Reflective optical element for grazing incidence

Abstract

A reflective optical element (101) for grazing incidence of radiation with a working wavelength from the EUV wavelength range is proposed, comprising a reflective layer (107) on a substrate (103), wherein a filter layer (105) is arranged between the reflective layer (107) and the substrate (103), the filter layer (105) having a larger real part of the refractive index in the DUV wavelength range than the reflective layer (107).

Description

[0001] The present invention relates to a reflective optical element for grazing incidence of radiation with an operating wavelength from the EUV wavelength range, comprising a reflective layer on a substrate. The invention further relates to an optical system with at least two such reflective optical elements.

[0002] In EUV lithography systems, reflective optical elements for the extreme ultraviolet (EUV) wavelength range (e.g., wavelengths between approximately 5 nm and 20 nm) are used for the lithography of semiconductor devices. These elements include photomasks or multilayer mirrors for quasi-normal incidence or mirrors with metallic surfaces for grazing incidence. Plasma radiation sources are another possible type of radiation source. The most well-known plasma radiation sources are based on laser-generated plasma (LPP source) or gas-discharge-generated plasma.

[0003] Many known EUV radiation sources emit radiation not only in the operating wavelength range. To reduce the radiation dose on the wafer being exposed due to radiation with longer wavelengths than EUV radiation, so-called out-of-band or interference radiation, e.g., deep ultraviolet radiation, visible radiation, or infrared radiation, US patent 7,773,196 B2 discloses combining two mirrors. One mirror, located in the beam direction, has a higher reflectivity for out-of-band radiation, while the other, located in the beam direction, has a lower reflectivity for out-of-band radiation. The first mirror can have an EUV-radiation-reflecting multilayer system with a sawtooth structure and / or a special coating, so that EUV radiation and out-of-band radiation are reflected in different directions. The second mirror can have a special coating that absorbs out-of-band radiation.

[0004] Reflective optical elements for grazing radiation are based on the effect of total internal reflection. To achieve high reflectivity in the EUV wavelength range, a reflective layer is typically applied to a substrate, preferably containing one or more transition metals. Ruthenium, molybdenum, niobium, and palladium have proven particularly effective. A disadvantage is that reflective layers containing one or more of these transition metals also exhibit high reflectivity for interference radiation in the ultraviolet to DUV (deep ultraviolet) wavelength range (e.g., wavelengths between approximately 100 nm and 380 nm), based on total internal reflection.

[0005] It is an object of the present invention to demonstrate a way to reduce the proportion of interfering radiation in the working beam.

[0006] The problem is solved by a reflective optical element for grazing incidence of radiation with a working wavelength from the EUV wavelength range with a reflective layer on a substrate, in which a filter layer is arranged between the reflective layer and the substrate, wherein the filter layer has a larger real part of the refractive index in the DUV wavelength range than the reflective layer.

[0007] It has been found that despite total internal reflection, radiation penetrates the reflective layer via evanescent waves and interacts with the material there. The penetration depth of the radiation is wavelength-dependent, with DUV radiation penetrating deeper than EUV radiation. Providing a filter layer with a higher refractive index in the DUV wavelength range than the reflective layer has the positive effect of reducing the ratio of DUV to EUV radiation in the total radiation reflected at grazing incidence. It is assumed that DUV radiation incident through the filter layer and penetrating to the extent necessary is coupled below the reflective layer and propagates within the filter layer, similar to a waveguide, thus becoming virtually unavailable for reflection.

[0008] Due to the improved EUV to DUV radiation ratio, the proposed reflective optical element for grazing incidence is not only suitable for use in EUV lithography devices, but also for metrological applications, such as for measuring masks or other optical elements.

[0009] Advantageously, the filter layer exhibits a real part of the refractive index of at least 0.65 at wavelengths in the DUV wavelength range. This has proven particularly suitable for EUV wavelengths in the range of approximately 10 nm to 15 nm and DUV wavelengths in the range of approximately 120 nm to 200 nm, in order to couple incident DUV radiation particularly well into the filter layer without excessively reducing the intensity of the reflected EUV radiation.

