DE102013203746A1 - Reflective optical element for optical system for extreme ultraviolet lithography apparatus, has multilayer system whose substrate side is provided with two layers made from amorphous silicon, silicon nitride, ruthenium and molybdenum - Google Patents

Abstract

The optical element e.g. optical element (13) has a multilayer system composed of alternatively arranged layers of a material with a lower real unit and a high real unit of refraction index in the extreme ultraviolet wavelength range on a substrate. A substrate side of multilayer system is provided with two layers made from amorphous silicon having a thickness between 1-3nm, silicon nitride having a thickness between 1-2nm, ruthenium and molybdenum. The supplementary layer is made of metal tin, zinc, tungsten and rhenium. Independent claims are included for the following: (1) optical system for EUV lithography apparatus; and (2) EUV lithography apparatus.

Description

The present invention relates to an extreme ultraviolet wavelength region reflective optical element for the extreme ultraviolet wavelength region having a multilayer system of alternating layers of a material having a smaller real part of the refractive index in the extreme ultraviolet wavelength range and a material having a higher real part of the refractive index in the extreme ultraviolet wavelength range on a substrate. In addition, the present invention relates to an optical system for EUV lithography or to an EUV lithography apparatus with such a reflective optical element.

In EUV lithography devices, for the lithography of semiconductor devices, reflective optical elements are used for the extreme ultraviolet (EUV) wavelength range (eg, wavelengths between about 5 nm and 20 nm), such as photomasks or mirrors. Since EUV lithography devices generally have a plurality of reflective optical elements, they must have the highest possible reflectivity in order to ensure a sufficiently high overall reflectivity. Since a plurality of reflective optical elements are usually arranged one after the other in an EUV lithography device, even smaller reflectivity deteriorations in each individual reflective optical element have a greater effect on the overall reflectivity within the EUV lithography device.

In the operation of EUV lithography devices, reflective optical elements are exposed to intense EUV radiation in order to minimize the exposure time. In the interior of EUV lithography devices, especially in the interior of lighting and projection systems, vacuum conditions prevail in order to reduce possible contamination by the residual gas atmosphere.

Another source of contamination may be the source of radiation for the EUV radiation, particularly if it is a plasma source based on laser excitation (LPP source). For this purpose, material droplets are exposed to intense laser radiation in order to excite them to a plasma, the u. a. Radiation emitted by EUV or soft X-ray wavelength range. Preference is given to using metals such as tin. As a by-product of the plasma radiation, so-called debris also material atoms or molecules or material particles are emitted, the u. a. can also hit the optically used surfaces of reflective optical elements and deposit there, which can lead to significant reflectivity losses.

So far, in particular metallic deposits are thereby removed from the optically used surfaces of reflective optical elements, by z. B. subjected to hydrogen radicals. It is believed that the metal deposits are thereby converted to volatile metal hydrides. However, recent studies, such as by MMJW van Herpen et al., Chem. Phys. Lett. 484 (2010) 197-199 in that, in particular in the case of reflective optical elements having a topmost layer of ruthenium and tin deposits, complete removal of the tin deposits can not be readily achieved in this way. According to van Herpen et al. (supra), complete removal of the tin deposits is achieved in reflective optical elements in which a 2 nm thick silicon nitride layer is provided between the ruthenium layer and the tin deposit.

It is now an object of the present invention to provide a reflective optical element whose reflectivity does not suffer much from the growth of contamination.

This object is achieved by an extremely ultraviolet wavelength reflective optical element having a multilayer system of alternating layers of a material having a lower real part of the refractive index in the extreme ultraviolet wavelength range and a material having a higher real part of the refractive index in the extreme ultraviolet wavelength range on a substrate, in which two further layers are arranged on the side of the multilayer system facing away from the substrate, the further substrate layer being made of amorphous silicon having a thickness of between 1 nm and 3 nm and the further layer of silicon nitride further removed from the substrate having a thickness of between 1 nm and 2 nm ,

