Optical arrangement and EUV lithography system

Abstract

The invention relates to an optical arrangement (25) comprising: an optical element (Mi) for reflecting radiation, in particular for reflecting EUV radiation, which has a substrate (28) and a first surface (29a) for reflecting the radiation, onto which a reflective coating (30) is preferably applied, wherein the substrate (28) has at least one cavity (31) extending below the first surface (29a). The optical arrangement (25) also includes a cooling device (26) for dissipating heat from the substrate (28), preferably by flowing a cooling fluid (32) through the at least one cavity (31). The optical arrangement (25) further comprises a temperature control device (27) with one or more Peltier elements (33, 33') for locally temperature-controlling the substrate (28) and at least one temperature sensor (35) for determining a temperature (T), in particular a local temperature. S) of the substrate (28). The invention also relates to an EUV lithography system with at least one such optical arrangement (25).

Description

Background of the invention

[0001] The invention relates to an optical arrangement comprising: an optical element for reflecting radiation, in particular for reflecting EUV radiation, comprising a substrate and a first surface for reflecting radiation, preferably with a reflective coating applied to the substrate, wherein the substrate has at least one cavity extending below the first surface, and a cooling device for dissipating heat from the substrate, preferably by flowing a cooling fluid through the at least one cavity. The invention also relates to an EUV lithography system comprising at least one such optical arrangement.

[0002] For the purposes of this application, an EUV lithography system is understood to be an optical system that can be used in the field of EUV lithography. In addition to a projection exposure system for EUV lithography used for the fabrication of semiconductor devices, the lithography system may, for example, be an inspection system for inspecting a photomask (hereinafter also referred to as a reticle) used in such a projection exposure system, for inspecting a semiconductor substrate to be structured (hereinafter also referred to as a wafer), or a metrology system used for measuring a projection exposure system for EUV lithography or parts thereof, for example, for measuring a projection optic.

[0003] Optical elements for reflecting radiation (hereinafter also referred to as mirrors) used in lithography systems are typically exposed to inhomogeneous light at the optically used first surface (hereinafter also referred to as the mirror surface), onto which the reflective coating is applied. The location-dependent light exposure at the mirror surface of mirrors in lithography systems causes a location-dependent variation in heat input into the substrate material, resulting in location-dependent variation in substrate deformation and a concomitant deformation of the mirror surface. This deformation of the mirror surface typically leads to a loss in image quality. The variety of illumination settings that can be configured during the operation of a lithography system also means a variety in the location dependence of the light exposure at the mirror surface.

[0004] Various strategies exist to reduce the deformation of the mirror surface, but these generally have drawbacks. For example, the previously described flow of a cooling fluid, such as water, through cavities in the substrate can result in the pattern of the cavities being imprinted on the mirror surface as deformations. Furthermore, the heat generated at a hotspot (i.e., a location with a high local temperature or heat input) on the mirror surface is at least partially transferred from the hotspot to cooler areas of the substrate in the direction of fluid flow. Additionally, the arrangement of the cavities is fixed and not equally suitable for all lighting settings or light distributions on the mirror surface.

[0005] It is generally known to use manipulators to compensate for deformations on the mirror surface, which locally influence the temperature of the mirror's substrate. Such manipulators can, for example, consist of multiple piezoelectric elements that act on the substrate to deform it. However, piezoelectric elements exhibit hysteresis effects and are typically bonded to the substrate. Changes in the adhesive bond due to the absorption or release of water from the environment can also lead to undesirable deformations.

[0006] Manipulators in the form of heat emitters, such as IR light sources that emit heat radiation onto the substrate, are generally designed to generate only a predefined heating profile. Furthermore, the spatial resolution of the heating profile is limited due to blurring effects during projection with non-ideal optics. Another problem with heat emitters is the reflection of heat radiation from the mirror surface and scattering from internal walls, which can lead to heat transfer to adjacent mirrors or other components (crosstalk). The use of the previously described cavities, through which a cooling fluid flows, is also unsuitable for targeted local manipulation of the mirror surface shape or the mirror temperature, partly because controlling the fluid quantity or mass flow rate through the cavities is only possible mechanically. Object of the invention

[0007] The object of the invention is to provide an optical arrangement and an EUV lithography system that enable the most precise possible adjustment, in particular control of the surface shape or temperature of an optical element. Subject matter of the invention

[0008] This task is solved by an optical arrangement of the type mentioned above, further comprising: a temperature control device with one or more Peltier elements for locally temperature control of the substrate and at least one temperature sensor for determining a particularly local temperature of the substrate.

[0009] In the optical arrangement according to the invention, Peltier elements (thermoelectric coolers or TECs) are used in combination with temperature sensors and the cooling device described above for the precise adjustment and control of the surface shape of the first surface. Peltier elements can be installed almost seamlessly and with high spatial resolution, particularly near the first surface. Combined with at least one temperature sensor, and usually with multiple temperature sensors, the Peltier elements allow for purely electronic, precise control and adjustment of heat flows and temperature distributions in the substrate, and thus control over the shape of the first surface.

