Method and device for producing protective fluoride coatings for reflective optical elements

Abstract

The present invention relates to a method for preparing a fluoride protective coating (11) for protecting a metal reflective layer (12) of a reflective optical element (13) for the VUV wavelength range, comprising the step of irradiating a native oxide layer (16) of the metal reflective layer (12) with UV / VUV radiation (17) in the presence of a fluorinating agent (FW) to convert the native oxide layer (16) into a fluoride layer (18) of the fluoride protective coating (11). The present invention also relates to a reflective optical element (13) having a fluoride protective coating (11) prepared using this method, and to an optical setup having at least one such reflective optical element (13).

Description

[Technical Field]

[0001] This application claims priority from German Patent Application No. 102022210514.1, filed October 5, 2022, the entire disclosure content of which is incorporated by reference into the content of this application.

[0002] The present invention relates to a method and device for producing a fluoride protective coating for protecting a metallic reflective layer of a reflective optical element for use in the VUV wavelength range. The present invention also relates to a reflective optical element for use in the VUV wavelength range having a fluoride protective coating produced using this method, and to an optical setup for the VUV wavelength range comprising at least one such reflective optical element. [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, radiation in the VUV wavelength range is used, for example, in projection exposure apparatus and wafer or mask inspection apparatus.

[0004] Such devices often use reflective optical elements with a metallic reflective layer and a protective fluoride coating. To achieve high reflectivity, metallic reflective layers, particularly aluminum, are typically fabricated using physical vapor deposition, specifically electron beam evaporation. Since high temperatures adversely affect reflectivity, aluminum is typically deposited on an unheated substrate.

[0005] Fluoride protective coatings are formed at least partially, and particularly completely, from fluorides. Unlike oxides, fluorides have the advantage of being sufficiently transparent in the VUV wavelength range. In particular, fluoride protective coatings can serve to protect underlying metal reflective layers from oxidation. For example, fluoride protective coatings can be used to suppress the formation of a native oxide layer on a metal reflective layer made of aluminum, which would lead to a significant loss of reflectivity due to the high degree of extinction of aluminum oxide in the VUV wavelength range.

[0006] For example, WO 2006 / 053705 discloses the use of a protective layer made of thiolite to protect against deterioration of a reflective metal layer made of aluminum and serving to reflect wavelengths between 120 nm and 260 nm. Furthermore, German Patent Application Publication No. 102018211499 also 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 manufacturing the same.

[0007] Fluoride layers are typically deposited using physical vapor deposition (PVD). These methods include thermal evaporation, electron beam evaporation (EBE), and ion beam sputtering (IBS). Deposition is typically performed at high temperatures, and the substrate may be heated to obtain the densest possible protective coating with the lowest possible absorption. However, heating an unprotected aluminum layer can result in significant loss of reflectivity due to the formation of a highly absorbing native aluminum oxide layer, especially in the VUV wavelength range, and possibly contamination.

[0008] A method for fabricating MgF2-protected Al mirrors that partially overcomes this problem is disclosed in U.S. Patent Application Publication No. 2017 / 0031067 and in a paper titled "Enhanced MgF2 and LiF Over-Coated Al Mirrors for FUV Space Astronomy" by M.A. Quijada et al., Proc. SPIE 8450, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation II, 84502H (2012). The method described there consists of three steps. First, a substrate is coated with aluminum to the desired thickness at room temperature. Then, as quickly as possible, the aluminum layer is coated with a 4-5 nm thick MgF2 layer at room temperature to protect the aluminum from oxidation and contamination. Finally, the substrate is heated, and additional MgF2 is deposited on the thin MgF2 layer to the desired thickness.

[0009] However, the formation of an oxide layer cannot be completely avoided using this procedure, as can be seen, for example, from the following considerations: commercial PVD coating chambers without locks (typically called "load locks") are generally operated at high vacuum. In these cases, the residual gas pressure before coating is 10 -3 mbar~10 -8 mbar, typically about 10 -6 mbar~10 -7 According to the Langmuir adsorption model, the time period during which the formation of a monolayer of adsorbed material on the layer occurs is 10 -6 mbar~10 -7 Assuming a residual gas pressure of mbar, this is in the range of single digit seconds. Therefore, even at such low pressures, a monolayer of adsorbed material forms very quickly. Such adsorbed material (H2O and O2) oxidizes the aluminum layer, resulting in a significant loss of reflectivity despite the low temperature of the protective layer.

