Projection system with minimized thermally induced oscillating aberrations

Abstract

The present invention relates to a projection system for a projection exposure system for microlithography, in particular for EUV lithography, comprising an optical element. The objective of providing a projection system for a projection exposure system for microlithography, in particular for EUV lithography, in which thermally induced oscillating aberrations are minimized, is achieved by the projection system comprising a radiation-absorbing element which is configured to temporarily interrupt the beam path of the projection system behind the optical element.

Description

[0001] The present invention relates to a projection system for a projection exposure system for microlithography, in particular for EUV lithography, comprising an optical element. The invention further relates to a projection exposure system for microlithography and a method for fabricating integrated circuits using microlithography.

[0002] Projection systems for microlithography, according to the current state of the art, are subject to periodically changing thermal stress caused by the microlithography work cycle. During the exposure of a semiconductor wafer, the optical elements of the projection system heat up due to the comparatively high light intensity of the exposure light. This is because light absorption by the optical elements, while largely reduced, cannot be completely suppressed due to physical limitations. Conversely, to ensure that no light enters the wafer area during a wafer change, measures are typically taken to prevent the exposure light from penetrating the projection system during this time. Consequently, the optical elements cool down again during the wafer change.Within a microlithographic work cycle, which includes both a wafer exposure phase and a wafer change phase, periodic, thermally induced changes in size and / or deformation of the optical elements occur, also known as thermal vibrations. These thermal vibrations can lead to oscillating optical aberrations of the wavefront emerging from the projection system, which in turn negatively affect the optical performance during wafer exposure and thus the lithographic yield and throughput. This is a particularly critical problem in EUV lithography, as higher optical precision is generally required and the EUV projection systems used are especially susceptible to thermally induced, oscillating aberrations.However, this problem generally also arises with projection systems that are not used in microlithography, and always occurs when no light enters the projection system during a temporary dead time.

[0003] Against this background, the present invention aims to provide a projection system for a projection exposure system for microlithography, in particular for EUV lithography, in which thermally induced oscillating aberrations are minimized. Furthermore, the invention aims to provide a corresponding projection exposure system for microlithography and a corresponding method for the fabrication of integrated circuits by means of microlithography.

[0004] According to a first teaching of the present invention, the above-mentioned problem for a projection system for a projection exposure system for microlithography, in particular for EUV lithography, comprising an optical element, is solved by the projection system comprising a radiation-absorbing element which is configured to temporarily interrupt the beam path of the projection system behind the optical element.

[0005] In the context of the present invention, a projection system is understood to be an arrangement comprising at least one optical element that projects a beam of light emanating from a radiation source from an object plane onto an image plane. The projection system is a projection system for a projection exposure unit for microlithography. In particular, the projection system is a projection system for a projection exposure unit for EUV lithography. For example, the projection system is or comprises a projection lens. Furthermore, in this context, an optical element is understood to be, in particular, an imaging optical element, such as a lens, a mirror, a prism, a flat plate, or the like.

[0006] In the context of the present invention, the term 'light' is understood to mean electromagnetic radiation in general, without implying a restriction to the spectral range of visible light.

[0007] In the context of the present invention, interrupting the beam path behind the optical element means that the beam path is interrupted at a point that is struck or traversed by a light beam at a later time than the point where the optical element is located, according to the speed of propagation of light. In other words, the point where the beam path is interrupted is located in the beam path behind the point where the optical element is located.

[0008] As the inventors recognized, the measures to prevent light from entering the projection system, which are otherwise usually necessary during a dead time of the projection system, such as the time of a wafer change, can be omitted if a radiation-absorbing element according to the invention is present, which is configured to temporarily interrupt the beam path of the projection system behind the optical element. From the perspective of the projection system, therefore, no input-side measures are taken during a dead time to prevent light from entering the projection system; instead, the beam path is interrupted at the output side or within the projection system.This means that an optical element of the projection system, which is positioned in the beam path in front of the radiation-absorbing element, can be permanently exposed to the light beam emitted by the radiation source and accordingly absorbs light. Due to this permanent light absorption, a steady thermal state can be established for the optical element instead of thermal oscillations, since the optical element assumes a temperature corresponding to thermal equilibrium. Consequently, in a projection system according to the invention, the oscillating aberrations caused by thermal oscillations can be minimized for the optical element.Since the temperature of thermal equilibrium can be higher than the ambient temperature, and since this temperature increase can in turn lead to a change in the beam path at the optical element due to deformation, it is not impossible that time-constant optical aberrations may occur in a projection system according to the invention; however, such time-constant aberrations are significantly easier to correct than aberrations that oscillate over time. As described at the outset, oscillating optical aberrations caused by thermal vibrations pose a problem in microlithography, which is particularly critical in the field of EUV lithography. In these areas, the use of a projection system according to the invention is therefore particularly advantageous in order to minimize the thermally induced, oscillating aberrations.

