Holding device, optical assembly and optical system

Abstract

The invention relates to a holding device (28) for a mirror element, in particular for a mirror element (19) for the reflection of EUV radiation (16), which has a ratio of length (L) to breadth (B) of more than 2:1, preferably more than 3:1, particularly preferably more than 4:1, in particular more than 10:1. The holding device (28) has a frame (29) with a plurality of holding elements (30) for laterally clamping the mirror element (19). The holding elements (30) have protruding sprung portions (32) for resiliently supporting the mirror element (19). The invention also relates to an optical assembly (27) having such a mirror element (19) and having a holding device (28), designed as described above, for holding the mirror element (19). The invention also relates to an EUV lithography system having at least one such optical assembly (27).

Description

[0001] Mounting device, optical assembly and optical system

[0002] Reference to related application

[0003] This application claims priority from German patent application DE102023205748.4 filed on June 20, 2023, the entire disclosure of which is incorporated by reference into this application.

[0004] Background of the invention

[0005] The invention relates to a mounting device for a mirror element, in particular for a mirror element for reflecting EUV radiation, which has a length-to-width ratio of more than 2:1, preferably more than 3:1, particularly preferably more than 4:1, in particular more than 10:1. The invention also relates to an optical assembly comprising such a mounting device and such a mirror element. The invention further relates to an optical system, in particular an EUV lithography system, comprising at least one such optical assembly.

[0006] The EUV lithography system can be an EUV lithography system for exposing a wafer or another optical arrangement used for EUV lithography, for example, an inspection system, e.g., an arrangement for measuring or inspecting masks, wafers, or the like used in EUV lithography. The EUV lithography system is designed for operation with radiation in the EUV wavelength range. For the purposes of this application, the EUV wavelength range is understood to mean a wavelength range between approximately 5 nm and approximately 30 nm. The mirror element described above, or more precisely, the substrate of such a mirror element, is typically rectangular or cuboid-shaped.

[0007] Mirror elements for EUV lithography or X-ray optics are typically exposed to high radiation loads during operation, whereas this is not the case during downtime. This can lead to temperature fluctuations of the mirror elements on the order of, for example, ΔT = ±40 K. Temperature fluctuations of the order of ΔT = ±10 K can also occur during transport of such mirror elements.

[0008] Typically, mirror elements, or more precisely, mirror element substrates, for EUV lithography are made of a so-called zero-expansion material, i.e., a material with a very low coefficient of thermal expansion (CTE) that reaches a minimum at a temperature, the so-called zero-crossing temperature. The material of the support device is typically not made of a zero-expansion material, which is why its coefficient of thermal expansion differs from the coefficient of thermal expansion of the mirror element material. The difference between the coefficient of thermal expansion of the mirror element substrate and the support device material can lead to unacceptable deformations, stresses, and possibly fractures.

[0009] Mirror elements for EUV lithography systems are typically not mounted in a mount, as is usually the case with lens elements, but rather with a three-point support, in which the mirror element, or more precisely the substrate of the mirror element, is supported by a mounting device at three support points. However, this type of mounting also generally leads to intolerable deformations for mirror elements with a large aspect ratio, i.e., a large difference between their length and width.

[0010] A further problem with the mounting of such mirror elements, especially when it comes to spare parts, is the fact that the optical assembly, which consists of the mirror element and the mounting device, has to absorb a shock load or g-forces of up to 10g during handling and transport if it has high natural frequencies of, for example, more than approximately 100 Hz, without being damaged.

[0011] Object of the invention

[0012] The object of the invention is to provide a mounting device, an optical assembly and an EUV lithography system which enable the mounting of mirror elements with a large aspect ratio with the smallest possible deformations and stresses.

[0013] Subject of the invention

[0014] According to a first aspect, this object is achieved by a holding device of the type mentioned at the outset, which has a frame with a plurality of holding elements for laterally clamping the mirror element.

[0015] The mounting elements typically have resilient sections that protrude beyond the frame to provide resilient support for the mirror element.

[0016] In this aspect of the invention, it is proposed to use a mount for holding a generally rectangular mirror element with a high aspect ratio. This mount clamps the mirror element, or more precisely the substrate of the mirror element, laterally on a plurality of mounting elements, typically more than ten or more than twenty mounting elements, and in this way introduces lower local stresses into the mirror element than is the case with a three-point support. For clamping, the mounting elements engage the side surfaces of the mirror element; however, the mounting elements can also be designed to support the mirror element, i.e., they can clamp the mirror element laterally and also serve as a support for the mirror element.

