Method of processing reflective optical element, reflective optical element and optical device

Abstract

The invention relates to a method for treating a reflective optical element (1) for the VUV wavelength range having an aluminum surface (3). The treatment of the reflective optical element (1) comprises irradiating the reflective optical element (1) with a hydrogen plasma jet (10) to remove the aluminum oxide layer (4) formed on the aluminum surface (3). The invention also relates to a reflective optical element (1) for use in the VUV wavelength range treated by such a method, and to an optical arrangement comprising at least one such optical element (1).

Description

[0001] Cross-referencing related applications

[0002] This application claims priority to German Patent Application No. 102023204747.0, filed on May 22, 2023, the entire disclosure of which is considered part of the disclosure of this application and is incorporated herein by reference. Technical Field

[0003] This invention relates to a method for processing a reflective optical element having an aluminum surface for use in the VUV wavelength range. The aluminum surface may be formed on an aluminum substrate of the reflective optical element or on an aluminum layer applied to a substrate (e.g., to a quartz glass substrate). The invention also relates to an optical element processed by or having been processed by this method, and to an optical device for use in the VUV wavelength range, the optical device comprising at least one such optical element. Background Technology

[0004] VUV reflective optical elements, in the form of VUV mirrors, are required for applications including mask inspection, wafer inspection, and projection exposure equipment. These VUV mirrors typically must have a reflectivity exceeding 80% within the VUV wavelength range (i.e., wavelengths between 115 nm and 190 nm). A lifespan of over 10 years is required. For the desired application, only aluminum mirrors possess sufficient reflectivity for VUV radiation. However, the stability of aluminum mirrors is extremely critical because significant oxidation occurs when exposed to ambient air and short-wave radiation, potentially leading to a reduction in VUV reflectivity of up to 80%.

[0005] To increase the stability of mirrors or aluminum surfaces and prevent oxidation, several suggestions have been made to use a protective layer of metal fluorides (such as LiF, MgF2, AlF3) with relatively low absorption rates applied to the aluminum surface.

[0006] Examples of such protective layers are particularly described in G. Hass and R. Tousey’s “Reflecting Coatings for the Extreme UV” in JOSA, 49(6), 593-602(1959), J. Hennessy et al.’s “Performance and prospects of far ultraviolet aluminum mirrors protected by atomic layer deposition” in JOSA, 49(6), 593-602(1959), J. Hennessy et al.’s “Atomic layer deposition and etching methods for far ultraviolet aluminum mirrors” in JOSA, 2(4), 041206, J. Hennessy et al.’s “Atomic layer deposition and etching methods for far ultraviolet aluminum mirrors” in JOSA, 49(6), 593-602(1959), ...

[0007] In these articles, a metal fluoride protective layer acts as a barrier against oxidizing substances such as water and oxygen to prevent the aluminum surface from oxidizing under operating conditions. The latter article proposes removing the natural aluminum oxide layer on a mirror via atomic layer etching, followed by the application of a metal fluoride layer via atomic layer deposition.

[0008] Aluminum surfaces can also be protected by applying a protective layer that is not formed of metal fluorides before degradation or oxidation. For example, such a protective layer may contain silicon or carbon, or be composed of silicon or carbon. The protective layer is used to passivate the aluminum and is removed before the use of reflective optical elements.

[0009] When VUV reflectors are irradiated under different environmental conditions, degradation is observed even with a protective layer, resulting in a significant decrease in the reflectivity of the VUV reflectors.

[0010] The article "Plasma-assisted deposition of metal fluoride coatings and modeling the extinction coefficient of as-deposited single layers" published by M. Bischoff et al. in Appl. Opt. US 50, 232-238 (2010) describes the post-processing of metal fluoride layers deposited by plasma-assisted electron beam deposition with UV irradiation. The article states that this post-processing increases the initially poor transmittance of LaF3, MgF2, and AlF3 layers in the DUV wavelength range. Summary of the Invention

[0011] The purpose of this invention is to describe in detail a method for processing reflective optical elements in order to stabilize or improve their optical properties.

[0012] Invention Theme

[0013] This objective is achieved by a method of the type specified above, wherein the treatment of the reflective optical element includes irradiating the optical element with a hydrogen plasma jet to remove the aluminum oxide layer formed on the aluminum surface.

[0014] As further described above, a natural layer of alumina (Al2O3) quickly forms on the aluminum surface upon contact with ambient air. During operation in optical devices, irradiation of reflective optical elements with VUV radiation typically results in an increase in the thickness of the natural alumina layer with increasing irradiation time.

