Assembly of an optical system

Abstract

The invention relates to an assembly of an optical system, comprising at least one mirror with a mirror main body (200, 300), in which there is a fluid channel arrangement with at least one fluid channel (206, 306) through which a fluid can flow, wherein the fluid channel arrangement is coupled to a fluid line system via a detachable flange connection, wherein said flange connection comprises a flange interface (202, 302) formed on the mirror main body and a flange (203, 303) force-lockingly mounted on said flange interface (202, 302), wherein a seal (207, 307) is formed between the flange (203, 303) and the flange interface (202, 302) in order to provide a differential vacuum.

Description

[0001] Assembly of an optical system

[0002] The present application claims priority from German patent application DE 10 2022 210 087.5, filed on September 23, 2022. The content of this DE application is incorporated by reference into the present application text.

[0003] BACKGROUND OF THE INVENTION

[0004] Field of the invention

[0005] The invention relates to an assembly of an optical system. The optical system can, in particular, be a beamline of a synchrotron or a free-electron laser.

[0006] State of the art

[0007] For optical applications in the EUV range (e.g., wavelengths below 30 nm) or in the X-ray range (e.g., wavelengths below 0.1 nm), mirrors are used as optical components due to the lack of suitable transparent refractive materials. Examples include synchrotron mirrors and mirrors used in the illumination system or projection lens of a microlithographic projection exposure system.

[0008] A problem that arises in practice is that such mirrors experience heating and associated thermal expansion or deformation, partly due to the absorption of the incident radiation. The temperature profiles generated in the mirror substrate or at the optical effective surface can, particularly in the case of comparatively highly localized heat inputs from the incident electromagnetic radiation, exhibit pronounced inhomogeneity across the optically utilized area. Consequently, the thermally induced deformation profiles resulting from the respective temperature profiles cause optical aberrations during operation of the respective optical system that are difficult or impossible to correct.

[0009] This can be particularly true for a synchrotron mirror, where the current heat-affected zone, corresponding to the currently optically used area, is typically comparatively small relative to the entire mirror surface during operation and also varies locally during operation. This can result in an impairment of the imaging properties of the optical system comprising the respective mirror.

[0010] To avoid surface deformations caused by heat input and the associated optical aberrations, various approaches are known, for example active cooling using fluid channels through which a (cooling) fluid can flow.

[0011] In practice, however, depending on the implementation of the connection between the mirror and the cooling fluid supply system, further undesirable effects may occur due to mechanical, thermal, and / or dynamic stresses, which limit the performance and service life of the optical system. For example, leaks may require complex repair processes or even replacement of the entire mirror, which incurs considerable costs and, furthermore, impairs the achievable throughput due to the necessary operational interruptions. Furthermore, leaks also pose a risk of contamination of the optical system (typically operated under vacuum conditions) by escaping cooling fluid.Furthermore, the connection of the fluid channel arrangement and its operation can cause parasitic forces and associated deformations of the mirror surface, which in turn lead to optical aberrations.

[0012] The state of the art is only given as an example

[0013] US 10,955,595 B2 and US 2015 / 0083938 A1.

[0014] SUMMARY OF THE INVENTION

[0015] It is an object of the present invention to provide an assembly of an optical system which enables effective prevention of thermally induced deformations while mitigating the problems described above.

[0016] This object is achieved according to the features of independent claim 1.

[0017] An assembly of an optical system according to the invention comprises:

[0018] - at least one mirror with a mirror base body in which a fluid channel arrangement with at least one fluid channel through which a fluid can flow runs;

[0019] - wherein the fluid channel arrangement is coupled to a fluid line system via a detachable flange connection;

[0020] - wherein this flange connection comprises a flange interface formed on the mirror base body and a flange mounted force-fittingly on this flange interface;

[0021] - wherein a seal is formed between the flange and the flange interface to provide a differential vacuum.