[0009] In addition to minimizing thermally induced oscillating aberrations as described above, the radiation-absorbing element, which is designed to temporarily interrupt the beam path of the projection system behind the optical element, also ensures that no light escapes from the projection system during a dead time, i.e., that no light enters the wafer area during a wafer change, for example.

[0010] In one embodiment of the invention, the radiation-absorbing element is a beam dump. A beam dump is a radiation-absorbing element that is inserted into the beam path to interrupt the beam path and must be completely removed from the beam path to restore it. For example, the beam dump may have a sliding mechanism, a pivoting mechanism, or the like. In this way, the beam dump can be configured to temporarily interrupt the beam path of the projection system.

[0011] In an alternative embodiment of the invention, the radiation-absorbing element is a beam shutter. A beam shutter is understood to be a radiation-absorbing element that is permanently located in the beam path and has an aperture mechanism to temporarily interrupt and then release the beam path. In this way, the beam shutter can also be configured to temporarily interrupt the beam path of the projection system. The aperture mechanism can have a circular opening, a substantially circular opening in the form of a polygon, or a slit-shaped opening.

[0012] In a further alternative embodiment of the invention, the radiation-absorbing element is a combination of a movable mirror and a stationary beam trap. In this embodiment, the movable mirror can be inserted into the beam path to temporarily direct the beam path onto the stationary beam trap, through which the light radiation is then absorbed. In this way, the combination of movable mirror and stationary beam trap can also be configured to temporarily interrupt the beam path of the projection system.

[0013] In a preferred embodiment of the invention, the projection system comprises at least one further optical element. In this embodiment, the radiation-absorbing element is configured to interrupt the beam path of the projection system downstream of the at least one further optical element. In principle, each optical element of the projection system that is arranged in the beam path upstream of the radiation-absorbing element can be permanently exposed to the light beam emitted by the radiation source and can therefore absorb light. Thus, a steady thermal state can be established for each corresponding optical element instead of thermal oscillations.As a result, in this embodiment of the projection system according to the invention, thermally induced oscillating aberrations can be minimized for each optical element which is arranged in the beam path in front of the radiation-absorbing element, which overall leads to a particularly low-aberration projection system.

[0014] Notwithstanding the above, certain optical elements of the projection system can also be positioned in the beam path behind the radiation-absorbing element. While such optical elements are subject to periodically changing thermal stress, this may not be harmful or may even be desirable for certain optical elements.

[0015] In a further preferred embodiment of the invention, the radiation-absorbing element is configured to interrupt the beam path of the projection system behind the last, the penultimate, or the third-to-last optical element of the projection system. In the context of the present invention, the last optical element of the projection system is the optical element that a light beam, corresponding to the speed of propagation of light, strikes or passes through last, particularly at the latest time. Similarly, the penultimate optical element of the projection system is the optical element that a light beam, corresponding to the speed of propagation of light, strikes or passes through at the penultimate position, i.e., last before the last optical element.Furthermore, the third-to-last optical element of the projection system is the optical element that is hit or passed through by a light beam at the third-to-last position, i.e., last before the penultimate optical element, according to the propagation speed of light.

[0016] To interrupt the beam path of the projection system behind an optical element, the radiation-absorbing element can be temporarily inserted into the beam path behind the optical element for a desired duration of interruption. Particularly for space reasons, it can be advantageous to configure the radiation-absorbing element to interrupt the beam path of the projection system behind the penultimate or third-to-last optical element. More installation space is typically available behind the third-to-last optical element than, for example, behind the second-to-last optical element, and especially behind the last optical element, which is usually located close to the wafer.

[0017] Furthermore, thermal vibrations are less pronounced in optical elements located further back in the projection system, particularly the last or penultimate optical element. Optical elements that are struck or traversed by a light beam later than those located further forward in the projection system, according to the speed of light propagation, heat up comparatively less and, accordingly, cool down less when the beam path is interrupted in front of these elements.

