Measuring arrangement for interferometrically determining the surface shape of a test object, and method for producing an optical element for such a measuring arrangement

Abstract

The invention relates to a measuring arrangement for interferometrically determining the surface shape of a test object, wherein a test wave generated from electromagnetic radiation and reflected by the test object can be overlaid with a reference wave not reflected by the test object, having a reference element (110, 210, 310, 410) with a reference surface (112, 212, 312, 412) which splits the reference wave in reflection from the test wave in the optical beam path before said test wave hits the test object (120, 220, 320, 420), and having at least one beam shaping surface (111, 211, 311, 411, 413, 414) located in the optical beam path in front of the reference surface, wherein this beam shaping surface is designed as an aspherical distortion correction surface such that a distortion occurring in a predefined reference plane of the measuring arrangement is reduced. The invention also relates to a method for producing an optical element for such a measuring arrangement.

Description

[0001]

[0002] Measuring arrangement for the interferometric determination of the surface shape of a test specimen, as well as a method for manufacturing an optical element for such a measuring arrangement

[0003] The present application claims priority from German patent application 10 2024 209 094.8, filed on 23 September 2024. The content of this German application is incorporated into the present application text by reference.

[0004] BACKGROUND OF THE INVENTION

[0005] Field of invention

[0006] The invention relates to a measuring arrangement for the interferometric determination of the surface shape of a test specimen, and to a method for manufacturing an optical element for such a measuring arrangement. The test specimen can, in particular, be an optical element for microlithography, especially an optical element of a microlithographic projection exposure system.

[0007] State of the art

[0008] Microlithography is used to manufacture microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a projection exposure system, which includes an illumination unit and a projection lens. The image of a mask (= reticulum) illuminated by the illumination unit is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection lens, in order to transfer the mask structure onto the photosensitive coating of the substrate.

[0009] In projection lenses designed for the EUV range, i.e., at wavelengths of, for example, approximately 13 nm or approximately 7 nm, mirrors are used as optical components for the imaging process due to the lack of suitable translucent refractive materials. Typical EUV projection lenses, such as those known from US 2016 / 0085061 A1, can, for example, have an image-side numerical aperture (NA) in the range of NA = 0.55 and project an object field (e.g., ring-segment-shaped) onto the image plane or wafer plane.

[0010] Increasing the image-side numerical aperture (NA) typically necessitates an increase in the required mirror surface area of ​​the mirrors used in the projection exposure system. Given the high accuracy requirements in microlithography, this means that, in addition to manufacturing, the inspection of the surface shape, especially of EUV mirrors, presents a significant challenge. For high-precision mirror inspection, interferometric measurement methods are employed, for example, using diffractive optical elements such as computer-generated holograms (CGH). The determination of the surface shape of the respective mirror or test specimen is based on the interferometric superposition of a test wave, generated (e.g.) by the CGH and reflected from the test specimen, with a wavefront adapted to the desired surface shape of the test specimen, and a reference wave that is not reflected from the test specimen.

[0011] There are various approaches to generating the reference wave, such as placing a reference mirror in the optical beam path of a so-called reference mirror interferometer or using a Fizeau element in a Fizeau interferometer. Another approach, also known as "matrix testing," involves generating the reference wave required for interferometric measurement or superposition with the test wave reflected from the test specimen by means of a splitting of the test wave achieved through reflection via a reference surface of a reference element (also called a "matrix") located in the optical beam path in front of the test specimen.

[0012] As the size of the mirror to be tested increases, the need arises to produce ever larger CGHs (e.g., with diameters of several tens of centimeters). With conventional electron lithography, this requires extremely long writing times as well as the costly development or adaptation of the machine components used for production.

[0013] Another problem that arises in practice regarding the most reliable and accurate interferometric determination of mirror surfaces is distortion in the interferogram or camera image generated by the interferometer camera in the measurement setup. This distortion can be intrinsically caused by the optical system, particularly when imaging a non-spherical test object surface, if a non-spherical wave originating from such a surface (e.g., a freeform surface) is projected onto the interferometer camera. The result is a resolution that varies depending on the location on the interferometer camera, leading to an inaccurate determination of the surface shape, potentially resulting in incorrect processing of the test object and ultimately impairing the performance of the optical system using this mirror.the projection exposure system.

[0014] For examples of the state of the art, reference is made only to DE 10 2022 209 651 A1, DE 10 2011 004 376 B3, DE 10 2021 202 909 A1, US 6,312,373 B1, EP 1 682 851 B1 and the publication by X. Qiao et al.: “Measurement and correction of lateral distortion in a Fizeau interferometer based on the self-calibration technique”, Optics Express, Vol. 30, No. 20, pages 36134-36143, https: / / doi.org / 10.1364 / OE.467554.

[0015] SUMMARY OF THE INVENTION

[0016] Against the above background, it is an object of the present invention to provide a measuring arrangement for the interferometric determination of the surface shape of a test specimen and a method for producing an optical element for such a measuring arrangement, which enables reliable testing even of comparatively large - especially raised - mirror surfaces.

[0017] This problem is solved by the features of independent patent claims.

