Method and device for forming a fluoride or oxyfluoride layer for an optical element for the VUV wavelength range, and optical element including the fluoride or oxyfluoride layer

Abstract

A method and device for forming a fluoride or oxyfluoride layer for an optical element for the VUV wavelength range, and an optical element including the fluoride or oxyfluoride layer, are provided. The present invention relates to a method for forming a fluoride or oxyfluoride layer (1) for an optical element (2) for use in the VUV wavelength range, comprising depositing an oxide layer (4) and converting the oxide layer (4) into a fluoride or oxyfluoride layer (1) by irradiating said oxide layer (4) with UV / VUV radiation (6) in the presence of an active fluorinating agent (FW). The present invention also relates to an optical setup having at least one such optical element (2) and an associated device for forming a fluoride or oxyfluoride layer (1) for an optical element (2) designed for use in the VUV wavelength range.

Description

[Technical Field]

[0001] REFERENCE TO RELATED APPLICATIONS This application claims priority from German Patent Application Publication No. 102022210513.3, filed October 5, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0002] The present invention relates to a method for forming fluoride or oxyfluoride layers for optical elements for use in the VUV wavelength range. The present invention also relates to optical elements for use in the VUV wavelength range, comprising (at least) one fluoride or oxyfluoride layer formed using the method, and to optical systems for the VUV wavelength range having at least one such optical element. The present invention also relates to devices for forming fluoride or oxyfluoride layers for optical elements designed for use in the VUV wavelength range. [Background technology]

[0003] In this application, the VUV wavelength range is understood to mean the wavelength range of electromagnetic radiation between 115 nm and 190 nm. The VUV wavelength range is particularly important for microlithography. For example, the VUV wavelength range is used, for example, in projection exposure apparatus and wafer or mask inspection apparatus. Optical elements having at least one fluoride layer are often used in such apparatus. Highly reflective optical elements for the VUV wavelength range typically have a fluoride layer, for example, to protect the underlying metal reflective layer, from oxidation, through which the radiation is reflected.

[0004] For example, German Patent Application Publication No. 102018211499 discloses a reflective optical element having a substrate, a metal layer, a metal fluoride layer added to the metal layer, and an oxide layer added to the metal fluoride layer, as well as a method for producing the same. In this case, the oxide layer reduces degradation at the high radiation intensities used in lithography, thus extending the service life of the optical element. Layer stacks consisting of different fluorides or fluorides and oxides may also be used for reflective or antireflective coatings of optical elements.

[0005] However, the narrow band gap of most oxides results in high absorption in the VUV wavelength range, so fluorides are preferred as layer materials.

[0006] However, the high radiation intensities required in optical systems in the form of wafer or mask inspection apparatus and projection exposure apparatus lead to degradation of the fluoride and the optical elements as a whole, thereby shortening their service life, which can be suppressed using the deposition of high-density fluoride layers.

[0007] In general, dense solids, especially dense fluoride layers, can be produced using plasma-assisted deposition methods, for example, by sputter deposition, for example, by ion beam sputtering (IBS) or plasma ion-assisted deposition (PIAD). To compensate for the preferential sputtering of lighter elements (in this case, O, F), and thus maintain the stoichiometry of the deposited layer, reactive gases are typically provided during such deposition processes. In the context of oxide layer deposition, such reactive gases can be, for example, O. In the case of fluoride deposition, F is appropriately provided, as described, for example, in the article "Plasma-assisted deposition of metal fluoride coatings and modeling the extinction coefficient of as-deposited single layers," M. Bischoff et al., Appl. Opt. 50, 232-238 (2010), in which metal fluoride layers are deposited using plasma-assisted electron beam deposition.

[0008] Although F2 is an attractive active fluorinating agent from a scientific point of view, gaseous fluorine comes with significant challenges in practical use, making F2 more challenging as a process gas: for example, unlike O2, F2's corrosiveness and toxicity require a comprehensive safety infrastructure.

[0009] Other gaseous active fluorinating agents, such as SF6, CF4, or NF3, have the potential disadvantage of incorporating sulfur, carbon, or nitrogen into the deposited layer due to momentum transfer in plasma-assisted and / or ion-assisted processes. Optical performance, especially in the VUV wavelength range, suffers from the incorporation of these foreign atoms into the deposited layer. Specifically, this leads to increased absorption or extinction of the layer.

[0010] To reduce absorption and degradation of the fluoride layer, the prior art has proposed post-treatment of the fluoride layer. Post-treatment means that the treatment is carried out after the deposition of the fluoride layer is complete. However, many of the known post-treatment methods have the disadvantage that they are performed at high temperatures or require high temperatures for effective post-treatment.

[0011] For example, US Patent Application Publication No. 2004 / 0006249 describes a method for post-treatment of a fluoride layer. Preferably, the post-treatment is carried out at a temperature ranging from 10° C. to 150° C. and a fluorine concentration of 1000 ppm to 100%.

[0012] The literature also describes that post-treatment in the form of post-irradiation with UV light can improve the optical performance of plasma- (ion-)assisted deposited fluoride films deposited in the deep UV wavelength range. For example, the above-cited paper by M. Bischoff et al. also describes post-treatment of metal fluoride layers with UV radiation. This paper shows that this post-treatment improves the initially poor transmittance of LaF3, MgF2, and AlF3 layers in the deep UV wavelength range. During this post-irradiation, color centers are bleached and presumably unsaturated bonds are post-oxidized (superficially).

[0013] This procedure is feasible for oxides in general and for fluorides used in the DUV wavelength range (i.e., wavelengths above 190 nm), but generally excludes fluorides in the VUV wavelength range, where oxidation would result in a loss of optical performance.

[0014] German Patent Application Publication No. 102021200490 also describes post-treatment of metal fluoride layers by irradiation, using electromagnetic radiation having at least one wavelength less than 300 nm. The metal fluoride layer is applied to a metal layer of a reflective optical element for use in the UV wavelength range. Irradiation results in passivation of the metal fluoride layer, which inhibits deterioration of the metal layer. However, this passivation protective layer is typically an oxide layer, which leads to the aforementioned drawbacks.

[0015] UV light irradiation can also be used as an aid in conjunction with layer deposition for nanostructuring or in the operation of a microlithography device.

[0016] DE 102018221190 A1 discloses the nanostructuring of a substrate for the transmission of radiation in the FUV / VUV wavelength range by introducing an energy input, for example by irradiation with UV / VUV radiation, where the substrate is crystalline, for example the substrate is a MgF2 single crystal, which irradiation allows the surface of the MgF2 single crystal to be reorganized so that an anti-reflection effect occurs.

