Method and device for post-treatment of a fluoride layer for optical systems for the VUV wavelength range, and optical elements including said fluoride layer

Abstract

A method and device for post-treatment of a fluoride layer for optical systems for the VUV wavelength range, and optical elements including the fluoride layer, are provided. The present invention relates to a method for post-treating a fluoride layer (1) for an optical element (2) for use in the VUV wavelength range, comprising the step of irradiating the fluoride layer (1) with UV / VUV radiation (8) in the presence of an active fluorinating agent (FW). The present invention also relates to an optical element (2) having a fluoride layer (1) post-treated using this method, and to an optical setup comprising at least one such optical element (2).

Description

[Technical Field]

[0001] REFERENCE TO RELATED APPLICATIONS This application claims priority from German Patent Application Publication No. 102022210512.5, 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 post-treating a fluoride layer for an optical element for use in the VUV wavelength range. The invention also relates to an optical element having a fluoride layer post-treated using this method, and to an optical setup for the VUV wavelength range comprising at least one such optical element. The invention also relates to a device for post-treating a fluoride layer for an optical element designed for use in the VUV wavelength range. [Background technology]

[0003] In the present 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.

[0004] Optical elements with at least one fluoride layer are often used in such devices. 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 radiation is reflected. For example, U.S. Patent Application Publication No. 2017 / 0031067 describes an Al mirror protected by a MgF2 layer. WO 2006 / 053705 describes a protective layer made of thiolite that protects the reflective metal layer from degradation. German Patent Application Publication No. 102018211499 also discloses a reflective optical element and a method for fabricating the same, including a substrate, a metal layer, a metal fluoride layer added to the metal layer, and an oxide layer added to the metal fluoride layer. In this case, the oxide layer reduces degradation at the high radiation intensities used in lithography, thereby extending the service life of the optical element. For the reflective or anti-reflective coating of optical elements, layer stacks consisting of different fluorides or fluorides and oxides may additionally be used.

[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] During the production of optical elements for the VUV wavelength range, fluoride layers are typically deposited using physical vapor deposition (PVD). In particular, these methods include thermal evaporation, electron beam evaporation (EBPVD) and ion beam sputtering (IBS). Already during coating (due to residual oxygen and water in the chamber), or at the latest after the layer has been exposed to ambient air, previously unsaturated bonds, mainly at the surface and possibly at grain boundaries and / or pores, are oxidized by oxygen or water or saturated with OH groups. Thus, oxyfluorides M x O y F z (M = metal atom of fluoride), hydroxyfluoride Mx (OH) y F z Oxidation / hydroxylation, or a mixture of both, occurs near the surface and along grain boundaries and / or pores due to diffusion. This oxidation / hydroxylation generally leads to a narrowing of the band gap and / or the formation of localized defect states near the band edges. These two effects result in increased extinction in the VUV wavelength range.

[0007] To reduce absorption and degradation of the fluoride layer, the prior art proposes post-treatment of the fluoride layer, which means that the treatment is carried out after the deposition of the fluoride layer is completed.

[0008] For example, the paper "Postfluorination of fluoride films for vacuum-ultraviolet lithography to improve their optical properties" by Y. Taki et al., Appl. Opt. 45, 1380 (2006), discloses a method for post-treating fluoride layers (MgF2, AlF3, LaF3) for optical elements in the VUV wavelength range and the corresponding devices. The method described in this paper consists of two steps. In the first step, the fluoride layer is exposed to F2 gas at a temperature of 100 °C, resulting in refluorination of the low-fluorine regions. In the second step, the fluoride layer is re-densified at a higher temperature of 300 °C and a lower F2 concentration. As a result of this post-treatment, the unwanted absorption of VUV radiation in the fluoride layer is significantly reduced. Furthermore, the irradiation stability of the antireflection layer treated in this way, as tested using long-term laser irradiation at 157 nm, is improved.

[0009] A similar method is also disclosed in JP 11140617. In this process, a metal fluoride layer applied to a substrate is heated in a chamber to a temperature of 100°C to 700°C. Then, an inert gas and an activator for fluorination, such as gaseous NF3 or gaseous XeF2, are introduced into the chamber, where the concentration of the activator for fluorination is 1% to 20% by volume, and the metal fluoride layer is exposed to this atmosphere for a predetermined period of time.

