[0001] The invention relates to an optical arrangement for an EUV projection exposure apparatus, comprising an optical component, which has a substrate and a surface on a side of the substrate which surface is optically operative in the EUV spectral range, and comprising a cooling device for the optical component, said cooling device having a cooling medium.

[0002] The invention additionally relates to a method for cooling an optical component of an EUV projection exposure apparatus, which has a substrate and a surface on a side of the substrate which surface is optically operative in the EUV spectral range, by means of a cooling device having a cooling medium.

[0003] An optical arrangement and a method for cooling of the type mentioned in the introduction are known from US 2007/0084461 Al for example.

[0004] The optical component of the optical arrangement of the type mentioned in the introduction is, in the context of the present invention, by way of example, a mirror or a diffraction grating for EUV applications. [0005] In an EUV projection exposure apparatus, the optical components, in particular the collector of the EUV radiation source, are subject to high thermal loads, that is to say that the optical components can heat up to a very great extent during the operation of the EUV projection exposure apparatus. This gives rise to a plurality of technical problems.

[0006] Firstly, the optical component can become too hot, as a result of which the material of the substrate and the optical layers of the optically operative surface can be destroyed.

[0007] The optical component can be deformed by heat gradients, which can form in the optical component, to such a great extent that the optical performance of the optical system no longer corresponds to the specification. Heat gradients arise, in particular, as a result of the thermal loading not being uniform as viewed over the optical component, and also as a result of the optical component being constructed from different materials (by way of example, the mirror layer consists of a different material from the substrate) and the heat propagation in the optical component not being homogeneous. Moreover, heat gradients can arise as a result of the on the one hand high thermal loading on the optically operative surface and the heat outflow on the opposite side of the substrate.

[0008] Furthermore, the heat-governed deformation of the optical component during operation can change over time as a result of so-called transient effects. A single static correction of the imaging aberration resulting from the deformation in the optical system, for example with the aid of other optical components, is accordingly inadequate.

[0009] In order to solve the problems mentioned above, cooling concepts for cooling the optical components have been developed in the prior art. [00010] The document US 2007/0084461 Al cited in the introduction proposes a cooling concept in which one or more cooling channels through which cooled water flows as cooling medium are integrated into the substrate of the optical component. In this case, it is also possible to provide heat pipes in or on the optical component, which are thermally conductively connected to the cooling channels.

[0011] Further documents in which the cooling concept is based on the integration of cooling channels or cooling lines through which water flows in the optical component are US 2005/0099611 Al; US 2006/0093253 Al; WO 2007/051638 Al; JP 2005064391 A; US 6,822,251 Bl; US 2006/0227826 Al; US 2005/01218446 Al; US 7,329,014 B2.

[0012] The cooling of optical components by means of a cooling device having cooling channels or cooling lines which pass through the optical component, and through which water flows, has the disadvantage, however, that the structure of the cooling channels or cooling lines embosses on the optically operative surface of the optical component if the optical component is thermally loaded. An undulation arises in the optically operative surface with a wavelength corresponding to the distance between the channel centres of two adjacent channels. The amplitude of said undulation is dependent on the coefficient of thermal expansion of the substrate, on the thermal power to be absorbed (surface load), and on the heat gradient between the optically operative surface, at which the heat arises, and the central flow in the cooling channel or in the cooling line.

[0013] The undulations of the optically operative surface lead to optical aberrations which can considerably impair the optical performance of the optical component. Such optical aberrations include, for example, an impaired focusing effect of the optical component, and/or the homogeneity of the radiation in the far field can be corrupted. In the case where the optical component is a diffraction grating, the separability of the different wavelengths by the diffraction grating can be impaired. [0014] Alongside the abovementioned "short-wave" aberrations of the optically operative surface, a "long-wave" aberration can also occur by virtue of the optically operative surface being deformed overall by the total heating of the substrate, such that, by way of example, the focal length drifts thermally, or the actual form of the optically operative surface that arises at a corresponding temperature correspondingly deviates from the ideal form.

[0015] The invention is based on the object of improving an optical arrangement of the type mentioned in the introduction and a method for cooling of a type mentioned in the introduction to the effect that the abovementioned disadvantages of the known cooling concepts are avoided.

[0016] According to the invention, this object is achieved with regard to the optical arrangement mentioned in the introduction by virtue of the fact that the substrate comprises a material having at a temperature T _s « 0°C a thermal conductivity of > 50 Wm^K ^"1 and a coefficient of thermal expansion of < 3 * 10 ^"6 K ^_1 , and in that the cooling medium has a temperature of « 0°C in order to cool the substrate to the temperature T _s .

[0017] With regard to the method mentioned in the introduction, the object on which the invention is based is achieved by virtue of the fact that the substrate comprises a material having at a temperature T _s « 0°C a thermal conductivity of > 50 Wm^K ^"1 and a coefficient of thermal expansion of < 3 * 10 ^"6 K ^_1 , and in that the cooling medium is provided with a temperature of « 0°C, and in that the cooling medium cools the substrate to the temperature T _s .

[0018] The optical arrangement according to the invention and the method according to the invention for cooling an optical component provide a novel cooling concept in optical EUV systems. This novel cooling concept is based on operating the optical component at a temperature of significantly less than 0°C, to be precise in a low-temperature range in which the material of the substrate has a high thermal conductivity and a low thermal expansion.

[0019] The above-described problem of the deformation or distortion of the optically operative surface by the thermal loading is proportional to the quotient of coefficient of thermal expansion and thermal conductivity of the substrate. In order to obtain the least possible heat-governed distortion of the optical element, the thermal expansion has to be correspondingly very small and the thermal conductivity very high.

