Method for fabricating a reference element for an interferometer

Abstract

The present application relates to a method (10) of fabricating a reference element (14) of an interferometer configured to measure the surface shape of a test object, the reference element (14) including a first surface (16) that is transparent to measurement radiation of the interferometer and serves as a reference surface for the interferometric measurement of the test object. The method (10) includes 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 (70) the first measurement wave (64) generated by 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) passed by the second measurement wave, evaluating (68, 74) the first interferogram and the second interferogram, and processing (78) a first surface (16) and another surface (62) opposite the first surface (16) of the reference element (14) based on the evaluation. The invention further relates to a corresponding device (12) for processing the reference element (14).

Description

[Technical Field]

[0001] This application claims priority from German Patent Application No. 10 2022 209 651.7, filed September 14, 2022, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a method and apparatus for fabricating a reference element of an interferometer configured to measure the surface shape of a test object, the reference element including a first surface that is transparent to measurement radiation of the interferometer and serves as a reference surface for the interferometric measurement of the test object. [Background technology]

[0003] Interferometric measuring devices are known that generate a test wave whose wavefront matches the target shape of a surface, making it possible to measure test objects, such as aspheric optical surfaces of microlithography optical elements, with high precision. For example, computer-generated holograms (CGHs) are used as diffractive optical elements for this purpose. The diffractive structure required to generate the test wave is identified by computer-aided simulation of the interferometer together with the target surface of the test object, and then prepared on a substrate as a CGH. Superimposing the test wave reflected from the surface with a reference wave generates an interferogram, which allows the deviation of the surface from the target shape to be determined with high precision.

[0004] The reference wave can also be generated by a diffractive element and have a spherical or plane wavefront. For example, a complex-coded CGH can be used for this purpose, which includes a superimposed diffractive structure for simultaneously generating a test wave and a reference wave with different propagation directions. However, local structural errors in the diffractive pattern of the complex-coded CGH can lead to unwanted reflections, which affect one of the two waves generated at each location more than the other. This is particularly due to the very different diffractive structures used to generate each aspherical, spherical, or plane wavefront. The affected waves have phase disturbances, which can lead to distortions in the interference pattern and, ultimately, errors in determining the surface shape.

[0005] Therefore, it has been proposed to use a reference wave having a wavefront very similar to or identical to the test wave, and to generate this wave not by a diffractive element but by a reference element, also known as a matrix. The test wave, having a wavefront conforming to the target shape and generated by the diffractive element, first passes through an optically transparent reference element having an interface surface conforming to the target surface as a reference surface. At the reference surface, part of the test wave is reflected as a reference wave, while another part continues to the test object and is reflected by 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 surface of the detector. From the interference pattern thus generated and acquired, it is possible to very accurately determine the deviation of the surface from the target surface. Even if there is an unwanted reflection from the diffractive element, it will affect the test wave and the reference wave equally and will not lead to an error in the interference pattern.

[0006] However, a problem with such interferometric measurement devices is that the reference surface of the reference element must be highly dimensionally stable against temperature fluctuations. Even small shape deviations from the target reference surface can lead to measurement errors when determining the surface shape of the test object. Therefore, it is preferable to use a low-thermal-expansion material for the reference element. However, low-thermal-expansion materials have the disadvantage of being highly optically inhomogeneous. Their refractive index inhomogeneity can be 1,000 times greater than that of quartz. Because both the test and reference waves pass through the material, their effect is lost in the interferogram if the alignment is perfect. However, as a result of the inhomogeneity, even a small error in the alignment of the test object relative to the reference element can lead to measurement errors that make it extremely difficult or impossible to determine the surface shape of the test object down to the sub-nanometer range. Summary of the Invention [Problem to be solved by the invention]

[0007] SUMMARY OF THE INVENTION The object of the present invention is to provide a method and an apparatus which overcomes the above-mentioned problems and which improves the measurement accuracy, in particular of an interferometer having a reference element. [Means for solving the problem]

[0008] According to the present invention, the above-mentioned object is achieved, 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 includes a first surface that transmits measurement radiation of the interferometer and serves as a reference surface for interferometric measurement of the test object. The method includes a step of generating a first interferogram by superimposing a first measurement wave generated by reflection from the reference surface with a second measurement wave generated by interaction with a standard test object. Furthermore, a second interferogram is generated by superimposing the first measurement wave generated by reflection from the reference surface with a third measurement wave that passes through a beam path different from the beam path passed by the second measurement wave. Furthermore, the method includes a step of evaluating the interferogram and, based on the evaluation result, processing the first surface of the reference element and another surface of the reference element opposite the first surface.

[0009] The reference element is configured, in particular with an interferometer, to measure the surface of a microlithography optical element, for example a mirror of a microlithography projection exposure apparatus with extreme ultraviolet (EUV) radiation. The EUV radiation used has a wavelength of less than 100 nm, in particular a wavelength of about 13.5 nm or about 6.8 nm.