[0010] Finally, methods for post-treatment of fluoride layers are also described in US Patent Application Publication No. 2004 / 0006249. Preferably, the post-treatment is carried out at a temperature ranging from 10° C. to 150° C. and at a fluorine concentration of 1000 ppm to 100%.

[0011] However, known post-treatment methods have the disadvantage that they are typically performed at high temperatures or that effective post-treatment requires high temperatures. These high temperatures typically come with several problems. For example, they can result in interdiffusion of layers, which can result in a loss or change of optical performance. High temperatures can also lead to roughening of the metal layer (so-called hillock formation). Finally, high temperatures can also be accompanied by stress and / or relaxation. For example, cracks can occur due to different expansion coefficients of the individual layers and / or the substrate.

[0012] The literature also suggests that post-treatment in the form of post-irradiation with UV light can improve the optical performance of plasma- (ion-)assisted fluoride coatings deposited in the deep UV wavelength range. For example, the paper "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), describes the post-treatment of metal fluoride layers deposited by plasma-assisted electron beam deposition with UV radiation. The paper shows that this post-treatment improved 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 may be 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] Thus, U.S. Patent No. 7,798,096 describes the use of UV light to assist the deposition of high-k dielectrics using chemical vapor deposition or atomic layer deposition, where UV light is used to excite or ionize process gases and thereby initiate or enhance surface reactions during deposition.

[0017] DE 102018221190 A1 further discloses 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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]

[0023] In contrast, the object addressed by the present invention is to provide a method for post-treatment of fluoride layers for the VUV wavelength range and corresponding devices, in which the post-treated fluoride layers have high optical performance, in particular low, long-term stable absorption. [Means for solving the problem]

[0024] According to a first aspect, this object is achieved by a method for post-treating a fluoride layer for an optical element for use in the VUV wavelength range, the method comprising the step of irradiating the fluoride layer with UV / VUV radiation in the presence of an active fluorinating agent.

[0025] 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).

[0026] The active fluorinating agent is preferably a gaseous substance which, as a result of irradiation with UV / VUV radiation, forms molecules and / or atoms, in particular ionized and / or excited fluorine (hereinafter collectively referred to as fluorine species). The fluorine species thus formed cause refluorination of oxyfluorides / hydroxyfluorides at the surface or within the fluoride layer.

[0027] As a result of the refluorination, the reflectance (or transmittance) of the optical element is significantly improved and stabilized over time, which leads to a significant increase in the system transmittance of optical arrangements for the VUV wavelength range, which have at least one optical arrangement with a fluoride layer post-treated in this way, and thereby to higher throughput, for example in the case of VUV microlithography devices.

[0028] This also significantly reduces the heating of the optical elements during operation (also known as "lens heating"). This primarily leads to a reduction in imaging aberrations caused by lens heating. In addition, the thermal activation process that accelerates the degradation of optical elements proceeds more slowly, which concomitantly extends the useful life of the optical elements. Specifically, such thermal activation processes include Arrhenius activation, i.e., D∝exp(-E D / k B T), where D is the diffusion constant and E D is the activation energy for diffusion, and k B is the Boltzmann constant and T is the temperature.

[0029] Compared to known methods for post-treating fluoride layers, the method according to the invention is advantageous in that it can be carried out without high temperatures and the associated disadvantages can be avoided.

[0030] In a variation of this method, UV / VUV radiation is emitted with energy E ph is at least the dissociation energy E of the active fluorinating agent diss and a first spectral region for photodissociation of the active fluorinating agent that includes at least one wavelength equal to the dissociation energy E dissrepresents the energy required to split the chemical bond of the active fluorinating agent by electromagnetic radiation (light).

[0031] 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

[0032] 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 ≦E UP 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.

[0033] In a further development of this variant, the highest energy E UP is the dissociation energy E of the active fluorinating agent diss is no more than 100%, preferably no more than 50% greater than the

[0034] In a further variation of this method, the highest energy (E UP ) is the band gap energy E of the fluoride layer G Hereinafter, the band gap energy E of the fluoride layer is preferably G This reduces the light absorption in the solid.