[0010] The prior art discloses several additional methods for producing fluoride layers or optical elements containing fluoride layers.

[0011] 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.

[0012] 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.

[0013] Further variations of physical vapor deposition of fluoride layers are also described in US Patent Application Publication No. 2013 / 0122252, JP Patent Publication No. 11172421 and JP Patent Publication No. 2003193231.

[0014] In addition to methods for fabricating fluoride layers or optical elements containing fluoride layers, the prior art also proposes post-treatment of fluoride layers, which means that the treatment is carried out after the deposition of the fluoride layer is complete. The post-treatment typically serves to reduce the absorption of the fluoride layer and its degradation.

[0015] For example, U.S. Patent Application Publication No. 2004 / 0006249 describes a fluorination post-treatment method for fluoride layers, in which the fluoride layer is exposed to gaseous fluorine at temperatures between 10° C. and 150° C. and fluorine concentrations between 1000 ppm and 100%. As a result of the post-treatment, the composition of the fluoride layer approaches stoichiometry.

[0016] German Patent Application Publication No. 102021200490 also describes post-treatment of metal fluoride layers by irradiation, using electromagnetic radiation having at least one wavelength below 300 nm. The metal fluoride layer is applied to a metal layer of a reflective optical element for use in the VUV wavelength range. Irradiation results in passivation of the metal fluoride layer, which inhibits degradation of the metal layer. However, the passivation protective layer is typically an oxide layer, which leads to the aforementioned drawbacks.

[0017] Finally, the prior art proposes using UV light as assistance during the deposition of layers for nanostructuring or irradiation of optical elements during operation of a microlithography apparatus.

[0018] In this regard, U.S. Patent No. 7,798,096 describes the use of UV light to assist in the deposition of high-k dielectrics with good layer quality 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.

[0019] Furthermore, 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. The irradiation allows the surface of the MgF2 single crystal to be reorganized so that an anti-reflection effect occurs.

[0020] 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, in which the optical elements are irradiated with UV light having a wavelength greater than the wavelength of the operating light of the optical system of 300 nm or less in order to anneal fluoride defects. Summary of the Invention [Problem to be solved by the invention]

[0021] In light of the above background, it is an object of the present invention to provide a method for producing a fluoride protective coating for protecting a metallic reflective layer of a reflective optical element for the VUV wavelength range, which method achieves the highest possible reflectivity of the reflective optical element in the VUV wavelength range. [Means for solving the problem]

[0022] According to a first aspect, this object is achieved by a method for producing a fluoride protective coating for protecting a metallic reflective layer of a reflective optical element for the VUV wavelength range, the method comprising the step of irradiating a native oxide layer of the metallic reflective layer with UV / VUV radiation in the presence of an active fluorinating agent to convert the native oxide layer into a fluoride layer of the fluoride protective coating.

[0023] 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 native oxide 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).

[0024] The active fluorinating agent is preferably a gaseous substance that forms molecules and / or atoms, particularly ionized and / or excited fluorine (hereinafter collectively referred to as fluorine species), upon irradiation with UV / VUV radiation. The fluorine species thus formed replace oxygen molecules in the native oxide layer, thus resulting in the conversion of the native oxide layer to a fluoride layer. Thus, unlike the method described in the above-cited paper by A. Quijada et al., the formation of the native oxide layer is not minimized; instead, the native oxide layer is converted to a fluoride layer.

[0025] In addition to photodissociation of the active fluorinating agent, the UV / VUV radiation can also serve to provide the activation energy for the fluorination process, i.e., the fluorination of the native oxide layer. The spectrum of the UV / VUV radiation is preferably adapted to the activation energy of the fluorination process.

[0026] The method according to the invention leads to a significant increase in the reflectivity of the reflective optical element, especially in the VUV wavelength range, due to the conversion of the native oxide layer, which is highly absorbing in particular in the VUV wavelength range, into a fluoride layer which is significantly less absorbing.

[0027] The increased reflectivity is accompanied by a significant increase in system transmittance of optical setups for the VUV wavelength range having at least one reflective optical element with such a fluoride protective coating, thereby resulting in higher throughput, e.g., in the case of VUV microlithography equipment.

