Optical element for reflecting radiation in the VUV wavelength range

Abstract

To provide an optical element for reflecting radiation in a VUV wavelength region, which has low sensitivity with respect to environmental influences, in particular with respect to reactions with fluorine.SOLUTION: The invention relates to an optical component (1) for reflecting radiations (2) in the VUV range of wavelengths, comprising surfaces (4a, 4a ') and a dielectric multilayer coating (5) applied to the surfaces (4a, 4a '). The dielectric multilayer coating (5) is designed for reflection of radiation (2) in the VUV wavelength range, in particular for broadband reflection. The surfaces (4a, 4a ') are preferably formed from a metallic substrate having a reflectivity of less than or equal to 60%, in particular less than or equal to 50%, in the VUV range. It is preferred that the substrate for reflection of radiation (3) in the UV / VIS wavelength range has a reflectivity of more than 50%, preferably more than 70%, more preferably more than 80%, in particular more than 90%, at at least one wavelength (λ VIS) in the UV / VIS wavelength range.SELECTED DRAWING: Figure 1a

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority from German Patent Application No. 102024205788.6, filed June 21, 2024, the entire disclosure of which is considered part of the disclosure of the present application and is incorporated by reference into the present disclosure of the present application.

[0002] The present invention relates to an optical element for reflecting radiation in the VUV wavelength range, the optical element comprising a surface and a dielectric multilayer system applied to the surface. The present invention also relates to an optical assembly in the VUV wavelength range, in particular a VUV lithography apparatus or a wafer inspection system, comprising at least one optical element as described above. [Background technology]

[0003] Generally, reflective optical elements for the UV wavelength region, and in particular the VUV wavelength region, have metallic surfaces, i.e., surfaces formed from metallic materials, and it is true that it is the metallic material that primarily contributes to the reflectivity of the optical element. The metallic surface can be formed on a substrate of the reflective optical element made of a metallic material. It is also possible for the metallic surface to be formed on a metal layer applied to the substrate of the optical element, and the substrate itself does not necessarily have to be made of a metallic material.

[0004] In particular, for reflecting radiation in the VUV wavelength range, i.e., between 100 and 200 nm (VUV wavelength range according to DIN 5031 Part 7), metal layers made of aluminum are frequently used because aluminum has the highest reflectivity in this wavelength range. In particular, surfaces made of aluminum enable a high reflectivity of 90% over a broad wavelength range from approximately 115 nm to the infrared wavelength range. However, aluminum has very low stability against environmental influences and UV radiation and is prone to oxidation, which significantly reduces reflectivity. Due to aluminum's high tendency to oxidize, at least one protective layer is typically applied to the metal layer in the form of an aluminum layer. The protective layer is typically a fluoride layer, particularly a metal fluoride layer, such as MgF2 or AlF2.

[0005] DE102018211499 describes a reflective optical element for the VUV wavelength region, including a substrate and a metal layer. The optical element has at least one metal fluoride layer on the side of the metal layer facing away from the substrate and at least one oxide layer on the side of the metal fluoride layer facing away from the substrate. The electric field of the standing wave generated upon reflection should have a minimum in the region of the at least one oxide layer. The metal layer can be aluminum, an aluminum alloy, rhodium, ruthenium, palladium, osmium, iridium, platinum, magnesium, or a combination thereof.

[0006] WO2006053705 describes a method for protecting a metal mirror from degradation and a metal mirror dielectrically protected against UV radiation having wavelengths between 120 nm and 260 nm. The metal mirror comprises a substrate, a mirror layer having a reflective metal layer applied to the substrate, and a protective layer, e.g., of thiolite, applied to the mirror layer to protect it from degradation of its reflective properties due to environmental influences. The metal layer may consist essentially of a pure metal, in particular aluminum, an alloy, or a semimetal. A dielectric multilayer system may be applied to the protective layer.

[0007] In the operation of reflective optical elements for the VUV wavelength range with an aluminum layer, it has been shown that atoms from the gas phase are transported very efficiently to the surface of the aluminum layer, despite the protective layer. Therefore, the protective layer was found to be effective only when the intensity of the incident radiation is low, as in metrology instruments. If aluminum with a protective layer is to be used at wavelengths below 190 nm, for example, in exposure or inspection systems for wafers or masks, severe degradation can be expected. Because aluminum is a highly reactive metal, it forms oxides in contact with oxygen and fluorides in contact with fluorine. This can adversely affect the lifetime of the optical element, or the reflectivity of the optical element can be significantly impaired in parts of the VUV wavelength range. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] DE102018211499 [Patent Document 2] WO2006053705 Summary of the Invention [Problem to be solved by the invention]

