Optical element with at least one smoothing layer and optical arrangement

Abstract

The invention relates to an optical element (1), in particular a mirror, comprising: a substrate (2), and an optical coating (3), preferably for reflecting radiation (4), in particular for reflecting radiation (4) in the UV or VIS wavelength range, which has a plurality of successive layers (6, 7) extending from the direction of the substrate (2) towards a free surface (5) of the optical coating (3), the layer roughness (12) of which increases with increasing distance from the substrate (2). Within the coating (3), in addition to the plurality of layers (6, 7), at least one smoothing layer (9) is arranged, which has a lower layer roughness (12) than one of the layers (6, 7). The invention also relates to an optical arrangement comprising: at least one optical element (1) configured as described above.

Description

Background of the invention

[0001] The invention relates to an optical element, in particular a mirror, comprising: a substrate, and an optical coating, preferably for reflecting radiation, in particular for reflecting radiation in the UV or VIS wavelength range, which has a plurality of successive layers extending from the direction of the substrate towards a free surface of the optical coating, the roughness of which increases with increasing distance from the substrate. The invention also relates to an optical arrangement, preferably a laser or a semiconductor technology system, in particular a UV lithography system or a wafer inspection system, comprising at least one such optical element.

[0002] Optical coatings on optical elements can be used for various purposes. For example, an optical coating can serve as an anti-reflective coating, preventing reflections of incident radiation. The optical coating can also function as an optical filter, such as an edge filter, as used in spectacle lenses, or as a partially reflective coating. The following assumes that the optical coating serves to reflect incident radiation, particularly in the UV or visible wavelength range, but this is not mandatory.

[0003] Optical elements for reflecting radiation in the UV wavelength range, particularly in the form of mirrors, are used, for example, in semiconductor technology. The UV wavelength range encompasses wavelengths between 100 nm and 380 nm and includes, among others, the VUV wavelength range with wavelengths between 100 nm and 200 nm (VUV wavelength range according to DIN 5031 Part 7). The VIS wavelength range encompasses wavelengths between 380 nm and 780 nm. The coating of the optical element described here can be specifically designed for reflecting radiation in the VUV wavelength range.

[0004] Typically, the coating is designed as a highly reflective coating for reflecting radiation at a useful wavelength in the UV, VUV, or VIS wavelength range. In this case, the optical element is typically a mirror exhibiting a reflectivity of more than 80%, usually more than 90%, for radiation at a useful wavelength in the UV, particularly the VUV, or VIS wavelength range.

[0005] The layers of such coatings can be applied or deposited onto the substrate in various ways. If the layers are deposited using coating techniques that employ low-energy, thermal material particles, such as thermal evaporation or electron beam evaporation, the deposited layers typically exhibit increased surface roughness compared to other coatings or the substrate surface, leading to a significant loss of reflectivity. This problem is particularly pronounced with thick single layers and especially with coatings containing numerous thick single layers or layer pairs. Possible consequences include a reduction in the maximum achievable reflectivity due to increased scattered light and a reduction in lifetime due to porous layer growth.In a very simple growth model, the layer roughness increases linearly with the number of periods of layer pairs, or with the number of layer pairs located between the substrate and the surface of the respective layer. The layer roughness therefore depends not only on the layer material and the deposition method, but also on the position of each layer within the coating. The reflectivity loss of the coating due to scattered light, on the other hand, increases quadratically with the layer roughness.

[0006] To avoid problems with layer roughness, other coating techniques can, in principle, be used that employ higher-energy material particles, e.g., particles with energies above approximately 5 eV, such as magnetron sputtering or ion beam sputtering. However, fluoride materials, i.e., materials containing fluorine, cannot currently be deposited by sputtering without simultaneously accepting significant losses in the quality of the deposited layers. For the material MgF2, there are some approaches to this, see for example the article “Effect of sputtering power on MgF2 thin films deposited by sputtering technique under fluorine trapping”, R. De et al., AlP Conference Proceedings 1731, 080078 (2016) or the article “Far ultraviolet optical properties of MgF2 films deposited by ion-beam sputtering and their application as protective coatings for Al”, J. Larruquert et al., Optics Communications, Vol.215 Issues 1-3, pp. 93-99 (2003), but the coating technology described there is not yet fully developed.

[0007] Epitaxial growth can also be used to deposit layers, but this typically occurs at temperatures just below the decomposition temperature of the material being grown. This melting or sublimation temperature is above approximately 1000°C for most materials suitable for coating applications, exceeding the limits of current coating systems. Another challenge when using such high temperatures is the dimensional stability of the substrate, particularly the surface to which the coating is applied. With typical substrate materials like CaF2, significant deformation of the substrate's shape or surface can occur at such high temperatures. In the worst-case scenario, the substrate can become unusable for its intended application. Object of the invention

[0008] The object of the invention is to reduce the roughness of a coating of an optical element and to provide an optical arrangement with such an optical element. Subject matter of the invention

[0009] This problem is solved by an optical element of the type mentioned above, in which, in addition to the plurality of layers, at least one smoothing layer is arranged within the coating. This smoothing layer has a lower surface roughness than any of the layers. The term "within the coating" means that the smoothing layer is not located on the free surface, i.e., at the interface between the coating and the environment, or at the interface between the coating and the substrate. Rather, the smoothing layer is situated between two of the plurality of layers that have a higher surface roughness. The smoothing layer is made of a different material than the plurality of layers, whose surface roughness increases with increasing distance from the substrate. The inclusion of the smoothing layer made of a different material generally does not result in any (significant) loss of reflectivity due to the design.The smoothing layer thus helps to reduce the loss of reflectivity caused by roughening, without itself creating a loss of reflectivity that would then have to be recovered.

[0010] In one embodiment, the smoothing layer is applied by sputtering. Sputtering, particularly ion beam sputtering or magnetron sputtering, allows the deposition of layers with a comparatively low roughness. It is generally not possible or practical to post-process the smoothing layer by polishing to reduce the roughness of the smoothing layer and subsequent layers. It is understood that other coating processes that use higher-energy material particles for deposition and therefore enable non-roughening deposition can also be used to deposit the smoothing layer.

[0011] In contrast, the majority of layers whose roughness increases with increasing distance from the substrate, and which increase the overall roughness of the coating, are typically deposited using a coating process that employs low-energy material particles. This coating process could, for example, be thermal evaporation or electron beam evaporation. The majority of layers whose roughness increases with increasing distance from the substrate can, for instance, be composed of fluoride materials, since fluoride materials generally cannot be deposited by sputtering or similar processes without significant loss of quality.

