WO2013124224A1

WO2013124224A1 - Method for optimizing a protective layer system for an optical element, optical element and optical system for euv lithography

Info

Publication number: WO2013124224A1
Application number: PCT/EP2013/053091
Authority: WO
Inventors: Dirk Heinrich Ehm; Jeroen Huijbregtse; Arnoldus Jan Storm; Edwin Te Sligte; Tina GRABER; Hermanus Hendricus Petrus Theodorus BEKMANN
Original assignee: Carl Zeiss Smt Gmbh; Asml Netherlands B.V.
Priority date: 2012-02-24
Filing date: 2013-02-15
Publication date: 2013-08-29
Also published as: DE102012202850A1

Abstract

The invention relates to a method for optimizing a protective layer system (59) for an EUV radiation (6) reflecting multilayer system (51) of an optical element (50), comprising the following steps: selecting a material for a topmost layer (57) of the protective layer system (59) from a group of chemical compounds comprising: oxides, carbides, nitrides, silicates and borides, wherein selecting the material for the topmost layer (57) is effected depending on an enthalpy of formation of the respective chemical compound. The invention also relates to an optical element (50), comprising: an EUV radiation (6) reflecting multilayer system (51), and a protective layer system (59) having a topmost layer (57) composed of a material selected from a group of chemical compounds comprising: oxides, carbides, nitrides, silicates and borides, wherein the protective layer system (59) either consists of the topmost layer (57) having a thickness (d) of between 5 nm and 15 nm, or the protective layer system (59) has at least one further layer (58) below the topmost layer (57), the thickness (d₂) of which is greater than the thickness (d₁) of the topmost layer (57), and wherein the topmost layer (57) has a thickness (d-i) of not more than 5 nm and a thickness (d2) of the further layer (58) or of the further layers is greater than 5 nm.

Description

Method for optimizing a protective layer system for an optical element, optical element and optical system for EUV lithography

Cross-reference to Related Applications

This application claims priority to German Patent Application No. 10 2012 202 850.1 , filed on February 24, 2012, the entire contents of which are hereby incorporated by reference in the disclosure of this application.

Background of the Invention

The invention relates to a method for optimizing a protective layer system for an EUV radiation reflecting multilayer system of an optical element. The invention also relates to an optical element comprising an EUV radiation reflecting multilayer system and comprising a protective layer system, and to an optical system for EUV lithography comprising at least one such optical element.

In EUV lithography apparatuses, reflective optical elements for the extreme ultraviolet (EUV) wavelength range (at wavelengths of between approximately 5 nm and approximately 20 nm) such as, for instance, photo masks or mirrors based on reflective multilayer systems are used for producing semiconductor components. Since EUV lithography apparatuses generally have a plurality of reflective optical elements, the latter have to have a highest possible reflectivity in order to ensure a sufficiently high total reflectivity. The reflectivity and the lifetime of the reflective optical elements can be reduced by contamination of the optically used reflective surface (interface with the environment) of the reflective optical elements, which arises on account of the short-wave irradiation together with residual gases in the operating atmosphere. Since the plurality of reflective optical elements are usually arranged one behind another in an EUV lithography apparatus, even relatively small contaminations on each individual reflective optical element affect the total reflectivity to a relatively large extent.

Contamination can occur on account of moisture residues, for example. In this case, water molecules are dissociated by the EUV radiation and the resulting free oxygen radicals oxidize the optically active surfaces of the reflective optical elements. A further source of contamination is polymers, which can originate for example from the vacuum pumps used in EUV lithography apparatuses, or from residues of photoresists which are used on the semiconductor substrates to be patterned, and which lead, under the influence of the operating radiation, to carbon contaminations on the reflective optical elements. While oxidative contaminations are generally irreversible, carbon contaminations, in particular, can be removed, inter alia, by treatment with reactive hydrogen, by virtue of the reactive hydrogen reacting with the carbon-containing residues to form volatile compounds. The reactive hydrogen can be hydrogen radicals or else ionized hydrogen atoms or molecules. If the light source provided in the EUV

lithography apparatus generates EUV radiation on the basis of a tin plasma, tin and, if appropriate, zinc or indium compounds (or generally metal (hydride) compounds) occur in the vicinity of the light source and can be attached to the optically used surface. Since these substances generally have a high

absorption for EUV radiation, deposits of these substances on the optically used surfaces lead to a high loss of reflectivity, for which reason these substances should be removed with the aid of suitable cleaning methods.

In order to protect the reflective multilayer system against degradation, it is known to apply a protective layer system to the multilayer system. Degradation is understood to mean contamination effects such as e.g. the growth of a carbon layer, oxidation, metal depositions, etc., but also the delamination of individual layers, the etching-away or sputtering of layers, etc. In particular, it has been observed that under the influence of reactive hydrogen which is used for cleaning or which can arise on account of the interaction of the EUV radiation with hydrogen present in the residual atmosphere, detachment of individual layers, in particular close to the surface of the multilayer system, can occur.

US 2011/0228237 A1 discloses providing, for the purpose of protecting the reflective multilayer system, a protective layer system comprising at least two layers, of which one layer comprises a material selected from the group S1O2, Y2O3 and Zr0₂ and a further layer comprises a material selected from a group comprising silicon oxide (having different stoichiometric ratios), Y and ZrO.

Object of the Invention

It is an object of the invention to provide a method for optimizing a protective layer system for an EUV radiation reflecting multilayer system of an optical element, an associated optical element and an EUV lithography system comprising at least one such optical element.

Subject Matter of the Invention

This object is achieved by means of a method for optimizing a protective layer system for an EUV radiation reflecting multilayer system of an optical element, comprising the following steps: selecting a material for a topmost layer of the protective layer system from a group of chemical compounds comprising: oxides, carbides, nitrides, silicates and borides, wherein selecting the material for the topmost layer is effected depending on an enthalpy of formation of the respective chemical compound.

The inventors have recognized that the stability of a protective layer system essentially depends on whether the topmost layer of the protective layer system is inert toward reactions with contaminating substances present, if appropriate, in the vicinity of the optical element and, if appropriate, reactive hydrogen and toward oxidation by oxygen present in the residual gas, or water. In this case, the chemical stability of the compound used for the topmost layer essentially depends on the strength of the (covalent) bonds of the respective oxide, carbide, nitride, silicate or boride, the bond strength of which can be measured by the enthalpy of formation. In order to be able to compare the enthalpy of formation of compounds having a different number of atoms with one another, the enthalpy of formation is preferably normalized by the value of the enthalpy of formation being divided by the number of atoms of the respective compound. In this way, a material selection for the topmost layer can be effected, e.g. by the respective materials being ordered according to the (normalized) enthalpy of formation, wherein materials having a greater (negative) enthalpy of formation, i.e. having a stronger covalent bond, are rated more advantageously for the material for the topmost layer than materials having a lower enthalpy of formation. It goes without saying that, alongside the enthalpy of formation, further properties of the abovementioned compounds can also be taken into account for the material selection, as explained further below.