[0017] A lateral clamping of mirror elements for EUV lithography is described, for example, in DE102015219671 A1. The web-shaped fastening elements described therein for holding a mirror element each extend radially from the mount toward the circular mirror element. The forces that occur when the distance between the annular mount and the circular mirror element changes due to temperature fluctuations act in a radial direction and are evenly absorbed by the fastening elements described therein.

[0018] This is not the case with the mirror element described here, which has a high aspect ratio: Even with equal distances between adjacent mounting elements along the edge or lateral circumference of the mirror element, the directions of forces acting on the mounting elements when the distance between the mirror element and the mount changes are different. Accordingly, the type of mounting described in DE102015219671 A1 cannot be readily transferred to mirror elements with a high aspect ratio. Alternatively or in addition to lateral clamping, in a further aspect of the invention or in a further embodiment, the mirror element can also be mounted by a support on the bottom or top of the mirror element with at least three mounting elements.

[0019] In one embodiment, the mount is formed from a material with a linear thermal expansion coefficient of 2 ppm / K or less, preferably 1 ppm / K or less. As described above, the material of the mirror element, or more precisely the material of the substrate of the mirror element, is often a zero-expansion material. In order to minimize the change in the distance between the mirror element and the mount when the temperature changes, and ideally to avoid the resulting deformations and stresses, it is advantageous in this case for the mount to be formed from a material with the lowest possible linear thermal expansion coefficient.

[0020] It is not absolutely necessary for the substrate material to be a zero-expansion material. The substrate can instead be made of a different material, for example quartz glass or silicon. In this case too, the ratio of the linear thermal expansion coefficient of the material of the mount and the material of the substrate of the mirror element should be as close to one as possible, i.e. the linear thermal expansion coefficients of the two materials should differ as little as possible. In the event that the substrate material is not a zero-expansion material, the linear thermal expansion coefficient of the mount is generally greater than stated above in order to meet this requirement. In a further embodiment, the mount is made of Invar. Invar is an iron-nickel alloy which has a low linear thermal expansion coefficient of 2 ppm / K orof 1 ppm / K or less. It is understood that other materials having the lowest possible coefficient of linear thermal expansion can also be used as the holder for the mounting device described here. As described above, materials with a higher coefficient of linear thermal expansion can be used for the holder if the mounting device is intended to support a substrate that is not made of a zero-expansion material.

[0021] In another embodiment, the mounting elements are attached to the socket. For the application described here, it has proven advantageous if the mounting elements are not formed integrally with the socket, but are attached to the socket via a material-to-material, force-to-fit, and / or form-fit connection. For example, the mounting elements can be attached to the socket using screw connections.

[0022] For attachment, the mounting elements have a mounting section, the underside of which is typically attached to the top of the socket. The mounting section can have two, three, or more attachment points, e.g., in the form of holes, to which a respective mounting element is screwed to the socket. This enables precise alignment of each mounting element. It is understood that the mounting section can also be attached to the socket in other ways.

[0023] In a further embodiment, the holding elements have a section that projects beyond the frame and has a fastening surface for fastening the mirror element. Typically, the holding elements are formed in one piece and have the projecting section and the fastening section described above. The section that projects beyond the frame typically acts like a spring for the elastic or resilient mounting of the mirror element on the frame and can be web-shaped or designed in another way. The projecting section or, if applicable, the connection of the projecting section to the fastening section can have a very small thickness for the resilient mounting, as described, for example, in DE102015219671 A1, which is incorporated into this application in its entirety by reference.

[0024] At the fastening section, the mirror element, or more precisely the substrate of the mirror element, is generally connected to the respective support element via a material-to-material connection (see below). The fastening surfaces typically run laterally along the support elements and contact the substrate on its long side or wide side. In principle, it is also possible for the fastening surfaces to be designed to support the substrate, i.e., the fastening surfaces can have a section that serves as a support for the substrate.