[0028] Lower absorption also significantly reduces the thermal load, i.e., the heating of optical elements during operation (also called "lens heating"). This primarily leads to a reduction in imaging aberrations caused by lens heating. Furthermore, reduced lens heating concomitantly extends the useful life of reflective optical elements, since thermal activation processes that accelerate the degradation of optical elements proceed more slowly. In particular, such thermal activation processes include Arrhenius activation, i.e., D∝exp(-E D / k BT), 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. This type of diffusion process can increase the roughening of, for example, a metallic reflective layer, which leads to higher scattered light losses. Therefore, the reflective optical element can be replaced less frequently compared to the prior art.

[0029] A further advantage of the method according to the invention is that the fluoride layer has a higher density compared to fluoride layers deposited at room temperature, in particular compared to MgF2 layers deposited at room temperature as described in the above-cited paper by A. Quijada et al. This may be the result, in particular, of the increased atomic mobility due to UV / VUV radiation.

[0030] In a variation of this method, UV / VUV radiation contributes to the photodissociation of the active fluorinating agent, resulting in the formation of a fluorinated compound with 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 bonds 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. UPis 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 of the first spectral region is UP is the band gap energy E of the formed fluoride layer G Hereinafter, the band gap energy E of the fluoride layer preferably formed G This may reduce light absorption in the fluoride layer and thus reduce the formation of point defects (e.g., F centers).

[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 comprises a second spectral range for mobilizing atoms at the surface and / or in the volume (i.e. at the grain boundaries and / or in the grain volume) of the fluoride layer being formed, the second spectral range being in an energy range of 75% to 100%, preferably 80% to 95%, of the band gap energy of the fluoride layer.

[0037] UV / VUV radiation near the band edge of the corresponding fluoride allows surface atoms or atoms in the bulk to be mobilized without these atoms being desorbed, as described, for example, in the cited German patent application DE 102018221190. This increased mobility can contribute to the creation of dense and smooth fluoride layers.

[0038] In a further variant, the UV / VUV radiation or further electromagnetic radiation additionally used to irradiate the formed fluoride layer serves to anneal at least one crystalline defect in the formed fluoride layer and includes a spectral region that at least partially overlaps with the absorption region of said at least one crystalline defect, which spectral region preferably includes the absorption energy of the crystalline defect, the average energy of the 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 native oxide layer to the fluoride layer.

[0039] 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.

[0040] 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).

[0041] In a further variation of this method, the metal reflective layer is an aluminum layer. In this case, the fluoride layer is specifically formed from AlF3. AlF3 has the advantage that it typically forms a very smooth layer because it generally has an x-ray amorphous structure. Furthermore, the aluminum layer has high reflectivity in the VUV wavelength range.

[0042] In a further variant of this method, the native oxide layer is irradiated in a protective gas atmosphere, preferably in vacuum. The protective gas is preferably transparent to electromagnetic radiation in the UV / VUV wavelength range. Also, a protective gas that is relatively unreactive with optically relevant oxides and fluorides is 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. The irradiation of the native oxide layer is typically carried out in a process chamber filled with a protective gas atmosphere. The protective gas atmosphere can be generated by introducing (metered-in) an active fluorinating agent in the form of a protective gas, typically in the form of an inert gas, into the evacuated interior of the process chamber. Alternatively, the irradiation can be carried out in a process chamber with a prevailing protective gas atmosphere at higher pressure, for example, atmospheric pressure. In this case, an active fluorinating agent is additionally introduced into the process chamber at the existing atmospheric pressure.

[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, SF6.

[0044] In a further variation of this method, the partial pressure of the active fluorinating agent during irradiation of the native oxide layer is 10 -8 mbar to 10 mbar, preferably 10 -6 mbar~10 -1 mbar.

[0045] In a further variation of this method, the partial pressure of the active fluorinating agent is adjusted to a target value during irradiation of the native oxide layer.

[0046] In a further variation, the method further comprises depositing a further fluoride layer of the fluoride protective coating on the fluoride layer, the further fluoride layer being deposited after conversion of the native oxide layer to the fluoride layer is complete, and the further fluoride layer may be deposited, in particular, using physical vapor deposition.