[0017] DE 102021201477 A1 also discloses a method for operating a microlithography optical system having optical elements with a fluoride coating or consisting of a fluoride substrate, whereby during operation the optical elements are irradiated with UV light having a wavelength longer than the wavelength of the operating light of the optical system, which is 300 nm or less, in order to anneal fluoride defects.

[0018] Finally, further methods of producing fluoride layers or optical elements with fluoride layers are known in which other measures are used to reduce absorption and / or degradation.

[0019] For example, German Patent Application Publication No. 102005017742 discloses a method for coating a substrate by plasma-assisted deposition of a coating material, such as a fluoride material. The plasma contains ions with a relatively low effective ion energy, while the effective energy per molecule is relatively high, which is intended to result in a high packing density with low absorption and contamination of the deposited layer.

[0020] German Patent Application Publication No. 102020208044 discloses a method for producing optical elements for the VUV wavelength range, such as mirrors, windows, or beam splitters, with a coating of a fluorine scavenger layer that can be added to a fluoride layer. The purpose of the fluorine scavenger layer is to prevent degradation of the fluoride layer, resulting in a longer lifetime of the optical element. The underlying mechanism is a significant reduction in the mobility of interstitial fluorine atoms due to the so-called fluorine scavengers in the fluorine scavenger layer.

[0021] Further variations of physical vapor deposition of fluoride layers are also described in US Patent Application Publication No. 2013 / 0122252, JP 11172421 and JP 2003193231. Summary of the Invention [Problem to be solved by the invention]

[0022] In contrast, the object addressed by the present invention is to provide a method and a device for forming a fluoride or oxyfluoride layer, as well as an optical element and an optical arrangement comprising such an optical element, which allows or has high optical performance. [Means for solving the problem]

[0023] According to a first aspect, this problem is solved by a method of the type described at the beginning, which comprises depositing an oxide layer and converting it into a fluoride or oxyfluoride layer by irradiating it with UV / VUV radiation in the presence of an active fluorinating agent. The oxide layer may be deposited directly on the substrate. However, it is also possible to deposit the oxide layer on a further layer previously applied to the substrate (see below).

[0024] In the context of the present application, UV / VUV radiation refers to electromagnetic radiation in the wavelength range of 115 nm to 350 nm. Preferably, the fluoride layer is irradiated with VUV radiation in the wavelength range of 115 nm to 190 nm in the presence of an active fluorinating agent (see above).

[0025] In the method according to the present invention, a (typically dense) oxide layer is first deposited in a coating process. Subsequently, the oxide layer is converted into a fluoride layer or an oxyfluoride layer using UV / VUV radiation by providing a suitable active fluorinating agent. The active fluorinating agent is preferably a gaseous substance that photodissociates upon irradiation with UV / VUV radiation, forming molecules and / or atoms, particularly ionized and / or excited fluorides (hereinafter collectively referred to as fluorine species), in the process. The reactive fluorine species cause the chemical conversion of the oxide to a fluoride, at least in the surface region of the oxide layer. While the entire oxide layer may be converted into a fluoride layer, it is also possible that the oxide layer is converted into a fluoride layer only near the surface, with an oxyfluoride layer forming in the remaining volume. In the latter case, this layer is referred to as an oxyfluoride layer in the present application. Complete conversion of the oxide layer into a fluoride layer can be experimentally demonstrated for oxide layer thicknesses of less than about 10 nm. However, longer irradiation duration allows deeper penetration of the (stoichiometric) fluorinated tip, so that complete conversion can occur even for larger layer thicknesses.

[0026] Because the conversion of oxides to fluorides is chemically driven, it is generally possible to reduce the amount of active fluorinating agent required during post-treatment or conversion processes compared to coating processes that use less critical active fluorinating agents, such as NF3. Also, the amount of active fluorinating agent required can be reduced compared to coating processes. The reduction in the amount of active fluorinating agent can be compensated for by extending the fluorination process time. Both measures (the use of less critical active fluorinating agents and the reduction in the amount of active fluorinating agent) ensure that safety infrastructure requirements can be relaxed compared to fluorine-assisted coating processes that use F2.

[0027] In one variant, the oxide layer is preferably deposited using a PVD ("physical vapor deposition") deposition process selected from the group including sputter deposition, in particular magnetron sputtering, ion beam sputtering (IBS), ion beam assisted sputtering (IBAS), and plasma ion assisted deposition (PIAD). The oxide layer is typically deposited using a coating method that allows the deposition of dense layers; for this purpose, plasma-assisted or ion-assisted processes are advantageous. As detailed above, to maintain the stoichiometry of the deposited oxide layer, the oxide layer can be deposited in the presence of a gaseous oxidizing material. For example, reactive gases in the form of O2, HO, HO, ... can be used as the oxidizing material.

[0028] In a further variant, the oxide layer is deposited using CVD ("chemical vapor deposition"), in particular the atomic layer deposition process (ALD) or the plasma-enhanced atomic layer deposition process (PEALD). Atomic layer deposition allows for the deposition of extremely thin layers, down to a monolayer.

[0029] In a further variation, an oxide layer is deposited in a coating chamber and the oxide layer is converted to a fluoride or oxyfluoride layer in a fluorination chamber spatially separated from the coating chamber. For fluorination or conversion of the oxide layer, the substrate with the deposited oxide layer is transferred from the coating chamber to the fluorination chamber. Transfer may be performed manually or in an automated manner.

[0030] The assisted deposition of fluoride layers in a coating chamber in which an active fluorinating agent is supplied has the problem that the fluorine gas of the active fluorinating agent reacts strongly with hot components in the coating chamber (e.g., with the electrodes of the plasma source or the deposition boat used during deposition), resulting in generally significant contamination in the deposited layer. This problem is avoided by the fluorination process described herein, which is carried out after coating and generally does not use high temperatures. This variant for post-treatment or fluorination utilizes a dedicated fluorine-resistant fluorination chamber (see below), inside which a protective gas atmosphere is present and which is resistant to reactive fluorine species.

[0031] In a further variant, UV / VUV radiation contributes to the photodissociation of the active fluorinating agent, resulting in an energy E ph is at least the dissociation energy E of the active fluorinating agent diss In this case, the dissociation energy E diss represents the energy required to split the chemical bond of the active fluorinating agent by electromagnetic radiation (light).