[0025] In a further development of this embodiment, fastening surfaces of mounting elements mounted along a long side of the mirror element are arranged at equal distances from one another, and / or fastening surfaces of mounting elements mounted along a wide side of the mirror element are arranged at equal distances from one another. In this way, the most even distribution possible of the forces acting on the mounting elements can be achieved. The forces can be caused, for example, by temperature changes, vibrations, or during the initial insertion of the substrate or mirror element into the mount.The distances between the fastening surfaces of two adjacent support elements attached to the long side and the fastening surfaces of two adjacent support elements attached to the wide side are generally the same, but this is not mandatory.

[0026] In a further embodiment, the holding elements are formed, at least in the projecting section, in particular completely, from a material with a tensile strength of more than 800 MPa. As described above, the holding elements serve to absorb forces. It is also necessary to retract the holding elements or their sections projecting beyond the frame when inserting the substrate of the mirror element into the mount in order to enable contact-free insertion of the substrate into the mount. The retraction of the resilient holding elements is limited by the permissible stresses of the material of the holding elements. The holding elements should therefore be made from a material that has the highest possible breaking strength or tensile strength.

[0027] Materials with a low linear thermal expansion coefficient, e.g. Invar, typically have low tensile strength and are therefore not suitable as materials for the holding elements. In the holding device described here, a functional separation is therefore carried out in which the socket is made of a material with a linear thermal expansion coefficient that is adapted to the linear thermal expansion coefficient of the substrate in order to reduce thermal stresses, and in which the holding elements are made of a high-strength material with high tensile strength in order to be able to deflect or retract the holding elements by the necessary amount and to absorb forces. In a further embodiment, the holding elements are formed from a tool steel, at least in the projecting section.

[0028] Tool steels typically have a high tensile strength of 800 MPa or more. For example, the tool steel can be a stainless steel, e.g., a martensitic chromium steel with added nickel, particularly X 17 CrNi 16-2, or a high-strength steel such as Stavax ESR (Electro-Slag Refining), which has a tensile strength of more than 1000 MPa.

[0029] A further aspect of the invention relates to an optical assembly comprising: a mirror element having a length to width ratio greater than 2:1, preferably greater than 3:1, particularly preferably greater than 4:1, in particular greater than 10:1, and a holding device for holding the mirror element, which is designed as described above.

[0030] The mirror element can, for example, be designed in the form of a so-called vertical focusing mirror, which focuses incident radiation in a vertical direction. Such mirror elements generally have a high aspect ratio within the value range specified above. Such mirror elements are typically designed to reflect radiation in the EUV wavelength range or for radiation in the X-ray range and can be used, for example, in EUV light sources, in EUV lithography systems, or in synchrotron optics. Use in a reflectometer for measuring the reflectivity of a mirror element, e.g., an EUV mirror, is also possible. The mirror element has a substrate with a surface to which a reflective coating is applied. The reflective coating can be designed to reflect EUV radiation or, if necessary, to reflect X-ray radiation.The mirror element is typically operated under grazing incidence. The surface with the reflective coating is usually concavely curved.

[0031] In one embodiment, the ratio between a linear thermal expansion coefficient of the material of the mount and a linear thermal expansion coefficient of the material of a substrate of the mirror element is between 0.5 and 2.0, preferably between 0.8 and 1.25. As described above, it is advantageous if the linear thermal expansion coefficients of the mount and the substrate of the mirror element are, if possible, in the same order of magnitude in order to avoid deformations and stresses as much as possible.

[0032] In one embodiment, the projecting sections of the support elements bridge a gap between a substrate of the mirror element and the mount. The projecting sections of the support elements bridge a gap between the inner edge of the mount and the outer edge of the mirror element, more precisely a respective long side or wide side of the substrate of the mirror element. As described above, the support elements or their projecting sections serve to elastically mount the mirror element on the mount. The stiffness of the projecting sections can be selected such that the natural frequencies of the optical assembly are within a desired range. High natural frequencies are generally advantageous from a dynamic perspective; however, in certain situations, it may be advantageous if the natural frequencies are not selected too high, e.g.if the optical assembly is a spare part that needs to be transported.

[0033] In a further embodiment, the mirror element, or more precisely the substrate of the mirror element, is attached to the fastening surfaces of the support elements by means of a joining agent, in particular an adhesive. As described above, the mirror element is typically attached to the support elements via a material-to-material connection.

