Method for processing reflective optical elements, reflective optical elements, and optical apparatus

Abstract

The present invention provides a method for processing reflective optical elements, reflective optical elements, and optical devices. [Solution] The present 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) includes irradiating the reflective optical element (1) with a hydrogen plasma jet (10) to remove an aluminum oxide layer (4) formed on the aluminum surface (3). The present invention also relates to a reflective optical element (1) for use in the VUV wavelength range, the reflective optical element (1) treated by this method, and an optical device comprising at least one such optical element (1).

Description

[Technical Field]

[0001] Cross-reference of related applications This application claims priority to German Patent Application No. 102023204747.0, filed on 22 May 2023. The disclosure of this German Patent Application is deemed in whole to be part of the disclosure of this application and is incorporated into the disclosure of this application by reference.

[0002] The present invention relates to a method for processing a reflective optical element for the VUV wavelength range having an aluminum surface. The aluminum surface may be formed on an aluminum substrate of the reflective optical element, or on an aluminum layer coated on a substrate, such as a quartz glass substrate. The present invention also relates to an optical element processed by this method or an optical element processed by this method, and an optical device for the VUV wavelength range including at least one such optical element. [Background technology]

[0003] Reflective optical elements in the form of VUV mirrors for reflecting VUV ("vacuum ultraviolet") radiation are required for applications including mask inspection or wafer inspection systems, or within projection exposure equipment. Such VUV mirrors generally must have a reflectivity of over 80% in the VUV wavelength range, i.e., between 115 nm and 190 nm. The required lifespan for such mirrors is over 10 years. For the desired application, only aluminum mirrors possess sufficient reflectivity to VUV radiation. However, the stability of aluminum mirrors is extremely important because they undergo significant oxidation when exposed to ambient air and when irradiated with short-wavelength radiation, which can lead to a reduction in VUV reflectivity of up to 80%.

[0004] Various proposals have been made to use a protective layer of relatively low-absorbing metal fluorides (e.g., LiF, MgF2, AlF3) applied to the aluminum surface to increase the stability of mirror or aluminum surfaces and to prevent oxidation.

[0005] Examples of such protective layers are described, in particular, in the papers "Reflecting coatings for the extreme ultraviolet", G. Hass & R. Tousey, JOSA, 49(6), 593-602 (1959), "Performance and prospects of far ultraviolet aluminum mirrors protected by atomic layer deposition", J. Hennessy et al., Journal of Astronomical Telescopes, Instruments, and Systems, 2(4), 041206, and "Atomic layer deposition and etching methods for far ultraviolet aluminum mirrors", J. Hennessy et al., in Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems (Vol. 10401, p. 1040119), Int. Society for Optics and Photonics.

[0006] These papers suggest that a protective layer of metal fluoride acts as a barrier against oxidizing species such as water and oxygen to prevent oxidation of the aluminum surface under operating conditions. The last paper proposes removing the native aluminum oxide layer on the mirror by atomic layer etching, followed by coating with a metal fluoride layer by atomic layer deposition.

[0007] It is also possible to protect the aluminum surface by applying a protective layer that is not formed from metal fluorides before degradation or oxidation occurs. For example, such a protective layer may contain or consist of silicon or carbon. This protective layer acts to passivate the aluminum and is removed before use of the reflective optical element.

[0008] When VUV mirrors were exposed to radiation under different ambient conditions, degradation was observed even in the presence of a protective layer, resulting in a significant decrease in the reflectivity of the VUV mirrors.

[0009] The paper "Plasma-assisted deposition of metal fluoride coatings and modeling the extinction coefficient of as-deposited single layers", M. Bischoff et al., Appl. Opt. 50, 232-238 (2010) describes post-treatment of metal fluoride layers deposited by plasma-assisted electron beam deposition by irradiation with UV radiation. This paper states that this post-treatment can increase the initially insufficient transmission in the DUV wavelength range of LaF3, MgF2, and AlF3 layers. [Overview of the project] [Problems that the invention aims to solve]

[0010] The object of the present invention is to specify a method for processing a reflective optical element to stabilize or improve its optical properties. [Means for solving the problem]

[0011] This objective is achieved by a method of the type initially specified, in which 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.