[0032] Preferably, the first spectral region has an energy E ph is at least the dissociation energy E of the active fluorinating agent diss In this case, the following relationship applies to all photons in the first spectral region: E diss ≦E ph

[0033] It may be further advantageous to limit the first spectral region to higher energies so that the rate of potentially negative and / or competitive effects is suppressed or reduced. UP is preferably selected below a threshold energy above which negative and / or competitive effects are amplified or even occur, where E UP is the highest energy in the first spectral region, i.e., E ph ≦EUP Examples of such negative and / or competing effects include the absorption of light in solids and the photodissociation of potentially oxidizing species (e.g., O2 and H2O) in the gas phase.

[0034] In the developed form, the highest energy E of the first spectral region UP is no more than 100%, preferably no more than 50% greater than the dissociation energy of the active fluorinating agent.

[0035] In a further variation of this method, the highest energy of the first spectral region is equal to the band gap energy E of the fluoride or oxyfluoride layer formed. G Hereinafter, the band gap energy E of the fluoride or oxyfluoride layer preferably formed G This may reduce light absorption in the fluoride or oxyfluoride layer and thus reduce the formation of point defects (e.g., F centers).

[0036] To reduce the photodissociation of potentially oxidizing species in the gas phase (e.g., O and H O), e.g., F, F, F - , F * or the effective rate r of formation of potentially fluorinated species such as HF fluorinating But for example, O, O2, O * , O - , O.H. * and OH - The effective rate r of potential oxidizing species such as oxidizing Become bigger r fluorinating >r oxidizing Thus, it is further advantageous to limit the first spectral region to lower and higher energies.

[0037] In a further variation of this method, the UV / VUV radiation includes a second spectral region for mobilizing atoms at the surface, at the grain boundaries, and / or in the grain volume of the oxide layer, the second spectral region being in an energy range of 75% to 100%, preferably 80% to 95%, of the band gap energy of the fluoride or oxyfluoride layer. Specifically, the energy of the light in the second spectral region must be higher than the binding energy of the corresponding atoms in the solid.

[0038] High-energy electromagnetic radiation near the band edge of the oxide allows surface atoms or atoms to be mobilized without being desorbed, as described, for example, in DE 102018221190 cited in the summary. This enhanced mobility can also contribute to the transport of active fluorinating agents or fluorine species provided at the surface into the bulk of the oxide layer, presumably primarily via grain boundaries. This can therefore support deep fluorination, thereby improving optical performance. This enhanced diffusion can also address intrinsic point defects in the grain volume.

[0039] In a further variant, the UV / VUV radiation or the further electromagnetic radiation additionally used to irradiate the fluoride or oxyfluoride layer formed during conversion serves to anneal at least one crystalline defect in the fluoride or oxyfluoride layer and includes a spectral region that at least partially overlaps with the absorption region of said at least one crystalline defect, said spectral region preferably including the absorption energy of the crystalline defect, the average energy of this spectral region more preferably not deviating from the absorption energy of the crystalline defect by more than 0.5 eV, in particular not by more than 0.25 eV. The irradiation to anneal the at least one crystalline defect may be carried out during or after the conversion of the oxide layer to a fluoride or oxyfluoride layer.

[0040] A potential problem with irradiation with VUV radiation is that it can create crystalline defects, especially F / H central defect pairs, in fluorides or oxyfluorides by single-photon processes. These crystalline defects can be annealed using irradiation in a spectral region that at least partially overlaps with the absorption region.

[0041] The absorption energy of a crystal defect is understood to mean the energy or wavelength at which the absorption coefficient of the crystal defect has a maximum value. The absorption region of a crystal defect is understood to mean the region in which the absorption coefficient is greater than 1 / 100 of the value at the maximum absorption coefficient. The absorption energies of crystal defects of several fluorides relevant to this application are shown below as examples: MgF2: 260 nm (4.77 eV), AlF3: 190 nm (6.53 eV), 170 nm (7.29 eV), LaF3: 459 nm (2.7 eV), 564 nm (2.2 eV), 729 nm (1.7 eV).

[0042] In a further variant of this method, the oxide layer is irradiated in a protective gas atmosphere. Preferably, the protective gas is transparent to electromagnetic radiation in the UV / VUV wavelength range. Also, a protective gas that is relatively unreactive with optically relevant oxides and fluorides should be preferred. Inert gases in the form of light noble gases (helium, neon, and argon) are particularly suitable as protective gases, with argon being particularly well suited. Noble gases, particularly mixtures of the aforementioned noble gases, can also be used as protective gases. As detailed above, the oxide layer is typically irradiated in a fluorination chamber containing the protective gas atmosphere.

[0043] In a further variation of this method, the active fluorinating agent comprises at least one member of the group consisting of F2, HF, XeF2, NF3, CF4, and SF6. As detailed above, an active fluorinating agent that is less critical than F2, such as NF3, can be selected for conversion or fluorination.

[0044] In a further variant of this method, the partial pressure of the active fluorinating agent during irradiation of the oxide layer is 0.05-106 The partial pressure of the active fluorinating agent may be set or specified during irradiation. The partial pressure of the active fluorinating agent may also be controlled to a target value while the oxide layer is being irradiated.

[0045] In further variations, oxide layers are deposited in the form of MgO, Al2O3, La2O3, Gd2O3, CaO, SrO, or BaO layers. The first three oxides named here and their corresponding fluorides, MgF2, AlF3, and LaF3, are of great relevance to the optical industry. Further named oxides and their corresponding fluorides, GdF3, CaF2, SrF2, and BaF2, are also used in the optical industry. Chemically driven conversion of oxide layers to fluoride or oxyfluoride layers can be achieved by the deposition of the corresponding fluorides MgF3, AlF3, LaF3, and BaF2. X F Y (M=metal, F=fluorine) is oxide M a O b This works particularly well when the oxide is more stable than the MgF2 oxide. This is especially true for MgO, which has a significant energy driving force for converting the oxide (MgO) to the corresponding fluoride (MgF2). In the case of Al2O3 / AlF3, the enthalpy of formation of the oxide and fluoride is nearly the same, which is why the conversion is slightly less successful than the MgO / MgF2 conversion.

[0046] In a further variant, the oxide layer is a MgF2 layer, Mg x O y F z layer, AlF3 layer, Al x O y F z layer, LaF3 layer, La x O y F z layer, GdF3 layer, Gd x O y F z layer, CaF2 layer, Ca x O y F z layer, SrF2 layer, Sr x O y F z layer, BaF2 layer or Bax O y F z The oxide is typically converted to a fluoride or oxyfluoride layer in the form of a layer. As detailed above, the oxide is typically substantially completely converted to a fluoride at the surface of the oxide layer or at a short distance from the surface. The oxide layer is typically completely converted to a fluoride layer when the oxide layer has a small thickness, typically on the order of 5 nm to 10 nm. If the oxide layer is thicker, only the volume region of the oxide layer adjacent to the surface of the oxide layer where the VUV radiation is irradiated may be converted to fluoride, while oxyfluorides are formed at greater distances from the surface. In this case, the conversion may occur in the form of an oxyfluoride layer M with varying stoichiometry. x O y F z (M = metal), i.e., the ratios of x, y, and z vary globally depending on the distance from the surface of the oxyfluoride layer. By increasing the duration of irradiation, the (stoichiometric) fluorinated tip can be penetrated deeper. In this way, fluoride layers with thicknesses greater than 10 nm can be formed.