In one variant of the method, the group from which the material is selected comprises oxides, carbides, nitrides, silicates and borides of the following chemical elements: Y, Ce, Zr, Nb, Si, Ti, V, Mo, Mn, Al, W, Cr, La, Co, Ru, B, Hf, U, Be. The material selection can also involve comparing the enthalpy of formation of a respective compound of one of the abovementioned chemical elements with that of an associated oxide or hydride, which can be formed, if appropriate, by a chemical reaction of the compound with the residual gas or with contaminating substances. The enthalpy of formation of the respective compound (or the absolute value of the enthalpy of formation) should be greater than that of the respective oxide or hydride compound, in order to ensure that the topmost layer of the protective layer system is chemically inert in the vacuum environment. In one variant of the method, the group comprises the following chemical compounds: Y₂0₃, Ce₂0₃, Zr0₂, Ce0₂, Nb₂0₅, Nb0₂, NbO, Si0₂, ΤΊ3Ο5, V₃0₅, Ti₂0₃, Mo0₂, Mn0₂, Ti0₂, V₂0₅, Al₂0₃, V₂0₃, MoSi₂, Mn₂0₃, W0₃, Cr₃0₄, TiO, Mn₃0₄, Mo0₃, La₂0₃, Cr₂0₃, MnO, W0₂, Cr0_2> VO, AIN, Co₃0₄, Si₃N₄, Ru0₂, BN, SiC, Ru0₄. Compounds of the abovementioned elements can typically be applied to the reflective multilayer system with the aid of conventional coating methods (e.g. deposition from the gas phase, "chemical vapor deposition", CVD, "physical vapor deposition" PVD, sputtering, etc.).

In particular if the optical element in the EUV lithography system is arranged in proximity to the radiation source (e.g. in the case of a collector mirror), if appropriate only those materials which do not form intermetallic compounds with tin can be included in the selection. Ru0₂ and Ru0₄, for example, cannot be taken into account in the material selection for such an optical element since these compounds have an affinity for tin. The material selection can also take account of whether the respective compound enters into a reaction with hydrogen, e.g. forms a readily volatile hydride, absorbs metal hydrides or is etched away by a hydrogen plasma possibly present. Such etching-away has been observed e.g. in the case of Si₃N₄, in the case of BN and in the case of SiC, for which reason these compounds should generally be precluded from the selection or should be used only in special cases, e.g. if no cleaning is required, as materials for the topmost layer of the protective layer system. In the case of specific materials, e.g. in the case of Ce₂0₃ or in the case of Ce0₂, a change in the oxidation state has been observed during irradiation with intensive EUV radiation. These materials and Zr0₂ have a high conductivity for oxygen ions, which, if appropriate, fosters the oxidation of further layers situated below the topmost layer and can have a (generally disadvantageous) influence on the reflection properties thereof. Therefore, these materials can, if appropriate, likewise be precluded from the selection. However, these materials are suitable for further layers of the protective layer system situated below the topmost layer, since said further layers do not come directly into contact with oxygen ions from the environment.

In a further variant, the method additionally comprises: choosing a thickness of at least one layer of the protective layer system depending on a penetration depth of reactive hydrogen into the at least one layer. The penetration depth of reactive hydrogen, in particular of ionized hydrogen atoms or hydrogen radicals, depends on the kinetic energy of the respective ions, which can be

approximately 100 eV or higher in an EUV lithography apparatus. The penetration depth of hydrogen ions having these kinetic energies into the protective layer system or into the reflective multilayer system is material- dependent and is typically of the order of magnitude of approximately 10 nm to approximately 15 nm. If the hydrogen ions penetrate into the multilayer system situated below the protective layer system, this can lead to blistering and thus to the detachment of individual layers of the multilayer system. It is supposed that the incorporated hydrogen in silicon layers, for example, leads to the formation of silane compounds which can result in possibly locally delimited blistering or layer detachment.

A suitable choice of the thickness(es) and of the material of the layer(s) of the protective layer system can prevent or greatly reduce the penetration of hydrogen into the underlying multilayer system. In order to achieve the effect that the maximum reflectivity of the optical element is still sufficient for use in EUV lithography, the total thickness of the protective layer system should generally not exceed a value of 25 nm, the aim being to achieve a smallest possible thickness in conjunction with a high barrier effect for hydrogen ions. Materials which have a high diffusivity for hydrogen can also serve as barrier layers since the hydrogen is not incorporated in the respective material.

The protective layer system can have a single (topmost) layer, the thickness of which should be chosen such that the underlying multilayer system is protected against penetrating hydrogen. This is the case, for example, if the material of the topmost layer is selected from the group comprising: Nb02, NbO, ΤΊΟ2, BN, TiO, M0S12, T13O5, S13N4 and the topmost (i.e. the single) protective layer has a thickness of between 8 nm and 12 nm, preferably between 8 nm and 10 nm. In the case of the materials mentioned above, a comparatively small thickness is sufficient to prevent the penetration of hydrogen ions. With the use of Y2O3, Ce₂O₃, ZrO₂, La₂O₃, CeO₂, SiO₂, Nb₂O₅, V₂O₅, or ZrN as material for the topmost or single layer of the protective layer system, a larger thickness is typically required for preventing the penetration of hydrogen, which thickness can be e.g. in the range of between 10 nm and 18 nm, preferably between approximately 12 nm and approximately 15 nm. In this case, the penetration depth of the hydrogen ions into the respective material can be implemented experimentally or by means of simulation calculations e.g. on the basis of the Monte Carlo method (e.g. by means of so-called "Stopping and Range of Ions in Matter", SRIM simulations).

