Field of the invention

The invention relates to a method for avoiding a degradation of an optical element, wherein the optical element has a reflective surface at least regionally and wherein the optical element is arranged in a housing enclosing an interior.

The invention furthermore relates to a projection system for a projection exposure ap paratus for EUV lithography, an illumination system for a projection exposure appa ratus for EUV lithography, and a projection exposure apparatus.

Prior art

Projection exposure apparatuses for semiconductor lithography serve to produce mi- crostructured components by means of a photolithographic method. In this case, a structure-bearing mask, the so-called reticle, is imaged onto a photosensitive layer with the aid of a projection optical unit or a projection system. The minimum feature size that can be imaged with the aid of such a projection optical unit is determined by the wavelength of the imaging light used. The shorter the wavelength of the imaging light used, the smaller the structures that can be imaged with the aid of the projection optical unit. Nowadays, mainly imaging light of the wavelength 193 nm or imaging light of a wavelength in the extreme ultraviolet range (EUV), i.e. at least 5 nm and at most 30 nm, is used. When using imaging light of a wavelength of 193 nm, both refractive optical elements and reflective optical elements are used within the projection expo sure apparatus. When using imaging light of a wavelength in the EUV range, exclu sively reflective optical elements, in particular mirrors, are used, which are typically operated under vacuum conditions in a vacuum environment.

Optical elements of this type usually have a reflective surface, that is to say a reflective coating, which is arranged on a substrate of the optical element. If the wavelength of the imaging light used is in the EUV range of between 5 nm and 30 nm, the reflective coating typically comprises a plurality of individual layers that consist alternately of materials having different refractive indices. Such a multilayer system can comprise alternating silicon and molybdenum layers, for example. During the operation of the projection exposure apparatus, the reflective coating is exposed to EUV radiation that fosters a chemical reaction between the layer materials used and gaseous substances present in a residual gas atmosphere in an interior of the projection exposure appa ratus.

In order to protect the individual layers against degradation, a capping layer is typically applied on the reflective coating, which capping layer can consist of ruthenium, for example. However, such a capping layer may also be subject to a degradation, for example an oxidation, as a result of a chemical reaction with residual gas present in the vacuum environment, wherein the chemical reaction is initiated or at least fostered by the EUV radiation. A degradation, for example the oxidation, of the capping layer or of layers arranged below the capping layer during operation of the projection exposure apparatus leads in particular to an undesired reduction of a reflectivity of the respective optical element and thus to a reduction of the total transmission of said optical element.

Methods for avoiding a degradation of an optical element are known from DE 102011 079 450 A1 , DE 102 09 493 B4, DE 10061 248 A1 , DE 10 2006 042 987 A1 and US 9,335,279 B2.

Object of the invention

Against the above background, it is an object of the invention to provide a method, a projection system, an illumination system and a projection exposure apparatus whereby the problems mentioned above are solved, in particular whereby the degra dation of the optical element during operation can be effectively counteracted.

This object is achieved in accordance with the features of the independent patent claims.

Disclosure of the invention

The invention provides for carrying out the method for avoiding a degradation of the optical element with the following steps: a) determining a first degradation value; b) determining at least one second degradation value, wherein the first degradation value and the at least second degradation value are determined at different points in time; c) forming a degradation profile on the basis of the first degradation value and the at least second degradation value; d) calculating a temporal development of the degradation profile; e) determining at least one forecast degradation value on the basis of the cal culated temporal development of the degradation profile; f) comparing the at least one forecast degradation value with a predefinable first limit degradation value; g) moni toring for a predefinable first deviation between the at least one forecast degradation value and the first limit degradation value; h) feeding a first decontamination medium into the interior if attainment of the predefinable first deviation is identified. The method according to the invention having the features of Claim 1 has the advantage that a future degree of degradation or degradation state, in particular a degree of degradation or degradation state expectable at an arbitrarily predefinable point in time, of the optical element is ascertained or is ascertainable on the basis of the at least one forecast degradation value. Consequently, knowledge both regarding a current degree of deg radation and, depending on the forecast degradation value, regarding a future degree of degradation of the optical element is always available. Consequently, depending on the comparison between at least the one forecast degradation value and the first limit degradation value and a deviation that is determined or determinable by the compari son, the fact of whether or not the first decontamination medium ought to be fed to the interior is ascertainable early and in a targeted manner. This ensures reliable and effi cient operation of the optical element since the current degree of degradation of the optical element is regulatable or influenceable depending on the feed of the first de contamination medium. The formation of an undesired degradation or an undesired degree of degradation on the optical element can thus be reliably avoided and perma nent and continuous operation of the optical element can thus be ensured. In the pre sent case, “degradation” means a loss of desired material properties of the optical el ement resulting for example in a deterioration or a loss of a desired reflectivity. In the present case, “decontamination medium” means a medium that counteracts the deg radation.

In accordance with one embodiment, the predefinable first deviation is attained if the forecast degradation value is less than or equal to the first limit degradation value. In other words: The predefinable first deviation is not attained if the forecast degradation value is greater than the first limit degradation value. The first decontamination medium is thus fed to the interior if the forecast degradation value is less than or equal to the first limit degradation value.