[0015] The inventors discovered that, using a hydrogen plasma jet, an alumina layer is removed or reduced to metallic aluminum, thereby forming an exposed aluminum surface. The hydrogen plasma jet contains active hydrogen substances, such as in the form of atomic hydrogen or hydrogen radicals, as well as in the form of hydrogen in an excited electronic state, which reduces and removes the alumina layer from the alumina layer.

[0016] In a preferred embodiment, the hydrogen plasma jet includes at least one carrier gas selected from the group consisting of N2 and Ar. The active material generated by the plasma source is typically absorbed by the carrier gas and directed onto the aluminum surface in the form of a hydrogen plasma jet. The use of N2 and / or Ar as the carrier gas has proven advantageous for this purpose.

[0017] In another variation, the exposed aluminum surface is irradiated with a fluorine plasma jet after the alumina layer is removed. The active material generated by the plasma source is typically absorbed by a carrier gas and directed onto the aluminum surface in the form of a fluorine plasma jet.

[0018] The inventors have recognized that irradiating reflective optical elements, specifically aluminum surfaces, with a fluorine plasma jet can improve the optical properties of the elements. Specifically, irradiation with a fluorine plasma jet can increase the reflectivity of optical elements that have already been exposed to VUV radiation for an extended period and / or have been exposed to an oxygen-containing environment. Treatment with a fluorine plasma jet can regenerate reflective optical elements, particularly after prolonged VUV radiation irradiation. Ideally, the treatment can restore the reflectivity of the optical element to its state before VUV radiation irradiation.

[0019] The proposed treatment of reflective optical elements using fluorine plasma jets has the following advantages: the treatment can be performed quickly and requires very few technical prerequisites.

[0020] In one development of this variation of the method, an aluminum fluoride layer is formed by irradiating an aluminum surface with a fluorine plasma jet. In this case, the irradiation comprises two steps, typically performed consecutively. In the first step, the aluminum oxide layer is removed or reduced to metallic aluminum by means of a hydrogen plasma jet, thereby forming an exposed aluminum surface. In the second step, the aluminum at the exposed surface is oxidized by the active fluorine material of the fluorine plasma jet to form a passivated aluminum fluoride layer.

[0021] An aluminum surface is irradiated with a jet of fluorine plasma to oxidize the metallic aluminum on the surface and form aluminum fluoride, for example, according to the following chemical reaction equation:

[0022] Al + 3 F * → AlF3

[0023] Here, F* represents an active fluorine substance, such as fluorine radicals present in a fluorine plasma jet. As further described above, the metal fluoride layer in the form of an aluminum fluoride layer can act as a barrier against oxidizing substances (such as water and oxygen) to prevent the aluminum surface from oxidizing under operating conditions. The AlF3 protective layer has the advantage of relatively low absorption of VUV radiation. The AlF3 layer can be produced in a particularly simple manner by irradiating the aluminum surface of a reflective optical element with a fluorine plasma jet.

[0024] As further described above, the (natural) alumina layer formed on the aluminum surface is removed before the aluminum fluoride layer can be formed. If the aluminum surface is not otherwise protected (see below), the natural alumina layer must be removed before the aluminum fluoride layer can be formed on the aluminum surface. Besides irradiating the reflective optical element with a hydrogen plasma jet, there are other options for removing the alumina layer.

[0025] For example, the alumina layer can be removed by irradiating it with a fluorine plasma jet, preferably at least partially converting it into an aluminum fluoride layer. The alumina layer can also be removed by active etching with a fluorine plasma jet; see, for example, the article “Chemical sputtering of Al2O3 by fluorine-containing plasmas excited by electro cyclotron approximation” by YH Lee et al., published in the Journal of Applied Physics 68, 5329 (1990), which is incorporated herein by reference in its entirety. This article describes active ion etching of alumina (Al2O3) by CHF3 and SF6 plasmas generated by electron cyclotron resonance. This demonstrates that the etching products produced when alumina is etched solely by chemical reaction lack volatility, meaning that the etching products cannot desorb at room temperature. Therefore, it is noted that ion bombardment is necessary for etching Al2O3 by chemically enhanced physical sputtering. When irradiating the alumina layer with a fluorine plasma jet, the alumina layer is bombarded with an active fluorine substance, such as fluorine ions.