[0018] In a further preferred embodiment of the invention, a surface of the radiation-absorbing element has an absorptivity of at least 0.9, preferably at least 0.95, and particularly preferably at least 0.98. For example, the surface can have an absorptivity of 0.99 or more. In this context, the absorptivity is specifically the spectral absorptivity for any wavelength when light is incident perpendicular to the surface. This means that the surface of the radiation-absorbing element has a spectral absorptivity of at least 0.9, preferably at least 0.95, and particularly preferably at least 0.98, for example 0.99 or more, for at least one wavelength when light is incident perpendicularly.The at least one wavelength can, in particular, be a wavelength contained in the wavelength spectrum of a light beam passing through the projection system during normal operation. Furthermore, the surface of the radiation-absorbing element can, in particular, be a surface designed to be inserted into the beam path of the projection system in order to interrupt the beam path. Higher absorption coefficients of the surface generally result in higher light absorption by the surface, thus more effectively minimizing thermal vibrations and, consequently, the oscillating optical aberrations induced by them.

[0019] In a further preferred embodiment of the invention, a surface of the radiation-absorbing element has a reflectance of at most 0.1, preferably at most 0.05, and particularly preferably at most 0.02. For example, the surface can have a reflectance of 0.01 or less. In this context, the reflectance is specifically the spectral reflectance for any wavelength when light is incident perpendicular to the surface. This means that the surface of the radiation-absorbing element has a spectral reflectance of at most 0.1, preferably at most 0.05, and particularly preferably at most 0.02, for example 0.01 or less, for at least one wavelength when light is incident perpendicularly.The at least one wavelength can, in particular, be a wavelength contained in the wavelength spectrum of a light beam passing through the projection system during normal operation. Furthermore, the surface of the radiation-absorbing element can, in particular, be a surface designed to be inserted into the beam path of the projection system in order to interrupt the beam path. Higher surface reflectances can, in principle, reduce the light reflection from the surface of the radiation-absorbing element back towards the optical element(s), thus more effectively minimizing thermal vibrations and, consequently, the oscillating optical aberrations induced by them.

[0020] In a further preferred embodiment of the invention, the radiation-absorbing element comprises a body made of a metal or a metal alloy. Metallic materials are well suited in this context because they generally exhibit high light absorption, which, as described above, has a beneficial effect on minimizing thermally induced, oscillating aberrations. Furthermore, metallic materials also offer the advantages of high mechanical stability as well as sufficiently high temperature resistance and thermal shock resistance.

[0021] In a further preferred embodiment of the invention, the radiation-absorbing element has an antireflection coating. The antireflection coating can, for example, be made of acrylic paint, carbon nanotubes, anodized aluminum, and / or a chemically etched nickel-phosphorus alloy. Antireflection coatings enable particularly low reflectance values ​​to be achieved, which, as described above, has a beneficial effect on minimizing thermally induced oscillating aberrations.

[0022] In a further preferred embodiment of the invention, the radiation-absorbing element comprises means for active and / or passive cooling. Examples of means for active and / or passive cooling are cooling fins, one or more fans, or water cooling. In certain application areas of the projection system according to the invention, for example in microlithography, but also in laser technology, high light intensities may need to be absorbed by the radiation-absorbing element. Depending on the application, the light intensity can be, for example, a few watts, but in extreme cases also up to several kilowatts. This can cause the radiation-absorbing element to heat up considerably, for example to temperatures above 100 °C, which can reduce the durability of the radiation-absorbing element and, in extreme cases, even damage it.Furthermore, excessive heating of the radiation-absorbing element is detrimental to system safety. Active and / or passive cooling methods can safely dissipate the absorbed radiation energy as heat, thus eliminating these disadvantages.

[0023] According to a second teaching of the present invention, the aforementioned problem for a projection exposure system for microlithography is solved by the projection exposure system comprising a radiation source and a projection system according to the first teaching. In particular, the projection exposure system can be a projection exposure system for EUV lithography. With a projection exposure system according to the second teaching, the oscillating optical aberrations caused by thermal vibrations can be effectively minimized. Reference is made to the explanations relating to the first teaching.

[0024] According to a third teaching of the present invention, the aforementioned problem for a method for manufacturing integrated circuits by microlithography, in which photoresist-coated semiconductor wafers are exposed using a projection exposure system, is solved by the projection exposure system comprising a radiation source and a projection system according to the first teaching, wherein the radiation-absorbing element interrupts the beam path of the projection system at least for the duration of a dead time, in particular a wafer change. In the context of the present invention, the duration of a dead time can be understood, for example, as the duration of a wafer change, the duration of a wafer lot change, the duration of a die change, or the duration of a reticle change. A wafer lot can be understood as a batch or lot of wafers that are processed at the same time.A die can be understood as a single segment of a wafer. A reticle is understood to be a photomask. At least during a reticle change, a reflective optical element can be inserted into the beam path of the projection exposure system. This element is designed to continue directing light into the projection system during the change, instead of the reticle itself. In this way, light reaches the projection system even during the change, preventing unwanted cooling of the optical elements of the projection system. The reflective optical element can serve as a dummy reticle. This is particularly advantageous because the duration of a reticle change is typically longer than that of a wafer change.