[0010] The optical arrangement makes it possible, in particular, to generate an inhomogeneous temperature distribution in the substrate of the optical element and in optical layers.

[0011] The consequences are mechanical stress and a change in the refractive index of the substrate. The stress leads to deformation of the substrate and to layer stresses, which manifest themselves, among other things, as a change in surface shape, but can also lead to birefringence. The change in refractive index and the birefringence can influence the reflectivity of the optical element, for example, a mirror. Potential applications can exploit some of these effects. The optical element is frequently referred to as a mirror in the following, but it can also be any other optical element where precise control of a surface shape is critical or advantageous, e.g., a reflective reticle for EUV imaging systems, diffraction gratings for EUVR systems, etc. Another possible application is for controlled inhomogeneous heat transfer from the edge of the NA aperture, where the heat input depends on the illumination setting.

[0012] The cooling device for dissipating heat from the substrate can be designed to allow a cooling fluid to flow through at least one cavity and may, for this purpose, include one or more fluid lines. However, it is also possible that the cooling device for dissipating heat from the substrate or from the interior of a vacuum housing in which the mirror is located—for example, a vacuum housing of a projection lens for a lithography system—comprises at least one highly thermally conductive structure, e.g., made of metal, which is located in or near the substrate and transfers heat to an outer wall of the housing to cool the substrate. In this case, fluid cooling via the cooling device may be unnecessary.

[0013] In one embodiment, at least one of the Peltier elements is introduced into the substrate, in particular embedded in the substrate or arranged in the at least one cavity. Embedding in the substrate means that the Peltier element is arranged in the substrate material outside the cavity(ies) and surrounded by the substrate material. Alternatively, the Peltier element can, for example, be introduced into a cavity within the substrate. The cavity can be permeated by the cooling fluid, but this is not mandatory. In any case, care must be taken to ensure that the internal structure of the Peltier element is insulated from the penetration of the cooling fluid if such penetration would cause problems (e.g.,Corrosion, material dissolution and fluid contamination due to electrolysis, Peltier element becomes less efficient due to the electrical conductivity of the cooling fluid, the control of the Peltier element becomes more complex due to the thermal conductivity of the cooling fluid, total failure due to short circuit, etc.).

[0014] A Peltier element can be considered a heat pump whose output and pumping direction are controlled by the application of an electric current. Therefore, during operation, i.e., when the electric current is applied, a Peltier element has a first, cooler side and a second, warmer side. In this embodiment, the Peltier element can, in principle, face a cavity through which the cooling fluid flows, with either its first, cooler side or its second, warmer side. Typically, the warm side of the Peltier element faces the respective cavity to transfer heat to the cooling fluid. The cooling fluid flows through each cavity to remove the "used" (heated) cooling fluid from the substrate of the mirror and replace it, for example, with a new, cooler portion of the cooling fluid via a heat exchanger.

[0015] The shape and distribution of the Peltier elements in the substrate depend on the variety of light distributions that the optical arrangement must compensate for to prevent deformation of the mirror surface. In an optical system such as a projection exposure unit, these light distributions depend, among other things, on the lighting settings configured within the projection exposure unit's lighting system.

[0016] In a further development of this embodiment, at least one Peltier element arranged in the cavity is in thermally conductive contact with the cooling fluid on a second side and is in thermally conductive contact with the substrate on a first side, or at least on the first side, particularly as a whole, not in mechanical contact with the substrate. As described above, the first side of the Peltier element is typically the side that absorbs heat, and the second side is typically the side that dissipates heat. The second side can be in direct contact with the cooling fluid or indirectly, for example, if a cooling line is routed along the second side of the Peltier element. It is also possible for the thermally conductive contact with the cooling fluid to be established via thermally conductive structures or via thermally conductive, e.g., metallic, components.The first side of the Peltier element can be in contact with the material of the mirror's substrate. For example, the first side can be in direct contact with the substrate or the wall of the mirror's cavity, or it can be in thermally conductive contact with the substrate via a thermal paste or similar material.

[0017] Alternatively, the first side of the Peltier element may have no mechanical contact with the substrate or mirror. In this case, heat can be transferred via the residual gas atmosphere in the mirror cavity and / or by thermal radiation, especially if the distance or gap between the Peltier element and the substrate is narrow. However, the heat transfer efficiency is lower than with direct or indirect mechanical contact; that is, generally less heat can be dissipated. It can be advantageous, however, if the Peltier element(s) and the supporting structures or components are completely mechanically separated from the substrate or mirror. This eliminates various mechanical influences on the mirror, such as the thermal expansion of the Peltier element(s), vibrations caused by the cooling fluid, etc.

[0018] In this embodiment, one or more fluid lines can run within the cavity containing the at least one Peltier element, or extend into the cavity. For this purpose, a section of a cooling line, which otherwise runs outside the housing in which the mirror is located, can be routed through a wall of the housing into the interior of the housing and into the cavity. However, it is also possible that the cooling line does not run within the housing in which the mirror is located, and that heat is transferred to the wall of the housing via one or more thermally conductive components located within the cavity, or extending into it, which may also serve as a support structure for the Peltier element(s).