[0026] The article “Advances inprecision freeform manufacturing by plasma jet machining” by T. Arnold et al., published in EPJ Web Conf. 238, 2020, is incorporated herein by reference in its entirety. It describes the production of precise freeform optical elements by plasma jet machining under atmospheric pressure. Due to the unique chemical mechanism of material removal based on a dry etching process using fluorine-containing gases, the choice of materials that can be processed by plasma jet machining is limited. The aforementioned article and paper “Investigation on Reactivity Driven Etching Mechanism in Plasma Jet-based Precision Surface Machining of Borosilicate Crown Optical Glass” by F. Kazemi, Leibniz Institute for Surface Engineering (IOM), 2020, indicates that borosilicate crown glass can also be processed by active plasma jet machining, where etching produces a residual layer. The plasma jet machining described in the aforementioned article and paper can be used to remove alumina layers.

[0027] If a fluorine plasma jet is used to remove the alumina layer, the process can be performed in a single step. The active fluorine material from the plasma jet first removes the alumina layer through active etching, subsequently forming a passivation protective layer of aluminum fluoride. Plasma parameters are typically chosen such that the active etching of the alumina layer, i.e., its removal, occurs first. Different plasma parameters that do not cause material removal are usually established to form the aluminum fluoride layer.

[0028] If suitable plasma parameters are established during irradiation, the following chemical reactions may also occur in the alumina layer:

[0029] 2 Al₂O₃ + 12 F * → 4 AlF3 + 3 O2

[0030] Where F* represents an active fluorine substance. In this case, at least part of the alumina layer is removed because it transforms into an aluminum fluoride layer. In this case, the alumina layer can be selectively removed and an aluminum fluoride layer formed without changing the plasma parameters irradiated by the fluorine plasma jet.

[0031] In another variation, a protective layer applied to the aluminum surface is removed before the aluminum fluoride layer forms. This protective layer is not the (natural) alumina layer further described above, but a specially applied protective layer designed to protect the aluminum surface from the formation of a natural alumina layer. The material of the protective layer should be chosen so that it can be removed in a simple manner.

[0032] In one development of this variant, the protective layer is formed of at least one material that combines with fluorine to form a volatile fluorine substance. This protective layer can be removed in a simple manner, for example, by a fluorine plasma jet.

[0033] The protective layer can be formed, for example, from silicon or carbon. These materials prevent the formation of a natural alumina layer on the aluminum surface and are easier to remove from the aluminum surface than an alumina layer.

[0034] In one development, the protective layer is removed by irradiation with a fluorine plasma jet. In this case, the fluorine in the plasma jet etches the material of the protective layer, such as Si or C, resulting in the formation of volatile substances, such as SiF4 or CF4. To increase the etching rate, oxygen can be added to the fluorine plasma jet, for example by the additional formation of CO2, or the fluorine plasma jet can be used for irradiation in the presence of oxygen. In this development, subsequently, i.e., after the protective layer is removed, irradiation with the fluorine plasma jet forms an aluminum fluoride layer on the aluminum surface through the oxidation of metallic aluminum.

[0035] In an alternative variation, a protective layer of at least one metal fluoride applied to the aluminum surface is irradiated with a fluorine plasma jet for post-fluorination treatment. In this case, unlike the previous description, instead of forming an aluminum fluoride layer during fluorine plasma jet irradiation, an existing metal fluoride layer, such as a LiF layer, MgF2 layer, or AlF3 layer, is post-fluorinated. Clearly, the protective layer in the form of an aluminum fluoride layer can be manufactured, for example, by means of alternative variations of the methods further described above or by a thermal evaporation process.

[0036] When reflective optical elements are irradiated with VUV radiation under different environmental conditions, significant photon-induced damage (formation of color centers and oxidation) was detected in the protective layer in the form of metal fluoride. The damage may persist to the aluminum surface (forming the interface between the metal fluoride protective layer and the substrate composed of aluminum or aluminum layers), potentially causing the aluminum mirror to lose its function completely.

[0037] The inventors have recognized that when a protective layer of at least one metal fluoride is irradiated with a jet of fluorine plasma, the optical properties of a reflective optical element can be regenerated or maintained after irradiation with VUV radiation. This utilizes the fact that active fluorine substances in the fluorine plasma, for example in the form of fluorine radicals, convert the metal oxides formed in the protective layer during VUV irradiation into metal fluorides, and also satisfy or eliminate color centers formed during VUV irradiation. The conversion of metal oxides to metal fluorides increases the reflectivity of the reflective optical element because the metal fluorides used for the protective layer typically have lower VUV radiation absorption than the corresponding metal oxides.

