Optical Assembly

Abstract

The present invention relates to an optical system assembly including at least one mirror having a mirror body (200, 300) in which is present a fluid channel device having at least one fluid channel (206, 306) through which a fluid can flow, the fluid channel device being connected to a fluid line system by a removable flange connection, the flange connection including a flange interface (202, 302) formed on the mirror body and a flange (202, 303) mounted on the flange interface (202, 302) in a force-locking manner, and a seal (207, 307) formed between the flange (202, 303) and the flange interface (202, 303) to achieve a differential vacuum.

Description

[Technical Field]

[0001] This application claims the priority of German patent application DE102022210087.5, filed on September 23, 2022, the contents of which are incorporated herein by reference.

[0002] The present invention relates to an assembly for an optical system, in particular the optical system may be a beam guiding unit ("beamline") of a synchrotron or a free electron laser. [Background technology]

[0003] For optical applications in the EUV region (e.g., wavelengths below 30 nm) or in the X-ray region (e.g., wavelengths below 0.1 nm), mirrors are used as optical components because suitable optically transparent refractive materials are not available. Examples include synchrotron mirrors and mirrors used in illumination devices or projection lenses of microlithography projection exposure apparatus.

[0004] A practical problem that arises is that such mirrors undergo heating and accompanying thermal expansion or deformation, particularly as a result of absorption of incident radiation, and the temperature profiles generated in the mirror substrate or on the optically active surface in the process can consequently have significant non-uniformities over the optically used area—especially in the case of relatively strong, localized heat input of incident electromagnetic radiation—so that the thermally induced deformation profiles resulting from the respective temperature profiles give rise to optical aberrations that may be impossible or simply difficult to correct during operation of the respective optical system.

[0005] In particular, this may be the case for synchrotron mirrors, where during operation of the synchrotron the current heat affected zone, which corresponds to the area currently in optical use, is typically relatively small with respect to the entire mirror surface and further varies locally during operation, which may ultimately result in impairment of the imaging properties of the optical system including the respective mirror.

[0006] Various techniques are known that help to prevent surface deformations due to heat input and the associated optical aberrations, for example active cooling, which in each case uses fluid channels through which a (cooling) fluid can flow.

[0007] However, in practice, depending on the implementation of the connection between the mirror and the cooling fluid supply system, further undesirable effects may arise as a result of mechanical, thermal, and / or dynamic loads, limiting the performance and service life of the optical system. For example, in the case of a leak, a potentially complex repair process or even the replacement of the entire mirror may be required, resulting in considerable costs and even a reduction in the achievable throughput due to the necessary operational downtime. Furthermore, in the case of a leak, there is also a risk of contamination of the optical system (which is usually operated under vacuum conditions) by the leaking cooling fluid. Furthermore, the connection of the fluid channel device and its operation may cause parasitic forces and accompanying deformations of the mirror surface, which cause optical aberrations.

[0008] With respect to the prior art, reference is made, by way of example only, to U.S. Pat. No. 10,955,595 and U.S. Patent Application Publication No. 2015 / 0083938. Summary of the Invention [Problem to be solved by the invention]

[0009] The problem addressed by the present invention is to provide an assembly for an optical system, said optical system allowing effective avoidance of thermally induced deformations while mitigating the above mentioned problems. [Means for solving the problem]

[0010] This problem is solved by the features of independent claim 1.

[0011] - at least one mirror comprising a mirror body having a fluid channel device extending therethrough, the fluid channel device having at least one fluid channel through which a fluid can flow; An assembly according to the invention for an optical system, comprising: - the fluid channel device is connected to a fluid line system by a releasable flange connection; the flange connection includes a flange interface formed on the mirror body and a flange frictionally mounted on the flange interface; a seal is formed between the flange and the flange interface to establish a differential vacuum; An assembly according to the invention for an optical system.