[0047] In a further variant, a metal reflective layer, specifically an aluminum layer, is deposited on the substrate before the oxide layer is deposited. As mentioned above, the number of materials available for reflecting VUV radiation is limited. Typically, an aluminum layer is used as the metal reflective layer, which can be protected from environmental influences by a dense layer, as described, for example, in the aforementioned German Patent Application Publication No. 102018211499. In the examples described herein, a metal reflective layer, typically in the form of an aluminum layer, is first applied to the substrate. An oxide layer as dense as possible (e.g., MgO) is then deposited on the metal reflective layer by plasma or ion assistance. The metal reflective layer and the oxide layer are typically deposited in the same coating chamber. This oxide layer is then converted into a fluoride or oxyfluoride layer by the above-mentioned fluorination process, for which purpose the coated substrate is typically transferred from the coating chamber to a fluorination chamber. As detailed above, the optical performance of reflective optical elements produced using this method is significantly improved for the VUV wavelength range by converting the oxide into an (oxy)fluoride layer.

[0048] In a further development of this variant, a further fluoride layer is deposited on the substrate before the oxide layer is deposited. In this case, unlike the above, the oxide layer is not deposited directly on the metal reflective layer, but instead a fluoride layer (e.g. MgF2, AlF3...) is first deposited on the metal reflective layer. A thin ion-assisted or plasma-assisted deposited oxide layer is added to this layer stack and converted into a fluoride or oxyfluoride layer as detailed above.

[0049] In both of the above cases, this method can be used to realize optical elements in the form of broadband reflecting mirrors that are, firstly, well protected against ambient influences and, secondly, have high optical performance.

[0050] A further aspect of the present invention relates to an optical element for use in the VUV wavelength range, comprising a fluoride or oxyfluoride layer formed using the above-described method. The optical element can be a reflective optical element, for example in the form of a metal mirror protected or passivated with a fluoride as described above. However, the optical element can also be a purely fluoride system without any metallic reflective layer, for example a transmissive optical element with an anti-reflective or highly reflective coating for the VUV wavelength range, or a partially transmissive optical element, for example in the form of a beam splitter. The anti-reflective or highly reflective coating is typically a dielectric multilayer coating comprising one or more fluoride layers.

[0051] A further aspect of the invention relates to an optical setup for the VUV wavelength range, in particular a VUV lithography apparatus or a wafer inspection system, comprising at least one optical element as described above. The optical setup may for example be a (VUV) lithography system, a wafer or mask inspection system, a laser system, etc.

[0052] Finally, a further aspect of the present invention relates to a device for forming a fluoride or oxyfluoride layer for an optical element designed for use in the VUV wavelength range by converting an oxide layer into a fluoride or oxyfluoride layer, the device comprising: a fluorination chamber; a supply unit for supplying an inert gas and an active fluorinating agent to the fluorination chamber, the interior of which is resistant to the active fluorinating agent and its conversion products; and at least one UV / VUV radiation source for irradiating the oxide layer with UV / VUV radiation in the fluorination chamber in the presence of the active fluorinating agent to convert the oxide layer into a fluoride or oxyfluoride layer. The fluorination chamber typically comprises a substrate holder used to hold a substrate that is introduced into the fluorination chamber and onto which an oxide layer (optionally on a further underlayer) is applied.

[0053] With regard to the advantages obtained by this device, reference is made to the above description of the method according to the invention and its variants.

[0054] The UV / VUV radiation emitted by the UV / VUV radiation source is partially absorbed by the active fluorinating agent, and the resulting fluorine species formed by photodissociation of the active fluorinating agent result in the conversion of the oxide layer to a fluoride or oxyfluoride layer.

[0055] The UV / VUV radiation source is designed for photodissociation of the active fluorinating agent and typically emits radiation in the wavelength range of preferably 115 nm to 1000 nm, more preferably 120 nm to 170 nm, and particularly 140 nm to 170 nm. For example, the UV / VUV radiation source may be a D2 lamp (deuterium arc lamp), a Xe gas discharge lamp, or a Hg vapor lamp. Alternatively, an F2 excimer laser (with a wavelength of 157 nm) may be used.

[0056] The fluorination chamber can be hermetically sealed. The conversion products of the active fluorination agent refer to fluorine species and compounds formed therefrom (e.g., HF). Stability is specifically understood to mean the formation of a passivation layer on the inside of the fluorination chamber. In particular, there should be no formation of volatile fluorine compounds that could precipitate on optical elements processed in the fluorination chamber. The fluorination chamber, and in particular its interior, can be at least partially manufactured from a metallic material, which typically must be free of Cr and Ti, to prevent corrosion. The fluorination chamber may specifically be manufactured from Monel steel.

[0057] Alternatively, the inside of the fluorination chamber may have a fluorine-stable coating to prevent corrosion. Such a coating is preferably applied by a galvanic process. Suitable materials are, in particular, Ni, NiP, Pt or Ru / Rh mixtures.

[0058] The supply unit is preferably designed to introduce the active fluorination agent diluted in an inert gas into the fluorination chamber, and may include a suitable metering control valve, in particular a mass flow controller, to set the partial pressure of the active fluorination agent in the fluorination chamber to the desired value.

[0059] The device preferably further includes a fluorine gas sensor and / or a dedicated sensor for the active fluorinating agent. The device may further include a closed-loop controller for controlling the partial pressure of the active fluorinating agent, the control preferably being effected using measurements from the fluorine gas sensor and / or the dedicated sensor for the active fluorinating agent.

[0060] Thus, the partial pressure of the active fluorinating agent may first be set indirectly by setting the flow rate, or may be actively controlled using a sensor and closed-loop controller.

[0061] Preferably, the device may also include one or more additional fluorine-resistant sensors for determining O2 and / or H2O partial pressures within the post-treatment chamber.

[0062] The device may optionally include a second UV / VUV radiation source emitting UV / VUV radiation in a second spectral region for mobilizing atoms at the surface and / or in the volume of the oxide layer and / or newly formed fluoride or oxyfluoride layer as described above in the context of the method. In an alternative form of this, the device may include only one UV / VUV radiation source emitting UV / VUV radiation in the first and second spectral regions.