[0025] The method can, in particular, be a method for fabricating integrated circuits using EUV lithography. In such a method according to the invention, because the radiation-absorbing element interrupts the beam path of the projection system at least for the duration of a dead time, in particular a wafer change, the measures to prevent light from entering the projection system, which are otherwise usually necessary during the dead time, can be omitted. In this way, an optical element of the projection system, which is arranged in the beam path upstream of the radiation-absorbing element, can be permanently exposed to the light beam emitted from the radiation source. This effectively minimizes the oscillating optical aberrations caused by thermal vibrations, as already explained in connection with the first teaching.Nevertheless, the radiation-absorbing element, which interrupts the beam path of the projection system at least for the duration of a dead time, in particular a wafer change, ensures that no light enters the area of ​​the wafer during the dead time, in particular a wafer change.

[0026] Further embodiments and advantages of the invention can be found in the following detailed description of some exemplary embodiments of the present invention, particularly in conjunction with the drawing. The drawing shows in Fig. 1 a schematic view of a first embodiment of a projection system for a projection exposure system for microlithography according to the first teaching of the invention; Fig. 2 a schematic view of a second embodiment of a projection system for a projection exposure system for microlithography according to the first teaching of the invention; and Fig. 3 a schematic view of a third embodiment of a projection system for a projection exposure system for microlithography according to the first teaching of the invention.

[0027] Fig. Figure 1 shows a schematic view of a first embodiment of a projection system 100 for a projection exposure system for microlithography according to the first teaching of the invention. The projection system 100 comprises an optical element 110, which in this example is a lens 110. The beam path 130, originating, for example, from a light source (not shown), which runs from left to right in the figure, is indicated by the dashed lines.

[0028] How Fig. As also shown in Figure 1, the projection system 100 further comprises a radiation-absorbing element 120. This element is, for example, a beam trap 120. The beam trap 120 is arranged along the beam path 130 behind the lens 110. The beam trap 120 also has a sliding mechanism (not shown) and can thus, as indicated by arrow 140, be inserted into and subsequently removed from the beam path 130. In this way, the beam trap 120 is designed to temporarily interrupt the beam path 130 of the projection system 100 behind the lens 110. Alternatively, other mechanisms are conceivable besides the described sliding mechanism, such as a pivoting mechanism.

[0029] Since the beam path 130 can be interrupted by the beam trap 120 using the sliding mechanism behind the lens 110, measures to prevent light from entering the projection system 100, which are otherwise usually necessary during a dead time of the projection system 100, such as during a wafer change, are no longer required. In this way, the lens 110 is permanently exposed to the light beam 130 emanating from the radiation source (not shown) and therefore absorbs light. This allows the lens 110 to reach a steady thermal state instead of thermal vibrations, thus minimizing oscillating optical aberrations for the lens 110.Nevertheless, by temporarily interrupting the beam path 130 by the beam trap 120, it can be ensured that no light escapes from the projection system 100 during the dead time, so that, for example, in microlithographic applications, no light enters the area of ​​the wafer during a wafer change.

[0030] Fig. Figure 2 shows a schematic view of a second embodiment of a projection system 200 for a projection exposure system for microlithography according to the first teaching of the invention. The projection system 200 comprises a first optical element 210a and a second optical element 210b, which are again lenses 210a, 210b. The beam path 230, originating, for example, from a light source (not shown), which in the figure again runs from left to right, is as already shown in Fig. 1 indicated by the dashed lines.

[0031] How Fig. As also shown in Figure 2, the projection system 200 comprises a radiation-absorbing element 220, again exemplified here as a beam trap 220. The beam trap 220 is arranged along the beam path 230 both behind the first lens 210a and behind the second lens 210b. The beam trap 220 also has a sliding mechanism (not shown) and can thus, as indicated by the arrow 240, be inserted into and subsequently removed from the beam path 230. In this way, the beam trap 220 is designed to temporarily interrupt the beam path 230 of the projection system 200 behind the first lens 210a and behind the second lens 210b. As an alternative to the described sliding mechanism, other mechanisms are also conceivable, such as a pivoting mechanism.

