Method for interferometrically determining surface shape of test object and measurement assembly

Abstract

The invention relates to a method and a measuring assembly for interferometrically determining a surface shape of a test object, in particular an optical element of a microlithographic projection exposure system. In the method according to the invention, a test wave generated by diffracted electromagnetic radiation on a computer-generated hologram (CGH) (110, 210, 211, 310, 410) is reflected by a test object (120, 220, 320, 420) as a spherical wave, in which a sub-region of the spherical wave is reflected back to the test object (120, 220, 320, 420) by a spherical mirror (130, 230, 330, 430) during auto-collimation and overlaps a reference wave not reflected at the test object (120, 220, 320, 420), the surface shape of the test object (120, 220, 320, 420) is determined based on a plurality of interferometrics performed continuously and different from each other with respect to the sub-regions of the spherical waves reflected by the spherical mirror (130, 230, 330, 430) during the auto-collimation process.

Description

[0001] This application claims the priority of German patent application DE 10 2022 206650.2 filed on June 30, 2022. The contents of this DE application are simultaneously incorporated into the text of the present application by reference. Technical Field

[0002] The invention relates to a method and a measuring arrangement for interferometrically determining the surface shape of a test object. The test object can be, in particular, an optical element for microlithography, in particular an optical element of a microlithography projection exposure apparatus. Background Art

[0003] Microlithography is used to produce microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus, which comprises an illumination device and a projection lens. The image of the mask (=reticle) illuminated by the illumination device is projected by the projection lens onto a substrate (e.g. a silicon wafer) which is coated with a photosensitive layer (photoresist) and is arranged in the image plane of the projection lens, in order to transfer the mask structure to the photosensitive coating of the substrate.

[0004] In projection lenses designed for the EUV range, i.e. at wavelengths of, for example, about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process due to the lack of availability of suitable light-transmitting refractive materials. A typical projection lens designed for EUV, as known, for example, from US 2016 / 0085061 A1, may have, for example, an image-side numerical aperture (NA) in the region of NA=0.55 and images an object field (e.g. in the shape of a ring segment) into the image plane or wafer plane. An increase in the image-side numerical aperture (NA) is usually accompanied by an enlargement of the required mirror surface area of ​​the mirrors used in the projection exposure apparatus. This in turn has the consequence that, in addition to manufacturing, testing the surface shape of the mirrors also presents demanding challenges.

[0005] In this case, in particular, interferometric methods using computer-generated holograms (CGHs) are used for high-precision testing of mirrors. Here, the determination of the surface shape of the respective mirror or test object is based on the interference superposition of a test wave, which is generated by the CGH and has a wavefront adapted to the target shape of the test object surface, and a reference wave.

[0006] Furthermore, it is known here to perform a calibration in order to take into account wavefront aberrations caused by the measuring assembly or the CGH. Known methods in this respect include, for example, the use of additional calibration mirrors in conjunction with the CGH to generate corresponding additional calibration waves. For this purpose, the CGH can have a complex encoding in the form of diffraction structure patterns superimposed on each other in order to generate test waves, reference waves and calibration waves.

[0007] As the size of the mirrors to be tested increases, there is a need to realize larger CGHs. In the case of conventional production of CGHs by electron lithography, in which a hard mask applied to the CGH substrate is patterned by electron lithography using a suitable electron beam-sensitive resist, the production of larger CGHs (with a diameter of several tens of centimeters) requires very long writing times, which firstly leads in practice to problems with aging effects of the resists used and an increased probability of failure of the electron beam writer used, and secondly also requires cost-intensive development or adaptation of the machine components used.