[0019] It is also possible for active heat transfer to occur from the interior of the housing or vacuum chamber containing the mirror to a cooling line or a heat-conducting structure outside the housing. For this purpose, one or more Peltier elements can be used, for example, arranged on the side of the housing wall facing away from the interior and possibly separated from the cooling line or the environment by another wall.

[0020] In another embodiment, at least one of the Peltier elements is arranged between the cavities and the first surface. In this embodiment, the at least one Peltier element is typically controlled such that the heat generated at the first surface (i.e., the mirror surface) by a respective light distribution and absorbed by the substrate is absorbed on the cold side of the respective Peltier element facing the first surface and transferred to the cooling fluid, e.g., in the form of cooling water, via the second side of the Peltier element facing the cavities. The heat transfer through each Peltier element can be individually controlled or regulated in this case. It is also possible, in principle, to use the Peltier element for local heating of the mirror surface. In this case, the warm side of the Peltier element faces the mirror surface during operation.In this embodiment, the Peltier elements can be arranged particularly near the mirror surface and – individually controllable – distributed almost without gaps, i.e. with high spatial resolution, below the mirror surface.

[0021] In another embodiment, at least one of the Peltier elements is arranged between the cavity(ies) and a second surface of the substrate (on the back of the substrate) facing away from the first surface. The Peltier elements arranged between the cavities and the second surface are typically controlled such that their hot side faces the cavities and their cold side faces the second surface on the back of the substrate. In this way, heating of the mirror surface originating from the second surface can be avoided. The Peltier elements arranged between the cavities and the second surface also provide an additional means of manipulating the geometry or temperature of the mirror surface locally.

[0022] This embodiment is typically combined with the embodiment described above, in which at least one of the Peltier elements is arranged between the first surface and the cavity(ies). In this way, each cavity, which may be designed, for example, as a tubular cooling channel, can be surrounded by Peltier elements from both sides, i.e., from above and from below.

[0023] In another embodiment, at least one of the Peltier elements has a curvature that is preferably adapted to the curvature of the first surface of the substrate. Typically, the first and second sides of the Peltier element are parallel to each other, and the Peltier elements are (essentially) planar or flat. The flat shape of the Peltier elements is generally advantageous because it exhibits better homogeneity compared to the cavities through which the cooling fluid flows. However, it is also possible for the Peltier elements to deviate from a flat shape and have a curvature.

[0024] The shape of the Peltier elements can also be adapted to the geometry or shape of the first surface, which has the reflective coating. If the first surface has a concave or convex curvature, the respective Peltier element can also have a concave or convex curvature. The radius of curvature of each Peltier element can match the radius of curvature of the first surface, but this is not mandatory.

[0025] In a further embodiment, the at least one temperature sensor, and in particular a plurality of temperature sensors, is attached to or introduced into the substrate, in particular embedded in the substrate or arranged in the at least one cavity. The at least one temperature sensor can, in particular, be arranged near one or a group of Peltier elements to measure the local temperature of the substrate. For this purpose, the at least one temperature sensor can be arranged in a cavity through which the cooling fluid may flow; however, it is also possible for the temperature sensor to be embedded in the substrate and not in direct contact with the respective cavities.It is also possible, in principle, not to integrate the temperature sensor into the substrate, but to position it on the outside of the substrate or elsewhere, provided that the (local) temperature of the substrate can be inferred from the temperature measured by the temperature sensor. It is also generally possible to use the Peltier elements themselves as temperature sensors for the temperature difference between their hot and cold sides. For controlling Peltier elements, data from other sensor types (e.g., position, deformation, pressure, shear forces, centrifugal forces of flowing fluid, vibrations, sound, thermal radiation, electromagnetic fields) or information from control signals (e.g., from actuators, steppers, light sources, IR mirror heaters) can be taken into account.

[0026] The use of temperature sensors makes it possible to operate the optical arrangement in a controlled manner. For example, each Peltier element or group of Peltier elements can be assigned a temperature sensor located in the immediate vicinity of the respective Peltier element to allow for local temperature control of the substrate; however, such an arrangement is not strictly necessary.

[0027] In another embodiment, the temperature control device is designed to regulate a local temperature of the substrate by acting on at least one of the Peltier elements, in particular for the dynamic adjustment of a predetermined deformation of the first surface in the case of a non-steady-state temperature distribution in the substrate. As described above, a local (actual) temperature can be measured using the temperature sensors, and the temperature control device, which acts on the Peltier elements, can regulate the actual temperature to a predetermined target temperature. Such regulation allows the shape of the mirror surface to be controlled, for example, to reduce the deformation of the mirror surface due to the location-dependent variation in the amount of light applied (so).Reducing the deformation of the mirror surface can, for example, serve to compensate for temperature inhomogeneities caused by different lighting settings. Temperature inhomogeneities on the mirror surface resulting from the arrangement of cavities or cooling channels in the substrate can also be compensated for by the Peltier elements. In this case, the arrangement of the cavities is not visible as an inhomogeneous temperature distribution on the mirror surface.