[0018] The invention relates in particular to a measuring arrangement for the interferometric determination of the surface shape of a test object, wherein a test wave generated from electromagnetic radiation and reflected at the test object can be superimposed with a reference wave not reflected at the test object, with

[0019] - a reference element with a reference surface which, in the optical beam path, splits off the reference wave from the test wave by reflection before it strikes the test object; and

[0020] - at least one beam-shaping surface located in the optical beam path in front of the reference surface;

[0021] - wherein this beam shaping surface is designed as an aspherical distortion correction surface such that distortion occurring in a given reference plane of the measuring arrangement is reduced.

[0022] Distortion can be understood here as a variation—generally undesirable—in the respective area of ​​pixels generated by the mapping onto a reference plane, whereby said mapping is based on a Cartesian grid of pixels (with identical areas) on the object plane of the mapping (i.e., in this case, on the surface of the test specimen). Distortion can, for example, be defined using pixel areas as follows: First, the area of ​​pixels is determined when mapping a given ideal pixel grid onto a reference plane (differentially geometrically via the infinitesimal area as the root of the determinant of the metric tensor). From these areas, an effective pixel size is obtained. eff= F (side length of a square with the same area). The distortion V is then the (dimensionless) ratio of maximum to minimum effective pixel size:

[0023] Maximum(s e y) Maximum( F) Minimum(s e / / ) Minimum( F) J Minimum(F)

[0024] Alternatively, when defining distortion via the areas of pixels, it is also possible to define distortion as the ratio between the maximum area of ​​a pixel and the minimum area of ​​a pixel in the reference plane (which corresponds to the above definition except for the calculation of the square root).

[0025] According to a second definition, distortion can also be defined using pixel edge lengths: In this case, s will be used in the following. x and s yThe edge lengths of pixels in two orthogonal directions x and y for mapping a given ideal pixel grid onto a reference plane (differentially geometrically via the infinitesimal lengths of the tangent vectors). For these two directions, distortions are defined as the ratio between the maximum and minimum edge lengths:

[0026] T , Maximum(s x ) T , Maximum

[0027] 14 = - Minimum( - —s x -) and 14 y , = - Minimum - — - A distortion as a measure for both directions combined can be further defined as the maximum of these two distortions:

[0028] V = Maximum^ , . Maximum^ Maximum (see v ) v , ) with V x = V v = — — . - yJ Minimum(s x ) y minimum

[0029] The distortion according to this second definition (over edge lengths of pixels) usually yields slightly larger values ​​than the distortion according to the previously described first definition (as the square root of the ratio of maximum to minimum area of ​​pixels).

[0030] For the purposes of the present application, an “aspherical surface” (i.e., in particular the aspherical distortion correction surface provided for in the invention) is understood to be a surface that deviates from a sphere, whereby this surface may, but need not, have rotational symmetry.

[0031] The distortion correction surface according to the invention can, in particular, serve to correct an “intrinsic distortion” as defined below: This “intrinsic distortion” also exists in a completely flawless, perfectly adjusted, and ideal test setup (with an ideal reference element without inhomogeneities in the material and an ideal test specimen) and results from the fact that the wavefront emanating from an ideal test specimen with a surface deviating from a sphere (i.e., an aspherical surface or a freeform surface) is no longer an exact spherical wavefront. The intrinsic distortion means that determining the distortion on a reference surface between the test specimen and the back side of the reference element would yield a distortion value greater than one (where a distortion value of one represents “distortion-free” or corresponds to a scenario without distortion).

[0032] The invention differs initially from embodiments of a reference element in which the back surface of the reference element (in combination with another optical element such as a CGH) serves only to transmit unchanged (i.e., not to shape) the wave incident on the reference surface of the reference element (and subsequently also incident unchanged on the test specimen), and which is hereinafter also referred to as a "reference meniscus" (for the purpose of defining a reference for defining or delimiting the embodiment according to the invention): Such a conventional reference element has the shape of a meniscus, in which both surfaces of the reference element are similar to the test specimen, and the illumination wave incident on the back surface passes through both surfaces without light deflection or refraction in the direction of the local surface normals. The back surface can be coated to prevent reflected light.

[0033] When using the back surface of the reference element as a distortion correction surface, the area removed during the fabrication of this back surface, starting from the back surface of the reference meniscus (with the same center thickness), can be more than 100 pm or even several millimeters (mm). In the case of the distortion correction surface, the principal curvature of the ground sphere or best-fit sphere can even have a different sign compared to the back surface of the reference meniscus (i.e., the reference element with distortion correction can take the shape of, for example, a biconvex or biconcave lens instead of a meniscus).

[0034] The invention also differs from the conventional approach according to DE 10 2022 209 651 A1 cited in the introduction, in which the processing of the back surface of a reference element is carried out according to a correction determined on the basis of an interferogram to compensate for inhomogeneities in the material of the reference element. While according to DE 102022 209651 A1 the back surface is used only for the transmission or shaping (normally in combination with other elements) of the illumination wave incident on the reference surface, the back surface of the reference element according to the invention (i.e., the beam shaping surface according to the invention) serves not only to shape the illumination wave incident on the reference surface but also to shape the imaging wave traveling from the test specimen to the (interferometer) camera.Such simultaneous shaping of the illumination wave incident on the mirror under test and the imaging wave traveling to the (interferometer) camera is only possible if the shaping of the illumination wave incident on the test object is caused not only by the back surface of the reference element (= beam shaping surface) but also by other elements such as a CGH.