[0008] Regarding the prior art, reference is made, by way of example only, to the following publications: “Convex Aspherical Surface Testing Using Catadioptric Partial Compensating System, 2016 J. Phys.: Conf. Ser. 680 012036” by Jingxian Wang et al., and B. H.Müller, "Metrology of Optical Components and Systems, Chapter 46-Interferometry, Wiley-VCH, 1st edition, Berlin 2012" by H.Gross, DE 10 2015 202 695 A1 and US7,728,987B2. Summary of the invention

[0009] Against the above background, the object of the present invention is to provide a method and a measuring assembly for interferometrically determining the surface shape of a test object, in particular an optical element of a microlithography projection exposure device, which enables reliable testing even for relatively large mirror surfaces while at least partially avoiding the above-mentioned problems.

[0010] This object is achieved by the features of the independent patent claims.

[0011] In particular, the invention relates to a method for interferometrically determining the surface shape of a test object.

[0012] - wherein the test waves generated by diffraction of electromagnetic radiation at a computer generated hologram (CGH) are in each case reflected by the test object as spherical waves;

[0013] - wherein a sub-region of the spherical wave is reflected back to the test object by the spherical reflector during the autocollimation process and is superimposed with a reference wave that is not reflected at the test object;

[0014] - wherein the surface shape of the test object is determined based on a plurality of interferometric measurements which are performed successively and differ from one another with respect to a sub-region of a spherical wave reflected by the spherical mirror during the autocollimation process.

[0015] The invention is based in particular on the concept that during the interferometric determination of the surface shape of a test object (optionally with even larger dimensions, for example greater than 0.3 m or 0.5 m), a computer generated hologram (CGH) arranged in the beam path is used in combination with a spherical mirror arranged downstream of the test object with respect to the beam path, and in the process the CGH adapted to the target shape of the surface of the test object is configured with respect to the test wave generated by the CGH by diffraction of electromagnetic radiation in such a way that the test wave is reflected by the test object as a spherical wave. The spherical wave emanating from the test object is then reflected back from the spherical mirror in at least one sub-region in a self-collimation process and is subsequently superimposed with a reference wave (which is generated in each case in a manner known per se without reflection at the test object, for example in a Fizeau arrangement or in a reference mirror arrangement).

[0016] The above-described configuration according to the invention makes it possible to configure the CGH for generating the test wave so that it is relatively small even in the case of a test object having a relatively large size, for example greater than 0.3 m or 0.5 m, thereby avoiding the production engineering problem described in the introduction in the configuration in which the test wave is reflected by the test object as a spherical wave and then only reflected back by the spherical reflector during the autocollimation process. In addition, the invention also includes the further concept that the size of the spherical reflector required according to the invention is also kept relatively small by performing a plurality of interferometric measurements on sub-regions of the spherical wave reflected by the spherical reflector during the autocollimation process that are different from each other, in particular due to the fact that the test wave reflected by the test object as a spherical wave is gradually scanned using the spherical reflector and the surface shape of the test object is then determined using the stitching principle.

[0017] In general, according to the invention, the additional expenses associated with the use of an additional spherical mirror and the additional expenses associated with the control for scanning the test waves reflected by the test object as spherical waves are accepted in order to in turn achieve, in particular, the advantage of significantly smaller dimensions of the CGH compared to the test object.

[0018] With regard to the use of a plurality of interferometric measurements performed successively on respective different sub-regions of a spherical wave reflected by a spherical mirror in the autocollimation process, the invention is based on a further consideration: the "stitching problem" which arises fundamentally in this case can be solved by computational correction and use of the overlapping regions of the sub-regions detected in the different interferometric measurements and the solution of an optimization problem in which the corresponding positioning degrees of freedom are determined precisely by calculation such that the relative deviations arising in the overlapping regions are minimized. Furthermore, deviations of the spherical mirrors used from a perfect sphere can be compensated.

[0019] The invention advantageously exploits the fact that additional information in the form of the interferogram phase or wavefront values, respectively provided by the individual interferometric measurements, is available in the overlap region, wherein this additional information in turn allows the fundamentally unavoidable uncertainty regarding the alignment of the spherical mirror relative to the test object to be based on variables that can be determined by optimization. Depending on the overlap of the subapertures, the optimization process yields the best matching positions of the subaperture segments relative to each other, information about the deviation of the spherical mirror from a perfect sphere, and the elimination of the fundamentally unavoidable positional uncertainty of the spherical mirror.