[0028] In principle, the combination of Peltier elements and temperature sensors allows for purely electronic, precise control of heat flows and temperature distributions within the optical element. This can also be used to employ the optical arrangement as an optical manipulator for the targeted adjustment of the wavefront of the incident light. The response time of the Peltier elements is on the order of seconds. The response time to the deformation of the mirror surface depends on the thermal conductivity, the mirror material, and the distance between the mirror surface and the respective Peltier element.It is also fundamentally possible to set the desired, predetermined shape of the first surface of the mirror significantly faster than the longest reaction time, and to keep this shape constant by continuously controlling the temperature control device, while internally a steady distribution of heat flows and temperatures can establish itself in the substrate over a longer period of time, i.e., while the substrate has an inhomogeneous temperature distribution, which is also referred to as dynamic shape retention or dynamic temperature control for fast profile manipulation.

[0029] The three-dimensional temperature distribution within the substrate, and consequently the surface temperature distribution, can be inhomogeneous to establish a target temperature profile at the first surface. There are infinitely many three-dimensional temperature distributions that establish or generate the same two-dimensional temperature profile at the first surface. This fact can be used for rapid profile manipulation in the dynamic temperature control described above, where the heat flows and the temperature distribution within the substrate are initially unsteady.

[0030] In another embodiment, a plurality of Peltier elements are arranged longitudinally in a staggered arrangement within at least one cavity through which the cooling fluid flows. If a large amount of heat is introduced locally at a specific location (hotspot) on the mirror surface, this leads to a strong local heating of the cooling fluid in the underlying cavity; that is, the cooling fluid heats up locally. The heat introduced into the respective cavity at this location is transported by the cooling fluid along its flow direction; that is, the heat is "smeared" in the direction of the cooling fluid flow.In the event that several individually controllable Peltier elements are arranged offset from each other in the longitudinal direction of the respective cavity, the effect of heat exchange in the direction of flow of the cooling fluid on the local temperature at the mirror surface can be suppressed by arranging the Peltier elements, which are laterally offset to the hotspot and in the direction of flow of the cooling fluid, to take into account the effect of heat transport during cooling of the mirror surface in order to homogenize the temperature at the mirror surface.

[0031] Another aspect of the invention relates to an EUV lithography system comprising: at least one optical arrangement as described above. The EUV lithography system may be an EUV lithography system for exposing a wafer or another optical arrangement that uses EUV radiation, for example, an EUV inspection system, e.g., for inspecting masks, wafers, or the like used in EUV lithography. As described above, the optical arrangement can be used to compensate for deformations on the mirror surface and thus improve the image quality of the EUV lithography system.

[0032] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures in the drawing, which show details essential to the invention, and from the claims. The individual features can be implemented individually or in any combination in a variant of the invention. drawing

[0033] Examples of implementation are shown in the schematic drawing and are explained in the following description. It shows Fig. 1. Schematic representation in meridional section of a typical projection exposure system for EUV projection lithography, Fig. 2 A schematic representation of an optical arrangement with a directly cooled mirror having a substrate with cavities through which a cooling fluid in the form of cooling water flows, in a cross-section Fig. 3 a schematic representation of the directly cooled mirror of Fig. 2 in a longitudinal section of one of the cavities, Fig. 4a-c Schematic representations of the distribution of heat input at the mirror surface, the temperature of the cooling fluid and the temperature of the mirror surface, each in the longitudinal direction of the cavity of Fig. 3, Fig. 5a,b a schematic representation of an optical arrangement analogous to Fig. 2, in which Peltier elements are embedded in the substrate in addition to the cavities, Fig. 6 a schematic representation of the directly cooled mirror of Fig. 5a in a longitudinal section of one of the cavities, Fig. 7a-c schematic representations of the distribution of heat input onto the surface of the mirror, the temperature of the cooling fluid and the temperature at the surface of the mirror, each in the longitudinal direction of the cavity, Fig. 8a,b schematic representations of an optical arrangement in which the mirror has a cavity in which a Peltier element is arranged, the first side of which is in thermally conductive contact with the substrate and the second side of which is in thermally conductive contact with a cooling fluid, Fig. 8c,d schematic representations analogous to Fig. 8a,b, in which the first side of the Peltier element or the Peltier element as a whole is spaced away from the substrate, as well as Fig. 9a-c schematic representations of an optical arrangement with a mirror having a cavity in which several Peltier elements consisting of the substrate are attached to a common support structure, wherein a fluid line runs in the support structure ( Fig. 9a) or wherein the support structure is designed as a thermally conductive structure which is in thermally conductive contact with a cooling fluid ( Fig. 9b,c).

[0034] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0035] The following will refer to Fig. 1. The essential components of an optical arrangement for EUV lithography, in the form of a projection exposure system 1 for microlithography, are described as an example. The description of the basic structure of the projection exposure system 1 and its components is not to be understood as restrictive.

[0036] One embodiment of a lighting system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, a lighting optic 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the lighting system. In this case, the lighting system does not include the light source 3.

[0037] A reticule 7 located in the object field 5 is illuminated. The reticule 7 is held by a reticule holder 8. The reticule holder 8 can be moved, particularly in one scanning direction, via a reticule displacement drive 9.