[0038] These two processes—the conversion of metal oxides into metal fluorides and the elimination of color centers through active fluoride substances—increase the VUV reflectivity of reflective optical elements to up to 80%, ensuring the functionality of the reflective optical elements for their intended applications. Treatment with fluorine plasma jets can also convert the alumina formed on the aluminum surface into aluminum fluoride, which similarly leads to increased reflectivity.

[0039] In another variation, the reflective optics are irradiated with VUV radiation prior to treatment. Irradiation with VUV radiation typically occurs during the operation of the optical device integrating the reflective optics. As further described above, irradiation with VUV radiation leads to a degradation process, for example, in a protective layer containing at least one metal fluoride, and also results in an increase in the thickness of the (natural) alumina layer, which reduces the reflectivity of the optics. It is advantageous or recommended that the reflective optics be treated periodically, particularly periodically, after irradiation with VUV radiation for a specific duration. Alternatively, the reflectivity of the optics to VUV radiation can be monitored, and treatment can be performed when the reflectivity falls below a critical value.

[0040] The fluorine plasma jet is generated by a plasma source, such as a high-frequency plasma source or a microwave plasma source. This process is typically carried out in a processing chamber containing reflective optical elements for the process. Other components within the processing chamber must be compatible with the fluorine-containing gas, i.e., they cannot be corroded by active fluorine.

[0041] In one variant, the hydrogen plasma jet and / or fluorine plasma jet move across the aluminum surface during the processing of the reflective optics. For the processing, it may not be sufficient for the hydrogen and / or fluorine plasma jets to be aligned to only one location (e.g., the center of the aluminum surface). Instead, it has been found advantageous for the hydrogen and / or fluorine plasma jets to move across the aluminum surface. To achieve this, the plasma processing system in which reflective optics are introduced for processing can have a position control system. It is also advantageous when the plasma processing system has a temperature monitoring or control system to monitor or control the temperature in the processing chamber. It has also been found advantageous or necessary to have a detection system for detecting fluorine gas (F2) or a protective system to prevent fluorine gas from escaping from the processing chamber.

[0042] The movement of hydrogen plasma jets and / or fluorine plasma jets across the aluminum surface allows for spatial resolution processing of reflective optical elements by varying the duration for which the hydrogen / fluorine plasma jets are guided to corresponding positions on the surface. Thus, the thickness of the alumina layer removed by the hydrogen plasma jet can be locally varied across the entire surface. Furthermore, the fluorination of the aluminum surface by the fluorine plasma jet, and the thickness of the resulting aluminum fluoride layer, can also be locally varied across the entire surface.

[0043] Specifically, radial variation in fluorination via the fluorine plasma jet can be generated by radially altering the duration of fluorination on the aluminum surface. This allows the formation of an aluminum fluoride layer with a radially varying thickness distribution on the aluminum surface. This aluminum fluoride layer with such a radially varying thickness distribution can advantageously be used as a gray filter with a radially varied filter profile. For example, this type of gray filter can be used to generate or compensate for apodization effects. It should be understood that the duration of fluorination via the fluoride plasma jet and / or reduction via the hydrogen plasma jet can be selectively varied across the entire surface in a non-rotationally symmetric manner.

[0044] In another variation, the processing of reflective optical elements is carried out under vacuum conditions. The proposed processes for processing reflective optical elements do not necessarily require performance under vacuum conditions. However, it has been found advantageous to process them under vacuum conditions using fluorine plasma jets for better environmental control and reduced risk of surface contamination. The same applies to processing using hydrogen plasma jets.

[0045] In a further variation, the fluorine plasma jet includes at least one reactive gas selected from the group consisting of CF4, CHF3, C2F6, NF3, SF6, and F2. Obviously, other reactive gases, such as XeF2, XeF4, and XeF6, may also be present in the plasma jet. The reactive gas is excited in the plasma source to generate reactive fluorine gaseous substances, for example, in the form of fluorine radicals, fluoride ions, or fluorine in an excited electronic state. A small amount of oxygen (O2) may also be added to the fluorine plasma jet to improve the etching rate, for example, when removing a protective layer of a material that combines with oxygen to form volatile substances. This is the case, for example, with carbon protective layers, because carbon reacts with oxygen to form volatile carbon dioxide.

[0046] In a further variation, the fluorine plasma jet includes at least one carrier gas preferably selected from the group consisting of N2, He, and Ar. The active material generated by the plasma source is typically absorbed by the carrier gas and directed onto the aluminum surface or protective layer in the form of a fluorine plasma jet. The carrier gas is typically an inert gas, such as nitrogen or a rare gas; in addition to He and Ar, Ne and Kr can be used selectively.