[0020] According to one embodiment, the test object to be characterized with regard to its surface shape has an optically active surface in the form of a free-form surface without rotational symmetry.

[0021] According to one embodiment, the interferometric measurements differ from one another with respect to the rotational position and / or the thrust position of the spherical mirror.

[0022] According to one embodiment, at least two sub-regions of the spherical wave detected in different interferometry measurements have a common overlapping region.

[0023] According to one embodiment, the surface shape of the test object is determined taking into account relative deviations of the measurement data obtained for the overlapping region in the interferometric measurement.

[0024] According to one embodiment, the number of interferometric measurements that differ from one another with respect to sub-regions of the spherical wave reflected by the spherical mirror during the autocollimation process is at least three, in particular at least four.

[0025] According to one embodiment, the test object is a mirror or a lens element.

[0026] According to one embodiment, the test object is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

[0027] According to one embodiment, the test object is an optical element for microlithography, in particular an optical element of a microlithography projection exposure apparatus.

[0028] The invention furthermore relates to a method for interferometrically determining the surface shape of a test object in the form of an optical element of a microlithographic projection exposure apparatus.

[0029] - wherein the test waves generated by diffraction of electromagnetic radiation at a computer generated hologram (CGH) are in each case reflected by the test object as spherical waves;

[0030] - wherein at least a sub-region of the spherical wave is reflected by the spherical mirror back to the test object during the autocollimation process and is superimposed with a reference wave which is not reflected at the test object; and

[0031] wherein the surface shape of the test object is determined interferometrically based on the superposition.

[0032] The invention furthermore relates to a measuring arrangement for interferometrically determining the surface form of a test object, in particular an optical element of a microlithography projection exposure apparatus, comprising:

[0033] a computer generated hologram (CGH) which is arranged in the beam path and generates a test wave by diffraction of electromagnetic radiation which is adapted to a target shape of the surface of the test object in such a way that the test wave is reflected by the test object as a spherical wave;

[0034] - a spherical mirror which reflects a sub-region of the spherical wave reflected by the test object back to the test object during the autocollimation process; and

[0035] - an evaluation device for determining the surface shape of the test object based on a plurality of interferometric measurements performed in succession, in each case superimposing a sub-region of the wave reflected back to the test object during the autocollimation process with a reference wave not reflected at the test object, wherein the interferometric measurements differ from one another with respect to the sub-region of the spherical wave reflected by the spherical mirror during the autocollimation process.

[0036] According to one embodiment, the first sub-region and the second sub-region have a common overlapping area.

[0037] According to one embodiment, the measuring assembly has means for manipulating the rotational position and / or the thrust position of the spherical mirror.

[0038] According to one embodiment, the CGH has a complex encoding for generating different output waves.

[0039] According to one embodiment, the measuring assembly has at least one calibration mirror, wherein the output wave comprises at least one calibration wave for interferometric superposition with the reference wave after reflection of the calibration wave at the calibration mirror.

[0040] Further configurations of the invention can be gleaned from the description and the dependent claims.

[0041] The invention is explained in more detail below based on the exemplary embodiments shown in the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In the attached picture:

[0043] Figure 1-2 A schematic diagram for illustrating the basic principle of the present invention is shown;

[0044] Figure 3-4 shows schematic diagrams for explaining the arrangement and the function of the measuring assembly according to the invention in different embodiments;

[0045] Figure 5 shows a schematic diagram for illustrating a splicing process that may be employed in the context of the present invention; and

[0046] Figure 6 Shown is a schematic diagram of a projection exposure apparatus designed for operation in EUV. DETAILED DESCRIPTION

[0047] The embodiments of the invention described below have in common that, in each case, for interferometrically determining the surface shape of a test object, a computer-generated hologram (CGH) arranged in the beam path is used in combination with a spherical reflector arranged downstream of the test object with respect to the beam path, wherein the CGH adapted to the target shape of the surface of the test object is configured with respect to the test waves generated by the CGH by diffraction of electromagnetic radiation in such a way that the test waves are reflected by the test object as spherical waves.