[0034] In a further embodiment, the mirror element has a substrate made of a zero-expansion material. As described above, a zero-expansion material is understood to be a material that has a so-called zero-crossing temperature, at which the linear thermal expansion coefficient of the substrate is at a minimum. The zero-expansion material can be, for example, titanium-doped quartz glass or a glass ceramic. Alternatively, the substrate of the mirror element can be formed not from a zero-expansion material, but from another material, for example, quartz glass or silicon.

[0035] A further aspect of the invention relates to an optical system, in particular an EUV lithography system, comprising: at least one optical assembly as described above. Mirror elements with a high aspect ratio can be used, for example, in the illumination system of an optical system in the form of an EUV lithography system. It is also possible for the optical assembly to be used in an optical system only for measuring purposes, for example in a reflectometer for measuring the reflectivity of a mirror, e.g. an EUV mirror. As described above, the use of the optical assembly is not limited to EUV lithography systems; rather, the optical assembly can also be used in other optical systems, for example in synchrotron optics.

[0036] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which illustrate details essential to the invention, and from the claims. The individual features can be implemented individually or in combination in a variant of the invention.

[0037] drawing

[0038] Examples of embodiments are shown in the schematic drawing and are explained in the following description.

[0039] Fig. 1 shows a meridional section of a projection exposure system for EUV projection lithography,

[0040] Fig. 2a is a schematic representation of a mirror element in the form of a vertical focusing mirror, and

[0041] Fig. 2b is a schematic representation of an optical assembly with the mirror element of Fig. 2a and with a holding device which has a mount and a plurality of holding elements for laterally clamping the mirror element.

[0042] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0043] The following describes, by way of example, the essential components of an optical arrangement for EUV lithography in the form of a projection exposure system 1 for microlithography with reference to Fig. 1. The description of the basic structure of the projection exposure system 1 and its components is not to be understood as restrictive. One embodiment of an illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, an illumination optics 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 module separate from the rest of the illumination system. In this case, the illumination system does not comprise the light source 3.

[0044] A reticle 7 arranged in the object field 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be displaced, in particular in a scanning direction, via a reticle displacement drive 9.

[0045] For illustrative purposes, a Cartesian xyz coordinate system is shown in Fig. 1. The x-direction runs perpendicular to the drawing plane. The y-direction runs horizontally, and the z-direction runs vertically. The scanning direction in Fig. 1 runs along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0046] The projection exposure system 1 comprises a projection system 10. The projection system 10 is used to image the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, in particular along the y-direction, via a wafer displacement drive 15. The displacement of the reticle 7, on the one hand, via the reticle displacement drive 9, and the displacement of the wafer 13, on the other hand, via the wafer displacement drive 15, can be synchronized with each other.

[0047] Radiation source 3 is an EUV radiation source.

[0048] Radiation source 3 emits, in particular, EUV radiation 16, which is also referred to below as useful radiation, illumination radiation, or illumination light. The useful radiation has, in particular, a wavelength in the range between 5 nm and 30 nm. Radiation source 3 can be a plasma source, for example, an LPP source (laser produced plasma) or a DPP source (gas discharged produced plasma). It can also be a synchrotron-based radiation source. Radiation source 3 can be a free-electron laser (FEL).

[0049] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 can be a collector mirror with one or more ellipsoidal and / or hyperboloidal reflection surfaces. The at least one reflection surface of the collector mirror 17 can be exposed to the illumination radiation 16 at grazing incidence (Gl), i.e., at angles of incidence greater than 45°, or at normal incidence (NI), i.e., at angles of incidence less than 45°. The collector mirror 17 can be structured and / or coated, on the one hand, to optimize its reflectivity for the useful radiation and, on the other hand, to suppress stray light.

[0050] After the collector mirror 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector mirror 17, and the illumination optics 4.

[0051] The illumination optics 4 comprises a deflecting mirror 19 and, downstream of this in the beam path, a first facet mirror 20. The deflecting mirror 19 can be a flat deflecting mirror or, alternatively, a mirror with a beam-influencing effect beyond the pure deflection effect. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter that separates a useful light wavelength of the illumination radiation 16 from stray light of a different wavelength. The first facet mirror 20 comprises a plurality of individual first facets 21, which are also referred to below as field facets. Only a few of these facets 21 are shown in Fig. 1 as examples. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optics 4. The second facet mirror 22 comprises a plurality of second facets 23.