[0047] Due to the high reactivity of active fluorine materials, the proposed treatment is stable and robust under various plasma conditions. These plasma conditions include, for example, the gas flow rate of the plasma jet, the pressure in the treatment chamber, the power of the plasma source, the distance between the plasma source and the aluminum surface, the treatment time, and the temperature.

[0048] From the description of the operational examples of the invention, with reference to the accompanying drawings showing details essential to the invention, and from the claims, further features and advantages of the invention will become apparent. In variations of the invention, each feature may be implemented individually or in any combination thereof. Attached Figure Description

[0049] A working example is shown in the diagram and illustrated in the description below. The attached diagram shows:

[0050] Figure 1a , 1b This is a schematic diagram of a reflector used in the VUV wavelength range, which has an exposed aluminum surface during and after VUV irradiation.

[0051] Figure 2a , 2b for Figure 1a , 1b A schematic diagram of the reflector during and after treatment with a fluorine plasma jet to form an aluminum fluoride layer.

[0052] Figures 3a-3c for Figure 1a , 1bSchematic diagram of the reflector during and after treatment with a hydrogen plasma jet, and during and after treatment with a fluorine plasma jet.

[0053] Figure 4a , 4b This is a schematic diagram of a mirror used in the VUV wavelength range, which has a protective layer during and after treatment with a fluorine plasma jet to form an aluminum fluoride layer.

[0054] Figure 5a , 5b This is a schematic diagram of a reflector used in the VUV wavelength range, which has a protective layer of metal fluoride during and after VUV irradiation.

[0055] Figure 6a , 6b for Figure 5a , 5b A schematic diagram of the reflector during and after the post-fluorination process of the protective layer, using a fluorine plasma jet treatment.

[0056] Figure 7 A schematic diagram of an optical device for the VUV wavelength range in the form of a VUV lithography equipment, and Figure 8 This is a schematic diagram of an optical device for the VUV wavelength range, in the form of a wafer inspection system. Detailed Implementation

[0057] In the following description of the accompanying drawings, the same reference numerals are used for the same or having the same function.

[0058] Figure 1a , 1b This diagram shows a reflective optical element in the form of a mirror 1 for the VUV wavelength range. The mirror 1 has an aluminum substrate 2 and an aluminum surface 3 that serves as the mirror surface. Figure 1a It is evident that the reflective optical element 1 is subjected to VUV radiation 5 during operation in the optical device, which is reflected at the aluminum surface 3 of the reflector 1. Alternatively, the reflective aluminum surface 3 may be formed on an aluminum layer applied to a substrate, such as quartz glass or other materials. Figure 1a As shown, a natural aluminum oxide layer 4 is formed on the aluminum surface 3.

[0059] Figure 1b This shows mirror 1 after being irradiated with VUV radiation 5 for a limited time. (Example) Figure 1b As shown, irradiation with VUV radiation 5 increases the thickness of the alumina layer 4. To increase the reflectivity of the mirror 1 after VUV radiation 5 irradiation to approximately 80% as required for use in, for example, wafer inspection systems, the mirror 1 is introduced into the processing chamber 6 after a defined time of VUV radiation 5 irradiation and subjected to a fluorine plasma jet 7, as shown. Figure 2aAs shown. Treatment with the fluorine plasma jet 7 can be performed periodically whenever the irradiation of the VUV radiation 5 within the optical device reaches a specific duration. Treatment with the fluorine plasma jet 7 can also be performed whenever the reflectivity of the reflector 1 falls below a critical value.

[0060] When the reflector 1 is treated with a fluorine plasma jet 7, the fluorine plasma jet 7 is guided onto the aluminum surface 3 of the reflector 1 and moved on the surface 3 by means of a position control system (not shown). The fluorine plasma jet 7 is generated by a plasma source 8, which may be, for example, an RF plasma source or a microwave plasma source. The fluorine plasma jet 7 is a gas jet in which active fluorine substances are present in an inert carrier gas, such as N2, He or Ar, Ne, Kr, etc. Active fluorine substances, for example, in the form of fluorine radicals, are generated by an active fluorine gas in the plasma source 8. The active fluorine gas may be, for example, a gas selected from the group consisting of: CF4, CHF3, C2F6, NF3, SF6, and F2. It may also be a fluorine-containing gas, such as XeF2, XeF4, XeF6, etc. Oxygen (O2) may also be added to the fluorine plasma jet 7.