[0009] It is an object of the present invention to provide an optical element for reflecting radiation in the VUV wavelength range that has low susceptibility to environmental influences, in particular to reaction with fluorine, and a further object of the present invention to provide an optical assembly including such an optical element. [Means for solving the problem]

[0010] This object is achieved in a first aspect by an optical element of the type detailed at the beginning, in which the dielectric multilayer coating is designed for reflecting radiation in the VUV wavelength range, in particular for broadband reflection, and the surface is preferably formed from a metal substrate having a reflectivity of 60% or less, in particular 50% or less, in the VUV wavelength range. A reflectivity of 60% or less or 50% in the VUV wavelength range means that the substrate does not have a reflectivity of more than 60% or 50% over a wavelength range in the VUV wavelength range that is more than 30 nm wide.

[0011] The inventors have recognized that an optical element for reflecting VUV radiation does not necessarily require a metallic substrate surface to provide the majority of the reflectivity of the optical element for reflection in the VUV wavelength region. Indeed, a (highly) reflective multilayer coating can also produce a high reflectivity for VUV radiation even if the substrate itself has a relatively low reflectivity. In particular, the substrate surface does not need to be made of aluminum.

[0012] Due to the fact that the surface substrate has a relatively low reflectivity in the VUV wavelength region, it is possible to use materials other than aluminum, in particular materials that are more stable than aluminum with respect to reaction with fluorine (see below). In this way, it is possible to limit degradation of the optical element and save costs. The surface metal substrate is generally a pure metal, but in principle it can also be an alloy or a semi-metal. Alternatively, the surface substrate can be a non-metallic substrate, such as a semiconductor or an insulator.

[0013] Dielectric multilayer coatings allow for high reflectivity and, at the same time, good stability against ambient conditions, especially against incident VUV radiation. Because only a limited number of dielectrics are available, high reflectivity can only be achieved over one or more narrow wavelength ranges. However, dielectric multilayer coatings can also be designed for broadband reflection of VUV radiation.

[0014] The dielectric multilayer coating can have a reflectivity of more than 60% or more than 70%, preferably more than 80% or more than 85%, for example, in a wavelength range in the VUV wavelength range having a width of at least 20 nm, preferably at least 30 nm, particularly at least 60 nm, or having a total width as the sum of subranges of at least 20 nm, preferably at least 30 nm, more preferably at least 60 nm. In combination with the reflectivity of the surface from the substrate, the optical element can have a reflectivity in the VUV wavelength range of 60% or more, 70% or more, 80% or more, or 85% or more over a wavelength range having a width or sum of subranges with a total width of at least 20 nm, 30 nm, or 60 nm.

[0015] For example, the reflective optical element may have a substrate, such as a metallic material, surface that has a reflectivity of at least 30% over a wavelength range of at least 15 nm within the VUV wavelength range. The reflectivity over this wavelength range and / or other wavelength ranges within the VUV wavelength range may be increased by a dielectric multilayer coating such that a reflectivity of 60% or more, 70% or more, 80% or more, or 85% or more of the optical element is achieved over a total wavelength range of at least 20 nm, preferably at least 30 nm, and especially at least 60 nm within the VUV wavelength range.

[0016] In one embodiment, the substrate for reflecting radiation in the UV / VIS wavelength range has a reflectivity of more than 50%, preferably more than 70%, more preferably more than 80%, and particularly more than 90% at at least one wavelength in the UV / VIS wavelength range. In the context of the present application, the UV / VIS wavelength range refers to the wavelength range between 200 nm and 830 nm. For example, to adjust a reflective optical element with light in the visible spectrum range, it has been found to be advantageous if the optical element not only reflects radiation in the VUV wavelength range, but also has a high reflectivity in the UV / VIS wavelength range, i.e., the visible wavelength range and / or the ultraviolet range. The high reflectivity of the optical element in the VUV wavelength range and / or the (near) UV wavelength range between 200 nm and 360 nm is largely generated by the substrate, which means that a dielectric multilayer coating is not required for this purpose and contributes little or nothing to the reflectivity of the optical element in the UV / VIS wavelength range. The surface is affected by radiation in the UV / VIS wavelength range, in other words the dielectric multilayer coating and further layers of coating applied to the surface are transparent to radiation in the UV / VIS wavelength range, so that radiation in the UV / VIS wavelength range reaches the surface and is reflected there.

[0017] In one development of the invention, the wavelength in the UV / VIS wavelength range at which the substrate has a reflectance of more than 50%, preferably more than 70%, more preferably more than 80%, in particular more than 90%, is 650 nm or more, preferably 550 nm or more, more preferably 360 nm or more, in particular 260 nm or more. In this way, the adjustment of the optical element is simplified and is carried out with the help of radiation in the UV / VIS wavelength range.

