Optical system and method for operating an optical system

Abstract

The invention relates to an optical system, in particular for microlithography, having at least one optical element (100, 200, 300, 400) that comprises a substrate (105, 205, 305, 405) and at least one optically effective layer system (110, 210, 310, 311, 410) situated on this substrate (105, 205, 305, 405), and at least one monitoring unit (120, 220, 240, 320, 340) for monitoring a degradation state of this layer system (110, 210, 310, 311, 410), wherein the monitoring unit (120, 220, 240, 320, 340) comprises a detector (121, 221, 241, 321, 341) for detecting non-directed radiation (130, 230, 330, 430) that emanates from the optical element (100, 200, 300, 400) during the operation of the optical system, wherein the non-directed radiation (130, 230, 330, 430) detected by the detector comprises fluorescence radiation that is caused or modified in the event of an interaction of the layer system of the optical element with used light pre-sent during the operation of the optical system.

Description

[0001]Optical system, in particular for microlithography, and method for operating an optical systemThe present application claims the priority of the German patent application DE 102024 129451.5, filed on October 11, 2024. The content of this DE application is incorporated by reference in the present application text. BACKGROUND OF THE INVENTION Field of the invention The invention relates to an optical system, in particular for microlithography, and to a method for operating an optical system. The invention can be advantageously appliedinter alia in a laser light source having an optical pulse stretcher, and it can be usedin a microlithographic projection exposure apparatus in particular. Prior art Microlithography is used to produce microstructured, electronic components. The microlithography process is performed in what is known as a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (= reticle) illuminated by means of the illumination device is in this case projected by means of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the pro- jection lens, in order to transfer the mask structure to the light-sensitive coating on the substrate. During the operation of a microlithographic projection exposure apparatus, degrada- tion or defects may arise in the respective optically effective layer systems of optical elements (e.g. in highly reflective or partially reflective layers of mirrors or beam split- ters or in antireflection layers of lens elements) on account of the respective energy input as a result of the incident electromagnetic radiation. Said degradation or defects include e.g. changes in the respective layer materials by chemical reaction (e.g. oxi- dation) or an incorporation of contaminants (e.g. hydrocarbons), but also a partial de- tachment of the layer materials. As a result, such degradation or defects lead to a significant impairment of the optical properties (e.g. reduction in reflectivity, spectral shift or else increased absorption or transmission). This in turn may lead to a deterior-ation in the optical performance of the respective optical element or of an optical sys-tem comprising this element, up to a complete failure of the (overall) system. In a projection exposure apparatus designed for operation in the DUV range (e.g. atoperating wavelengths of less than 250 nm, in particular less than 200 nm), use istypically made of laser light sources in the form of excimer lasers, in particular kryptonfluoride excimer lasers at an operating wavelength of 248 nm or argon fluorideexcimer lasers at an operating wavelength of 193 nm. The aforementioned opticalelements exposed to a high radiation load may be mirrors or beam splitters in partic-ular, which are used in such a laser light source. This application may be implemented in an optical pulse stretcher or else as a deflection mirror or output coupling mirror within such a laser light source. A possible approach to avoiding a degradation-related system outage, for instance in the aforementioned laser light source or a microlithographic projection exposure apparatus, includes the preventative replacement of optical elements within the scope of regular servicing intervals and / or a monitoring of the performance of the overall system. However, in addition to the danger of a belated replacement and a system outage caused thereby, there is also the risk of a possibly premature and unnecessary replacement of still functional optical elements. Then again, monitoring the perfor- mance of the overall system does not yet allow a targeted assignment of a possibly arising reduction in performance to individual optical elements and hence does not yet allow for a selective replacement of the same. Regarding the prior art, reference is made purely by way of example to DE 102010 006326 A1, WO 2010 / 047768 A1, DE 102017202850 A1 and the articles J. Heber et al.: "Deep UV laser induced fluorescence in fluoride thin films", Appl. Phys. A 76, 123–128 (2003), DOI: 10.1007 / s00339-002-1502-9, Ch. Mühlig et al.: "Laser induced fluorescence and absorption measurements for DUV optical thin film characteriza- tion", Proc. of SPIE Vol.7101 (2008), 71011R-1 to 71011R-9 and W. Triebel: "Char- acterization of DUV optical materials by direct absorption measurements and LIF", Proc. of SPIE