[0061] The power and other plasma parameters of plasma source 8 are adjustable, allowing the alumina layer 4 to be removed first by reactive (ionic) etching, as described in the further cited article or F. Kazemi's paper. After removing the alumina layer 4, metallic aluminum can be applied to the alumina surface 3 using a fluorine plasma jet 7 to form, as described in the following article. Figure 2b The aluminum fluoride layer 9 is shown.

[0062] Alternatively, the plasma parameters can be adjusted to allow the following chemical reactions to occur during the irradiation of the alumina layer 4 with the fluorine plasma jet 7:

[0063] 2 Al₂O₃ + 12 F * → 4 AlF3 + 3 O2.

[0064] In the above reaction formula, F* represents an active fluorine substance.

[0065] In this case, the alumina layer 4 is transformed by the treatment with fluorine plasma jet 7. Figure 2b The aluminum fluoride layer 9 is shown. The aluminum fluoride layer 9 forms a passivation protective layer, and its absorption of VUV radiation 5 is significantly lower than that of the aluminum oxide layer 4. For example... Figure 2b As shown, the processed mirror 1 has sufficient reflectivity to be introduced (and possibly reintroduced) into an optical device and operated therein.

[0066] As further described above, the treatment using fluorine plasma jet 7 in Figure 2aThe treatment is carried out in the treatment chamber 6 shown. In the example shown, the treatment is performed under vacuum conditions to minimize the risk of surface contamination. Alternatively, the treatment can be carried out in the treatment chamber 6 at a higher pressure, such as atmospheric pressure, using a fluorine plasma jet 7.

[0067] The components within processing chamber 6 must be able to withstand the corrosive effects of active fluorine substances or fluorine gases. Monitoring for fluorine gas leakage from processing chamber 6 or conducting appropriate safety checks is generally also necessary. The temperature within processing chamber 6 should also be monitored and adjusted or regulated if necessary.

[0068] Figures 3a-3c Showing Figure 1a An alternative method to restore the reflectivity of mirror 1 in the process. In this process, in Figure 3a In the first step shown, the alumina layer 4 is irradiated with a hydrogen plasma jet 10 to remove it from the aluminum surface 3. The hydrogen plasma jet 10 is a gaseous jet in which active hydrogen substances are present in an inert carrier gas, such as N2, He or Ar, Ne, Kr, etc. The active hydrogen substances are generated in another plasma source 11, for example by the movement of molecular hydrogen through a heated filament. Obviously, the alumina layer 4 can also be removed from the aluminum surface 3 in a different manner than by means of the hydrogen plasma jet 10, if appropriate.

[0069] In the second step of processing mirror 1, such as Figure 3b As shown, an exposed aluminum surface 3 is irradiated with a fluorine plasma jet 7, such as in combination with... Figure 2a As described. At point 3 on the aluminum surface, metallic aluminum is oxidized to aluminum fluoride. In this case, for example, the following chemical reaction can occur:

[0070] Al + 3 F * → AlF3.

[0071] exist Figure 3a , 3b After the treatment described above, the reflective optical element 1 has a passivated aluminum fluoride layer 9 (see [reference]). Figure 3c It can also be used in optical devices. Processing with hydrogen plasma jet 10 and fluorine plasma jet 7 can be carried out in the same processing chamber 6, but it can also be carried out in two different processing chambers or two different plasma processing systems.

[0072] By varying the duration for which the fluorine plasma jet 7 is directed to a specific location on the exposed aluminum surface 3, a passivated aluminum fluoride layer 9 with a position-dependent thickness distribution can be obtained. For example, when the irradiation duration of the fluorine plasma jet 7 is varied in a rotationally symmetric manner, a passivated aluminum fluoride layer 9 with a rotationally symmetric thickness distribution can be produced. For example, this rotationally symmetric thickness profile can be used as a gray filter to produce or compensate for apodization effects.

[0073] Figure 4a , 4b This illustrates the processing of a reflector 1 for the VUV wavelength range, wherein at least one volatile fluorine substance M is formed with fluorine F. a F b A special protective layer 12 composed of material M is applied to the aluminum surface 3. The protective layer 12 may be composed of, for example, silicon or carbon. Immediately after aluminum is deposited on the aluminum surface 3, the protective layer 12 is deposited in the processing chamber 6 by a conventional coating method in a previous step (not shown) to prevent the formation of a natural alumina layer 12.

[0074] like Figure 4a As shown, the protective layer 12 is removed by active etching using a fluorine plasma jet 7, simultaneously or subsequently forming Figure 4b The aluminum fluoride layer 9 is shown. In, as... Figure 4a In the processing of the silicon protective layer 12 shown, for example, the following chemical reaction may be performed:

[0075] Si + 4 F * → SiF4,

[0076] Al + 3 F * → AlF3.