[0018] The reflectivity of a metal substrate depends on factors including the plasma frequency of the electron gas of the metal material or the photon energy at the plasma frequency. It is advantageous if the plasma frequency or photon energy of the metal material is greater than or equal to about 1.9 eV (corresponding to a wavelength of 650 nm), or greater than or equal to about 2.25 eV (corresponding to a wavelength of 550 nm).

[0019] In a further embodiment, the dielectric multilayer coating comprises at least two layers of different materials, preferably fluorine-based or oxide materials, and the dielectric multilayer coating preferably consists of a layer of fluorine-based material. The two or more materials of the dielectric multilayer coating should have sufficient transmittance or as low absorption as possible in the VUV wavelength region. Additionally, the two or more materials should have sufficient contrast or difference in refractive index to generate maximum reflection intensity for radiation in the VUV wavelength region through interference. The manufacture or design of a dielectric multilayer coating to generate high reflectivity in the VUV wavelength region is known in principle and therefore will not be described in detail here. To generate high reflectivity in the VUV wavelength region, it is advantageous if the dielectric multilayer coating contains only fluoride layers and no oxide layers, because oxide layers generally have greater absorption for VUV radiation than fluoride layers.

[0020] In one development, the fluorine-based materials are MgF2, AlF3, LaF3, GdF3, LiF, YF3, SrF2, BaF2, ErF3, PrF3, NdF3, CaF2, NaF, and thiolite (Na5Al3F 14 ), cryolite (Na2NaAlF6), and mixtures thereof. These materials have relatively low absorption in the VUV wavelength range and can be easily separated. It has been found to be advantageous if the first layer of the dielectric multilayer coating, i.e., the layer of the dielectric multilayer coating closest to the surface of the substrate, is formed from AlF3.

[0021] In a further development, the oxide material is selected from the group comprising Al2O3, SiO2, and mixtures thereof, which have relatively low absorption in the VUV wavelength region.

[0022] It is preferable if the dielectric coating, generally a broadband reflective multilayer coating, especially the material at the interface with the environment, is designed so that high energy photons (short wavelengths) are reflected especially near the interface of the multilayer coating with the environment. In this way, the surface is exposed to fewer high energy photons, which increases the lifetime of the optical element.

[0023] In a further embodiment, the surface substrate forms a stable fluorine compound, preferably a stable metal fluoride, with fluorine, and preferably a passivating fluoride layer, particularly a passivating metal fluoride layer, is formed on the surface.For the purpose of this application, a stable fluorine compound, particularly in the form of a metal fluoride, means a fluorine compound that is in a solid state at atmospheric pressure and at 25°C, preferably 150°C, particularly 250°C.The indicator of whether this criterion is met can be the melting temperature of the fluorine compound or metal fluoride, and one-third of the melting temperature should be above 25°C, preferably above 150°C, particularly above 250°C.

[0024] The passivating fluoride layer can occur during the operation of the optical element or, in some cases, even during its manufacture, for example, if (post)fluorination is performed therein. Generally, it is advantageous to operate fluorine-based optical systems exposed to radiation in the VUV wavelength range in a fluorine-containing atmosphere or environment, since in this way it is possible to counteract the loss of fluorine in the solid body caused by the action of radiation in the VUV wavelength range. Furthermore, as indicated above, in the optical elements described herein, the aluminum surface can be replaced with the surface of a fluorine-stable substrate, for example in the form of a metal, which leads to a significant increase in the lifetime of optical elements operating in a fluorine-containing atmosphere or environment.

[0025] Furthermore, as mentioned above, in a fluorine-containing environment, fluorine reaches the surface despite the dielectric multilayer coating. If a stable fluorine compound is formed, a thin passivating fluorine compound is formed on the surface. It is advantageous if the fluorine compound of the substrate has polar covalent properties (i.e., a small band gap) and does not have strong ionic bonding properties.

[0026] In a further embodiment, the difference between the electronegativity of fluorine and the electronegativity of the substrate is 2.2 or less. The electronegativity value is determined according to Pauling. The electronegativity contrast ΔEN, i.e., the difference between the electronegativity of the anion and the electronegativity of the cation, determines the bonding characteristics. A value of ΔEN=1.7 is considered to be the boundary between polar covalent bonds (with ΔEN≦1.7) and ionic bonding systems or bonds (with ΔEN>1.7). An electronegativity contrast of 2.2 or less has the effect that fluorine compounds formed in the reaction with fluorine present in the environment, for example in the form of metal fluorides, have polar covalent rather than ionic bonding characteristics, which is favorable for the formation of a passivation layer.