[0048] To illustrate this basic principle, Figure 1 First, a schematic diagram is shown, in which a CGH generating a test wave is indicated by "110", a (particularly aspherical) test object is indicated by "120", and a spherical reflector is indicated by "130". In this case, as also described below, Figure 1 As already indicated in , it may also be sufficient for the spherical mirror 130 in its specific position to reflect only a relatively small sub-region of the spherical wave emanating from the test object 120 during the autocollimation process, wherein the remaining sub-region can then be scanned based on a travel movement along the dash-dot line.

[0049] Figure 2 A schematic diagram is shown for illustrating another possible embodiment, wherein Figure 1 Similar or functionally substantially identical components are indicated by reference numerals incremented by "100". Figure 2In addition to the first CGH 210, a second CGH 211 at a different position is used, wherein the second CGH 211 makes it possible to determine the surface shape of the test object 220 in areas that cannot be detected during testing with the first CGH 210 (because the spherical mirror 230 will block the corresponding light beam or test wave).

[0050] Figure 3 A schematic diagram is shown for explaining the arrangement and function of the measuring assembly according to the invention in a so-called Fizeau arrangement. In this case, the electromagnetic radiation generated by a light source 301 is passed via a (micro) lens 302 and a pinhole 303 to a beam splitter 304 and from there via a collimator 305 to a Fizeau element 306 which first generates the reference waves required for the interferometric measurement by reflection in a manner known per se. Figure 3 The radiation transmitted through the Fizeau element 306 is incident on the CGH 310, which generates a test wave for testing the surface shape of a downstream (particularly aspherical) test object 320. The test wave does not correspond directly to the "test object shape" mathematically with respect to its wavefront, but rather corresponds to the "test object shape" according to the above reference. Figure 1 The principle described is configured as a spherical wave and in the self-collimation process the test wave is reflected back by a spherical mirror 330 arranged downstream in the beam path.

[0051] Furthermore, in a manner not shown but known per se, the CGH 310 can generate, by means of a suitable complex coding, further output waves, in particular in the form of one or more calibration waves, which are then reflected in a manner known per se at a calibration mirror (not shown) and are likewise interferometrically superimposed with the reference wave in the measuring assembly.

[0052] The test wave reflected by the spherical mirror 330 in the autocollimation process returns from the test object 320 via the CGH 310 and the Fizeau element 306 to the beam splitter 304 and, like the reference wave, is guided by the beam splitter to the interferometer camera 360, in which case the test wave passes through the aperture 340 and the eyepiece 350. The interferometer camera 360 captures the interference pattern generated by the interference wave, from which the surface shape of the test object 320 is determined by an evaluation device (not shown).

[0053] Figure 4 Similarly, a schematic diagram is shown for explaining the arrangement and function of a measuring arrangement according to the invention in another exemplary embodiment, wherein Figure 3 Similar or functionally substantially identical components are indicated by reference numerals incremented by "100". Figure 4The individual interferometric measurements performed differ with respect to the corresponding detected sub-areas of the spherical waves reflected by the test object 420, wherein a “switch” from one sub-area to another is achieved by changing the relative position of the spherical mirror 430 with respect to the test object 420, preferably by manipulating the rotational position and / or push position of the spherical mirror 430.