[0020] At room temperature, however, none of the materials customary for the production of optical components meets these requirements. On the one hand, there are materials which have a very small thermal expansion at room temperature, in particular glasses and glass ceramics, such as, for example, quartz or Zerodur having a coefficient of thermal expansion of approximately 2 * 10 ^"8 K ^_1 . However, such materials have an extremely low thermal conductivity, such that they have low suitability for dissipating heat well. On the other hand, there are materials having a very high thermal conductivity at room temperature, such as, for example, copper or aluminium having a thermal conductivity of greater than 200 Wm^K ^"1 , but such materials, because they are metals, have a high coefficient of thermal expansion.

[0021] While there are some materials, such as silicon carbide (SiC), silicon or beryllium, for example, which have both a very small coefficient of thermal expansion and a high thermal conductivity, these materials are very expensive in terms of processing and material costs. Silicon carbide, for example, is additionally very hard and the processing time for manufacture by machining is correspondingly very long and the manufacturing processes are thus highly cost-intensive.

[0022] The cooling concept according to the invention, which consists in operating the optical element at a temperature T _s of significantly less than 0°C, has the advantage, by contrast, that the optical element, in particular the substrate, can be manufactured from a material which, for example, at room temperature does not meet the requirements in respect of a high thermal conductivity and a low coefficient of thermal expansion, but in return can be machined and processed well at room temperature. By way of example, many metals exhibit the property that they have a low coefficient of thermal expansion and a high thermal conductivity at very low temperatures. On the other hand, metals can be processed well at room temperature.

[0023] The cooling of components in the low-temperature range is described in the articles: C. Shawn Rogers et al., "The Cryogenic Cooling Program at the Advanced Photon Source", Argonne National Laboratory, Illinois, USA, June 1994, and in the article by G. Marot et al., "Cryogenic cooling of monochromators", Rev. Sci. Instrum. 63(1), January 1992, pages 477 to 480, but exclusively for systems that are operated with synchrotron radiation. Optical EUV systems, however, always adhere to the above-described cooling concepts which are based on cooling channels or cooling lines which pass through the substrate of the optical component and through which cooled, but liquid, water flows.

[0024] In one preferred configuration of the optical arrangement, the material of the substrate has a coefficient of thermal expansion of < 1 * 10 ^"6 K ^_1 , with further preference of < 0.1 * 10 ^"6 K ^_1 , at the temperature T _s « 0°C.

[0025] The choice of a material for the substrate having such low coefficients of thermal expansion at low temperatures is particularly advantageous with regard to avoiding heat-induced deformations of the optical component.

[0026] In a further preferred configuration of the optical arrangement, the temperature T _s is less than -50°C, with further preference less than -100°C, with further preference less than -150°C, with further preference less than -200°C. [0027] The temperature T _s with which the optical component is operated is chosen in accordance with the material of the substrate and the coefficient of thermal expansion and/or thermal conductivity thereof.

[0028] Preferably, the material of the substrate is a metal, a metal alloy or a crystalline material.

[0029] In this case, it is advantageous that these materials can be processed very simply and cost-effectively at room temperature and have a very small thermal expansion and a very high thermal conductivity at the low operating temperatures according to the invention of the optical component.

[0030] In a further preferred configuration of the optical arrangement, the cooling medium is a liquefied gas.

[0031] A liquefied gas is correspondingly preferably used in the method according to the invention.

[0032] The use of a liquefied gas, wherein the gas is preferably selected from the group consisting of the noble gases, nitrogen, hydrogen, has the advantage that such liquefied gases thusly are relatively expedient in terms of production, have a self- cooling effect and in most cases there are relatively low safety risks, particularly if liquid nitrogen or a liquid noble gas is used as the cooling medium.

[0033] The use of a liquefied gas as the cooling medium has the advantage, in principle, that the temperature of the liquid phase of the gas is already very low. Moreover, the cooling medium can be supplied in a simple manner in liquid phase.

[0034] In the method according to the invention, the cooling medium is preferably operated in the region of a phase transition from liquid phase, in which the cooling medium is supplied, to gaseous phase, in which the cooling medium is discharged after absorbing heat from the optical component.

[0035] In this case, it is advantageous that, by means of the cooling medium, the heat from the optical component can be particularly effectively absorbed and dissipated by the use of latent heats, here of heat of evaporation.

[0036] In accordance with a further preferred configuration of the method, the gaseous phase of the cooling medium is preferably used for purging and/or cleaning the optically operative surface.

[0037] Optical systems, particularly in EUV applications, are usually operated using a purge gas, wherein the abovementioned configuration of the method affords the advantage that the gaseous phase of the cooling medium is simultaneously used as a purge gas.

[0038] With regard to the way in which the optical component is cooled by means of the cooling medium, one preferred configuration of the method provides for the cooling medium to be directly applied to the substrate, and/or for the substrate to be cooled by the cooling medium indirectly via at least one thermally conductive element, in particular via a heat pipe.

[0039] In one preferred configuration of the optical arrangement for carrying out the method in accordance with the first variant mentioned above, it is provided that the cooling device has at least one chamber which is formed in the substrate as a cavity, and/or is arranged in direct proximity to the substrate on that side of the substrate which faces away from the optically operative surface, and which at least partly accommodates the optical element, wherein the at least one chamber has at least one inlet and at least one outlet for the cooling medium. [0040] In this configuration of the cooling device, therefore, the cooling medium is directly applied to the substrate.

[0041] While it is conceivable for the cooling medium to cool the optical component without a phase transition in this case, it is preferred if the cooling medium enters into the chamber in liquid phase and exits from the chamber in gaseous phase.

[0042] This measure has the advantage over cooling without a phase transition that it is possible to dissipate significantly larger amounts of heat on account of the phase transition in the form of latent heats. This also requires a smaller amount of cooling medium than if the cooling medium cools the optical component without a phase transition.

[0043] Moreover, this configuration of the cooling device has the advantage already mentioned above that the gaseous phase of the cooling medium can be used as a purge or cleaning gas for the optical component.