In accordance with a further embodiment, the first limit degradation value is chosen in such a way that a critical oxidation of the reflective surface is present in the event of attainment of the predefinable deviation, wherein if attainment of the predefinable de viation is identified, a reducing medium as first decontamination medium is fed to the interior. The advantage here is that attainment or formation of a critical oxidation and thus of a critical degradation state of the optical element is identified or is identifiable early. Actual attainment of this critical degradation state, i.e. such that a current deg radation state is or becomes a critical oxidation state, can thus be counteracted in a timely and targeted manner by the feed of the first decontamination medium. In the present case, “critical oxidation” means a state of the reflective surface in which the latter is irreversibly oxidized. The reflective surface is irreversibly oxidized for example if a quartz and/or silicon layer that cannot be removed or can be removed only with great effort has formed on said surface. The irreversible oxidation leads to a reduced reflectivity of the optical element and thus to an adverse influence on the lifetime and functionality of said optical element. An oxidation is usually caused by oxygen-contain ing molecules present in a residual gas atmosphere in the interior of a projection ex posure apparatus. The reducing medium is preferably gaseous hydrogen (hte).

In accordance with one development, provision is made for the feed of the first decon tamination medium to be stopped if it is detected that the forecast degradation value is greater than the first limit degradation value. This affords the advantage of a particu larly efficient and economical use of the first decontamination medium. The first de contamination medium is thus fed exclusively if it is ascertained that the forecast deg radation value is less than or equal to the first limit degradation value.

In accordance with a further embodiment, provision is made for the forecast degrada tion value to be compared with a predefinable second limit degradation value, wherein monitoring for a predefinable second deviation between the at least one forecast deg radation value and the second limit degradation value is effected, and wherein a sec ond decontamination medium is fed to the interior if attainment of the predefinable second deviation is identified. The advantage here is that the forecast degradation value is now compared with two limit degradation values and even more accurate mon itoring of the optical element for a degradation is thus ensured. Preferably, the second limit degradation value is greater than the first limit degradation value. Preferably, the first and second decontamination media are different.

In accordance with a further embodiment, attainment of the predefinable second devi ation is identified if the forecast degradation value is greater than or equal to the second limit degradation value. In other words: The predefinable second deviation is not at tained if the forecast degradation value is less than the second limit degradation value. The second decontamination medium is thus fed to the interior if the forecast degra dation value is greater than or equal to the second limit degradation value.

In accordance with a further embodiment, the second limit degradation value is chosen in such a way that a critical reduction of the reflective surface is present in the event of attainment of the predefinable second deviation, wherein if attainment of the predefin able deviation is identified, an oxidizing medium as second decontamination medium is fed to the interior. The advantage here is that attainment or formation of a critical reduction and thus of a critical degradation state of the optical element is identified or is identifiable early. Actual attainment of this critical degradation state, i.e. such that the current degradation state is or becomes a critical reduction state, can thus be coun teracted in a timely and targeted manner by the feed of the second decontamination medium. In the present case, “critical reduction” means a state of the reflective surface in which, on account of a reaction between hydrogen atoms or hydrogen molecules and the reflective surface, a layer removal occurs or has occurred, in particular on account of an etching process resulting from the reaction, or a delamination of the reflective surface, that is to say a delamination of a reflective coating and/or of a cap ping layer applied on the reflective coating, of the optical element occurs or has oc curred. A layer removal or a delamination leads to a loss of desired material properties of the optical element and thus to an adverse influence on the lifetime and functionality of said optical element. A reduction is usually caused by hydrogen present in the re sidual gas atmosphere in the interior of the projection exposure apparatus. The oxidiz ing medium is preferably gaseous oxygen (O2), water (H2O) or carbon dioxide (CO2). In accordance with a further embodiment, provision is made for the feed of the second decontamination medium to be stopped if it is detected that the forecast degradation value is less than the second limit degradation value. This affords the advantage of a particularly efficient and economical use of the second decontamination medium. The second decontamination medium is thus fed exclusively if it is ascertained that the forecast degradation value is greater than or equal to the second limit degradation value.

In accordance with a further embodiment, provision is made for a reflectivity value, a polarization value or a phase value to be determined as degradation value. The ad vantage here is that the degradation value is determined on the basis of a parameter which is influenced by the degradation. In this regard, the reflectivity of the optical ele ment changes for example with increasing degradation. The reflectivity or a reflectivity value can be determined for example by determining a ratio between a light intensity of a light beam incident on the optical element and a light intensity of the light beam reflected at the optical element.

In accordance with a further embodiment, provision is made for the reflectivity value to be determined depending on a determined temperature of the surface of the optical element, a detected duty cycle of a light source that generates working light, a detected clock rate or clock frequency of the light source, a detected pulse energy of at least one, in particular a respective, light pulse generated by the light source and/or a de tected partial pressure in the interior. The advantage here is that the reflectivity value is determined depending on one parameter or a plurality of parameters which is/are detectable in a simple manner during EUV operation of a projection exposure appa ratus. This enables the reflectivity value and/or degradation value to be determined indirectly, in particular in a model-based manner. The temperature of the surface can be determined for example by a temperature sensor or an infrared camera configured to detect radiation signals in the infrared range. The partial pressure in the interior is preferably determined by a residual gas analyser. The partial pressure in the interior is, in particular, the partial pressure of gaseous substances present in the residual gas atmosphere, for example water, oxygen, hydrogen, nitrogen, helium, neon, argon, krypton, xenon, methane (Ch ) and/or carbon dioxide (CO2). The duty cycle is prefer- ably determined by detecting a ratio of a pulse duration, in particular the temporal du ration of an EUV light pulse, to a pulse period duration, that is to say a temporal interval between two EUV light pulses. In order to be able to determine the duty cycle, provision is preferably made for the light source to be an EUV light source which is operated or operable in a pulsed manner. The pulse energy is preferably detected by a pulse en ergy detecting unit, for example a photodiode. Alternatively, the pulse energy is prede fined. The clock rate or clock frequency, that is to say the number of light pulses gen erated per second by the light source or EUV light source operated in a pulsed manner, is preferably predefined or predefinable. Optionally, a power of the light source is as certained depending on the pulse energy and the clock rate. The reflectivity value is then determined depending on the ascertained power.