In a further variant, the material of the topmost layer of the protective layer system is selected on the basis of the reflectivity and/or the thickness- dependent change in reflectivity of the topmost layer at the wavelength to be reflected by the multilayer system. In order to achieve the highest possible reflectivity of the optical element, it is advantageous in particular in the case of single-layered protective layer systems to select for the topmost layer a material having a high reflectivity or a low absorption at the wavelength to be reflected. Additionally or alternatively, it can be advantageous if the change in reflectivity in the case of a (possibly infinitesimal) change in thickness is taken into account in the material selection, in order that differences in thickness that possibly occur cannot have a disadvantageous effect on the behavior of the reflectivity of the optical element. The reflectivity or the change thereof can be used, if appropriate, together with the enthalpy for forming a figure of merit for assessing the suitability of a respective material as topmost layer of the protective layer system. As an alternative to the variants which are described further above and in which the protective layer system has only a single layer, the protective layer system can also have at last one further layer below the topmost layer, the thickness of which is chosen to be greater than the thickness of the topmost layer. Such an at least two-layered protective layer system has proved to be particularly advantageous in order to optimize the requirements made of the protective layer system: for the topmost layer it is possible to choose a material which is inert or resistant toward all degradation processes at the interface with the vacuum environment. The material of the at least one underlying layer is chosen such that said material has a good barrier effect for high-energy hydrogen ions in conjunction with good transmission for EUV radiation at the wavelengths to be reflected. In this case, the thickness of the topmost layer which is required to protect the further layer(s) against degradation is generally smaller than the thickness of the further layer(s) required to stop the

penetrating hydrogen ions.

In one variant, a thickness of not more than 5 nm, preferably of not more than 2 nm (and typically greater than 1 nm), is chosen for the topmost layer. Such a thickness generally suffices to protect the underlying further layer(s) against degradation. In particular, for the topmost layer it is also possible to use particularly inert materials which, on account of their high absorption for EUV radiation, are typically not used in EUV lithography, e.g. tungsten oxide (W₂0_3> W0₂ or W0₃), tungsten carbide (WC), titanium carbide (TiC) and aluminum oxide (Al₂0₃). A (total) thickness of more than 5 nm, in particular of more than 10 nm (and typically not greater than 15 nm), is typically chosen for the further layer (or the further layers). These thicknesses (together with the topmost layer) generally suffice to protect the underlying multilayer system against the penetration of hydrogen ions. In a further variant, the method comprises: selecting the material of the at least one further layer depending on the reflectivity of the optical element provided with the protective layer system for the wavelength to be reflected by the multilayer system. The material of the at least one further layer is in this case preferably chosen such that the reflectivity at the wavelength to be reflected is as high as possible. It has been found that there are material combinations for a protective layer system having two (or more) layers in which the reflectivity of the optical element can be significantly increased compared with a protective layer system having a single layer (and comparable thickness). Although the reflectivity of the optical element is also dependent on the underlying multilayer system or on the optimization thereof, it is typically advantageous if the material of the at least one further layer has a lower absorption coefficient (i.e. a lower absolute value of the imaginary part of the refractive index) for the EUV radiation than the topmost layer. It goes without saying that, despite the comparatively small thickness of the topmost layer of a two- or multilayered protective layer system, the material of the topmost layer can also be selected, if appropriate, depending on the respective achievable reflectivity or absorption of the topmost layer for EUV radiation at the wavelength to be reflected.

The protective layer system optimized in the manner described above can be applied to the underlying reflective multilayer system by means of a

conventional coating process, which multilayer system can likewise be applied to a suitable underlying substrate by means of a conventional coating method.

A further aspect of the invention is realized in an optical element, comprising: an EUV radiation reflecting multilayer system, and a protective layer system applied to the reflective multilayer system and having a topmost layer composed of the material selected from the group comprising: oxides, carbides, nitrides, silicates and borides, wherein the protective layer system either consists of the topmost layer having a thickness of between 5 nm and 15 nm, or the protective layer system has at least one further layer below the topmost layer, the thickness of which is greater than the thickness of the topmost layer.

As explained further above, the protective layer system can either consist of one layer, the thickness of which is chosen such that the penetration of hydrogen ions into the underlying multilayer system can be prevented, or a protective layer system having at least two layers can be provided, of which layers the topmost layer serves as protection against degradation and has a comparatively small thickness and the other layer(s) are substantially used for stopping hydrogen ions.

In one embodiment, the material of the topmost layer is selected from the oxides, carbides, nitrides, silicates and borides of the following chemical elements: Y, Ce, Zr, Nb, Si, Ti, V, Mo, Mn, Al, W, Cr, La, Co, Ru, B, Hf, U, Be. In particular oxides and nitrides of a number of the elements specified above have proved to be substantially resistant to reactive hydrogen (and to tin deposits).

The material of the topmost layer can be selected, in particular, from the group comprising: Y₂0₃, Ce₂0_3> Zr0₂, Ce0₂, Nb₂0_5> Nb0₂, NbO, Si0₂, Ti₃0₅, V₃0₅, Ti₂0₃, Mo0₂, Mn0₂, Ti0₂, V₂0₅, Al₂0₃, V₂0₃, MoSi₂, Mn₂0₃, W0₃, Cr₃0₄, TiO, Mn₃0₄, Mo0₃, La₂0₃, Cr₂0₃, MnO, W0₂, Cr0₂, VO, AIN,Co₃0₄, Si₃N₄, Ru0₂, BN, SiC and Ru0₄, wherein in this group in particular Y₂0₃, Ce₂0₃, Zr0₂, Ce0₂, Nb₂0₅ and NbO have proved to be particularly advantageous materials for the topmost layer of the protective layer system on account of their material properties. Some chemical compounds from the above list cannot be used, or can be used only under specific conditions, for the topmost layer on account of known disadvantages. This applies to Si₃N₄, BN and SiC, for example, in which etching-away by a hydrogen plasma has been observed, and so these should not be used if hydrogen cleaning is intended to be carried out. In an embodiment in which the protective layer system consists only of the topmost layer, the material of the topmost layer is selected from the group comprising: Nb02, NbO, TiO₂, BN, TiO, MoSi₂, Ti₃O₅, Si₃N₄ and has a thickness of between 8 nm and 12 nm, preferably between 8 nm and 10 nm. Alternatively, the material of the topmost (single) layer of the protective layer system can also be selected from the group comprising: Y2O3, Ce2O3, ΖΓΟ₂, La2O3, CeO2, S1O2, Nb₂O5, V2O5, ZrN and in this case has a thickness of between approximately 10 nm and approximately 18 nm, preferably between 12 nm and 15 nm.