Vol.5991, 59911P-1 to 59911P-16. SUMMARY OF THE INVENTION A problem addressed by the present invention is that of providing an optical system, in particular for microlithography, and a method for operating an optical system that allow detection or monitoring of a degradation state existing within the optical system while at least partly avoiding the aforementioned predicaments. This problem is solved by the features of the independent claims. According to the invention, an optical system, in particular for microlithography, com- prises: -at least one optical element that comprises a substrate and at least one opticallyeffective layer system situated on this substrate; and -at least one monitoring unit for monitoring a degradation state of this layer sys-tem, -wherein the monitoring unit comprises a detector for detecting non-directedradiation that emanates from the optical element during the operation of theoptical system; -wherein the non-directed radiation detected by the detector comprises fluores-cence radiation that is caused or modified in the event of an interaction of the layer system of the optical element with used light present during the operation of the optical system. Within the meaning of the present application, the term "non-directed radiation" com- prises both electromagnetic radiation that has the same intensity in all directions from the optical element and radiation whose intensity varies depending on the direction. In this context and within the meaning of the present application, the term "non-di- rected radiation" in particular comprises radiation that deviates from the main light path of an optical system containing the optical element. In particular, the concept on which the invention is based is that of realizing detection or monitoring of a degradation state in an optical system or for an optical element situated therein on the basis of the detection of non-directed radiation that emanates from the relevant optical element during operation. In particular, this non-directedradiation may be fluorescence radiation. In this context, the invention makes use ofthe fact that such a fluorescence radiation may be caused by defects in the layer material, and the intensity and the spectral position of a detected fluorescence signal accordingly may be representative of contamination present and the type and concen- tration thereof. In further embodiments, other non-directed radiation, in particular in the form of stray light, may be detected in addition to the detection of fluorescence radiation. As yet to be described in detail below, the present invention comprises both embodi- ments in which the aforementioned non-directed radiation (fluorescence radiation in particular) is caused by the layer degradation to be detected, or defects accompanying this, itself and embodiments in which said non-directed radiation or fluorescence radi- ation is generated in any other way (in particular by a fluorescence layer introduced into the layer system of the optical element precisely for this purpose) and then is modified or rendered visible (optionally to a greater extent) when degradation occurs. In particular, use may also be made here of a fluorescence layer which initially fluo- resces but then (for instance as a consequence of the layer degradation on the optical element) no longer illuminates in the event of increased irradiation (e.g. a fluorophore which degrades significantly as a result of a two-photon process or else in the event of contact with oxygen, etc.). The specific placement of the fluorescence layer may be suitably chosen depending on the operating wavelength of the optical system and in a manner dependent on the material combination of the layer system of the optical element. In particular, the fluo- rescence layer may be incorporated into the optically effective layer system as an intermediate layer but may also be placed e.g. on the back side of the optical element in further embodiments. Furthermore, the fluorescence layer may be incorporated as a layer provided in addition to the actual layer structure of the optical element or may else replace a layer provided as a standard (i.e. be provided in place of this layer) in the layer structure of the optical element. Monitoring layer degradation on the basis of the detection of non-directed radiation according to the invention is advantageous in particular in that a detector used to this end may be placed outside of the optical beam path of used light that passes through the optical system during operation. As a consequence, the actual operation of the optical system is not disturbed, and it is possible to realize monitoring during operation ("in operando"). According to the invention, the disadvantage is accepted that the detected non-di-rected radiation has relatively low intensities in comparison with directed radiation.However, this disadvantage is accepted deliberately in order in return to obtain the aforementioned advantages of the "in operando" monitoring in particular. Moreover, embodiments are also described below, in which the comparatively low intensities of the non-directed radiation are accounted for by a particularly advantageous placement of the detector (for instance on the back side of a substrate that is transparent to the aforementioned radiation). According to an embodiment, the monitoring unit is designed