In accordance with a further embodiment, the temperature is determined depending on the duty cycle and/or depending on the clock rate and the pulse energy. The tem perature is thus ascertained taking account of the EUV light energy absorbed by the optical element. The advantage here is that there is no need for additional components for direct temperature measurement, for example a temperature sensor or an infrared camera. In order to determine the temperature depending on the duty cycle, it is pref erably provided that a predefinable temperature is or has been assigned to a predefin able ratio of pulse duration to pulse period duration. In order to determine the temper ature depending on the clock rate and the pulse energy, it is preferably provided that a predefinable temperature is or has been assigned to a respective detected clock rate in combination with a respective detected pulse energy. For the optional case where a power of the light source is ascertained depending on the pulse energy and the clock rate, that is to say from the product of pulse energy and clock rate, the temperature is determined depending on the power. In order to determine the temperature depending on the power, it is preferably provided that a temperature is or has been assigned to a respective ascertained power. Optionally, a time duration for which the pulsed EUV light source has already been in operation can be taken into account when determining the temperature depending on the duty cycle and/or depending on the clock rate and the pulse energy.

In accordance with a further embodiment, provision is made for the reflectivity value to be determined depending on a predefinable temperature/partial pressure relationship, a predefinable duty cycle/partial pressure relationship, a predefinable duty cycle/tem perature/partial pressure relationship, a predefinable clock rate/pulse energy/partial pressure relationship and/ora predefinable clock rate/pulse energy/temperature/partial pressure relationship. The advantage here is that the reflectivity value is determined particularly reliably on the basis of at least two parameters. A respective relationship or a respective diagram is preferably ascertained experimentally or is predefined. For the optional case where a power of the light source is ascertained depending on the pulse energy and the clock rate, the reflectivity value is determined depending on a predefinable power/partial pressure relationship or a predefinable power/tempera ture/partial pressure relationship.

In accordance with a further embodiment, the polarization value is determined by el- lipsometry. In this case, the optical element is irradiated for example with predefinably polarized light, for example linearly polarized light, and the reflected light is detected by a detector. Afterwards, the polarization state of the reflected light is ascertained and monitored for a change in this polarization state in comparison with the predefinably polarized light. If a change is detected which is greater than a predefinable limit change or maximum allowed change, then a degradation of the surface of the optical element is identified.

In accordance with a further embodiment, the phase value or a phase shift is deter mined by interferometry. To that end, for example, a predefinable reference interfer ence pattern is compared with an interference pattern determined during the operation of the optical element. Depending on the comparison, in particular depending on a determined deviation of the determined or forecast interference pattern with respect to the reference interference pattern, a degradation of the surface of an optical element can be determined. In the present case, the degradation can be determined particularly advantageously by means of an in-situ measurement of a wavefront, in particular an amplitude of the wavefront.

In accordance with one development, it is provided that the degradation profile formed on the basis of the first degradation value and the at least second degradation value is extrapolated in order to calculate the temporal development of the degradation profile. In this case, “extrapolated” means that the degradation profile is extrapolated into the future, in particular to an arbitrarily predefinable future point in time. By way of example, the degradation profile can be extrapolated by a linear function or a polynomial func tion. Alternatively, the degradation profile can be extrapolated on the basis of a prede finable, preferably experimentally determined, model function. One or else more than one forecast degradation value can be ascertained on the basis of the extrapolated degradation profile.

In accordance with a further embodiment, a point in time until the predefinable first or second deviation is attained is ascertained on the basis of the extrapolated degradation profile. Preferably, for this purpose, the point in time until the predefinable first and/or second deviation or the first or second limit degradation value is attained is ascertained on the basis of a slope or a gradient of the extrapolated degradation profile. Preferably, the feed of the first or second decontamination medium is regulated or adapted de pending on the ascertained point in time.

In accordance with a further embodiment, the forecast degradation value is determined in such a way that it has a predefinable temporal distance with respect to the at least second degradation value. This affords the advantage that the forecast degradation value is known for a predefinable or fixedly ascertained point in time. The temporal distance is for example a maximum of one second, in particular a maximum of five seconds or a maximum of 10 seconds or more. In the present case, “second degrada tion value” means any degradation value determined last. Depending on the number of degradation values determined, the degradation value determined last can thus also be a third, fourth or a further degradation value.

The projection system according to the invention for a projection exposure apparatus for EUV lithography having the features of Claim 18 is distinguished by a specially prepared control device configured, when used as intended, to carry out the above- described method according to any of Claims 1 to 17. The advantages already men tioned are afforded thereby. Further advantages and preferred features are evident from the description above and from the claims.

In accordance with one development, provision is made for the optical element to com prise a capping layer at least regionally. The capping layer can be formed from zircon, titanium, yttrium, cerium, niobium, molybdenum, vanadium, lanthanum, boron, and/or their oxides, nitrides, carbides, borides and/or silicides and/or from ruthenium, plati num, palladium, iridium, rhodium, gold, silver, osmium, nickel, cobalt, chromium, cop per, tungsten and/or molybdenum. The optical element thus comprises a capping layer which ensures additional protection against a degradation of the reflective surface.

The illumination system according to the invention for a projection exposure apparatus for EUV lithography having the features of Claim 19 is distinguished by a specially prepared control device configured, when used as intended, to carry out the above- described method according to any of Claims 1 to 17. The advantages already men tioned are afforded thereby. Further advantages and preferred features are evident from the description above and from the claims.