[0044] In a further preferred configuration of the optical arrangement, the cooling device has at least one supply line for the cooling medium which leads into the chamber, and which has at least one exit opening from which the cooling medium enters into the chamber and flows against the substrate from the side facing away from the optically operative surface.

[0045] Preferably, the exit opening of the supply line is configured such that the cooling medium, preferably in liquid phase, flows as uniformly as possible against that side of the substrate which faces away from the optically operative surface, and the cooling medium then evaporates as a result of heat absorption and is discharged from the chamber or is used further as a cleaning or purge gas for the optical component. [0046] The at least one exit opening can be embodied as a slot extending approximately over the width of the substrate, or it is possible to form a multiplicity of exit openings in the form of small slots or small openings in the supply line.

[0047] The substrate can have, on the side facing away from the optically operative surface, at least one projection which dips into the cooling medium in liquid phase, wherein the cooling medium in this case fills the chamber up to a level, such that the at least one projection can dip by its end into the cooling medium, or against which the cooling medium flows.

[0048] In the configuration in which the at least one projection dips into the liquid phase of the cooling medium, a separate supply line leading into the chamber is not necessary.

[0049] In this configuration, however, care should be taken to ensure that the amount of liquid cooling medium in the chamber is kept at as constant a level as possible by means of the setting of the corresponding cooling medium supply.

[0050] In a further preferred configuration, the at least one projection can have a flow-guiding effect, such that the at least one projection guides the cooling medium to the at least one outlet.

[0051] This ensures that the cooling medium that undergoes transition to the gas phase as a result of the thermal expansion can be effectively discharged from the chamber and does not make dead spaces in which the cooling is inadequate as a result of gas accumulations in the chamber.

[0052] Thus, the at least one projection can be embodied in spiral fashion, for example, in the case of a rotationally symmetrical substrate, such that the gas that arises is guided to the outlet in a manner following the spiral projection. [0053] The substrate can also have a plurality of projections in the form of ribs, pins or the like on the side facing away from the optically operative surface.

[0054] In this case, care must also again be taken to ensure that the gaseous phase of the cooling medium that arises as a result of the heat absorption can escape well between the ribs, pins or the like.

[0055] In further preferred configurations, the at least one chamber is liquid- and/or gas-tight with respect to the optically operative surface, which has the advantage of ensuring that the liquid phase of the cooling medium cannot be applied to the optically operative surface, or the at least one chamber is gas-permeable with respect to the optically operative surface, which is advantageous if the gaseous phase of the cooling medium that arises as a result of heat absorption is intended to be used as a purge or cleaning gas.

[0056] In connection with the latter configuration, gas passage openings can be present in the optically operative surface, such that the gaseous phase of the cooling medium that arises as a result of heat absorption can be fed as a purge and/or cleaning gas to the optically operative surface not only from the side, but in a manner distributed at a plurality of locations on the optical surface, in order thus to enable uniform, effective purging over the entire region of the optically operative surface.

[0057] In further preferred configurations of the optical arrangement in which the method for cooling is carried out such that the substrate is cooled indirectly by the cooling medium via at least one thermally conductive element, it is provided that the cooling device has at least one cooling container and/or at least one cooling line which is arranged outside a chamber, which accommodates the optical element, wherein the cooling medium is present in the at least one cooling container and/or the at least one cooling line, and in that the substrate is thermally conductively connected to the at least one cooling container and/or the at least one cooling line via at least one thermally conductive element. [0058] In these configurations, therefore, the cooling medium does not come directly into contact with the optical element. What is advantageous about this configuration is that the cooling can take place at a location relatively far away from the optical component, to be precise outside the chamber in which the optical component is usually arranged. The at least one thermally conductive element can, for example, also be embodied in a flexible fashion in order to bring about a mechanical decoupling between the cooling container or the cooling line and the optical component, which in turn has the advantage that the optical component experiences no or only small mechanical loads. Furthermore, the chamber in which the optical component is arranged can be surrounded by the cooling container over part of or the whole periphery, and it is possible to provide a corresponding number of thermally conductive elements or an individual thermally conductive element extending over the whole periphery or over part of the periphery for the purpose of thermal coupling between the optical component and the cooling container or the cooling line.

[0059] In the case where the cooling device has at least one cooling container, the latter preferably has at least one inlet for the cooling medium and at least one outlet for the cooling medium, wherein the cooling medium enters into the at least one cooling container in liquid phase and exits from the at least one cooling container in gaseous phase.

[0060] This configuration has the advantage that the cooling is effected with a phase transition of the cooling medium, such that the configuration of the cooling device with a cooling container may be preferable to the configuration with a cooling line in which the cooling medium flows only in liquid phase.

[0061] In one preferred configuration of the at least one thermally conductive element, the latter is embodied as a heat pipe. [0062] The use of a heat pipe as the at least one thermally conductive element has the advantage that a heat pipe enables significantly higher heat transport from the warm end to the cold end by comparison with, for example, a metallic wire.

[0063] In accordance with a further aspect, the invention relates to an optical arrangement for an EUV projection exposure apparatus, comprising an optical component, which has a substrate and a surface on a side of the substrate which surface is optically operative in the EUV spectral range, and comprising a cooling device for the optical component, wherein the cooling device has a heat pipe having a tubular container, in which a cooling medium circulates between an evaporation end and a condensing end, wherein the entire surface of the substrate that faces away from the optically operative surface forms the evaporation end of the tubular container of the heat pipe.

[0064] Although the document US 2007/0084461 Al cited in the introduction already describes the use of heat pipes for cooling optical components in optical EUV systems, the heat pipes therein are embodied as thin small tubes which extend circumferentially around the optical component and therefore do not enable the optical component to be cooled over the whole area.