It has been found that reflective optical elements having on their multilayer system a layer of amorphous silicon having a thickness between 1 nm and 3 nm, preferably 1.5 nm to 3 nm, particularly preferably 1.55 nm to 2.5 nm and above another layer of silicon nitride having a thickness between 1 nm and 2 nm, in the presence of an overlying additional layer of material, as may occur in contaminations show lower reflectivity losses than conventional reflective optical elements, in which the layer of contaminant material directly on the multi-layer system grows up. The two additional layers allow the reflection of the standing wave field or the position of the reflective surface optical element with respect to the standing wave field before a contamination layer grows, so influence that a layer thickness variation of the contaminant causes minimal changes in reflectivity. In this way, even with laterally inhomogeneous contamination, the imaging properties of an optical system in which the reflective optical element is used can be maintained. Especially with small thicknesses of the layer of contamination material in the range of a few nanometers, sufficient reflectivities may still be present, in which, for example, an EUV lithography device can continue to be operated. As a result, in the case of the reflective optical elements proposed here, the cleaning cycles for removing the topmost layer of contamination material can be selected to be further apart in terms of time than in the case of conventional reflective optical elements. In addition, the reflective optical element proposed here has a comparable good cleanability, as in van Herpen et. al. (ibid.) described reflective optical elements having a 2 nm thick Siliziumnitridlage on a ruthenium layer.

The object is further achieved by an extreme ultraviolet wavelength reflective optical element having a multilayer system of alternating layers of a lower refractive index material in the extreme ultraviolet wavelength range and a higher refractive index real material in the extreme ultraviolet wavelength range on a substrate in which the top layer of the multi-layer system is of the higher refractive index material having a different thickness than the other layers of the multi-layer system of the higher refractive index material, and the other side of the multilayer system is another layer of ruthenium and titanium further away from the substrate an additional layer of metal is arranged.

It has been found that adjusting the thickness of the uppermost layer of the multi-layer system of the higher refractive index material results in the additional metal layer resulting in only minor reflectivity variations, especially at lower metal layer thicknesses. Complete removal of an additional layer of metal that could be due to contamination, as in van Herpen et al. (ibid.) would not be necessary for applications in, for example, EUV lithography.

Advantageously, the uppermost layer of the multilayer system of higher refractive index material has a thickness of between 1 nm and 5 nm, in order to obtain low reflectivity variations.

Preferably, a barrier layer is arranged between the uppermost layer of the multi-layer system of the material with a higher real part of the refractive index and the further layer of ruthenium. This serves to suppress interdiffusion of the ruthenium layer and the uppermost layer of the multi-layer system from the material with a higher real part of the refractive index and to avoid a consequent drop in the reflectivity.

In preferred embodiments, the first-mentioned reflective optical element on the side facing away from the substrate of the further layer of silicon nitride has an additional layer which contains carbon, silicon dioxide or a metal. In the second significant reflective optical element, the additional layer is metal. The metal is particularly preferably tin, zinc, tungsten or rhenium. These contaminants may typically occur on reflective optical elements used in EUV lithography devices. Carbon contaminants can result from various organic substances, for example, for sealing vacuum pumps or photoresist on objects to be exposed. Silica contamination can arise if water or oxygen is contained in the residual gas atmosphere and comes into contact with silicon, for example from the multilayer system, or if silanes are present in the residual gas atmosphere which can evaporate from other components of the EUV lithography apparatus. Zinc, rhenium and especially tin can be from laser plasma sources. Tungsten can come from heating wires used to process hydrogen radicals to remove metallic contamination. The reflective optical elements with the additional layer can already be prepared by conventional coating methods prior to installation in an optical system of an EUV lithography device. Particularly preferably, the thickness of the additional layer is chosen so that no additional reflectivity losses are to be expected by additional growth or slight cleaning of contamination. A so prepared reflective optical element has the advantage that the additional layer serves as a kind of protective layer. In other variants, the additional layer can also be applied only after installation in the EUV lithography device by conventional contamination processes, which are specifically triggered for this purpose.

Advantageously, the additional layer has a thickness between 0.1 nm and 2 nm. In this thickness range, the additional layer already has a certain protection against contamination by each other contaminants and against mechanical effects, but still shows neither reflectivity losses or deterioration of imaging properties that would significantly affect operation of the EUV lithography device.

Advantageously, the multilayer system has molybdenum or ruthenium as material with a higher real part of the refractive index silicon and as a material with the lower real part in order to achieve high reflectivities, in particular in the wavelength range around 12.5 nm to 15 nm.