[0022] The invention is based in particular on the concept of coupling, in an assembly of an optical system, a fluid channel arrangement in a mirror base body of a mirror, which serves to prevent or reduce thermally induced deformations and through which a fluid can flow, to a fluid line system without a material-to-material connection (in particular "solder-free") via a fundamentally detachable flange connection. By dispensing with the implementation of a material-to-material connection (in particular the use of a soldering technique), a comparatively soft interface ("separating layer") along the joining area is avoided, which in turn avoids the problems associated with such a separating layer, both from a thermal and strength perspective. Furthermore, if a leak occurs as a result of the detachable connection, the corresponding component can be easily replaced.Maintenance work is also considerably simplified with the assembly according to the invention, since the flange can simply be unscrewed and screwed back on if necessary (e.g. after cleaning and, if necessary, using a new seal) and then the optical system can be immediately put back into operation.

[0023] The invention deliberately accepts disadvantages, which include, on the one hand, the increased installation space required to implement the screw connection compared to a soldered connection, and, on the other hand, the problem of introducing undesirable mechanical stresses into the mirror. Fundamentally, the coupling according to the invention via a flange leads to an increase in the acting forces, among other things due to the attachment of corresponding masses for the flange, any threaded plates, etc., which in turn increases the risk of fractures in the mirror base body as well as the risk of deformation.

[0024] However, the acceptance of these disadvantages according to the invention is done deliberately, on the one hand, in order to achieve the previously described advantages of a solderless connection and, on the other hand, due to the consideration that the above-mentioned parasitic forces and the associated deformations can be significantly reduced or minimized by a suitable design of the flange connection and in particular of the flange interface formed on the mirror base body.

[0025] According to the invention, a seal is formed between the flange and the flange interface to provide a differential vacuum.

[0026] For the purposes of the present application, the term "differential vacuum" refers to a vacuum located between a first vacuum and a second vacuum, in which a vacuum pressure exists whose value lies between the vacuum pressure present in the first vacuum and the vacuum pressure present in the second vacuum. The second vacuum can be present in the fluid channel arrangement, and the first vacuum can be present in an external environment of the mirror.

[0027] Quantitatively, the vacuum pressure present in the first vacuum or in the outer environment of the mirror can be in the range of (10“ 9 -10 -12 ) mbar, the vacuum pressure in the second vacuum or in the fluid channel arrangement in the range of (0.1 -10) bar and the vacuum pressure in the differential vacuum in the range of (10' 3 -10' 4 ) mbar.

[0028] The differential vacuum (which can also be referred to as a "support vacuum") enables leakage control in the assembly according to the invention. Any leaks can be detected in real time by permanently monitoring the differential vacuum. As soon as contamination is detected in this vacuum, the associated system or machine can be shut down. This allows for the fact that the mirror, when used as a synchrotron mirror, is used in a UHV environment (in the range of 10 -1 ° mbar to 10 -12 mbar) and the sealing requirements are lower than those for EUV applications (10 -9 mbar) are even higher.

[0029] According to one embodiment, the seal is a seal based on the double O-ring principle. For the purposes of the present application, the term "seal based on the double O-ring principle" is intended to also encompass a seal in which at least one of the two sealing rings or O-rings is replaced by a molded seal. Furthermore, the term "O-ring" for the purposes of the present application is also intended to encompass seals with a square cross-sectional profile.

[0030] According to one embodiment, a monitoring device is provided for monitoring the tightness of the flange connection against escaping fluid during operation of the optical system.

[0031] The invention further relates to an assembly of an optical system, with

[0032] - at least one mirror with a mirror base body in which a fluid channel arrangement with at least one fluid channel through which a fluid can flow runs;

[0033] - wherein the fluid channel arrangement is coupled to a fluid line system via a detachable flange connection;

[0034] - wherein this flange connection comprises a flange interface formed on the mirror base body and a flange mounted on this flange interface in a force-locking manner; and

[0035] - wherein a monitoring device is provided for monitoring the tightness of the flange connection against escaping fluid during operation of the optical system.

[0036] According to one embodiment, the monitoring device is configured to realize the monitoring of a tightness of the flange connection without influencing an external vacuum environment of the mirror.

[0037] According to one embodiment, the monitoring device is configured to detect fluid entering a region between the flange and the flange interface.

[0038] According to one embodiment, the monitoring device comprises a mass spectrometer, a gas detector or a humidity sensor.