[0047] As mentioned above, the fluoride layer that results from irradiating a native oxide layer with UV / VUV radiation in the presence of an active fluorinating agent is particularly dense and smooth. Both high density and low roughness promote the growth of this further fluoride layer. In particular, this further fluoride layer is consequently particularly dense and smooth as well.

[0048] A further aspect of the invention relates to a reflective optical element for the VUV wavelength range that includes a protective fluoride coating made using the method described above.

[0049] 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 reflective 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.

[0050] Finally, a further aspect of the present invention relates to a device for producing a fluoride protective coating for protecting a metal reflective layer of a reflective optical element for the VUV wavelength range, the device comprising: a process chamber; a supply unit for supplying an inert gas and an active fluorinating agent into the process chamber, the interior of which is resistant to the active fluorinating agent and its conversion products; and a UV / VUV radiation source for irradiating the native oxide layer of the metal reflective layer with UV / VUV radiation in the presence of the active fluorinating agent in order to convert the native oxide layer into a fluoride layer of the fluoride protective coating. Regarding the advantages obtained by this device, reference is made to the above description of the method according to the present invention and its variants.

[0051] The UV / VUV radiation emitted by the UV / VUV radiation source is partially absorbed by the active fluorinating agent. The resulting fluorine species formed by photodissociation of the active fluorinating agent result in the conversion of the native oxide layer to a fluoride layer. Furthermore, the UV / VUV radiation source can also contribute to providing activation energy for the fluorination process and / or emitting UV / VUV radiation for UV / VUV radiation-driven mobilization of atoms at the surface of the resulting fluoride layer. However, the purpose of the UV / VUV radiation source does not necessarily have to be to perform functions other than photodissociation of the active fluorinating agent. Also, a respective additional UV / VUV radiation source can be used to provide activation energy and mobilize atoms. Therefore, in addition to the UV / VUV radiation source, the device preferably has one or two additional UV / VUV radiation sources and / or additional radiation sources for generating additional electromagnetic radiation, for example, to anneal crystal defects or to provide activation energy for the fluorination process.

[0052] The process 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 to be interpreted in particular as meaning the formation of a passivation layer inside the process chamber. In particular, there must be no formation of volatile fluorine compounds that can precipitate on the reflective optical elements. Therefore, the materials used must be free of Cr and Ti. The process chamber may in particular be made of Monel steel.

[0053] Alternatively, the inside of the process 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.

[0054] The supply unit is preferably designed to introduce the active fluorinating agent diluted in an inert gas into the process chamber, and may include a suitable metering control valve, in particular a needle valve or a mass flow controller (MFC), to set the partial pressure of the active fluorinating agent in the process chamber to the desired value.

[0055] The device preferably further includes a fluorine gas sensor (e.g., a residual gas analyzer) 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.

[0056] 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.

[0057] In one embodiment, the device further comprises at least one coating source for depositing a metal reflective layer and / or a further fluoride layer of the fluoride protective coating. It is advantageous if the creation of the fluoride protective coating and the deposition of the metal reflective layer are carried out in one and the same process chamber, since in this case there is no need to transfer the substrate on which the deposition is carried out.

[0058] Further features and advantages of the present 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.

[0059] Exemplary embodiments are shown in the schematic drawings and explained in the following description. [Brief explanation of the drawings]

[0060] [Figure 1] FIG. 1 is a schematic diagram illustrating the preparation of a protective coating of MgF2 for an aluminum mirror using methods known in the prior art. [Figure 2] FIG. 1 shows the calculated reflectance of a fluoride-protected aluminum mirror with and without a native aluminum oxide layer as a function of wavelength. [Figure 3] FIG. 1 is a schematic diagram illustrating the creation of a fluoride protective coating for protecting a metal reflective layer of a reflective optical element for the VUV wavelength range using a method according to the present invention, which comprises irradiating a native oxide layer of the metal reflective layer with VUV radiation. [Figure 4] 1 is a schematic diagram showing absorption and spectral regions particularly relevant to the irradiation of a native oxide layer during use of a method according to the invention; FIG. [Figure 5] FIG. 1 is a schematic diagram showing a device according to the invention for producing a fluoride protective coating for protecting a metal reflective layer of a reflective optical element for the VUV wavelength range. [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