[0010] It has proven advantageous if the reflective layer has a real part of the refractive index less than 0.95 and the reflective layer has an imaginary part of the refractive index less than 0.05 at the operating wavelength. This ensures good reflectivity at grazing incidence with minimal absorption at the operating wavelength in the EUV wavelength range.

[0011] In a preferred embodiment, the reflective layer has a thickness between approximately 4 nm and approximately 8 nm and the filter layer has a thickness between approximately 4 nm and approximately 34 nm in order to allow the most efficient coupling of DUV radiation into the filter layer with the lowest possible loss of EUV radiation.

[0012] Preferably, the filter layer comprises one or more materials from the group consisting of carbon, calcium fluoride, and magnesium fluoride. The carbon can be amorphous carbon, graphite-like carbon, or diamond-like carbon. These filter layer materials exhibit significantly different refractive indices than transition metals in both the DUV and EUV wavelength ranges, particularly than ruthenium, molybdenum, niobium, and palladium, which are especially suitable for the total internal reflection of EUV radiation.

[0013] Advantageously, the ratio of reflected intensity of radiation in the DUV wavelength range to reflected intensity at the operating wavelength is at a maximum of 0.8 for an incidence angle in the range of 65° to 85°, preferably 75° to 85°. At these incidence angles, not only is there a sufficient proportion of total internal reflection, but corresponding reflective optical elements can also be integrated into optical systems for EUV lithography or measuring devices without excessive design effort.

[0014] In a particularly preferred embodiment, a further layer is arranged between the filter layer and the substrate. Providing this additional layer allows for the control of any potential unwanted coupling of DUV radiation coupled into the filter layer. In particular, the influence of the substrate material is reduced, and the additional layer allows for a wider selection of substrate materials without the coupled-in DUV radiation being coupled out again to such an extent that it would contribute to interference radiation.

[0015] Advantageously, the additional layer has the same or a larger real part of the refractive index as the reflection layer, in order to propagate the coupled DUV radiation as far as possible within the filter layer.

[0016] It is particularly advantageous if the reflective layer and the subsequent layer are made of identical material. This not only increases the propagation range of the DUV radiation coupled into the filter layer, but also simplifies the manufacturing process of the reflective optical element.

[0017] Preferably, the additional layer has a thickness between approximately 25 nm and approximately 60 nm in order to limit the influence of the substrate on the total reflection of the grazing incident radiation and the coupling in and out of the DUV radiation as much as possible.

[0018] In a preferred embodiment, the reflective layer and / or the filter layer and / or optionally the further layer are formed as multiple layers. By making at least one of the layers as multiple layers, it is possible to selectively influence, in particular, the actual refractive index, namely both the real part and the imaginary part at a specific wavelength, of the respective layer in order to further improve the ratio of EUV to DUV radiation in the radiation reflected by the reflective optical element.

[0019] Furthermore, the problem is solved by an optical system with at least two reflective optical elements as described. The major advantage of such an optical system is that, by appropriately designing the respective reflective optical elements proposed here, the suppression of DUV radiation in the reflected radiation can be made wavelength-dependent, thus enabling efficient suppression of DUV radiation over a relatively wide wavelength range for the entire optical system. Such optical systems are well suited, among other things, as components of EUV lithography devices as well as wafer and / or mask inspection systems.

[0020] Preferably, the at least two reflective optical elements have different filter layers that differ in material, thickness and / or distance to the substrate and / or interface of the reflective layer to the vacuum, in order to suppress the DUV radiation in the optical system in a particularly broadband and / or particularly efficient manner.