[0035] In other words, in the measuring arrangement according to the invention, the back surface of the reference element (= beam shaping surface) is characterized in particular by the fact that this back surface or beam shaping surface shapes the imaging wave running from the test object to the interferometer camera and that, on the other hand, the shaping of the illumination wave incident on the test object is additionally carried out via at least one further element (e.g. a CGH).

[0036] The invention further differs from a process known as ICA (intrinsically corrected asphere), which is conventionally used to adapt a real system to a predetermined design without regard to the resulting distortion. This conventional adaptation of a real system to a predetermined design does not necessarily lead to a reduction or correction of the distortion and, depending on the defects present in the real system (e.g., material inhomogeneities), can even ultimately lead to an increase in distortion.

[0037] It should also be noted that – while the reference surface of the reference element or matrix (i.e., the surface of the reference element facing the test specimen) is a surface similar to the test specimen or mirror – the distortion correction surface provided according to the invention (the CGH-side surface of the matrix) generally has nothing in common with the surface shape of the test specimen or mirror. The principal curvature of the basic sphere or best-fit sphere can even have a different sign than the surface of the test specimen (i.e., the reference element can assume the shape of, for example, a biconvex or biconcave lens instead of a meniscus).

[0038] For the purposes of this application, "distortion correction" preferably refers to the correction of distortion in the image by the optical subsystem of the test setup, consisting of the test specimen and the reference element ("matrix"). However, it is also conceivable to implement distortion correction by including further optical elements of the test setup; that is, according to the invention, the distortion correction surface can also be used to correct the distortion for the entire image, including the interferometer camera. Thus, according to the invention, distortion caused, for example, by the CGH used to generate the test wave and the eyepiece in the interferometer, and even distortion caused by the interferometer camera (if known), can also be corrected.

[0039] The reference plane or surface can be located, for example, in the area of ​​a CGH (constant wave generator) that may be used to generate the test wave, or in the area of ​​the interferometer camera. However, the position and shape of the reference plane or surface for distortion determination can, in principle, be chosen arbitrarily. According to the invention, it is also conceivable to correct or reduce the distortion on a spherical reference surface (e.g., for an interferometer camera with a spherical sensor).

[0040] The invention is based on a known approach in a measuring arrangement for the interferometric determination of the surface shape of a test object (in particular EUV mirrors). The reference wave required for the interferometric measurement or its superposition with a test wave reflected from the test object is generated by means of a splitting from the test wave via a reference surface located in the optical beam path in front of the test object. By designing said reference surface to be similar to the surface of the test object, it is ensured that the wavefronts of the test wave and the reference wave are configured similarly or identically to each other (except for a constant optical path or phase difference).In this approach, the invention initially differs from "reference mirror arrangements," such as those known from DE 10 2015 209 490 A1, in which the reference wave required for interferometric superposition is already generated by the diffractive optical element or CGH and directed from there in a direction different from the test wave onto a typically planar reference mirror located at a certain distance. This difference initially offers the advantage (given that the test wave is typically generated by diffraction at a diffractive optical element or CGH) that structural defects in the diffractive optical element affect both the test wave and the reference wave equally, thus eliminating the need for separate functionalities of the CGH (such as a corresponding complex coding for generating both the reference wave and the test wave).Furthermore, when the reference surface is placed according to the inventive approach, the resulting cavity or gap length (corresponding to the remaining path to be traveled by the electromagnetic radiation between the reference surface and the test specimen) can be kept comparatively small, with the consequence that pressure or temperature fluctuations (especially refractive index inhomogeneities occurring in the form of "air streaks" when measuring in air) play only a subordinate role.

[0041] Starting from the approach chosen according to the invention, also referred to as "matrix testing technique," the invention is based in particular on the concept of designing a beam-shaping surface located in the optical beam path in front of the aforementioned reference surface as an aspherical distortion correction surface, by which a distortion occurring in a predetermined reference plane of the measuring arrangement is reduced. The aforementioned reference plane can, in principle, be located at any position within the measuring arrangement, in particular, for example, in the area of ​​a CGH (constant wave generator) that may be present for generating the test wave, or also in the area of ​​the interferometer camera. Furthermore, the beam-shaping surface designed aspherically according to the invention can, in particular, reduce the distortion generated by the aforementioned CGH, but also, depending on the embodiment, distortion occurring in the rest of the optical beam path.Distortion generated along the light path of the measurement setup up to the interferometer camera is corrected. It should be noted that the CGH structure must then be adapted according to the design of the distortion correction surface (e.g., within optimization steps).

[0042] The invention is based, among other things, on the consideration that a beam-shaping surface located in the optical beam path in front of the reference surface (and thus outside the "cavity" defined by said reference surface and the surface of the test specimen) is comparatively uncritical with regard to its surface accuracy and the applicable tolerance values, since the beam-shaping surface in question is traversed equally by both the test wave reflected back from the test specimen and the reference wave split off by reflection from the reference surface. In other words, according to the invention, the beam-shaping surface is used to a particularly advantageous extent as an additional degree of freedom for distortion correction.