[0010] The reference element transmits the measurement 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 measurement radiation, for example, 80% or less or 60% or less of the intensity of the measurement radiation. That is, the reference element is an element that transmits the measurement radiation, and a portion of the intensity of the measurement radiation can be absorbed by the reference element. The portion of the intensity that is not absorbed or that is transmitted through the material of the reference element is reflected from a first surface or reference surface of the reference element as a first measurement wave. For example, the reflected portion may constitute only about 4% of the unabsorbed intensity. According to one embodiment, the measurement radiation passes through the reference element as a test wave, and a portion of the test wave is reflected from the first surface as a first measurement wave, while the non-reflected portion of the test wave is incident on a standardization test object downstream of the reference element and reflected from the standardization test object as a second measurement wave. According to yet another embodiment, the reference element has a maximum thickness or optical path length of 200 mm or more.

[0011] A standardized test object, also referred to as a master test object, is a test object whose surface corresponds with a very high accuracy to a target shape. This target shape may correspond to the target shape of a test object to be measured using a reference element after processing the reference element. In particular, the master test object is manufactured using measurement methods known from the prior art, which are often very complex and serve for high-precision measurement of surface shapes. For example, these measurement methods can include the use of a multiply-coded diffractive optical element, which can generate not only test and reference waves but also calibration waves for high-precision characterization of defects in the diffraction pattern of the diffractive optical element. In this case, measurements can be performed in a high-vacuum or low-pressure vacuum region. Such a multiply-coded diffractive optical element in the form of a quadruple-coded CGH for generating two calibration waves is described, for example, in U.S. Pat. No. 10,337,850. Similarly, a five-fold-coded CGH can be used to generate three calibration waves.

[0012] For example, the calibration element may take the form of a spherical or plane mirror and may also be referred to as an internal reference. According to one embodiment of the present invention, measurement radiation as a test wave passes through a beam splitter, which directs one part of the test wave to the reference element and another part to the calibration element. At the reference element, the part of the test wave first passes through the reference element and is subsequently reflected by a reference surface as a first measurement wave. The other part of the test wave is reflected by the reference element as a third measurement wave.

[0013] The interferogram is preferably evaluated using known computer-aided calculation methods or simulations. For example, the deviation of the reference surface from the target surface, the influence of optical inhomogeneity of the reference element on its optical properties, further optical properties of the reference element, or any combination of these features, can be determined. In particular, the reference element can be processed using equipment and methods for mechanical removal of material from optical surfaces, such as those used in the manufacture of inherently corrected aspheric surfaces (ICAs). Such removal devices use, for example, ion beams to incorporate any desired correction profile into the optical element.

[0014] In the interferometric measurement of a test object, the solution according to the invention makes it possible to avoid errors that can be caused by the optical inhomogeneity of the material of the reference element and thus by high refractive index inhomogeneity, in particular retrace errors and errors caused by displacement of the point of incidence of the test beam during the interferometric measurement as a result of the refractive index inhomogeneity, in particular caused by a slight tilt of the test object relative to the reference element during the interferometric measurement.

[0015] According to the method of the present invention, both the first surface and another surface opposite the first surface of the reference element are processed based on the evaluation results. The first surface can also be referred to as the front side or reference surface, and the other surface can also be referred to as the back side of the reference element. For example, the reference element is constructed and arranged so that measurement radiation enters the reference element from the other surface as a test wave, is partially reflected by the first surface as a first measurement wave, and the first measurement wave exits the reference element again through the other surface. Thus, the optical properties of the reference element relative to the first measurement wave, and possibly to further measurement waves from the test object, are affected by both the processing of the first surface and the processing of the other surface.

[0016] According to yet another embodiment, both the first and second measurement waves pass through the reference element, and the third measurement wave is generated by interaction with the calibration element, and the third measurement wave does not pass through the reference element.

[0017] According to yet another embodiment, the third measurement wave is also generated by interaction with a standardization object which is in a different tilt position at the time of generation of the third measurement wave than at the time of generation of the second measurement wave.

[0018] According to yet another embodiment, the first surface of the reference element is processed based on the evaluation result of the first interferogram. The first interferogram is generated by superimposing a first measurement wave reflected by the first surface or the reference surface with a second measurement wave generated by interaction with the standardized test object, e.g., by reflection at the surface of the standardized test object. Thus, the deviation of the first surface of the reference element from the accurately known surface of the standardized test object can be determined using the first interferogram. In particular, the first surface of the reference element is processed based on the evaluation result to match the target surface. For example, the wavefront of the measurement radiation at the surface of the standardized test object or the reference surface of the reference element is used as the target surface.