[0012] Furthermore, as explained above, when the aluminum surface comes into contact with ambient air, a natural layer of aluminum oxide (Al2O3) forms very quickly. When a reflective optical element is irradiated with VUV radiation while it is operating within an optical device, the thickness of the natural aluminum oxide layer typically increases as the duration of irradiation increases.

[0013] The inventors of this invention have found that a hydrogen plasma jet can be used to remove or reduce an aluminum oxide layer to metallic aluminum in order to form an exposed aluminum surface. The hydrogen plasma jet includes reactive hydrogen species, such as reactive hydrogen species in the form of atomic hydrogen or hydrogen radicals and reactive hydrogen species in the form of hydrogen in an excited electronic state, and the reactive hydrogen species reduce the aluminum oxide in the aluminum oxide layer and remove the aluminum oxide layer.

[0014] In a preferred modified embodiment, the hydrogen plasma jet contains at least one carrier gas selected from the group including N2 and Ar. Reactive species generated by the plasma source are typically incorporated into the carrier gas and guided to the aluminum surface in the form of a hydrogen plasma jet. Using N2 and / or Ar as carrier gases has been found to be advantageous for this purpose.

[0015] In an additional modified embodiment, after the removal of the aluminum oxide layer, a fluorine plasma jet is irradiated onto the exposed aluminum surface. Reactive species generated by the plasma source are typically incorporated into the carrier gas and guided onto the aluminum surface in the form of a fluorine plasma jet.

[0016] The inventors of this invention have recognized that irradiating reflective optical elements, more specifically aluminum surfaces, with a fluorine plasma jet can improve the optical properties of the optical elements. In particular, irradiating optical elements with a fluorine plasma jet when they have been exposed to VUV radiation for a long period of time and / or when they have been exposed to an oxygen-containing environment can increase the reflectivity of the optical elements. Treatment with a fluorine plasma jet can regenerate reflective optical elements, especially after prolonged exposure to VUV radiation. In an ideal case, this treatment can restore the reflectivity of the optical elements to what it was before the VUV radiation exposure.

[0017] The proposed treatment of reflective optical elements using a fluorine plasma jet has the advantage that the treatment can be performed rapidly and requires only minor technical prerequisites for implementation.

[0018] In one evolved embodiment of this modified example of the method, irradiation with a fluorine plasma jet forms an aluminum fluoride layer on an aluminum surface. In this case, the irradiation typically involves two steps that proceed sequentially. In the first step, a hydrogen plasma jet is used to remove the aluminum oxide layer or reduce the aluminum oxide layer to metallic aluminum in order to form an exposed aluminum surface. In the second step, the aluminum on the exposed surface is oxidized by a reactive fluorine species of the fluorine plasma jet in order to form a passivated aluminum fluoride layer.

[0019] Irradiation of an aluminum surface with a fluorine plasma jet oxidizes the metallic aluminum on the aluminum surface to form aluminum fluoride, for example, according to the following chemical reaction equation. Al+3F * →AlF3 In the above equation, F *represents reactive fluorine species present in the fluorine plasma jet, such as fluorine radicals. As further explained above, the metal fluoride layer in the form of an aluminum fluoride layer may serve as a barrier against oxidizing species such as water and oxygen to prevent oxidation of the aluminum surface under operating conditions. The protective layer of AlF3 has the advantage of having a relatively low absorption to VUV radiation. Irradiation of the aluminum surface of the reflective optical element with a fluorine plasma jet can generate the AlF3 layer in a particularly simple manner.

[0020] As further explained above, before forming the aluminum fluoride layer, the (natural) aluminum oxide layer formed on the aluminum surface is removed. If the aluminum surface is not protected by some other method (see below), it is necessary to remove the natural aluminum oxide layer before forming the aluminum fluoride layer on the aluminum surface. There are alternatives to removing the aluminum oxide layer other than irradiating the reflective optical element with a hydrogen plasma jet.