[0043] In addition to the greater accuracy achievable according to the invention as a result of the distortion correction, a further advantage of the inventive design is that even when testing comparatively large test specimen or mirror surfaces, the diffractive optical element or CGH used to generate the corresponding test wave can be designed to be comparatively small, since, as described below, the particularly effective beam shaping via the inventively aspherically designed beam shaping surface can be used to adapt the optical beam path accordingly. This avoids the problems (e.g., manufacturing-related) that arise when using comparatively large CGHs, as described in the introduction.

[0044] Furthermore, according to the invention, it is also possible to test different types of test objects or mirrors simply by exchanging the reference element and the diffractive optical element (e.g. CGHs) used to generate the test wave, while otherwise maintaining the same distance between the CGH and the reference element providing the reference surface (referred to as "matrix"), thus ultimately using a uniform design of the test tower.

[0045] The inventive concept of distortion correction via the targeted aspherical design of a beam shaping surface located in the optical beam path in front of the reference surface has the further advantage over a possible computational distortion correction that an undesirable variation in resolution (as is not eliminated in said computational distortion correction) is avoided.

[0046] According to one embodiment, the measuring arrangement includes an interferometer camera, wherein the beam shaping surface causes beam shaping of an imaging wave running in the optical beam path from the test object to the interferometer camera.

[0047] According to one embodiment, the beam shaping surface is designed such that the value of the distortion is reduced compared to a corresponding reference measuring arrangement with a reference element, in which the reference surface and beam shaping surface are designed such that the reference element allows a wavefront adapted to the test specimen to pass through unchanged both on the way to the test specimen and on the way back from the test specimen.

[0048] According to one embodiment, the beam shaping surface is designed as an aspherical distortion correction surface such that the distortion occurring in the given reference plane of the measuring arrangement is reduced compared to a corresponding reference measuring arrangement with the beam shaping surface designed as a sphere (as, for example, in the measuring arrangement known from DE 10 2021 202 909 A1 cited above).

[0049] According to one embodiment, the beam shaping surface is designed as an aspherical distortion correction surface such that the distortion occurring in the given reference plane of the measuring arrangement is reduced compared to a corresponding reference measuring arrangement with the beam shaping surface designed as a sphere best adapted to the aspherical distortion correction surface.

[0050] According to one embodiment, the relationship with p > 0.2, where V2 specifies the distortion occurring in a given reference plane of the measuring arrangement, and where V specifies the distortion occurring in the given reference plane of the reference measuring arrangement.

[0051] According to one embodiment, the above-mentioned relationship (1 ) is satisfied with p > 0.4, in particular with p > 0.5, and further in particular with p > 0.6.

[0052] According to one embodiment, the beam shaping surface is a freeform surface without rotational symmetry.

[0053] According to one embodiment, the beam shaping surface deviates from the beam shaping surface of the reference element of the reference measuring arrangement by more than 100 pm.

[0054] According to one embodiment, the beam shaping surface is arranged on the side of the reference element facing away from the reference surface.

[0055] According to one embodiment, the measuring arrangement features a computer-generated hologram (CGH) which generates the test wave by diffraction of electromagnetic radiation.

[0056] According to one embodiment, the CGH has an optically effective surface whose area is at least 1.2 times smaller, in particular at least 2 times smaller, and further, in particular at least 5 times smaller, than the area of ​​a surface of the test specimen to be tested in the measuring arrangement. The beam shaping surface, designed according to the invention as an aspherical distortion correction surface, is used to achieve a significantly smaller dimension for the CGH compared to the test specimen (or the reference element), whereby the particularly effective beam shaping is utilized via the beam shaping surface according to the invention.

[0057] According to one embodiment, the distance between the CGH and the surface of the test specimen is greater than the minimum cavity radius of the test specimen. The minimum cavity radius is sometimes also referred to as the "caustic distance".

[0058] According to one embodiment, at least one further refractive optical element is arranged between the CGH and the reference element. This design has the advantage that the refractive power required in the optical beam path of the measuring arrangement for beam shaping or adaptation to a CGH that may be used to generate the test wave can be distributed across several optical elements.

[0059] According to one embodiment, the minimum distance between the reference surface and the test specimen is less than 10 mm.

[0060] According to one embodiment, there is no optical medium with a refractive index n greater than 1.1 between the reference surface and the test specimen.

[0061] According to one embodiment, the test specimen to be characterized with regard to its surface shape has an optical effective surface in the form of a freeform surface without rotational symmetry.

[0062] According to one embodiment, the test object is a mirror or a lens. According to one embodiment, the test object is designed for a working wavelength of less than 30 nm, in particular less than 15 nm.

[0063] According to one embodiment, the surface shape of the reference surface is identical to the surface shape of the test specimen, except for a constant propagation distance in the direction of the respective local surface normals.

[0064] According to one embodiment, the test object is an optical element for microlithography, in particular for a microlithographic projection exposure system.