In a particularly preferred embodiment, the reflective optical element is designed as a collector mirror. In EUV lithography, collector mirrors are often used as the first mirror in the beam direction behind the radiation source, in particular plasma radiation sources, in order to collect the radiation emitted by the radiation source in different directions and to reflect it in bundles to the next mirror. Because of the proximity to the radiation source, contamination, in particular due to debris from the radiation source, can deposit there on the optically used surface of the reflective optical element with a particularly high degree of probability, which can lead in some cases to very high reflectivity losses in conventional reflective optical elements. When the collector mirror is an extreme ultraviolet wavelength reflective optical element having a multilayer system of alternately arranged layers of a material having a lower refractive index extreme ultraviolet wavelength region and a material having a higher real refractive index extreme ultraviolet wavelength region Substrate in which two further layers are arranged on the substrate side facing away from the Viellagensystem, wherein the substrate nearer further layer of amorphous silicon has a thickness between 1 nm and 3 nm and the further removed from the substrate further layer of silicon nitride of a thickness between 1 nm and 2 nm is, especially at contamination thicknesses to about 1.5 nm to 2 nm significantly lower reflectivity losses are observed. Such a collector mirror has the advantage that its contamination layer thickness can be in equilibrium between debris separation and EUV-induced hydrogen purge without exhibiting variations in local reflectivities that are detrimental to local variations around this equilibrium position.

Furthermore, the object is achieved by an optical system for EUV lithography or by an EUV lithography apparatus having at least one reflective optical element as described above.

The present invention will be explained in more detail with reference to preferred embodiments. Show this

1 schematically an embodiment of an EUV lithography device with a lighting system with a proposed here reflective optical element as a collector mirror;

2a a schematic representation of a first exemplary embodiment of the reflective optical element;

2 B a schematic representation of a second exemplary embodiment of the reflective optical element;

2c a schematic representation of a third exemplary embodiment of the reflective optical element

3 the reflectivity of an embodiment of the reflective optical element according to 2 B depending on the thickness of the additional layer for different materials;

4 the reflectivity of a conventional reflective optical element as a function of the thickness of the uppermost layer for different materials;

5 - 10 the course of the reflectivity for different variants of the reflective optical element in embodiments according to 2 B depending on the thickness of the additional layer and the thickness of the layer of amorphous silicon; and

11 the course of the reflectivity for a variant of the reflective optical element in embodiments according to 2c depending on the thickness of the additional layer and the thickness of the uppermost layer of the multi-layer system of the material with a higher real part of the refractive index.

In 1 schematically is an EUV lithography device 10 shown. Essential components are the lighting system 14 , the photomask 17 and the projection system 20 , The EUV lithography device 10 is operated under vacuum conditions so that the EUV radiation is absorbed as little as possible in its interior.

As a radiation source 12 For example, a plasma source or a synchrotron can serve. The example shown here is a plasma source. The emitted radiation in the wavelength range of about 5 nm to 20 nm is first from the collector mirror 13 bundled. The operating beam is then in the lighting system 14 introduced. Im in 1 example shown has the lighting system 14 two mirrors 15 . 16 on. The mirror 15 . 16 direct the beam onto the photomask 17 that has the structure on the wafer 21 should be displayed. At the photomask 17 it is also a reflective optical element for the EUV wavelength range, which is replaced depending on the manufacturing process. With the help of the projection system 20 becomes that of the photomask 17 reflected beam on the wafer 21 projected and thereby imaged the structure of the photomask on him. The projection system 20 In the example shown, there are two mirrors 18 . 19 on. It should be noted that both the projection system 20 as well as the lighting system 14 each may have only one or even three, four, five or more mirrors. In modifications, the radiation source 12 also outside the lighting system 14 be arranged.

In order to ensure the highest possible and constant reflectivity even with increasing contamination, one or more of the mirrors or the photomask are designed such that they are a multilayer system of alternately arranged layers of a material with a lower real part of the refractive index in the extreme ultraviolet wavelength range and of a material having a higher real part of the refractive index in the extreme ultraviolet wavelength range on a substrate and that on the substrate side facing away from the Viellagensystem two further layers are arranged, wherein the substrate closer further layer of amorphous silicon has a thickness between 1 nm and 3 nm and further from the substrate removed further layer of silicon nitride of a thickness between 1 nm and 2 nm. It is particularly advantageous for the collector mirror 13 because of its proximity to the radiation source 12 the risk of contamination with carbon, silica, tin, zinc, tungsten or rhenium-containing debris is particularly high.