[0021] Advantageously, the optical system comprises an even number of the grazing incidence reflective optical elements proposed herein, wherein half of these reflective optical elements have a first filter layer and the other half have a second filter layer. Alternatively, the optical system advantageously comprises at least three of the grazing incidence reflective optical elements proposed herein, at least one of which has a different filter layer than the other reflective optical elements. By also providing identically designed grazing incidence reflective optical elements, the manufacturing effort can be reduced while still achieving broadband suppression of DUV radiation in the optical system.

[0022] The present invention will be explained in more detail with reference to preferred embodiments. To this end, we will show... Fig. 1 schematically the structure of a first embodiment of the reflective optical element for grazing incidence of radiation with a working wavelength from the EUV wavelength range; Fig. 2 schematically the structure of a second embodiment of the reflective optical element for grazing incidence of radiation with a working wavelength from the EUV wavelength range; Fig. 3 the reflectivity at an EUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for a first reflective optical element for grazing incidence; Fig. 4 the reflectivity at a DUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for the first reflective optical element for grazing incidence; Fig. 5 the reflectivity at a DUV wavelength as a function of the angle of incidence and the thickness of the filter layer for the first reflective optical element at a thickness of the reflection layer; Fig. 6 the reflectivity at an EUV wavelength as a function of the angle of incidence and the thickness of the filter layer for the first reflective optical element at a thickness of the reflection layer; Fig. 7 the reflectivity at an EUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for a second reflective optical element for grazing incidence; Fig. 8 the reflectivity at a DUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for the second reflective optical element for grazing incidence; Fig. 9 the reflectivity at an EUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for a third reflective optical element for grazing incidence; Fig. 10 the reflectivity at a DUV wavelength as a function of the thickness of the reflection layer and the thickness of the filter layer for the third reflective optical element for grazing incidence; Fig. 11 schematically the structure of a first embodiment of an optical system with at least one reflective optical element for grazing incidence of radiation with a working wavelength from the EUV wavelength range; Fig. 12 schematically the structure of a second embodiment of an optical system with at least one reflective optical element for grazing incidence of radiation with a working wavelength from the EUV wavelength range; Fig. 13. The reflectivity at an angle of incidence as a function of wavelength in the DUV wavelength range for different thicknesses of the reflection layer and the filter layer; and Fig. 14 the reflectivity as a function of wavelength in the DUV wavelength range for various optical systems with at least one reflective optical element for grazing incidence of radiation with a working wavelength from the EUV wavelength range.

[0023] In Fig. Figure 1 schematically depicts the structure of a first embodiment of a reflective optical element 101 for grazing incidence of radiation with a working wavelength in the EUV wavelength range. The reflective optical element 101 has a reflective layer 107 on a substrate 103, with a filter layer 105 arranged between the reflective layer 107 and the substrate 103. The reflective layer 107 and the filter layer 105 differ in that the filter layer 105 has a larger real part of the refractive index in the DUV wavelength range than the reflective layer 107.

[0024] When used with conventional EUV radiation sources, such as plasma radiation sources, the reflective optical element is exposed not only to radiation in the EUV wavelength range but also to longer-wavelength interference radiation, particularly from the DUV wavelength range. Transition metals, among other materials, have proven effective for reflective coatings in grazing-incidence reflective optical elements at wavelengths in the EUV range. The reflective coating can be single-layered or multi-layered. Multi-layered reflective coatings can be optimized compared to single-layered reflective coatings with regard to, for example, lifetime. Multi-layered reflective coatings can have two, three, four, five, or more layers, in which all layers can be made of different materials, or two or more layers can be made of the same material.Especially for operating wavelengths in the EUV range, ruthenium, molybdenum, niobium, and palladium have proven to be effective materials for the reflective layer, in order to achieve total internal reflection over the widest possible angle of incidence. For high reflectivity, the reflective layer exhibits a real refractive index of less than 0.95 and an imaginary refractive index of less than 0.05 at the operating wavelength.