[0012] In another embodiment, the majority of layers comprise first and second layers, wherein the first layers consist of a first material and the second layers consist of a second material whose refractive index for radiation is lower than that of the first material. In particular, highly reflective coatings for optical elements used to reflect radiation typically have repeatedly formed double layers or layer pairs consisting of a material with a very high refractive index and a partner material with a low refractive index, where the refractive index refers to the real part of the refractive index. Such a coating with layer pairs of materials with higher and lower refractive indices enables the reflection of radiation based on interference effects.The difference in refractive index and the number of periods of the layer pairs determine the final reflectivity of the coating, with maximum reflectivity reaching saturation after a certain number of layer pair periods. Antireflective coatings also typically consist of repeatedly layered pairs made of two different materials.

[0013] In this advanced training, the first and / or second material is / are a fluoride material, preferably selected from the group comprising: AlF3, MgF2, LiF, CaF2, YF3, YbF3, GdF3, LaF3, CeF3, SmF3, NdF3. The fluoride materials listed here, and possibly others, exhibit a comparatively low absorption coefficient in the VUV wavelength range, particularly at a wavelength of approximately 193 nm, and can therefore be used as materials for a reflective coating in the VUV wavelength range.

[0014] Typically, the first and second fluoride materials are chosen to have the greatest possible difference in refractive index. For example, the first material with the high refractive index could be LaF3 and the second material with the lower refractive index could be MgF2, but of course other material combinations are also possible.

[0015] In another embodiment, the material of the at least one smoothing layer is an oxide material, preferably selected from the group comprising: SiO2, Al2O3, TiO2, ZrO, HfO2, and Ta2O5. SiO2, in particular, has proven advantageous as a material for the smoothing layer because it exhibits a very low absorption coefficient in the VUV wavelength range, only slightly higher than that of most fluoride materials typically used to produce the coating. Although Al2O3 has a higher absorption coefficient, it can also be used as the oxide material for the smoothing layer. The other oxide materials, namely TiO2, ZrO, HfO2, and Ta2O5, can also be used as materials for the smoothing layer.

[0016] The inventors recognized that for the reflection of radiation in the UV wavelength range, particularly in the VUV wavelength range, not only fluoride materials are suitable, but also certain oxide materials that exhibit a suitable refractive index and a comparatively low absorption coefficient. Accordingly, in reflective coatings that necessarily contain fluoride materials to ensure their optical functionality, the additional oxide layer(s) can be non-roughening, or they can reduce the existing roughness of the coating, so that the formation of the layer roughness starts again from the smoothing layer with a lower surface roughness.

[0017] Coatings that rely solely on fluoride materials for optical functionality lack oxide layers that can be deposited with low roughness. However, provided there are no adverse reasons affecting the optical element's lifetime, individual oxide layers can be introduced into such coatings as smoothing layers. In this case, the oxide smoothing layer(s) should have minimal, ideally no, impact on the coating's optical functionality. Any theoretical disadvantages resulting from the additional oxide smoothing layer, such as a slight reduction in reflectivity, can be more than compensated for or even overcome by the improved coating roughness.

[0018] In an alternative embodiment, the material of the at least one smoothing layer is a nitride material, preferably selected from the group comprising: Si3N4, GaN, and AlN. Nitride materials can also be deposited with low layer roughness, for example, by sputtering. However, due to their comparatively high absorption, smoothing layers made of nitride materials are generally not suitable for use in reflective coatings for the UV or VUV wavelength range. Smoothing layers made of nitride materials can, however, be used in reflective coatings, e.g., for the VIS wavelength range, which have a plurality of layers whose layer roughness increases with increasing distance from the substrate. These layers can be fluoride or, if applicable, oxide materials, provided the latter are deposited not by sputtering but, for example, by thermal evaporation.

[0019] In another embodiment, the smoothing layer has a physical thickness of more than 2 nm, preferably more than 10 nm and / or not more than 100 nm. Sufficient smoothing of the coating by the smoothing layer typically requires a minimum thickness of the smoothing layer, which depends on the type of material used and the roughness of the layer to be smoothed. The same applies to the maximum thickness of the smoothing layer. If a large number of smoothing layers are used, the individual smoothing layers typically have a comparatively small thickness.

[0020] In another embodiment, the coating comprises a sequence of layer groups or periods, preferably each having the same optical thickness. The optical thickness of the layer groups is typically an integer multiple of half the useful wavelength of the radiation that is to be reflected by the coating or for which the coating exhibits maximum reflectivity. A coating with a sequence of layer groups of the same optical thickness is typically a reflective coating. It is understood that the coating need not necessarily have a periodic sequence of layers. This is generally not the case, for example, if the coating is designed as a partially reflective coating.

[0021] When layer groups have identical optical thicknesses, they are typically half the useful wavelength λ, meaning the thickness of the layer groups is usually λ / 2. Layer groups with identical optical thicknesses are generally adjacent to each other, resulting in a periodic sequence. This is advantageous for achieving the highest possible reflectivity of the coating. The term "identical optical thickness" means that, ideally, the layer groups have exactly the same optical thickness. It is understood that minor variations in the optical thicknesses of the layer groups may occur unintentionally due to the manufacturing process.

[0022] In a further development of this embodiment, the sequence includes layer groups that do not have a smoothing layer, wherein the layer groups that do not have a smoothing layer are preferably designed as layer pairs, each consisting of one of the first layers and one of the second layers, wherein the first layer and the second layer of a respective layer pair particularly have the same optical thickness.

[0023] The high reflectivity of the coating for the operating wavelength, typically exceeding 90%, 95%, or 97%, can be achieved through the periodic arrangement or sequence of layer pairs or bilayers. This usually requires a number of layer pairs on the order of approximately 10, 20, or more. As described above, the reflectivity of the optical element cannot be further increased beyond a certain number of layer pairs, which is typically on the order of 40–50. The two layers of each layer pair generally have the same optical thickness, typically λ / 4, but this is not strictly necessary.The optical thicknesses of the two layers can differ from each other – especially slightly – without affecting the maximum reflectivity.

[0024] In another further development, the sequence of layer groups contains at least one layer group with a smoothing layer. This at least one layer group with the smoothing layer is arranged within the sequence of layer pairs and enables smoothing that reduces the layer roughness for all layer groups following or deposited on top of the layer group with the smoothing layer, for example, in the form of the layer pairs described above, as well as for the interfaces between these layers. Due to the identical thickness of the layer group containing the smoothing layer and the other layer groups that do not contain a smoothing layer, the periodicity of the sequence of layer groups is maintained.