[0038] In Fig. Figure 1 shows a Cartesian xyz coordinate system for illustrative purposes. The x-direction runs perpendicular to the plane of the drawing. The y-direction runs horizontally, and the z-direction runs vertically. The scan direction runs in the Fig. 1 along the y-direction. The z-direction runs perpendicular to the object plane. 6.

[0039] The projection exposure system 1 comprises a projection system 10. The projection system 10 serves to image the object field 5 onto an image field 11 in an image plane 12. A structure on the reticulum 7 is imaged onto a light-sensitive layer of a wafer 13 located in the image plane 12 within the area of ​​the image field 11. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, particularly along the y-direction, via a wafer transfer drive 15. The displacement of the reticulum 7 via the reticulum transfer drive 9 and of the wafer 13 via the wafer transfer drive 15 can be synchronized with each other.

[0040] Radiation source 3 is an EUV radiation source. Specifically, radiation source 3 emits EUV radiation 16, which is also referred to below as useful radiation, illumination radiation, or illumination light. The useful radiation has a wavelength in the range between 5 nm and 30 nm. Radiation source 3 can be a plasma source, for example, an LPP source (laser-produced plasma) or a DPP source (gas-discharged produced plasma). It can also be a synchrotron-based radiation source. Radiation source 3 can be a free-electron laser (FEL).

[0041] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 can be a collector mirror with one or more ellipsoidal and / or hyperboloid reflective surfaces. The at least one reflective surface of the collector mirror 17 can be illuminated by the illumination radiation 16 at grazing incidence (GI), i.e., with angles of incidence greater than 45°, or at normal incidence (NI), i.e., with angles of incidence less than 45°. The collector mirror 17 can be structured and / or coated to optimize its reflectivity for the useful radiation and to suppress stray light.

[0042] After the collector mirror 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector mirror 17, and the illumination optics 4.

[0043] The illumination optics 4 comprise a deflecting mirror 19 and, downstream in the beam path, a first faceted mirror 20. The deflecting mirror 19 can be a planar deflecting mirror or, alternatively, a mirror with a beam-shaping effect in addition to its deflecting function. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter that separates a useful wavelength of the illumination radiation 16 from stray light of a different wavelength. The first faceted mirror 20 comprises a plurality of individual first facets 21, which are hereinafter also referred to as field facets. Of these facets 21, the Fig. 1 Only a few examples are shown. In the beam path of the lighting optics 4, a second faceted mirror 22 is arranged downstream of the first faceted mirror 20. The second faceted mirror 22 comprises a plurality of second facets 23.

[0044] The illumination optics 4 thus form a double-faceted system. This basic principle is also known as a honeycomb condenser (fly's eye integrator). With the aid of the second faceted mirror 22, the individual first facets 21 are imaged into the object field 5. The second faceted mirror 22 is the last beam-shaping, or indeed the last, mirror for the illumination radiation 16 in the beam path before the object field 5.

[0045] The projection system 10 comprises a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the projection exposure system 1.

[0046] In the Fig. In the example shown, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have an aperture for the illumination radiation 16. The projection system 10 is a double-obscured optic. The projection optic 10 has an image-side numerical aperture greater than 0.4 or 0.5, and which can also be greater than 0.6, for example, 0.7 or 0.75.

[0047] The mirrors Mi, just like the mirrors of the lighting optics 4, can have a highly reflective coating for the lighting radiation 16.

[0048] Fig. Figure 2 shows an example of an optical arrangement 25 comprising a mirror Mi (i = 1, ..., 6) of the projection optics 10 and a cooling device 26, which, for the sake of simplicity, is shown in Fig. 2 is represented in the form of a square. The mirror Mi has a substrate 28 made of a material with a low coefficient of thermal expansion, which in the example shown is ULE®. The substrate 28 has a first (upper) surface 29a (hereinafter: mirror surface) onto which a reflective coating 30 is applied, which reflects the EUV radiation 16 (cf. Fig. 1) reflected.

[0049] The cooling device 26 is designed to supply a cooling fluid 32 to a plurality of cavities 31 formed in the substrate 28 and to discharge the cooling fluid 32 from the cavities 31. In the example shown, the cooling fluid 32 is water, which has a high heat capacity and is therefore particularly well suited to dissipating the heat introduced into the substrate 28 when the mirror surface 29a is exposed to EUV radiation 16. In the example shown, the cavities 31 are designed as straight, tubular cooling channels with a constant, circular cross-section, as shown by Fig. 2, which define the cavities 31 in a cross-section in an XZ-plane of an XYZ coordinate system as well as by means of Fig. Figure 3 shows one of the cavities 31 in a longitudinal section in an XY plane of the XYZ coordinate system.

[0050] As in Fig. 2 indicated by double arrows and by dashed lines on the mirror surface 29a, the pattern of the cavities 31 spaced apart from each other in the X direction is imprinted on the mirror surface 29a in the form of deformations when the cooling fluid 32 flows through it, which has an adverse effect on the image quality of the projection optics 10.