The projection exposure apparatus according to the invention for EUV lithography hav ing the features of Claim 20 is distinguished by a specially prepared control device configured, when used as intended, to carry out the above-described method accord ing to any of Claims 1 to 17. The advantages already mentioned are afforded thereby. Further advantages and preferred features are evident from the description above and from the claims.

The invention will be explained in greater detail below with reference to the drawings. In this respect:

Figure 1 shows a schematic illustration of a projection exposure apparatus for EUV lithography in accordance with a first exemplary embodiment,

Figure 2 shows an exemplary illustration of an optical element,

Figure 3 shows a flow diagram for carrying out a method for avoiding a degrada tion of the optical element,

Figure 4 shows a schematic illustration of a projection exposure apparatus for EUV lithography in accordance with a second exemplary embodiment, Figure 5 shows a time/reflectivity value diagram in accordance with a first exem plary embodiment,

Figure 6 shows a time/reflectivity value diagram in accordance with a second ex emplary embodiment, and

Figure 7 shows a duty cycle/partial pressure relationship in accordance with one exemplary embodiment. Figure 1 shows a projection exposure apparatus 1 for EUV lithography or an EUV li thography apparatus in accordance with one exemplary embodiment. The projection exposure apparatus 1 comprises a beam generating system 2 having a controllable light source 3, an EUV light source 3 in the present case, which generates working light, an illumination system 4 and a projection system 5.

In accordance with the present exemplary embodiment, the beam generating system 2 comprises a first housing 7, which encloses an interior 6 of the beam generating system 2 at least regionally, the illumination system 4 comprises a second housing 9, which encloses an interior 8 of the illumination system 4 at least regionally, and the projection system 5 comprises a third housing 11 , which encloses an interior 10 of the projection system 5 at least regionally. The first, second and/or third housing 7, 9, 11 are/is embodied in each case as a partial housing of an overall housing 12 of the pro jection exposure apparatus 1 , said overall housing being illustrated merely in a simpli fied way here.

The projection exposure apparatus 1 , in particular the overall housing 12 or the partial housings 7, 9, 11 forming the overall housing 12, is/are operated under vacuum con ditions. In accordance with the present exemplary embodiment, the EUV light source 3 is an EUV light source 3 operated in a pulsed manner, for example a plasma source oper ated in a pulsed manner or a free electron laser operated in a pulsed manner. “In a pulsed manner” means that the controllable EUV light source 3 is controllable in such a way that it is switched on and off at predefinable, and in particular uniform, time intervals. In the present case, a duty cycle of the EUV light source 3 is detected, that is to say the ratio of a duration of an emitted EUV light pulse to a light pulse period duration, i.e. a temporal distance from one emitted EUV light pulse to a next. For de tecting the duty cycle, the projection exposure apparatus 1 or the beam generating device 2 comprises a duty cycle detecting device 13.

As an alternative or in addition to detecting the duty cycle, a clock rate or clock fre quency, that is to say the number of EUV light pulses emitted per second by the EUV light source 3, and a pulse energy are detected. In order to detect the clock rate, the projection exposure apparatus 1 or the beam generating device 2 comprises a clock rate detecting device, not illustrated here. Alternatively, the clock rate is predefined or predefinable. Preferably, the clock rate is at least 0 kHz and at most 100 kHz. In order to detect the pulse energy, in particular a respective EUV light pulse emitted by the EUV light source 3, the projection exposure apparatus 1 or the beam generating device 2 comprises a pulse energy detecting device, not illustrated here. The pulse energy detecting device is embodied as a photodiode, for example, wherein the pulse energy is detected depending on a light intensity of at least one EUV light pulse, in particular of a respective EUV light pulse, which light intensity is detected by the photodiode. Alternatively, the pulse energy is predefined or predefinable. Preferably, the pulse en ergy is at least 0 mJ and at most 100 mJ.

In the present case, the EUV light emitted by the EUV light source 3 and having a wavelength of at least 5 nm and at most 30 nm in the present case is focused in a collector mirror 14 of the beam generating device 2 and is then guided into the illumi nation system 4.

Without being restricted thereto, the illumination system 4 comprises a vacuum gener ating unit 15 for generating a vacuum in the interior 8 of the partial housing 9, optionally for generating a vacuum in two or all partial housings 7, 9, 11 , in particular for gener ating a vacuum in the overall housing 12. In addition, the illumination system 4 com prises a first controllable decontamination medium reservoir 16 for feeding a first de contamination medium and optionally a second controllable decontamination medium reservoir 17 for feeding a second decontamination medium into at least the interior 8. The first decontamination medium is a reducing medium, for example hydrogen, and the second decontamination medium is an oxidizing medium, for example oxygen, or vice versa.

In addition, the illumination system 4 comprises at least one residual gas analyser 18 and also at least one first and one second optical element 19, 20 embodied as mirrors. The residual gas analyser 18 is for example a mass spectrometer, for example a quad- rupole spectrometer. The residual gas analyser 18 is configured for detecting a partial pressure in an interior, or one partial pressure or a plurality of partial pressures of gas eous substances present in the residual gas atmosphere, in the present case the re sidual gas atmosphere in the interior 8 of the housing 9. The gaseous substances are, in particular, substances which can provide for a degradation of the optical elements 19, 20. Substances which can provide for a degradation are for example water or water vapour, hydrocarbons, in particular carbon dioxide, oxygen, hydrogen, nitrogen, he lium, neon, argon, krypton, xenon, methane (CFU) and/or carbon dioxide (CO2).