In 2a is schematically the structure of a reflective optical element 50 shown. The illustrated example is a reflective optical element placed on top of a substrate 52 applied multi-day system 51 based. As substrate materials, materials with a low thermal expansion coefficient are preferably selected. In the multi-day system 51 These are essentially alternating layers of a material with a higher real part of the refractive index at the operating wavelength at which, for example, the lithographic exposure is carried out (also spacer 55 called) and a material with a lower real part of the refractive index at the operating wavelength (also absorber 54 called), wherein in the example shown here, an absorber-spacer pair a stack 53 forms, which corresponds in periodic multilayer systems of a period. This somehow simulates a crystal whose lattice planes correspond to the absorber layers at which Bragg reflection occurs. The thicknesses of the individual layers 54 . 55 as well as the repeating stack 53 can over the entire multi-day system 51 be constant or vary, depending on which spectral or angle-dependent reflection profile is to be achieved. The reflection profile can also be selectively influenced by the basic structure of absorber 54 and spacers 55 is supplemented by more more and less absorbing materials to increase the maximum possible reflectivity at the respective operating wavelength. For this purpose, in some stacks absorber and / or spacer materials can be exchanged for each other or additional layers of other materials can be provided. The absorber and spacer materials can have constant or varying thicknesses over all stacks in order to optimize the reflectivity. Furthermore, in individual or all stacks, additional layers can also be used, for example, as diffusion barriers between spacer and absorber layers 55 . 54 be provided. At the first to the substrate 52 adjacent location may be an absorber, a spacer or an additional layer.

At the in 2a exemplified first embodiment of the reflective optical element 50 is on the multi-day system 51 on the substrate side facing away from a situation 57 made of amorphous silicon having a thickness of between 1 nm and 3 nm, preferably 1.5 nm to 3 nm, more preferably 1.55 nm to 2.5 nm, and thereon, on the side of the silicon layer facing away from the substrate 57 another layer of silicon nitride 56 a thickness between 1 nm and 2 nm. In another, in 2 B illustrated embodiment of the reflective optical element 50 is located on the side of the silicon nitride layer facing away from the substrate 56 an additional location 58 which comprises carbon, silicon, silicon dioxide or another silicon compound or a metal, such as tin, zinc, tungsten or rhenium, or any other material having high absorption in the EUV wavelength range. At the additional location 58 it may be a native Kontaminationslage or even a targeted before or after installation of the reflective optical element 50 placed in the optical system of an EUV lithography device layer. In further variants can between the multi-day system 51 and the amorphous silicon layer one or more layers may be additionally provided.

In 2c a third embodiment of the reflective optical element is shown schematically. As in the other examples presented, a multi-layer system 51 on a substrate 52 arranged at the Spacerlagen 55 and absorber layers 54 alternately arranged and one stack each 53 form. The topmost spacer layer 55 ' of the multi-day system 51 has a different thickness than the other Spacerlagen 55 of the multi-day system 51 , On the multi-day system 51 is on the substrate side facing away from another location 59 made of ruthenium. Between the topmost Spacerlage 55 ' and the other location 59 Ruthenium is a barrier layer 60 , the usual from barrier layers materials such as boron carbide, carbon and. Ä., Or may be from the absorber material. On the side of the ruthenium layer facing away from the substrate 59 is an additional location 58 made of metal, which is like in the in the 2a . 2 B As illustrated examples, for example, tin, zinc, tungsten or rhenium or may be any other material with high absorption in the EUV wavelength range.

In 3 is the dependence of the reflectivity of the thickness of the additional layer for here proposed reflective optical elements, as shown by 2 B discussed. These are reflective optical elements which have a multilayer system based on molybdenum as absorber and silicon as spacer material and can therefore be used particularly well in the wave range between 12.5 nm and 15.0 nm. In the present example, the multilayer system was optimized for maximum reflectivity at normal incidence of 13.5 nm. For this purpose, the multi-layer system usually has 50 stacks of molybdenum and silicon layers, wherein a stack has a thickness of 6.93 nm and the ratio of the absorber layer thickness of molybdenum to the total thickness of the stack, also called gamma, is 0.4. On this multilayer system, in the present example, a layer of amorphous silicon having a thickness of 2.2 nm and above a silicon nitride layer having a thickness of 1.5 nm is provided. Depending on the material of the additional layer, both the thickness of the layer of amorphous silicon and the thickness of the silicon nitride layer may vary slightly.

In 3 are the reflectivity curves depending on their thickness for additional layers of tin (dash-dot), zinc (long dashed), carbon (dashed short), tungsten (dash-dotted), rhenium (dashed) and silicon dioxide (solid line) for thicknesses between 0.1 nm and 2 nm applied. For additional layers of tin and zinc, the reflectivity remains constant within 2% up to a thickness of 1.6 nm. For comparison, in 4 the reflectivity loss for comparable reflective optical elements without the layer of amorphous silicon and the layer of silicon nitride on the multilayer system are also plotted for increasing thicknesses of layers of tin (dashed) and zinc (solid) of an additional layer. There, even at a thickness of 1 nm, a reflectivity loss of well over 15% can be observed. Similar results have also been observed for reflective optical elements based on multilayer systems with ruthenium as the absorber material.