[0051] As a rule, the heat input into the substrate 28 or onto the mirror surface 29a is not homogeneous, but varies depending on location. Fig. 4a shows, by way of example, the heat input W along the longitudinal direction Y of the in Fig. 3 of the cavity shown 31. As in Fig. As can be seen in Figure 4a, the spatial distribution of the heat input W exhibits a maximum ("hotspot") at a position P in the Y direction, as shown in Fig. 3 is indicated by a dashed line. At position P of the hotspot in the longitudinal direction Y of the cavity 31, there is a significant increase in the temperature Tw of the cooling fluid 32. As in Fig. As can be seen in Figure 4b, the heat introduced into the respective cavity 31 at position P is transported by the cooling fluid 32 along its flow direction (negative Y direction), i.e., the heat is "smeared" in the flow direction of the cooling fluid 32. The inhomogeneous temperature distribution Tw of the cooling fluid 32 results in an inhomogeneous temperature distribution Ts on the mirror surface 29a along the longitudinal direction Y of the cavity 31, which is Fig. 4c is shown.

[0052] The one related to Fig. 2, Fig. 3 and Fig. The negative effects of cooling with the cooling fluid 32 described in 4a-c can be counteracted by using an optical arrangement 25 as described in Fig. 5a,b is designed and, in addition to the cooling device 26, has a temperature control device 27, which is shown in the form of a square for the sake of simplicity.

[0053] The temperature control device 27 has a plurality of Peltier elements 33, 33' which are embedded in the material of the substrate 28. In the case of the Fig. In the example shown in 5a, the Peltier elements 33 are arranged in the thickness direction Z of the substrate 28 between the mirror surface 29a and the plurality of cavities 31. In the example shown in Fig. In the example shown in Figure 5b, first Peltier elements 33 are arranged in the thickness direction Z of the substrate 28 between the mirror surface 29a and the cavities 31, and second Peltier elements 33' are arranged between the cavities 31 and a second surface 29b on the back side of the substrate 28, which faces away from the mirror surface 29a. In the example shown in Fig. In the example shown in 5b, the cavities 31 are practically enclosed by the Peltier elements 33, 33'.

[0054] The Peltier elements 33, 33' are essentially flat and have a first side 34a, 34a' facing away from the cavities 31 and a second side 34b, 34b' facing the cavities 31. The Peltier elements 33 act as a heat pump and, in the example shown, typically transport heat from the respective first side 34a, 34a' to the respective second side 34b, 34b'. In other words, the first Peltier elements 33 serve to cool the mirror surface 29a and transfer heat from the mirror surface 29a to the cooling fluid 32, which flows through the cavities 31. Accordingly, the second Peltier elements 33' serve to cool the second surface 29b on the back of the mirror Mi. Due to the planar design of the Peltier elements 33, 33', which are arranged at a small distance from each other, the imprinting of a pattern on the mirror surface 29a, which is created by the cavities 31, can be avoided.Nevertheless, the Peltier elements 33, 33' can be used to efficiently influence the surface shape of the mirror surface 29a, as shown in . Fig. 5a,b is indicated by dashed lines.

[0055] The respective Peltier elements 33, 33' can be planar, as shown in Fig. 5a. However, it is also fundamentally possible that the Peltier elements 33, 33' have a curvature, as shown in Fig. Figure 5b shows one of the four first Peltier elements 33 and one of the four second Peltier elements 33'. The concave curvature of the respective Peltier element 33, 33' is adapted to the concave curvature of the mirror surface 29a.

[0056] As in Fig. As can also be seen in Figures 5a and 5b, the optical arrangement 25 in the example shown includes a temperature sensor 35 embedded in the substrate 28. The temperature sensor 35 is located near the mirror surface 29a and serves to determine the local temperature of the substrate 28 near the mirror surface 29a. It is understood that, unlike in Figure 5a and 5b, the optical arrangement 25 in the example shown includes a temperature sensor 35 embedded in the substrate 28. The temperature sensor 35 is located near the mirror surface 29a and serves to determine the local temperature of the substrate 28 near the mirror surface 29a. Fig. As shown in Figures 5a and 5b, a plurality of temperature sensors 35 can be embedded in the substrate 28 or, if necessary, introduced into the cavities 31. Each temperature sensor 35 can, for example, be assigned to one of the Peltier elements 33, 33' or to a group of Peltier elements 33, 33' in order to determine the local temperature of the substrate 28 with spatial resolution. Based on the spatially resolved actual temperature distribution in the substrate 28, the temperature control device 27 can individually control or act upon the respective Peltier elements 33, 33' to regulate the local temperature of the substrate 28 to a local setpoint temperature. The control can be used to compensate for deformations of the mirror surface 29a that are due to locally varying heat input into the substrate 28 by the EUV radiation 16 (as shown). Fig. The optical arrangement 25 shown in Figures 5a and 5b can also be used to selectively deform the shape of the mirror surface 29a locally in order to manipulate the wavefront of the incident EUV radiation 16 and to use the optical arrangement 25 as a thermal manipulator. The temperature control device 27 can optionally also act on the cooling device 26 to regulate the temperature and / or the flow rate of the cooling fluid 32 through the cavities 31. In this way, even with a non-steady-state temperature distribution in the substrate 28, a dynamic adjustment of a predetermined shape of the first surface 29a or dynamic temperature control for rapid profile manipulation can be achieved.