Furthermore, the illumination system 4 comprises a control device 21 . The control de vice 21 is connected to the residual gas analyser 18, the EUV light source 3 and the first and second decontamination medium reservoirs 16, 17 in terms of signalling, that is to say for example in a wire-based or wireless manner. In the present case, the control device 21 is configured to control the EUV light source 3 and also at least one decontamination medium reservoir, that is to say the first and/or the optional second decontamination medium reservoir 16, 17. Preferably, the control device 21 is config ured in such a way that it fulfils the function of the duty cycle detecting device 13, additionally or alternatively the function of the clock rate detecting device and/or of the pulse energy detecting device. Alternatively, the duty cycle detecting device 13, addi tionally or alternatively the clock rate detecting device and/or the pulse energy detect ing device, is/are a separate ascertaining device connected to the control device 21 in terms of signalling.

In the present case, the EUV light introduced into the illumination system 4 is guided by the optical elements 19, 20 onto a photomask 22, or a reticle, having a structure that is imaged onto a wafer 23 on a reduced scale by means of the projection system 5. For this purpose, the projection system 5 comprises a third and a fourth optical ele ment 24, 25, which are likewise embodied as mirrors. Optionally, the projection system 5 and also the illumination system 4 each comprise only one or three, four, five or more optical elements 19, 20, 24, 25.

Additionally or alternatively, the projection system 5 comprises one or a plurality of the components described in association with the illumination system 4, that is to say the residual gas analyser 18, the first and the optional second decontamination medium reservoir 16, 17, the vacuum generating unit 13 and/or the control device 21 .

Additionally or alternatively, the beam generating system 2 comprises one or a plurality of the components described in association with the illumination system 4.

In accordance with an alternative embodiment, the control device 21 is embodied as a separate control device 21 and thus in particular as a control device 21 that is assign able to the beam generating system 2, to the illumination system 4 or to the projection system 5.

Alternatively or additionally, the residual gas analyser 18 is configured for determining a partial pressure in the interior 10 of the projection system 5 or in an interior 40 of the overall housing 12. Alternatively or additionally, the first controllable decontamination medium reservoir 16 is configured for feeding the first decontamination medium and the second controllable decontamination medium reservoir 17 is configured for feeding the second decontamination medium into the interior 10 of the projection system 5.

In the present case, the optical elements are not restricted to mirrors. The photomask 22, or the reticle, a mask holding device, not illustrated here, for holding the photomask 22, for example a mask stage or a so-called reticle stage, and/or the collector mirror 14 should also be understood as optical elements in the present case.

Figure 2 shows the structure of one or more of the optical elements 19, 20, 24, 25 of the projection exposure apparatus 1 in accordance with one exemplary embodiment. In the present case, the structure is described on the basis of the example of the optical element 19. The optical element 19 comprises a substrate 26, which consists of a glass material such as, for example, quartz glass or a glass ceramic material, for example ULE® (ultra low expansion) glass, produced by Corning. Alternatively, the substrate 26 con sists of a metal, for example copper or aluminium. In the present case, a reflective coating or a multilayer system 27 is applied to the substrate 26. The multilayer system 27 forms the reflective surface 28 of the optical element 19. The multilayer system 27 is applied on that side of the reflective optical element 19 which is exposed to the EUV light.

The multilayer system 27 comprises alternating silicon and molybdenum layers. Alter natively, the multilayer system 27 comprises for example alternating molybdenum and beryllium layers, ruthenium and silicon layers or molybdenum carbide and silicon lay ers.

The optical element 19 furthermore comprises a capping layer 29, which is applied over the multilayer system 27. In the present example, the capping layer 29 is a ruthe nium layer. Alternatively, the capping layer 29 is formed from zircon, titanium, yttrium, cerium, niobium, molybdenum, vanadium, lanthanum, boron, and/or their oxides, ni trides, carbides, borides and/or silicides and/or from platinum, palladium, iridium, rho dium, gold, silver, osmium, nickel, cobalt, chromium, copper, tungsten and/or molyb denum. The capping layer 29 is transmissive or transparent to the EUV radiation. Op tionally, the multilayer system 27 and the capping layer 29 thus form the reflective surface 28 of the optical element 19.

The capping layer 29 serves to protect the multilayer system 27 against a degradation, for example against an oxidation as a result of oxygen contained in the residual gas atmosphere, for example the residual gas atmosphere in the interior 8 of the housing 9. Furthermore, the capping layer 29 protects the multilayer system 27 against a re duction, in particular delamination, for example as a result of hydrogen contained in the residual gas atmosphere.

Figure 3 shows a flow diagram for carrying out a method for avoiding a degradation of the optical element in accordance with one exemplary embodiment. The method is preferably carried out by the control device 21 . For this purpose, the control device 21 preferably comprises a microprocessor, in particular for executing a computer pro gram, the program code of which causes the method described to be carried out, and also a RAM component and a ROM component, wherein preferably data, for example predefinable relationships or diagrams, and programs, for example algorithms and computing programs, are stored on the ROM component. For the sake of simplicity, the method will be described with reference to the optical element 19 of the illumination system 4, without being restricted thereto. The method can alternatively or additionally also be carried out on any other optical element described.

A first step S1 involves determining a first degradation value.

In order to determine the first degradation value, a duty cycle of the EUV light source 3 is detected in accordance with the present exemplary embodiment. A temperature of the surface 28 of the optical element 19 is determined depending on the detected duty cycle. The determination is preferably effected by means of a predefinable duty cy cle/temperature relationship stored in the control device 21 , in particular, by means of which relationship a temperature is assigned to the detected duty cycle. As an alterna tive or in addition to detecting the duty cycle, both a clock rate of the EUV light source 3 and a pulse energy of at least one EUV light pulse emitted by the EUV light source 3 are detected in order to determine the first degradation value. Preferably, a power of the EUV light source 3 is ascertained depending on the pulse energy and the clock rate. A temperature of the surface 28 of the optical element 19 is determined depend ing on the detected clock rate and the detected pulse energy, in particular the power ascertained therefrom. The determination is preferably effected by means of a prede finable clock rate/pulse energy/temperature relationship stored in the control device 21 , in particular, by means of which relationship a temperature is assigned to the de tected clock rate in combination with the detected pulse energy. Optionally, the deter mination is effected by means of a predefinable power/temperature relationship stored in the control device 21 , in particular, by means of which relationship a temperature is assigned to the power ascertained.