[0063] To anneal at least one crystalline defect in the fluoride or oxyfluoride layer, the UV / VUV radiation source and / or the second UV / VUV radiation source can further optionally emit in a spectral region that at least overlaps with the absorption region of the at least one crystalline defect. Alternatively, the device may include one or more additional radiation sources that serve to anneal at least one crystalline defect in the fluoride layer and that also emit electromagnetic radiation in a spectral region that at least partially overlaps with the absorption region of the at least one crystalline defect. Preferably, therefore, at least one radiation source is suitable for bleaching crystalline defects that occur in the fluoride or oxyfluoride layer during conversion. The spectral region can preferably be adapted to the absorption region or regions of the crystalline defects. In particular, the additional radiation source or sources can be one or more tunable radiation sources (e.g., based on a broadband primary light source with downstream wavelength selection). Alternatively or additionally, dedicated radiation sources can be used for the oxide to be converted or for each oxide to be converted.

[0064] 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 of the drawings which show the details essential to the invention, and from the claims, wherein the individual features can be realized in the variants of the invention, each individually alone or as several features in any desired combination.

[0065] Exemplary embodiments are shown in the schematic diagrams and explained in the following description, with details as follows: [Brief explanation of the drawings]

[0066] [Figure 1] FIG. 1 is a schematic diagram showing three snapshots of how a dense oxide layer is deposited and converted to a fluoride or oxyfluoride layer by subsequent exposure to VUV radiation in the presence of an active fluorinating agent. [Figure 2] FIG. 1 is a schematic diagram illustrating the absorption and spectral regions associated with irradiation of an oxide layer. [Figure 3] FIG. 1 is a schematic diagram illustrating a device for forming a fluoride or oxyfluoride layer by converting an oxide layer in the presence of an active fluorinating agent. [Figure 4a-4b] FIG. 1 shows the enthalpy of formation or difference in enthalpy of formation of three selected oxides and fluorides. [Figure 5a-5b] 2A-2C show two examples of procedures for making an optical element for reflecting VUV radiation using the method shown in FIG. 1; [Figure 6] 1 is a schematic diagram showing an optical setup for the VUV wavelength range in the form of a VUV lithography apparatus; [Figure 7] FIG. 1 is a schematic diagram showing an optical setup for the VUV wavelength range in the form of a wafer inspection system. DETAILED DESCRIPTION OF THE INVENTION

[0067] 1 shows a schematic diagram of the formation of a fluoride or oxyfluoride layer 1 of an optical element 2 for use in the VUV wavelength range. Three snapshots M1, M2, M3 of the substrate 3 of the optical element 2 during the formation of the fluoride or oxyfluoride layer 1 are shown.

[0068] The first snapshot M1 on the left of Figure 1 shows the substrate 3 after deposition of a dense oxide layer 4, which in the illustrated example consists of MgO. The dense oxide layer 4 is deposited by a PVD deposition process known per se, for example by sputter deposition, in particular magnetron sputtering, ion beam sputtering (IBS), ion beam assisted sputtering (IBAS) or plasma ion assisted deposition (PIAD). Deposition in a CVD process is also possible, which may include for example atomic layer deposition (ALD) or plasma enhanced atomic layer deposition (PEALD).

[0069] The oxide layer 4 is deposited in a coating chamber not shown in Figure 1. The substrate 3 with the oxide layer 4 deposited thereon is removed from the coating chamber, as indicated by the arrow in Figure 1, and transferred to a fluorination chamber (also not shown in Figure 1).

[0070] The second snapshot M2 shown in the center of Figure 1 shows the oxide layer 4, more precisely the surface 5 of the oxide layer 4 exposed to the ambient environment, during irradiation with UV / VUV radiation 6 in the presence of the active fluorinating agent FW. As a result of the irradiation, the active fluorinating agent FW dissociates, giving rise to the fluorine species F, F2, F * The fluorine species F, F2, and F * reacts with the oxides of the oxide layer 4 present on the surface 5, converting them to fluorides (in this case MgF2). For example, the active fluorinating agent FW is NF3, which upon photodissociation produces fluorine species F, F2, F * The gas may be a different substance capable of supplying the gas, in particular at least one substance from the group including F2, HF, XeF2, CF4 and SF6.

[0071] The third snapshot M3 on the right of Figure 1 shows the fluoride layer 1 converted from the oxide layer 4 during fluorination. The fluoride layer 1 is (at least approximately) stoichiometric fluoride. The conversion of the dense oxide layer 4 to the fluoride layer 1 significantly improves the optical performance of the optical element 2. Contrary to what was described above, an oxyfluoride layer may be formed during conversion instead of the fluoride layer 1. This occurs when not all of the oxides in the oxide layer are converted to fluorides. The degree of conversion of the oxides in the oxide layer 4 to fluorides depends on various influencing factors, particularly the thickness of the oxide layer 4 and the duration of irradiation.

[0072] The fluoride layer 1 is also, although not necessarily, irradiated with further electromagnetic radiation 7 during post-treatment in the illustrated embodiment, for the purpose of annealing crystalline defects 8 in the fluoride layer 1. Alternatively or additionally, irradiation with further electromagnetic radiation 7 may occur during irradiation of the oxide layer 4, for the purpose of annealing crystalline defects 8 in the already converted portions of the fluoride layer 1.

[0073] 1, the oxide layer 4 may also be heated during irradiation, but heating is not a necessary component of the method.

[0074] Figure 2 shows the absorption and spectral regions associated with irradiation of the oxide layer 4. Energy is plotted on the horizontal axis and absorption cross section is plotted on the vertical axis. The dissociation energy E of the active fluorinating agent FW is diss Schematically shown are the absorption cross section 12 of the newly formed fluoride or oxyfluoride layer 1 including the Urbach tail 12', and the absorption cross section 13 of the crystal defects 10 within the fluoride or oxyfluoride layer 1.