[0021] For example, the aluminum oxide layer can be removed by irradiating it with a fluorine plasma jet, preferably by irradiating it with a fluorine plasma jet and converting the aluminum oxide layer to an aluminum fluoride layer, at least partially. The aluminum oxide layer can also be removed by reactive etching using a fluorine plasma jet. For example, see the paper “Chemical sputtering of Al2O3 by fluorine-containing plasmas excited by electron cyclotron resonance”, YH Lee et al., Journal of Applied Physics 68, 5329 (1990), which is incorporated in its entirety by reference in this application. This paper describes reactive ion etching of aluminum oxide (Al2O3) by CHF3 and SF6 plasmas generated by electron cyclotron resonance. This paper states that the lack of volatility of the etching product produced in etching aluminum oxide by chemical reaction alone means that the etching product cannot be desorbed at room temperature. Therefore, this paper states that ion bombardment is essential for etching Al2O3 by chemically enhanced physical sputtering. In irradiation of an aluminum oxide layer with a fluorine plasma jet, the aluminum oxide layer is subjected to an impact by a reactive fluorine species, such as fluorine ions.

[0022] The paper "Advances in precision freeform manufacturing by plasma jet machining", T. Arnold et al., EPJ Web Conf. 238, 2020, which is incorporated herein by reference in its entirety, describes the manufacturing of precision freeform optical components by plasma jet machining at atmospheric pressure. Due to the purely chemical mechanism of material removal based on a dry etching process using a fluorine-containing gas, the options for materials that can be machined by plasma jet machining are limited. The above-mentioned paper and the dissertation "Investigation on Reactivity Driven Etching Mechanism in Plasma Jet-based Precision Surface Machining of Borosilicate Crown optical Glass", F. Kazemi, Leibnitz Institute of Surface Engineering (IOM), 2020 also state that the machining of borosilicate crown glass where etching produces a residual layer is also possible by reactive plasma jet machining. Using the plasma jet machining described in this paper and dissertation, an aluminum oxide layer can be removed.

[0023] When a fluorine plasma jet is used for the removal of the aluminum oxide layer, this treatment can be carried out in a single step where the reactive fluorine species of the fluorine plasma jet first removes the aluminum oxide layer by reactive etching and subsequently forms a passivation protection layer of aluminum fluoride. At this time, the plasma parameters are usually selected such that the reactive etching of the aluminum oxide layer is first carried out, i.e., the aluminum oxide layer is first removed. Different plasma parameters that do not cause material removal are generally established for the purpose of forming the aluminum fluoride layer.

[0024] If appropriate plasma parameters are established during irradiation, the following chemical reactions may proceed within the aluminum oxide layer. 2Al2O3+12F * →4AlF3+3O2 In the above equation, F * represents a reactive fluorine species. In this case, the aluminum oxide layer is converted to an aluminum fluoride layer, so the aluminum oxide layer is removed at least partially. In this case, optionally, it is possible to remove the aluminum oxide layer and form an aluminum fluoride layer without having to change the plasma parameters of the fluorine plasma jet irradiation.

[0025] In an additional modified embodiment, a protective layer applied to the aluminum surface is removed before the formation of the aluminum fluoride layer. This protective layer is not the (natural) aluminum oxide layer described further above, but rather a specially applied protective layer intended specifically to protect the aluminum surface from the formation of the natural aluminum oxide layer. The material of the protective layer should be chosen so that the protective layer material can be removed in a simple manner.

[0026] In one advanced embodiment of this modified model, the protective layer is formed from at least one material that forms volatile fluorine species with fluorine. Such a protective layer can be removed in a simple manner, for example, by a fluorine plasma jet.

[0027] The protective layer may be formed from, for example, silicon or carbon. These materials prevent the formation of a native aluminum oxide layer on the aluminum surface and can be removed from the aluminum surface more easily than the aluminum oxide layer.

[0028] In one advanced embodiment, the protective layer is removed by irradiation with a fluorine plasma jet. In this case, the fluorine species in the fluorine plasma jet etch the protective layer material, for example, Si or C, to form volatile species, for example, SiF4 or CF4. To increase the etching rate, for example, by the formation of additional CO2, it is possible to add oxygen to the fluorine plasma jet or irradiate with the fluorine plasma jet in an oxygen-containing environment. In this advanced embodiment as well, the irradiation with the fluorine plasma jet subsequently forms an aluminum fluoride layer on the aluminum surface by oxidation of metallic aluminum, i.e., after the removal of the protective layer.