to detect a change in the non-directed radiation over time. In this case, this change over time may relate to the intensity (optionally also the intensity normalized to a current source power in the optical system) of the non-directed radiation, but in addition to that or in an alternative also to other parameters characteristic of this radiation. To this end, spectral changes in particular may be detected and monitored, for example a transition from one fluo- rescence state to another. According to an embodiment, the non-directed radiation comprises fluorescence radi- ation. According to an embodiment, the non-directed radiation comprises stray light. According to an embodiment, the non-directed radiation is caused by defects that are generated in the event of an interaction of the layer system with used light present during the operation of the optical system. According to an embodiment, the layer system comprises a fluorescence layer that emits fluorescence radiation in the event of an interaction with used light present dur- ing the operation of the optical system. According to the invention, a loss of used light accompanied by such a fluorescence layer and an increased complexity of the layer structure are deliberately accepted in order in return to allow a particularly reliable diagnosis of an incipient layer degradation. According to an embodiment, the monitoring unit comprises a spectral filter that is designed in accordance with a wavelength or a wavelength range of the non-directed radiation. According to an embodiment, the detector is arranged outside of the optical beam path of used light that passes through the optical system during operation. According to an embodiment, the substrate is produced from a material that is trans- parent to the non-directed radiation. According to an embodiment, the detector is arranged on the side of the substrate distant from the layer system. According to an embodiment, the optical system is designed for an operating wave-length of less than 250 nm, in particular less than 200 nm. However, the invention isnot limited thereto, wherein the optical system may in particular also be designed for longer operating wavelengths (e.g. also in the visible wavelength range). According to an embodiment, the optical element is a mirror, wherein the optically effective layer system is a reflection layer system of this mirror. According to an embodiment, the optical element is a lens element or an output cou- pling element ("output coupler"), wherein the optically effective layer system is an anti- reflection layer of this lens element or of this output coupling element. According to an embodiment, the optical element is a beam splitter, wherein the opti- cally effective layer system is an antireflection layer or a partially reflective layer of this beam splitter. According to an embodiment, the optical system comprises a laser light source, wherein the optical element is arranged in this laser light source. According to an embodiment, the optical system is a microlithographic projectionexposure apparatus.The invention also relates to a method for operating an optical system, wherein the optical system comprises at least one optical element having a substrate and at least one optically effective layer system situated on this substrate, wherein non-directed radiation that emanates from the optical element during the operation of the optical system is detected by at least one monitoring unit for monitoring a degradation state of the layer system, wherein the non-directed radiation detected by the detector com- prises fluorescence radiation that is caused or modified in the event of an interaction of the layer system of the optical element with used light present during the operation of the optical system. According to an embodiment, said monitoring is performed during an ongoing opera- tional mode of the optical system. According to an embodiment, the optical element is replaced depending on a change in the detected, non-directed radiation over time. For instance, the optical element may be replaced should a specified threshold value for the intensity of the non-di- rected radiation be exceeded. In this case, the optical system may be configured with the above-described features in particular. Further configurations of the invention can be gathered from the description and the dependent claims. The invention is explained in detail below with reference to an exemplary embodiment shown in the attached figures. BRIEF DESCRIPTION OF THE DRAWINGS In the figures:Figures 1-4 show schematic illustrations for explaining exemplary embodimentsof an optical system according to the invention;Figure 5 shows a diagram for explaining the mode of operation of the inven-tion in an exemplary embodiment;Figure 6 shows a schematic illustration of the possible basic structure of alaser light source as an application example of the present invention; andFigure 7 shows a schematic illustration of the possible basic structure of a microlithographic projection exposure apparatus designed for oper- ation in the DUV range. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS What is common to the embodiments described below with reference to Figs 1-5 is that on the basis of the measurement of non-directed radiation (in particular fluores- cence radiation and / or stray light) there is a detection or monitoring of layer degrada-tion that arises within an optical system or on an optical element situated thereon.Advantageously, this may in particular also realize monitoring during the ongoingoperation of the optical system, with the consequence that a replacement of a signifi-cantly degraded component, which might possibly become necessary, can be initiatedin a targeted manner and at a suitable time – i.e., in particular, neither too late (i.e.once an outage of the optical system has already occurred) nor too early (i.e. in the event of still sufficient optical performance or a functional optical element). It is also possible to plan the time of future servicing works should the time curve of the intensity of the non-directed radiation be known. In merely a schematic illustration, Fig.1 shows an optical element 100 having a sub- strate 105 and an optically effective layer system 110 situated on the substrate 105. In the exemplary embodiment of Fig.1, the optical element 100 is a mirror, in which the layer system 110 is configured as a reflection layer system. In an exemplary application, the mirror may in turn be arranged in a laser light source (e.g. with the structure shown schematically in Fig.6 and yet to be described in detail). In further applications, the optical element 100 may also be a beam splitter (e.g. for the optical pulse stretcher of a laser light source) or a lens element (in which the optically effective layer system 110 then is configured as an antireflection layer), e.g. in the microlitho- graphic projection exposure apparatus with the structure shown schematically in Fig. 7 and yet to be described in detail. The layer system 110 of the optical element 100 in the form of a mirror according to Fig.1 may e.g. comprise a plurality of alternately arranged individual layers within a multilayer structure in a manner known per se, wherein these individual layers may for example comprise a fluoridic material. In an exemplary embodiment, the layer structure may be made of alternating lanthanum fluoride (LaF3) and magnesium fluo- ride (MgF2) layers. In embodiments, the material of the respective individual layers may be selected from the group consisting of AlF3, MgF2, LiF, CaF2, YF3, YbF3, GdF3, LaF3, CeF3, SmF3, NdF3, SiO2, Al2O3, TiO2, ZrO, HfO2, Ta2O5, Si3N4. Typically (and without the invention being restricted thereto), the number of individual layers may be at least twenty, in particular at least thirty, and further in particular at least forty. Moreover, functional layers, e.g. in the form of barrier layers or adhesion layers, may also be provided for in the layer structure. Electromagnetic radiation or used light (denoted by "101" in Fig.1) is incident on the optical element 100 or the layer system 110 during operation. In particular (and without the invention being restricted thereto), this may be DUV radiation having a wavelengthof less than 250 nm.In Fig.1, "120" denotes a monitoring unit that comprises a detector 121 for detecting non-directed radiation 130 that emanates from the optical element 100 during theoperation of the optical system. In the exemplary embodiment of Fig. 1, the monitoringunit 120 also comprises a spectral filter 122 for selecting the spectral range detected by the detector 121, and a converging lens element 123. Both the spectral filter 122 and the converging lens element 123 are optional and may also be omitted in further embodiments of the invention. In particular, the non-directed radiation 130 may be fluorescence radiation that is gen- erated by defects which arise in the layer system 110 of the optical element 100 during operation and accompany the layer degradation to be detected. By way of the moni- toring unit 120, it is possible in particular to detect a change in the intensity of this non- directed radiation 130 over time, in order to recognize a significant degradation and a correspondingly incipient outage of the optical element 100 in timely fashion. For instance, the optical element 100 may be replaced in a targeted manner should a specified threshold value for the intensity of the non-directed radiation 130 be exceeded. Non-directed radiation generated in any other way, in particular in the form of stray light, may also be detected in addition to the detection of fluorescence radiation or in an alternative. Fig.2 schematically shows a further embodiment, wherein components that are anal- ogous or essentially functionally the same by comparison with Fig.1 are labelled by reference numerals increased by “100”. According to Fig. 2, a further monitoring unit 240 (once again having detector 241, spectral filter 242 and converging lens element 243) is provided in addition to the monitoring unit 220 (possibly also as an alternative in further embodiments notdepicted here) and placed on the (back) side of the substrate 205 or of the opticalelement 200 distant from the layer system 210. In order to allow the detection of non- directed (e.g. fluorescence) radiation 230 by the detector 241 in this case, the sub- strate 205 is produced from a material transparent to said radiation 230. In comparison with the embodiment of Fig.1, the embodiment of Fig.2 in particular has the advan- tage of detecting a larger solid angle range for the non-directed radiation 230, whereby it is possible to account for the possibly comparatively low intensity of