[0025] Since total internal reflection of EUV radiation typically also reflects DUV radiation, which can distort measurements or lead to exposure errors in lithography, the reflective optical elements proposed here incorporate a filter layer that has a higher real refractive index in the DUV wavelength range than the reflective layer. Particularly good results were obtained when the filter layer had a real refractive index of at least 0.65 at wavelengths in the DUV range. Especially in combination with reflective layers containing ruthenium, molybdenum, niobium, and palladium, good results were achieved with filter layers containing one or more materials from the group consisting of carbon, calcium fluoride, and magnesium fluoride.

[0026] By exploiting the fact that the penetration depth of DUV radiation is greater than that of EUV radiation, the DUV radiation incident on the reflective optical element can be coupled into the filter layer thanks to its higher refractive index than that of the reflective layer. Within the filter layer, the DUV radiation can propagate laterally and is no longer reflected along with the EUV radiation. This can be achieved, in particular, without negatively affecting the reflectivity of the EUV radiation, by positioning the filter layer at a distance from the surface relative to the surrounding environment. Due to the different penetration depths in matter, specifically in the reflective layer, this distance results in a greater proportion of incident DUV radiation than incident EUV radiation. It has proven advantageous for the reflective layer to have a thickness between approximately 4 nm and 8 nm, and for the filter layer to have a thickness between approximately 4 nm and 34 nm.

[0027] By extracting DUV radiation, the ratio of EUV radiation to DUV radiation in the radiation reflected by the reflective optical element can be improved. In preferred embodiments of the reflective optical element, the ratio of reflected intensity in the DUV wavelength range to reflected intensity at the operating wavelength is at a maximum of 0.8 for an incidence angle in the range of 65° to 85°, preferably 75° to 85°.

[0028] In a further embodiment, a reflective optical element 201 proposed here, as in Fig. Figure 2 schematically depicts the structure, which, in addition to a reflective layer 207 and a filter layer 205, includes a further layer 209 arranged between the filter layer 205 and the substrate 203. The provision of this additional layer allows for the control of any potential unwanted coupling out of the DUV radiation coupled into the filter layer. In particular, the influence of the substrate material is reduced, and the additional layer permits a wider selection of substrate materials without causing a significant amount of coupled-in DUV radiation to be re-coupled and thus contribute to interference radiation. It has proven advantageous for the additional layer to have the same or a larger real part of the refractive index as the reflective layer. From a manufacturing perspective, it is particularly advantageous if the reflective layer and the additional layer are made of identical material.Furthermore, the adhesion to the substrate 203 and the roughness of the reflective optical element 201 can be influenced by means of the additional layer 209. Advantageously, the additional layer has a thickness of between approximately 25 nm and approximately 60 nm.

[0029] It should be noted that not only the reflective layer, but also the filter layer and / or any subsequent layers may be multilayered. In particular, this allows for influencing the actual complex refractive index in order to further improve the coupling of interfering DUV radiation into the filter layer and thus remove it from the radiation reflected at the reflective optical element during grazing incidence.

[0030] In Fig. Figure 3 shows the reflectivity at an EUV wavelength of 13.5 nm as a function of the thickness of the reflective layer and the thickness of the filter layer for a first reflective optical element. This reflective optical element consists of a reflective layer of ruthenium of variable thickness, a filter layer of diamond-like carbon of variable thickness, and a further layer of 30 nm ruthenium on a quartz glass substrate. The thickness dependence was investigated at a wavelength of 13.5 nm and an incidence angle of 75° to the surface normal. Fig. 4. Analogously, the reflectivity at a wavelength of 120 nm was investigated. The reflectivity is plotted in contour lines, where in Fig. 3. The lines of equal reflectivity are spaced approximately 0.0080 to 0.0085 apart, with a value of 0.800 for the line of highest reflectivity and a value of 0.675 for the line of lowest reflectivity. Fig. 4. The lines of equal reflectivity have a distance of approximately 0.0085, with a value of 0.599 for the line of highest reflectivity and a value of 0.333 for the line of lowest reflectivity.