[0065] The invention further relates to an optical element for use in a measuring arrangement with the features described above, wherein the optical element has a beam shaping surface designed as an aspherical distortion correction surface such that distortion occurring in a predetermined reference plane of the measuring arrangement is reduced.

[0066] According to one embodiment, the optical element is a reference element having the reference surface, wherein the beam shaping surface is formed on the side of the reference element facing away from the reference surface.

[0067] The invention further relates to a method for manufacturing an optical element for a measuring arrangement with the features described above, wherein a beam shaping surface is formed on the optical element as an aspherical distortion correction surface in such a way that a distortion occurring in a predetermined reference plane of the measuring arrangement is reduced.

[0068] The invention further relates to a method for manufacturing an optical element for a measuring arrangement for the interferometric determination of the surface shape of a test object, wherein a beam shaping surface is formed on the optical element, which causes a beam shaping of an imaging wave running in the optical beam path from the test object to an interferometer camera of the measuring arrangement in such a way that a predetermined function for the local variation of the image scale between the test object and the interferometer camera is realized.

[0069] Thus, the back surface of the reference element (= beam shaping surface) can be used not only for distortion correction, but also additionally for realizing certain imaging properties or alternatively for realizing arbitrary imaging properties (with respect to a given reference surface).

[0070] In further embodiments, the beam-shaping surface can be designed such that the image scales are homogeneous everywhere but scaled by a specific factor, for example, to achieve a specific value for the image scale on the interferometer camera. In further embodiments, the beam-shaping surface can also be designed such that the image scales are not homogeneous everywhere, but follow a predetermined function for locally varying image scales.

[0071] In further embodiments, the beam shaping surface can also be designed in such a way that certain imaging properties are fulfilled according to certain evaluation criteria or to realize any imaging properties.

[0072] Further embodiments of the invention can be found in the description and the dependent claims.

[0073] The invention is explained in more detail below with reference to exemplary embodiments illustrated in the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] They show:

[0075] Figure 1 is a schematic representation to illustrate the concept of the invention in an exemplary embodiment;

[0076] Figures 2-4 are schematic representations to illustrate further possible embodiments;

[0077] Figure 5 is a schematic representation to illustrate a possible setup for a measuring arrangement for the interferometric determination of the surface shape of a test specimen; and

[0078] Figure 6 shows a schematic representation of a projection exposure system designed for operation in the EUV.

[0079] DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS

[0080] Fig. 5 shows a schematic representation illustrating a possible setup for the interferometric determination of the surface shape of a test specimen. Electromagnetic radiation generated by a light source 501 travels via an optical fiber 502 to a beam splitter 503 and from there via a deflecting mirror 504 to a diffractive optical element in the form of a CGH 505. The CGH 505, with a diffractive structure 505a located on it, generates a test wave for examining the surface shape of a test specimen 520, where this test specimen 520 may, in particular, have an optical surface or test specimen surface 521 in the form of a freeform surface without rotational symmetry. According to Fig. 5, the radiation passes through a reference element 510 on its way from the CGH 505 to the test specimen 520. This reference element has an entry surface 511 and an exit surface 512.At the aforementioned exit surface 512, a reference wave is split off from the test wave by reflection. Unlike the test wave, this reference wave is not reflected by the test specimen 520, but travels back along the original beam path without reflection. The exit surface of the reference element 510 is therefore hereinafter also referred to as the "reference surface" 512.

[0081] The reference surface 512 and the test object surface 521 are "similar surfaces" in that a beam exiting the test object surface 521 perpendicularly also strikes the reference surface 512 perpendicularly. According to Fig. 5, the test object wave and the reference wave travel back through the reference element 510, the deflecting mirror 504, and the beam splitter 503 along the original beam path of the test wave. From this beam splitter 503, the reference wave and the test wave pass through an aperture 507 and an eyepiece 508 to an interferometer camera 509. The interferometer camera 509 records an interferogram generated by the interfering waves, from which the surface shape of the test object 520 is determined by an evaluation unit (not shown).

[0082] Starting from the known setup of an interferometric measuring arrangement according to Fig. 5, different embodiments of the invention are now described with reference to the merely schematic representations of Figs. 1-4.

[0083] These embodiments, which are merely exemplary and simplified, have in common that at least one beam shaping surface located in the beam path in front of the reference surface is designed as an aspherical distortion correction surface in such a way that any distortion occurring in a given reference plane of the measuring arrangement is reduced.

[0084] In particular, the beam shaping surface can be designed in such a way that the value of the distortion is reduced compared to a corresponding reference measuring arrangement with a reference element, in which the reference surface and beam shaping surface are designed in such a way that the reference element allows a wavefront adapted to the test specimen to pass through unchanged both on the way to the test specimen and on the way back from the test specimen.

[0085] Furthermore, the beam shaping surface can also be designed as an aspherical distortion correction surface in such a way that the distortion occurring in the specified reference plane of the measuring arrangement is reduced compared to a corresponding reference measuring arrangement with the beam shaping surface designed as a sphere, and / or that the distortion occurring in the specified reference plane of the measuring arrangement is reduced compared to a corresponding reference measuring arrangement with the beam shaping surface designed as a sphere best adapted to the aspherical distortion correction surface.