[0061] FIG. 1 shows a schematic diagram of the fabrication of a protective coating 1 made of MgF2 for an aluminum mirror 2 using the method described in U.S. Patent Application Publication No. 2017 / 0031067, cited in the Abstract, and in the paper by MA Quijada et al., also cited in the Abstract. The protective coating 1 disclosed therein includes a first MgF2 layer 3 and a second MgF2 layer 4. FIG. 1 shows an aluminum mirror 2 including an aluminum layer 6 applied to a substrate 5 in four snapshots MS1, MS2, MS3, and MS4, each showing only a small portion near the surface 7 of the aluminum layer 6.

[0062] The first snapshot MS1 shows the aluminum mirror 2 after the deposition of the aluminum layer 6 on the substrate 5 and before the formation of the native oxide layer 8.

[0063] The second snapshot MS2 shows the deposition of the first MgF2 layer 3 of the protective coating 1, which is carried out as quickly as possible after the deposition of the aluminum layer 6 at room temperature. However, this means that the formation of a native oxide layer 8 cannot be completely avoided. Furthermore, the first MgF2 layer 3 deposited is typically a porous layer.

[0064] The third snapshot MS3 shows the aluminum mirror 2 after the deposition of the first MgF2 layer 3 has been completed. In the example shown, the first MgF2 layer 3 has a thickness of 4 nm to 5 nm.

[0065] The fourth snapshot MS4 shows the aluminum mirror 2 after the deposition of the second MgF2 layer 4 is completed. The substrate 5 is heated for the deposition of the second MgF2 layer 4. The higher the temperature, the more dense the second MgF2 layer 4 becomes.

[0066] The formation of the native oxide layer 8 cannot be completely avoided, in particular because it forms within just a few seconds even at relatively low residual gas pressures in the PVD coating chamber. The formation of the native oxide layer 8 results in a loss of reflectivity of the aluminum mirror 2, as will be explained below with reference to FIG. 2.

[0067] In Figure 2, the calculated reflectance 9 of a fluoride-protected aluminum mirror with a native oxide layer is compared to the calculated reflectance 10 of a fluoride-protected aluminum mirror without a native oxide layer. Reflectances 9 and 10 are each shown as a function of wavelength. The native oxide layer is located at the aluminum-fluoride transition and has a thickness of approximately 1 nm.

[0068] The results shown reveal that even a native oxide layer with a relatively small thickness of 1 nm causes a loss of reflectivity of up to 10% in the VUV wavelength range.

[0069] 3 shows a schematic diagram of the fabrication of a fluoride protective coating 11 for protecting a metallic reflective layer 12 of a reflective optical element 13 for the VUV wavelength range, using the method detailed above to avoid reflection losses due to the native oxide layer 8. The figure shows four snapshots M1, M2, M3, M4, each showing only a small portion near the surface 14 of the metallic reflective layer 12.

[0070] The first snapshot M1 shows a reflective optical element 13 including a metal reflective layer 12 and a substrate 15 prior to the creation of a fluoride protective coating 11. In this exemplary illustration, the native oxide layer 16 has not yet formed. In the illustrated embodiment, the metal reflective layer 12 is an aluminum layer, although this is not required.

[0071] If the coating methods for depositing the metal reflective layer 12 and the fluoride protective coating 11 are incompatible, the fluoride protective coating 11 can be made by transferring the reflective optical element 13 to a process chamber (not shown here). If the coating methods are compatible, the reflective optical element 13 is typically left in the coating chamber in which the metal reflective layer 12 is deposited. In both cases, as shown in the second snapshot M2, the native oxide layer 16 of the metal reflective layer 12 is irradiated with UV / VUV radiation 17 in the presence of an active fluorinating agent FW. As a result of this irradiation, the active fluorinating agent FW dissociates, producing fluorine species F, F2, F * The fluorine species F, F2, and F * reacts with the native oxide layer 16 to form a fluoride layer 18 thereon, which is part of the fluoride protective coating 11 .

[0072] For example, the active fluorinating agent FW is NF3, which photodissociates to produce 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 comprising F2, HF, XeF2, NF3, CF4 and SF6.

[0073] Also, for example, but not necessarily, irradiation with UV / VUV radiation 17 results in mobilization of atoms at surface 18' and in volume 18'' of the formed fluoride layer 18.