In a further, alternative embodiment, the topmost layer has a thickness of not more than 5 nm, preferably of not more than 2 nm (and generally of more than approximately 1 nm) and the thickness of the further layer (or the total thickness of the further layers) is greater than 5 nm, in particular greater than 10 nm (and typically not greater than 15 nm). As explained further above, in this way it is possible to obtain a protective layer system having a high reflection for EUV radiation, which system effectively prevents the penetration of hydrogen ions into the underlying multilayer system.

In a further embodiment, the material of the at least one further layer has a lower absorption for EUV radiation at the wavelength to be reflected by the multilayer system than the topmost layer. This is advantageous for increasing the reflectivity of the optical element for EUV radiation at the operating wavelength.

In order to produce a high reflectivity at the operating wavelength, the multilayer system typically has alternately arranged layers of a material having a lower real part of the refractive index in the EUV wavelength range and of a material having a higher real part of the refractive index in the EUV wavelength range. In order to achieve a high reflectivity in a wavelength range of around 12.5 nm to 14.5 nm, typically silicon is used as material having the higher real part of the refractive index and molybdenum is used as the material having the lower real part of the refractive index. As explained further above, it has been found that in particular layers composed of pure silicon are attacked by penetrating reactive hydrogen to a particularly great extent even if further layers composed of different material are arranged thereabove. The protective layer system proposed here protects even silicon layers well against high-energy reactive hydrogen.

In a further embodiment, the material of the at least one further layer is selected from the group comprising: Y, Ce, Zr, Nb, Si, Ti, V, Mo, Mn, Al, W, Cr, La, Co, Ru, B, Hf, U, Be and the oxides, carbides, nitrides, silicates and borides thereof. In particular materials which have a good barrier effect for hydrogen ions, on the one hand, and have a sufficient transmission for EUV radiation, on the other hand, have proved to be advantageous as materials for the further layers. It goes without saying that the considerations which are relevant to the selection of the material of the topmost layer of the protective layer system (in particular with regard to sufficient stability or small tendency toward

degradation) do not apply, or apply only in a restricted manner, to the further layers and that, therefore, if appropriate, materials or compounds other than those presented further above are also suitable for use in the further layer or further layers.

In a further embodiment, the material of the at least one further layer is selected from the group comprising: B4C, S13N4, Mo, Ru, Zr, Si. These materials have proved to be particularly advantageous for use as further layers of the protective layer system.

In one preferred further embodiment the topmost layer of the protective layer system has a thickness of 2 nm or less, in particular of approximately 1.5 nm or less, and the material of the topmost layer is selected from the group

comprising: tungsten oxide (W2O3, WO2 and WO3), tungsten carbide (WC), titanium carbide (TiC) and aluminum oxide (AI2O3). These materials are particularly inert, but have a high absorption for EUV radiation. On account of the small thickness of the topmost layer, these materials can nevertheless be used in the protective layer system proposed here.

In one preferred embodiment, the reflective optical element is embodied as a collector mirror. In EUV lithography, collector mirrors are often used as the first mirror in the ray direction downstream of the radiation source, in particular a plasma radiation source, in order to collect the radiation emitted in different directions by the radiation source and to reflect it in a concentrated fashion to the next mirror. Owing to the high radiation intensity in the vicinity of the radiation source, it is particularly highly likely there that molecular hydrogen present in the residual gas atmosphere can be converted into atomic hydrogen having high kinetic energy, such that precisely collector mirrors are particularly at risk of exhibiting detachment phenomena at the upper layers of their multilayer system on account of penetrating reactive hydrogen.

In particular if the plasma radiation source is operated on the basis of tin plasma, tin contaminants can possibly deposit at the interface between the topmost layer of the protective layer system and the vacuum, which possibly cannot be completely prevented even by a suitable selection of a material for the topmost layer. However, it generally suffices if the material of the topmost layer is inert toward the substances (typically cleaning gases) used when cleaning away tin from the surface.

A further aspect of the invention relates to an optical system for EUV

lithography comprising at least one optical element as described above. The optical system can be an EUV lithography apparatus for exposing a wafer or some other optical system which uses EUV radiation, for example a system for measuring masks used in EUV lithography. Further features and advantages of the invention are evident from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can each be realized individually by themselves or as a plurality in arbitrary combination in a variant of the invention.

Drawing

Exemplary embodiments are illustrated in the schematic drawing and are explained in the description below. In the figures:

Figure 1 shows a schematic illustration of an EUV lithography apparatus,

Figures 2a, b show schematic illustrations of an optical element for the EUV lithography apparatus from figure 1 , which optical element comprises a protective layer system having one layer and having two layers, respectively,

Figure 3 shows a table with a multiplicity of materials and with a figure of merit assigned thereto for assessing their suitability as topmost layer of the protective layer system,

Figure 4 shows a bar chart of the figures of merit of the materials from the table from figure 3,

Figures 5a, b show a schematic illustration of the reflectivity of an optical

element with a protective layer system having a single layer as a function of the thickness of the layer for different materials,

Figures 6a, b show illustrations analogous to figures 5a, b for a protective layer system having a first layer composed of Y203 (Figure 6a) and composed of V205 (Figure 6b) and a second layer from a group of further materials,

Figures 7a, b show illustrations of the penetration depth of hydrogen ions into an individual layer of a protective layer system composed of Ce₂03 and composed of M0S12, respectively, and

Figures 8a-h show a plurality of illustrations of the penetration depth analogous to figures 7a, b for protective layer systems having a topmost layer composed of Y2O3 and a further layer from a group of further materials.

In the following description of the drawings, identical reference signs are used for identical or functionally identical components.

Figure 1 schematically shows an optical system for EUV lithography in the form of a projection exposure apparatus . The projection exposure apparatus 1 comprises a ray generating system 2, an illumination system 3 and a projection system 4, which are accommodated in separate vacuum housings and arranged successively in a beam path 6 proceeding from an EUV light source 5 of the ray shaping system 2. By way of example, a plasma source or a synchrotron can serve as EUV light source 5. The radiation emerging from the light source 5 in the wavelength range of between approximately 5 nm and approximately 20 nm is firstly concentrated in a collector mirror 7 and the desired operating wavelength AB, which is approximately 13.5 nm in the present example, is filtered out by means of a monochromator (not shown).

