomogeneities can be determined. By means of the evaluation device 50, a correction for the back side 62 of the reference element 14 to compensate for the optical inhomogeneities 76 is thus determined and transferred to the processing device 52.

Finally, the reference surface 14 or first surface is processed 78 and the back side 62 or further surface of the reference element 14 is processed 80 with the aid of the processing device 52. The reference surface 14 is processed 78 on the basis of the correction determined by means of the first interferogram Adaptation to the target area or surface 36 of the standard test specimen. The processing 80 of the back side 62 is carried out according to the correction determined by means of the second interferogram to compensate for inhomogeneities 76 in the material of the reference element 14. This post-processing of optical surfaces is also known as ICA (intrinsically corrected asphere).

According to one exemplary embodiment, the generation 60, 70 and evaluation 68, 74 of the two interferograms and the processing 78, 80 of the surfaces 16, 62 of the reference element 14 can be carried out iteratively several times one after the other. After a final processing 78, 80 of the surfaces 16, 62, a new generation 60, 70 and evaluation 68, 74 of interferograms can also take place in order to create a calibration deviation for subsequent measurements of test specimens using the reference element 14 receive. The calibration deviation can, for example, be taken into account when evaluating measurements on test specimens.

With the corrections made, the reference element 14 can now be used as an optical matrix for very precise measurement of test objects, for example EUV mirrors, in an interferometer despite the material inhomogeneities 76. The device 12 can also be used as such an interferometer with the corrected reference element 14. For this purpose, the closure element 48 must be closed and the standard test specimen 32 is replaced by a test specimen 82, as illustrated in FIG. 3. Analogous to the first interferogram, a 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 interferogram now recorded.

3 shows a partial representation of the device 12 according to FIG. 3 serves to illustrate the effect of the processing of the reference element 14 described with reference to FIG. 1 on beam paths that are relevant when measuring the surface 84 of the test specimen 82. The processing of the back 62 and the reference surface 16 of the reference element 14 based on the measuring method described in FIG. 1 is also referred to as the first embodiment of the processing of the reference element 14.

When measuring the surface 84, part of the irradiated test wave 34 is reflected on the reference surface 16 of the reference element 14 as a reference wave 86. The part of the radiation of the test wave 34 that passes through the reference element 14 is reflected on the surface 84 of the test object as a test object wave 88. The rays 34k, 86k and 88k shown with solid lines in FIG. 3 in the area of the reference element 14 and a space between the reference element 14 and the test specimen 82 show the respective (corrected) optical beam path for the reference element 14 after processing the reference element 14 in the first embodiment.

The space between the reference element 14 and the test specimen 82 is also referred to as the interferometer cavity 90. The interferometer cavity 90 is to be understood as meaning that area of the interferometer in which the test object wave 88 and the reference wave 86 do not run in the same beam path.

The dotted rays 34i represent the course of the test wave 34 in the case in which the optical material of the reference element 14 is completely homogeneous, i.e. has no inhomogeneities, and accordingly no correction of the back 62 of the reference element 14 was made. In this ideal case, the rays 34i of the test shaft 34 strike the reference surface 16 and the surface 84 of the test object 82 perpendicularly, without taking into account its deviations from its target shape. The reference wave and test object wave generated in this case would travel back in the beam path of the dotted beams 34i.

The dashed rays 34r, 86r and 88r represent an example of real beam paths when the reference element 14 is not processed. In the case shown, these are the beam paths which are in the presence of inhomogeneities 76 in the refractive index of the optical material of the reference element 14, for which incoming test wave 34, the reference wave 86 and the test specimen wave 88 result without correction of the back 62 of the reference element 14. The rays 34r of the incoming test wave 34 bend to the right in the area of the inhomogeneities 76 and therefore no longer strike the reference surface 16 perpendicularly. This has the result that the beams 86r of the reference wave 86 do not travel back in the beam path 34r of the incoming test wave 34, but are tilted towards it.

Furthermore, the rays 34r of the test wave 34 passing through the reference surface 16 also do not strike the surface 84 of the test object 82 perpendicularly. This means that the beams 88r of the test specimen wave 88 also run back tilted towards the beam path 34r of the incoming test wave 34. In the present case, the beams 86r and 88r are parallel but offset from one another. This causes errors in the measurement result of the surface shape of the test object 82.

The beam offset in particular violates the advantageous common path principle. In an interferometer based on the common path principle, errors outside the interferometer cavity 90 (e.g. in the beam splitter 26 or in the collimator 42 of the detection module 38) do not lead to a measurement error, since the test object wave 88 and reference wave 86 are influenced equally. However, due to the violation of the common path principle in the beam path represented by the dashed rays, errors outside the interferometer cavity 90 lead to errors in the measurement result of the surface shape of the test specimen 82.