[0027] In a further embodiment, the substrate forms a fluoride with fluorine, having a melting temperature of above 800°C, preferably above 900°C, more preferably above 1000°C, in particular above 1200°C. In particular, when the optical element is operated in a fluorine-containing atmosphere, it is favorable for the life of the optical element if the substrate forms a non-volatile fluoride with fluorine, in particular a non-volatile metal fluoride. Metal fluorides with a high melting temperature are generally non-volatile (see above). In the case where the metal substrate forms, for example, several fluorides with fluorine, as in the case of copper, which forms CuF and CuF2, the melting temperature refers to the fluoride with the lowest melting temperature.

[0028] In a further variant, the substrate has a melting temperature above 700° C., preferably above 800° C., more preferably above 1000° C. When the substrate has a high melting temperature, the dielectric multilayer coating can be deposited at high temperature and therefore at high density.

[0029] The substrate should also be easily processable, and in particular polishable to high optical quality.

[0030] The metal substrate is preferably selected from the group consisting of Cu, Ni, Co, Fe, Hf, Y, Sc, Cd, Zn, lanthanides, and mixtures thereof. Each of these metal (base) materials forms a metal fluoride with a melting temperature above 800°C, and therefore meets the requirement that volatile metal fluorides are not formed on the metal surface in a fluorine-containing environment. With the secondary condition that the metal substrate itself should have a high melting temperature, the following metal materials remain from the above list: Cu, Ni, Co, Fe, Hf, Y, Sc, and possibly lanthanides. Additionally, if it is taken into account that the bonding properties of the metal fluoride should be more polar covalent than strong ionic, i.e., less bonding properties, the following metal substrates remain from the above list: Cu, Co, Fe, Ni.

[0031] It is also possible for the metal substrate to be selected from the group comprising Ru, Cr, Ta, Rh, metals of main groups I and II, i.e., Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, and mixtures thereof. These materials are less successful in meeting the fluorine stability requirements further specified above, but may be used in optical elements where sufficient stability is achieved in combination with a dielectric layer applied to the substrate.

[0032] In a further embodiment, the substrate is selected from the group comprising TiO2, TiN, SiC, VC, Si. These materials are further possible substrates that are particularly attractive with regard to fluorine stability.

[0033] In a further embodiment, the surface is formed on a substrate made of a base material. In this case, the substrate itself forms the base reflector, the surface of which is polished to a high surface quality. In this case, the bulk substrate can have one or more cavities through which a cooling medium can flow to reduce the thermal load of the optical element. The substrate can be made of, for example, copper, which has a high thermal conductivity and is therefore well suited to removing heat from the optical element.

[0034] In an alternative embodiment, the surface is formed on a layer of a substrate that is applied to the substrate. In this case, the substrate is applied to the substrate in a coating process. The substrate itself may be formed from a substrate, for example a metallic material, although this is not absolutely necessary, i.e., the substrate may also be formed from glass, ceramic, etc.

[0035] In a further embodiment, at least one functional layer is disposed between the substrate and the base layer. The functional layer can be, for example, a polishing layer, an adhesive or adhesion promoter layer, a protective layer, a diffusion barrier layer, or one or more layers or layer stacks for adjusting the layer stress between the substrate and the base layer.

[0036] In a further embodiment, at least one functional layer is disposed between the surface and the dielectric multilayer coating. The functional layer can be a smoothing or polishing layer, an adhesion promoter layer, a diffusion barrier layer, a protective layer, etc. In addition to the layers further described above, the dielectric multilayer coating can also have further functional layers, which can be, for example, the types of functional layers further described above.

[0037] A further aspect of the invention relates to an optical assembly for the VUV wavelength region, in particular a VUV lithography system or a wafer inspection system, comprising at least one optical element designed as described above, the optical element preferably being exposed to a fluorine-containing environment during operation of the optical assembly. The optical assembly can be a (VUV) lithography system in the form of a VUV lithography assembly, a wafer or mask inspection system, a laser system, etc.

[0038] Furthermore, as mentioned above, in order to counteract the loss of fluorine in the generally fluorine-based layers of the dielectric multilayer coating, it is preferred if the optical elements in the optical assembly are operated in a fluorine-containing environment or atmosphere, and in particular in this case it is preferred if the substrate is fluorine-stable at the surface.

[0039] Furthermore, the optical elements described above can, in principle, be initially designed to reflect wavelengths outside the VUV wavelength range and coated with a coating system. The optical performance of such optical elements in the VUV wavelength range can then be improved by post-fluorination. For example, the optical elements can be irradiated with UV / VUV radiation and / or heated in the presence of a fluorinated active material for this purpose. When optical elements are manufactured in this manner, it is also necessary or preferable that the surface substrate be fluorine-stable, even if the optical elements in the optical assembly are not operated in a fluorine-containing environment.