[0035] 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.

[0036] 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 fluoride 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 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.

[0037] High-energy electromagnetic radiation near the band edge of fluorides allows surface atoms or atoms to be mobilized without being desorbed, as described, for example, in DE 102018221190 cited in the Abstract. 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 fluoride 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.

[0038] In a further variant of this method, the fluoride layer is an AlF3 layer. While this method is in principle suitable for the post-treatment of any type of fluoride layer (e.g., MgF2, LaF3, ...), it has been found to be particularly advantageous in the case of AlF3 layers. The reason for this appears to be the special structure of thin AlF3 layers. AlF3 is one of the few fluorides that form an x-ray amorphous structure. This structure consists locally of different, energetically similar AlF3 polymorphs. These various structural motifs are connected to each other via their edges. The resulting significant disorder leads to many unsaturated bonds, which are passivated by oxygen or hydroxyl groups. These AlF3 molecules, which are intrinsically present in the deposited AlF3 thin film after the coating process, are then transferred to the AlF3 film. x O y F z Agents and Al x OH y F z The agent can be substantially converted to AlF3 using the method according to the invention, with a corresponding improvement in optical performance in the VUV wavelength range.

[0039] The method is also suitable for post-treatment of fluoride layers containing at least one fluoride from the following group: magnesium fluoride, aluminum fluoride, sodium fluoride, lithium fluoride, thiolite, cryolite, calcium fluoride, erbium fluoride, neodymium fluoride, gadolinium fluoride, dysprosium fluoride, samarium fluoride, holmium fluoride, hafnium fluoride, lanthanum fluoride, europium fluoride, lutetium fluoride, cerium fluoride, barium fluoride, strontium fluoride, yttrium fluoride.

[0040] In a further variant of this method, the UV / VUV radiation or further electromagnetic radiation additionally used to irradiate the fluoride layer comprises a spectral region that at least partially overlaps with the absorption region of at least one crystalline defect for annealing at least one crystalline defect of the fluoride layer, this spectral region preferably comprising the absorption energy of the crystalline defect, the average energy of this spectral region 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.

[0041] A potential problem with irradiation with VUV radiation is that it can create crystalline defects in fluorides by single-photon processes, particularly F / H central defect pairs, which can be annealed using irradiation in a spectral region that at least partially overlaps with the absorption region.

[0042] 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).

[0043] In a further variant of this method, the fluoride 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, especially mixtures of the above noble gases, can also be used as protective gases. The fluoride layer is typically irradiated in a post-treatment chamber with a protective gas atmosphere inside.

[0044] In a further variant of this method, the oxygen concentration around the fluoride layer upon irradiation is less than 10 ppmV, preferably less than 1 ppmV, more preferably less than 100 ppbV, especially less than 50 ppbV. Upon irradiation, the oxygen concentration around the fluoride layer should be as low as possible.

[0045] In a further variation of this method, the HO concentration at the time of irradiation of the fluoride layer is less than 10 ppmV, preferably less than 1 ppmV, more preferably less than 500 ppbV, especially less than 100 ppbV, around the fluoride layer. The problem with higher HO concentrations is that water is a source of hydrogen under photodissociation which reacts with fluorine formed from the active fluorinating agent to form HF, for example:

[0046]

number

[0047] Also, sources of light contamination are preferably eliminated from the process to the greatest extent possible, this being especially true for carbon compounds and silanes.

[0048] 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, SF6.

[0049] In a further variant of this method, the partial pressure of the active fluorinating agent during irradiation of the fluoride layer is 0.05 to 10 6 ppmV, preferably 0.075 ppmV to 50 ppmV, and more preferably 0.1 ppmV to 10 ppmV.

[0050] In a further variant of this method, the partial pressure of the active fluorinating agent is adjusted to a target value during irradiation of the fluoride layer.

[0051] In a further variant of this method, the fluoride layer is heated during irradiation. The refluorination of the fluoride layer is assisted by heating. However, compared to the known methods for refluorination detailed above, a better fluorination effect is obtained at the same temperature, or the temperature may be selected to be substantially lower.

[0052] Also, the fluorination process is preferably assisted by using a plasma.