Even with additional layers of tungsten or rhenium, only minor reflectivity losses at thicknesses of up to 2 nm of the respective additional layer are observable. At a thickness of 2 nm, the reflectivity loss is less than 2%. In the case of an additional layer of silicon dioxide, the reflectivity loss at a position of up to 2 nm is even significantly below 1%, while with an additional layer of carbon even an increase in reflectivity of up to 2 nm can be observed. The reflective optical elements with additional layers of said materials on a silicon nitride and amorphous silicon pillar system arranged on a multilayer system are therefore relatively insensitive in their reflectivity to the growth of common types of contamination within an EUV lithography device. Therefore, they have a longer life than conventional reflective optical elements and need to be cleaned less often, so that less life is caused in EUV lithography devices. In addition, it can be removed without residue especially in an additional layer of tin. But because of the insensitivity to the thickness of the additional layer can be readily accepted if the additional layer is not completely removed during cleaning. This has the advantage that during cleaning, not the underlying further layer of silicon nitride is attacked, since it is protected by the additional layer.

In the 5 to 10 is corresponding to the embodiments described above 2 B the reflectivity not only as a function of the thickness of the uppermost layer, but also as a function of the thickness of the amorphous silicon layer shown as contour line graph. It is at 5 the additional layer of tin, at 6 from zinc, at 7 from carbon, at 8th from tungsten, at 9 from rhenium, and at 10 made of silicon dioxide. At the contour lines, the respective maximum reflectivity is at 13.5 nm and normal incidence. In particular, at higher thicknesses of the amorphous silicon layer can be found in all examined materials for the top layer areas in which the reflectivity remains constant with increasing thickness of the top layer. By way of example, a thickness of the amorphous silicon layer of 2.4 nm under a 1.5 nm thick silicon nitride thickness will be considered in more detail here: In this thickness of the amorphous silicon layer, the range from about 0.1 nm to about 0.9 nm tin ( 5 ), from about 0.1 nm to about 0.5 nm zinc ( 6 ), from about 0.1 nm to about 0.6 nm or about 1.1 nm to about 2 nm amorphous carbon ( 7 ), from about 0.1 nm to about 1.1 nm tungsten ( 8th ), from about 0.1 nm to about 1.3 nm rhenium ( 9 ) and from about 0.1 nm to about 1.3 nm silica ( 10 ) has a relatively constant reflectivity. Areas of both relatively constant and sufficiently high reflectivity z. B. from about 0.685 can also be from so small thicknesses of the amorphous silicon of about 1.7 nm for tin, about 1.65 nm for zinc, about 1.55 nm for amorphous carbon, about 1.7 nm at tungsten, about 1.7 nm at rhenium and about 1.55 nm at silicon dioxide. Preference is given to layer thicknesses of amorphous silicon of 2 nm and up to 3 nm, more preferably 2 nm to 2.5 nm, in which depending on the thickness of the additional layer reflectivities of just below 0.7 and higher can be achieved.

The silicon nitride layer between the amorphous silicon layer and the additional layer had a thickness of 1.5 nm in all examples shown. Similar results were also found for silicon nitride layers with thicknesses of 1 nm, 1.25 nm, 1.75 nm, 1.9 nm and 2 nm. Overall, the silicon nitride layer can have any thickness between 1 nm and 2 nm.