[0075] The UV / VUV radiation 6 used to irradiate the oxide layer 4 has a first spectral region 14 for photodissociation of the active fluorinating agent FW. For example, the first spectral region 14 has an energy E ph is the dissociation energy E of the active fluorinating agent FW. diss and at least one wavelength at least equal to

[0076] Also, the highest energy E UP is the dissociation energy E of the active fluorinating agent FW. diss is less than 50% greater than (i.e., 1.5 × E diss (See FIG. 2 below). This is by way of example only and is not required. This minimizes potentially negative and / or competitive effects. The highest energy E UP is the band gap energy E of the oxide layer 4 Gand preferably the band gap energy E G is less than 75% of the

[0077] Furthermore, the fluoride or oxyfluoride layer 1 is further irradiated with, for example, electromagnetic radiation 7, which serves the purpose of annealing at least one crystalline defect 10 in the fluoride or oxyfluoride layer 1. To this end, the further electromagnetic radiation 7 has a spectral region 16 that overlaps with an absorption region 17 of the at least one crystalline defect 10. In this exemplary embodiment, the spectral region 16 of the further electromagnetic radiation 9 lies within the absorption region 17 of the F-center crystalline defect 8, although this is not necessary. In this alternative, the UV / VUV radiation 6 may also include the corresponding spectral region.

[0078] In the illustrated embodiment, the spectral region 16 of the further electromagnetic radiation 9 is the region of the crystal defect 10 where the absorption cross section is greatest, i.e., the absorption energy E A However, this is not necessarily limited to this. The absorption region 17 of the crystal defect 10 includes the absorption energy E A The absorption cross section is defined by the maximum absorption cross section down to 1 / 100 of its maximum value (FWHM) in the spectral region 16. m (arithmetic mean) is the absorbed energy E of 10 crystal defects. A It is advantageous if the deviation does not exceed 0.5 eV, and in particular does not exceed 0.25 eV.

[0079] The UV / VUV radiation 8 also includes a second spectral region 18 for mobilizing atoms at the surface 5, grain boundaries 5' and / or grain volume 5" of the newly formed fluoride or oxyfluoride layer 1, which correspond to the grain boundaries 5' and grain volume 5" of the oxide layer 4. In the illustrated embodiment, this second spectral region 18 is in the energy range of 75% to 100% of the bandgap energy E of the fluoride or oxyfluoride layer 1. Preferably, the second spectral region 18 is in the energy range of 75% to 100% of the bandgap energy E of the fluoride or oxyfluoride layer 1. G It may be 80% to 95% of the above.

[0080] 3 shows a device 60 for forming the fluoride or oxyfluoride layer 1 of the optical element 2 of FIG. 1 by converting the oxide layer 4 to the fluoride or oxyfluoride layer 1. The device 60 includes a fluorination chamber 61, a supply unit 62, and a UV / VUV radiation source 63.

[0081] In this case, by way of example, optical element 2, which includes oxide layer 4 applied to substrate 3, is mounted on substrate holder 64, which is rotatable about axis of rotation 65 within fluorination chamber 61. However, deviating from the example shown, device 60 may not include rotatable substrate holder 64.

[0082] The supply unit 62 serves the purpose of supplying the fluorination chamber 61 with a protective gas in the form of an inert gas IG and an active fluorinating agent FW. The supply unit 62 includes a first valve 66 for the controlled supply of the inert gas IG and a second valve 67 for the controlled supply of the active fluorinating agent FW. The second valve 67 is a controllable metering valve. Thus, the oxide layer 4 can be irradiated in the protective gas atmosphere in the fluorination chamber 61 in the presence of the active fluorinating agent FW. The device 60 further includes a gas outlet 68 for discharging the inert gas IR and reaction products formed during fluorination. In the illustrated embodiment, the inert gas IR is argon, but other inert gases IR, such as other light noble gases like helium or neon, can also be used. Noble gases, in particular mixtures of the above-mentioned noble gases, can also be used as the inert gas IR.

[0083] The UV / VUV radiation source 63 serves to irradiate the oxide layer 4 with UV / VUV radiation 6 in the presence of the active fluorinating agent FW in the fluorination chamber 61. For example, in the illustrated embodiment, the UV / VUV radiation 6 enters the fluorination chamber 61 through an MgF2 window 69. The VUV radiation source 63 serves to generate UV / VUV radiation 6 in the first spectral region 14 mentioned above.

[0084] For example, device 60 also includes in this case, although this is not required, a second UV / VUV radiation source 70 for irradiating oxide layer 4 with UV / VUV radiation 7 in the above-mentioned second spectral region 18 in order to mobilize atoms at the surface 5, grain boundaries 5′ and / or grain volume 5″ of oxide layer 4. In the illustrated embodiment, UV / VUV radiation 7 from second UV / VUV radiation source 70 enters fluorination chamber 61 through an MgF2 window 69′. Device 60 also includes a further radiation source 71 for irradiating fluoride or oxyfluoride layer 1 formed during conversion with further electromagnetic radiation 7 in the spectral region 17 as described above in the context of FIG. 2 in order to anneal at least one crystalline defect 10 in fluoride or oxyfluoride layer 1. The further electromagnetic radiation 7 from further radiation source 71 enters fluorination chamber 61 through a further MgF2 window 69″.

[0085] Instead of at least one of the MgF2 windows 69, 69', 69", it is in principle also possible to use windows made of other materials, for example CaF2, SrF2 and / or BaF2, in which case it is crucial that they have sufficient transparency at the wavelengths used.

[0086] The fluorination chamber 61 can be sealed gas-tight. Furthermore, the inside 72 of the fluorination chamber 61 is resistant to the active fluorinating agent FW and its conversion products. To this end, in the illustrated embodiment, the fluorination chamber 61, or at least its inside 72, is made of metal in the form of Monel steel, which forms a passivation layer to prevent corrosion. In principle, the fluorination chamber 61 can also be made of other corrosion-resistant materials, provided they do not contain Cr and Ti.

[0087] Alternatively, a corrosion-resistant coating, for example, made of NiP, Pt or a Ru / Rh mixture, can be applied to the interior 72 of the fluorination chamber 61. The corrosion-resistant coating can be applied to the interior 72 of the fluorination chamber 61, for example, by a galvanic process. Components located within the fluorination chamber 61 that come into contact with the active fluorinating agent FW are likewise resistant to the active fluorinating agent FW and its conversion products.

[0088] The device 60 shown here also includes, for example, but not necessarily, a sensor 73 for measuring the oxygen concentration, CO, in the fluorination chamber 61 and the H2O concentration, C, in the fluorination chamber 61. H2O and a further sensor 74 for measuring

[0089] For example, the oxygen concentration c in the fluorination chamber 61 during irradiation or conversion of the oxide layer 4 O2 is less than 50 ppbV. Oxygen concentration c O2 The oxygen concentration c must be as low as possible, but may be higher than 50 ppbV. O2 is advantageously less than 10 ppmV, preferably less than 1 ppmV, more preferably less than 100 ppbV.