[0031] To suppress DUV radiation to a greater extent, it is generally advantageous for the reflective layer to be as thin as possible and the filter layer as thick as possible. Furthermore, the filter layer should not only have a refractive index greater than that of the reflective layer in the DUV wavelength range, but also a high imaginary refractive index to effectively absorb DUV radiation. Conversely, to avoid significantly impairing the reflectivity of EUV radiation, the reflective layer should ideally have a minimum thickness on the order of the operating wavelength, and the filter layer should be as thin as possible. Additionally, the reflective layer should have the smallest possible real and imaginary refractive index components in the EUV wavelength range.

[0032] Despite seemingly conflicting requirements, particularly regarding the thickness of the reflective layer and the filter layer, solutions can be found that achieve both sufficient EUV reflectivity and DUV suppression, as exemplified by the following: Fig. 3 and Fig. 4. This can be observed in the overall view. For example, in the thickness range between approximately 4 nm and approximately 10 nm of the amorphous carbon filter layer, the reflectivity in the DUV range decreases significantly, without the EUV reflectivity being significantly affected.

[0033] Comparing a conventional reflective optical element for grazing incidence of EUV radiation with a 30 nm monolayer as the reflective layer on a substrate with a reflective optical element proposed here, featuring a 5 nm thick ruthenium reflective layer on a 6 nm thick amorphous carbon filter layer, which in turn is coated with another 30 nm thick ruthenium layer, we obtain, at an incidence angle of 75° in each case, a ratio of DUV radiation at 160 nm to EUV radiation at 13.5 nm of 0.868 for the conventional reflective optical element and a value of 0.773 for the reflective optical element with the filter layer. The DUV radiation is thus suppressed by approximately 10% more effectively. If diamond-structured carbon is used instead for the filter layer, the value is even lower, at only 0.439.

[0034] This relationship also applies when the angle of incidence changes. In the Fig. 5 and Fig. Figure 6 shows the reflectivity at a DUV wavelength and an EUV wavelength as a function of the angle of incidence and the thickness of the filter layer for a comparable reflective optical element at a given thickness of the reflection layer. The DUV wavelength is 160 nm ( Fig. 5) and the EUV wavelength is 13.5 nm ( Fig. 6) The thickness of the ruthenium reflective layer is 5 nm, the thickness of the diamond-like carbon filter layer is 6 nm, and the thickness of the next layer, also made of ruthenium, is 30 nm. The reflectivity is plotted as a solid line. For comparison, the reflectivity of a reflective optical element without a filter layer, i.e., with a 35 nm thick ruthenium layer, is plotted as a dashed line. Particularly at higher angles of incidence, approaching 70° and above, the influence of DUV radiation suppression is significantly greater than the reflection loss in the EUV wavelength range.

[0035] In the Fig. 7 and Fig. 8 are the reflectivity at an EUV wavelength of 13.5 nm ( Fig. 7) or a DUV wavelength of 120 nm ( Fig. 8) at an angle of incidence of 75° and the thickness of the filter layer for a second reflective optical element with a variable thickness of the reflection layer and a variable thickness of the filter layer made of calcium fluoride, wherein this reflective optical element has a further layer of 30 nm thickness. The reflection layer and the further layer are made of ruthenium as in the first example. The reflectivity is plotted in contour lines, where in Fig. 7. The lines of equal reflectivity are spaced approximately 0.075 apart, with a value of 0.788 for the line of highest reflectivity and a value of 0.413 for the line of lowest reflectivity. In Fig. The lines of equal reflectivity are spaced approximately 0.02 apart, with a value of 0.56 for the line of highest reflectivity and a value of 0.22 for the line of lowest reflectivity. For a reflective optical element with a reflective layer of 6 nm ruthenium, a filter layer of 30 nm calcium fluoride, and another layer of 30 nm ruthenium, the ratio of DUV radiation at 160 nm to EUV radiation at 13.5 nm is 0.183, both at an angle of incidence of 75°.