[0040] To improve the lifetime of the optical element against irradiation with radiation in the VUV wavelength range, it may be advantageous if the crystalline defects are bleached, i.e., repaired, during the operation of the optical element. To this end, the optical element may be irradiated with UV / VIS radiation in an appropriate wavelength range during operation of the optical assembly, as described, for example, in WO 2022 / 214376, which is incorporated herein by reference in its entirety.

[0041] Further features and advantages of the invention will become apparent from the following description of an embodiment of the invention, which refers to the figures of the drawing which show the details essential to the invention, and from the claims. The individual features can be realized each, separately or together in any combination in a variant of the invention.

[0042] Examples are shown in the schematic drawings and explained in the following description. [Brief explanation of the drawings]

[0043] [Figure 1a] 1 is a schematic diagram of an optical element having a substrate of a metallic material for reflecting radiation in the UV / VIS wavelength range and having a dielectric multilayer coating for reflecting radiation in the VUV wavelength range. [Figure 1b] FIG. 1b is a schematic diagram of an optical element similar to FIG. 1a, in which a metal layer is applied to the substrate to reflect radiation in the UV / VIS wavelength range. [Figure 2] FIG. 1C is a schematic diagram of the reflectivity of the optical element of FIGS. 1a and 1b as a function of wavelength. [Figure 3a] 1 is a schematic diagram of the reflectivity of ruthenium, the reflectivity of a dielectric multilayer coating, and the reflectivity of a combination of ruthenium and a dielectric multilayer coating; [Figure 3b] 1 is a schematic diagram of the reflectivity of ruthenium, the reflectivity of a dielectric multilayer coating, and the reflectivity of a combination of ruthenium and a dielectric multilayer coating; [Figure 3c] 1 is a schematic diagram of the reflectivity of ruthenium, the reflectivity of a dielectric multilayer coating, and the reflectivity of a combination of ruthenium and a dielectric multilayer coating; [Figure 4] 1 is a schematic diagram of an optical assembly for the VUV wavelength region in the form of a VUV lithography apparatus. [Figure 5] FIG. 1 is a schematic diagram of an optical assembly for the VUV wavelength region in the form of a wafer inspection system. DETAILED DESCRIPTION OF THE INVENTION

[0044] In the following description of the drawings, the same reference numbers are used for identical or functionally identical components.

[0045] Figure 1a shows an optical element 1 designed to reflect radiation 2 in the VUV wavelength region and also to reflect radiation 3 in the UV / VIS wavelength region. Optical element 1 comprises a metal substrate 4 having a metal surface 4a on which a dielectric multilayer coating 5 is applied. As shown on the right side of Figure 1a, the dielectric multilayer coating 5 serves to reflect radiation 2 in the VUV wavelength region, while the metal substrate 4, and more specifically the metal surface 4a of the metal substrate 4, serves to reflect radiation 3 in the UV / VIS wavelength region.

[0046] The substrate 4, and therefore also the surface 4a, is formed from a metal substrate having a reflectivity of 60% or less, typically 50% or less, in the VUV wavelength region, i.e. the surface 4a of the substrate 4 only contributes slightly to the reflectivity of the optical element 1 in the VUV wavelength region. The metal material of the substrate 4 is copper in the illustrated example, and as is evident from the dashed line in Figure 2 which corresponds to the reflectivity R of the metal material as a function of wavelength λ, the reflectivity R of the metal material in the UV / VIS wavelength region of 650 nm is VIS 2, the reflectivity R of the metallic material of the substrate 4 is also greater than 90% at wavelengths in the UV / VIS wavelength range above 650 nm. This makes it possible to modulate the optical element 1 with radiation 3 in the UV / VIS wavelength range.

[0047] 2 shows, as a solid line or curve, the reflectivity R of the optical element 1 resulting from the combination of a metal substrate and a dielectric multilayer coating 5. As can be seen from the solid curve, the optical element 1 for radiation 2 in the VUV wavelength range has a high reflectivity R of more than 70% in a wavelength range having a width of about 60 nm, and even more than 80% in a wavelength range having a width of about 30 nm. The optical element 1 is therefore designed for broadband reflection of radiation 2 in the VUV wavelength range.