[0090] In the illustrated embodiment, the H2O concentration c H2O In principle, the H2O concentration c in the fluorination chamber 61 during irradiation of the oxide layer 4 is less than 100 ppbV. H2O should be as low as possible, but the H2O concentration c H2O may be greater than 100 ppbV. However, the H2O concentration c H2O It is advantageous if the V is less than 10 ppmV, preferably less than 1 ppmV, more preferably less than 500 ppbV.

[0091] Although this is not required, the device 60 of the illustrated embodiment operates by controlling the partial pressure c of the active fluorinating agent FW in the fluorination chamber 61. FWand a sensor 75 for measuring the partial pressure c of the active fluorination agent FW in the fluorination chamber 61. FW and a closed-loop controller 76 for adjusting the partial pressure c of the active fluorinating agent FW in the fluorination chamber 61 to a target value. FW This is done using the actual measurement M from the sensor 75 to measure the partial pressure c of the active fluorinating agent FW and the control of the second valve 67. FW The second valve 67 may be designed to measure only the partial pressure of the active fluorinating agent FW in the fluorination chamber 61, but may also be a residual gas analyzer capable of determining the partial pressure of other gases contained within the fluorination chamber 21. Such a residual gas analyzer may perform the functions of the three sensors 73, 74, and 75 shown in Figure 3. If the second valve 67 is a metering control valve, e.g., a mass flow regulator valve, the partial pressure c of the active fluorinating agent FW in the fluorination chamber 61 may be determined. FW It is possible to omit the use of the sensor 75 for measuring

[0092] The active fluorinating agent FW is added to the inert gas IR in the supply unit 62. The partial pressure c of the active fluorinating agent FW in the post-treatment chamber 61 during irradiation of the oxide layer 4 FW is typically 0.05 to 10 6 ppmV, preferably 0.075 ppmV to 50 ppmV, and more preferably 0.1 ppmV to 10 ppmV.

[0093] In principle, the chemically and energetically driven conversion of oxides to fluorides is x F y (M=metal, F=fluorine) is oxide M a O b This works particularly well whenever oxides are more stable than MgO / MgF2, Al2O3 / AlF3, and La2O3 / LaF3. Figure 4a shows the enthalpies of formation (in kJ / mol) for three example pairs of substances: MgO / MgF2, Al2O3 / AlF3, and La2O3 / LaF3. Figure 4b shows the enthalpies of formation Δ f H 0 ox and the corresponding enthalpy of formation of fluoride Δ f H 0fl The difference Δ f H 0 ox -Δ f H 0 fl For positive values ​​of , the fluoride of each material pair is more stable than the oxide. As is evident from Figure 6b, this is true only for Mg among the three elements shown, i.e., there is a significant driving force for converting the oxide to the corresponding fluoride. Experiments demonstrate a significant reduction in the quenching of Al2O3 by the fluorination step described above. Experimental evaluation suggests that the conversion of Al2O3 to AlF3 is slightly less successful than the conversion of MgO to MgF2, consistent with predictions from the enthalpy of formation. The conversion of other oxides to fluorides or oxyfluorides is also possible, such as Gd2O3 to GdF3, CaO to CaF2, SrO to SrF2, or BaO to BaF2.

[0094] The method described above in the context of Figure 1 may be used for the fabrication of different optical elements. In the following, two examples for the fabrication of an optical element 2 in the form of a mirror with broadband reflection of radiation in the VUV wavelength range will be described based on Figures 5a and 5b.

[0095] In the embodiment shown in FIG. 5a, the substrate 3 is introduced into a coating chamber 59 for a coating process in which, in a first step, a metallic reflective layer in the form of an aluminum layer 10 is deposited on the substrate 3 using conventional deposition methods. In a subsequent step on the aluminum layer 10, a dense oxide layer 4, as described in the context of FIG. 1, is deposited on the aluminum layer 10 using a plasma-assisted coating method (see above). Since the aluminum layer 10 is protected from environmental influences by the oxide layer 4, the substrate 3 with the aluminum layer 10 and the oxide layer 4 deposited thereon can be removed from the coating chamber 59 and introduced into a fluorination chamber 61, which is part of a device 60 shown in FIG. 3. After the coating process, the fluorination process described in the context of FIG. 3 takes place in the fluorination chamber 61, in which the oxide layer 4, for example in the form of an MgO layer, is converted into a fluoride or oxyfluoride layer 1.

[0096] In the example shown in FIG. 5b, the deposition of the dense oxide layer 4 is preceded by the deposition of a (further) fluoride layer 11, possibly consisting of, for example, MgF2, on the aluminum layer 10. As explained in the context of FIG. 1, a relatively thin dense oxide layer 4 is deposited on the fluoride layer 11. In a manner similar to the example described in FIG. 5a, the substrate 3 is removed from the coating chamber 59 and transferred to a fluorination chamber 61, where a fluorination process occurs that converts the oxide layer 4 into a fluoride layer 1. In the example shown in FIG. 5b, the oxide layer 4 deposited on the further fluoride layer 11 is relatively thin, typically 5-10 nm, and therefore, unlike the example described in FIG. 5a, is not converted into an oxyfluoride layer but into a fluoride layer 1. However, thicker embodiments of the oxide layer 4 are also possible.

[0097] The optical element 2 described above comprising a fluoride or oxyfluoride layer 1 may be used in different optical setups for the VUV wavelength range.

[0098] 6 shows an optical setup for the VUV wavelength range 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 further comprises a radiation source 24, which may be, for example, an excimer laser.

[0099] Radiation 25 emitted by radiation source 24 is conditioned using illumination system 22 so that a mask 26, also called a reticle, is illuminated by radiation 25. In the illustrated embodiment, illumination system 22 includes a housing 32 in which both transmissive and reflective optical elements are disposed. The illustration representatively shows transmissive optical element 27, which focuses radiation 25, and reflective optical element 28, which deflects the radiation.

[0100] The mask 26 includes a structure on its surface that is to be transferred onto an optical element 29, e.g., a wafer, that is exposed using the projection system 23 to produce semiconductor components. In the illustrated embodiment, the mask 26 is configured as a transmissive optical element. In alternative embodiments, the mask 26 can also be configured as a reflective optical element.

[0101] The projection system 22, in the illustrated embodiment, comprises at least one transmissive optical element, which representatively shows two transmissive optical elements 30, 31 that serve, for example, to reduce structures on the mask 26 to a size desired for exposure of the wafer 29.

[0102] In both illumination system 22 and projection system 23, a variety of transmissive, reflective or other optical elements can be combined with each other as required, including more complex schemes. Optical setups without transmissive optics can also be used for VUV lithography.