[0074] The third snapshot M3 shows the reflective optical element 13 after irradiation of the native oxide layer 16. As a result of irradiation with UV / VUV radiation 17 in the presence of an active fluorinating agent FW, the native oxide layer 16 has been converted into a fluoride layer 18. Since the metallic reflective layer 12 is an aluminum layer in the illustrated embodiment, the fluoride layer 18 here consists, for example, mainly of AlF3. The fluoride layer 18 is furthermore particularly dense and smooth.

[0075] Although not necessarily, in the illustrated embodiment, a further fluoride layer 19 of the fluoride protective coating 11 is deposited on the fluoride layer 18 in a subsequent step.

[0076] A fourth snapshot M4 shows the reflective optical element 13 after deposition of a further fluoride layer 19 of the fluoride protective coating 11. Here, for example, but not necessarily, the further fluoride layer 19 is formed from AlF and has been deposited using physical vapor deposition, and the substrate 15 has been heated.

[0077] Figure 4 shows the absorption and spectral regions particularly relevant to UV / VUV radiation 17 of a native oxide layer 16 in the presence of an active fluorinating agent FW. 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 61 of the formed fluoride layer 18 including the Urbach tail 61' and the absorption cross section 65 of the crystal defects in the fluoride layer.

[0078] The UV / VUV radiation 17 used to irradiate the native oxide layer 16 has a first spectral region 62 for photodissociation of the active fluorinating agent FW. For example, the first spectral region 62 has an energy E ph is the dissociation energy E of the active fluorinating agent FW. diss , and includes at least one wavelength at least equal to

[0079] 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. 4 below). This is by way of example only and is not required. This minimizes potentially negative and / or competitive effects. UP is the band gap energy E of the fluoride layer 18 G and preferably the band gap energy EG is less than 75% of the

[0080] The UV / VUV radiation 17 also includes a second spectral region 63 for mobilizing atoms at the surface 18′ and / or in the volume 18″ of the fluoride layer 18 to be formed. Here, this second spectral region 63 is within the band gap energy E G Preferably, the second spectral region 63 is in the energy range of 75% to 100% of the band gap energy E G The energy may be within a range of 80% to 95% of the above.

[0081] Furthermore, the formed fluoride layer 18 is irradiated with, for example, further electromagnetic radiation 20 in order to anneal the at least one crystalline defect 10 in the fluoride layer 18. For this purpose, the further electromagnetic radiation 20 includes a spectral region 64 that overlaps with an absorption region 66 of the at least one crystalline defect. In the illustrated embodiment, the spectral region 64 of the further electromagnetic radiation 20 lies within the absorption region 66 of the crystalline defect, which is an F-center, although this is not necessary. Alternatively, the UV / VUV radiation 17 may also include the corresponding spectral region.

[0082] In the illustrated embodiment, the spectral region 64 of the further electromagnetic radiation 20 is the absorption energy E A However, this is not necessarily limited to this. The absorption region 66 of the crystal defect has an absorption energy E A It is defined as the 100-fold decrease in the absorption cross section maximum (FWHM) in the spectral region 64. m is the absorption energy of the crystal defect E A It is advantageous if the deviation does not exceed 0.5 eV, and in particular does not exceed 0.25 eV.

[0083] Figure 5 shows an apparatus 70 for producing a fluoride protective coating 11 for protecting a metal reflective layer 12 of a reflective optical element 13 for the VUV wavelength range using the method detailed above in relation to Figure 3. The apparatus includes a process chamber 71, a supply apparatus 72 and a UV / VUV radiation source 73.

[0084] A reflective optical element 13, including a metallic reflective layer 12 and a native oxide layer 16, is mounted on a substrate holder 74 that is rotatable about an axis of rotation 75 within a process chamber 71. However, deviating from the embodiment shown here, device 70 need not include a rotatable substrate holder 74.