this radiation 230. In further embodiments, the monitoring unit according to the invention may also be placed only on the (back) side of the substrate 205 or of the optical element 200 distant from the layer system 210. The detector of the monitoring unit according to the invention may be a photodetector, or else a CCD camera or a multiple diode or quadrant diode for detecting and analys- ing the two-dimensional intensity distribution in further embodiments. The detector may also be placed directly on the back side of the optical element or mirror. In that case, a possible criterion for the replacement of the optical element may also be e.g. a local increase in the light intensity. In this case, reliable monitoring may even be ensured in the event of an (intended or else undesirable) variation in the intensity of the primary radiation. Moreover, an automatic calibration of the monitoring according to the invention may be realized. Fig. 3 in turn schematically shows a further embodiment, wherein components that are analogous or essentially functionally the same by comparison with Fig. 2 arelabelled by reference numerals increased once again by “100”. In the embodiment ofFig.3 and in contrast to Fig.1 and Fig.2, the optical element 300 is a beam splitter that has a respective optically effective layer system 310 or 311 on opposite sides of the substrate 305. Purely by way of example, this beam splitter may be a beam splitter present in the optical pulse stretcher of Fig.6. Fig.4 in turn shows a schematic illustration for explaining a further embodiment. The embodiment in Fig.4 differs from those in Figs 1-3 in that, in particular, a fluorescence layer 450 is introduced into the layer structure of the layer system (denoted by "410" and having a multiplicity of alternating layers 410a, 410b), wherein said fluorescence layer (which replaces a layer 410a in the example) may e.g. be produced from doped or modified SiO2 and emits fluorescence radiation as non-directed radiation 430 in the event of interaction with used light present during the operation of the optical system. In the exemplary embodiment according to Fig. 4 (but without the invention beingrestricted thereto), the layer material of the layers 410a is lanthanum fluoride (LaF3),and the layer material of the layers 410b is magnesium fluoride (MgF2). In further ex- amples, provision might also be made for different layer materials and optionally also for more than two different layer materials in the layer structure.As indicated in Fig. 5, the non-directed radiation 430 – which is incidentally detectedin a manner analogous to the embodiments of Fig. 1 to Fig. 3 – is modified in terms ofits intensity in the event of layer degradation (detachment of some of the upper six layers in the example) and thus allows the timely recognition of layer degradation. In contrast to the embodiments of Figs 1-3, the non-directed radiation 430 or fluores- cence radiation in Figs 4-5 is therefore generated by the fluorescence layer 450 intro- duced into the layer system 410 of the optical element 400 precisely for this purpose and is then rendered visible (optionally to greater extent) when degradation occurs. In a modification of the example from Fig. 5, the layer degradation may also relate to more or fewer layers, wherein these layers may in each case be partly or else com- pletely degraded (e.g. detached or modified in terms of their composition). In further embodiments, the fluorescence layer may also be configured such that it initially fluoresces but then (for instance as a consequence of the layer degradation on the optical element) no longer illuminates in the event of increased irradiation (e.g. a fluorophore which degrades significantly as a result of two-photon processes or else in the event of contact with oxygen, etc.). The specific placement of the fluorescence layer 450 may be suitably chosen depend- ing on the operating wavelength of the optical system and in a manner dependent on the material combination of the optical element. In particular, the fluorescence layer 450 according to the exemplary embodiment of Fig.4 may be incorporated into the optically effective layer system but may also be placed e.g. on the back side of the optical element in further embodiments. Furthermore, the fluorescence layer 450 may be incorporated as a layer provided in addition to the actual layer structure of theoptical element or – like in the exemplary embodiment of Fig. 4 – may replace a layerin the layer structure of the optical element (one layer 410a made of lanthanumfluoride (LaF3) in the example).Optionally – as yet to be explained below – the fluorescence layer 450 may also beused for adjustment purposes, for instance in an optical pulse stretcher, e.g. with the structure described below with reference to Fig.6. In the schematic illustration of Fig.5, the layer system denoted by "510" corresponds to the non-degraded state, whereas "510'" symbolizes the layer system in an at least spatially locally degraded state. On the basis of the simulation using the example of a LaF3 / MgF2 multilayer system, the graphic makes clear that, in the event of a degrada- tion, e.g. the intensity of the used light incident on the introduced fluorescence layer is subjected to corresponding variations. The distribution of the electric field strength of the used light (plotted in