[0025] As described above, the smoothing layer should contribute little or nothing to the optical functionality of the coating. Therefore, a characteristic of the coating described here is that, apart from the increased roughness, the coating exhibits essentially the same optical performance when the smoothing layer is omitted—more precisely, when the layer group containing the smoothing layer is omitted.

[0026] In a further development of this embodiment, at least a number of two layer groups, preferably at least a number of five layer groups, particularly preferably at least a number of ten layer groups, and in particular at least a number of fifteen layer groups, are arranged between the substrate and the at least one layer group containing the smoothing layer and / or between a free surface of the coating and the at least one layer group, and these layer groups do not contain a smoothing layer.

[0027] As described above, assuming a simple growth model for layer growth, the roughness increases linearly with the number of layer groups or layer pairs, and the reflectivity loss increases quadratically with the number of layer pairs or layer groups if no smoothing layer is present. Depending on the extent of the increase in roughness with the number of layer pairs, it is therefore advantageous to apply a layer group with a smoothing layer after a certain number of, for example, 5, 10, or possibly 15 layer groups, particularly in the form of layer pairs, applied to the substrate. Similarly, it is advantageous to have, for example, 5, 10, 15, or more layer groups or layer pairs without a smoothing layer between the layer group with the smoothing layer and the free surface of the coating exposed to the environment.

[0028] In a further development of this embodiment, the sequence of layer groups comprises exactly one layer group with a smoothing layer, wherein the number of layer groups that do not contain a smoothing layer and are arranged between the layer group with the smoothing layer and the substrate differs by fewer than four layer groups, preferably by fewer than two layer groups, from the number of layer groups that do not contain a smoothing layer and are arranged between the layer group with the smoothing layer and the free surface. In particular, the number of layer groups that do not contain a smoothing layer and are arranged between the layer group with the smoothing layer and the substrate can correspond to the number of layer groups that do not contain a smoothing layer and are arranged between the layer group with the smoothing layer and the free surface.

[0029] If the coating has only a single layer group with a smoothing layer, it is generally advantageous for this layer group to be positioned approximately in the middle of the sequence of layer groups. Assuming that the smoothing layer restores the original roughness, the sub-stacks of the sequence located between the substrate and the layer group with the smoothing layer, as well as between the layer group with the smoothing layer and the free surface or the interface with the environment, will each exhibit at most half the roughness they would otherwise have. The reflectivity loss due to roughness is therefore reduced to approximately one-quarter of the reflectivity loss that would occur without the smoothing layer.

[0030] In a further development, the sequence of layer groups comprises at least two layer groups with a smoothing layer, wherein at least a number of two layer groups, preferably at least a number of five layer groups, particularly preferably at least a number of ten layer groups, and especially at least a number of fifteen layer groups, are arranged between two adjacent layer groups in the sequence containing the smoothing layer, and these layer groups do not contain a smoothing layer. The use of two or more additional layer groups with a smoothing layer is advantageous, for example, if the increase in roughness occurs too rapidly, such that a single smoothing layer, which is typically arranged substantially in the middle of the sequence of layer groups (as shown), does not provide sufficient roughness reduction, or if the roughness reduction is not possible in one step, i.e., with the aid of a single smoothing layer.

[0031] In one embodiment, the layer group with the smoothing layer forms a further layer pair that, in addition to the smoothing layer, comprises either a first layer or a second layer, wherein the smoothing layer and the first layer or the second layer preferably have the same optical thickness. In this case, typically in addition to the layer pairs that generate the reflectivity of the coating, at least one further layer pair containing the smoothing layer is added. In the further layer pair, either the first layer or the second layer is replaced by the smoothing layer, while the other layer of the layer pair is retained. The thickness of the smoothing layer typically corresponds to the thickness of the replaced first layer or the replaced second layer. Typically, the smoothing layer has the same thickness as the layer that is replaced by the smoothing layer.If one of the two layers is replaced by an oxide material with a small absorption coefficient, the maximum achievable reflectivity remains practically unchanged, and depending on the oxide material used, only the spectral width of the coating is slightly altered.

[0032] In an alternative development, each layer group, including the smoothing layer, consists solely of the smoothing layer itself; that is, the layer group is formed as a single layer. It is also possible for one or more layer groups, or one or more periods within the sequence of layer groups, to be formed as a smoothing layer. It has been shown that even in this case, with an oxide material for the smoothing layer exhibiting a low absorption coefficient, such as SiO2, the maximum reflectivity of the coating remains practically unchanged, with only the spectral width of the reflectivity profile decreasing slightly.

[0033] If an oxide material with a higher absorption coefficient is used for the smoothing layer, it is advantageous for this material, or the layer group containing the smoothing layer, to be located not too far from the substrate in the sequence. This is because the majority of the reflection of the incident radiation occurs in the layer pairs adjacent to the free surface of the coating, which forms the interface with the environment. Consequently, the electric field of the standing wave formed during the reflection of the radiation in the coating is stronger there than in layer pairs near the substrate. Therefore, it is advantageous if, in this case, the layer group containing the smoothing layer is located in the middle of the sequence of layer groups or between the middle of the sequence of layer groups and the substrate.

[0034] In a further embodiment, the at least one layer group with the smoothing layer comprises the smoothing layer as well as one of the first layers and one of the second layers, wherein the smoothing layer is arranged between the first layer and the second layer, wherein the first layer is located closer to the substrate than the second layer, and wherein the smoothing layer preferably has an optical thickness that is less than 25%, in particular less than 15%, of the optical thickness of the layer group with the smoothing layer.

[0035] In particular, if the oxide material of the smoothing layer has a comparatively large absorption coefficient, it is advantageous if the smoothing layer(s) in the coating is / are arranged at positions where the higher absorption has as little effect as possible, which is the case at positions where the electric field of the reflected radiation has a minimum.

[0036] The inventors have recognized that, in the coating described here, the minima of the electric field occur at the interfaces between the first layer with the higher refractive index and the second layer with the lower refractive index when the first layer is located closer to the substrate than the second layer. Conversely, the maxima of the electric field occur at the interfaces between the second and first layers when the second layer is located closer to the substrate than the first layer. It is therefore advantageous to place the smoothing layer between the first and second layers of each layer group, with their optical layer thicknesses adjusted accordingly to ensure a uniform thickness across the layer groups.To reduce absorption, the smoothing layer in the respective layer group has a comparatively small optical thickness, which is usually on the order of less than approximately 25% or less than approximately 15% of the optical thickness of the layer group.