[0052] The illumination optics 4 thus form a double-faceted system. This basic principle is also referred to as a fly's-eye integrator. With the help of the second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-forming mirror, or indeed the last mirror for the illumination radiation 16 in the beam path before the object field 5.

[0053] The projection system 10 comprises a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the projection exposure system 1.

[0054] In the example shown in Fig. 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or a different number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection system 10 is a doubly obscured optics system. The projection optics 10 has an image-side numerical aperture that is greater than 0.4 or 0.5 and can also be greater than 0.6 and can be, for example, 0.7 or 0.75. The mirrors Mi, just like the mirrors of the illumination optics 4, can have a highly reflective coating for the illumination radiation 16.

[0055] Fig. 2a shows a mirror element 19 in the form of the deflecting mirror of the illumination optics 4 of Fig. 1, which in this case has an additional bundle-influencing effect. The mirror element 19 has a substrate 25 with a concavely curved surface 26, to which a reflective coating (not shown) for reflecting EUV radiation 16 is applied. In the example shown in Fig. 2a, the substrate 25 is cuboid-shaped and has a length L of approximately 60 cm, a width B of approximately 13 cm and a height H of approximately 7 cm. The ratio of length L to width B of the substrate 25 of the mirror element 19 is thus more than 4:1, i.e. the mirror element 19 has a large aspect ratio.

[0056] The substrate 25 is formed from a zero-expansion material, i.e., a material with a very low linear thermal expansion coefficient that reaches a minimum at a temperature, the so-called zero-crossing temperature. The substrate 25 can be, for example, titanium-doped quartz glass or a glass ceramic. The substrate 25 has a linear thermal expansion coefficient O2 that is less than approximately 0.6 ppm / K in the zero-crossing temperature range.

[0057] Fig. 2b shows an optical assembly 27 comprising the mirror element 19 of Fig. 2a and a mounting device 28 for the mirror element 19. The mounting device 28 has a mount 29 in the form of a rectangular frame, which in the example shown is made of Invar. The mount 29 can also be made of a different material that has the smallest possible linear thermal expansion coefficient ai, which is 2 ppm / K or less, in the example shown approximately 1 ppm / K or less. The ratio ai / O2 between the linear thermal expansion coefficient ai of the material of the mount 29 and the linear thermal expansion coefficient O2 of the material of the substrate 25 is approximately 1.66 in the example shown.

[0058] A plurality of support elements 30 are attached to the mount 29 and surround the mirror element 19. The support elements 30 have a support section 31 in the form of a holding block and a substantially triangular section 32 that projects beyond the mount 29 toward the mirror element 19. The support elements 30 are attached to the top of the mount 29 by the underside of the support section 31. In the example shown, the attachment is achieved using three screws 33a-c, which are arranged at equal distances from one another. The projecting sections 32 of the support elements 30 bridge a gap 34 between the mount 29 and the mirror element 19.

[0059] On the end face of each projecting section 32, a fastening surface 35 is provided for fastening the mirror element 19 to the mount 29 of the mounting device 28. The mirror element 19, or more precisely the substrate 25, is firmly bonded to the fastening surface 35 of each mounting element 30 by means of a joining agent in the form of an adhesive. The fastening surface 35 may optionally also have a section that serves as a support for the substrate 25 of the mirror element 19.

[0060] In the example shown, the support elements 30 are formed in one piece and consist of a material with a high fracture strength or tensile strength of more than 800 MPa. In the example shown, the support elements 39 are made of tool steel or stainless steel in the form of X 17 CrNi 16-2. It is understood that the support elements 30 can also be made of other materials with high tensile strength, in particular from other tool steels, e.g., from Stavax ESR, which has a tensile strength of more than 1000 MPa.

[0061] The projecting section 32 acts in the manner of a spring element for elastically supporting the mirror element 19. For the resilient support, the projecting section 32 can, for example, have a partial area in which it has a smaller thickness than the remaining projecting section 32.

[0062] As can be seen in Fig. 2b, fastening surfaces 35 of support elements 30, which are mounted along a long side 36 of the mirror element 19, are arranged at equal distances A from one another. Furthermore, fastening surfaces 35 of support elements 30, which are mounted along a wide side 37 of the mirror element 19, are arranged at equal distances A from one another. The support elements 30 themselves are also distributed evenly or equidistantly over the circumference of the mirror element 19. In this way, the force absorption is distributed as evenly as possible across all support elements 30.