[0012] In particular, within the assembly of the optical system, the present invention is based on the concept of connecting a fluid channel device to a fluid line system, in principle, by a releasable flange connection without integral joints (specifically, "solder-free"), which helps to avoid or reduce thermally induced deformations through which fluids can flow and which are found within the mirror body of the mirror. In this case, a relatively soft interface ("separation layer") along the joining area is avoided by somehow achieving this without the implementation of integral joints (specifically, without the application of soldering techniques), thereby avoiding the problems associated with such separation layers, both in terms of thermal and strength. Furthermore, as a result of the releasable connection, the corresponding components can be easily replaced in the event of a leak. Since the flanges can be easily unscrewed and rescrewed (e.g., after cleaning and optional insertion of a new seal) if necessary, maintenance work is also greatly simplified in the assembly according to the present invention, and the optical system can then be quickly returned to operation.

[0013] According to the invention, firstly, with regard to the increased installation space required to realize a screw connection compared to a soldered connection, and secondly, but also with regard to the problem of introducing undesirable mechanical stresses into the mirror, disadvantages in this respect are intentionally accepted. In principle, the connection according to the invention by means of a flange leads to increased acting forces, inter alia, as a result of the attachment of the flange, any corresponding mass such as a threaded plate, and therefore to an increased risk of fractures occurring in the mirror body and of deformations occurring.

[0014] However, according to the present invention, these drawbacks are intentionally accepted, firstly, in order to achieve the aforementioned advantages of a solderless connection, and secondly, due to the consideration that the above-mentioned parasitic forces and accompanying deformations can be significantly reduced or minimized by appropriate embodiments of the flange connection and in particular of the flange interface formed on the mirror body.

[0015] In accordance with the present invention, a seal is formed between the flange and the flange interface to provide a differential vacuum.

[0016] For the purposes of the present application, the term "differential vacuum" should be understood in this case to mean a vacuum that is located between a first vacuum and a second vacuum, the vacuum pressure of said differential vacuum having a value between the vacuum pressures present in the first vacuum and the vacuum pressures present in the second vacuum. In this context, the second vacuum may be present in the fluid channel device, and the first vacuum may be present in the external environment of the mirror.

[0017] Quantitatively, in detail, the vacuum pressure present in the first vacuum or in the external environment of the mirror is (10 -9 ~10 -12 ) mbar, the vacuum pressure present in the second vacuum, i.e. in the fluid channel device, may be in the range of (0.1-10) bar, and the vacuum pressure present in the differential vacuum may be in the range of (10 -3 ~10 -4 ) mbar.

[0018] The differential vacuum (sometimes also called "support vacuum") allows the tightness to be monitored within the assembly according to the invention. Any leaks can be detected in real time as a result of the permanent monitoring of the differential vacuum. The associated system or machine can be stopped as soon as contamination of this vacuum is detected. In particular, this is the case when the mirror is used as a synchrotron mirror (10 -10 mbar~10 -12 The fact that it is operated in a UHV environment (in the range of 10 mbar) and the hermeticity requirements are more stringent than for example EUV applications (10 -9 mbar).

[0019] According to one embodiment, the seal is a seal according to the double O-ring principle. Within the meaning of the present application, the expression "seal according to the double O-ring principle" should also include seals in which at least one of the two sealing rings or O-rings is replaced by a molded seal. Furthermore, the term "O-ring" within the meaning of the present application should also include seals with a polygonal cross-sectional profile.

[0020] According to one embodiment, a monitoring device is provided to monitor the tightness of the flange connection for fluid leakage during operation of the optical system.

[0021] The present invention also provides - at least one mirror comprising a mirror body having a fluid channel device extending therethrough, the fluid channel device having at least one fluid channel through which a fluid can flow; 1. An assembly for an optical system, comprising: - the fluid channel device is connected to a fluid line system by a releasable flange connection; the flange connection includes a flange interface formed on the mirror body and a flange frictionally mounted on the flange interface; - a monitoring device is provided to monitor the tightness of the flange connections with regard to fluid leakage during the operation of the optical system; assemblies for optical systems.