[0032] Since the beam path 230 can be interrupted by the beam trap 220 using the sliding mechanism behind the first lens 210a and behind the second lens 210b, analogous to the above statements in connection with Fig. 1. For both the first lens 210a and the second lens 210b, a steady thermal state is established instead of thermal vibrations, so that oscillating optical aberrations for both lenses 210a and 210b are minimized accordingly. This results in an overall projection system with particularly low aberrations.

[0033] Fig. Figure 3 shows a schematic view of a third embodiment of a projection system 300 for a projection exposure system for microlithography according to the first teaching of the invention. The projection system 300 shown can be used in particular in EUV lithography. The projection system 300 comprises a total of six optical elements 310a, 310b, 310c, 310d, 310e, 310f, which are mirrors 310a, 310b, 310c, 310d, 310e, 310f. The beam path 330, which runs from top to bottom in the figure, is indicated by the dashed lines. In this embodiment, the beam path 330 shown originates from a reflecting mask (not shown) which is located in the beam path emanating from an EUV light source (not shown).

[0034] How Fig. As also shown in Figure 3, the projection system 300 also includes a radiation-absorbing element 320, again exemplified here by a beam trap 320. The beam trap 320 is arranged along the beam path 330 both behind the first mirror 310a and behind the subsequent mirrors 310b, 310c, 310d, 310e, and 310f. The beam trap 320 also has a sliding mechanism (not shown) and can thus, as indicated by the arrow 340, be inserted into the beam path 330 and subsequently removed from it. In this way, the beam trap 320 is designed to temporarily interrupt the beam path 330 of the projection system 300 behind the first mirror 310a and behind the subsequent mirrors 310b, 310c, 310d, 310e, 310f. As an alternative to the described sliding mechanism, other mechanisms are also conceivable, such as a pivoting mechanism.

[0035] Since the beam path 330 can be interrupted by the beam trap 320 using the sliding mechanism behind the first mirror 310a and behind the further mirrors 310b, 310c, 310d, 310e, 310f, analogous to the above statements in connection with Fig. 1 and Fig. 2. For both the first mirror 310a and the further mirrors 310b, 310c, 310d, 310e, 310f, a steady thermal state is established instead of thermal oscillations, so that oscillating optical aberrations for all mirrors 310a, 310b, 310c, 310d, 310e, 310f are minimized accordingly, resulting in a projection system with particularly low aberrations.

[0036] The one in the Fig. 1, Fig. 2 and Fig. The three beam traps 120, 220, 320 shown have in common that their surfaces 121, 221, 321 exhibit an absorptivity greater than 0.99 and a reflectance less than 0.01. This not only ensures that the surfaces 121, 221, 321 absorb light particularly well, but also that they reflect very little light back towards the optical elements 110, 210a, 210b, 310a, 310b, 310c, 310d, 310e, 310f. Both the high absorptivity and the low reflectance effectively minimize thermal vibrations and, consequently, the oscillating optical aberrations they induce.

[0037] Furthermore, the in the Fig. 1, Fig. 2 and Fig. The three beam traps shown, 120, 220, and 320, have in common that they feature a metallic body and an anti-reflective coating. While the use of a metallic material for the body offers the advantages of high light absorption, high mechanical stability, sufficiently high temperature resistance, and sufficient resistance to thermal shock, the anti-reflective coating allows for particularly low reflectance values.

[0038] Furthermore, the data in the Fig. 1, Fig. 2 and Fig. The three radiation traps shown (120, 220, 320) each incorporate means for active and / or passive cooling (not shown). This allows even high light outputs, ranging from a few watts to several kilowatts, to be absorbed by the radiation traps (120, 220, 320) without them overheating. For light outputs of a few watts or less, purely passive cooling fins are generally sufficient. Light outputs in the range of several hundred watts, however, typically require one or more active fans, possibly in combination with passive cooling fins. At even higher light outputs of several kilowatts, air-based cooling is often no longer adequate, necessitating the use of active water cooling.For applications in EUV lithography, it should also be noted that the projection systems are usually located in a vacuum, so fans are not a suitable means of active cooling.