[0053] A further aspect of the invention relates to an optical element for use in the VUV wavelength range that includes a fluoride layer that has been or will be post-treated using the method described above.

[0054] The optical element is specifically a reflective optical element. For example, the optical element may include a metal layer applied to the substrate to reflect electromagnetic radiation in the VUV wavelength range, with a fluoride layer then being applied to the metal layer to protect it. Post-treatment using the post-treatment method according to the present invention results in improved optical performance of the optical element during processing. Specifically, its reflectance in the VUV wavelength range is significantly higher than before post-treatment, and it is stable against environmental influences. However, the optical element may also be a transmissive optical element to which a dielectric multilayer coating containing one or more fluoride layers is applied. For example, the multilayer coating may have an anti-reflection function.

[0055] 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.

[0056] Finally, a further aspect of the present invention relates to a device for post-treating a fluoride layer for an optical element designed for use in the VUV wavelength range, comprising a post-treatment chamber, a supply unit for supplying an inert gas and an active fluorinating agent to the post-treatment 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 fluoride layer with UV / VUV radiation in the post-treatment chamber in the presence of the active fluorinating agent. Regarding the advantages obtained by this device, reference is made to the above description of the method and its variants.

[0057] 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 refluorination of the fluoride layer.

[0058] The UV / VUV radiation source preferably has a first spectral region for photodissociation of the active fluorinating agent. The wavelength of the radiation emitted by the UV / VUV radiation source is preferably 115 nm to 1000 nm, more preferably 120 nm to 170 nm, and particularly preferably 140 nm to 170 nm. For example, the UV / VUV radiation source may include 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.

[0059] The post-treatment chamber can be hermetically sealed. The conversion products of the active fluorinating 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 post-treatment chamber. In particular, there should be no formation of volatile fluorine compounds that could precipitate on the optical elements. The post-treatment chamber, and in particular its interior, can be at least partially manufactured from a metallic material, which should typically be free of Cr and Ti, for example, to prevent corrosion. The post-treatment chamber may specifically be manufactured from Monel steel.

[0060] Alternatively, the inside of the post-treatment 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, NiP, Pt or Ru / Rh mixtures.

[0061] The supply unit is preferably designed to introduce the active fluorinating agent diluted in an inert gas into the after-treatment chamber, and may include a suitable metering control valve, in particular a mass flow controller, to set the partial pressure of the active fluorinating agent in the after-treatment chamber to the desired value.

[0062] 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.

[0063] 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.

[0064] Preferably, the device also includes one or more additional fluorine-resistant sensors for determining the O2 and / or H2O partial pressure in the post-treatment chamber.

[0065] 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 fluoride layer as detailed 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.

[0066] To anneal at least one crystalline defect in the fluoride 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 the at least one crystalline defect and that further 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 layer and / or substrate during refluorination. 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 may 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 may be used for the fluoride to be refluorinated or for each of the fluorides to be refluorinated.

[0067] 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.

[0068] Exemplary embodiments are shown in the schematic diagrams and explained in the following description, with details as follows: [Brief explanation of the drawings]

[0069] [Figure 1] FIG. 1 is a schematic diagram illustrating the post-treatment of a fluoride layer for an optical element for use in the VUV wavelength range using a post-treatment method according to the present invention. [Figure 2] FIG. 1 is a schematic diagram showing the absorption and spectral regions associated with the emission of a fluoride layer using a post-treatment method. [Figure 3]FIG. 1 is a schematic diagram illustrating a device for post-treating a fluoride layer for an optical element designed for use in the VUV wavelength range. [Figure 4] 1 is a schematic diagram showing an optical setup for the VUV wavelength range in the form of a VUV lithography apparatus; [Figure 5] FIG. 1 is a schematic diagram showing an optical setup for the VUV wavelength range in the form of a wafer inspection system. [Figure 6] 1 is a schematic diagram showing an optical element having a fluoride layer post-treated using a method according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0070] 1 shows a schematic diagram of the post-treatment of a fluoride layer 1 of an optical element 2 for use in the VUV wavelength range. The fluoride layer 1 of the optical element, which has been applied to a substrate 3, is illustrated in three snapshots M1, M2, M3 before (snapshot M1), during (snapshot M2), and after (snapshot M3) the post-treatment, in each case showing only a small portion of the surface 4 of the fluoride layer 1 that is exposed to the environment.