[0065] By contrast, in the present aspect according to the invention, the optical component itself is part of a heat pipe by virtue of the back surface of the substrate forming the evaporation end (warm end) of the tubular container of the heat pipe. In this case, the cooling medium situated in the tubular container circulates from the back surface of the substrate to the condensing end (cold end), condenses there and passes to the evaporation end again either by gravity, but preferably by the capillary effect of a capillary substance present in the tubular container or by means of a corresponding surface structure of the walls of the container. [0066] In this case, it is preferred if the tubular container has at its condensing end on the inner side a conical surface having a tapering facing towards the optical element.

[0067] This advantageously achieves a circulation of the cooling medium which is directed in the centre from the condensing end to the evaporation end, and is directed there uniformly along the surface of the substrate to the outer regions of the substrate and to the condensing end again.

[0068] Further advantages and features will become apparent from the following description of the accompanying drawing.

[0069] It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the respectively specified combination, but also in other combinations or by themselves, without departing from the scope of the present invention.

[0070] Exemplary embodiments of the invention are illustrated in the drawing and are described hereinafter with reference thereto. In the figures:

Fig. 1 shows an optical arrangement comprising a cooling device in accordance with a first exemplary embodiment;

Fig. 2 shows an optical arrangement comprising a cooling device in accordance with a further exemplary embodiment;

Fig. 3 shows an optical arrangement comprising a cooling device in accordance with a further exemplary embodiment;

Fig. 4 shows an optical arrangement comprising a cooling device in accordance with a further exemplary embodiment; Fig. 5 shows an optical arrangement comprising a cooling device in accordance with a further exemplary embodiment;

Fig. 6 shows an optical arrangement comprising a cooling device in accordance with a further exemplary embodiment;

Fig. 7 shows an optical arrangement comprising a cooling device in accordance with a further exemplary embodiment; and

Fig. 8 shows an optical arrangement comprising a cooling device in accordance with yet another exemplary embodiment.

[0071] Fig. 1 illustrates an optical arrangement provided with the general reference sign 10, this optical arrangement being used in an EUV projection exposure apparatus (not illustrated) for lithographic applications.

[0072] The optical arrangement 10 generally has an optical component 12. The component 12 has a substrate 14 and a surface 16 that is optically operative in the EUV spectral range, on a side of the substrate 14, which is arranged on the top side of the substrate 14 in Fig. 1. The surface 16 is a reflective surface here and in the further exemplary embodiments.

[0073] Generally, the optical component 12 is a mirror or a diffraction for lithographic EUV applications.

[0074] During the operation of the optical component 12, EUV radiation 18 is applied to the optically operative surface 16. In this case, part of the incident radiation energy is absorbed by the optically operative surface 16 and the substrate 14. As a result, heat is generated in the optical component 12, and propagates in the substrate 14. However, the heat propagating in the substrate 14 usually cannot be dissipated to a sufficient extent, not even via the holding elements (not illustrated), by means of which the optical component 12 is held in a supporting structure of the overall optical system.

[0075] The consequence of the heating is that the substrate 14 expands, such expansion being partly impeded by the abovementioned holding elements and the supporting structure, as a result of which deformations are caused in the optical component 12, in particular in the optically operative surface 16, whether they be local deformations or a global deformation of the entire optical component 12. Such deformations can impair the optical performance of the optical component 12.

[0076] Such deformations can also vary in the course of operation, that is to say that transient effects can occur.

[0077] However, not just deformations can impair the optical performance of the optical component 12, rather the heat that arises in the optical component 12 can also damage or even destroy the material of the substrate 14 and the optical layers forming the optically operative surface 16.

[0078] The problem of the optical component 12 being heated to a great extent is considerable particularly in the case where the optical component 12 is used as a collector mirror of the EUV radiation source, since the collector mirror collects the entire radiation power of the EUV radiation source, and the original EUV radiation also contains infrared components of the radiation, which lead to the optical component 12 being heated to a particularly great extent.

[0079] In order to prevent the optical component 12 from being heated, a cooling device 20 is present.

[0080] In the exemplary embodiment shown, the cooling device 20 has at least one chamber 22 embodied as a cavity in the substrate 14. A cooling medium 24 enters into the chamber 22 via an inlet 26 in accordance with an arrow 28, flows through the chamber 22 and exits from the chamber 22 again via an outlet 30 in accordance with an arrow 32. The cooling medium 24 thus flows through the chamber 22. The chamber 22 preferably has an extent over substantially the areal extent of the optically operative surface 16.

[0081] In the prior art, liquid, cooled water or some other liquid comparable therewith was used as cooling medium 24. In the optical arrangements in accordance with the prior art, the chamber 22 was embodied as a narrow cooling line or narrow cooling channel. The heat dissipation from the optical component 12 was inadequate in this case. Furthermore, on account of the cooling lines or cooling channels in the substrate undulations in the optically operative surface arose, which impaired the optical performance of the optical component.

[0082] By contrast, the invention provides for the substrate 14 to comprise a material which has at a temperature T _s « 0°C a thermal conductivity of > 50 Wm 'K ^"1 and a coefficient of thermal expansion of < 3*10 ^"6 K ^_1 . The cooling medium 24 has a temperature of « 0°C, such that the substrate 14 is cooled to the temperature T _s .

[0083] Therefore, according to the invention, the optical component 12 is operated at an operating temperature that is much less than 0°C. Materials used for the substrate 14 can be metals, metal alloys or crystalline materials which correspondingly have a thermal conductivity and a coefficient of thermal expansion in the abovementioned ranges at very low temperatures. Such materials have the advantage that they can be processed very simply and cost-effectively at room temperature, in order to produce the optical component 12.

[0084] By way of example, aluminium or copper can be used as metals.

[0085] The lower the coefficient of thermal expansion of the material used for the substrate 14, the lesser the extent to which heat-induced deformations of the optical component 12 occur. It is thus preferably provided that the material of the substrate has a coefficient of thermal expansion of < 1 * 10 ^~6 Κ ^_1 , with further preference of < 0.1 * 10- ⁶ K \ at a temperature T _s « 0°C.