[0057] With the help of the temperature control unit 27, it is also possible to do this in connection with Fig. 4a-c to reduce the smearing of heat generated at a hotspot at position P on the mirror surface 29a in the flow direction (negative Y direction) of the cooling fluid 32. For this purpose, a group of Peltier elements 33 can be used, arranged side by side in the longitudinal direction Y of a respective cavity 31, as described in Fig. 6 is shown. As in Fig. As can be seen in 7b, the temperature profile Tw of the cooling fluid 32, influenced by the hotspot, corresponds to that in Fig. The temperature profile Tw of the cooling fluid 32 shown in 4b is consistent.

[0058] With the aid of the individually controllable Peltier elements 33, the effect of the location-dependent variation in the temperature Tw of the cooling fluid 32 in the longitudinal direction of the cavity 31 on the temperature Ts at the mirror surface 29a can be compensated, as shown in Fig. Figure 7c shows that, despite the heat transfer in the flow direction of the cooling fluid 32, a substantially homogeneous temperature Ts can be generated at the mirror surface 29a in this way. The Peltier elements 33 thus compensate for temperature differences at the mirror surface 29a caused by the heat transfer in the flow direction of the cooling fluid 32.

[0059] Fig. Figures 8a-d show an optical arrangement 25 in which a Peltier element 33 is arranged in a cavity 31 of the substrate 28. The second side 34b of the Peltier element 33 is in thermal contact with the cooling fluid 32, which flows along the second surface 29b on the underside of the substrate 28.

[0060] At the in Fig. In the optical arrangement 25 shown in Figure 8a, the first side 34a of the Peltier element 33 is in thermal contact with the substrate 28 via a thermal paste 36, which can be, for example, in the form of an adhesive. The cavity 31 containing the Peltier element 33 is in Fig. 8a is closed by means of a cover 37, which is sealed by means of adhesive beads 38. In this way, it can be prevented that the cooling fluid 32 can penetrate into the cavity 31.

[0061] At the in Fig. In the example shown in 8b, the cover 37 is screwed onto the substrate 28 via a thread 40, and the cavity 31 is sealed against the environment and against the cooling fluid 32 by means of annular seals 39. In the example shown in Fig. In the example shown in Figure 8c, the first side 34a of the Peltier element 33 is spaced away from the substrate 28, and heat transfer takes place via a narrow gap 41 between the first side 34a of the Peltier element 33 and the substrate 28 by thermal radiation or via the residual gas present in the vacuum housing. In this case, the Peltier element 33 is attached to the cover 37 by means of an adhesive bead 38. The cover 37 is as shown in Figure 8c. Fig. 8b is screwed onto the substrate 28 via a thread 40.

[0062] At the in Fig. In the optical arrangement 25 shown in Figure 8d, the first side 34a of the Peltier element 33 is also spaced away from the substrate 28. The second side 34b of the Peltier element 33 is attached to a wall 46 of a vacuum housing that surrounds the mirror Mi. The vacuum housing is a housing for the projection optics 10. The mirror Mi is spaced from the wall 46 of the vacuum housing by a gap 45, through which heat transfer occurs by thermal radiation or via the residual gas present in the vacuum housing to a cooling fluid 32 that flows along the side of the wall 46 facing away from the mirror Mi. The wall 46 can also be an outer wall of a fluid line in which the cooling fluid 32 flows. The Peltier element 33 as a whole is shown in the Fig. The example shown in Figure 8d is not connected to the substrate 28 of the mirror Mi. In this way, various mechanical influences on the mirror Mi can be eliminated, such as the thermal expansion of the Peltier element 33, vibrations caused by the cooling fluid 32, etc.

[0063] Fig. Figures 9a-c show optical arrangements 25 in which four Peltier elements 33 are attached to a common support structure 47 and positioned in a cavity 31 of the substrate 28. The support structure 47 is attached to the wall 46 of the vacuum housing in which the mirror Mi is located. The Peltier elements 33 are positioned as shown in Fig. 8d is not in mechanical contact with the substrate 28, since the mirror Mi is separated from the wall 46 of the vacuum housing by a slit 45 and the Peltier elements 33 are separated from the substrate 28 by a slit 41. In the optical arrangements 25 of Fig. Thermally conductive plates 48 are attached to the first sides 34a of the Peltier elements 33 in sections 9a-c to improve heat conduction to the substrate 28 and to adapt the flat shape of the heat-absorbing surface of the Peltier elements 33, as shown in the example, to the shape of the back of the substrate 28 facing away from the mirror surface 29a. For this purpose, some of the plates 48 have a curvature on their side facing the substrate 28 that matches the curvature of the back of the substrate 28. The plates 48 also extend laterally beyond the Peltier elements 33 to achieve the most complete possible coverage of the back of the substrate 28 and thus enable optimal heat transfer from the substrate 28 to the Peltier elements 33. Adapting the geometry of the Peltier elements 33 to the geometry of the substrate 28 would also be possible in principle, but is more complex to implement.Each pair of adjacent Peltier elements 33 is separated from each other by a heat-insulating component 49 to prevent crosstalk when adjusting the temperature.