The radiation treated with regard to wavelength and spatial distribution in the ray generating system 2 is introduced into the illumination system 3, which has a first and second reflective optical element 9, 10 in the present example. The two reflective optical elements 9, 10 guide the radiation onto a photo mask 1 as further reflective optical element, which has a structure which is imaged onto a wafer 12 on a reduced scale by means of the projection system 4. For this purpose, a third and fourth reflective optical element 13 and 14 are provided in the projection system 4. It should be pointed out that both the illumination system 3 and the projection system 4 can in each case have only one or else three, four, five or more reflective optical elements.

The structure of two optical elements 50 such as can be realized on one or more of the optical elements 7, 9, 10, 11 , 13, 14 of the projection exposure apparatus 1 from figure 1 is illustrated by way of example below with reference to figures 2a, b. The optical elements 50 each have a substrate 52 consisting of a substrate material having a low coefficient of the thermal expansion, e.g. composed of Zerodur®, ULE® or Clearceram®.

In the case of the reflective optical elements 50 illustrated in figures 2a, b, a multilayer system 51 is applied to the substrate 52. The multilayer system 51 has alternately applied layers of a material having a higher real part of the refractive index at the operating wavelength λ_Β (also called spacer 55), and of a material having a lower real part of the refractive index at the operating wavelength λβ (also called absorber 54), wherein an absorber-spacer pair forms a stack 53. This construction of the multilayer system 51 in a certain way simulates a crystal whose lattice planes correspond to the absorber layers at which Bragg reflection takes place. The thickness of the individual layers 54, 55 and of the repeating stacks 53 can be constant or else vary over the entire multilayer system 51 , dependent on what spectral or angle-dependent reflection profile is intended to be achieved. The reflection profile can also be influenced in a targeted manner by the basic structure composed of absorber 54 and spacer 55 being supplemented by further more and less absorbent materials in order to increase the possible maximum reflectivity at the respective operating wavelength λβ. For this purpose, absorber and/or spacer materials can be exchanged for one another in some stacks 53 or the stacks can be constructed from more than one absorber and/or spacer material. The absorber and spacer materials can have constant or else varying thicknesses over all the stacks 53 in order to optimize the reflectivity. Furthermore, it is also possible to provide additional layers for example as diffusion barriers between spacer and absorber layers 55, 54.

In the present example, in which the optical element 50 was optimized for an operating wavelength λβ of 13.5 nm, i.e. in the case of an optical element 50 which has the maximum reflectivity for substantially normal incidence of radiation at a wavelength of 13.5 nm, the stacks 53 of the multilayer system 51 have alternate silicon and molybdenum layers. In this case, the silicon layers correspond to the layers 55 having a higher real part of the refractive index at 13.5 nm, and the molybdenum layers correspond to the layers 54 having a lower real part of the refractive index at 13.5 nm. Other material combinations such as e.g. molybdenum and beryllium, ruthenium and beryllium or lanthanum and B₄C are likewise possible. In the present example, the multilayer system 51 has a molybdenum layer as topmost layer 54.

The reflective optical elements 50 from figures 2a, b each have an optical surface 56 forming the interface with the vacuum environment. In the projection exposure apparatus 1 the optical elements 50 are operated under vacuum conditions in a residual gas atmosphere in which typically a small proportion of oxygen, a proportion of reactive hydrogen and, if appropriate, a proportion of tin are present. Tin compounds (or generally metal hydride compounds) can occur, in particular, if the light source 5 generates EUV radiation on the basis of a tin plasma.

In order to protect the optical elements 50 against these and possible further contaminating substances, in the example shown in figure 2a, a protective layer system 59 formed from a single (and thus topmost) layer 57 (having thickness d), is applied to the multilayer system 51. The example of an optical element 50 as shown in figure 2b differs from that shown in figure 2a merely in that the protective layer system 59 has two layers 57, 58. The topmost layer 57 (having thickness d1) in this case has a surface 56 directed toward the environment or toward the vacuum, and the lower layer 58 (having thickness d2) is arranged adjacent to the topmost layer 54 of the multilayer system 51. It should be pointed out that the protective layer system 59 can also have more than two layers, for example three, four, five or more layers. It should also be pointed out that between the layers of the protective layer system 59 if appropriate additional (thin) layers can be arranged which counteract a mixing of two adjacent layers 57, 58, for example by performing the function of a diffusion barrier.

Depending on the choice of the material of the topmost layer 57 and the number and type of the further layer(s) 58, the topmost layer of the multilayer system 51 adjoining the protective layer system 59 can be a spacer layer 55 or an absorber layer 54. Preferably, a topmost absorber layer 54 is adjoined by a protective layer 58 having a higher real part of the refractive index and a topmost spacer layer 55 is adjoined by a protective layer 58 having a lower real part of the refractive index at the wavelength for which the multilayer system 51 is deigned, in order to obtain a reflectivity that is as high as possible. It is further advantageous if the topmost layer of the multilayer system 51 adjoining the protective layer system 59 is an absorber layer, in order to additionally protect the topmost spacer layer of the multilayer system against reactive hydrogen, in particular in the case of spacer layers 55 composed of silicon.

A material which is used for the topmost layer 57 of the protective layer system 59 is subject to a number of requirements: the material firstly should be chemically stable and as far as possible not enter into any reactions with reactive hydrogen and secondly should be suitable for coating by means of a conventional coating method. It is also advantageous if the material of the topmost layer 57 has a good barrier effect for hydrogen ions and - in particular in the case of optical elements 50 arranged in proximity to the radiation source 5 - has a high resistance toward a tin sputtering process.

In order to select suitable materials from a multiplicity of materials which are appropriate, in principle, for application as thin layers or plies by means of conventional coating methods, a multiplicity of these materials were assessed with regard to their suitability as topmost layer 57 on the basis of an

assessment scheme which is described in greater detail below with reference to the table shown in figure 3. The materials presented in the table from figure 3 are chemical compounds in the form of oxides, carbides, nitrides, silicates and borides of the chemical elements Y, Ce, Zr, Nb, Si, Ti, V, Mo, Mn, Al, W, Cr, La, Co, Ru, B, Hf, U, Be. Although a multiplicity of compounds of these chemical elements have already been investigated in the table from figure 3, it goes without saying that the selection given in the table is not exhaustive and that there are further chemical compounds which, if appropriate, are likewise suitable as materials for the topmost layer 57 of the protective layer system 59.