[0037] It is possible that all layer groups in the sequence of layer groups exhibit the structure described here, with the smoothing layer as well as the first and second layers. To reduce the reflectivity loss due to the oxide smoothing layer, alternatively, only some layer groups can exhibit the structure described here, with the smoothing layer and the first and second layers, while other layer groups can, for example, be formed as layer pairs that only have the first and second layers or consist only of the first and second layers. For instance, only every Nth layer group in the sequence can have a smoothing layer, where N is a non-one integer, i.e., N = 2, 3, 4, 5, ...

[0038] Alternatively or additionally, the adjustment of the optical thicknesses of the two layers in a given layer group can be asymmetrical; that is, the thickness of the first and second layers in the respective layer group with the smoothing layer can be deliberately deviated from. For example, the optical thickness of only one of the two layers can be reduced by the optical thickness of the smoothing layer, while the optical thickness of the other layer remains unchanged. In this way, in addition to a correctable spectral shift, it can be ensured that the smoothing layer does not cause any significant change in the maximum achievable reflectivity or spectral width of the coating.

[0039] Generally, thin barrier layers may be placed between the first and second layers of the coating to prevent diffusion of materials at the interface between the two layers. Such barrier layers are typically present in every period or layer group, but they generally do not contribute to smoothing, i.e., they do not reduce the coating's surface roughness.

[0040] In another embodiment, the electric field of a standing wave formed by the reflection of radiation at a useful wavelength in the UV wavelength range exhibits a minimum in the region of the at least one smoothing layer. The useful wavelength typically represents the wavelength at which the coating exhibits its maximum reflectivity, or the useful wavelength deviates only slightly from the useful wavelength.

[0041] Regardless of the coating design, reducing absorption is generally advantageous if the oxide smoothing layer is located within the coating where the electric field of the forming standing wave has a minimum, as described, for example, in DE102018211499A1. This patent describes a reflective optical element for the VUV wavelength range, comprising a substrate and a metal layer. On the side of the metal layer facing away from the substrate, the optical element has at least one metal fluoride layer, and on the opposite side, at least one oxide layer. The electric field of a standing wave formed during reflection is intended to have a minimum in the region of the at least one oxide layer.

[0042] Another aspect of the invention relates to an optical arrangement, preferably a semiconductor lithography system, particularly preferably a UV lithography system, especially a VUV lithography system, or a wafer inspection system, comprising: at least one optical element as described above. The semiconductor lithography system can be a UV lithography system or a VUV lithography system, i.e., a projection exposure system for exposing a wafer, or a part of such a projection exposure system, for example, an illumination system or a projection lens. The semiconductor lithography system can also be an inspection system, e.g., for inspecting masks, wafers, or the like used in semiconductor lithography.The optical arrangement may also be a laser, for example an excimer laser for a wavelength of 193 nm or 248 nm, or another optical arrangement that may not be used in semiconductor technology.

[0043] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures in the drawing, which show details essential to the invention, and from the claims. The individual features can be implemented individually or in any combination in a variant of the invention. drawing

[0044] Examples of implementation are shown in the schematic drawing and are explained in the following description. It shows Fig. 1a a schematic representation of an optical element in the form of a mirror having a sequence of layer groups in the form of layer pairs of identical optical thickness, each consisting of two fluoride layers, Fig. 1b,c schematic representations of the mirror of Fig. 1a, which includes an additional layer pair with an oxide smoothing layer and with one of the two fluoride layers of Fig. 1a shows, Fig. 2 a schematic representation of the refractive indices and absorption coefficients of various fluoride and oxide layer materials for a wavelength of 193 nm, Fig. 3a,b schematic representations of the reflectivity of the mirror of Fig. 1a-c depending on the wavelength, Fig. 4a,b schematic representations of the reflectivity of a mirror analogous to Fig. 1a-c, which has layer pairs of oxide layers and a nitride smoothing layer, Fig. 5a,b a schematic representation of the mirror of Fig. 1a and a mirror which has an additional layer group in the form of an oxide smoothing layer, Fig. 6a,b schematic representations of the reflectivity of the mirror of Fig. 5a,b depending on the wavelength with SiO2 as the material of the smoothing layer, Fig. 7a,b schematic representations analogous to Fig. 6a,b with Al2O3 as the material of the smoothing layer, Fig. 8a,b a schematic representation of the mirror of Fig. 1a and a mirror which has two additional layer groups in the form of oxide smoothing layers, Fig. 9a,b schematic representations of the reflectivity of the mirror of Fig. 8a,b depending on the wavelength with SiO2 or with Al2O3 as the material of the smoothing layer, Fig. 10a,b a schematic representation of a mirror with a coating similar to the mirror of Fig. 8b has two additional layer groups in the form of smoothing layers, as well as the layer roughness of the coating layers, Fig. 11 A schematic representation of the electric field of a standing wave of radiation reflected from the coating as a function of the coating thickness, Fig. 12a,b Schematic representations of the reflectivity of a mirror whose coating has a sequence of layer groups, each with two fluoride layers of equal thickness and an oxide smoothing layer, as a function of wavelength, Fig. 13a,b schematic representations analogous to Fig. 12a,b, in which the fluoride layers have different thicknesses, Fig. 14 a schematic representation of a semiconductor technology plant in the form of a VUV lithography plant, Fig. 15 a schematic representation of a semiconductor technology plant in the form of a wafer inspection system, as well as Fig. 16 a schematic representation of a detail of a laser with an end mirror.

[0045] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0046] Fig. Figures 1a-c show a highly schematic optical element in the form of a mirror 1, which has a substrate 2 and a coating 3. The coating 3 is designed to reflect radiation 4 in the VUV wavelength range, more precisely at a useful wavelength λ of 193 nm. For all subsequent considerations, it is assumed that the radiation 4, unlike in Fig. Figures 1a-c show the radiation striking coating 3, or more precisely, the free surface 5 of coating 4, which forms the interface with the environment of mirror 1, at an angle of 0° to the surface normal. It is understood that the radiation 4 does not travel as shown in Figure 1a-c. Fig. 1a-c shown is not reflected at the surface 5 of the coating 3, but penetrates the coating 3 and forms a standing wave there, as described in more detail below.