[0086] Referring initially to Fig. 1, a possible conventional setup and optical beam path for that section of the interferometric measuring arrangement is shown on the left, which includes a CGH 55 with CGH structure 55a, a reference element 60 with spherical beam shaping surface 61 and reference surface 62, and a test specimen 70 with test specimen surface 71.

[0087] In contrast, an embodiment according to the invention is shown on the right in Fig. 1, wherein, compared to the conventional arrangement shown on the left, analogous components are designated with reference numerals increased by "50". In contrast to the conventional embodiment, in the embodiment according to the invention, the beam-shaping surface 111 of the reference element 110 on the right in Fig. 1 is designed as an aspherical distortion correction surface, with the result that a distortion visible on the left in Fig. 1, which occurs in the area of ​​the CGH 55 or in a reference plane located there, is eliminated.

[0088] As can be seen from Fig. 1 (as well as in Fig. 2-4), the surface shape of the reference surface 62 or 112 can be identical to the surface shape of the test specimen 70 or 120 up to a constant propagation distance in the direction of the respective local surface normals.

[0089] According to one embodiment, the relationship with p > 0.2, where V2 specifies the distortion occurring in a given reference plane of the measuring arrangement, and where V specifies the distortion occurring in a corresponding measuring arrangement with the beam shaping surface configured as a sphere best adapted to the aspherical distortion correction surface.

[0090] In embodiments, the aforementioned relationship (1) can be satisfied in particular with p > 0.4, in particular with p > 0.5, and further in particular with p > 0.6.

[0091] To optimize the distortion correction surface according to the invention (which can in particular be a freeform surface, e.g., on the rear surface of the reference element 110 facing away from the reference surface 112), the following procedure can be used: a) Specification of pixels with a regular Cartesian grid on a first reference plane on the test object 120 (e.g., a mirror); this first reference plane can be defined in its position by the centroid of the test object surface 121 and in its orientation by the mean surface normal; b) Definition or selection of a second reference plane (e.g., at the location of the CGH 105); this second reference plane can be perpendicular to a reference ray emanating from the centroid of the test object surface 121 and with a direction corresponding to the direction of the above.a) the mean surface normal on the test specimen surface 121 and at the intersection of this reference ray with the surface of the CGH 105 exhibiting the CGH structure 105a; c) calculation of rays in the normal direction on the test specimen surface 121 and originating from this test specimen surface 121; d) determination of the ray penetration points on the aforementioned second reference plane (e.g. at the CGH 105); e) determination of the local imaging scales for the imaging between the first (test specimen) reference plane and the aforementioned second reference plane (e.g. at the CGH 105) in two sections (x- and y-directions); and f) optimization of the polynomial coefficients of the distortion correction surface or freeform surface (= beam shaping surface 111 ) such that the ratio of the imaging scales between the first (test object) reference plane and the above second reference plane (e.g. at CGH 105) is as homogeneous as possible everywhere.

[0092] In further embodiments, it is also possible, using the described method, to determine the ray points on a reference surface with any surface shape (e.g., a sphere) and then, for the analogous calculation of distortion, to project the ray points onto the vertex plane of the reference surface; the surface normal of the vertex plane coincides with the reference ray. Thus, distortion calculation can also be extended to images onto arbitrary surfaces, such as a spherical sensor in a camera.

[0093] Fig. 2 shows, again in a schematic and simplified representation, a further embodiment, wherein, compared to the arrangement shown on the right in Fig. 1, analogous or essentially functionally identical components are designated with reference numerals increased by "100". As can be seen from Fig. 2, the beam shaping surface 211, which according to the invention is again designed as an aspherical distortion correction surface, is additionally used to achieve a significantly smaller dimension for the CGH 205 compared to the test specimen 220 or the reference element 210, whereby the particularly effective beam shaping via the beam shaping surface 211 according to the invention is utilized.

[0094] The invention is not fundamentally limited to the use of a CGH for generating the test wave. Fig. 3 shows a further possible embodiment with the generation of the test wave via a refractive optical element 305, wherein its light-emitting surface 305a is also designed as an aspherical surface, in particular a freeform surface. In the embodiment of Fig. 3, the distortion correction according to the invention is thus achieved by two beam-shaping surfaces, namely the light-emitting surface 305a of the refractive optical element 305 and the beam-shaping surface 311 of the reference element 310.

[0095] Fig. 4 shows another possible embodiment, in which, compared to Fig. 1, analogous or essentially functionally identical components are designated with reference numerals increased by "300". In contrast to Fig. 1, Fig. 4 shows an additional refractive optical element 410a arranged in front of the reference element 410, which has the reference surface 412, with respect to the optical beam path. This design has the advantage that the refractive power required for beam shaping or adaptation to the CGH 405 can be distributed between both optical elements 410 and 410a. In the embodiment of Fig. 4, the distortion correction according to the invention is carried out via the light-entry surface 411 of the refractive optical element 410a, whereas the light-emission surface 413 of this element 410a, as well as the light-entry surface 414 of the reference element 410, are spherically shaped.In further embodiments (not shown), the distortion correction according to the invention can also be implemented on a different surface (e.g., the light-emitting surface 413 of element 410a). Furthermore, analogous to Fig. 4, the refractive power can also be distributed across more than two optical elements.