In the first embodiment of the processing of the reference element 14 described above, the shape of the back side 62 of the reference element 14 is corrected in such a way that the rays 34k of the test shaft 34 take the course in the reference element 14 shown with the solid lines. This course is configured in such a way that the rays 34k of the test wave 34 each strike the reference surface 16 of the reference element 14 and the surface 84 of the test object 82 perpendicularly. The result of this is that the beams 86k and 88k of the reference wave 86 and the test object wave 88 each travel back in the beam path of the incoming test wave and thus lie on top of each other, whereby the previously mentioned errors in the measurement result of the surface shape of the test object 82 are avoided.

The common path principle is thus retained and errors outside the interferometer cavity 90 do not lead to errors in the measurement result of the surface shape of the test object 82. An advantage of the correction carried out in the first embodiment of the processing of the reference element 14 lies in particular in maintaining the common path. Principle and thus leads to a strong reduction Error influences caused by interferometer components outside the interferometer cavity 90. These include particles, high-frequency surface defects of the optics and also the inhomogeneity of the matrix material of the reference element 14. These effects are also referred to as retrace errors.

The influence of the retrace errors 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 carried out 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 specimen 82, for example at distances > 1 mm, in particular > 10 mm.

2 shows a second exemplary embodiment of a method and a device 10 for processing a reference element 14. This method differs from the method shown in FIG. 1 only in the generation 170 of the second interferogram shown in the lower section of the figures. This is done analogously to the generation 60 of the first interferogram, with the difference that the standard test specimen 32 is tilted relative to the position assumed during the generation 60 of the first interferogram.

In this tilt position 171, the third measurement wave 172 is generated, which is superimposed on the first measurement wave 64 generated by reflection on the reference surface 16 to generate the second interferogram. The back side 62 of the reference element 14 is processed on the basis of a correction determined by evaluation 74 of the second interferogram to compensate for the inhomogeneities.

4 shows, analogously to FIG. 3, the partial representation of the device 12 used for surface measurement of the test specimen 81 according to the upper representation of 1 to illustrate the effect of the processing of the reference element 14 described with reference to FIG Reference element 14 referred to.

The beam path of the test shaft 34 shown in FIG. 4 with dotted lines 34i in the ideal case, as well as the beams 34r, 86r and 88r shown as dashed lines in the real case correspond to the corresponding lines in FIG The beam paths resulting from the exemplary embodiment are shown in solid lines. Due to the corrected back 62, the beam path of the test wave 34 in the reference element 14 is adapted in such a way that the rays 34k of the test wave 34, after passing through the reference surface 16, strike the same point on the surface 84 of the test object 82 as rays of the test wave 34i in the ideal case. In other words, rays 34k and 34i emanating from the same beam of the test wave 34 strike the same point on the surface 84. In contrast to the exemplary embodiment shown in FIG. 3, however, the angle of incidence is not identical, which is why a beam offset still occurs between the test object wave 88k and the reference wave 86k.

However, the second embodiment of the processing of the reference element 14 illustrated in FIG .

This causes two subsequent errors: Firstly, the subsequent processing of the surface 84 of the test specimen 82 in the manufacturing process takes place at a slightly incorrect position. This reduces the convergence of the processing, which results in greater manufacturing effort for the test specimen. On the other hand, in the event that the test specimen 82 during the measurement is slightly tilted and this tilting should be corrected mathematically, an incorrect calculation due to the unknown point of impact. This reduces the measurement accuracy, which also results in greater manufacturing effort for the test specimen 82. These sources of error can be avoided by processing the reference element 14 in the second embodiment.

The greater the tilting of the test specimen 82 during the measurement, the greater the errors corrected by means of the correction illustrated in FIG. 4. Therefore, the correction carried out in the second embodiment of the processing of the reference element 14 is particularly advantageous in the case of large tilts of the test object 82 during the measurement, for example in the case of tilts > 10 prad, in particular > 100 prad.

The above description of exemplary embodiments, embodiments or embodiment variants is to be understood as an example. The resulting disclosure enables the person skilled in the art, on the one hand, to understand the present invention and the advantages associated therewith, and, on the other hand, also includes, in the understanding of the person skilled in the art, obvious changes and modifications to the structures and methods described. Therefore, all such alterations and modifications to the extent that they come within the scope of the invention as defined in the appended claims, and equivalents, are intended to be covered by the protection of the claims.