[0047] At the in Fig. In the mirror 1 shown in Figure 1a, the coating 3 has a plurality of first fluoride layers 6 and second fluoride layers 7, which together form a layer group in the form of a layer pair 8. In the example shown, the first layers 6 have a first fluoride material with a first refractive index that, for the useful wavelength λ of the radiation 4 to be reflected, is greater than a second refractive index of the second fluoride material of the second layers 7. The order of the two layers 6, 7 in the respective layer pair 8 can also be reversed, i.e., instead of the first layer 6, the second layer 7 can be positioned closer to the substrate 2. The layer pairs 8 of the coating 3 have the same optical thickness, which is λ / 2, in the example shown. The first layer 6 and the second layer 7 of a respective layer pair 8 have the same thickness, which is λ / 4, in the example shown.

[0048] In the Fig. In the examples shown in 1a-c, the first fluoride material in the first layer 6 is LaF3 and the second fluoride material in the second layer 7 is MgF2. It is understood that the fluoride materials in both layers 6 and 7 could also be other materials, for example, the other materials shown in Fig. The fluoride materials AlF3, LiF, CaF2, YF3, YbF3, GdF3, CeF3, SmF3, NdF3 are shown. A respective pair of layer materials for the coating 3 is selected based on the refractive index n, more precisely based on the difference in the refractive indices of the two fluoride materials of layers 6, 7, as well as on the absorption coefficient k of the respective material, in order to generate the highest possible reflectivity of the mirror 1.

[0049] At the in Fig. In the mirror 1 shown in Figure 1a, the fluoride layers 6, 7 were deposited by electron beam evaporation or by thermal evaporation, resulting in a coating 3 with a high roughness. The roughness of the deposited fluoride layers 6, 7 increases linearly with increasing thickness of the coating 3, assuming a simple growth model. The reduction in reflectivity due to scattered light caused by the roughness of the layers 6, 7 increases quadratically with the roughness.

[0050] Fig. Figures 1b and 1c show a mirror 1 in which a smoothing layer 9, consisting of an oxide material, is arranged within the coating 3 to reduce roughness. In the example shown, the oxide material is SiO2, which has a comparatively low absorption coefficient k (see Figure 1b). Fig. 2) In the example shown, the smoothing layer 9 made of SiO2 is applied by sputtering, more precisely by magnetron sputtering, and has a comparatively low roughness, which ideally corresponds to the initial roughness of the substrate 2, more precisely to the surface 2a of the substrate 2 onto which the coating 3 is applied, or is lower than the initial roughness of the substrate 2.

[0051] In the example shown, the physical thickness d of the smoothing layer 9 results from the condition that the optical thickness of the smoothing layer 9 is λ / 4. The physical thickness d phys is determined by the optical thickness d opt Taking into account the refractive index n of SiO2 of 1.56 at the useful wavelength λ of 193 nm, and at perpendicular incidence of the radiation 4 at approximately 31 nm (it applies: d phys = d opt / n). It is understood that the physical thickness d of the smoothing layer 9 can also assume other values, which are typically on the order of between approximately 2 nm or 10 nm and approximately 100 nm.

[0052] The smoothing layer 9 should have as little influence as possible on the optical functionality of the mirror 1. This is achieved in the Fig. The smoothing layer 9 shown in 1b,c is achieved by containing the smoothing layer 9 in a further layer group 10 with an optical thickness of λ / 2, so that the periodicity of the coating 3, more precisely the sequence of layer groups 8, 10, is not disturbed by the further layer group 10.

[0053] The further layer group 10 with the smoothing layer 9 is found in the Fig. In the examples shown in 1b and c, the layer pairs 8 are arranged in the middle of the coating 3 and the sequence of layer pairs 8, respectively. That is, the same number of layer pairs 8 are arranged between the substrate 2 and the layer group 10 with the smoothing layer 9 as between the layer group 10 with the smoothing layer 9 and the free surface 5, which forms the interface with the environment. Due to the arrangement of the layer group 10 with the smoothing layer 9 in the middle of the coating 3, the coating 3 has two sub-stacks of layer pairs 8, each representing half the thickness of the coating 3. Fig. 1a correspond. The coating 3 of Fig. 1b,c therefore exhibits at most only half the roughness of Fig. 1a occurs when the initial roughness is restored by the smoothing layer 9. The reflectivity loss due to the roughness of layers 6, 7 of coating 3 of Fig. 1b,c is therefore reduced compared to coating 3 of Fig. 1a to about a quarter.

[0054] At the in Fig. In the example shown in Figure 1b, the further layer group 10 has, in addition to the smoothing layer 9, a second fluoride layer 7 made of MgF2. The smoothing layer 9 made of SiO2 replaces the first fluoride layer 6 of the layer pairs 8, which do not have a smoothing layer 9, and has an optical thickness of λ / 4.

[0055] At the in Fig. In the example shown in Figure 1c, the further layer group 10 has, in addition to the smoothing layer 9, a first fluoride layer 6 made of LaF3. In this case, the smoothing layer 9 made of SiO2 replaces the second fluoride layer 7 of the layer pairs 8, which do not have a smoothing layer 9, and also has an optical thickness of λ / 4.

[0056] Fig. Figures 3a and 3b show the reflectivity of coating 3 for the three in Fig. The cases described in 1a-c depend on the wavelength. Fig. The reflectivity profiles shown in Figures 3a and 3b, as well as those shown in the other figures, are based on a numerical simulation in which additional losses due to absorption and transmission were neglected. The simulation is based on a sequence of twenty-four layer pairs 8, of which in Fig. For clarity, only sixteen layer pairs 8 are shown in Figures 1a-c. The reflectivity loss due to the roughness of layers 6 and 7, as well as its reduction by the smoothing layer 9, were not considered in the numerical simulation. The numerical simulation assumed idealized, smooth layers; that is, the reflectivity profiles shown correspond to the maximum achievable reflectivity without reflectivity losses due to layer roughness.

[0057] As in Fig. As can be seen in 3b, the addition of the extra layer group 10 with the smoothing layer 9 does not result in any loss of reflectivity compared to the one in Fig. 1a shown coating 3, whose reflectivity profile in Fig. 3a,b is represented by a solid line. The reflectivity curve of the in Fig. The coating 3 shown in 1b is represented by a narrow dashed line, the reflectivity profile of which is shown in Fig. Coating 3 shown in Figure 1c is represented by a broad dashed line. As in Fig. As can be seen in 3a, when MgF2 is replaced by SiO2, only the spectral width decreases slightly, since there is a lower optical contrast to LaF3.