[0039] According to one embodiment, the assembly has at least one further joint in addition to the flange connection, wherein the monitoring device is further provided for monitoring the tightness of this further joint against escaping fluid.

[0040] According to one embodiment, the flange interface forms at least one decoupling structure for reducing force transmission from the flange to the mirror base body.

[0041] According to one embodiment, this decoupling structure is formed by a tapered section of the flange interface.

[0042] According to one embodiment, in this tapered section, the fluid channel runs along an axial direction, wherein the ratio between an axial extension of this tapered section and a wall thickness of the tapered section remaining towards the fluid channel is in the range of 1 to 5.

[0043] According to one embodiment, the flange is mounted to the flange interface via a screw connection.

[0044] According to one embodiment, this screw connection is made along a section of the flange interface, wherein for this section the ratio between its extension in a direction perpendicular to the screwing direction and the extension in the screwing direction is in the range of 2 to 10, in particular in the range of 2 to 4.

[0045] According to one embodiment, the mirror has a cover plate bonded to the mirror base body, on which a reflective layer system is formed. The term "reflective layer system" is intended to encompass both a single layer and a multi-layer system. According to one embodiment, the cover plate is monolithically bonded to the mirror base body via direct bonding or fusion bonding. These bonding processes eliminate the need for auxiliary materials, and the elimination of a physical joint results in a monolithic component with optimized mechanical properties.

[0046] According to one embodiment, the mirror base body is made of a silicon-containing material, in particular a material from the group containing monocrystalline silicon (Si), silicon dioxide (SiO2) and titanium dioxide-doped quartz glass.

[0047] According to one embodiment, the mirror is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

[0048] According to one embodiment, the mirror is designed for an operating wavelength of less than 0.1 nm.

[0049] According to one embodiment, the optical system is a beamline of a synchrotron or a free-electron laser.

[0050] The invention further relates to an optical system with an assembly having the features described above.

[0051] According to one embodiment, the optical system is a synchrotron.

[0052] According to a further embodiment, the optical system is a projection lens or an illumination device of a microlithographic projection exposure apparatus.

[0053] Further embodiments of the invention can be found in the description and the dependent claims. The invention is explained in more detail below with reference to exemplary embodiments illustrated in the accompanying drawings.

[0054] BRIEF DESCRIPTION OF THE DRAWINGS

[0055] They show:

[0056] Figure 1 is a schematic diagram to explain a possible

[0057] Application of an assembly according to the invention in a synchrotron;

[0058] Figure 2 is a schematic representation to explain a possible embodiment of an assembly according to the invention;

[0059] Figures 3a-3b are schematic representations to explain another possible embodiment of an assembly according to the invention; and

[0060] Figure 4 shows a schematic representation of the possible structure of a microlithographic projection exposure system designed for operation in the EUV.

[0061] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0062] According to the invention, in an assembly of an optical system, a fluid channel arrangement in a mirror base body of a mirror, which serves to prevent thermally induced deformations and through which a cooling fluid can flow, is coupled to a fluid line system without a material connection (in particular "solderless") via a detachable flange connection. The mirror present in an assembly according to the invention can be used, for example, as a deflecting mirror or beam-guiding optical component in a synchrotron, as is shown only schematically in Fig. 1 for a mirror 100. According to Fig. 1, in such a synchrotron, the electromagnetic radiation 150 (in the example in the form of a divergent X-ray beam) generated by the acceleration or deflection of an electron beam 170 strikes the mirror 100.The mirror 100 can in particular also be positioned in a so-called "beamline" onto which the electromagnetic radiation generated in the synchrotron strikes. In this example, an elliptical "footprint" generated on the mirror 100 is designated as the optically utilized region with "101", and the electromagnetic radiation emanating from the mirror 100 after reflection (in the example in the form of a convergent X-ray beam) is designated with "160". In the scenario shown, the optically utilized region or "footprint" is a comparatively highly localized region of the mirror surface, the current position of which can, however, "wander" or vary on the mirror surface. An assembly according to the invention serves in particular to prevent or reduce deformations thermally induced by electromagnetic radiation.