[0077] SiF4 is a volatile fluoride and does not remain on the aluminum surface 3, allowing the silicon-based protective layer 12 to be removed using a fluorine plasma jet 7 until the aluminum surface 3 is exposed. Correspondingly, if the protective layer 12 is also composed of carbon, volatile CF4 is formed by irradiation with the fluorine plasma jet 7. During irradiation, oxygen can be added as an active gas to promote the formation of volatile CO2 and improve the etching rate. Carbon is removed until the aluminum surface 3 is exposed. The reaction between the metallic aluminum at the exposed aluminum surface 3 and the active fluorine in the fluorine plasma jet 7 forms a passivated aluminum fluoride layer 9 on the reflector 1, such as... Figure 4b As shown.

[0078] Figure 5a , 5b Showing mirror 1, where... Figure 1a , 1b Compared to the reflector 1 shown, a protective layer 13 in the form of a metal fluoride layer is applied before irradiation with VUV radiation 5. The metal fluoride can be, for example, LiF, MgF2, or AlF3. If the metal fluoride is AlF3, the protective layer 13 can be formed by the process of the reflector 1 described above, and can correspond to the aluminum fluoride layer 9.

[0079] The metal fluoride in protective layer 13 reacts with oxidizing gases in the environment of reflector 1 when irradiated with VUV radiation 5, such as O2, or possibly with O3, H2O, N2O, O*, OH*, NO*, O( 1 D) and other reactions occur, forming metal oxides. Additionally, at the surface 3 where the protective layer 13 forms the interface between the aluminum substrate 2, aluminum also transforms into aluminum oxide (Al2O3), such as... Figure 5b As shown. Similarly, as Figure 5b As shown, when irradiated with VUV radiation 5, color centers 14 are formed in the protective layer 13.

[0080] The metal fluoride in the protective layer 13 is converted into metal oxide and forms color center 14, and the metal aluminum at surface 3 is oxidized to Al2O3, resulting in a significant reduction in the reflectivity of mirror 1.

[0081] Therefore, as Figure 6a As shown, Figure 5b The reflector 1 shown is treated by the fluorine plasma jet 7. The reflector 1, or more specifically the protective layer 13, is... Figure 6a The plasma is irradiated in a processing chamber (not shown). The processing chamber or plasma coating system can be combined as follows: Figure 2a Design it as described further.

[0082] The power and other plasma parameters of plasma source 8 can be adjusted to allow the following chemical reactions to occur in protective layer 13 or on surface 3:

[0083] 2 MO + F * → 2 MF + O2

[0084] 2 Al₂O₃ + 12 F * → 4 AlF3 + 3 O2

[0085] In the above reaction equations, MO represents metal oxide and MF represents metal fluoride. It is clear from the reaction equations that the reaction with fluorine substance F* leads to the refluorination of the protective layer 13, where the formed metal oxide MO is converted into the metal fluoride MF. When treated with fluorine plasma jet 7, the aluminum oxide formed on the aluminum surface 3 can also be converted into aluminum fluoride, which similarly leads to increased reflectivity.

[0086] Treatment with fluorine plasma jet 7 also eliminated color centers 14, such as Figure 6b As shown. These treatments resulted in an increase in the VUV reflectivity of mirror 1 to up to 80%, and also led to an increase in the lifespan of mirror 1.

[0087] The mirror 1, which has been treated in the above manner, can be used in different optical devices in the VUV wavelength range.

[0088] Figure 7 This describes an optical device for the VUV wavelength range, in the form of a VUV lithography apparatus. The VUV lithography apparatus 21 includes two optical systems: an illumination system 22 and a projection system 23. The VUV lithography apparatus 21 also has a radiation source 24, which may be, for example, an excimer laser.

[0089] The radiation 25 emitted by the radiation source 24 is modulated by means of the illumination system 22, thereby illuminating the mask 26 (also called the mask master). In the example shown, the illumination system 22 has a housing 32 in which transmissive and reflective optical elements are arranged. In a representative manner, the figure shows the transmissive optical element 27 that focuses the radiation 25 and the reflective optical element 28 that deflects the radiation.

[0090] The mask 26 has a structure on its surface that is transferred to the optical element 29 to be exposed, such as a wafer, by means of a projection system 23 for the production of semiconductor components. In the example shown, the mask 26 is designed as a transmissive optical element. In an alternative embodiment, the mask 26 may also be designed as a reflective optical element.