[0019] According to yet another embodiment, the other surface of the reference element is processed based on the evaluation result of the second interferogram. The second interferogram is generated by superimposing the first measurement wave reflected by the first or reference surface with a third measurement wave generated as an internal reference by interaction with the calibration element, e.g., by reflection at the surface of the calibration element. In this situation, unlike the third measurement wave, the first measurement wave passes through the reference element. The second interferogram can thus be used to determine optical properties of the reference element that adversely affect the first measurement wave when passing through the reference element. In particular, the other surface of the reference element is processed based on the evaluation result to compensate for optical inhomogeneities of the reference element, such as refractive index inhomogeneities.

[0020] According to one embodiment of the present invention, the reference element has a temperature range of 5°C to 35°C and a resolution of 200 x 10 -6 K -1 Low thermal expansion materials include those having an average coefficient of thermal expansion of less than or equal to the absolute value of -200 x 10 over the specified temperature range. -6 K -1 Over 200 x 10 -6 K -1 In particular, the material has a melting point of 50×10 -6 K -1It has an average coefficient of thermal expansion of the following absolute value: For example, the low thermal expansion material includes silicate glass, such as ULE® glass. ULE® glass refers to "ultra-low expansion" glass, a product of Corning, Inc., designated Corning Code 2972. Alternatively or additionally, the silicate glass may be Zerodur®, a product of Schott. Due to the low thermal expansion, high thermal stability of the reference surface of the reference element is achieved.

[0021] According to one embodiment of the present invention, the first surface of the reference element has an aspherical shape. The first surface, also referred to as a reference surface, may particularly correspond to an aspherical surface of a standardization test object or an aspherical target surface of a test object. In this context, an aspherical surface is understood to be an aspherical surface or a free-form surface. An aspherical surface is understood to be a rotationally symmetric surface that deviates from a given spherical surface by 0.05 mm or more, particularly 0.1 mm or more, 1 mm or more, or 5 mm or more. In this specification, such an aspherical surface is also referred to as a rotationally symmetric aspherical surface or simply aspherical surface. A free-form surface is understood to be a shape that deviates from a given rotationally symmetric aspherical surface by 5 μm or more, particularly 10 μm. Furthermore, a free-form surface deviates from a given spherical surface by 0.05 mm or more, particularly 0.1 mm or more, 1 mm or more, or 5 mm or more.

[0022] According to yet another embodiment of the present invention, the first and second measurement waves are generated from a test wave whose wavefront is adapted to a non-spherical target shape of the surface of the standardized test object. In particular, the wavefront of the test wave at the location of the surface of the standardized test object differs only slightly 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 from the standardized test object essentially unchanged. Preferably, the wavefront of the test wave is adapted to the target shape by a diffractive optical element. The diffractive optical element is, for example, configured as a computer-generated hologram (CGH) having a diffraction pattern appropriately formed to generate the above-mentioned wavefront.

[0023] In one embodiment of the present invention, the closing device for the third measurement wave is opened to generate the second interferogram. In particular, the open closing device allows the third measurement wave from the calibration element to pass, but closing the closing device blocks the beam path downstream of the calibration element from the third measurement wave. After the second interferogram is acquired, the closing device can be closed. Alternatively or additionally, the closing device can be used to pass or block the measurement radiation upstream of the calibration element. For example, the closing device can be configured as a shutter. In addition to opening the closing device, the standardization specimen can be removed from the beam path of the measurement radiation, or another closing device can be closed to block the second measurement wave. Thus, when the second interferogram is generated by the superposition of the first and third measurement waves, the second measurement wave is blocked.

[0024] According to one embodiment of the invention, there is a final generation and evaluation of the interferogram, the result of which is stored as a calibration deviation of the measurement of the test object. In particular, the generation and evaluation of the interferogram and the processing of the surface of the reference element based on the evaluation result are first performed iteratively. For example, the calibration deviation takes into account the deviation of the reference surface of the reference element from a target shape during the measurement of the test object, as well as the influence of optical inhomogeneities of the reference element on the measurement wave when passing through the reference element.

[0025] According to yet another embodiment, a test object is then placed in place of the standard test object, and deviations of the surface of the test object from the target surface are determined by generating and evaluating an interferogram. Such measurement of the test object can be performed immediately after processing of the reference element or storage of the calibration deviations. Preferably, the test object is placed in the interferometer in the same position and with the same alignment as the standard test object, and an interferogram is acquired, which is formed when a measurement wave generated by reflection at the reference surface is superimposed with a measurement wave generated by interaction with the test object.

[0026] Furthermore, the above-mentioned object can be achieved by, for example, an apparatus for processing a reference element of an interferometer configured to measure the surface shape of a test object, the reference element including a first surface that is transparent to measurement radiation of the interferometer and serves as a reference surface for interferometric measurement of the test object. The apparatus includes a standardized test object having a standardized surface and an acquisition module configured to acquire a first interferogram formed by superimposing a first measurement wave generated by reflection from the reference surface with a second measurement wave generated by interaction with the standardized surface. The acquisition module is further configured to acquire a second interferogram formed by superimposing the first measurement wave generated by reflection from the reference surface with a third measurement wave passing through a beam path different from the beam path passed by the second measurement wave. The apparatus further includes an evaluation device for evaluating the interferogram and a processing device for processing at least the first surface of the reference element based on the evaluation results. In particular, the standardized surface corresponds to a target shape with high precision.