[0022] According to one embodiment, the monitoring device is configured to provide monitoring of the tightness of the flange connection without affecting the external vacuum environment of the mirror.

[0023] According to one embodiment, the monitoring device is configured to detect fluid entering the area between the flange and the flange interface.

[0024] According to one embodiment, the monitoring device includes a mass spectrometer, a gas detector, or a humidity sensor.

[0025] According to one embodiment, the assembly comprises, in addition to the flange connection, at least one further connection point, and a monitoring device is also provided for monitoring the tightness of this further connection point with regard to leaking fluid.

[0026] According to one embodiment, the flange interface takes the form of at least one decoupling structure that serves to reduce the transmission of forces from the flange to the mirror body.

[0027] According to one embodiment, the decoupling structure is formed by a tapered portion of the flange interface.

[0028] According to one embodiment, the fluid channel extends axially within the tapered section, and the ratio between the axial extent of the tapered section and the wall thickness of the tapered section remaining towards the fluid channel is in the range of 1-5.

[0029] According to one embodiment, the flange is attached on the flange interface by means of a threaded connection.

[0030] According to one embodiment, the threaded connection is implemented along a portion of the flange interface, for which the ratio of its extent in the direction perpendicular to the thread direction to its extent in the thread direction is in the range of 2 to 10, in particular in the range of 2 to 4.

[0031] According to one embodiment, the mirror comprises a cover plate bonded to the mirror body and on which a reflective layer system is formed, in this context the expression "reflective layer system" should include both single-layer systems and multi-layer systems.

[0032] According to one embodiment, the cover plate is monolithically connected to the mirror body by direct bonding or fusion bonding. These bonding processes can be accomplished without auxiliary materials and may result in a monolithic component with optimized mechanical properties due to the lack of a physical separating joint.

[0033] According to one embodiment, the mirror body is manufactured from a silicon-containing material, in particular from a group comprising monocrystalline silicon (Si), silicon dioxide (SiO2), and titanium dioxide-doped quartz glass.

[0034] According to one embodiment, the mirror is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

[0035] According to one embodiment, the mirror is designed for an operating wavelength of less than 0.1 nm.

[0036] According to one embodiment, the optical system is a beam guiding unit ("beamline") of a synchrotron or a free electron laser.

[0037] The invention also relates to an optical system having an assembly with the above-mentioned features.

[0038] According to one embodiment, the optical system is a synchrotron.

[0039] According to a further embodiment, the optical system is a projection lens or an illumination device of a microlithographic projection exposure apparatus.

[0040] Further embodiments of the invention are evident from the description and the dependent claims.

[0041] The invention will be elucidated in more detail below with reference to exemplary embodiments illustrated in the accompanying drawings. [Brief explanation of the drawings]

[0042] [Figure 1] 1 is a schematic diagram for illustrating a possible application of an assembly according to the invention in a synchrotron; [Figure 2] 1A-1C are schematic diagrams illustrating possible embodiments of an assembly according to the invention. [Figure 3a] 10A-10C are schematic diagrams for explaining further possible embodiments of the assembly according to the invention; [Figure 3b] 10A-10C are schematic diagrams for explaining further possible embodiments of the assembly according to the invention; [Figure 4] 1 is a schematic diagram of a possible architecture of a microlithography projection exposure apparatus designed for operation in EUV; DETAILED DESCRIPTION OF THE INVENTION

[0043] According to the present invention, a fluid channel device, which helps to avoid thermally induced deformations within the optical system assembly, allows cooling fluid to flow through, and is found within the mirror body of the mirror, is connected to the fluid line system by a releasable flange connection without integral joints (particularly in a "solder-free" manner).