[0048] In the example shown in FIG. 1a, the dielectric multilayer coating 5 comprises multiple alternating first and second layers 6a and 6b of fluorine-based materials, with the fluorine-based material of the first layer 6a being different from the fluorine-based material of the second layer 6b. The two fluorine-based materials have a relatively large refractive index difference and sufficient transmittance in both the VUV and UV / VIS wavelength regions. The fluorine-based materials of the dielectric multilayer coating 5 can be selected from a group including, for example, MgF2, AlF3, LaF3, GdF3, LiF, YF3, SrF2, BaF2, ErF3, PrF3, NdF3, CaF2, NaF, thiolite, and cryolite, but may also be other fluorine-based materials. The thicknesses of the dielectric multilayer coating 5 and the fluorine-based material layers 6a and 6b are selected according to known optical design principles to establish the desired high reflectivity R of the optical element 1 in the VUV wavelength region. As an alternative or in addition to a fluorine-based material, the dielectric multilayer coating 5 may further comprise an oxide material, for example Al2O3 or SiO2.

[0049] The optical element 1 is, in particular, a fluorine-containing optical element, which is composed of molecular fluorine F2 or, for example, fluorine radicals F * It is preferred if the metallic material of surface 4a or substrate 4 forms stable metal fluorides, since when operated in an environment containing active fluorine species in the form of fluorine, fluorine can penetrate the reflective dielectric multilayer coating 5 and cause degradation of the metallic surface 4a. Therefore, it is preferred if the metallic material of surface 4a is fluorine-stable, i.e., forms stable metal fluorides, which leads to the formation of a thin passivating metal fluoride layer 7 on the metallic surface 4a, as shown on the right side of FIG. 1a. In contrast, if the metallic material is not fluorine-stable at surface 4a, fluorine, F, F * When exposed to radiation in the VUV wavelength range 2 in an environment containing volatile metal fluorides can form, which can, for example, promote delamination.

[0050] For a metallic material to be fluorine stable, it is first necessary that the metallic material does not form volatile metal fluorides with fluorine, which generally applies when the metallic material forms metal fluorides with high melting temperatures of over 800°C, over 900°C, over 1000°C, or over 1200°C.

[0051] Metallic materials that satisfy the above-mentioned conditions regarding the melting temperature of metal fluorides are listed in Table 1 below, and the values ​​in Table 1 were obtained from "https: / / www.webelement.com" or "https: / / www.wikipedia.org". [Table 1]

[0052] It would be advantageous if the metallic material could be heated during the deposition process, so that the dielectric multilayer coating 5 could be deposited with maximum density. Therefore, the metallic material preferably has a melting temperature of above 700° C., above 800° C., or above 1000° C. Among the metallic materials in Table 1, this condition is met by the following metals: Cu, Ni, Co, Fe, Hf, Y, Sc, and the lanthanides: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.

[0053] To achieve the above-described effect of forming a passivating metal fluoride layer 7 on the metal surface 4a, it has been found preferable that the bonding properties of the metal fluoride be polar covalent rather than ionic, i.e., the difference ΔEN(Pauling) between the electronegativity of fluorine EN(F) and the electronegativity EN(Me) of the metallic material is 2.2 or less (see Table 1). Since a ΔEN(Pauling) value of 1.7 corresponds to the boundary between polar covalent and ionic bonding systems, the bonding properties in the right-hand column are defined as the difference ΔEN(Pauling) - 1.7. Among the metallic materials listed in Table 1, this additional condition is met by Cu, Co, Fe, and Ni.

[0054] In the example shown in Fig. 1a, copper has been chosen as the material for the substrate 4. Copper has a high thermal conductivity and can therefore particularly effectively remove heat from the optical element 1. In order to remove heat particularly efficiently, the substrate 4 made of copper can have one or more cavities through which a temperature-controlling fluid, for example water, can flow.

[0055] The optical element 1 shown in Figure 1b differs from the optical element 1 shown in Figure 1a in that the metal surface 4a' is formed on a thin metal layer 4' applied to a substrate 8. In the example shown, the metal layer 4' typically has a thickness of the order of about 50-100 nm. The substrate 8 is not made of a metal material, but in the example shown consists of quartz glass.

[0056] As can also be seen in Figure 1b, a functional layer 9 is arranged between the metal layer 4' and the substrate 8. The functional layer 9 in the example shown is a polishing layer, but may also be an adhesive layer, or an adhesion-promoting layer, or one or more layers or layer stacks for adjusting layer stresses between the metal material of the metal layer 4' and the material of the substrate 8, or a diffusion barrier layer, or a protective layer.

[0057] In the optical element 1 of Fig. 1b, the further functional layer 10 arranged between the metal surface 4a' and the dielectric multilayer coating 5 is a smoothing layer. However, the functional layer 10 can also perform another function. It will be clear that in the example shown in Fig. 1a, a further functional layer may also be arranged between the dielectric multilayer coating 5 and the metal surface 4a.

[0058] Furthermore, in addition to the metal substrates mentioned above, it is also possible to use other metal substrates for the surfaces 4, 4' that are less successful in meeting the above-stated requirements for fluorine stability, but that nevertheless achieve sufficient fluorine stability in combination with the dielectric layers 6a, 6b applied to the substrates. These metal materials may be, in particular, Ru, Cr, Ta or Rh.