[0063] Fig. 2 shows a purely schematic representation to explain one possible embodiment of an assembly according to the invention. According to Fig. 2, the assembly comprises at least one mirror with a mirror base body 200. Furthermore, the mirror comprises a cover plate 200a bonded to the mirror base body 200, on which a reflective layer system (not shown) is formed. Depending on the specific application scenario, the mirror can be a plane mirror or have any other geometry (e.g., spherical or cylindrical).

[0064] In the specific embodiment, the mirror base body 200 is made of monocrystalline silicon (Si), and the reflective layer system in the example comprises 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 can also be made of another precious metal, for example platinum (Pt), rhodium (Rh), silver (Ag), ruthenium (Ru), palladium (Pd), osmium (Os), or iridium (Ir). Furthermore, the reflective layer can also be made of an organic material, for example carbon (C), boron carbide (B4C), or silicon carbide (SiC).

[0065] Depending on the intended use, the mirror can be a mirror designed for operation under grazing incidence or a mirror designed for operation under normal incidence (English: "normal incidence"). In the latter case, the mirror typically has a multi-layer system as a reflective layer system in the form of, for example, an alternating sequence of individual layers made of, for example, at least two different layer materials. Furthermore, in further embodiments, the mirror base body 200 or the mirror substrate can also be made of a different substrate material, e.g. one that also contains silicon, for example silicon dioxide (SiO2) or Zerodur® (from Schott AG). Furthermore, depending on the intended use, a titanium silicate glass marketed under the name ULE® by Corning Inc. can also be used as the substrate material.

[0066] Without suitable countermeasures, the electromagnetic radiation incident on the mirror during operation leads to an undesirable temperature or deformation profile (possibly with high gradients in the optically used region) and associated optical aberrations of the optical system comprising the mirror. In order to counteract such an undesirable temperature or deformation profile, a fluid channel arrangement 205 with at least one fluid channel 206 through which a fluid (e.g. water) can flow runs in the mirror base body 200, as indicated in Fig. 2. The fluid channel arrangement 205 is coupled to a fluid line system via a detachable flange connection, said flange connection comprising a flange interface 202 formed on the mirror base body 200 and a flange 203 mounted on this flange interface 202 in a force-fitting manner. The flange 203 can be made, for example, of stainless steel.In the exemplary embodiment (but without the invention being limited thereto), the detachable flange connection is realized as a screw connection, for which purpose screws designated “204” in Fig. 2 are fixed to threaded plates designated “208” (also made of stainless steel in the exemplary embodiment).

[0067] As a result of the inventive implementation of the required coupling between the fluid channel arrangement running in the mirror base body 200 and the external fluid line system via a detachable flange connection with force-locking mounting of the flange 203 on the flange interface 202, as already described above, the problems associated with a soldered connection regarding thermal connection and leaks are avoided, and maintenance work is also simplified. In the event of a leak, the relevant detachably coupled component can be easily replaced.

[0068] In return, however, the invention—as already described at the beginning—accepts problems associated with the addition of additional masses due to the flange 203, the flange interface 202, and the threaded plates 208, as well as with the generation of additional forces (e.g., screw or clamping) to establish the force-fitting coupling. These forces act as disturbing forces or parasitic forces on the mirror in addition to the forces caused by the fluid flow in the fluid channel arrangement and, without appropriate countermeasures, in turn result in undesirable deformations of the optical effective surface.

[0069] In order to reduce or minimize the influence of the creation of additional parasitic forces on the mirror deformations, which is accepted according to the invention, the flange connection according to the invention is preferably designed specifically with appropriate design or optimization of the parameters described below.

[0070] In the following (without the invention being limited thereto), it is assumed that the flange 203 is mounted to the flange interface 202 via a screw connection. In further embodiments, the force-fitting mounting of the flange 203 to the flange interface 202 can also be realized in another suitable manner, in particular via a clamp connection.