[0029] In an alternative modified embodiment, a fluorine plasma jet is irradiated onto a protective layer of at least one metal fluoride coated on the aluminum surface for post-fluorination. In this case, unlike the further description above, instead of irradiating with a fluorine plasma jet to form an aluminum fluoride layer, post-fluorination is performed on an existing metal fluoride layer, such as a LiF layer, MgF2 layer, or AlF3 layer. It is clear that a protective layer in the form of an aluminum fluoride layer can be produced, for example, using the alternative modified embodiment of the method further described above, or otherwise by a thermal deposition process.

[0030] When reflective optical elements were irradiated with VUV radiation under different ambient conditions, significant photon-induced damage (formation of color centers and oxidation) to the protective layer in the form of a metallic fluoride was detected. The damage may extend to the aluminum surface forming the interface between the metallic fluoride protective layer and the aluminum substrate or aluminum layer, to such an extent that the functionality of the aluminum mirror may be completely lost.

[0031] The inventors of this invention have recognized that irradiating a protective layer of at least one type of metal fluoride with a fluorine plasma jet can restore or maintain the optical properties of a reflective optical element after irradiation with VUV radiation. This utilizes the fact that active fluorine species in the fluorine plasma, for example, active fluorine species in the form of fluorine radicals in the fluorine plasma, convert the metal oxides formed in the protective layer to metal fluorides during VUV irradiation, and fill or eliminate color centers formed during VUV irradiation. Since the metal fluorides used in the protective layer generally have lower absorption to VUV radiation than the corresponding metal oxides, the conversion of metal oxides to metal fluorides increases the reflectivity of the reflective optical element.

[0032] Both processes, namely the conversion of metal oxides to metal fluorides and the elimination of color centers by active fluoride species, lead to an increase of up to 80% in the VUV reflectivity of the reflective optical element, which ensures the functionality of the reflective optical element for the desired application. Treatment using a fluorine plasma jet can further convert aluminum oxide formed on the aluminum surface into aluminum fluoride, which also leads to an increase in reflectivity.

[0033] In additional modified embodiments, the reflective optical element is irradiated with VUV radiation before processing. VUV radiation irradiation is typically performed during the operation of the optical device incorporating the reflective optical element. As further described above, VUV radiation irradiation leads to a degradation process, for example, the degradation of a protective layer containing at least one type of metal fluoride, but also to an increase in the thickness of the (spontaneous) aluminum oxide layer, which reduces the reflectivity of the optical element. It is advantageous or advisable to perform processing of the reflective optical element occasionally, especially regularly, after a certain period of VUV radiation irradiation. Alternatively, it is also possible to monitor the reflectivity of the optical element to VUV radiation and perform processing when the reflectivity falls below a threshold.

[0034] The fluorine plasma jet is generated by a plasma source, which may be, for example, a high-frequency plasma source or a microwave plasma source. The process is typically carried out in a process chamber where reflective optical elements are arranged for the process. Other components placed in the process chamber must be compatible with the fluorine-containing gas, i.e., they must not be attacked by reactive fluorine.

[0035] In one modified embodiment, a hydrogen plasma jet and / or fluorine plasma jet are moved across the aluminum surface while processing a reflective optical element. For processing, it may be insufficient for the hydrogen plasma jet and / or fluorine plasma jet to be aligned to a single position, for example, to the center of the aluminum surface. Instead, it has been found to be advantageous to move the hydrogen plasma jet and / or fluorine plasma jet across the aluminum surface. To enable this, the plasma processing system into which the reflective optical element is introduced for processing may have a position control system. It is also advantageous for the plasma processing system to have a temperature monitoring or control system to monitor or control the temperature within the process chamber. It has also been found to be advantageous or necessary to have a detection system for detecting fluorine gas (F2) or a safety protection system to prevent fluorine gas leakage from the process chamber.