[0059] FIG. 3a shows the reflectance R of ruthenium as a metallic material of surface 4 in FIG. 1a or surface 4' in FIG. 1b as a function of wavelength. As can be seen in FIG. 3a, ruthenium in the VUV wavelength region has a reflectance R (at normal incidence) of greater than 50% and less than 60% at wavelengths greater than 115 nm. FIG. 3b shows the reflectance R (again at normal incidence) of a dielectric multilayer coating 5 formed from three layers of Al2O3, AlF3, and LaF3. The reflective multilayer coating 5 is optimized for a high reflectance R in the wavelength range between 140 nm and 170 nm to produce a high broadband reflectance R along with the reflectance R of the metallic surface 4 or 4' shown in FIG. 3a. The high reflectance R is greater than 60% or close to 70% in the wavelength range between approximately 140 nm and 220 nm, as can be seen in FIG. 3c. Comparing FIG. 3b and FIG. 3c shows that the metallic surface 4 or 4' significantly contributes to the reflectance of the optical element 1.

[0060] Further, in the above-mentioned examples, the surfaces 4, 4' are formed from a metallic substrate having a reflectivity of 60% or less, or 50% or less, in the VUV wavelength region. However, it is also possible for the surfaces 4, 4' to be formed from a non-metallic substrate having this property. For example, the substrate may be selected from the group including TiO2, TiN, SiC, VC, Si, and mixtures thereof. These substrates may also be fluorine-stable or have sufficient fluorine stability in combination with a dielectric layer applied to the substrate.

[0061] 4 shows an optical assembly for the VUV wavelength region in the form of a VUV lithography apparatus 21. The VUV lithography apparatus 21 comprises two optical systems: an illumination system 22 and a projection system 23. The VUV lithography apparatus 21 additionally comprises a radiation source 24, which may be, for example, an excimer laser.

[0062] Radiation 25 emitted by radiation source 24 is conditioned with the aid of illumination system 22, so that a mask 26, also called a reticle, is illuminated by illumination system 22. In the example shown, illumination system 22 has a housing 32 in which both transmissive and reflective optical elements are arranged. In an exemplary manner, the figure shows a transmissive optical element 27 that focuses radiation 25 and a reflective optical element 28 that deflects the radiation.

[0063] The mask 26 has on its surface a structure that is to be transferred onto an optical element 29, e.g., a wafer, which is exposed with the aid of the projection system 23 in order to manufacture semiconductor components. In the example shown, the mask 26 is designed as a transmissive optical element. In alternative embodiments, the mask 26 may also be designed as a reflective optical element.

[0064] The projection system 23 includes at least one transmissive optical element in the illustrated example, which shows in an exemplary manner two transmissive optical elements 30, 31, which serve, for example, to reduce the structures on the mask 26 to a size desired for exposure of the wafer 29.

[0065] In both illumination system 22 and projection system 23, a wide variety of transmissive, reflective, or other optical elements may be combined with each other as desired, including in more complex ways. Optical assemblies without transmissive optical elements may also be used for VUV lithography.

[0066] FIG. 5 shows an optical assembly for the VUV wavelength region in the form of a wafer inspection system 41, which could also be a mask inspection system. The wafer inspection system 41 comprises an optical system 42 with a radiation source 54, from which radiation 55 is directed onto a wafer 49 by 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, the wafer 49 can be replaced by the mask to be inspected. The radiation reflected, diffracted, and / or refracted by the wafer 49 is directed onto a detector 50 via a transmissive optical element 47 by a further concave mirror 48, also associated with the optical system 42, for further evaluation. The wafer inspection system 41 further comprises a housing 52 in which the two mirrors 46, 48 and the transmissive optical element 47 are arranged. The radiation source 54 can be exactly one radiation source or a combination of several individual radiation sources, for example to provide a substantially continuous radiation spectrum. In a variant, one or more narrowband radiation sources 54 can also be used.