[0085] The supply unit 72 serves to supply the inert gas IG and the active fluorinating agent FW into the process chamber 71. The supply unit 72 includes a first valve 76 for the controlled supply of the inert gas IG and a second valve 77 for the controlled supply of the active fluorinating agent FW. The second valve 77 is also a controllable metering valve. The device 70 further includes a vacuum pump 78 for evacuating the process chamber 71. The active fluorinating agent FW is metered into the evacuated interior of the process chamber 71 with the inert gas IG acting as a carrier gas. Alternatively, the irradiation of the native oxide layer 16 in the presence of the active fluorinating agent FW can be carried out in a protective gas atmosphere at a higher pressure, for example, atmospheric pressure. In this case, the process chamber 71 is filled with an inert gas, and the active fluorinating agent FW is additionally metered and controlled.

[0086] The UV / VUV radiation source 73 serves to irradiate the native oxide layer 16 with UV / VUV radiation 17 in the presence of an active fluorinating agent FW within the process chamber 71. This serves to convert the native oxide layer 16 into a fluoride layer 18 (not shown here). For example, the UV / VUV radiation 17 enters the process chamber 71 through a MgF window 79 in this case.

[0087] For example, in this case the device 70 also includes, although this is not essential, a second UV / VUV radiation source 80 for irradiating the native oxide layer 16 with UV / VUV radiation 17 in order to mobilize atoms at the surface 18′ and / or in the volume 18″ of the fluoride layer 18 to be formed. For example, here a further MgF2 window 81 for the second UV / VUV radiation source 80 is provided.

[0088] Instead of at least one of the MgF2 windows 79, 81, it is in principle also possible to use windows made of other materials, for example CaF2, SrF2 and / or BaF2, but for this sufficient transparency at the wavelengths used is essential.

[0089] Deviating from the above, device 70 may also have a different number of UV / VUV radiation sources. In particular, a single UV / VUV radiation source may serve both to convert native oxide layer 16 into fluoride layer 18 and to mobilize atoms at surface 18′ and / or in volume 18″ of formed fluoride layer 18. Device 70 may further include a further radiation source (not shown) for irradiating fluoride layer 18 formed during conversion with further electromagnetic radiation 20 in the spectral region 64 detailed above in relation to FIG. 4 in order to anneal at least one crystalline defect in fluoride layer 18. The further radiation source for annealing crystalline defects may, for example, be designed as a further UV / VUV radiation source.

[0090] The process chamber 71 is hermetically sealable, and the interior 82 of the process chamber 71 is resistant to the active fluorinating agent FW and its conversion products.

[0091] In the illustrated embodiment, although not necessarily, the device 70 further controls the partial pressure p of the active fluorinating agent FW in the process chamber 71. FW The sensor 83 measures the partial pressure p of the active fluorinating agent FW upon irradiation of the native oxide layer 16. FW is preferably 10 -8mbar to 10 mbar, more preferably 10 -6 mbar~10 -1 mbar.

[0092] The device 70 further controls the partial pressure p of the active fluorinating agent FW in the process chamber 71. FW to a target value, and the control is performed by a closed-loop control device 84 for controlling the partial pressure p FW This is done by controlling the second valve 77 using the actual measured value M of the sensor 83 to measure the partial pressure p of the active fluorinating agent FW. FW The residual gas analyzer may be designed to measure only the partial pressure of the other gases contained within the process chamber 71, but may also be capable of determining the partial pressure of other gases contained within the process chamber 71.

[0093] In the illustrated embodiment, although not necessarily, device 70 further comprises a coating source 85 used to deposit metal reflective layer 12 and further fluoride layer 19. For example, coating source 85 here is a unit for electron beam evaporation, but it could also be some other coating source, in particular a sputtering source or a thermal evaporator.

[0094] Alternatively, device 70 may have zero or multiple coating sources, specifically, device 70 may have dedicated coating sources for depositing metal reflective layer 12 and further fluoride layer 19, respectively.

[0095] The device 70 further includes a plasma source 86 to assist in the activation of the active fluorinating agent FW. Alternatively, the device 70 can include some other ion source or no plasma source. The active fluorinating agent FW can also be activated by increasing the temperature of the substrate 15 during or after the conversion of the oxide layer 16 to the fluoride layer 18.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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 having a radiation source 54, from which radiation 55 is directed onto a wafer 49. To this end, 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 the mask to be inspected. The radiation reflected, deflected, and / or refracted by the wafer 49 is directed via a transmissive optical element 47 to a detector 50 for further evaluation by a further concave mirror 48, also associated with the optical system 42. 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. For example, the radiation source 54 can be 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 can also be used.