[0039] Alternatively to the ones in the Fig. 1, Fig. 2 and Fig. In the three beam traps 120, 220, 320 shown, other radiation-absorbing elements are also conceivable in all three embodiments. For example, a radiation-absorbing element in the form of a beam shutter could be used in each case. This shutter is permanently located in the beam path 130, 230, 330 and has an aperture mechanism to temporarily interrupt and then reopen the beam path 130, 230, 330. As a further alternative, a radiation-absorbing element in the form of a combination of a movable mirror and a stationary beam trap could also be used. In such a combination, the movable mirror can be inserted into the beam path 130, 230, 330 to temporarily direct the beam path 130, 230, 330 onto the stationary beam trap, where the light radiation is absorbed.

[0040] To implement a projection exposure system according to the invention and the second teaching, any of the elements described in the Fig. 1, Fig. 2 and Fig. The embodiments of a projection system 100, 200, 300 shown in Figure 3 are combined with a suitable light source. In microlithography, for example, mercury vapor lamps or excimer lasers are used as light sources. In EUV lithography, for example, light sources are used in which tin droplets are converted into a plasma by laser pulses, which then emits EUV light with a wavelength of, for example, 13.5 nm.

[0041] A projection exposure system just described, which is one of those in the Fig. 1, Fig. 2 and Fig.The projection system 100, 200, 300, as shown in Figure 3, and a suitable light source, can advantageously be used to implement a method for the fabrication of integrated circuits by microlithography, in which photoresist-coated semiconductor wafers are exposed using the projection exposure system. Here, the radiation-absorbing element 120, 220, 320 is inserted into the beam path 130, 230, 330 of the projection system 100, 200, 300 by means of the described sliding mechanism (not shown) at least for the duration of a dead time, in particular a wafer change or a reticle change, so that the beam path 130, 230, 330 is interrupted.This eliminates the need for measures to prevent light from entering the projection system 100, 200, 300, which are otherwise typically required during the dead time, particularly during wafer changes. As described above, this also effectively minimizes the oscillating optical aberrations caused by thermal vibrations for the optical elements 110, 210a, 210b, 310a, 310b, 310c, 310d, 310e, 310f. Thus, in a corresponding method according to the invention, the optical performance, and therefore the lithographic yield and throughput, can be improved during wafer exposure, while still ensuring that no light enters the wafer area during the dead time, particularly during wafer changes.

Claims

[1] Projection system (100, 200, 300) for a projection exposure apparatus for microlithography, in particular for EUV lithography, comprising an optical element (110, 210a, 310a), characterized by that the projection system comprises a radiation-absorbing element (120, 220, 320) which is designed to temporarily interrupt the beam path (130, 230, 330) of the projection system behind the optical element (110, 210a, 310a). [2] Projection system according to claim 1, wherein the projection system comprises at least one further optical element (210b, 310b, 310c, 310d, 310e, 310f), wherein the radiation-absorbing element (220, 320) is configured to interrupt the beam path (230, 330) of the projection system behind the at least one further optical element (210b, 310b, 310c, 310d, 310e, 310f). [3] Projection system according to claim 2, wherein the radiation-absorbing element (220, 320) is arranged to interrupt the beam path (230, 330) of the projection system behind the third-to-last, the second-to-last or the last optical element (210b, 310b, 310c, 310d, 310e, 310f). [4] Projection system according to one of claims 1 to 3, wherein a surface (121, 221, 321) of the radiation-absorbing element has an absorption coefficient of at least 0.9, preferably at least 0.95, particularly preferably at least 0.

98. [5] Projection system according to one of claims 1 to 4, wherein a surface (121, 221, 321) of the radiation-absorbing element has a reflectance of at most 0.1, preferably at most 0.05, particularly preferably at most 0.

02. [6] Projection system according to one of claims 1 to 5, wherein the radiation-absorbing element (120, 220, 320) comprises a body made of a metal or a metal alloy. [7] Projection system according to one of claims 1 to 6, wherein the radiation absorbing element (120, 220, 320) has an anti-reflection coating. [8] Projection system according to one of claims 1 to 7, wherein the radiation-absorbing element comprises means for active and / or passive cooling. [9] Projection exposure system for microlithography, especially for EUV lithography, characterized by that the projection exposure apparatus comprises a radiation source and a projection system (100, 200, 300) according to one of claims 1 to 8. [10] Method for producing integrated circuits by means of microlithography, in particular by means of EUV lithography, in which semiconductor wafers coated with photoresist are exposed using a projection exposure apparatus, characterized bythat the projection exposure system comprises a radiation source and a projection system (100, 200, 300) according to one of claims 1 to 8, wherein the radiation-absorbing element (120, 220, 320) interrupts the beam path (130, 230, 330) of the projection system at least for the duration of a dead time, in particular a wafer change.

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