[0071] With regard to the reduction or minimization of deformations due to the parasitic forces generated by the screw connection, in the embodiment of Fig. 2 the flange interface 202 is designed in certain regions to form a decoupling structure (or a decoupling joint) 201. The decoupling structure 201 is formed here by a tapered section of the flange interface 202. This is achieved according to Fig. 2 in that the wall thickness or wall thickness (hi-d) / 2 remaining towards the fluid channel 206 is reduced by "cutting free" the flange interface 202 in the corresponding region 209 facing the mirror base body 200 in comparison to the remaining, non-tapered section of the flange interface 202, which faces the flange 203. As a result of the small wall thickness in the region 209, deformations orTransverse contractions are reduced in their influence on the optical effective surface of the mirror.

[0072] At the same time, the stiffness of the flange interface 202 in the "axial" direction (i.e., along the x-direction in the drawn-in coordinate system) is increased by appropriately selecting a comparatively large dimension t2. Additionally, this stiffness, which is primarily relevant for the effects of preload forces of the screw connection, can also be increased by increasing the dimension h2 (although the parameter h2 is less influential than the parameter t2). The ratio h2 / t2 can be optimized depending on the specific application scenario and is preferably in the range of 2 to 10, more preferably in the range of 2 to 4.

[0073] In the embodiment of Fig. 2, the screw connection thus takes place along a section of the flange interface 202, wherein preferably (but without the invention being limited thereto) for this section the ratio between its extension in a direction perpendicular to the screwing direction and the extension in the screwing direction is in the range of 2 to 10, in particular in the range of 2 to 4.

[0074] The aforementioned decoupling effect of the decoupling structure 201 along the x-direction in the drawn-in coordinate system can additionally be achieved by suitably increasing the dimension ti to influence the flexural rigidity of the decoupling structure 201, although this parameter has a comparatively smaller influence than the aforementioned wall thickness (hi-d) / 2. The ratio ti / [(hi-d) / 2] can be optimized depending on the specific application scenario and is preferably in the range from 1 to 5. In other words, according to the exemplary embodiment of Fig. 2, the decoupling structure 201 is formed by a tapered section of the flange interface 202, wherein preferably (but without the invention being limited thereto) the ratio between an axial extension of this tapered section and a wall thickness of the tapered section remaining towards the fluid channel is in the range from 1 to 5.With increasing value of this ratio ti / [(hi-d) / 2] the risk of breakage increases, whereas with decreasing value of this ratio the stiffness and thus the penetration of parasitic deformations onto the optical effective surface increases.

[0075] As for the reduction or minimization of deformations caused by unavoidable forces within the fluid channel arrangement due to the fluid flowing through it, this reduction or minimization can also be achieved, on the one hand, by appropriately reducing the wall thickness or wall thickness (hi-d) / 2 and the decoupling structure introduced thereby. On the other hand, the stiffness of said decoupling structure 201 for a given force generated in the fluid channel arrangement can be minimized by appropriately increasing the parameter ti.

[0076] When dimensioning the flange connection according to the invention, and in particular the flange interface, as described above, it should be noted that the rigidity of the decoupling structure 201 cannot be reduced to an absolute minimum. Depending on the specific load spectrum (pressure load from the fluid flowing in the fluid channel arrangement, acting external forces, e.g., from the fluid line system, etc.) and the fracture stress of the material used, the remaining wall thickness (hi-d) / 2 must still be large enough to ensure that any mechanical stresses that occur do not lead to damage to the mirror.

[0077] A further characteristic of the force-locking flange connection according to the invention is the differential vacuum, which will be described in more detail with reference to Figs. 3a-3b and, in embodiments of the invention, prevails throughout the entire assembly (including the mirror base body, cover plate, flange, and fluid line system). This differential vacuum is channeled through the entire flange and transferred at the interface between the flange and mirror. This is achieved by a gap between the two O-rings of a seal 207 based on the double O-ring principle. The differential vacuum (= "support vacuum") serves to check the tightness of the assembly according to the invention and to check the tightness of the mirror. Any leaks can be detected in real time by continuously monitoring the differential vacuum.As soon as contamination is detected in this differential vacuum, the associated system or machine can be shut down. This allows for the fact that the mirror, when used as a synchrotron mirror, operates in a UHV environment (in the range of 1 O). -10 mbar to 10 -12 mbar) and the sealing requirements are lower than those for EUV applications (10 -9 mbar) are even higher.