[0102] At least one of the reflective optical elements 28 of the VUV lithography apparatus 21 shown in Figure 6 and at least one of the reflective optical elements 46, 48 of the wafer inspection system 41 shown in Figure 7 are now designed as detailed above. Accordingly, at least one of the reflective optical elements 28, 46, 48 includes at least one fluoride layer that has been converted using the method described above in connection with Figure 3.

Claims

1. A method for making a fluoride protective coating (11) for protecting a metal reflective layer (12) of a reflective optical element (13) for the VUV wavelength range, comprising the steps of: - irradiating said native oxide layer (16) of said metallic reflective layer (12) with UV / VUV radiation (17) in the presence of an active fluorinating agent (FW) in order to convert said native oxide layer (16) into a fluoride layer (18) of said fluoride protective coating (11).

2. The UV / VUV radiation (17) has a first spectral region (62) that contributes to the photodissociation of the active fluorinating agent (FW), and has an energy (E ph ) is the dissociation energy (E diss 2. The method of claim 1, wherein the wavelengths of the light beams are at least equal to one another.

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 (17) comprises a second spectral region (63) for mobilizing atoms at the surface (18') and / or in the volume (18") of the fluoride layer (18) to be formed, the second spectral region (63) 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 UV / VUV radiation (17) or the further electromagnetic radiation (20) additionally used to irradiate the formed fluoride layer (18) serves to anneal at least one crystalline defect of the fluoride layer (18) and includes a spectral region (64) that at least partially overlaps with an absorption region (65) of the at least one crystalline defect, the spectral region (64) preferably having an absorption energy (E A ), and the average energy (E m ) is more preferably the absorption energy (E A 6. The method according to claim 1, wherein the .lambda. dc.sub.1 / 2 .lambda. dc.sub.2 / 2 .lambda. dc.sub.3 / 2 .lambda. dc.sub.4 / 2 .lambda. dc.sub.5 / 2 .lambda. dc.sub.6 / 2 .lambda. dc.sub.7 / 2 .lambda. dc

7. Method according to any one of claims 1 to 6, characterized in that the metallic reflective layer (12) is an aluminium layer.

8. Method according to any one of claims 1 to 7, characterized in that the irradiation of the native oxide layer (16) is carried out in a protective gas atmosphere, preferably in vacuum.

9. The active fluorinating agent (FW) is F 2 , HF, XeF 2 , N.F. 3 , C.F. 4 , S.F. 6 9. The method according to claim 1, further comprising the step of:

10. The partial pressure (p FW ) but 10 -8 mbar to 10 mbar, preferably 10 -6 mbar~10 -1 10. The method according to claim 1, wherein the temperature is 2000 KPa (1000 KPa) or 2000 KPa (1000 KPa).

11. The partial pressure (p FW 11. The method according to claim 1, wherein the temperature of the native oxide layer (16) is adjusted to a desired value during irradiation.

12. A method according to any one of claims 1 to 11, characterized in that it further comprises the step of depositing a further fluoride layer (19) of said fluoride protective coating (11) on said fluoride layer (18).

13. A reflective optical element (13) for use in the VUV wavelength range, comprising a fluoride protective coating (11) made using a method as claimed in any one of claims 1 to 12.

14. 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 reflective optical element (28, 46, 48) as claimed in claim 13.

15. 1. An apparatus (70) for producing a fluoride protective coating (11) for protecting a metallic reflective layer (12) of a reflective optical element (13) for the VUV wavelength range, comprising: a process chamber (71), a supply unit (72) for supplying an inert gas (IG) and an active fluorinating agent (FW) to a process chamber (71) whose inside (82) is resistant to said active fluorinating agent (FW) and its transformation products; a UV / VUV radiation source (73) for irradiating the native oxide layer (16) of the metal reflective layer (12) with UV / VUV radiation (17) in the presence of the active fluorinating agent (FW) in order to convert the native oxide layer (16) of the metal reflective layer (12) into a fluoride layer (18) of the fluoride protective coating (11); A device (70) comprising:

16. at least one coating source (85) for depositing said metallic reflective layer (12) and / or a further fluoride layer (19) of said fluoride protective coating (11); The device (70) of claim 15 further comprising:

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