[0044] The mirrors present in the assembly according to the invention can be used, for example, as deflection mirrors or beam-guiding optics in a synchrotron, as shown only diagrammatically in FIG. 1 for mirror 100. According to FIG. 1, in such a synchrotron, electromagnetic radiation 150 (in the form of a diverging X-ray beam in this example) generated by the acceleration or deflection of an electron beam 170 is incident on mirror 100. In particular, mirror 100 can also be arranged in what is known as a "beamline" on which the electromagnetic radiation generated in the synchrotron is incident. In this case, the elliptical "footprint" generated on mirror 100 in this example is designated "101" as the optically used area, and the electromagnetic radiation (in the form of a converging X-ray beam in this example) emerging from mirror 100 after reflection is designated "160". In this case, the optically used area or "footprint" in the illustrated situation is a relatively intense local area on the mirror surface, although its current position can "move" or change on the mirror surface. The assembly according to the invention particularly serves to prevent or reduce thermally induced deformations due to electromagnetic radiation.

[0045] Figure 2 shows a purely schematic diagram for illustrating a possible embodiment of an assembly according to the invention. According to Figure 2, the assembly comprises at least one mirror with a mirror body 200. Furthermore, the mirror comprises a cover plate 200a bonded to the mirror body 200 and on which a reflective layer system (not shown here) is formed. Depending on the particular application, the mirror may be a flat mirror or may have any other (e.g. spherical or cylindrical) geometric shape.

[0046] In a particular exemplary embodiment, the mirror body 200 is fabricated from monocrystalline silicon (Si), and the reflective layer system in this example includes a single layer of gold (Au) with an exemplary thickness in the range of 20 nm to 50 nm. In further embodiments, the reflective layer may also be fabricated from another noble metal, such as platinum (Pt), rhodium (Rh), silver (Ag), ruthenium (Ru), palladium (Pd), osmium (Os), or iridium (Ir). Additionally, the reflective layer may also be fabricated from an organic material, such as carbon (C), boron carbide (B4C), or silicon carbide (SiC).

[0047] Depending on the intended application, the mirror may be a mirror designed for operation under grazing incidence or a mirror designed for operation under normal incidence. As a reflective layer system, the mirror in the latter case usually comprises a multilayer system, for example made of at least two different layer materials, e.g., an alternating sequence of individual layers. Furthermore, in further embodiments, the mirror body 200 or mirror substrate can also be manufactured from another substrate material, for example, a substrate material that also contains silicon, such as silicon dioxide (SiO2) or Zerodur® (by Schott AG). Furthermore, depending on the intended application, titanium silicate glass sold by Corning Inc. under the trademark ULE® can also be used as a substrate material.

[0048] Without appropriate countermeasures, electromagnetic radiation incident on the mirror during operation causes undesirable temperature or deformation profiles (possibly with large gradients in the optically used region) and associated optical aberrations in the optical system containing the mirror. To counter such undesirable temperature or deformation profiles, as shown in FIG. 2 , a fluid channel device 205 having at least one fluid channel 206 through which a fluid (e.g., water) can flow extends through the mirror body 200. The fluid channel device 205 is connected to the fluid line system by a releasable flange connection, which includes a flange interface 202 formed on the mirror body 200 and a flange 203 frictionally mounted on the flange interface 202. For example, the flange 203 can be made of stainless steel. In this exemplary embodiment, the releasable flange connection is realized as a screw connection (but the invention is not limited thereto), and for this purpose, a screw indicated as "204" in FIG. 2 is fixed to a threaded plate indicated as "208" (and also made of stainless steel in this embodiment).

[0049] As a result of the realization according to the invention of the required connection between the fluid channel device extending in the mirror body 200 and the external fluid line system by means of a releasable flange connection using frictional attachment of the flange 203 on the flange interface 202, firstly, as already mentioned at the outset, problems associated with solder connections and occurring leaks are avoided with regard to the thermal connection, and secondly, maintenance operations are also facilitated, in the event of a leak, a simple replacement of the affected releasably connected component can furthermore be carried out.