[0096] Fig. 6 schematically shows, in meridional section, the possible setup of a microlithographic projection exposure system designed for operation in EUV, which includes mirrors that can be tested using a method according to the invention. For example, a test specimen examined with regard to its surface shape within the scope of the invention can be any mirror of this projection exposure system 1. However, the invention is not limited to this and is also advantageously applicable for determining the surface shape of other elements. In particular, the invention can also be used to determine the surface shape of optical elements (e.g., mirrors or lenses) in a system designed for operation in DUV (i.e., at wavelengths of less than 250 nm, particularly less than 200 nm) or of other elements.

[0097] According to Fig. 6, the projection exposure system 1 comprises a lighting device 2 and a projection lens 10. In one embodiment of the lighting device 2 of the projection exposure system 1, in addition to a light or radiation source 3, there is a lighting optic 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the lighting device. In this case, the lighting device does not include the light source 3.

[0098] In this process, a reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be moved, particularly in a scanning direction, via a reticle displacement drive 9. For illustrative purposes, a Cartesian xyz coordinate system is shown in Fig. 6. The x-direction runs perpendicular to the plane of the drawing. The y-direction runs horizontally, and the z-direction runs vertically. In Fig. 6, the scanning direction runs along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0099] The projection lens 10 serves to image the object field 5 onto an image field 11 in an image plane 12. A structure on the reticulum 7 is imaged onto a light-sensitive layer of a wafer 13 located in the image field 11 within the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be moved, particularly along the y-direction, via a wafer transfer drive 15. The movement of the reticulum 7 via the reticulum transfer drive 9 and of the wafer 13 via the wafer transfer drive 15 can be synchronized.

[0100] Radiation source 3 is an EUV radiation source. Radiation source 3 emits, in particular, EUV radiation, which is also referred to below as useful radiation or illumination radiation. The useful radiation has a wavelength in the range between 5 nm and 30 nm. Radiation source 3 could, for example, be a plasma source, a synchrotron-based radiation source, or a free-electron laser (FEL). The illumination radiation 16, emanating from the radiation source 3, is focused by a collector 17 and propagated through an intermediate focus in an intermediate focal plane 18 into the illumination optics 4. The illumination optics 4 has a deflecting mirror 19 and, downstream in the beam path, a first faceted mirror 20 (with schematically indicated facets 21) and a second faceted mirror 22 (with schematically indicated facets 23).

[0101] The projection lens 10 has a plurality of mirrors Mi (i = 1, 2, ... ), which are numbered according to their arrangement in the beam path of the projection exposure unit 1. In the example shown in Fig. 6, the projection lens 10 has six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have an aperture for the illumination radiation 16. The projection lens 10 is a double-obscured optical system. The projection lens 10 has an image-side numerical aperture greater than 0.5, and which can also be greater than 0.6, for example, 0.7 or 0.75.

[0102] Even though the invention has been described with reference to specific embodiments, numerous variations and alternative embodiments are apparent to the person skilled in the art, for example, through the combination and / or exchange of features of individual embodiments. Accordingly, it is understood to the person skilled in the art that such variations and alternative embodiments are included in the present invention and that the scope of the invention is limited only to the extent of the appended claims and their equivalents.

Claims

26 Patent claims 1. Measuring arrangement for the interferometric determination of the surface shape of a test object, wherein a test wave generated from electromagnetic radiation and reflected by the test object can be superimposed with a reference wave not reflected by the test object, with • a reference element (110, 210, 310, 410) with a reference surface (112, 212, 312, 412) which, in the optical beam path, reflects the reference wave from the test wave before it strikes the test specimen (120, 220, 320, 420); and • at least one beam shaping surface (111 , 211 , 311 , 411 , 413, 414) located in the optical beam path in front of the reference surface (112, 212, 312, 412); • wherein this beam shaping surface (111 , 211 , 311 , 411 , 413, 414) is designed as an aspherical distortion correction surface such that distortion occurring in a given reference plane of the measuring arrangement is reduced.

2. Measuring arrangement according to claim 1, characterized in that it has an interferometer camera (509), wherein the beam shaping surface (111 , 211 , 311 , 411 , 413, 414) causes beam shaping of an imaging wave running in the optical beam path from the test object (120, 220, 320, 420) to the interferometer camera (509).

3. Measuring arrangement according to claim 1 or 2, characterized in that the beam shaping surface (111 , 211 , 311 , 411 , 413, 414) is designed such that the value of the distortion is reduced compared to a corresponding reference measuring arrangement with a reference element in which the reference surface and beam shaping surface are designed such that the reference element allows a wavefront adapted to the test specimen to pass through unchanged both on the way to the test specimen and on the way back from the test specimen.

4. Measuring arrangement according to claim 1 or 2, characterized in that the beam shaping surface is designed as an aspherical distortion correction surface such that the distortion occurring in the predetermined reference plane of the measuring arrangement is reduced compared to a corresponding reference measuring arrangement with the beam shaping surface designed as a sphere.