[0078] Figs. 3a-3b show schematic representations to explain a possible embodiment of an assembly according to the invention. Compared to Fig. 2, analogous or essentially functionally identical components are designated by reference numerals increased by "100."

[0079] According to Fig. 3a-3b, bores "312a" and "312b" are designated within the mirror base body 300 and the flange 303, respectively, which each extend to a space 313 located between the two O-rings of the seal 307 (and essentially surrounding the fluid channel 306 in a tubular manner). The bore 312a provided in the mirror base body 300 extends to a groove 311 (in the exemplary embodiment, merely designed with a rectangular cross-section, by way of example), which surrounds a fluid volume 310 that receives the fluid.

[0080] Quantitatively, the vacuum pressure present in the outer environment of the mirror (labeled “350” in Fig. 3a) can be estimated in the range of (10 9 -10 -12 ) mbar, the vacuum pressure present in the fluid volume 310 in the range of (0.1 -10) bar and the vacuum pressure present within the bores 312a, 312b, the intermediate space 313 and the groove 311 (ie in the differential vacuum) in the range of (103 -10" 4 ) mbar.

[0081] The configuration described above has the consequence that, in the event of a leak, fluid escaping from the fluid channel 306 or the fluid volume 310, before reaching the (ultra-high) vacuum present in the external environment of the mirror, first enters the region of the differential vacuum provided according to the invention - i.e., depending on the location of the leak, into the groove 311 or the intermediate space 313 - and can thus be detected via a monitoring device 320 connected to the bore 312b. The monitoring device 320 can be configured in any suitable manner and can, for example, have a mass spectrometer, a gas detector or a humidity sensor. If fluid entering one of the regions of the differential vacuum described above is detected, the entire system ora fluid supply connected to the fluid channel 306, so that undesirable contamination of the (ultra-high) vacuum present in the outer surrounding area of ​​the mirror is reliably avoided. In embodiments, the assembly according to the invention can also have at least one further joint in addition to the flange connection, wherein the monitoring device can then also serve to monitor the tightness of this further joint(s) against escaping fluid.

[0082] Although reference was made to a synchrotron mirror in the embodiments described above, the invention can also be implemented in other optical systems, in particular, for example, in an illumination device or a projection lens of a microlithographic projection exposure system.

[0083] Fig. 4 shows a schematic meridional section of the possible structure of a microlithographic projection exposure system designed for operation in the EUV. According to Fig. 4, the projection exposure system 1 has an illumination device 2 and a projection lens 10. One embodiment of the illumination device 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 other illumination device. In this case, the illumination device does not include the light source 3. 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 displaced, in particular in a scanning direction, via a reticle displacement drive 9. In Fig.For illustrative purposes, a Cartesian xyz coordinate system is shown in Figure 4. The x-direction runs perpendicular to the drawing plane. The y-direction runs horizontally, and the z-direction runs vertically. The scanning direction in Figure 4 runs along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0084] The projection lens 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.

[0085] 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, in particular, a wavelength in the range between 5 nm and 30 nm. 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 emanating from the radiation source 3 is bundled by a collector 17 and propagates through an intermediate focus in an intermediate focal plane 18 into the illumination optics 4. The illumination optics 4 has a deflection mirror 19 and, downstream of this in the beam path, a first facet mirror 20 (with schematically indicated facets 21) and a second facet mirror 22 (with schematically indicated facets 23).

[0086] 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 system 1. In the example shown in Fig. 4, the projection lens 10 has 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 lens 10 is a doubly obscured optic. The projection lens 10 has a numerical aperture on the image side that is greater than 0.5 and can also be greater than 0.6 and can be, for example, 0.7 or 0.75.During operation of the microlithographic projection exposure system 1, the electromagnetic radiation impinging on the optical effective surface of the mirrors is partially absorbed and, as explained above, leads to heating and a concomitant thermal expansion or deformation, which in turn can impair the imaging properties of the optical system. The inventive concept can thus also be advantageously applied to any mirror of the microlithographic projection exposure system 1 of Fig. 4.

[0087] The invention can also be advantageously implemented in a projection exposure system designed for operation in the DUV (ie at wavelengths less than 250 nm, in particular less than 200 nm) or in another optical system.