arbitrary units) along the structure of the layer system is shown in spatially resolved fashion. Comparison of this distribution in the layer sys- tems 510 and 510' renders the increase in the electric field strength within the layer system should a degradation be present, in particular the increase within the fluores- cence layer 550, clearly evident. Thus, the effect of the degradation in this example is that the uppermost layers "disappear" (e.g. locally due to the formation of holes in the layer). Fig.6 shows a purely schematic and simplified illustration of the possible basic struc- ture of a laser light source 600 as a possible application of the present invention. In particular, the laser light source 600 comprises a seed laser 601 and an amplification stage 620 having a beam reversal module 621 that is indicated purely schematically. Fig.6 also indicates a relay optics unit 605 for adapting or aligning the output signal of the seed laser 601 with respect to the amplification stage 620. A unit for output coupling part of the laser beam is denoted by "606". "610" denotes an optical pulse stretcher that comprises a beam splitter 615 and a plurality of mirrors 611-614 (only four mirrors of which are plotted in Fig.6 for the sake of simplicity) for generating a multiplicity of light pulses. As a consequence of the repeated reflection of the light component output coupled by the beam splitter 615 (dashed arrows) at the respective optically effective surfaces of the mirrors 611-614, as indicated in Fig.6, this output coupled light component experiences a temporal delay in a manner known per se in relation to the light component transmitted through the beam splitter 615 before said output coupled light component follows the transmitted light component followingreflection at the beam splitter 615. The illustration of further details was dispensedwith in Fig.6 for the sake of simplicity. In this context, reference is made to the prior art in WO 2010 / 047768 A1 and DE 102022005007 A1 merely by way of example. Monitoring for layer degradation according to the invention may be implemented in particular for a mirror (e.g. deflection mirror or output coupling mirror) or a beam split- ter of the above-described laser light source 600, e.g. in the optical pulse stretcher610 (in one or more of the mirrors 611-614 or in the beam splitter 615), in the relayoptics unit 605 or else in the unit 606 for output coupling part of the laser beam. In this case, the procedure when realizing the invention may be analogous to the embodi- ments according to Figs 1-3 or else analogous to the embodiment according to Figs 4-5 (i.e. using an additional fluorescence layer 450). In the latter case, the fluores- cence layer 450 may optionally also be used for adjustment purposes. In this case, use is made of the fact that the fluorescence layer 450 optionally already emits fluores- cence radiation (albeit with comparatively low intensity) even in the case of an intact layer system 410 of the respective optical element 400 in the event of incident used light or a correctly adjusted optical element, and hence on the basis of the fluores-cence signal the correct alignment of the optical elements in the optical beam pathmay in each case be inferred during the adjustment. As an application example of the invention, Fig. 7 shows a structure, possible in principle, of a microlithographic projection exposure apparatus 700 designed for op- eration in the DUV. The projection exposure apparatus 700 according to Fig.7 comprises an illumination device 710 and a projection lens 720. The illumination device 710 serves for illuminat- ing a structure-bearing mask (reticle) 715 with light from a light source unit 705comprising a laser light source for example in the form of an ArF excimer laser for anoperating wavelength of 193 nm (or else in the form of an XeF excimer laser for anoperating wavelength of approximately 351 nm, in the form of a KrF excimer laser foran operating wavelength of approximately 248 nm or in the form of an F2 excimer laserfor an operating wavelength of approximately 157 nm) and also a beam shaping opti-cal unit that generates a parallel light beam.The illumination device 710 comprises an optical unit 711 which, inter alia, comprises a deflection mirror 712 in the example illustrated. The optical unit 711 may comprise for example a diffractive optical element (DOE) and a zoom-axicon system for produc- ing different illumination settings (i.e. intensity distributions in a pupil plane of the illumination device 710). A light mixing device (not illustrated) is situated in the beam path downstream of the optical unit 711 in the light propagation direction, which light mixing device may for example have, in a manner known per se, an arrangement composed of micro-optical elements which is suitable for attaining light mixing, and a lens-element group 713, downstream of which there is a field plane with a reticle masking system (REMA), which is imaged by a REMA lens 714, disposed down- stream in the light propagation direction, onto the structure-bearing mask (reticle) 715 arranged in a further field plane and which thereby delimits the illuminated region on the reticle. The structure-bearing mask 715 is imaged using the projection lens 720 onto a lens element substrate or a wafer 730 provided with a light-sensitive layer (pho- toresist). In particular, the projection lens 720 may be designed for immersion opera- tion, in which case an immersion medium is situated upstream of the wafer, or the light-sensitive layer thereof, in relation to the light propagation direction. Furthermore, it may have for example a numerical aperture NA greater than 0.85, in particular greater than 1.1. Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments will be apparent to a person skilled in the art, for example by combining and / or exchanging features of individual embod- iments. Accordingly, it goes without saying for a person skilled in the art that such variations and alternative embodiments are also included by the present invention, and the scope of the invention is restricted only within the meaning of the accompa- nying claims and the equivalents thereof. The present application encompasses aspects defined in the following clauses, which form part of the description but do not constitute any claims, pursuant to the decision J15 / 88 of the European Patent Office's Legal Board of Appeal. 1. Optical system, in particular for microlithography, having at least one optical ele- ment that comprises a substrate and at least one optically effective layer system situ- ated on this substrate, and at least one monitoring unit for monitoring a degradation state of this layer system, wherein the monitoring unit comprises a detector for detect- ing non-directed radiation that emanates from the optical element during the operation of the optical system. 2. Optical system according to Clause 1, characterized in that the monitoring unit is designed to detect a change in the non-directed radiation over time. 3. Optical system according to Clause 1 or 2, characterized in that the non-directed radiation comprises fluorescence radiation. 4. Optical system according to any of Clauses 1 to 3, characterized in that the non- directed radiation comprises stray light. 5. Optical system according to any of the preceding clauses, characterized in that the non-directed radiation is caused by defects that are generated in the event of an inter- action of the layer system with used light present during the operation of the optical system. 6. Optical system according to any of the preceding clauses, characterized in that the layer system comprises a fluorescence layer that emits fluorescence radiation in the event of an interaction with used light present during the operation of the optical system. 7. Optical system according to any of the preceding clauses, characterized in that the monitoring unit comprises a spectral filter that is designed in accordance with a wave- length or a wavelength range of the non-directed radiation. 8. Optical system according to any of the preceding clauses, characterized in that the detector is arranged outside of the optical beam path of used light that passes through the optical system during operation. 9. Optical system according to any of the preceding clauses, characterized in that the substrate is produced from a material that is transparent to the non-directed radiation. 10. Optical system according to any of the preceding clauses, characterized in that the detector is arranged on the side of the substrate distant from the layer system. 11. Optical system according to any of the preceding clauses, characterized in that itis designed for an operating wavelength of less than 250 nm, in particular less than200 nm. 12. Optical system according to any of Clauses 1 to 11, characterized in that the opti- cal element is a mirror, and the optically effective layer system is a reflection layer system of this mirror. 13. Optical system according to any of Clauses 1 to 11, characterized in that the opti- cal element is a lens element or an output coupling element, wherein the optically effective layer system is an antireflection layer of this lens element or of this output coupling element. 14. Optical system according to any of Clauses 1 to 11, characterized in that the opti- cal element is a beam splitter, wherein the optically effective layer system is an anti- reflection coating or a partially reflective layer of this beam splitter. 15. Optical system according to any of Clauses 1 to 14, characterized in that the opti- cal system comprises a laser light source, wherein the optical element is arranged in this laser light source. 16. Optical system according to any of Clauses 1 to 15, characterized in that it is a microlithographic projection exposure apparatus. 17. Method for operating an optical system, wherein the optical system comprises at least one optical element having a substrate and at least one optically effective layer system situated on this substrate, characterized in that non-directed radiation thatemanates from the optical element during the operation of the optical system isdetected by at least one monitoring unit for monitoring a degradation state of the layersystem. 18. Method according to Clause 17, characterized in that said monitoring is performed during an ongoing operational mode of the optical system. 19. Method according to Clause 17 or 18, characterized in that the optical element is replaced depending on a change in the detected, non-directed radiation over time. 20. Method according to any of Clauses 17 to 19, characterized in that the optical system is configured according to any of Clauses 1 to 16.