[0071] Prior to post-processing (initial snapshot M1), a mixture of oxyfluorides and / or hydroxyfluorides is present on the surface 4 and along the grain boundaries 5 of the fluoride layer 1 due to, for example, exposure of the fluoride layer 1 to ambient air. Specifically, the fluoride layer 1 is superficially oxidized due to saturation of previously unsaturated bonds, and has defect-rich grain boundaries 5 with unsaturated bonds and / or bonds saturated by O or OH from the atmosphere. These at least partially oxidized or defect-rich regions 6 adversely affect the optical performance of the optical element 1. In contrast, stoichiometric fluoride is typically present in the grain volume 7.

[0072] For post-treatment, the fluoride layer 1 or the substrate 3 to which the fluoride layer 1 has been applied is first transferred into a post-treatment chamber not shown in FIG. 1. Then, as shown in the second snapshot M2, the fluoride layer 1 is irradiated with UV / VUV radiation 8 in the presence of an active fluorinating agent FW. As a result of the irradiation, the active fluorinating agent FW dissociates, forming fluorine species F, F2, F * The fluorine species F, F2, and F * reacts with at least partially oxidized or highly defective regions 6, forming fluorides therein. For example, the active fluorinating agent FW is NF3, which upon photodissociation produces fluorine species F, F2, and 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.

[0073] The fluoride layer 1 is also irradiated with further electromagnetic radiation 9 during post-treatment in the illustrated embodiment, although this is not essential, which serves the purpose of annealing crystalline defects 10 in the fluoride layer.

[0074] 1, the fluoride layer 1 may also be heated during irradiation, but heating is not an essential component of this method.

[0075] The at least partially oxidized or highly defective regions 6 of the fluoride layer 1 are refluorinated after post-treatment (third snapshot M3). Now, (at least approximately) stoichiometric fluoride is present there as well. As a result, the optical performance of the optical element 2 is significantly improved and relatively stable with respect to environmental influences.

[0076] Figure 2 shows the absorption and spectral regions associated with the irradiation of the fluoride layer 1. 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 , the absorption cross section 12 of the fluoride layer 1 including the Urbach tail 12', and the absorption cross section 13 of the crystal defect 10 within the fluoride layer 1 are shown schematically.

[0077] The VUV radiation 8 used to irradiate the fluoride layer 1 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 at least the dissociation energy E of the active fluorinating agent FW diss and the highest energy E of the first spectral region 14. UP is used here for example, but not necessarily, to denote the dissociation energy E of the active fluorinating agent FW. diss The highest energy E of the first spectral region 14 is less than 50% greater than the UP is the band gap energy E of the fluoride layer 1 G and preferably the band gap energy E G is less than 75% of the

[0078] The fluoride layer 1 is further irradiated with electromagnetic radiation 9, for example to anneal at least one crystalline defect 10 in the fluoride layer 1. To this end, the further electromagnetic radiation 9 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 crystalline defect 10, which is an F-center, although this is not necessary. In this alternative, the UV / VUV radiation 8 may include the corresponding spectral region.

[0079] 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 is the absorbed energy E of the crystal defect 10. AIt is advantageous if the deviation does not exceed 0.5 eV, and in particular does not exceed 0.25 eV.

[0080] The UV / VUV radiation 8 also includes a second spectral region 18 for mobilizing atoms at the surface 4, grain boundaries 5 and / or grain volume 7 of the fluoride layer 1. 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 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 layer 1. G It may be 80% to 95% of the above.

[0081] Figure 3 shows a device 60 for post-treating a fluoride layer 1 for an optical element 2 of Figure 1 using the post-treatment method described above. The device 60 includes a post-treatment chamber 61, a supply unit 62 and a first UV / VUV radiation source 63.

[0082] In this case, by way of example, an optical element 2 comprising a fluoride layer 1 applied to a substrate 3 is mounted on a substrate holder 64 that is rotatable about an axis of rotation 65 within a post-treatment chamber 61. However, deviating from the example shown, the device 1 may not include a rotatable substrate holder 64.