[0058] Fig. Figures 4a and 4b show the reflectivity of a mirror 1, which is as shown in Figure 1. Fig. The mirror 1 is designed as shown in Figures 1a-c, but is not designed for the reflection of radiation 4 in the VUV wavelength range, but rather in the VIS wavelength range at a useful wavelength λ of 500 nm. In this mirror 1, the first layers 6 consist of an oxide material, more precisely TiO2, and the second layers 7 also consist of an oxide material, more precisely HfO2. The smoothing layer 9 is shown in Figure 1a-c. Fig. The example shown in 1a-c is not formed from an oxide material, but from a nitride material, specifically Si3N4. As in Fig. As can be seen in Figures 4a and 4b, replacing a layer 6 of TiO2 or a layer 7 of HfO2 with the smoothing layer 9 of Si3N4 does not decrease the reflectivity of mirror 1, but rather increases it slightly. The spectral bandwidth of mirror 1 changes only slightly.

[0059] Fig. Figure 5a shows again mirror 1 with coating 3 of Fig. 1a, Fig. Figure 5b shows the mirror 1 with a coating 3 in which an additional layer group 10 has been inserted, consisting of a single smoothing layer 9. The smoothing layer 9, like the sequence of layer pairs 8, has an optical thickness of λ / 2. As in Fig. As can be seen in 6a,b, the maximum reflectivity is practically unchanged by the additional smoothing layer 9; only the spectral width of the reflectivity profile is slightly reduced.

[0060] The fact that the smoothing layer 9 has practically no effect on the reflectivity of the mirror 1 is due to the fact that this oxide material has a very low absorption coefficient k (cf. Fig. 2), so that the smoothing layer 9 is practically invisible. If Al2O3 is used instead of SiO2 as the oxide material, the following results are obtained for the in Fig. 5a,b examples shown in Fig. The reflectivity profiles shown in Figures 7a and 7b illustrate this. Besides a further reduction in spectral bandwidth, a decrease in reflectivity is also discernible, at least to some extent. This decrease is only so small because there is only a single smoothing layer 9 made of Al₂O₃ in the coating 3, and this layer is located in the center of the coating 3. Since the majority of the reflection is generated at the layer pairs 8 located near the free surface 5, which forms the interface with the environment, the effect on the reflectivity increases the closer the smoothing layer 9 made of Al₂O₃ is to the free surface 5 of the coating 3.

[0061] Fig. Figures 8a and 8b show a representation of a mirror 1 analogous to Fig. 5a,b, in which, in addition to the smoothing layer 9 in the middle of the coating 3, a further smoothing layer 9a is arranged within the coating 3, extending from the substrate 2 to approximately 3 / 4 of the total thickness of the coating 3. The second smoothing layer 9a also has an optical thickness of λ / 2. Fig. Figures 9a and 9b show the reflectivity profile for the two cases where the smoothing layers 9 and 9a are made of SiO2 or Al2O3. As can be seen, the second smoothing layer 9a made of SiO2 is still practically invisible with respect to maximum reflectivity, while two smoothing layers 9 and 9a made of Al2O3 lead to a significant drop in reflectivity in addition to reducing the spectral bandwidth. When smoothing layers 9 and 9a made of SiO2 are used in the coating 3, their arrangement within the coating 3 plays a minor role with regard to the decrease in reflectivity.

[0062] If only one smoothing layer 9 is used in the coating 3, it is advantageous for it to be positioned approximately in the center of the coating 3, as this results in a reduction of roughness for half of the total number of layer pairs 8. It can be advantageous to have at least two, five, ten, or fifteen layer pairs 8 between the substrate 3 and the smoothing layer 9. The distance between the substrate 3 and the smoothing layer 9 depends on the degree of smoothing effect of the smoothing layer 9. The number of layer pairs 8 that should be positioned between the smoothing layer 9 and the substrate 3 also depends on the degree of roughness increase with the thickness of the coating 3. If two or more smoothing layers 9, 9a, ...within the coating 3, it may be advantageous if at least a number of two, five, ten or fifteen layer groups 8 are arranged between two adjacent layer groups 9, 9a in the sequence, which do not contain a smoothing layer 9, 9a, ...

[0063] Due to the problem described above, it is particularly advantageous for a smoothing layer 9 made of an oxide material with a comparatively large absorption coefficient if it is not located too close to the free surface 5, which forms the interface with the environment. Specifically, there should be at least two, five, ten, or fifteen layer pairs 8 between the smoothing layer 9 and the free surface 5 that do not contain a smoothing layer 9.

[0064] Fig. Figures 10a and 10b show the layer roughness 12 of layers 6, 7, 9, and 9a of a coating 3 of a mirror 1, which is like the one in Fig. Mirror 1 shown in Figure 8b has two smoothing layers 9, 9a. Unlike mirror 1 of Fig. 8b The thickness of the first layers 6 made of LaF3 of a respective layer pair 8 differs from the thickness of the second layers 7 made of MgF2 of a respective layer pair 8, with each layer pair 8 having a total optical thickness of λ / 2. The two smoothing layers 9, 9a replace, as in the mirror of Fig. 1c the second layer 7 of each layer pair 8.

[0065] In the representation of Fig. Figure 10a shows coating 3 with the physical thicknesses of layers 6, 7, 9, and 9a. Fig. 10a is shown to the right of layers 6, 7, 9, 9a of coating 3, showing the respective layer roughness 12, i.e. the roughness on the respective layer 6, 7, 9, 9a. Fig. Figure 10b shows the layer roughness as a function of the number of the respective layer in the layer sequence starting from substrate 2, in arbitrary units. The layer roughness of Fig. 10b can be expressed, for example, in the form of the square roughness R q (rms value) or measured in other ways.

[0066] As in Fig. As can be seen in Figures 10a and 10b, the layer roughness 12 increases essentially linearly from substrate 2 with the number of layers 6 and 7 up to the first smoothing layer 9. The first smoothing layer 9 reduces the layer roughness 12 to almost the same value as the roughness at the surface 2a of substrate 2. After the first smoothing layer 9, the layer roughness 12 again increases essentially linearly with the increasing number of layers 6 and 7 up to the second smoothing layer 9a, which reduces the layer roughness 12 again. The layer roughness 12 then increases essentially linearly again from the second smoothing layer 9a up to the interface 5 with the environment. The layer roughness 12 of the coating 3 at the interface 5 with the environment is therefore, at the level shown in Figure 10a, 10b, the layer roughness 12 is approximately 12. Fig. 10a mirror 1 is significantly less than would be the case with a coating 3 without smoothing layers 9, 9a.