Claims

Claims1. Optical system, in particular for microlithography, comprising^ at least one optical element (100, 200, 300, 400) that comprises a substrate(105, 205, 305, 405) and at least one optically effective layer system (110, 210, 310, 311, 410) situated on this substrate (105, 205, 305, 405); and ^at least one monitoring unit (120, 220, 240, 320, 340) for monitoring adegradation state of this layer system (110, 210, 310, 311, 410),^ wherein the monitoring unit (120, 220, 240, 320, 340) comprises a detector(121, 221, 241, 321, 341) for detecting non-directed radiation (130, 230,330, 430) that emanates from the optical element (100, 200, 300, 400) during the operation of the optical system;^ wherein the non-directed radiation (130, 230, 330, 430) detected by thedetector (121, 221, 241, 321, 341) comprises fluorescence radiation that iscaused or modified in the event of an interaction of the layer system of the optical element (100, 200, 300, 400) with used light present during the operation of the optical system.

2. Optical system according to Claim 1, characterized in that the monitoring unit(120, 220, 240, 320, 340) is designed to detect a change in the non-directed radiation (130, 230, 330, 430) over time.

3. Optical system according to Claim 1 or 2, characterized in that the non-directedradiation (130, 230, 330, 430) comprises stray light.

4. Optical system according to any of Claims 1 to 3, characterized in that the non-directed radiation (130, 230, 330, 430) is caused by defects that are generated in the event of an interaction of the layer system (110, 210, 310, 311, 410) with used light present during the operation of the optical system.

5. Optical system according to any of the preceding claims, characterized in that thelayer system (410) comprises a fluorescence layer (450) that emits fluorescenceradiation in the event of an interaction with used light present during the operation of the optical system.

6. Optical system according to any of the preceding claims, characterized in that themonitoring unit (120, 220, 240, 320, 340) comprises a spectral filter (122, 222, 242, 322, 342) that is designed in accordance with a wavelength or a wavelength range of the non-directed radiation.

7. Optical system according to any of the preceding claims, characterized in that thedetector is arranged outside of the optical beam path of used light that passes through the optical system during operation.

8. Optical system according to any of the preceding claims, characterized in that thesubstrate (105, 205, 305, 405) is produced from a material that is transparent to the non-directed radiation (130, 230, 330, 430).

9. Optical system according to any of the preceding claims, characterized in that thedetector (241, 341) is arranged on the side of the substrate (205, 305) distant from the layer system (210, 310).

10. Optical system according to any of the preceding claims, characterized in that itis designed for an operating wavelength of less than 250 nm, in particular lessthan 200 nm.

11. Optical system according to any of Claims 1 to 10, characterized in that the opticalelement (100, 200, 300, 400) is a mirror, and the optically effective layer system (110, 210, 310, 311, 410) is a reflection layer system of this mirror.

12. Optical system according to any of Claims 1 to 10, characterized in that the opticalelement is a lens element or an output coupling element, wherein the optically effective layer system is an antireflection layer of this lens element or of this out- put coupling element.

13. Optical system according to any of Claims 1 to 10, characterized in that the opticalelement is a beam splitter, wherein the optically effective layer system is an anti- reflection layer or a partially reflective layer of this beam splitter.

14. Optical system according to any of Claims 1 to 13, characterized in that the opticalsystem comprises a laser light source (600), wherein the optical element is ar- ranged in this laser light source.

15. Optical system according to any of Claims 1 to 14, characterized in that it is amicrolithographic projection exposure apparatus (700).

16. Method for operating an optical system, wherein the optical system comprises atleast one optical element (100, 200, 300, 400) having a substrate (105, 205, 305, 405) and at least one optically effective layer system (110, 210, 310, 311, 410) situated on this substrate (105, 205, 305, 405), characterized in that non-directed radiation (130, 230, 330, 430) that emanates from the optical element (100, 200, 300, 400) during the operation of the optical system is detected by at least one monitoring unit (120, 220, 240, 320, 340) for monitoring a degradation state of the layer system (110, 210, 310, 311, 410), wherein the non-directed radiation (130, 230, 330, 430) detected by the detector (121, 221, 241, 321, 341) com- prises fluorescence radiation that is caused or modified in the event of an inter- action of the layer system of the optical element (100, 200, 300, 400) with used light present during the operation of the optical system.

17. Method according to Claim 16, characterized in that said monitoring is performedduring an ongoing operational mode of the optical system.

18. Method according to Claim 16 or 17, characterized in that the optical element(100, 200, 300, 400) is replaced depending on a change in the detected, non- directed radiation (130, 230, 330, 430) over time.

19. Method according to any of Claims 16 to 18, characterized in that the opticalsystem is configured according to any of Claims 1 to 15.

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