[0086] The temperature T _s at which the optical component 12 is operated by cooling by means of the cooling medium 24 is preferably less than -50°C, with further preference less than -100°C, with even further preference less than -150°C, and with even further preference less than -200°C.

[0087] A liquefied gas, for example liquid nitrogen, can be used as cooling medium 24, in particular in order to obtain the very low temperatures mentioned above. Nitrogen is liquid in a temperature range of -195.8°C to -210.1°C (at normal pressure).

[0088] As an alternative to liquid nitrogen, however, it is also possible to use a liquefied noble gas such as argon or helium or else some other cryogenic gas such as liquid hydrogen as cooling medium 24.

[0089] In the exemplary embodiment shown in Fig. 1, the cooling medium 24 in liquid phase flows through the at least one chamber 22 and in the process absorbs heat from the substrate 14. In this case, the chamber 22 is arranged at a small distance from the optically operative surface 16.

[0090] In the exemplary embodiment shown in Fig. 1, the cooling medium 24 flows through the at least one chamber 22 without a phase transition liquid/gaseous. In this case, care must be taken to ensure that no gas bubbles arise in the cooling medium 24 in the at least one chamber 22, or such gas bubbles which arise have to be discharged at least from the chamber 22 in order to avoid flow dead spaces. Moreover, the arising of gas bubbles can excite the optical component 12 to effect oscillations that can impair the optical performance of the optical component 12. [0091] Since the use of the cooling medium 24 exclusively in liquid phase is restricted to a small temperature range, for example to a temperature range of approximately 14 K in the case of liquid nitrogen, a description is given below of further exemplary embodiments in which the cooling medium is operated with a phase transition liquid/gaseous.

[0092] Fig. 2 illustrates an optical arrangement 40 for an EUV projection exposure apparatus for semiconductor lithography, comprising an optical component 42, which has a substrate 44 and an optically operative surface 46, wherein the optically operative surface 46 receives EUV radiation 48. The optical component 42 is a mirror or a diffraction grating, for example.

[0093] The optical arrangement 40 has a cooling device 50, by means of which the optical component 42 is operated at a very low temperature, for example a temperature in the range of approximately -195°C to approximately -200°C.

[0094] With regard to the choice of material for the substrate, the same statements as for the substrate 14 of the optical component 12 in Fig. 1 hold true, particularly with regard to the coefficient of thermal expansion and the thermal conductivity of the material at very low temperatures.

[0095] The cooling device 50 has a chamber 52, which is arranged in direct proximity to the substrate 44 on that side of the substrate 44 which faces away from the optically operative surface 46, and which at least partly accommodates the optical component 42.

[0096] In the exemplary embodiment shown, the chamber 52 is sealed, for example by means of a membrane 54, relative to the optically operative surface 16, wherein, as will also be described later, such sealing is not absolutely necessary. [0097] The cooling device 50 furthermore has at least one supply line 56 for a cooling medium 58. The cooling medium 58 is a liquefied gas, for example liquid nitrogen. The cooling medium 58 is supplied in liquid phase in accordance with an arrow 60 through the supply line 56, which projects into the chamber 52 and extends at least in one dimension over the corresponding dimension of the optical component 42.

[0098] The supply line 56 has at least one, here a multiplicity of exit openings 60. The exit openings 60 can be embodied in the form of small holes, slots, or the supply line 56 can have the at least one exit opening in the form of a slot or gap extending in the longitudinal direction of the supply line 56.

[0099] The supply line 56 thus forms the inlet for the cooling medium 58 into the chamber 52.

[0100] The cooling medium 58 is supplied in liquid phase by the supply line 56, wherein it then enters into the chamber 52 through the exit openings 60, as is indicated by flow arrows 62. In this case, the cooling medium 58 flows against the substrate 44 over a large area on the side facing away from the optically operative surface 46, as is indicated by flow arrows 64. In this case, the cooling medium 58 evaporates, that is to say undergoes transition from the liquid phase to the gaseous phase, and the cooling medium 58 then exits from the chamber 52 again in gaseous phase through one or more outlets 66, as is indicated by flow arrows 68 and arrows 70.

[0101] The chamber 52 for the cooling device 50 can be, for example, a vacuum chamber or else a gas container in which the optical component 42 is usually arranged in EUV applications, and which contains a gas that is used for cleaning and/or purging the optically operative surface 46. In connection with the cooling device 50, provision can now advantageously be made for using the gaseous phase of the cooling medium 58 as the purge or cleaning gas for purging or cleaning the optically operative surface 46, wherein, in such a case, the membrane 54 is either not provided or is gas-permeable at least to a sufficient extent.

[0102] In the exemplary embodiment in Fig. 2, provision could likewise be made for not allowing the cooling medium 58 to evaporate at that side of the substrate 44 which faces away from the optically operative surface 46; rather, this could also be effected in corresponding caverns in the substrate 44 (see also Fig. 4).

[0103] In the arrangement in accordance with Fig. 2 care must be taken to ensure that the correct amount of coolant 58 is applied to the optical component 52. If the amount of coolant is too low or the heat to be absorbed is too high, the temperature rises in the optical component 42, and if the amount of coolant is too high or the heat to be absorbed is too low, the coolant 58 remains substantially liquid and it can happen that the liquid coolant fills the region towards the optical component 42. At all events the liquid coolant 58 must be prevented from flooding the optical component 52, which could destroy the optical component 42. This could happen if the chamber 52 is not sealed relative to the optically operative surface 46 by means of the membrane 54, for example.