[0036] Another example is in the Fig. 9 and Fig. 10 the reflectivity at an EUV wavelength of 13.5 nm ( Fig. 9) or a DUV wavelength of 160 nm ( Fig. 10) at an angle of incidence of 75° as a function of the thickness of the reflective layer and the thickness of the filter layer for a third reflective optical element, which has a filter layer of magnesium oxide and another layer of molybdenum with a thickness of 50 nm. The reflective layer is again made of ruthenium. The reflectivity is plotted in contour lines, where in Fig. 9. The lines of equal reflectivity have a distance of approximately 0.054, with a value of 0.792 for the line of highest reflectivity and a value of 0.328 for the line of lowest reflectivity. In Fig. The lines of equal reflectivity are spaced approximately 0.026 apart, with a value of 0.695 for the line of highest reflectivity and a value of 0.326 for the line of lowest reflectivity. For a reflective optical element with a 7 nm ruthenium reflective layer, a 14 nm magnesium oxide filter layer, and a 50 nm molybdenum filter layer, the ratio of DUV radiation at 160 nm to EUV radiation at 13.5 nm is 0.657, both at an angle of incidence of 75°.

[0037] In Fig. Figure 11 schematically depicts an optical system 1101 which, in the present example, has four reflective optical elements 1103, 1105, 1107, 1109 for grazing incidence of a working wavelength in the EUV wavelength range. Fig. Figure 12 schematically depicts an optical system 1201, which in this example comprises five reflective optical elements 1203, 1205, 1207, 1209, 1211 for grazing incidence of a working wavelength in the EUV wavelength range. The radiation incident on each optical system 1101, 1201 is symbolized by a wavy arrow. Optical systems with two, three, six, seven, or more reflective optical elements for grazing incidence of a working wavelength in the EUV wavelength range are also possible. Furthermore, any number of additional optical elements, not shown here, can be provided, in particular reflective optical elements for smaller angles of incidence, e.g., for quasi-normal incidence. At each reflective optical element for grazing incidence, the proportion of interfering radiation in the DUV wavelength range is further reduced. Such optical systems are suitable, among other things, for...good as components of lithography devices as well as wafer and / or mask inspection systems.

[0038] In both embodiments shown here according to Fig. 11 or Fig. 12. At least two reflective optical elements have different filter layers. This has the advantage that the suppression of interference radiation in the DUV wavelength range can be adjusted depending on the wavelength or angle of incidence. Thus, with appropriate tuning of the reflective optical elements, DUV radiation can be suppressed more efficiently over a wider wavelength range or for different angles of incidence. The filter layers can differ, in particular, in their material and thickness. By varying the thickness of the reflective layer, the distance of the filter layers from the surface to the surroundings can also be adjusted. Additionally, the suppression of interference radiation can be influenced by the choice of material for the second layer and its thickness.

[0039] In the Fig. In the example shown in Figure 11, the optical system 1101 has an even number of reflective optical elements 1103, 1105, 1107, 1109 for grazing incidence, wherein half of these reflective optical elements have a first filter layer and another half of these reflective optical elements have a second filter layer.

[0040] In the Fig. In the example shown in Figure 12, the optical system 1201 has at least three reflective optical elements for grazing incidence, specifically five elements 1203, 1205, 1207, 1209, 1211, at least one of which has a different filter layer than the other reflective optical elements in order to suppress the DUV radiation penetrating the optical system particularly strongly over different wavelength or angle of incidence ranges.

[0041] In Fig. Figure 13 shows the reflectivity as a function of wavelength for three proposed reflective optical elements and one conventional reflective optical element at an angle of incidence of 75°. The conventional reflective optical element has a ruthenium monolayer 35 nm thick (solid line). The proposed reflective optical elements have a ruthenium reflective layer 5 nm thick and another ruthenium layer 30 nm thick. They also have a filter layer of 1.4 nm carbon (dotted line), 3.6 nm carbon (dashed line), or 5.6 nm carbon (dash-dotted line). These are all diamond-like carbons. Up to approximately 105 nm, the DUV radiation is reflected less the thinner the filter layer. From approximately 140 nm onward, the DUV radiation is reflected less the thicker the filter layer.