[0054] With the help of a suitable rotation device, the spherical mirror is moved on a circular or spiral path around the center of the spherical wave. The subapertures picked up on the way must overlap so that the position error of the spherical mirror and its deviation from a perfect sphere can be calculated and compensated by an optimization process. To clarify, Figure 5 Only schematically indicated subapertures 501-504 are shown, as is a spherical wave 500 reflected by the test object. For the stitching principle, refer to B. "Metrology of Optical Components and Systems, Chapter 46-Interferometry, Wiley-VCH, 1st edition, Berlin 2012" by H.Müller and H.Gross.

[0055] In this case, the stitching method also includes a "self-calibration" of the spherical mirrors used. Constant deviations can be distinguished from variable deviations due to the fact that the spherical mirrors are used in multiple positions, but their footprint remains the same.

[0056] In further embodiments, another CGH can also be used in a conventional manner to perform interferometric determination of the surface shape of a sub-region of the test object, so that for the other CGH or the relevant sub-region, the relevant test waves generated by the other CGH are directly reflected back by the test object during the self-collimation process.

[0057] In a further embodiment, the measuring assembly according to the invention can also have a so-called reference mirror arrangement instead of a reference mirror from Figure 3-4 The Fizeau device, in this case, generates the reference wave required for the interferometric measurement in a manner known per se, with a reference mirror arranged downstream of the respective CGH in the beam path.

[0058] Figure 6 A schematic diagram showing an exemplary projection exposure apparatus which is designed for operation in EUV and comprises a mirror which can be tested by the method according to the invention. Figure 6, the illumination device in the projection exposure apparatus 610 designed for EUV comprises a field facet mirror 603 and a pupil facet mirror 604. The light from the light source unit comprising a plasma light source 601 and a collector mirror 602 is directed at the field facet mirror 603. A first telescope mirror 605 and a second telescope mirror 606 are arranged downstream of the pupil facet mirror 604 in the optical path. A deflection mirror 607 is arranged downstream in the optical path, which directs the radiation incident thereon to the object field in the object plane of the projection lens comprising six mirrors 621-626. At the location of the object field, a mask 631 carrying a reflective structure is arranged on a mask stage 630 and is imaged into an image plane by means of a projection lens, wherein a substrate 641 coated with a photosensitive layer (photoresist) is located on a wafer stage 640.

[0059] The test object investigated in the context of the present invention with respect to its surface shape may be, for example, any mirror of the projection exposure apparatus 610. However, the present invention is not limited thereto and is also advantageously applicable to determining the surface shape of other elements. In particular, the present invention can also be used to determine the surface shape of optical elements (e.g. mirrors or lens elements) or other elements operating in DUV (i.e. at wavelengths of less than 250 nm, in particular less than 200 nm). As an example, the present invention can also be used to determine the surface shape of a matrix array, which itself is used for measuring the surface shape (and optionally also has large aspherical surfaces).

[0060] Although the present invention has been described based on specific embodiments, many variations and alternative embodiments are obvious to those skilled in the art, such as by combining and / or exchanging the features of the various embodiments. Therefore, it is self-evident to those skilled in the art that these variations and alternative embodiments are simultaneously included in the present invention, and the scope of the present invention is limited only to the meaning of the attached patent claims and their equivalents.

Claims

1. A method for interferometrically determining the surface shape of a test object, -in, a test wave generated by diffraction of electromagnetic radiation at a computer generated hologram (CGH) (110, 210, 211, 310, 410) which is reflected in each case by the test object (120, 220, 320, 420) as a spherical wave; - wherein a sub-region of the spherical wave is reflected back to the test object (120, 220, 320, 420) by a spherical reflector (130, 230, 330, 430) during the self-collimation process and is superimposed with a reference wave that is not reflected at the test object (120, 220, 320, 420); - wherein the surface shape of the test object (120, 220, 320, 420) is determined based on a plurality of interferometric measurements, which are performed consecutively and differ from one another with respect to sub-regions of the spherical waves reflected by the spherical mirror (130, 230, 330, 430) during the autocollimation process.

2. The method according to claim 1, characterized in that The test object (120, 220, 320, 420) to be characterized with regard to its surface shape has an optically active surface in the form of a free-form surface without rotational symmetry.