[0050] However, according to the present invention—also as initially described—there is an acceptance of the problems associated with the attachment of additional mass due to flange 203, flange interface 202 and threaded plate 208, which are also associated with the generation of additional (e.g., screw or clamp) forces to create a clamping connection. These forces, in addition to the forces due to the fluid flow in the fluid channel device, act on the mirror as disturbance or parasitic forces and, in the absence of appropriate countermeasures, cause undesired deformation of the optically active surface.

[0051] In order to reduce or minimize the influence of the generation of additional parasitic forces on the mirror deformation, which is accepted according to the invention, there is preferably according to the invention a targeted embodiment of the flange connection according to the invention, using a suitable design or optimization of the parameters as described below.

[0052] In this case, it is considered below (without the invention being limited thereto) that the flange 203 is mounted on the flange interface 202 by means of a screw connection. In further embodiments, the frictional mounting of the flange 203 on the flange interface 202 can also be achieved in another suitable way, in particular, for example, by means of a clamp connection.

[0053] First, with regard to reducing or minimizing deformations due to parasitic forces generated by the screw connection, the flange interface 202 in the embodiment of FIG. 2 is embodied in a region for the formation of a decoupling structure (or decoupling joint) 201. The decoupling structure 201 is here formed by a tapered portion of the flange interface 202. According to FIG. 2, the decoupling structure is implemented by "breaking off" the flange interface 202 in the corresponding region 209 facing the mirror body 200, thanks to the remaining wall thickness (h1-d) / 2 towards the fluid channel 206, which is reduced compared to the remaining non-tapered portion of the flange interface 202 facing the flange 203. As a result of the thin walls in region 209, deformations or lateral contractions that occur when the flange 203 is screwed in are reduced in terms of their impact on the optically active surface of the mirror.

[0054] At the same time, the stiffness of the flange interface 202 in the "axial" direction (i.e., in the x-direction of the indicated coordinate system) is increased by appropriate selection of a relatively large dimension t2. Furthermore, this stiffness, which is mainly related to the influence of the prestressing forces of the screw connection, is also increased by increasing the dimension h2 (whereby the parameter h2 has less influence in this respect than the parameter t2). The ratio h2 / t2 may be optimized according to the specific application circumstances and is preferably in the range of 2 to 10, more preferably in the range of 2 to 4.

[0055] In the exemplary embodiment of FIG. 2, the threaded connection is made along a portion of the flange interface 202, for which the ratio of its extent in the direction perpendicular to the thread direction to the extent in the thread direction is preferably in the range of 2 to 10 (but the invention is not limited thereto), in particular in the range of 2 to 4.

[0056] The above-mentioned decoupling effect of the decoupling structure 201 in the x-direction of the indicated coordinate system can be further achieved by appropriately increasing the dimension t1 in order to affect the bending stiffness of the decoupling structure 201, although this parameter has a relatively smaller influence in this respect than the wall thickness (h1-d) / 2. The ratio t1 / [(h1-d) / 2] may be optimized depending on the specific application and is preferably within the range of 1 to 5. In other words, according to the exemplary embodiment of FIG. 2, the decoupling structure 201 is formed by a tapered portion of the flange interface 202, and the ratio between the axial extent of this tapered portion and the wall thickness of the tapered portion remaining toward the fluid channel is preferably within the range of 1 to 5 (but the invention is not limited thereto). In this case, the risk of fracture increases with an increase in the value of this ratio t1 / [(h1-d) / 2], while the stiffness, and therefore the pressure of parasitic deformations on the optically active surface, increases with a decrease in the value of this ratio.

[0057] Furthermore, with regard to the reduction or minimization of deformations due to unavoidable forces in the fluid channel device due to the fluid flowing therethrough, this reduction or minimization can also be carried out firstly by an appropriate reduction of the wall thickness (h1-d) / 2 and the resulting introduced decoupling structures, and secondly, the stiffness of said decoupling structures 201 for a given force generated in the fluid channel device can be minimized by an appropriate increase of the parameter t1.