[0027] According to one embodiment of the apparatus, a closure device is arranged in the beam path of the third measurement wave to block or pass the third measurement wave. For example, the closure device takes the form of a shutter, and preferably passes the third measurement wave only during acquisition of the second interferogram. A different embodiment includes a separate closure device for passing or blocking the second measurement wave. This closure device passes the second measurement wave from the standardization object, for example, only during acquisition of the first interferogram.

[0028] The features shown with respect to the above-described embodiments, exemplary embodiments, or variants of the method according to the invention can be applied, as appropriate, 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 drawings and in the claims. The individual features can be implemented separately or in combination as embodiments of the invention. Furthermore, these features can represent advantageous embodiments that are independently protectable and, if necessary, protection can be sought only during the pendency of the present application or after its decision.

[0029] The above and further advantageous features of the present invention are illustrated in the following detailed description of exemplary embodiments or embodiments according to the present invention, with reference to the accompanying schematic drawings. [Brief explanation of the drawings]

[0030] [Figure 1] 1 shows an exemplary embodiment of a method for fabricating a reference element in a schematic diagram together with an exemplary embodiment of a corresponding apparatus. [Figure 2] 10 shows a schematic diagram of yet another exemplary embodiment of a method for fabricating a reference element together with yet another exemplary embodiment of a corresponding apparatus. [Figure 3] 2 shows a diagram of beam paths of a test wave and a reference wave when measuring the surface shape of a test object using a reference element processed by the method shown in FIG. 1. [Figure 4] 3 shows a diagram of beam paths of a test wave and a reference wave when measuring the surface shape of a test object using a reference element processed by the method shown in FIG. 2. DETAILED DESCRIPTION OF THE INVENTION

[0031] In the exemplary embodiments or embodiments or variants described below, functionally or structurally similar elements are denoted by the same or similar reference numerals as far as possible. Therefore, to understand the characteristics of the individual elements of a particular exemplary embodiment, please refer to the description or overview of other exemplary embodiments of the present invention.

[0032] 1 shows a schematic diagram of a first exemplary embodiment of a method 10 and an apparatus 12 for fabricating a reference element 14. The reference element 14 is used in an interferometer for measuring a test object and includes a first surface 16, also referred to as a reference surface for this purpose. In this exemplary embodiment, the reference element 14 is configured to measure the surface of a microlithography optical element as the test object, for example, a mirror of a microlithography projection exposure apparatus with extreme ultraviolet (EUV) radiation. In this case, the EUV radiation used for exposure has a wavelength of less than 100 nm, in particular a wavelength of about 13.5 nm or about 6.8 nm.

[0033] The reference surface 16 of the reference element 14 should correspond as closely as possible to the target surface of the specimen, i.e., in this example, the aspherical target surface of a microlithography EUV mirror. For high thermal stability of the reference surface 16, the reference element 14 contains a low thermal expansion material. For example, this material has a thermal expansion coefficient of 50×10 in the temperature range of 5°C to 35°C. -6 K -1 The reference element 14 has an average coefficient of thermal expansion of the following absolute value: 0.05 .005 .005 .005 .005 . Furthermore, this material or reference element 14 is transparent to the measurement radiation used, i.e., it absorbs only about 80% of the intensity of the measurement radiation, and the reference element 14 can have a maximum thickness or optical path length of 200 mm or more. The reference element 14 can therefore also be referred to as an optical matrix. If the reference element 14 is used in an interferometer for measuring a test object, the measurement wave of the interferometer passes through the reference element 14.

[0034] In addition to the reference element 14, the device 12 contains an illumination module 18 having a radiation source 20 and a waveguide 22 that supplies measurement radiation 24 that is sufficiently coherent for interferometric measurements. For example, a laser, e.g. a helium-neon laser with a wavelength of about 633 nm, is provided as radiation source 20. However, measurement radiation 24 may also have a different wavelength in the visible or invisible wavelength range of electromagnetic radiation. In an alternative embodiment, an optical device with lens elements, mirror elements, etc. may be provided instead of the waveguide 24.

[0035] The apparatus 12 further comprises a beam splitter 26, a deflecting mirror 28, a diffractive optical element 30, and a standardized test object 32. The diffractive optical element 30 is in the form of a computer-generated hologram (CGH) and includes a diffractive structure that converts the measurement radiation 24 into a test wave 34 having a wavefront that matches the aspherical target shape of the surface 36 of the standardized test object 32. Thus, at the location of the surface 36, the wavefront of the test wave 34 differs only slightly from the target shape. The surface 36 of the standardized test object 32 corresponds with high precision to the target shape of the test object that will be measured at a later stage using the reference element 14. Thus, the surface 36 in this exemplary embodiment corresponds with high precision to the aspherical surface of the EUV mirror to be measured.