[0104] In addition to the membrane 54 or instead of the latter, a regulation could also be provided for the cooling device 50, for example a temperature measurement of the optical component 42 and, depending thereon, a corresponding dosing of the cooling medium 58. On account of the fact that the thermal expansion of the substrate 44 is very small at very low temperatures, such as are provided here for cooling, in most materials in a relatively large low-temperature range, a corresponding temperature increase in the range of very low temperatures has no adverse influence on the thermal expansion of the optical component 42 and therefore no adverse influence on the optical performance of the optical component 42. [0105] Another possibility for a regulation of the amount of cooling medium 58 consists, for example, in measuring the weight-force of the chamber 52 or the filling level of the cooling medium 58 in the chamber 52.

[0106] Fig. 3 shows an optical arrangement 80 for an EUV projection exposure apparatus for lithographic applications, comprising an optical component 82, which has a substrate 84 and a surface 86 that is optically operative in the EUV spectral range, wherein EUV radiation 88 is applied to the optically operative surface 86 during operation. The optical component 82 is a mirror or a diffraction grating, for example.

[0107] With regard to the choice of material for the substrate 84 in respect of the thermal conductivity and coefficient of thermal expansion thereof at very low temperatures, the same statements as for the substrate 14 of the optical component 12 hold true.

[0108] The optical arrangement 80 has a cooling device 90, which has a chamber 92, which at least partially accommodates the optical component 82. The cooling device 90 has a cooling medium 94, which is a liquefied gas, for example liquid nitrogen.

[0109] The cooling medium 94 is introduced directly into the chamber 92 via an inlet 96, such that the cooling medium 94 has a predetermined filling level in the chamber 92, said filling level being indicated by a line 98 in Fig. 3. The chamber 92 is sealed relative to the optically operative surface 86, once again for example by means of a membrane 100, which can be completely gas-tight or can be gas- permeable, but should at least be liquid-tight.

[0110] The substrate 84 has, on the side facing away from the optically operative surface 86, at least one, in the exemplary embodiment shown a plurality of projections 102. The cooling medium 94 is introduced in liquid phase into the chamber 92 through the inlet 96. In this case, the projections 102 dip into the liquid cooling medium 94, as can be gathered from Fig. 3. As a result of heat absorption via the projections 102, the cooling medium 94 evaporates, as is indicated by gas bubbles 104. Consequently, the phase transition liquid-gaseous of the cooling medium 94 is utilized also in the cooling device 90 for cooling the optical component 82. The evaporation of the cooling medium 94 is indicated by arrows 106 in Fig. 3. The cooling medium 94 then exits in gaseous phase from the chamber 92 from one or a plurality of outlets 108. In this case, too, the gaseous phase of the cooling medium 94, given a corresponding permeability of the membrane 100 or absence thereof, can be used for purging or cleaning the optically operative surface 86.

[0111] In this case, the projection or projections 102 can have, in particular, a flow-guiding effect in order to guide the gaseous cooling medium that arises as a result of the phase transition to the outlet or outlets 108, in order to ensure that the gas that arises and hence the absorbed heat is reliably discharged from the chamber 92 and dead spaces in which a gas accumulation occurs are avoided.

[0112] The projections 102 can be embodied in the form of ribs or pins, for example, wherein the interspaces 110 between the projections 102 should be connected to the outlets 108 in a gas-guiding manner. This is ensured in the case of the configuration of the projections 102 in the form of pins. In the case of projections 102 in the form of ribs, the latter could be arranged in spiral fashion, particularly in the case of rotationally symmetrical optical components, such that the cooling medium that evaporates as a result of the phase transition can be guided to the outlets 108 in a manner following the spiral.

[0113] In the cooling device 90, it is possible to provide a measurement of the filling level (line 98) of the cooling medium 94 in the chamber 92. Alternatively or additionally, it is possible to perform a measurement of the hydrostatic pressure of the cooling medium 94 (liquid phase) in the chamber 92, or a weight measurement of the container of the chamber 92 could be performed, as has already been described with reference to Fig. 2. [0114] Fig. 4 shows an optical arrangement for an EUV projection exposure apparatus for use in lithography, comprising an optical component 122, which has a substrate 124 and a surface 126 that is optically operative in the EUV spectral range on the substrate 124. EUV radiation 128 is applied to the optically operative surface 126 during the operation of the optical arrangement 120.

[0115] The substrate 124 comprises a material which corresponds to the substrate 14 of the optical component 12 in Fig. 1 with regard to thermal conductivity and coefficient of thermal expansion at low temperatures.

[0116] The optical arrangement 120 constitutes a combination of features of the optical arrangement 80 and of the optical arrangement 40 in the aspects to be described below.

[0117] The optical arrangement 120 has a cooling device 130, which, like the cooling device 50 and the cooling device 90, has a chamber 132, in which the optical element 122 is arranged. The chamber 132 is, for example, a vacuum chamber or a gas container such as is usually provided for accommodating optical components in EUV applications.

[0118] Like the cooling device 50, the cooling device 130 has a supply line 134, which is embodied like the supply line 56 in Fig. 2. The supply line 134 projects into the chamber 132, wherein a cryogenic cooling medium 136, for example a liquefied gas, for example liquid nitrogen, is supplied by the supply line 134 in accordance with an arrow 138. The supply line 134 thus serves as an inlet for the cooling medium 134 into the chamber 132.

[0119] Like the supply line 56, the supply line 134 has a plurality of exit openings 140, through which the cooling medium supplied in liquid phase by the supply line 134 enters into the chamber 132. [0120] Like the optical component 82 in accordance with Fig. 3, the substrate 124 of the optical component 122 has a plurality of projections 142 in the form of ribs, pins or the like as has already been described above with reference to Fig. 3.

[0121] In contrast to the optical arrangement 80 in Fig. 3, the projections 142 do not dip into the liquid phase of the cooling medium 136, rather the liquid phase of the cooling medium 136 flows areally against them through the exit openings 140, to be precise also in the cavern-like interspaces between the projections 142, as is indicated by flow arrows 144 in Fig. 4. If the cooling medium 136 comes into contact with the projections 142, it evaporates on account of heat absorption and correspondingly undergoes transition to the gaseous phase.