In order to determine the first degradation value, a partial pressure in the interior 8 of the second housing 9, that is to say the housing of the illumination system 4, is addi tionally determined in accordance with the present exemplary embodiment. However, the partial pressure can also be determined in the interior of the first and/or third hous ing 7, 11 , optionally in the interior 40 of the overall housing 12.

A first reflectivity value is determined depending on the determined temperature and the determined partial pressure. The first reflectivity value is the first degradation value in the present case. The first reflectivity value is determined depending on a predefin- able temperature/partial pressure relationship stored in the control device 21, in par ticular. Alternatively, the reflectivity value is determined depending on a predefinable duty cycle/partial pressure relationship or a predefinable duty cycle/temperature/partial pressure relationship. The temperature/partial pressure relationship, the duty cy cle/partial pressure relationship and the duty cycle/temperature/partial pressure rela tionship are based in each case on experimentally ascertained or predefinable data. Alternatively, firstly a rate of change in the reflectivity over time is determined on the basis of the determined temperature and the determined partial pressure. The first re flectivity value is then determined by integration of the rate of change over time. If, alternatively or additionally, the clock rate and the pulse energy are detected, then the first reflectivity value is determined depending on a predefinable clock rate/pulse en ergy/partial pressure relationship and respectively power/partial pressure relationship or a predefinable clock rate/pulse energy/temperature/partial pressure relationship and respectively power/temperature/partial pressure relationship.

A second step S2 involves determining a second degradation value, a second reflec tivity value in the present case. The second degradation value is determined at a tem poral distance with respect to the first degradation value. The temporal distance is arbitrarily predefinable. It is possible to determine additionally as many further degra dation values as desired, i.e. at least one third, at least one fourth and so on. The second degradation value or reflectivity value is determined analogously to the first degradation value.

A third step S3 involves forming a degradation profile or reflectivity profile on the basis of the first and second degradation values or reflectivity values. A fourth step S4 involves calculation a temporal development of the degradation pro file. In order to calculate the temporal development of the degradation profile, the deg radation profile formed on the basis of the first and second degradation values is ex trapolated to an arbitrarily predefinable future point in time. The degradation profile can be extrapolated for example by a linear function, a polynomial function or a predefina ble model function.

A fifth step S5 involves determining a forecast degradation value or a reflectivity value on the basis of the calculated temporal development of the degradation profile. The forecast degradation value is determined in particular in such a way that it has a pre definable temporal distance with respect to the second degradation value. The tem poral distance can be one or more milliseconds or one or more seconds. If more than two degradation values or reflectivity values are determined, then the forecast degra dation value is determined in particular in such a way that it has a predefinable tem poral distance with respect to the degradation value or reflectivity value determined last in the process.

A sixth step S6 involves comparing the forecast degradation value with a predefinable first limit degradation value or limit reflectivity value. In accordance with the present exemplary embodiment, the first limit degradation value is or has been chosen in such a way that at this value a critical degree of oxidation of the reflective surface 28 or of the capping layer 29 is present. Optionally, provision is made for the forecast degra dation value to be compared with two predefinable limit degradation values, that is to say with the first limit degradation value and additionally with a second limit degrada tion value. Preferably, the second limit degradation value is or has been chosen in such a way that at this value a critical degree of reduction of the reflective surface 28 or of the capping layer 29 is present.

A seventh step S7 involves monitoring for a predefinable first deviation between the at least one forecast degradation value and the first limit degradation value. If the forecast degradation value is additionally compared with the predefinable second limit degra dation value in accordance with the optional embodiment, then monitoring for a prede finable second deviation between the at least one forecast degradation value and the second limit degradation value is additionally effected. If attainment of the predefinable first deviation is detected or identified, then a first de contamination medium is fed to the interior 8 in an eighth step S8. In the present case, attainment of the predefinable first deviation is identified if the forecast degradation value is less than or equal to the first limit degradation value. Since, in accordance with the exemplary embodiment, the first limit degradation value is chosen in such a way that a critical oxidation of the reflective surface 28 or of the capping layer 29 is present in the event of attainment of the predefinable deviation, a reducing medium as first decontamination medium is fed to the interior 8. In accordance with the optional case where the forecast degradation value is compared with two predefinable limit degra dation values, the first decontamination medium is fed to the interior 8 if attainment of the predefinable first deviation is detected. If attainment of the predefinable second deviation is detected or identified, then a second decontamination medium, in particu lar an oxidizing medium, is fed to the interior 8. Attainment of the predefinable second deviation is identified in particular if the forecast degradation value is greater than or equal to the second limit degradation value. Both the first decontamination medium and the second decontamination medium are fed in each case with a predefinable partial pressure.

If it is detected that the forecast degradation value is greater than the first limit degra dation value, then the feed of the first decontamination medium is stopped in a step S9. The method is then preferably continued at step S1 .

The above-described targeted feed of the first and/or second decontamination medium has the advantage of minimizing an adverse effect on optical properties of the optical element 19 on account of an influence by the respective decontamination medium. Said adverse effect can result, in particular, from EUV light being absorbed by the respective decontamination medium. The targeted feed of the respective decontami nation medium thus ensures that absorption on account of the decontamination me dium that is fed is particularly short.