[0036] The apparatus 12 further comprises an acquisition module 38 having a scattered light diaphragm 40, a collimator 42, and a detector 44, e.g., a digital camera, for acquiring an interferogram, a calibration element 46 having a closure device 48 as an internal reference, an evaluation device 50 for evaluating the acquired interferogram, 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 for evaluating the interferogram. The processing device 52 is configured to mechanically remove material from the first surface 16 or a reference surface and from other surfaces of the reference element 14. For this purpose, the processing device 52 uses, for example, an ion beam.

[0037] Below, the function of the apparatus 12 and the individual components of the apparatus 12 are described, along with a method 10 for fabricating a reference element 14.

[0038] First, a first interferogram is generated (60) and acquired. For this purpose, the reference element 14 is placed in the apparatus 12 at the location of the standardization specimen 52. In this case, the first surface 16 or reference surface of the reference element 14 faces the surface 36 of the standardization specimen 32, while the other surface 62, the back side of the reference element opposite the first surface 36, faces the diffractive optical element. The accurate positioning of the reference element 14 in the apparatus 12 can be achieved by adjustment elements not shown in FIG. 1 .

[0039] Measurement radiation 24 emerging from the exit aperture of waveguide 22 is incident on beam splitter 26. One portion of measurement radiation 24 is deflected by beam splitter 26 towards calibration element 46 in the form of a spherical mirror and blocked by closed closure device 48. Another portion of measurement radiation 24 continues towards optional deflection mirror 28 and from there towards diffractive optical element 30. Upon passing through diffractive optical element 30, measurement radiation 24 with a spherical wavefront is converted into test wave 34 with a wavefront adapted to aspheric surface 36 of standardization test object 32.

[0040] The test wave 34 enters the reference element 14 from its back side 62. At the front side or reference surface 16 of the reference element 14, part of the test wave 34 is reflected as a first measurement wave 64, while another part of the test wave 34 continues to the standardization object 32 and reflects off its surface 36 back 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 converted back while passing through the diffractive optical element 30. Both measurement waves 64, 66 enter the acquisition module 38 via the deflecting mirror 28 and the beam splitter 26. The superposition of the first measurement wave 64 and the second measurement wave 66 produces a first interferogram at the acquisition plane of the detector 44.

[0041] The acquired first interferogram is subsequently evaluated by the evaluation device 50. The evaluation process is indicated in Fig. 1 by means of arrow 68. Since the first measurement wave 64 was reflected by the reference surface 16 of the reference element 14 and the second measurement wave 66 was reflected by the surface 36 of the standardized test object 32, it is possible to determine, in particular, the deviation of the reference element 14 with respect to the surface 36 of the standardized test object from the first interferogram. Corrections of the reference surface 14 for adaptation to the target shape are therefore determined by means of the evaluation device 50 and transmitted to the processing device 52.

[0042] According to the method 10, a second interferogram is also generated (70) and acquired. This can be done before or after the generation 60 of the first interferogram or the evaluation 68 of the first interferogram. First, the closure element 48 is released, and the standardization specimen 32 is removed from the beam path of the test wave 34. Alternatively, a further closure element can be occluded in the beam path of the test wave 34 upstream of the standardization specimen 32. A portion of the measurement radiation 24 from the illumination module 18 is then deflected by the beam splitter 26 towards the calibration element 46 and reflected from the calibration element back to the beam splitter 26 as a third measurement wave 72. The calibration element 46 can therefore also be referred to as an internal reference.

[0043] Another part of the measurement radiation 24 travels from the beam splitter 26 via the deflecting mirror 28 and the diffractive optical element 30 to the reference element 14, where it enters the reference element and is reflected by the reference surface 16 of the reference element 14 as a first measurement wave 64. Via the diffractive optical element 30 and the deflecting mirror 28, the first measurement wave 64 returns to the beam splitter 26 and then enters the acquisition module 38 together with a third measurement wave 72. A second interferogram, generated by the superposition of the first measurement wave 64 and the third measurement wave 72, is acquired by the detector 44 and transmitted to the evaluation device 50.

[0044] The evaluation device 50 subsequently evaluates a second interferogram, which is indicated in Figure 1 by arrow 74. As will be clear from the above description, the second measurement wave 66 traverses a beam path 67 that runs from the surface 36 of the standardization object 32 via the deflection mirror 28, reflects off the beam splitter 26, and reaches the detector 44. The third measurement wave 72 traverses a beam path 73 that runs from the calibration element 46 to the detector 44 and passes through the beam splitter 26. The beam path 67 of the second measurement wave 66 is therefore different from the beam path 73 of the third measurement wave 72.