[0083] The supply unit 62 serves the purpose of supplying the after-treatment 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 fluoride layer 1 can be irradiated in the protective gas atmosphere in the after-treatment 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 after-treatment. 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.

[0084] The first UV / VUV radiation source 63 serves to irradiate the fluoride layer 1 with UV / VUV radiation 8 in the presence of the active fluorinating agent FW in the post-treatment chamber 61. For example, in the illustrated embodiment, the UV / VUV radiation 8 enters the post-treatment chamber 61 through an MgF2 window 69. The first UV / VUV radiation source 63 serves to generate UV / VUV radiation 8 in the first spectral region 14 detailed above.

[0085] For example, device 60 in this case may, but does not necessarily, include a second UV / VUV radiation source 70 for irradiating fluoride layer 1 with UV / VUV radiation 8 in the second spectral region 18 detailed above to mobilize atoms at surface 4, grain boundaries 5, and / or grain volume 7 of fluoride layer 1. In the illustrated embodiment, UV / VUV radiation from second UV / VUV radiation source 8 enters post-treatment chamber 61 through MgF2 window 69'. Device 60 also includes a further radiation source 71 for irradiating fluoride layer 1 with further electromagnetic radiation 9 in the spectral region 17 detailed above in the context of FIG. 2 to anneal at least one crystalline defect 10 in fluoride layer 1. Further electromagnetic radiation 9 from further radiation source 71 enters post-treatment chamber 71 through a further MgF2 window 69''.

[0086] 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.

[0087] The post-treatment chamber 61 can be sealed in an airtight manner. Furthermore, the interior 72 of the post-treatment chamber 61 is resistant to the active fluorinating agent FW and its conversion products. To this end, in the illustrated embodiment, the post-treatment chamber 61, or at least its interior 72, is made of metal in the form of Monel steel, which forms a passivation layer to prevent corrosion. In principle, the post-treatment chamber 61 can also be made of other corrosion-resistant materials, provided they do not contain Cr and Ti.

[0088] 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 post-treatment chamber 61. The corrosion-resistant coating can be applied to the interior 72 of the post-treatment chamber 61, for example, by a galvanic process. Components located within the post-treatment 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.

[0089] Also, the device 60 shown here may, for example, but not necessarily, reduce the oxygen concentration c O2 and a sensor 73 for measuring the H2O concentration c H2O and a further sensor 74 for measuring

[0090] For example, the oxygen concentration c in the post-treatment chamber 61 during irradiation of the fluoride layer 1 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. O2is advantageously less than 10 ppmV, preferably less than 1 ppmV, more preferably less than 100 ppbV.

[0091] In the illustrated embodiment, the H2O concentration c H2O is less than the concentration of H2O in the post-treatment chamber 61 during irradiation of the fluoride layer 1. 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.

[0092] The device 60, in the illustrated embodiment, but not necessarily, controls the partial pressure c of the active fluorinating agent FW in the post-treatment chamber 61. FW and a sensor 75 for measuring the partial pressure c of the active fluorinating agent FW in the post-treatment chamber 61. FW and a closed-loop controller 76 for controlling the partial pressure c of the active fluorinating agent FW in the post-treatment 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 only to measure the partial pressure of the active fluorinating agent FW in the after-treatment chamber 61, but may also be a residual gas analyzer capable of determining the partial pressure of other gases contained in the after-treatment chamber 21. Such a residual gas analyzer may perform the functions of the three sensors 73, 74, and 75 shown in FIG. 3. If the second valve 67 is a metering control valve, e.g., a mass flow control valve, the partial pressure c of the active fluorinating agent FW in the after-treatment chamber 61 may be determined. FW It is possible to omit the use of the sensor 75 for measuring

[0093] 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 fluoride layer 1 FW is typically 0.05 to 10 6ppmV, preferably 0.075 ppmV to 50 ppmV, and more preferably 0.1 ppmV to 10 ppmV.

[0094] 4 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] FIG. 5 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.

[0100] At least one of the optical elements 27, 28, 30, 31 of the VUV lithography apparatus 21 shown in Figure 5 and at least one of the optical elements 46, 47, 48 of the wafer inspection system 41 shown in Figure 6 are now designed as detailed above. Accordingly, at least one of the optical elements 27, 28, 30, 31 includes at least one fluoride layer that has been post-treated using the method described above.