[0058] It should be noted that within the scope of the above-mentioned dimensioning of the flange connection according to the invention, and in particular of the flange interface, the stiffness of the decoupling structure 201 cannot be reduced to a minimum. Depending on the specific load spectrum (pressure load due to the fluid flowing in the fluid channel device, external forces acting as a result of, for example, a fluid line system, etc.) and the fracture stress of the materials used, the remaining wall thickness (h1-d) / 2 must still be sufficiently thick to ensure that the resulting mechanical stresses do not cause damage to the mirror.

[0059] A further characteristic of the friction flange connection according to the invention is the differential vacuum, which will be explained in more detail with reference to Figures 3a-3b and which, in an embodiment of the invention, exists within the entire assembly (including the mirror body, the cover plate, the flange and the fluid line system). This differential vacuum is supplied throughout the entire flange and is passed at the interface between the flange and the mirror. This is achieved by the intermediate space between the two O-rings of the seal 207 according to the double O-ring principle. The differential vacuum (= "support vacuum") serves to monitor the tightness within the assembly according to the invention or to monitor the tightness of the mirror. Any leaks can be detected in real time as a result of the permanent monitoring of the differential vacuum. The associated system or machine can be stopped as soon as contamination of this differential vacuum is detected. This is particularly useful when the mirror is used as a synchrotron mirror (10 -10 mbar~10 -12 The fact that it is operated in a UHV environment (in the range of 10 mbar) and the hermeticity requirements are more stringent than for example EUV applications (10-9 mbar).

[0060] 3a-3b show schematic diagrams for illustrating possible embodiments of an assembly according to the invention, where similar or substantially functionally identical components compared to FIG. 2 are designated here with reference numerals increased by "100."

[0061] 3a-3b, "312a" and "312b" designate drilled holes extending in the mirror body 300 and the flange 303, each extending to an intermediate space 313 located between two O-rings of the seal 307 (and substantially tubularly surrounding the fluid channel 306). In this case, the drilled hole 312a provided in the mirror body 300 extends to a groove 311 (formed with a rectangular cross section in this exemplary embodiment purely by way of example), which groove surrounds the fluid-receiving volume 310.

[0062] Quantitatively, the vacuum pressure present in the mirror's external environment (designated "350" in Figure 3a) is (10 -9 ~10 -12 ) mbar, the vacuum pressure present in the fluid volume 310 may be in the range of (0.1-10) bar, and the vacuum pressure present in the holes 312a, 312b, the intermediate space 313, and the groove 311 (i.e., in the differential vacuum) may be in the range of (10 -3 ~10 -4 ) mbar.

[0063] The result of the above-described embodiment is that, in the event of a leak, fluid leaking from fluid channel 306 or fluid volume 310 will first enter the region of the differential vacuum provided according to the invention—i.e., into groove 311 or intermediate space 313, depending on the location of the leak—before it can reach the (ultra-high) vacuum present in the mirror's external environment and be detected by monitoring device 320 connected to drilled hole 312b. Said monitoring device 320 may be embodied in any suitable way and may, for example, comprise a mass spectrometer, a gas detector, or a humidity sensor. In the event of corresponding detection of fluid entering one of the aforementioned regions of differential vacuum, the entire system or the fluid supply connected to fluid channel 306 can be shut down, and unwanted contamination of the (ultra-high) vacuum present in the mirror's external environment region is preferably avoided.

[0064] In an embodiment, the assembly according to the invention may also include at least one further connection point in addition to the flange connection, and the monitoring device then also serves to monitor the tightness of this / these further connection points with regard to leaking fluid.

[0065] Although the embodiments described above refer to synchrotron mirrors, the invention may also be implemented in other optical systems, in particular in illumination devices or projection lenses of, for example, microlithography projection exposure systems.