[0045] Due to differences between the two beam paths 67 and 73, in particular since the first measurement wave 64, unlike the third measurement wave 72, has passed through the reference element 14 twice, optical properties of the reference element 14, such as local refractive index inhomogeneities 76 or other material inhomogeneities, can be determined in particular from the second interferogram. A correction for the back side 62 of the reference element 14 to compensate for the optical inhomogeneity 76 is thus determined by the evaluation device 50 and transmitted to the processing device 52.

[0046] Finally, processing 78 of the reference surface 14 or first surface and processing 80 of the backside 62 or another surface of the reference element 14 are performed using the processing device 52. In this case, the reference surface 14 is processed (78) based on corrections determined from the first interferogram that serve to match the target surface or surface 36 of the standardized specimen. The backside 62 is processed (80) according to corrections determined from the second interferogram that serve to compensate for material inhomogeneities 76 of the reference element 14. This post-processing of optical surfaces is also known as ICA (Inherently Corrected Aspheric).

[0047] 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 may be performed iteratively multiple times in succession according to an exemplary embodiment. Furthermore, after the final processing 78, 80 of the surfaces 16, 62, a new generation 60, 70 and evaluation 68, 74 of the interferograms may be performed to obtain a calibration deviation for a subsequent measurement of the test object using the reference element 14. For example, the calibration deviation may be taken into account when evaluating the measurement of the test object.

[0048] By performing the correction, the reference element 14 can be used in an interferometer as an optical matrix for highly accurate measurements of a test object, for example, an EUV mirror, even in the presence of material inhomogeneities 76. The apparatus 12 can also be used as such an interferometer with the corrected reference element 14. For this, the closure element 48 must be closed, and the standardization test object 32 is replaced by a test object 82, as shown in FIG. 3. As with the first interferogram, 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 accurately from the interferogram acquired at this time.

[0049] Figure 3 shows a partial view of the apparatus 12 shown in Figure 1 used for measuring the surface of a test object 82, the partial view showing only the test object 82 placed in place of the reference element 14 and the standardization test object 32. Figure 3 serves to illustrate the influence of the processing of the reference element 14 described with reference to Figure 1 on the beam path associated with measuring the surface 84 of the test object 82. The processing of the back side 62 and the reference surface 16 of the reference element 14 performed according to the measurement method described in Figure 1 is also referred to as a first embodiment of the processing of the reference element 14.

[0050] When measuring the surface 84, a portion of the irradiated test wave 34 is reflected by the reference surface 16 of the reference element 14 as a reference wave 86. A portion of the radiation of the test wave 34 that passes through the reference element 14 is reflected by the surface 84 of the test object as a test object wave 88. Beams 34k, 86k, and 88k, shown using solid lines in the area of ​​the reference element 14 and in the intermediate space between the reference element 14 and the test object 82 in Figure 3, represent each (corrected) light beam profile of the reference element 14 after processing of the reference element 14 in the first embodiment.

[0051] The intermediate space between the reference element 14 and the test object 82 is also referred to as an interference cavity 90. Interference cavity 90 should be understood to mean a region of the interferometer in which the test object wave 88 and the reference wave 86 do not travel along the same beam path.

[0052] The dotted beam 34i represents the path of the test wave 34 if the optical material of the reference element 14 were perfectly uniform, i.e., without any inhomogeneities, and therefore no correction was made for the backside 62 of the reference element 14. In this ideal case, ignoring deviations of the test object 82 from its target shape, the beam 34i of the test wave 34 is perpendicularly incident on the reference surface 16 and the surface 84 of the test object 82. The reference and test wave generated in this case would return along the beam path of the dotted beam 34i.

[0053] The dashed beams 34r, 86r, and 88r represent examples of actual beam paths without processing of the reference element 14. The illustrated case relates to the beam paths of the incident test wave 34, reference wave 86, and test wave 88 that would occur without correcting for the backside 62 of the reference element 14 when there is a refractive index inhomogeneity 76 in the optical material of the reference element 14. Each beam 34r of the incident test wave 34 bends to the right in the region of the inhomogeneity 76 and is no longer perpendicular to the reference surface 16. As a result, the beam 86r of the reference wave 86 returns at an angle relative to the beam path 34r of the incident test wave 34 rather than along it.

[0054] Furthermore, the beam 34r of the test wave 34 that passes through the reference surface 16 is also not incident perpendicularly to the surface 84 of the test object 82. Therefore, the beam 88r of the test wave 88 also returns at an angle relative to the beam path 34r of the incident test wave 34. In this case, the beams 86r and 88r are not parallel to each other but are offset from each other. This causes an error in the measurement results of the surface shape of the test object 82.

[0055] In particular, the advantageous common-path principle is violated by beam offset. Errors outside the interferometric cavity 90 (e.g., in the beam splitter 26 or the collimator 42 of the acquisition module 38) do not lead to measurement errors in an interferometer based on the common-path principle because they have equal effect on the test wave 88 and the reference wave 86. However, because the common-path principle is violated in the beam paths shown by the dashed beams, errors outside the interferometric cavity 90 lead to errors in the measurement results of the surface shape of the test object 82.