[0088] Although the invention has been described with reference to specific embodiments, numerous variations and alternative embodiments will become apparent to those skilled in the art, e.g., by combining and / or interchanging features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are encompassed by the present invention, and the scope of the invention is limited only by the appended claims and their equivalents.

Claims

Patent claims 1 . Assembly of an optical system, with • at least one mirror with a mirror base body (200, 300) in which a fluid channel arrangement with at least one fluid channel (206, 306) through which a fluid can flow runs; • wherein the fluid channel arrangement is coupled to a fluid line system via a detachable flange connection; • wherein this flange connection comprises a flange interface (202, 302) formed on the mirror base body (200, 300) and a flange (203, 303) mounted on this flange interface (202, 302) in a force-fitting manner; and • wherein a seal (207, 307) is formed between the flange (203, 303) and the flange interface (202, 302) to provide a differential vacuum.

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

3. Assembly according to claim 1 or 2, characterized in that a monitoring device (320) is provided for monitoring a tightness of the flange connection against escaping fluid during operation of the optical system.

4. Assembly of an optical system, with • at least one mirror with a mirror base body (200, 300) in which a fluid channel arrangement with at least one fluid channel (206, 306) through which a fluid can flow runs; • wherein the fluid channel arrangement is coupled to a fluid line system via a detachable flange connection; • wherein this flange connection comprises a flange interface (202, 302) formed on the mirror base body (200, 300) and a flange (203, 303) mounted on this flange interface (202, 302) in a force-fitting manner; and • wherein a monitoring device (320) is provided for monitoring the tightness of the flange connection against escaping fluid during operation of the optical system. Assembly according to claim 3 or 4, characterized in that the monitoring device (320) is configured to monitor the tightness of the flange connection without influencing an external vacuum environment of the mirror. Assembly according to claim one of claims 3 to 5, characterized in that the monitoring device (320) is configured to detect fluid entering a region between the flange (203, 303) and the flange interface (202, 302). Assembly according to one of claims 4 to 6, characterized in that the monitoring device (320) comprises a mass spectrometer, a gas detector, or a humidity sensor.An assembly according to any one of claims 3 to 7, characterized in that, in addition to the flange connection, it has at least one further joint, wherein the monitoring device (320) is further provided for monitoring the tightness of this further joint against escaping fluid. An assembly according to any one of the preceding claims, characterized in that the flange interface (202) forms at least one decoupling structure (201, 301) for reducing force transmission from the flange (203, 303) to the mirror base body (200, 300). The assembly according to claim 9, characterized in that this decoupling structure (201, 301) is formed by a tapered section of the flange interface (202, 302). The assembly according to claim 10, characterized in that in this tapered section, the fluid channel (206, 306) runs along an axial direction, wherein the ratio between an axial extension of this tapered section and a wall thickness of the tapered section remaining toward the fluid channel (206, 306) is in the range of 1 to 5. The assembly according to one of the preceding claims, characterized in that the flange (203, 303) is mounted on the flange interface (202, 302) via a screw connection.Assembly according to claim 12, characterized in that this screw connection takes place along a section of the flange interface (202, 302), wherein for this section the ratio between its extension in a direction perpendicular to the screw direction and the extension in the screw direction is in the range from 2 to 10, in particular in the range from 2 to 4. Assembly according to one of the preceding claims, characterized in that the mirror has a cover plate (200a, 300a) bonded to the mirror base body (200, 300), on which cover plate a reflective layer system is formed. Assembly according to claim 14, characterized in that the cover plate (200a, 300a) is monolithically connected to the mirror base body (200, 300) via direct bonding or fusion bonding. Assembly according to one of the preceding claims, characterized in that the mirror base body (200, 300) is made of a silicon. containing material, in particular a material from the group containing monocrystalline silicon (Si), silicon dioxide (SiO2) and titanium dioxide-doped quartz glass.

17. Assembly according to one 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 one 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 one 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. Optical system comprising an assembly according to one of the preceding claims.

21. Optical system according to claim 20, characterized in that it is a synchrotron.

22. Optical system according to claim 20, characterized in that it is a projection lens (10) or an illumination device (2) of a microlithographic projection exposure system (1).

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