5. Measuring arrangement according to claim 1 or 2, characterized in that the beam shaping surface is designed as an aspherical distortion correction surface such that the distortion occurring in the predetermined reference plane of the measuring arrangement is reduced in comparison to a corresponding reference measuring arrangement with the beam shaping surface configured as a sphere best adapted to the aspherical distortion correction surface.

6. Measuring arrangement according to one of the preceding claims, characterized in that the relationship V2= l + (l - p) - (V1- l) (1 ) with p > 0.2 is satisfied, where V2 indicates the distortion occurring in a given reference plane of the measuring arrangement, and where V indicates the distortion occurring in the given reference plane of the reference measuring arrangement.

7. Measuring arrangement according to claim 6, characterized in that the relationship (1 ) is satisfied with p > 0.4, in particular with p > 0.5, further in particular with p > 0.

6.

8. Measuring arrangement according to one of the preceding claims, characterized in that the beam shaping surface (111 , 211 , 311 , 411 , 413, 414) is a freeform surface without rotational symmetry.

9. Measuring arrangement according to one of the preceding claims, characterized in that the beam shaping surface (111 , 211 , 311 , 411 , 413, 414) deviates from the beam shaping surface of the reference element of the reference measuring arrangement by more than 100 pm.

10. Measuring arrangement according to one of the preceding claims, characterized in that the beam shaping surface (111 , 211 , 311 ) is arranged on the side of the reference element (110, 210, 310) facing away from the reference surface (112, 212, 312).

11. Measuring arrangement according to one of the preceding claims, characterized in that it has a computer-generated hologram (CGH) (105, 205, 405) which generates the test wave by diffraction of electromagnetic radiation.

12. Measuring arrangement according to claim 11, characterized in that the CGH (105, 205, 405) has an optically effective surface, the area of ​​which is smaller by at least a factor of 1.2, in particular at least a factor of 2, and further in particular at least a factor of 5, than the area of ​​a surface of the test specimen (120, 220, 320, 420) to be tested in the measuring arrangement.

13. Measuring arrangement according to claim 11 or 12, characterized in that a distance between the CGH (105, 205, 405) and the surface (121, 221, 421) of the test specimen (120, 220, 320, 420) to be tested is greater than a minimum hollow radius of the test specimen (120, 220, 320, 420).

14. Measuring arrangement according to one of the preceding claims, characterized in that at least one further refractive optical element (410a) is arranged between the CGH (405) and the reference element (410). 29 15. Measuring arrangement according to one of the preceding claims, characterized in that a minimum distance between reference surface (112, 212, 312, 412) and test specimen (120, 220, 320, 420) is less than 10 mm.

16. Measuring arrangement according to one of the preceding claims, characterized in that there is no optical medium with a refractive index n greater than 1.1 between the reference surface (112, 212, 312, 412) and the test specimen (120, 220, 320, 420).

17. Measuring arrangement according to one of the preceding claims, characterized in that the test specimen (120, 220, 320, 420) to be characterized with respect to its surface shape has an optical effective surface in the form of a freeform surface without rotational symmetry.

18. Measuring arrangement according to one of the preceding claims, characterized in that the test object (120, 220, 320, 420) is a mirror or a lens.

19. Measuring arrangement according to one of the preceding claims, characterized in that the test specimen (120, 220, 320, 420) is designed for a working wavelength of less than 30 nm, in particular less than 15 nm.

20. Measuring arrangement according to one of the preceding claims, characterized in that the surface shape of the reference surface (112, 212, 312, 412) is identical to the surface shape of the test specimen (120, 220, 320, 420) up to a constant propagation distance in the direction of the respective local surface normals.

21. Measuring arrangement according to one of the preceding claims, characterized in that the test specimen (120, 220, 320, 420) is an optical element for microlithography, in particular a microlithographic projection exposure system (1). 30 22. Optical element for use in a measuring arrangement according to one of claims 1 to 21, wherein the optical element has a beam shaping surface (111 , 211 , 311 , 411 , 413, 414) designed as an aspherical distortion correction surface such that distortion occurring in a predetermined reference plane of the measuring arrangement is reduced.

23. Optical element according to claim 22, characterized in that the optical element is a reference element (110, 210, 310) having the reference surface (112, 212, 312, 412), wherein the beam shaping surface (111 , 211 , 311 ) is formed on the side of the reference element (110, 210, 310) facing away from the reference surface (112, 212, 312).

24. Method for manufacturing an optical element for a measuring arrangement for the interferometric determination of the surface shape of a test specimen according to one of claims 1 to 21, wherein a beam shaping surface (111 , 211 , 311 , 411 , 413, 414) on the optical element is designed as an aspherical distortion correction surface such that a distortion occurring in a predetermined reference plane of the measuring arrangement is reduced.

25. Method for manufacturing an optical element for a measuring arrangement for the interferometric determination of the surface shape of a test object, wherein a beam shaping surface (111 , 211 , 311 , 411 , 413, 414) is formed on the optical element, which shapes a beam wave running in the optical beam path from the test object (120, 220, 320, 420) to an interferometer camera (509) of the measuring arrangement in such a way that a predetermined function for the local variation of the image scale between the test object (120, 220, 320, 420) and the interferometer camera (509) is realized.

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