[0056] In the first embodiment of processing the reference element 14, the shape of the back side 62 of the reference element 14 is corrected so that the beam 34k of the test wave 34 follows the path indicated by the solid line within the reference element 14. This path is configured so that the beam 34k of the test wave 34 is perpendicularly incident on the reference surface 16 of the reference element 14 and the surface 84 of the test object 82, respectively. As a result, the beams 86k and 88k of the reference wave 86 and the test object wave 88, respectively, return to and overlap with the beam paths of the incident test wave, thereby avoiding the above-mentioned errors in the measurement results of the surface shape of the test object 82.

[0057] The common-path principle is thus maintained, and errors external to the interferometric cavity 90 do not lead to errors in the measurement results of the surface shape of the test object 82. The advantage of the correction performed in the first embodiment of the processing of the reference element 14 is in particular the adherence to the common-path principle, and therefore a significant reduction in the error effects due to interferometer components external to the interferometric cavity 90. This also includes particles, high-frequency surface defects of the optical unit, and inhomogeneities in the matrix material of the reference element 14. These effects are also referred to as retrace errors.

[0058] The influence of the retrace error on the measurement result increases with the distance between the reference element 14 and the object 82 to be tested, i.e., with the length of the interferometric cavity 90. In particular, it is proportional to the distance. Therefore, the correction performed by the first embodiment of the processing of the reference element 14 is particularly advantageous when the distance between the reference element 14 and the object 82 is large, for example, greater than 1 mm, in particular greater than 10 mm.

[0059] Figure 2 shows a second exemplary embodiment of a method and apparatus 10 for fabricating a reference element 14. This method differs from the method shown in Figure 1 only in the generation of a second interferogram 170 shown in the lower part of the figure, which is performed similarly to the generation of the first interferogram 60, except that the standardization specimen 32 is now tilted relative to the position taken in the generation of the first interferogram 60.

[0060] A third measurement wave 172 is generated in this tilted position 171. To generate a second interferogram, said third measurement wave is superimposed with the first measurement wave 64 generated by reflection at the reference surface 16. The back side 62 of the reference element 13 is processed based on the corrections determined by evaluation 74 of the second interferogram to compensate for non-uniformities.

[0061] Figure 4, like Figure 3, shows a partial view of the device 12 used for the surface measurement of the test object 81 according to the above view of Figure 1, in order to illustrate the effect of the processing of the reference element 14 described with reference to Figure 2. The processing of the back side 62 and the reference surface 16 of the reference element 14 carried out on the basis of the measurement method described in Figure 2 is also referred to as a second embodiment of the processing of the reference element 14.

[0062] The dotted line 34i in FIG. 4 , which is used to indicate the beam path of the test wave 34 in the ideal case, and the dashed lines, which are used to indicate the beams 34r, 86r, and 88r in the actual case, correspond to the corresponding lines in FIG. 3 . The beam profile resulting in the second exemplary embodiment after processing the backside 62 is indicated using a solid line. As a result of the corrected backside 62, the beam path of the test wave 34 in the reference element 14 is adapted so that the beam 34k of the test wave 34 is incident on the surface 84 of the test object 82 at the same point as the beam 34i of the test wave in the ideal case after passing through the reference surface 16. In other words, the beams 34k and 34i emerging from the same beam of the test wave 34 are each incident on the same point on the surface 84. However, unlike the exemplary embodiment shown in FIG. 3 , the angles of incidence are not identical, so a beam offset still occurs in this case between the test wave 88k and the reference wave 86k.

[0063] However, the second embodiment of the processing of the reference element 14 shown in FIG. 4 eliminates errors that occur when non-uniformities in the optical material of the reference element 14 prevent accurate determination of the points of incidence of the individual beams of the test wave 34 on the surface 84 of the test object 82.

[0064] This error results in two tracking errors. First, subsequent machining of the surface 84 of the test object 82 is performed at a slightly inaccurate position during the manufacturing process. This reduces the convergence of the machining, thereby increasing the manufacturing cost of the test object 82. Second, if the test object 82 is slightly tilted during measurement and this tilt should be corrected by calculation, the calculation will be inaccurate because the point of incidence is unknown. This reduces the measurement accuracy, thereby increasing the manufacturing cost of the test object 82. These error sources can be avoided by machining the reference element 14 of the second embodiment.

[0065] The greater the tilt of the test object 82 during measurement, the greater the error that is corrected by the correction shown in Fig. 4. Therefore, the correction performed by the second embodiment of the processing of the reference element 14 is particularly advantageous in the case of a large tilt of the test object 82 during measurement, for example a tilt of more than 10 μrad, in particular a tilt of more than 100 μrad.