In 11 is for an example of a reflective optical element according to the in 2c structure shown the reflectivity not only as a function of the thickness of the uppermost layer, but also as a function of the thickness of the top Spacerlage the Viellagensystems shown. Reflective optical elements were investigated in which a multilayer system comprising fifty stacks with molybdenum as absorber and amorphous silicon as spacer was applied to a silicon dioxide substrate, the absorber layers having a thickness of 2.76 nm and the spacer layers a thickness of 4.14 nm exhibited. The thickness of the topmost spacer layer, ie, the spacer layer farthest from the substrate, was varied. More specifically, the cases were measured to have a thickness of 1 nm (solid diamond), 1.5 nm (open diamond), 2.0 nm (x-shaped), 3.0 nm (+ -shaped), 4.0 nm (double cross ) and 5.0 nm (open triangle). Above this, in all cases, a barrier layer of molybdenum of 2 nm thickness and a ruthenium layer of 1.5 nm thickness were arranged. As an additional layer, a tin layer of thickness between 0.1 nm and 2.0 nm was grown. The behavior of the reflectivity at quasi-normal incidence and a wavelength of 13.5 nm was investigated. While with tin layer thicknesses of up to 1 nm both at thicknesses of the top silicon layer from 1.0 nm to 2.0 nm only reflectivity fluctuations of up to 1 , 4% at 1.0 nm, 4.1% at 1.5 nm and 8.2% at 2.0 nm top silicon layer must be accepted, are at thicknesses of the top silicon layer of more than 2.0 nm, although the fluctuations the reflectivity with increasing tin layer thickness higher. But when the reflective optical elements are cleaned to a tin layer thickness of less than 1.0 nm, a much higher reflectivity of up to about 0.7 than in the case of the reflective optical elements with a thinner topmost silicon layer, where the maximum achievable reflectivity at about 0.65. Depending on the requirement of the desired application of the reflective optical element, the choice of the thickness of the uppermost spacer layer of the multilayer system under a ruthenium layer can thus be used to determine whether the reflectivity is to be optimized or the period of time it can be used without cleaning.

LIST OF REFERENCE NUMBERS

10

EUV lithography device

12

EUV radiation source

13

collector mirror

14

lighting system

15

first mirror

16

second mirror

17

mask

18

third mirror

19

fourth mirror

20

projection system

21

wafer

50

reflective optical element

51

Multilayer system

52

substratum

53

stack

54

absorber

55

spacer

56

silicon nitride layer

57

amorphous silicon layer

58

additional location

59

Rutheniumlage

60

barrier layer

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• MMJW van Herpen et al., Chem. Phys. Lett. 484 (2010) 197-199 [0005]
• van Herpen et. al. (loc. cit.) [0008]
• van Herpen et al. (loc. cit.) [0010]

Claims (11)

Reflective optical element for the extreme ultraviolet wavelength range with a multilayer system of alternating layers of a material with a lower real part of the refractive index in the extreme ultraviolet wavelength range and a material with a higher real part of the refractive index in the extreme ultraviolet wavelength range on a substrate, characterized in that substrate side of the multi-layer system ( 51 ) two more layers ( 56 . 57 ) are arranged, wherein the substratnähere another layer ( 57 ) of amorphous silicon having a thickness between 1 nm and 3 nm and the further layer removed from the substrate ( 56 ) of silicon nitride of a thickness between 1 nm and 2 nm. Reflective optical element according to claim 1, characterized in that it is on the substrate side facing away from the further layer ( 56 ) of silicon nitride an additional layer ( 58 ) comprising carbon, silica or a metal. Reflective optical element for the extreme ultraviolet wavelength range with a multilayer system of alternating layers of a material with a lower real part of the refractive index in the extreme ultraviolet wavelength range and a material with a higher real part of the refractive index in the extreme ultraviolet wavelength range on a substrate, characterized in that the uppermost Location ( 55 ' ) of the multi-tier system ( 51 ) of the material of higher refractive index a different thickness than the other layers ( 55 ) of the multi-tier system ( 51 ) of the material having a higher real part of the refractive index, that on the side of the multilayer system facing away from the substrate ( 51 ) another location ( 59 ) of ruthenium and further from the substrate an additional layer ( 58 ) is arranged of metal. Reflective optical element according to claim 3, characterized in that the uppermost layer ( 55 ' ) of the multi-tier system ( 51 ) of higher refractive index material has a thickness of between 1 nm and 5 nm. Reflective optical element according to claim 3 or 4, characterized in that between the uppermost layer ( 55 ' ) of the multi-tier system ( 51 ) of the material having a higher real part of the refractive index and the further layer ( 59 ) of ruthenium a barrier layer ( 60 ) is arranged. Reflective optical element according to one of claims 2 to 5, characterized in that the additional layer ( 58 ) as metal tin, zinc, tungsten or rhenium. Reflective optical element according to one of claims 2 to 6, characterized in that the additional layer ( 58 ) has a thickness between 0.1 nm and 2 nm. Reflective optical element according to one of Claims 1 to 7, characterized in that the multilayer system ( 51 ) as a material with a higher real part of the refractive index silicon and as a material with the lower real part molybdenum or ruthenium. Reflective optical element according to one of claims 1 to 8, characterized in that it is used as a collector mirror ( 13 ) is trained. Optical system for EUV lithography with a reflective optical element ( 50 ) according to one of claims 1 to 9. EUV lithography apparatus with a reflective optical element ( 50 ) according to one of claims 1 to 9.

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