For a material respectively indicated in the first column of the table, for a thickness of 10 nm (cf. second column) the third column indicates the maximum reflectivity R (in %) of this material or of the optical element 50 if this material is applied as (single-layered) protective layer system 51 to the multilayer system 59. The fourth column indicates the relative change in reflectivity R in the case of a change in thickness DR/R, i.e. the change in reflectivity R in the case of a thickness d of 10 nm compared with a thickness d of 0 nm (i.e. without a protective layer system). The fifth column indicates the rank order of the materials presented in the table with regard to the change in thickness DR/R. The rank order indicates the suitability of the respective material as topmost layer 57 of the protective layer system 59 with regard to the change in thickness DR R in comparison with the further materials presented in the table. The suitability of a respective material increases, the smaller the resulting change in reflectivity. The sixth column of the table indicates the enthalpy of formation (in kJ/mol) for forming a solid of the respective compound under standard or normal conditions (i.e. at a temperature of 298.15 K and a pressure of 1013 mbar), the enthalpy of formation being normalized to the number of atoms of the

respective compound. This means e.g. in the case of Nb02 as compound, which has three atoms and an enthalpy of formation of approximately -796 kJ/mol, that the normalized value entered into the table is -796 kJ/mol/3 = -265 kJ/mol. The seventh column of the table indicates the rank order of the presented materials with regard to the enthalpy of formation, wherein materials having (in terms of the absolute value) greater enthalpy of formation on account of the stronger bond have a greater suitability as topmost layer 57 of the protective layer system 51 than materials having lower enthalpy of formation.

The eight column of the table presents a rating with regard to the penetration depth of reactive hydrogen on a rating scale or a rating factor of 1 to 3 points, wherein a small penetration depth is rated with one point and a high penetration depth with three points.

The ninth column of the table indicates the sum of the ranks of the respective material in the rating of the change in reflectivity (fifth column) and in the rating of the enthalpy (seventh column). The tenth column indicates notes relating to known disadvantages of the respective materials, e.g. whether a specific material is hygroscopic or can be etched away easily. The rating takes account of such disadvantage with a factor of 0.3, while materials for which no such disadvantages are known are rated with a factor of 1.0 (eleventh column). In the present example, an affinity for tin also has a negative effect on the respective factor in the tenth column. However, the affinity for tin may be of secondary importance, if appropriate, in the case of optical elements far away from the radiation source 5, and so the factor in the eleventh column should be suitably adapted, if appropriate. Finally, the twelfth and last column indicates a total rating characteristic figure (figure of merit) for a respective material, which is calculated as follows in the present example: the value for the enthalpy (sixth column) multiplied by the rating characteristic figure for the penetration depth (eighth column), multiplied by the rating characteristic figure for known disadvantages (eleventh column), divided by the value for the change in reflectivity (fourth column). Illustrated on the basis of the example of Y203, which has the highest figure of merit, this results in: (- 381.06 [kJ/mol] / -14.34 ) x 3 x 1 = 79.7.

The figures of merit presented in the twelfth column of the table from figure 3 are illustrated in a rank order in a bar chart in figure 4, with figure of merit being plotted on a logarithmic scale. In this case it is evident that the highest figures of merit are achieved by the materials Y2O3, Ce203, Zr02, Ce02, Nb202 and NbO such that these materials, on the basis of the rating method described above, have proved to be particularly advantageous materials for the topmost layer 57 of the protective layer system 59. It goes without saying that the rating scheme described above can also be modified, e.g. by the results contained in the individual columns of the table from figure 3 being incorporated into the figure of merit with a different weighting, whereby the rank order shown in figure 4 possibly changes. It holds true in principle, however, that materials which acquire a comparatively high figure of merit in the case of the rating method described above maintain this figure of merit even in the case of (slight) modifications of the rating scheme, and so the method described above is accorded high meaningfulness regarding the suitability of materials for the topmost layer 57 of the protective layer system 59.

As explained further above, for the selection of a suitable material for the topmost layer 57 of the protective layer system 59, the reflectivity or the transmission of the respective material for the EUV radiation to be reflected or the change thereof is also of importance. Figures 5a, b respectively show the reflectivity R (in %) of an optical element having a single-layered protective layer system 59 as a function of the thickness d of the layer 57 for different materials up to a maximum thickness of 10 nm (figure 5a) and up to a maximum thickness of 15 nm (figure 5b), thereby giving rise to the values - indicated in the third column of the table from figure 3 - for the reflectivity for the operating wavelength AB in the case of a thickness d of 10 nm. The change in thickness DR/R as indicated in the fourth column corresponds to the relative change in reflectivity in the case of a thickness of 10 nm compared with a thickness of 0 nm of the protective layer system 59. This change dR/R approximately corresponds to the gradient of the respective reflectivity curve in the case of a thickness d of the protective layer system 59 of 10 nm.

Alongside the selection of a material for the topmost layer 57, the stipulation of a thickness d of the topmost layer (and, if appropriate, of further layers 58 situated underneath) is also necessary for the optimization of the protective layer system 59. In order to effectively protect the multilayer system 51 situated below the protective layer system 59 against hydrogen ions and to prevent blistering, the protective layer system 59 should have a sufficient total thickness. However, said thickness should not be chosen to be excessively large (typically not greater than approximately 25 nm), in order to prevent an excessive loss of reflectivity of the optical element 50.

The penetration depth of hydrogen ions having energies in the range of approximately 100 eV or higher is material-dependent and is in the range of between approximately 10 nm and approximately 15 nm for most of the materials investigated here. The penetration depth of various materials was investigated with the aid of computer simulations, where it was found that, in the case of a single-layered protective layer system 59 for the materials Y2O3, Ce₂0₃, Zr0₂, La₂0₃, Ce0₂, Si0₂, Nb₂0₅, V₂0₅, and ZrN, a thickness of approximately 15 nm suffices to virtually completely prevent the penetration of hydrogen ions into the underlying multilayer system 51. A result of such a simulation for Ce₂0₃ is illustrated by way of example in figure 7a. It has therefore proved to be advantageous if a single-layered protective layer system 59 using the materials mentioned above has a thickness d of between approximately 10 nm and approximately 18 nm, preferably between

approximately 12 nm and approximately 15 nm.

Corresponding simulations for a second group of materials, namely Nb0₂, NbO, ΊΠΟ2, BN, TiO, MoS^'i2, ΤΊ3Ο5, S13N4, revealed that in the case of these materials a smaller thickness d of typically a maximum of approximately 10 nm suffices to effectively prevent the penetration of hydrogen ions into the multilayer system 51. The result of such a simulation is illustrated by way of example for MoSi₂ in figure 7b. Comparison of figure 7a and figure 7b clearly shows that in the case of M0S12 practically no hydrogen ions penetrate in a range of the penetration depth of between approximately 10 nm and approximately 15 nm

(corresponding to 100 and 150 Angstrom), while this is not the case for Ce₂O₃. Accordingly, in the case of the second group of materials it is advantageous if the single-layered protective layer system 59 or the layer 57 has a thickness d of between approximately 8 nm and approximately 12 nm, preferably between approximately 8 nm and approximately 10 nm.