Reference symbol list

10 methods for editing a reference element

12 Device for processing a reference element

14 reference element

16 reference surface, first surface of the reference element

18 lighting module

20 radiation source

22 waveguides

24 measuring radiation

26 beam splitters

28 deflection mirrors

30 diffractive optical element

32 standard test specimen

34 test wave

34i beams of the test wave ideally

34k rays of the test wave in the corrected case

34r beams of the test wave in the real case

36 Surface of the standard test specimen

38 acquisition module

40 lens hood

42 collimator

44 detector

46 calibration element

48 locking device

50 evaluation device

52 processing device

60 Create 1 . Interferogram

62 Back, further surface of the reference element

64 first measurement wave

66 second measurement wave

68 Evaluate 1 . Interferogram Generate 2nd interferogram third measurement wave Evaluate 2nd interferogram Inhomogeneities Processing Reference surface Processing Back of test object Surface Reference wave i Rays of the reference wave in the ideal case k Rays of the reference wave in the corrected caser Rays of the reference wave in the real case Test wave i Rays of the test sample wave in the ideal case k Rays of the test wave in the corrected case Fallr rays of the test object wave in the real case Interferometer cavity 0 Generate 2nd interferogram 1 Tilt position 2 third measuring wave

Claims

Expectations

1. Method (10) for processing a reference element (14) for an interferometer configured for measuring a surface shape of a test object, wherein the reference element (14) is transparent to a measuring radiation of the interferometer and has a first surface (16), which serves as a reference surface for the interferometric measurement of the test object is used, with the following steps:

- Generating (60) a first interferogram by superimposing a first measuring wave (64) generated by reflection on the reference surface (16) with a second measuring wave (66) generated by interaction with a standard test specimen (32),

- Generating (70) a second interferogram by superimposing the first measuring wave (64) generated by reflection on the reference surface (16) with a third measuring wave (72; 172), the third measuring wave passing through a beam path (73; 173), which differs from a beam path (67) passed through by the second measuring wave, as well as

- Evaluating (68, 74) the interferograms and 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) based on the evaluation result.

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

3. The method according to claim 1, in which the third measuring wave (172) is also generated by interaction with the standard test specimen (32), the standard test specimen in the generation of the The third measuring shaft has a different tilting position than when the second measuring shaft (66) was generated.

4. Method according to one of the preceding claims, in which the first surface (16) of the reference element (14) is processed (78) based on an evaluation result of the first interferogram.

5. Method according to one of the preceding claims, in which the further surface (62) of the reference element (14) is processed (80) based on an evaluation result of the second interferogram.

6. Method according to one of the preceding claims, in which the reference element (14) has a material of low thermal expansion with an average coefficient of thermal expansion in the temperature range from 5°C to 35°C of at most 200x1 O' ⁶ K ¹ .

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

8. Method according to one of the preceding claims, in which the first measuring wave (64) and the second measuring wave (66) are generated from a test wave (34), the wave front of which is attached to a non-spherical target shape of the surface (36) of the standard test specimen (32 ) is adapted.

9. Method according to one of the preceding claims, in which a closure device (48) for the third measuring shaft (72) is opened in order to generate (70) the second interferogram.

10. Method according to one of the preceding claims, in which a final generation (60, 70) and evaluation (68, 74) of interferograms takes place, the result being saved as a calibration deviation for a measurement of a test object.

11. Method according to one of the preceding claims, in which a test object is subsequently arranged instead of the standard test object (32) and a deviation of a surface of the test object from a target surface is determined by generating and evaluating an interferogram.

12. Device (12) for processing a reference element (14) for an interferometer configured for measuring a surface shape of a test object, wherein the reference element (14) is transparent to a measuring radiation of the interferometer and has a first surface (16), which serves as a reference surface for the interferometric measurement of the test specimen is used, comprising:

- a standard test specimen (32) with a standard surface (36),

- a detection module (38) for detecting a first interferogram, which is formed by superimposing a first measurement wave (64) generated by reflection on the reference surface (16) with a second measurement wave (66) generated by interaction with the standard surface (36), wherein the detection module (38) is further configured to detect a second interferogram, which is formed by superimposing the first measurement wave (64) generated by reflection on the reference surface (16) with a third measurement wave (72), the third measurement wave (72) being one Beam path (73) passes through, which differs from a beam path (67) passed through by the second measuring wave, and

- an evaluation device (50) for evaluating (68, 74) the interferograms, 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 which a closure device (48) is arranged in the beam path of the third measuring shaft (72) for blocking or allowing the third measuring shaft (72) to pass through.