3. The method according to claim 1 or 2, characterized in that: The interferometric measurements differ from one another with respect to the rotational position and / or the thrust position of the spherical mirror (130, 230, 330, 430).

4. The method according to any one of claims 1 to 3, characterized in that At least two sub-regions of the spherical wave detected in different interferometric measurements have a common overlapping region.

5. The method according to claim 4, characterized in that The surface shape of the test object (120, 220, 320, 420) is determined taking into account position-dependent relative deviations of the measurement data obtained for the overlapping region in the interferometric measurement.

6. The method according to any one of the preceding claims, characterized in that The number of interferometric measurements that differ from one another with respect to sub-regions of the spherical wave reflected by the spherical mirror (130, 230, 330, 430) during the autocollimation process is at least three, in particular at least four.

7. The method according to any one of the preceding claims, characterized in that The measured data of the sub-area reflected during the autocollimation process are used to determine the deviation of the spherical mirror from a perfectly spherical shape by means of an optimization algorithm.

8. The method according to any one of the preceding claims, characterized in that The test object (120, 220, 320, 420) is a reflector or a lens element.

9. The method according to any one of the preceding claims, characterized in that The test object (120, 220, 320, 420) is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

10. The method according to any one of the preceding claims, characterized in that The test object (120, 220, 320, 420) is an optical element for microlithography, in particular an optical element of a microlithography projection exposure apparatus.

11. A method for interferometrically determining the surface shape of a test object in the form of an optical element of a microlithographic projection exposure apparatus, -in, a test wave generated by diffraction of electromagnetic radiation at a computer generated hologram (CGH) (110, 210, 211, 310, 410) which is reflected in each case by the test object (120, 220, 320, 420) as a spherical wave; - wherein at least a sub-region of the spherical wave is reflected back to the test object (120, 220, 320, 420) by a spherical reflector (130, 230, 330, 430) during the autocollimation process and is superimposed with a reference wave that is not reflected at the test object (120, 220, 320, 420); and - wherein the surface shape of the test object (120, 220, 320, 420) is determined interferometrically based on the superposition.

12. A measuring assembly for interferometrically determining the surface shape of a test object, in particular the surface shape of an optical element of a microlithography projection exposure apparatus, the measuring assembly comprising: a computer generated hologram (CGH) (110, 210, 211, 310, 410) which is arranged in the beam path and generates a test wave by diffraction of electromagnetic radiation, which is adapted to a target shape of the surface of the test object (120, 220, 320, 420) in such a way that the test wave is reflected by the test object as a spherical wave; a spherical reflector (130, 230, 330, 430) which reflects a sub-region of the spherical wave reflected by the test object (120, 220, 320, 420) back to the test object during the autocollimation process; and Evaluation device for determining the surface shape of the test object based on a plurality of interferometric measurements performed in succession, in each case superimposing a sub-region of the wave reflected back to the test object (120, 220, 320, 420) during the autocollimation process with a reference wave not reflected at the test object (120, 220, 320, 420), wherein the interferometric measurements differ from one another with respect to the sub-region of the spherical wave reflected by the spherical mirror (130, 230, 330, 430) during the autocollimation process.

13. The measuring assembly according to claim 12, characterized in that The first sub-region and the second sub-region have a common overlapping area.

14. The measuring assembly according to claim 12 or 13, characterized in that The measuring assembly has a device for manipulating the rotational position and / or the thrust position of the spherical mirror (130, 230, 330, 430).

15. The measuring assembly according to any one of claims 12 to 14, characterized in that The CGH (110, 210, 211, 310, 410) has complex coding for generating different output waves.

16. The measuring assembly according to claim 15, wherein: The measuring assembly has at least one calibration mirror, wherein the output wave comprises at least one calibration wave for interferometric superposition with a reference wave after the calibration wave is reflected at the calibration mirror.

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