[0063] When inserting the substrate 25 of the mirror element 19 into the mount 29, the support elements 30, or more precisely, their sections 32 projecting beyond the mount 29, are retracted to enable contact-free insertion of the substrate 25 into the mount 29. The retraction of the support elements 30 or the projecting, resilient sections 32 is limited by the permissible stresses of the material of the support elements 30.

[0064] In the mounting device 28 shown in Fig. 2a, b, a functional separation is therefore carried out in such a way that the mount 29 is made of a material with a linear thermal expansion coefficient ai that is as equal as possible to the linear thermal expansion coefficient θ2 of the substrate 25 in order to reduce thermal stresses, while the mounting elements 30 are made of a material with a high breaking or tensile strength for force absorption or for retracting the mounting elements 30 during the initial clamping of the substrate 25 in the mount 29. This makes it possible to reduce deformations and stresses when mounting the mirror element 19.

[0065] The optical assembly 27 described above can also be used in optical systems other than in an EUV lithography system 1, for example in a synchrotron optics or in a reflectometer for measuring the reflectivity of EUV mirrors or their coatings.

Claims

Patent claims 1. A mounting device (28) for a mirror element, in particular for a mirror element (19) for reflecting EUV radiation (16), which has a length (L) to width (B) ratio of more than 2:1, preferably of more than 3:1, particularly preferably of more than 4:1, in particular of more than 10:1, wherein the mounting device (28) has a mount (29) with a plurality of mounting elements (30) for laterally clamping the mirror element (19), characterized in that the mounting elements (30) have projecting, resilient sections (32) for resiliently mounting the mirror element (19).

2. Mounting device according to claim 1, wherein the socket (29) is formed from a material having a linear thermal expansion coefficient (cn) of 2 ppm / K or less, preferably 1 ppm / K or less.

3. Mounting device according to claim 1 or 2, wherein the socket (29) is made of Invar.

4. Mounting device according to one of the preceding claims, wherein the mounting elements (30) are attached to the socket (29).

5. Mounting device according to one of the preceding claims, in which a respective projecting portion (32) of a mounting element (30) has a fastening surface (35) for fastening the mirror element (19).

6. Mounting device according to claim 5, in which fastening surfaces (35) of mounting elements (30) which are arranged along a longitudinal side (36) of the Mirror element (19) are arranged at equal distances (A) from one another, and / or that fastening surfaces (35) of holding elements (30) which are arranged along a broad side (37) of the mirror element (19) are arranged at equal distances (A) from one another.

7. Holding device according to one of the preceding claims, in which the holding elements (30) are formed at least in the projecting section (32), in particular completely, from a material with a tensile strength of more than 800 MPa.

8. Holding device according to one of the preceding claims, in which the holding elements (30) are formed at least in the projecting section (32), in particular completely, from a tool steel.

9. Optical assembly (27), comprising: a mirror element, in particular a mirror element (19) for reflecting EUV radiation (16), which has a ratio of length (L) to width (B) that is greater than 2:1, preferably greater than 3:1, particularly preferably greater than 4:1, in particular greater than 10:1, and a holding device (28) for holding the mirror element (19), which is designed according to one of the preceding claims.

10. Optical assembly according to claim 9, wherein the ratio between a linear thermal expansion coefficient (ai ) of the material of the mount (29) and a linear thermal expansion coefficient (02) of the material of a substrate (25) of the mirror element (19) is between 0.5 and 2, preferably between 0.8 and 1.

25.

11. Optical assembly according to claim 9 or 10, wherein the projecting portions (32) of the support elements (30) form a gap (34) bridge between the substrate (25) of the mirror element (19) and the socket (3).

12. Optical assembly according to one of claims 9 to 11, wherein the substrate (25) of the mirror element (19) is fastened to the fastening surfaces (35) of the holding elements (13) by means of a joining agent, in particular by means of an adhesive.

13. Optical assembly according to one of claims 9 to 12, wherein the mirror element (19) comprises a substrate (25) made of a zero-expansion material.

14. Optical system, in particular EUV lithography system (1 ), comprising: at least one optical assembly (27) according to one of claims 9 to

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