[0066] In this regard, FIG. 4 shows, in meridional section, a schematic representation of a possible structure of a microlithography projection exposure apparatus designed for operation in the EUV. According to FIG. 4, the projection exposure apparatus 1 comprises an illumination device 2 and a projection lens 10. An embodiment of the illumination device 2 of the projection exposure apparatus 1 comprises, in addition to a light source or radiation source 3, an illumination optical unit 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 may also be provided as a module separate from the rest of the illumination device. In this case, the illumination device does not include the light source 3. What is exposed here is a reticle 7 that is arranged in the object field 5. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable by a reticle displacement drive 9, in particular in the scanning direction. For illustrative purposes, a Cartesian xyz coordinate system is shown in FIG. 4. The x-direction extends perpendicularly to the plane of the drawing, the y-direction extends horizontally, and the z-direction extends vertically. The scanning direction extends in the y direction in Figure 4. The z direction extends perpendicular to the object plane 6.

[0067] The projection lens 10 serves to image the object field 5 into an image field 11 in an image plane 12. Structures on the reticle 7 are imaged onto a photosensitive layer of a wafer 13, which is 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 by a wafer displacement drive 15, in particular in the y direction. The displacements, firstly of the reticle 7 by the reticle displacement drive 9 and secondly of the wafer 13 by the wafer displacement drive 15, can be performed to be synchronized with each other.

[0068] The radiation source 3 is an EUV radiation source. The radiation source 3 in particular emits EUV radiation, which is also referred to below as used radiation or illumination radiation. In particular, the used radiation has a wavelength in the range of 5 nm to 30 nm. The radiation source 3 can be, for example, a plasma source, a synchrotron-based radiation source, or a free electron laser (FEL). The illumination radiation 16 emitted by the radiation source 3 is focused by a collector 17, propagates through an intermediate focus in an intermediate focal plane 18, and enters the illumination optical unit 4. The illumination optical unit 4 comprises a deflection mirror 19 and, arranged downstream in the beam path, a first facet mirror 20 (with a facet 21 shown diagrammatically) and a second facet mirror 22 (with a facet 23 shown diagrammatically).

[0069] The projection lens 10 comprises a number of mirrors Mi (i=1, 2, ...), which are consecutively numbered according to their position in the beam path of the projection exposure apparatus 1. In the embodiment shown in FIG. 4, the projection lens 10 comprises six mirrors M1 to M6. Alternatives using 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 through-opening for the illumination radiation 16. The projection lens 10 is a 2x obscured optical unit. The projection lens 10 has an image-side numerical aperture greater than 0.5, possibly greater than 0.6, e.g., 0.7 or 0.75.

[0070] During operation of the microlithography projection exposure apparatus 1, electromagnetic radiation incident on the optically active surface of the mirror is partially absorbed, which, as explained in the introduction, leads to heating and associated thermal expansion or deformation, which may consequently cause impairment of the imaging properties of the optical system. The concept according to the invention can therefore be advantageously applied to any desired mirror of the microlithography projection exposure apparatus 1 of FIG.

[0071] The invention can furthermore be advantageously implemented in projection exposure apparatus or else in other optical systems designed for operation in the DUV (i.e. at wavelengths below 250 nm, in particular below 200 nm).

[0072] While the present invention has also been described in terms of specific embodiments, numerous variations and alternative embodiments may be realized by those skilled in the art, for example, by combining and / or substituting the features of the individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are also encompassed by the present invention, the scope of which is limited only in terms of the appended claims and their equivalents.

Claims

1. At least one mirror comprising a mirror body (200, 300) through which extends a fluid channel device having at least one fluid channel (206, 306) through which a fluid can flow.

1. An assembly for an optical system, comprising: - the fluid channel device is connected to a fluid line system by a releasable flange connection; the flange connection includes a flange interface (202, 302) formed on the mirror body (200, 300) and a flange (203, 303) frictionally mounted on the flange interface (202, 302); A seal (207, 307) is formed between the flange (202, 303) and the flange interface (202, 302) to create a differential vacuum; Assembly for optical system.