[0067] In the event that the oxide material of the smoothing layer 9 has a comparatively large absorption coefficient, the absorption has a less disruptive effect if an electric field, more precisely an electric field strength, of a standing wave 11 forming in the region of the smoothing layer 9 during the reflection of radiation 4 at the useful wavelength λ, which in Fig. Figure 11 shows a minimum. The minimum electric field strength, at which it is zero, is located in the region shown in Fig. 11 dashed line showing the interface between the first fluoride layer 6 and the second fluoride layer 7 of the respective layer pair 10. In Fig. Figure 11 shows the two fluoride layers 6, 7, each with a layer thickness of λ / 4, wherein in Fig. 11. On the left is the substrate 2 and on the right the surface 5 of the coating 3. The oxide smoothing layer 9, which is in Fig. 11, indicated by an arrow, is arranged between the first layer 6 and the second layer 7 of a respective layer group 10, whereby the optical thickness of the two layers 6, 7 is reduced by the corresponding amount.

[0068] Table 1 below shows the layer thicknesses of the fluoride layers 6, 7 made of LaF3 and MgF2 respectively, and the oxide smoothing layer 9 made of Al2O3 for three versions of the coating 3, each comprising twenty-four layer groups 10 with the first fluoride layer 6, the second fluoride layer 7, and the oxide smoothing layer 9 made of Al2O3 positioned between them. The smoothing layer 9 has an optical thickness that is less than 25% of the optical thickness λ / 2 of the layer group 10 containing the smoothing layer 9 in versions 2 and 3, and less than 15% in version 1. Table 1 Anzahl Material SchichtdickeVers. 1[λ / 4] SchichtdickeVers. 2[λ / 4] SchichtdickeVers. 3[M4] 1 LaF3 1 1 0,8 MgF2 0,95 0,9 1 24 Al2O3 0,1 0,2 0,2 LaF2 0,95 0,9 0,8 1 CaF2-Substrat sehr dick sehr dick sehr dick

[0069] Fig. Figures 12a and 12b show the reflectivity profile for the case where coating 3 is formed according to version 1 or version 2. "Standard" refers to... Fig. Figures 10a and 10b describe the case where layer groups 10 consist only of the first fluoride layer 6 and the second fluoride layer 7, each with an optical thickness of λ / 4. As can be seen from the reflected reflective profiles, the spectral width of the reflectivity is reduced only minimally by the smoothing layer 9; however, the decrease in reflectivity depends on the thickness of the smoothing layer 9. To reduce the loss of reflectivity, the number of smoothing layers 9 can be reduced. For example, only every Nth layer group 10 can be provided with a smoothing layer 9, while the other layer groups 8 consist only of the two fluoride layers 6 and 7, but no smoothing layer 9.

[0070] Alternatively or additionally, the optical thicknesses of the two fluoride layers 6, 7 can be adjusted asymmetrically, as is the case with version 3 in Table 1 above. As can be seen from the in Fig. As shown in the reflectivity curves in Figures 13a and 13b, in this case, apart from a correctable spectral shift, no significant change in the maximum achievable reflectivity or the spectral width of the reflectivity profile of mirror 1 can be observed.

[0071] It is understood that the methods described above for reducing reflectivity loss can also be combined in one and the same coating 3 to obtain an optical element 1 or a mirror with the highest possible reflectivity, ideally exceeding 95% or 97%. It is understood that the coating 3 or the optical element 1 can also be designed for wavelengths other than 193 nm by adjusting the optical thicknesses and, if necessary, the layer materials accordingly, as described in connection with Fig. 4a,b was described. The optical element 1 described above, with at least one smoothing layer 9, can be used in different optical arrangements, for example in semiconductor technology systems.

[0072] Fig. Figure 14 shows a semiconductor technology system for the VUV wavelength range in the form of a VUV lithography system 21. The VUV lithography system 21 comprises two optical systems, namely an illumination system 22 and a projection system 23. The VUV lithography system 21 also has a radiation source 24, which can be, for example, an excimer laser.

[0073] The radiation 25 emitted by the radiation source 24 is processed by the illumination system 22 to illuminate a mask 26, also called a reticle. In the example shown, the illumination system 22 has a housing 32 in which both transmitting and reflecting optical elements are arranged. A transmitting optical element 27, which focuses the radiation 25, and a reflecting optical element 28, which deflects the radiation, are shown as examples.

[0074] The mask 26 has a structure on its surface that is transferred to an optical element 29 to be exposed, for example a wafer, for the fabrication of semiconductor devices, using the projection system 23. In the example shown, the mask 26 is configured as a transmitting optical element. In alternative embodiments, the mask 26 can also be configured as a reflecting optical element.

[0075] The projection system 22 has at least one transmitting optical element in the illustrated example. Two transmitting optical elements 30 and 31 are shown as representatives; these serve, for example, to reduce the structures on the mask 26 to the size required for the exposure of the wafer 29.

[0076] In both the illumination system 22 and the projection system 23, a wide variety of transmitting, reflecting, or other optical elements can be combined in any desired, even complex, way. Optical arrangements without transmissive optical elements can also be used for VUV lithography.

[0077] Fig. Figure 15 shows a semiconductor technology system for the VUV wavelength range in the form of a wafer inspection system 41, although it could also be a mask inspection system. The wafer inspection system 41 has an optical system 42 with a radiation source 54, the radiation 55 of which is directed onto a wafer 49 by means of the optical system 42. For this purpose, the radiation 55 is reflected onto the wafer 49 by a concave mirror 46. In a mask inspection system, a mask to be inspected could be arranged instead of the wafer 49. The radiation reflected, diffracted, and / or refracted by the wafer 49 is directed by another concave mirror 48, also belonging to the optical system 42, via a transmitting optical element 47 to a detector 50 for further evaluation. The wafer inspection system 41 also has a housing 52 in which the two mirrors 46, 48 and the transmissive optical element 47 are arranged.The radiation source 54 can, for example, be exactly one radiation source or a combination of several individual radiation sources to provide an essentially continuous radiation spectrum. In variations, one or more narrowband radiation sources 54 can also be used.

[0078] At least the optical element 28 of the in Fig. 14 VUV lithography system 21 shown and at least one of the optical elements 46, 48 of the in Fig. The wafer inspection system 41 shown in 15 includes, like the optical element 1 of Fig. 1b,c of Fig. 5b, of Fig. 8b or as in Fig. 11 are formed and exhibit low layer roughness and high reflectivity.