[0101] 6 shows an optical element 2 for the VUV wavelength range in the form of an Al mirror, comprising an Al layer 90 applied to a substrate 3 and protected by a fluoride layer 1 in the form of an AlF3 layer. The fluoride layer 1 has been post-treated using the method described above, resulting in an improved reflectivity of the optical element 2 and reduced degradation during operation of the optical element 2.

Claims

1. A method for post-treating a fluoride layer (1) for an optical element (2) for the VUV wavelength range, comprising the steps of: a method comprising a step of irradiating said fluoride layer (1) with UV / VUV radiation (8) in the presence of an active fluorinating agent (FW).

2. The UV / VUV radiation (8) has an energy (E ph ) is at least the dissociation energy (E diss 2. The method of claim 1, further comprising a first spectral region (14) for photodissociation of the active fluorinating agent (FW) comprising at least one wavelength equal to .lambda.

3. The highest energy (E UP ) is the dissociation energy (E diss 3. The method of claim 2, wherein the saturation voltage is not more than 100%, preferably not more than 50%, greater than the saturation voltage.

4. The highest energy (E UP ) is the band gap energy (E G ) or less, preferably the band gap energy (E G 4. The method according to claim 2 or 3, characterized in that the surface roughness is 75% or less of the surface roughness.

5. 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 (7) of the fluoride layer (1), the second spectral region (18) being within the band gap energy (E G 5. 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 that of the hydroxyl group.

6. The fluoride layer (1) is AlF 3 The method according to any one of claims 1 to 5, characterized in that it is a layer.

7. The UV / VUV radiation (8) or the further electromagnetic radiation (9) additionally used to irradiate the fluoride layer (1) comprises a spectral region (16) that at least partially overlaps with an absorption region (17) of the at least one crystalline defect (10) for annealing the at least one crystalline defect (10) of the fluoride layer (1), and the spectral region (16) preferably has an absorption energy (E A ) and the mean energy (E m ) is more preferably the absorption energy (E A 7. The method according to claim 1, wherein the α-value of the α-value of the β ...

8. 8. The method according to any one of claims 1 to 7, characterized in that the fluoride layer (1) is irradiated in a protective gas atmosphere.

9. The oxygen concentration (C o2 9. The method according to claim 1, wherein the ion concentration of the ion source is less than 10 ppmV, preferably less than 1 ppmV, more preferably less than 100 ppbV, in particular less than 50 ppbV.

10. H during the irradiation of the fluoride layer (1) 2 O concentration (c H2o 10. The method according to claim 1, wherein the ion concentration of the ion source is less than 10 ppmV, preferably less than 1 ppmV, more preferably less than 500 ppbV, in particular less than 100 ppbV.

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. The partial pressure (c) of the active fluorinating agent (FW) during the irradiation of the fluoride layer (1) 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 partial pressure (c) of the active fluorinating agent (FW) during the irradiation of the fluoride layer (1) FW 13. The method according to claim 1, wherein the temperature is adjusted to a target value.

14. Method according to any one of the preceding claims, characterized in that the fluoride layer (1) is heated during the irradiation.

15. An optical element (2) for use in the VUV wavelength range, comprising a fluoride layer (1) post-treated using the method according to any one of claims 1 to 14.

16. An optical setup for the VUV wavelength range, in particular a VUV lithography apparatus (21) or a wafer inspection system (41), comprising at least one optical element (27, 28, 30, 31, 46, 47, 48) according to claim 14.

17. A device (60) for post-treating a fluoride layer (1) for an optical element (2) designed for use in the VUV wavelength range, comprising: - a post-treatment chamber (61), a supply unit (62) for supplying an inert gas (IG) and an active fluorinating agent (FW) to said post-treatment chamber (61) whose inside (72) is resistant to said active fluorinating agent (FW) and its transformation products; at least one UV / VUV radiation source (63, 70) for irradiating said fluoride layer (1) with UV / VUV radiation (8) in the presence of said active fluorinating agent (FW) in said post-treatment chamber (61); A device (60) comprising:

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