[0042] The resulting reflectivity of an optical system with four reflective optical elements at an angle of incidence of 75° is given by Fig. Figure 14 shows an optical system with four conventional reflective optical elements as in Figure 14. For reference, a solid line shows an optical system with four conventional reflective optical elements as in Figure 14. Fig. Figure 13 is used to illustrate this. Furthermore, the resulting reflectivity for two reflective optical elements with a 1.4 nm carbon filter layer and two with a 5.6 nm carbon filter layer is shown as a dashed line, and the resulting reflectivity for four reflective optical elements with a 3.6 nm carbon filter layer is shown as a dotted line, corresponding to the reflective optical elements as for Fig. 13. From a wavelength of 110 nm, the reflectivity decreases significantly compared to the optical system without any filter layers, even though the reflectivity increases from about 175 nm.

[0043] Comparable results can also be obtained with reflective layers made of molybdenum, niobium, and palladium. Due to their improved transmission and reflection properties, as well as their lifetime, the optical elements described here are particularly suitable for use in optical systems for lithography devices or mask and wafer inspection systems. Reference sign QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] US 7,773,196 B2

[0003]

Claims

[1] Reflective optical element for grazing incidence of radiation with a working wavelength from the EUV wavelength range with a reflective layer on a substrate, characterized by , that a filter layer is arranged between the reflective layer and the substrate, wherein the filter layer has a larger real part of the refractive index in the DUV wavelength range than the reflective layer. [2] Reflective optical element according to claim 1, characterized by that the filter layer has a real part of the refractive index of at least 0.65 at a wavelength in the DUV wavelength range. [3] Reflective optical element according to claim 1 or 2, characterized by that the filter layer contains one or more of the materials of the group formed from carbon, calcium fluoride and magnesium fluoride. [4] Reflective optical element according to any one of claims 1 to 3, characterized by, that the reflective layer has a real part of the refractive index less than 0.95 and an imaginary part of the refractive index less than 0.05 at the operating wavelength. [5] Reflective optical element according to any one of claims 1 to 4, characterized by , that the reflective layer has a thickness between approximately 4 nm and approximately 8 nm and the filter layer has a thickness between approximately 4 nm and approximately 34 nm. [6] Reflective optical element according to any one of claims 1 to 5, characterized by , that the ratio of reflected intensity in the DUV wavelength range to reflected intensity at the working wavelength is at most 0.8 at an angle of incidence in the range of 65° to 85°, preferably 75° to 85°. [7] Reflective optical element according to any one of claims 1 to 6, characterized by that another layer is arranged between the filter layer and the substrate. [8] Reflective optical element according to claim 7, characterized bythat the further layer has the same or a larger real part of the refractive index as the reflection layer. [9] Reflective optical element according to claim 7 or 8, characterized by that the reflective layer and the subsequent layer are made of identical material. [10] Reflective optical element according to any one of claims 7 to 9, characterized by , that the next layer has a thickness between approximately 25 nm and approximately 60 nm. [11] Reflective optical element according to any one of claims 1 to 10, characterized by that the reflective layer and / or the filter layer and / or possibly the further layer are formed in multiple layers. [12] Optical system comprising at least two reflective optical elements according to any one of claims 1 to 11. [13] Optical system according to claim 12, characterized by that the at least two reflective optical elements have different filter layers. [14] Optical system according to claim 12 or claim 13, characterized by , that it has an even number of reflective optical elements according to any one of claims 1 to 10, wherein half of these reflective optical elements have a first filter layer and another half of these reflective optical elements have a second filter layer. [15] Optical system according to claim 12 or claim 13, characterized by that it has at least three reflective optical elements according to one of claims 1 to 10, at least one of which has a different filter layer than the other reflective optical elements.

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