[0064] At the in Fig. The optical arrangement 25 shown in Figure 9a has a fluid line 50 that runs outside the vacuum housing containing the mirror Mi, and a section that runs inside the support structure 47 and serves to dissipate heat from the Peltier elements 33. The wall 46 of the vacuum housing has two openings through which the fluid line 50 enters and exits the vacuum housing.

[0065] At the in Fig. In the optical arrangement 25 shown in 9b, the wall 46 is closed and the cooling fluid 32 flows along the outside of the wall 46 of the vacuum housing, as is also shown in Fig. Figure 8d is shown. The wall 46 can also form the outer wall of a fluid line that runs outside the vacuum housing. In the example shown, the support structure 47 is solid and made of a thermally conductive metallic material to dissipate heat from the Peltier elements 33. The in Fig. 9b The cooling device not shown in the illustration can also be designed to dissipate heat from the substrate 28 not by means of the cooling fluid 32, but by means of a further heat-conducting structure which is attached outside the vacuum housing or on the side of the wall 46 facing away from the mirror Mi.

[0066] As in Fig. As shown in Figure 9c, active heat transfer from the vacuum housing containing the mirror Mi to the surroundings can occur. For this purpose, additional Peltier elements 33' are attached to the side of the wall 46 of the vacuum housing facing away from the mirror Mi. The first side 34a' of these elements faces the wall 46 of the vacuum chamber, and the second side 34b' of these elements faces another wall 50, along the outer surface of which the cooling fluid 32 flows. The double-walled design allows heat to be dissipated from the substrate 28, or from the vacuum housing containing the mirror Mi, in a particularly efficient manner.

[0067] To simplify the presentation, 25 of the optical arrangements are shown. Fig. 8a-d and of Fig. No temperature sensors are shown in sections 9a-c, but these are also present in these optical arrangements 25 and can be found in connection with the above. Fig. 2 to Fig.It must be designed as described in section 6. The temperature can also be set or regulated as described above.

[0068] In summary, the (local) temperature of the mirror Mi, in particular the temperature T, is efficiently controlled or regulated in the manner described above. S on the mirror surface 30, enables.

Claims

[1] Optical arrangement (25) comprising: an optical element (Mi) for reflecting radiation, in particular for reflecting EUV radiation (16), which has a substrate (28) and a first surface (29a) for reflecting the radiation, to which a reflective coating (30) is preferably applied, wherein the substrate (28) has at least one cavity (31) which runs below the first surface (29a), and a cooling device (26) for dissipating heat from the substrate (28), preferably by flowing through the at least one cavity (31) with a cooling fluid (32), characterized by a temperature control device (27) with one or a plurality of Peltier elements (33, 33') for locally controlling the temperature of the substrate (28) and at least one temperature sensor (35) for determining a particularly local temperature (Ts) of the substrate (28). [2] Optical arrangement according to claim 1, in which at least one of the Peltier elements (33, 33') is introduced into the substrate (28), in particular embedded in the substrate (28) or arranged in at least one cavity (31). [3] Optical arrangement according to claim 2, wherein at least one Peltier element (33) arranged in the cavity (31) is in heat-conducting contact with the cooling fluid (32) on a second side (34b) and is either in heat-conducting contact with the substrate (28) on a first side (34a) or is not in mechanical contact with the substrate (28) at least on the first side (34a), in particular as a whole. [4] Optical arrangement according to one of the preceding claims, in which at least one of the Peltier elements (33) is arranged between the at least one cavity (31) and the first surface (29a). [5] Optical arrangement according to one of the preceding claims, in which at least one of the Peltier elements (33') is arranged between the at least one cavity (31) and a second surface (29b) of the substrate (28) facing away from the first surface (29a). [6] Optical arrangement according to one of the preceding claims, in which at least one of the Peltier elements (33) has a curvature which is preferably adapted to a curvature of the first surface (29a) of the substrate (28). [7] Optical arrangement according to one of the preceding claims, in which the at least one temperature sensor (35), in particular a plurality of temperature sensors, are attached to the substrate (28) or introduced into the substrate (28), in particular embedded in the substrate (28) or arranged in the at least one cavity (31). [8] Optical arrangement according to one of the preceding claims, in which the temperature control device (27) is designed to regulate a preferably local temperature (Ts) of the substrate (28) by acting on at least one of the Peltier elements (33, 33'), in particular for dynamically adjusting a predetermined shape of the first surface (29a) in the case of a non-stationary temperature distribution in the substrate (28). [9] Optical arrangement according to one of the preceding claims, in which a plurality of Peltier elements (33) are arranged offset from one another along a longitudinal direction (Y) of at least one cavity (31) through which the cooling fluid (32) flows. [10] EUV lithography system (1), comprising: at least one optical arrangement (25) according to one of the preceding claims.

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