Instead of a single-layered protective layer system 59 as shown in figure 5a, it is also possible to use a multilayered, e.g. a two-layered protective layer system 59, as illustrated in figure 5b. In this case, the material of the first layer 57 can be chosen in the manner described above in order to impart to the protective layer system 59 a resistance toward all degradation processes at the interface 56 with the vacuum environment. The material of the at least one underlying layer 58 can be chosen such that said material has a high transmission for EUV radiation at the operating wavelength λ_Β (and also a good barrier effect for high- energy hydrogen ions in order to keep the thickness of the further layer 58 as small as possible). In this case, the material used for the at least one further layer 58 can have, in particular, a lower absorption coefficient at the operating wavelength λβ than the material of the topmost layer 57, whereby the reflectivity of a two-layered protective layer system 59 can be increased compared with a single-layered protective layer system 59 having comparable thickness.

Typically, in the case of a multilayered protective layer system 59, the thickness d1 of the topmost layer which is required to protect the further layer(s) 58 against degradation is less than the (total) thickness of the further layer(s) 58 which is required to effectively stop the hydrogen ions penetrating into the protective layer system 59. In particular, the thickness di of the topmost layer 57 can be not more than 5 nm, if appropriate not more than 2 nm, wherein the topmost layer 57 should typically not fall below a thickness di of approximately 1 nm. In the case of a two-layered protective layer system 59, the underlying layer 58 has a thickness d2 of typically more than approximately 5 nm (and generally a maximum thickness d2 of approximately 15 nm). It goes without saying that, in the case of protective layer systems 59 comprising three or more layers, the thickness d2 can be divided among a plurality of the further layers 58.

Since the material of the further layer 58 does not come directly into contact with the vacuum environment or the interface 56, the number of materials which can be used for the at least one further layer 58 is typically greater than for the topmost layer 57. The material of the further layer 58 can be selected, for example, from the group comprising: Y, Ce, Zr, Nb, Si, Ti, V, Mo, Mn, Al, W, Cr, La, Co, Ru, B, Hf, U, Be and the oxides, carbides, nitrides, silicates and borides thereof.

For the use of a topmost layer 57 composed of Y2O3, i.e. that material which proved to be particularly advantageous in the selection method described above in association with figure 3 and figure 4, the penetration depth was investigated in the case of a single-layer protective layer system (figure 8a) and in the case of two-layer protective layer systems 59 having a second layer composed of B₄C (figure 8b), S13N4 (figure 8c), Mo (figure 8d), Ru (figure 8e), Pt (figure 8f), Zr (figure 8g) and Si (figure 8h), wherein the transition between the topmost layer 57 and the further layer 58 in figures 8b-h occurred at a penetration depth of 2 nm, i.e. the thickness di of the topmost layer 57 composed of Y2O3 was 2 nm. A comparison with the single-layer protective layer system 59 from figure 8a or from figures 7a, b shows that, with regard to the penetration depth of hydrogen ions, two-layer protective layer systems 59 behave similarly, in principle, to single-layer protective layer systems 59 having comparable thickness.

Therefore, suitable choice of a material for the further layer 58 makes it possible to optimize or increase the reflectivity of the two-layer protective layer system 59 compared with a single-layer protective layer system 59. In this regard, figure 6a shows the reflectivity of the optical element 50 with the use of the materials shown in figures 8b-h for the second layer 58 having a thickness d2 of between 6 nm and 9 nm. It can clearly be discerned that the use of a second layer composed of silicon yields the highest reflectivity for the protective layer system 59 in the case of the materials investigated. With the exception of the use of ruthenium or platinum as second layer 58, for all materials

investigated here, in the case of a two-layered protective layer system 59 the reflectivity is increased compared with a single-layered protective layer system 59 composed of Y2O3, i.e. a two-layered protective layer system 59 having a topmost layer 57 composed of Y2O3 having a thickness di of 2 nm and a second layer 58 having a thickness d2 of approximately 7-8 nm composed of Si, Zr, B₄C, Mo or S13N4 has a higher reflectivity than a single-layered protective layer system 59 having a layer 57 composed of Y2O3.

As can be discerned with reference to figure 6b, this situation changes only insignificantly if V2O5 is used as material for the topmost layer 57, although this material has a significantly higher absorption for EUV radiation at the operating wavelength λ_Β than Y2O5. In this case, with the exception of platinum, for all materials investigated Si, Zr, B₄C, Mo, Si3N₄ and Ru the reflectivity of a two- layer protective layer system 59 is higher than with the use of a single layer 57 composed of V2O5. It is also evident that, on account of the comparatively small thickness of the topmost layer 57, the influence thereof on the reflectivity R of the optical element 50 is rather small.

Therefore, for the topmost layer 57, in particular if the latter has a thickness d-i of approximately 2 nm or less, it is also possible to use particularly inert materials which are generally not used in EUV lithography on account of their high absorption for EUV radiation. By way of example, it is possible to use a protective layer system 59 comprising two or more layers for an optical element 50 whose topmost layer 57 is formed from tungsten oxide (W2O3, WO2 or W0₃), tungsten carbide (WC), titanium carbide (TiC), aluminum oxide (Al₂0₃) or from other particularly inert materials.

To summarize, in the manner described above it is possible to provide an optimized protective layer system for optical elements which can be used in optical systems for EUV lithography, in particular in the presence of reactive hydrogen and/or tin or tin compounds. It should be pointed out that typically a change in the stoichiometry of the above-described chemical compounds, i.e. a slight change in the proportion of the individual atoms in the respective compound, influences the above-described results only slightly - if at all.

It should also be pointed out that, in the selection process described above, those materials which have a high affinity for tin deposits were ruled out or given a lower rating. If a protective layer system is optimized for an optical element which is arranged far away from the radiation source, such that the tin concentration in the environment is low or virtually no tin is present, it is the case that for those materials which acquired a low figure of merit in the above selection process on account of the affinity for tin, this rating may change.