2. Assembly according to claim 1, characterized in that the seal is a seal according to the double O-ring principle.

3. 3. An assembly according to claim 1 or 2, characterized in that a monitoring device (320) is provided for monitoring the tightness of the flange connection with respect to fluid leakage during operation of the optical system.

4. At least one mirror comprising a mirror body (200, 300) through which extends a fluid channel device having at least one fluid channel (206, 306) through which a fluid can flow.

1. An assembly for an optical system, comprising: - the fluid channel device is connected to a fluid line system by a releasable flange connection; the flange connection includes a flange interface (202, 302) formed on the mirror body (200, 300) and a flange (203, 303) frictionally mounted on the flange interface (202, 302); a monitoring device (320) is provided to monitor the tightness of the flange connection for fluid leakage during operation of the optical system; Assembly for optical system.

5. 5. An assembly according to claim 3 or 4, characterized in that the monitoring device (320) is configured to achieve the monitoring of the tightness of the flange connection without affecting the external vacuum environment of the mirror.

6. 6. The assembly according to claim 3, wherein the monitoring device (320) is configured to detect fluid entering the area between the flange (203, 303) and the flange interface (202, 302).

7. Assembly according to any of claims 4 to 6, characterized in that the monitoring device (320) comprises a mass spectrometer, a gas detector or a humidity sensor.

8. 8. An assembly according to any one of claims 3 to 7, characterized in that the assembly comprises, in addition to the flange connection, at least one further connection point, and the monitoring device (320) is also provided for monitoring the tightness of this further connection point with respect to leaking fluid.

9. 9. An assembly according to any one of claims 1 to 8, characterized in that the flange interface (202) is in the form of at least one decoupling structure (201, 301) that serves to reduce the transmission of forces from the flange (203, 303) to the mirror body (200, 300).

10. 10. An assembly according to claim 9, characterized in that the decoupling structure (201, 301) is formed by a tapered portion of the flange interface (202, 203).

11. 11. The assembly of claim 10, wherein the fluid channel (206, 306) extends axially within the tapered section, and wherein a ratio between the axial extent of the tapered section and the wall thickness of the tapered section remaining towards the fluid channel (206, 306) is in the range of 1 to 5.

12. Assembly according to any one of claims 1 to 11, characterized in that the flanges (203, 303) are mounted on the flange interfaces (202, 302) by means of a threaded connection.

13. Assembly according to claim 12, characterized in that the threaded connection is carried out along a portion of the flange interface (202, 302), for which the ratio of its extent in a direction perpendicular to the thread direction to its extent in the thread direction is in the range of 2 to 10, in particular in the range of 2 to 4.

14. Assembly according to any of the preceding claims, characterized in that the mirror comprises a cover plate (200a, 300a) bonded to the mirror body (200, 300) and on which a reflective layer system is formed.

15. 15. An assembly according to claim 14, characterized in that the cover plate (200a, 300a) is monolithically connected to the mirror body (200, 300) by direct bonding or fusion bonding.

16. The mirror body (200, 300) is made of a silicon-containing material, in particular, single crystal silicon (Si), silicon dioxide (SiO 2 Assembly according to any one of claims 1 to 15, characterized in that it is made from a material from the group comprising titanium dioxide-doped quartz glass.

17. Assembly according to any of the preceding claims, characterized in that the mirror is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

18. Assembly according to any of the preceding claims, characterized in that the mirror is designed for an operating wavelength of less than 0.1 nm.

19. Assembly according to any of the preceding claims, characterized in that the optical system is a beam guiding unit ("beamline") of a synchrotron or a free electron laser.

20. An optical system comprising an assembly according to any one of claims 1 to 19.

21. 21. The optical system of claim 20, wherein the optical system is a synchrotron.

22. 21. The optical system according to claim 20, characterized in that the optical system is a projection lens (10) or an illumination device (2) of a microlithography projection exposure apparatus (1).

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