[0067] At least optical element 28 of VUV lithography apparatus 21 shown in Figure 4 and at least one of optical elements 46, 48 of wafer inspection system 41 shown in Figure 5 are here designed similarly to optical element 1 of Figures 1a and 1b. VUV lithography system 21 and wafer inspection system 41 may be specifically designed to operate optical elements 28, 46, 48 in a fluorine-containing environment or atmosphere, or optical elements 28, 46, 48 may be located in a fluorine-containing environment. [Explanation of symbols]

[0068] 1 Optical element 2. Emission in the VUV wavelength range 3. Emission in the UV / VIS wavelength range 4 Metal substrate, substrate 4' metal layer 4a, 4a' metal surface 5. Dielectric multilayer coating 6a First layer 6b Second layer 7 Passivation metal fluoride layer 8 PCB 9 Functional Layer 10 Additional Functionality Layers 21 Lithography equipment, lithography system 22 Lighting system 23 Projection system 24 Radiation source 25 Radiation 26 Mask 27 Transmissive optical element 28 Reflective optical elements 29 Optical elements, wafers 30, 31 Transmissive optical elements 32 Housing 41 Wafer Inspection System 42 Optical system 46, 48 Concave mirror 47 Transmissive optical element 49 wafers 50 detectors 52 Housing 54 Radiation source 55 Radiation

Claims

1. An optical element (1) for reflecting radiation (2, 25, 55) in the VUV wavelength range, comprising: Surfaces (4a, 4a'), a dielectric multilayer coating (5) applied to said surfaces (4a, 4a'); An optical element (1) comprising: the dielectric multilayer coating (5) is designed for the reflection of radiation in the VUV wavelength range (2, 25, 55), in particular for broadband reflection, The surfaces (4a, 4a') are preferably formed from a metal substrate having a reflectivity of 60% or less, in particular 50% or less, in the VUV wavelength range. An optical element characterized by:

2. The substrate for reflecting radiation (3) in the UV / VIS wavelength range is at least one wavelength (λ VIS 2. The optical element according to claim 1, having a reflectivity at 1000 nm of more than 50%, preferably more than 70%, more preferably more than 80%, in particular more than 90%.

3. The wavelengths (λ) in the UV / VIS wavelength range at which the substrate has a reflectance of more than 50%, preferably more than 70%, more preferably more than 80%, especially more than 90%. VIS 3. The optical element according to claim 2, wherein the wavelength of the light emitting element (L) is 650 nm or more, preferably 550 nm or more, more preferably 360 nm or more, in particular 260 nm or more.

4. 4. An optical element according to claim 1, wherein the dielectric multilayer coating (5) comprises at least two layers (6a, 6b) of different materials, preferably fluorine-based or oxide materials, the dielectric multilayer coating (5) preferably consisting of layers (6a, 6b) of fluorine-based materials.

5. The fluorine-based material is MgF 2 , AlF 3 , LaF 3 , GdF 3 , LiF, YF 3 , SrF 2 , BaF 2 , ErF 3 , PrF 3 , NdF 3 , CaF 2 5. The optical element of claim 4, wherein the fluorine-containing silica is selected from the group comprising NaF, thiolite, cryolite, and mixtures thereof.

6. The oxide material is Al 2 O 3 , SiO 2 6. The optical element according to claim 4, wherein the compound is selected from the group comprising:

7. 7. The optical element according to claim 1, wherein the substrate of the surface (4a, 4a') forms a stable fluorine compound with fluorine, preferably a stable metal fluoride, and preferably a passivating fluoride layer, in particular a passivating metal fluoride layer (7), is formed on the surface (4a, 4a').

8. 8. An optical element according to any one of claims 1 to 7, wherein the difference between the electronegativity of fluorine and the electronegativity of the substrate is 2.2 or less.

9. 9. Optical element according to any one of claims 1 to 8, wherein the substrate forms a fluoride with fluorine having a melting temperature above 800°C, preferably above 900°C, more preferably above 1000°C, in particular above 1200°C.

10. Optical element according to any of the preceding claims, wherein the substrate has a melting temperature above 700°C, preferably above 800°C, more preferably above 1000°C.

11. 11. The optical element according to any one of claims 1 to 10, wherein the metal substrate is selected from the group comprising Cu, Ni, Co, Fe, Hf, Y, Sc, Cd, Zn, lanthanides, and mixtures thereof.

12. The substrate is TiO 2 11. The optical element according to any one of claims 1 to 10, wherein the material is selected from the group comprising TiN, SiC, VC, Si, and mixtures thereof.

13. An optical element according to any one of claims 1 to 12, wherein the surface (4a) is formed on a substrate (4) made from the base material.

14. Optical element according to any of the preceding claims, wherein said surface (4a') is formed on a layer (4') of said base material applied to a substrate (8).

15. 15. Optical element according to claim 14, wherein at least one functional layer (9) is arranged between the layer (4') of the substrate and the base (8).

16. Optical element according to any of the preceding claims, wherein at least one functional layer (10) is arranged between said surface (4a, 4a') and said dielectric multilayer coating (5).

17. 17. An optical assembly for the VUV wavelength region, in particular a VUV lithography system (21) or a wafer inspection system (41), comprising at least one optical element (1, 27, 28, 30, 31; 46, 47, 48) according to any of claims 1 to 16, which is preferably exposed to a fluorine-containing environment during operation of the optical assembly.

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