Claims

Patent Claims

1. Method for optimizing a protective layer system (59) for an EUV radiation (6) reflecting multilayer system (51 ) of an optical element (50), comprising the following steps:

selecting a material for a topmost layer (57) of the protective layer system (59) from a group of chemical compounds comprising: oxides, carbides, nitrides, silicates and borides, wherein selecting the material for the topmost layer (57) is effected depending on an enthalpy of formation of the respective chemical compound.

2. Method according to Claim 1 , wherein the group comprises oxides,

carbides, nitrides, silicates and borides of the following chemical elements: Y, Ce, Zr, Nb, Si, Ti, V, Mo, Mn, Al, W, Cr, La, Co, Ru, B, Hf, U, Be.

3. Method according to Claim 1 or 2, wherein the group comprises the

following chemical compounds: Y2O3, Ce₂0₃, Zr0₂, Ce0₂, Nb₂0₅, Nb0₂, NbO, Si0_2l ΤΊ3Ο5, V3O5, ΤΊ2Ο3, M0O2, Mn0₂, Ti0₂, V₂0₅, Al₂0₃, V₂0₃, MoSi₂, Mn₂0₃, W0₃, Cr₃0₄, TiO, Mn₃0₄, M0O3, La₂0₃, Cr₂0₃, MnO, W0₂, Cr0₂, VO, AIN, Co₃0₄, Si₃N₄, Ru0₂, BN, SiC, Ru0₄.

4. Method according to any of the preceding claims, further comprising:

choosing a thickness (d, d-i , d₂) of at least one layer (57, 58) of the

protective layer system (59) depending on a penetration depth of reactive hydrogen into the at least one layer (57, 58).

5. Method according to any of the preceding claims, wherein the protective layer system (59) consists of the topmost layer 57 having a thickness (d) of between 8 nm and 12 nm, wherein the material of the topmost layer (57) is selected from the group comprising: Nb02, NbO, ΤΊΟ2, BN, TiO, M0S12,

6. Method according to any of claims 1 to 4, wherein the protective layer

system (59) consists of the topmost layer (57) having a thickness (d) of between 10 nm and 18 nm, wherein the material of the topmost layer (57) is selected from the group comprising: Y2O3, Ce203, Zr02, La203, Ce02, S1O2, Nb₂0_5> V₂0_5> ZrN.

7. Method according to any of the preceding claims, wherein the material of the topmost layer (57) of the protective layer system (59) is selected on the basis of the reflectivity (R) and/or the thickness-dependent change in reflectivity (DR/R) of the topmost layer (57) at the wavelength (λ_Β) to be reflected by the multilayer system (51 ).

8. Method according to any of Claims 1 to 4 or 7, wherein the protective layer system (50) has at least one further layer (58) below the topmost layer (57), the thickness (d2) of which is chosen to be greater than the thickness (d1 ) of the topmost layer (57).

9. Method according to Claim 8, wherein a thickness (d1 ) of not more than 5 nm, preferably of not more than 2 nm, is chosen for the topmost layer (57), and wherein a thickness (d2) of more than 5 nm is chosen for the at least one further layer (58).

10. Method according to either of Claims 8 or 9, further comprising:

selecting the material of the at least one further layer (58) depending on the reflectivity (R) of the optical element (50) provided with the protective layer system (50) for the wavelength (AB) to be reflected by the multilayer system (51 ).

11. Optical element (50), comprising:

an EUV radiation (6) reflecting multilayer system (51), and

a protective layer system (59) having a topmost layer (57) composed of a material selected from a group of chemical compounds comprising: oxides, carbides, nitrides, silicates and borides, wherein the protective layer system (59) either consists of the topmost layer (57) having a thickness (d) of between 5 nm and 15 nm, or

the protective layer system (59) has at least one further layer (58) below the topmost layer (57), the thickness (d2) of which is greater than the thickness (d-i) of the topmost layer (57), wherein the topmost layer (57) has a thickness (d-i) of not more than 5 nm and a thickness (d2) of the further layer (58) or of the further layers is greater than 5 nm.

12. Optical system according to Claim 11 , wherein the material of the topmost layer (57) is selected from the oxides, carbides, nitrides, silicates and borides of the following chemical elements: Y, Ce, Zr, Nb, Si, Ti, V, Mo, Mn, Al, W, Cr, La, Co, Ru, B, Hf, U, Be.

13. Optical element according to Claim 12, wherein the material of the topmost layer (57) is selected from the group comprising: Y2O3, Ce₂O₃, ZrO₂, CeO₂, Nb₂O₂ and NbO.

14. Optical element according to any of Claims 11 to 13, wherein the protective layer system (59) consists of the topmost layer (57) having a thickness (d) of between 8 nm and 12 nm and the material of the topmost layer (57) is selected from the group comprising: NbO2, NbO, T1O2, BN, TiO, MoS^'12,

15. Optical element according to any of Claims 11 to 13, wherein the protective layer system (59) consists of the topmost layer (57) having a thickness (d) of between 10 nm and 18 nm and the material of the topmost layer (57) is selected from the group comprising: Y2O3, Ce2O3, ZrO2, La₂Os, CeO2, SiO₂, Nb₂O₅₍ V₂O₅₎ ZrN.

16. Optical element according to any of Claims 1 1 to 13, wherein the topmost layer (57) has a thickness (di) of not more than 2 nm.

17. Optical element according to Claim 16, wherein the material of the at least one further layer (58) has a lower absorption coefficient for EUV radiation at the wavelength (AB) to be reflected by the multilayer system (51 ) than the topmost layer (57).

18. Optical element according to either of Claims 16 and 17, wherein the

material of the at least one further layer (58) is selected from the group comprising:

Y, Ce, Zr, Nb, Si, Ti, V, Mo, Mn, Al, W, Cr, La, Co, Ru, B, Hf, U, Be and the oxides, carbides, nitrides, silicates and borides thereof.

19. Optical element according to Claim 18, wherein the material of the at least one further layer (58) is selected from the group comprising: B₄C, Si₃N₄, Mo, Ru, Zr, Si.

20. Optical element according to any of Claims 16 to 19, wherein the topmost layer (57) has a thickness (di) of 2 nm or less and the material of the topmost layer (57) is selected from the group comprising: tungsten oxide, tungsten carbide, titanium carbide and aluminum oxide.

21 . Optical element according to any of Claims 1 1 to 20, which is embodied as a collector mirror (7).

22. Optical system (1 ) for EUV lithography, comprising: at least one optical

element (7, 9, 10, 1 1 , 13, 14, 50) according to any of Claims 1 1 to 21 .