[0122] In contrast to the optical arrangements 40 and 80, the chamber 132 is not sealed with respect to the optically operative surface 126, rather the evaporated gaseous cooling medium 136 flows laterally against the optical component 122 and through gas passage openings 146 provided in the optically operative surface over to the side of the optically operative surface 126. In this way, the entire optically operative surface 126 can be purged or cleaned effectively and uniformly with the gaseous phase of the cooling medium 136.

[0123] The gas passage openings 146 are adjacent here to the interspaces between the projections 142 of the substrate 124. The gas passage openings 146 themselves are as small as possible in order not to appreciably reduce the optically operative surface 126 for the EUV radiation 128.

[0124] As in the cooling device 50 of the optical arrangement 40 in Fig. 2, the liquid cooling medium 136 thus flows areally against the substrate 124 of the optical component 122, then evaporates with heat absorption and passes laterally and through the gas passage openings 146 over to the side of the optically operative surface 126, as is indicated by arrows 148. The cooling medium 136 then leaves the chamber 132 in gaseous phase through one or more outlets 150, as is indicated by arrows 152, wherein the outlets 150, in contrast to Figs. 2 and 3, are arranged on the other side of the optically operative surface 126 with respect to the inlet (supply line 134).

[0125] It should be noted here that the cooling medium 136 as in the optical arrangement 80 in Fig. 3, can also be introduced in liquid phase directly into the chamber 132 and fills the chamber 132 up to a specific filling level, such that the projections 142 dip into the liquid phase of the cooling medium 136.

[0126] Fig. 5 shows a further exemplary embodiment of an optical arrangement 160 of an EUV projection exposure apparatus for lithographic applications.

[0127] The optical arrangement 160 comprises an optical component 162 having a substrate 164 and a surface 166 that is optically operative in the EUV spectral range and to which EUV radiation 168 is applied during the operation of the optical arrangement 160.

[0128] The substrate 164 comprises a material which corresponds to the substrate 14 of the optical component 12 of the optical arrangement 10 in Fig. 1 with regard to coefficient of thermal expansion and thermal conductivity.

[0129] The optical arrangement 160 comprises a cooling device 170, which has a chamber 172 for a cooling medium 174.

[0130] Similarly to the optical arrangement 10 in Fig. 1, the chamber 172 is integrated into the optical component 162 or into the substrate 164 thereof. Via an inlet 176, the cryogenic cooling medium 174, for example a liquefied gas, for example liquid nitrogen, is introduced into the chamber 172 in accordance with an arrow 178. In the chamber 172, the cryogenic cooling medium 174 in liquid phase comes into contact over the whole area with that side of the substrate 164 which faces away from the optically operative surface 166. As a result of heat absorption from the substrate 164, the cooling medium 174 evaporates partly, as is indicated by gas bubbles 180. In this case, too, the cooling medium 174 is operated in the phase transition liquid-gaseous. The gaseous phase of the cooling medium 174 then flows, in accordance with flow arrows 186, to one or more outlets 182 and leaves the chamber 172 via the latter in accordance with an arrow 184.

[0131] The arrangement in accordance with Fig. 5 is suitable, in particular, for a spatial arrangement of the optical component 162 in which the EUV radiation 168 is applied to the optically operative surface 166 of the optical component 162 from below. Generally, the arrangement in accordance with Fig. 5 can also be taken into consideration when the optical axis of the optical component 162 forms an angle of less than 30° with the vertical. In this arrangement, the gaseous phase of the cooling medium 174 can be very efficiently guided away from the optical component 162 and transported away upwards.

[0132] Whereas in the previous exemplary embodiments in accordance with Figs. 1 to 5 the optical component is brought directly into contact with the cooling medium, a description is given below, with reference to Figs. 6 and 7, of a cooling concept in which the optical component is cooled only indirectly by the cooling medium. Nevertheless, the cooling concept in the exemplary embodiments in Figs. 6 and 7 is also based on the fact that the optical component is cooled to very low temperatures or is operated at very low temperatures « 0°C at which thermal expansion of the optical component virtually does not occur.

[0133] Fig. 6 shows an optical arrangement 190 for an EUV projection exposure apparatus for lithographic applications, comprising an optical component 192, which has a substrate 194 and a surface 196 that is optically operative in the EUV spectral range. The optical component 192 is a mirror or a diffraction grating, for example. [0134] The substrate 194 comprises a material which has at low temperatures « 0°C a thermal conductivity and a coefficient of thermal expansion as was described with regard to the substrate 14 of the optical component 12 of the optical arrangement 10 in Fig. 1.

[0135] EUV radiation 198 is applied to the optically operative surface 196 during the operation of the optical arrangement 190.

[0136] The optical arrangement 190 comprises a cooling device 200 having a cooling container 202. A cryogenic cooling medium 204, for example a liquefied gas, for example liquid nitrogen, is introduced in liquid phase into the cooling container 202 via an inlet 206 in accordance with an arrow 208. The optical component 192 is accommodated in a chamber 210, for example in a vacuum chamber or in a gas container, wherein, in contrast to the previous exemplary embodiments, the cooling medium 204 is not introduced into the chamber 210.

[0137] In the optical arrangement 190, rather, the heat that arises on account of the EUV radiation 198 in the substrate 194 is transported out of the chamber 210 by at least one thermally conductive element 212 and is fed to the cooling container 202. In this case, the cooling medium 204 is operated with phase transition between the liquid phase and the gaseous phase. As a result of heat absorption from the at least one thermally conductive element 112, the cooling medium 204 in the cooling container 202 evaporates partly, as is indicated by gas bubbles 214. The gaseous phase of the cooling medium 204 is then transported out of the cooling container 202 via an outlet 216 in accordance with an arrow 218. What is advantageous about the optical arrangement 190 is that the cooling of the optical component 192 can take place far away from the optical component 192, and even outside the chamber 210. [0138] The at least one thermally conductive element 212 can be a heat pipe, in particular, but it is likewise possible for the at least one thermally conductive element 212 to be a solid part composed of a material having high thermal conductivity.