[0091] In the example, the projection system 22 has at least one transmissive optical element. The example shows two transmissive optical elements 30, 31 in a representative manner, which are used, for example, to reduce the structure on the mask 26 to the size required to expose the wafer 29.

[0092] In both the illumination system 22 and the projection system 23, various transmissive, reflective, or other optical elements can be combined with each other as needed, including in more complex ways. Optical devices without transmissive optical elements can also be used for VUV lithography.

[0093] Figure 8 The wafer inspection system 41 is an optical device for the VUV wavelength range, but it can also be a mask inspection system. The wafer inspection system 41 has an optical system 42 by which radiation 55 is directed from a radiation source 54 onto a wafer 49. For this purpose, the radiation 55 is reflected onto the wafer 49 by a concave mirror 46. In the case of a mask inspection system, the wafer 49 can be replaced by a mask to be inspected. The radiation reflected, diffracted, and / or refracted by the wafer 49 is directed to a detector 50 by another concave mirror 48 via a transmission optics element 47 for further evaluation; this concave mirror 48 is also associated with the optical system 42. The wafer inspection system 41 also has a housing 52 in which two mirrors 46, 48 and the transmission optics element 47 are disposed. The radiation source 54 can be, for example, exactly one radiation source or a combination of multiple individual radiation sources to provide a substantially continuous radiation spectrum. In variations, one or more narrowband radiation sources 54 may also be used.

[0094] Figure 7 At least one reflective optical element 28 of the VUV lithography apparatus 21 shown and Figure 8 At least one of the reflective optical elements 46, 48 of the wafer inspection system 41 shown may have been processed in the manner further described above and irradiated by the fluorine plasma jet 7. Specifically, at least one of the reflective optical elements 28, 46, 48 may have a protective layer 13 in the form of a metal fluoride layer, which has been bonded by... Figure 6a , 6b The described method involves post-fluorination treatment.

Claims

1. A method for processing a reflective optical element (1) for the VUV wavelength range, the reflective optical element having an aluminum surface (3), wherein the processing of the reflective optical element (1) comprises: The reflective optical element (1) is irradiated with a hydrogen plasma jet (10) to remove the aluminum oxide layer (4) formed on the aluminum surface (3).

2. The method of claim 1, wherein, The hydrogen plasma jet (10) includes at least one carrier gas selected from the group consisting of N2 and Ar.

3. The method of claim 1 or 2, wherein, After removing the aluminum oxide layer (4), the exposed aluminum surface (3) is irradiated with a fluorine plasma jet (7).

4. The method of claim 3, wherein, An aluminum fluoride layer (9) is formed on the aluminum surface (3) by irradiation with the fluorine plasma jet (7).

5. The method of claim 4, wherein, Before forming the aluminum fluoride layer (9), the protective layer (12) applied to the aluminum surface (3) is removed.

6. The method of claim 5, wherein, The protective layer (12) is composed of at least one volatile fluorine substance (M) that combines with fluorine (F) to form a volatile fluorine substance. a F b The material (M) is formed.

7. The method of claim 5 or 6, wherein, The protective layer (12) is formed of silicon or carbon.

8. The method of any one of claims 5 to 7, wherein, The protective layer (12) is removed by irradiation with the fluorine plasma jet (7).

9. The method of claim 3, wherein, The protective layer (13) of at least one metal fluoride applied to the aluminum surface (3) is irradiated with the fluorine plasma jet (7) to perform post-fluorination treatment.

10. The method as claimed in any of the preceding claims, wherein, The reflective optical element (1) was irradiated with VUV radiation (5) before processing.

11. The method as claimed in any of the preceding claims, wherein, The hydrogen plasma jet (10) and / or the fluorine plasma jet (7) move across the aluminum surface (3) during the processing of the reflective optical element (1).

12. The method as claimed in any of the preceding claims, wherein, The processing of the reflective optical element (1) is performed under vacuum conditions.

13. The method as claimed in any of the preceding claims, wherein, The fluorine plasma jet (7) includes at least one active fluorine gas selected from the group consisting of CF4, CHF3, C2F6, NF3, SF6 and F2.

14. The method as claimed in any of the preceding claims, wherein, The fluorine plasma jet (7) includes at least one carrier gas, preferably selected from the group consisting of N2, He and Ar.

15. An optical element (1) for use in the VUV wavelength range, processed by the method of any of the preceding claims.

16. An optical device for the VUV wavelength range, particularly a VUV lithography device (21) or a wafer inspection system (41), comprising at least one optical element (28, 46, 48) as claimed in claim 15.

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