Optionally, provision is made for a polarization value or a phase value to be determined as degradation value. The polarization value is determined by ellipsometry. In this case, the optical element 19 is irradiated for example with predefinably polarized light, for example linearly po larized light, and the reflected light is detected by a detector 30. Afterwards, the polar ization state of the reflected light is ascertained and a change in this polarization state in comparison with the predefinably polarized light is examined. A degradation of the surface 28 or of the capping layer 29 of the optical element 19 can be determined on the basis of this change. For the determination by ellipsometry, the projection exposure apparatus 1 preferably comprises a light source 31 that generates predefinably polar ized light, and the detector 30. The detector 30 is preferably connected to the control device 21 in terms of signalling.

The phase value is determined by interferometry. To that end, for example, a prede- finable reference interference pattern is compared with an interference pattern deter mined during the operation of the optical element 19. A degradation of the surface 28 or of the capping layer 29 of the optical element 19 can be determined depending on the comparison, in particular depending on a determined deviation of the determined or forecast interference pattern with respect to the reference interference pattern. For the determination by interferometry, the projection exposure apparatus 1 preferably comprises an interferometer 32 and a detector unit 33 for determining the interference patterns. The detector 33 is preferably connected to the control device 21 in terms of signalling.

A projection exposure apparatus 1 which enables the polarization values to be deter mined by ellipsometry and/or the phase value to be determined by interferometry is illustrated in a simplified manner in Figure 4.

Figure 5 shows, in order to elucidate ascertainment of a forecast degradation value, a time/reflectivity value diagram, not illustrated in a manner true to scale, in accordance with one exemplary embodiment. In this case, time t is plotted on the x-axis of the diagram and the reflectivity R is plotted on the y-axis of the diagram. In accordance with the exemplary embodiment, the reflectivity is a dimensionless number between zero, corresponding to zero per cent reflectivity, and one, corresponding to 100 per cent reflectivity. Alternatively, a change over time or a rate of change in the reflectivity, that is to say dR/dt, is plotted on the y-axis. Alternatively, the polarization or phase can be plotted on the y-axis.

In the present case, a first degradation value or reflectivity value, for example - and without being restricted thereto - 0.95, is determined at a first point in time ti and a second degradation value or reflectivity value, for example - and without being re stricted thereto - 0.94, is determined at a second point in time ii. A degradation profile or reflectivity profile is formed on the basis of the first and second degradation values. A temporal development of the degradation profile is subsequently calculated. In the present case, “temporal development” means a temporal development of the degra dation profile which goes beyond the point in time of the degradation value determined last, that is to say the degradation value at ii. The degradation profile is extrapolated in order to calculate the temporal development of the degradation profile.

A forecast degradation value or reflectivity value is determined on the basis of the ex trapolated degradation profile (illustrated in a dashed manner in the present case). The forecast degradation value can be determined in such a way that it has a prede- finable temporal distance with respect to the at least second degradation value or deg radation value determined last. In the present case, forecast degradation values are ascertained for the points in time ts, Ϊ4, ts. At least one, in particular each, of the forecast degradation values is compared with a predefinable first limit degradation value (illus trated in a dashed manner under reference sign 34) or limit reflectivity value, wherein monitoring for a predefinable deviation between the respective forecast degradation value and the first limit degradation value is effected.

In the present case, the limit degradation value is chosen in such a way that a critical, in particular irreversible, oxidation state of the reflective surface 28 or of the capping layer 29 is present in the event of attainment of the deviation, in particular if the forecast degradation value is less than or equal to the limit degradation value. In accordance with the present exemplary embodiment, a critical oxidation is present if the reflectivity of the optical element is 90 per cent or less. The first limit degradation value is thus 0.90 in the present case. However, it should be pointed out that the first limit degrada tion value or limit reflectivity value is arbitrarily selectable or predefinable. In this regard, the limit degradation value or limit reflectivity value can also be 0.95 or 0.85, for exam ple.

In the present case, the forecast degradation value or reflectivity value at the point in time Ϊ4 is equal to the limit degradation value. If attainment of the first deviation is iden tified, the first decontamination medium is fed to the interior 8.

Preferably, a time difference At or a time interval is ascertained on the basis of the point in time of the degradation value determined last, in the present case the point in time Ϊ2, and the point in time at which attainment of the deviation is identified, in the present case the point in time t4. In the present case, the time difference is At = t2 - 14. Preferably, the feed, in particular a metering and/or a feed partial pressure, of the first decontamination medium is regulated depending on the time difference At.

Optionally or additionally, the point in time at which the extrapolated degradation profile and thus a forecast degradation value attains the predefinable first limit degradation value is ascertained on the basis of the extrapolated degradation profile. To that end, the degradation profile is extrapolated for an arbitrarily predefinable time duration, for example a maximum of one second, a maximum of five seconds, a maximum of ten seconds or more. The point in time at which the forecast degradation value attains the predefinable first limit degradation value is then the point in time at which the forecast degradation profile intersects the limit degradation value. A targeted determination of the point in time at which the forecast degradation value attains the predefinable first limit degradation value is thus ensured.

Figure 6 shows a time/reflectivity value diagram, not illustrated in a manner true to scale, in accordance with a further exemplary embodiment. The difference with respect to the time/reflectivity value diagram from Figure 5 is that a predefinable second limit degradation value or limit reflectivity value (illustrated in a dashed manner under refer ence sign 35) is additionally depicted in the present case.