[0139] Furthermore, it is possible for the at least one thermally conductive element 212 to be configured in flexible fashion in order to realize a mechanical decoupling between the optical component 192 and the cooling container 202, in order to protect the optical component 192 against mechanical force influences.

[0140] Preferably, a plurality of thermally conductive elements 212 (not illustrated) are provided in a manner distributed circumferentially around the optical component 192, and the cooling container 202 can advantageously be embodied as a ring-shaped cooling container 202, for example, which surrounds the chamber 210 at least partly, but preferably over the whole periphery.

[0141] Fig. 7 shows a modification of the optical arrangement 190 in the form of an optical arrangement 220 for an EUV projection exposure apparatus for lithographic applications comprising an optical component 222, which has a substrate 224 and a surface 226 that is optically operative in the EUV spectral range and to which EUV radiation 228 is applied during the operation of the optical arrangement 220.

[0142] A cooling device 230 of the optical arrangement 220 is modified relative to the cooling device 200 of the optical arrangement 190 to the effect that it has, instead of the cooling container 202, a cooling line 232, through which a cryogenic cooling medium, for example a liquefied gas, for example liquid nitrogen, flows. As in the case of the cooling device 200, the cooling line 232 is thermally conductively connected to the optical component 222 via at least one thermally conductive element, for example a heat pipe, while the optical component 222 is accommodated in a chamber 236, and the at least one thermally conductive element 234 conducts the heat from the component 222 out of the chamber 236 to the cooling line 232. [0143] In contrast to the exemplary embodiment in accordance with Fig. 6, here the cryogenic cooling medium 234 is operated without a phase transition, similarly to the exemplary embodiment in Fig. 1.

[0144] A further cooling concept for an optical arrangement for an EUV projection exposure apparatus for lithographic applications is described below with reference to Fig. 8.

[0145] Fig. 8 shows an optical arrangement 240 for an EUV projection exposure apparatus for lithographic applications. The optical arrangement 240 comprises an optical component 242, which, in the exemplary embodiment shown, is embodied as a mirror, in particular as a collector mirror, but without being restricted thereto.

[0146] The optical component 242 has a substrate 244 and a reflective layer 245, the surface of which forms an optically operative surface 246.

[0147] In its function as a collector mirror, the optical component 242 collects radiation 248 emitted by a radiation source 247 and reflects it as usable EUV radiation 249. The optically operative surface 246 is correspondingly operative in the EUV spectral range, i.e. reflective in said spectral range.

[0148] The optical arrangement 240 furthermore comprises a cooling device 250 for cooling the optical component 242. The cooling concept of the cooling device 250 is based on the fact that the optical component 242 is part of a heat pipe 252 or is integrated into the heat pipe 252. The heat pipe 252 has a tubular container 254. In the container 254, a cooling medium 260 circulates between an evaporation end 256 (warm end) and a condensing end 258 (cold end), as is indicated by flow arrows 262, 264, 266, 268.

[0149] In this configuration, the entire surface 270 of a side of the optical component 242 that faces away from the optically operative surface 246 forms the evaporation end 256 of the heat pipe 252. The substrate 244 thus forms part of the heat pipe 252.

[0150] At the entire surface 270 of the substrate 244, the cooling medium 260 absorbs heat from the substrate 244 and evaporates. The cooling medium 260 is thus operated with a phase transition liquid-gaseous. The evaporated cooling medium 260 then flows to the condensing end 258, at which cooling blocks 272, 274 are arranged, which absorb and dissipate the heat from the gaseous cooling medium 260 at the condensing end 258. The cooling medium 260 correspondingly condenses again at the condensing end 258, and the liquid phase then returns to the evaporation end 256. In this case, the cooling medium 260 can also be cryogenic.

[0151] In order to support the circulation of the cooling medium 260 between the evaporation end 256 and the condensing end 258, in particular from the condensing end 258 to the evaporation end 256, a capillary material can be present in the container 254, or grooves or other roughnesses in the surface can be provided at the inner surface of the container 254, which support the return of the condensed cooling medium 260 to the evaporation end 256.

[0152] As is illustrated in Fig. 8, the tubular container 254 is embodied in conical fashion at its condensing end 258, to be precise with a tapering 278 facing towards the optical component 242. As a result, a circulation direction can be predetermined for the cooling medium 260, in accordance with which circulation direction the condensed cooling medium 260 returns substantially in the centre, i.e. near the optical axis of the optical component 242, to the evaporation end, where it is then distributed uniformly and areally towards the edge of the optical component 242, evaporates there again, etc.

[0153] With the cooling device 250, a substantially constant temperature can be obtained over the entire substrate 244. On account of the permanent phase transition liquid-gaseous or gaseous-liquid, the cooling medium 260 does not heat up. The cooling capacity which can be achieved with the cooling device 250 can attain more than 1 MW/m ² .

[0154] The cooling device 250 operates in particular without vibration, since the cooling medium film on the surface 270 is comparatively thin and formation of gas bubbles is thereby avoided to the greatest possible extent.

[0155] Since the heat pipe 252 operates near vacuum vapour pressures, the optical component 242 is not loaded by mechanical stresses.

[0156] The surface 270 can likewise be structured with grooves or capillaries in order to ensure a uniform distribution of the cooling medium 260 on the surface 270 and to make the cooled region of the surface 270 as large as possible. For this purpose, provision can also be made for configuring the surface 270 from a porous material.