[0066] The above description of exemplary embodiments, embodiments, or alternative embodiments should be understood as being given by way of example. The disclosure made thereby first enables those skilled in the art to understand the present invention and its associated advantages, and second encompasses changes and modifications of the described structures and methods that would be obvious to those skilled in the art. Accordingly, all such changes and modifications, and equivalents thereof, that fall within the scope of the present invention as set forth in the appended claims are intended to be protected by the scope of the claims. [Explanation of symbols]

[0067] 10. Method for fabricating reference elements 12 Reference element processing equipment 14 Reference Elements 16 Reference surface of the reference element, first surface 18 Lighting Module 20 Radiation source 22 Waveguide 24 Measuring Radiation 26 Beam Splitter 28 Deflecting mirror 30 Diffractive optical element 32 Standardized specimen 34 Test Wave 34i Ideal case of test wave beam 34k corrected beam of the test wave 34r Actual case of test wave beam 36 Surface of standardized specimen 38 Acquisition Module 40 Scattered light aperture 42 Collimator 44 detectors 46 Calibration element 48 Closure Device 50 evaluation devices 52 Processing equipment 60 Generation of the first interferogram 62 Back side of the reference element, another surface 64 First measurement wave 66 Second measurement wave 68 Evaluation of the first interferogram 70 Generation of the second interferogram 72 Third measurement wave 74 Evaluation of the second interferogram 76 Heterogeneity 78 Machining of Reference Surfaces 80 Back side processing 82 Test object 84 Surface 86 Reference wave 86i Ideal case reference wave beam 86k corrected reference beam 86r Reference wave beam in actual case 88 Object wave 88i Ideal case of test object beam Test object beam after 88k correction 88r Actual case of test object wave beam 90 Interference Cavity 170 Generation of the second interferogram 171 Tilt position 172 Third measurement wave

Claims

1. 1. A method (10) of fabricating a reference element (14) of an interferometer configured to measure a surface shape of a test object, the reference element (14) including a first surface (16) that is transparent to measurement radiation of the interferometer and serves as a reference surface for interferometric measurements of the test object, the method comprising: generating (60) a first interferogram 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 interferogram by superimposing a first measurement wave (64) generated by 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) passed by the second measurement wave; evaluating (68, 74) the first interferogram and the second interferogram, and processing (78) the first surface (16) of the reference element (14) and another surface (62) of the reference element (14) opposite the first surface (16) based on the evaluation results; A method comprising:

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

3. 10. The method of claim 1, A method in which the third measurement wave (172) is also generated by interaction with the standardization test object (32) that is in a different tilt position at the time of generating the third measurement wave than at the time of generating the second measurement wave (66).

4. The method of any one of claims 1 to 3, wherein the first surface (16) of the reference element (14) is processed (78) based on evaluation of the first interferogram.

5. The method according to any one of claims 1 to 4, The method further comprising: modifying (80) the other surface (62) of the reference element (14) based on evaluation of the second interferogram.

6. The method according to any one of claims 1 to 5, The reference element (14) has a temperature range of 5°C to 35°C of 200 x 10 －６ K －１ The method includes a low thermal expansion material having an average coefficient of thermal expansion of:

7. The method according to any one of claims 1 to 6, The method wherein the first surface (16) of the reference element (14) has an aspherical shape.

8. The method according to any one of claims 1 to 7, The method, wherein the first measurement wave (64) and the second measurement wave (66) are generated from a test wave (34) whose wavefront conforms to a non-spherical target shape of the surface (36) of the standardization test object (32).

9. The method according to any one of claims 1 to 8, A method in which a closure device (48) for said third measuring wave (72) is opened for said generating (70) of said second interferogram.

10. The method according to any one of claims 1 to 9, A method in which there is a final generation (60, 70) and evaluation (68, 74) of the interferogram, the result of which is stored as a calibration deviation of the measurement of the specimen.

11. The method according to any one of claims 1 to 10, A test specimen is then placed in place of said standardization test specimen (32) and deviations of the surface of said test specimen from the target surface are ascertained by generating and evaluating an interferogram.

12. 1. An apparatus (12) for processing a reference element (14) of an interferometer configured to measure a surface shape of a test object, the reference element (14) including a first surface (16) that is transparent to measurement radiation of the interferometer and serves as a reference surface for interferometric measurements of the test object, a standardized specimen (32) having a standardized surface (36); an acquisition module (38) configured to acquire a first interferogram 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 acquisition module (38) being further configured to acquire a second interferogram formed by superimposing the first measurement wave (64) generated by reflection at the reference surface (16) with a third measurement wave (72) passing through a beam path (73) different from the beam path (67) passed by the second measurement wave; an evaluation device (50) for evaluating (68, 74) the first interferogram and the second interferogram; a processing device (52) that processes (78) at least the first surface (16) of the reference element based on the evaluation result; A device comprising:

13. 13. The apparatus of claim 12, An apparatus in which a closure device (48) is arranged in the beam path of the third measurement wave (72) to block or pass the third measurement wave (72).

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