[0079] Fig. Figure 16 shows an optical element for reflecting radiation in the VUV wavelength range in the form of an end mirror 60 of a laser chamber 61, a resonator of an excimer laser 62. The laser light emitted by the excimer laser 62 is reflected at the end mirror 60, more precisely at a reflective coating 3 of the end mirror 60, which is applied to a substrate 2. The excimer laser 62 can, for example, power the radiation source 24 of the VUV lithography system 21. Fig. Form 14. 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] DE 102018211499A1

[0041] Zitierte Nicht-Patentliteratur

[0000] Effect of sputtering power on MgF2 thin films deposited by sputtering technique under fluorine trapping“, R. De et al., AlP Conference Proceedings 1731, 080078 (2016

[0006] Far ultraviolet optical properties of MgF2 films deposited by ion-beam sputtering and their application as protective coatings for Al“, J. Larruquert et al., Optics Communications, Vol. 215 Issues 1-3, pp. 93-99 (2003

[0006]

Claims

[1] Optical element (1), in particular mirror, comprising: a substrate (2), as well as an optical coating (3), preferably for reflecting radiation (4), in particular for the reflection of radiation (4) in the UV or VIS wavelength range, which has a plurality of successive layers (6, 7) from the direction of the substrate (2) towards a free surface (5) of the optical coating (3), the layer roughness (12) of which increases with increasing distance from the substrate (2), characterized by , that within the coating (3) in addition to the plurality of layers (6, 7) at least one smoothing layer (9, 9a) is arranged which has a lower layer roughness (12) than one of the layers (6, 7). [2] Optical element according to claim 1, wherein the at least one smoothing layer (9, 9a) is applied by sputtering. [3] Optical element according to claim 1 or 2, wherein the plurality of layers (6, 7) comprise first layers (6) and second layers (7), wherein the first layers (6) comprise a first material and the second layers (7) comprise a second material whose refractive index for the radiation (4) is smaller than the refractive index of the first material for the radiation (4). [4] Optical element according to claim 3, wherein the first material and / or the second material is / are a fluoride material preferably selected from the group comprising: AlF3, MgF2, LiF, CaF2, YF3, YbF3, GdF3, LaF3, CeF3, SmF3, NdF3. [5] Optical element according to any of the preceding claims, wherein the material of the at least one smoothing layer (9, 9a) is an oxide material, preferably selected from the group comprising: SiO2, Al2O3, TiO2, ZrO, HfO2, Ta2O5. [6] Optical element according to any one of claims 1 to 4, wherein the material of the at least one smoothing layer (9, 9a) is a nitride material, preferably selected from the group comprising: Si3N4, GaN, AIN. [7] Optical element according to one of the preceding claims, wherein the smoothing layer (9) has a physical thickness (d) of more than 2 nm, preferably more than 10 nm, and / or of not more than 100 nm. [8] Optical element according to one of the preceding claims, wherein the coating (3) comprises a sequence of layer groups (8, 10, 10a) which preferably each have the same optical thickness (λ / 2). [9] Optical element according to claim 8, wherein the sequence includes layer groups (8) that do not have a smoothing layer (9, 9a), wherein the layer groups are preferably formed as layer pairs (8) each consisting of one of the first layers (6) and one of the second layers (7), wherein the first layer (6) and the second layer (7) of a respective layer pair (8) in particular have the same optical thickness (λ / 4). [10] Optical element according to claim 8 or 9, wherein the sequence of layer groups (8, 10, 10a) includes at least one layer group (10, 10a) with a smoothing layer (9, 9a). [11] Optical element according to claim 10, in which at least a number of two layer groups (8), preferably at least a number of five layer groups (8), particularly preferably at least a number of ten layer groups (8), in particular at least a number of fifteen layer groups (8) are arranged between the substrate (3) and the at least one layer group (10, 10a) with the smoothing layer (9, 9a), which do not contain a smoothing layer (9, 9a). [12] Optical element according to claim 10 or 11, wherein the sequence of layer groups (8, 10) comprises exactly one layer group (10) with a smoothing layer (9), wherein a number of layer groups (8) that do not contain a smoothing layer (9) and are arranged between the layer group (8) with the smoothing layer (9) and the substrate (3) differs by less than four layer groups (8), preferably by less than two layer groups (8), from a number of layer groups (8) that do not contain a smoothing layer (9) and are arranged between the layer group (8) with the smoothing layer (9) and the free surface (5). [13] Optical element according to claim 10 or 11, wherein the sequence of layer groups (8, 10, 10a) comprises at least two layer groups (10, 10a) with a smoothing layer (9, 9a), wherein between two layer groups (10, 10a) adjacent in the sequence with the smoothing layer (9, 9a) there is at least a number of two layer groups (8), preferably at least a number of five layer groups (8), particularly preferably at least a number of ten layer groups (8), in particular at least a number of fifteen layer groups (8) which do not contain a smoothing layer (9, 9a). [14] Optical element according to one of claims 10 to 13, wherein the layer group with the smoothing layer (9) forms a layer pair (10) which, in addition to the smoothing layer (9), has either a first layer (6) or a second layer (7), wherein the smoothing layer (9) and the first layer (6) or the second layer (7) preferably have the same optical thickness (λ / 4). [15] Optical element according to one of claims 10 to 14, wherein the layer group (10, 10a) with the smoothing layer (9, 9a) consists of the smoothing layer (9, 9a). [16] Optical element according to any one of claims 10 to 15, wherein the at least one layer group (10) with the smoothing layer (9) comprises the smoothing layer (9) as well as one of the first layers (6) and one of the second layers (7), wherein the smoothing layer (9) is arranged between the first layer (6) and the second layer (7), wherein the first layer (6) is located closer to the substrate (7) than the second layer (7), and wherein the smoothing layer (9) preferably has an optical thickness that is less than 25%, in particular less than 15%, of the optical thickness (λ / 2) of the layer group (10) with the smoothing layer (9). [17] Optical element according to one of the preceding claims, wherein an electric field of a standing wave (11) formed during the reflection of radiation (4) at a useful wavelength (λ) in the UV wavelength range has a minimum in the region of the at least one smoothing layer (9). [18] Optical arrangement, preferably laser (62) or semiconductor technology system, in particular UV lithography system (21) or wafer inspection system (41), comprising: at least one optical element (1, 28, 46, 48, 60) according to any of the preceding claims.

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