In the present case, the second limit degradation value is chosen in such a way that a critical, in particular irreversible, reduction state of the reflective surface 28 or of the capping layer 29 is present if the forecast degradation value is greater than or equal to the second limit degradation value. In accordance with the present exemplary embod iment, a critical reduction is present if the reflectivity of the optical element is 100 per cent or more. The second limit degradation value or limit reflectivity value is thus 1.00 in the present case. If it is identified that the forecast degradation value is greater than or equal to the second limit degradation value, a second decontamination medium, in particular an oxidizing medium, is fed to the interior 8.

The use of two limit degradation values has the advantage that the degradation value of the optical element 19 is settable or regulatable in such a way that it lies between the first and second limit degradation values and thus in a defined process window or monitoring range. This ensures that the optical element 19 always remains in an ac ceptable range with regard to its degree of degradation.

It should be pointed out that the second limit degradation value or limit reflectivity value is arbitrarily selectable or predefinable and is not restricted to the exemplary embodi ment above. Preferably, provision is made for the second limit degradation value to be greater or to be chosen to be greater than the first limit degradation value.

Figure 7 shows, in order to elucidate ascertainment of a degradation value or reflectiv ity value, a duty cycle/partial pressure relationship or a duty cycle/partial pressure dia gram in accordance with one exemplary embodiment. In this case, the partial pressure p, for example of water or water vapour, is plotted on the x-axis of the diagram and the duty cycle DC of the EUV light source 3 operated in a pulsed manner is plotted on the y-axis of the diagram. A degradation value, in the present case a reflectivity value, is determined depending on the partial pressure and the duty cycle. Alternatively, a rate of change in the reflectivity is determined depending on the partial pressure and the duty cycle. A reflectivity value can then be determined by integration of the rate of change.

The partial pressure is specified in millibar, for example - and without being restricted thereto - in a range of from at least 10 ⁸ millibar to at most 10 ⁵ millibar. Alternatively, the partial pressure can lie for example in a range of from at least 10 ⁹ to at most 10 ⁴ millibar. The duty cycle is specified in a value range of 0 to 1 or 0 to 100 per cent. As already described, the temperature of the surface 28 of the optical element 19 can be ascer tained depending on the duty cycle. Alternatively or additionally, the temperature can thus also be plotted on the y-axis. Preferably, a temporal duration for which the EUV light source 3 operated in a pulsed manner has already been in operation is taken into account for ascertaining the temperature depending on the duty cycle. In particular, this takes account of the fact that with the duty cycle remaining constant, as the oper ating duration of the EUV light source 3 increases, the temperature of the surface 28 increases. The development of the temperature depending on both the operating du ration and the duty cycle is preferably ascertained experimentally or predefined.

A degradation value or reflectivity value is assignable to a respectively determined partial pressure/duty cycle combination. Depending on the partial pressure/duty cycle combination, the degradation values or reflectivity values can lie in an acceptable range, in particular an acceptable reductive range (illustrated at 36), a critical oxidative range (illustrated at 37), a transition range (illustrated at 38) between reductive and oxidative ranges, or a critical reductive range (illustrated at 39).

By way of example, a first partial pressure/duty cycle combination is determined in order to determine the first degradation value or reflectivity value and a second partial pressure/duty cycle combination is determined in order to determine the at least sec ond degradation value. On the basis of the first degradation value and the at least second degradation value, then the degradation profile is formed and a temporal de velopment of the degradation profile is calculated.

As an alternative or in addition to ascertaining the degradation value or reflectivity value depending on the duty cycle, the degradation value is ascertained depending on the power, that is to say the product of clock rate and pulse energy. In this case, the deg radation value is ascertained by means of a power/partial pressure relationship or a clock rate/pulse energy/partial pressure relationship. If the degradation value is ascer tained depending on the power, then the power is plotted on the y-axis.

In the present case, the power can be specified in a value range of zero watts to 10 000 watts. As already described, the temperature of the surface 28 of the optical element 19 can be determined depending on the power or the clock rate and the pulse energy. Consequently, the temperature can likewise be plotted on the y-axis in this case. Pref erably, a temporal duration for which the EUV light source 3 operated in a pulsed man ner has already been in operation is taken into account for determining the temperature depending on the power or the clock rate and the pulse energy. In particular, this takes account of the fact that with the power remaining constant, as the operating duration of the EUV light source 3 increases, the temperature of the surface 28 increases. The development of the temperature depending on both the operating duration and the power is preferably ascertained experimentally or predefined.

A degradation value or reflectivity value is assignable to a respectively determined power/partial pressure combination. By way of example, a first power/partial pressure combination is determined in order to determine the first degradation value or reflec tivity value and a second power/partial pressure combination is determined in order to determine the at least second degradation value. On the basis of the first degradation value and the at least second degradation value, then the degradation profile is formed and a temporal development of the degradation profile is calculated.

List of reference signs

1 Projection exposure apparatus

2 Beam generating system 3 EUV light source

4 Illumination system

5 Projection system

6 Interior

7 first housing 8 Interior

9 second housing

10 Interior

11 third housing

12 Overall housing 13 Duty cycle ascertaining device

14 Collector mirror

15 Vacuum generating unit

16 first decontamination medium reservoir

17 second decontamination medium reservoir 18 Residual gas analyser

19 first optical element

20 second optical element

21 Control device

22 Photomask 23 Wafer

24 third optical element

25 fourth optical element

26 Substrate

27 Multilayer system 28 Surface

29 Capping layer

30 Detector

31 light source that generates polarized light

32 Interferometer 33 Detector

34 first limit degradation value

35 second limit degradation value

36 reductive range 37 oxidative range

38 Transition range

39 critical reductive range

40 Interior