[0103] FIG. 7 shows an optical setup for the VUV wavelength range in the form of a wafer inspection system 41, which may 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. For this purpose, the radiation 55 is reflected from a concave mirror 46 onto the wafer 49. In a mask inspection system, the wafer 49 can be replaced by a mask to be inspected. The radiation reflected, deflected, and / or refracted by the wafer 49 is directed via a transmissive optical element 47 and by a further concave mirror 48, also associated with the optical system 42, to a detector 50 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 disposed. The radiation source 54 may be, for example, exactly one radiation source or a combination of several individual radiation sources to provide a substantially continuous radiation spectrum. In a variant, one or more narrowband radiation sources 54 may also be used.

[0104] At least one of the optical elements 27, 28, 30, 31 of the VUV lithography apparatus 21 shown in Figure 6 and at least one of the optical elements 46, 47, 48 of the wafer inspection system 41 shown in Figure 7 are now designed as described above. Thus, at least one of the optical elements 27, 28, 30, 31, 46, 47, 48 includes (at least) one fluoride or oxyfluoride layer formed or fabricated using the method described above.

Claims

1. A method for forming a fluoride or oxyfluoride layer (1) for an optical element (2) for use in the VUV wavelength range, comprising the steps of: depositing an oxide layer (4); converting said oxide layer (4) into said fluoride or oxyfluoride layer (1) by irradiating said oxide layer (4) with UV / VUV radiation (6) in the presence of an active fluorinating agent (FW); A method comprising:

2. 2. The method according to claim 1, characterized in that the oxide layer (4) is deposited using a PVD deposition process, preferably selected from the group comprising sputtering deposition, in particular magnetron sputtering, ion beam sputtering (IBS), ion beam assisted sputtering (IBAS) and plasma ion assisted deposition (PIAD).

3. 2. The method of claim 1, characterized in that the oxide layer (4) is deposited using a CVD deposition process, preferably comprising an atomic layer deposition process (ALD) or a plasma-enhanced atomic layer deposition process (PEALD).

4. 4. The method according to claim 1, wherein the oxide layer (4) is deposited in a coating chamber (59) and the oxide layer (4) is converted into the fluoride or oxyfluoride layer (1) in a fluorination chamber (61) spatially separated from the coating chamber (59).

5. The UV / VUV radiation (6) contributes to the photodissociation of the active fluorinating agent (FW), generating energy (E ph ) is at least the dissociation energy (E diss 5. The method according to claim 1, wherein the first spectral region (14) includes at least one wavelength equal to .lamda..times ...

6. The highest energy (E UP ) is the dissociation energy (E diss 6. The method of claim 5, wherein the saturation is greater than 100% or less, preferably 50% or less.

7. The highest energy (E UP ) is the band gap energy (E G ) or less, and preferably the band gap energy (E G 7. The method according to claim 5, wherein the surface area of ​​the sintered body is 75% or less of the surface area of ​​the sintered body.

8. The UV / VUV radiation (8) comprises a second spectral region (18) for mobilizing atoms at the surface (4), grain boundaries (5') and / or grain volume (5") of the fluoride or oxyfluoride layer (1), the second spectral region (18) being within the band gap energy (E G 8. The method according to claim 1, wherein the energy of the hydroxyl group is in the range of 75% to 100%, preferably 80% to 95%, of the hydroxyl group.

9. The UV / VUV radiation (8) or the further electromagnetic radiation (9) additionally used to irradiate the fluoride or oxyfluoride layer (1) formed during the conversion serves to anneal at least one crystalline defect (8) of the fluoride or oxyfluoride layer (1) and comprises a spectral region (16) which at least partially overlaps with an absorption region (17) of the at least one crystalline defect (8), the spectral region (16) preferably having an absorption energy (E A ) and the mean energy (E m ) is more preferably the absorption energy (E A 9. The method according to claim 1, wherein the α-value of the α-value of the β ...

10. 10. The method according to any one of claims 1 to 9, characterized in that the oxide layer (4) is irradiated in a protective gas atmosphere.

11. The active fluorinating agent (FW) is F 2 , HF, XeF 2 , N.F. 3 , C.F. 4 , SF 6 11. The method according to claim 1, further comprising at least one substance selected from the group consisting of:

12. During the irradiation of the oxide layer (4), the partial pressure (c FW ) is 0.05 to 10 6 The method according to any one of claims 1 to 11, characterized in that the concentration is in ppmV, preferably between 0.075 ppmV and 50 ppmV, more preferably between 0.1 ppmV and 10 ppmV.

13. The oxide layer (4) is a MgO layer, an Al 2 O 3 Layer, La 2 O 3 layer, Gd 2 O 3 13. The method according to claim 1, wherein the metal oxide is deposited in the form of a layer, a CaO layer, a SrO layer or a BaO layer.

14. The oxide layer (4) is MgF 2 layer, Mg x O y F z layer, AlF 3 layer, Al x O y F z layer, LaF 3 Layer, La x O y F z layer, GdF 3 layer, Gd x O y F z layer, CaF 2 layer, Ca x O y F z layer, SrF 2 layer, Sr x O y F z layer, BaF 2 layer or Ba x O y F z 14. The method according to claim 1, wherein the fluoride or oxyfluoride layer (1) is converted into a layer.

15. A method according to any one of the preceding claims, characterized in that a metallic reflective layer, in particular an aluminium layer (10), is deposited on the substrate (3) before the oxide layer (4) is deposited.

16. 12. A method according to claim 11, characterized in that a further fluoride layer (11) is deposited on the substrate (3) before the oxide layer (4) is deposited.

17. An optical element (2) for use in the VUV wavelength range, comprising a fluoride or oxyfluoride layer (1) formed using the method claimed in any one of claims 1 to 16.

18. 1. An optical arrangement for the VUV wavelength range, in particular a VUV lithography apparatus (21) or a wafer inspection system (41), characterized in that it comprises at least one optical element (27, 28, 30, 31, 46, 47, 48) as claimed in claim 12.

19. 1. A device for forming a fluoride or oxyfluoride layer (1) for an optical element (2) designed for use in the VUV wavelength range by converting an oxide layer (4) into said fluoride or oxyfluoride layer (1), comprising: a fluorination chamber (61), a supply unit (62) for supplying the fluorination chamber (61) with an inert gas (IG) and the active fluorination agent (FW), the inside (72) of the fluorination chamber (61) being resistant to the active fluorination agent (FW) and its transformation products; at least one UV / VUV radiation source (63, 70) for irradiating said oxide layer (4) with UV / VUV radiation (8) in the presence of said active fluorinating agent (FW) in said fluorination chamber (61) in order to convert said oxide layer (4) into said fluoride or oxyfluoride layer (1).

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