[0036] Moving a hydrogen plasma jet and / or fluorine plasma jet across the surface of an aluminum surface allows for spatially resolved processing of reflective optical elements by varying the duration for which the hydrogen / fluorine plasma jet is directed to each location on the surface. In this way, the thickness of the aluminum oxide layer removed by the hydrogen plasma jet can be locally varied across the surface. Furthermore, the fluorination of the aluminum surface by the fluorine plasma jet, and therefore the thickness of the aluminum fluoride layer formed on the aluminum surface, can also be locally varied across the surface.

[0037] In particular, by varying the duration of irradiation of the aluminum surface with a fluorine plasma jet along the radial direction, it is possible to radially vary the fluorination by the fluorine plasma jet. In this way, an aluminum fluoride layer with a radially varying thickness distribution can be formed on the aluminum surface. Such an aluminum fluoride layer with a radially varying thickness distribution can be advantageously used as a gray filter with a radially varying filter profile. This type of gray filter may be used, for example, to produce or compensate for an apodization effect. Alternatively, it is understood that the duration of fluorination by the fluoride plasma jet and / or reduction by the hydrogen plasma jet may be varied across the surface in a non-rotationally symmetric manner.

[0038] In additional modified embodiments, the processing of reflective optical elements is carried out under vacuum conditions. The proposed process for processing reflective optical elements does not necessarily require being carried out under vacuum conditions. However, it has been found that carrying out the processing, including fluorine plasma jets, under vacuum conditions is advantageous for better environmental control and to reduce the risk of surface contamination. The same is true for processing using hydrogen plasma jets.

[0039] In additional modified embodiments, the fluorine plasma jet includes at least one reactive gas selected from the group including CF4, CHF3, C2F6, NF3, SF6, and F2. It is also evident that other reactive gases, such as XeF2, XeF4, and XeF6, may be present in the plasma jet. The reactive gas is excited in the plasma source to generate reactive fluorine gas species, such as fluorine radicals, fluorine ions, or reactive fluorine gas species in the form of fluorine in an excited electronic state. It is also possible to add a small proportion of oxygen (O2) to the fluorine plasma jet to increase the etching rate, for example, when removing a protective layer of a material that forms volatile species with oxygen. This is the case, for example, in the case of a carbon protective layer, because carbon forms volatile CO2 with oxygen.

[0040] In additional modified embodiments, the fluorine plasma jet includes at least one carrier gas, preferably selected from the group including N2, He, and Ar. Reactive species generated by the plasma source are typically incorporated into the carrier gas and guided onto the aluminum surface or protective layer in the form of a fluorine plasma jet. The carrier gas is generally an inert gas, such as nitrogen or a noble gas, and the inert gas can optionally include Ne and Kr, as well as He and Ar.

[0041] Due to the higher reactivity of the activated fluorine species, the proposed process is stable and robust across a wide range of plasma conditions. These plasma conditions include, for example, the gas flow rate of the plasma jet, the process chamber pressure, the output power of the plasma source, the distance between the plasma source and the aluminum surface, the processing time, and the temperature.

[0042] Additional features and effects of the present invention will become apparent from the following description of embodiments of the invention with reference to the drawings, and from the claims, which illustrate details essential to the invention. Each feature can be implemented separately in its own right, or collectively in any combination of modified embodiments of the invention.

[0043] Examples are shown in the schematic diagrams, which will be clarified in the following description. [Brief explanation of the drawing]

[0044] [Figure 1a-1b] This is a schematic diagram of a mirror for the VUV wavelength range with an exposed aluminum surface, showing the conditions during and after irradiation with VUV radiation. [Figure 2a-2b] Figures 1a and 1b show schematic diagrams of the mirror during and after processing using a fluorine plasma jet to form an aluminum fluoride layer. [Figure 3a-3c] Figures 1a and 1b show schematic diagrams of the mirror during processing using a hydrogen plasma jet and during and after processing using a fluorine plasma jet. [Figure 4a-4b] This is a schematic diagram of a VUV wavelength range mirror with a protective layer during and after processing using a fluorine plasma jet to form an aluminum fluoride layer. [Figures 5a-5b] This is a schematic diagram of a VUV wavelength range mirror with a metal fluoride protective layer during and after irradiation with VUV radiation. [Figures 6a-6b] Figures 5a and 5b show schematic diagrams of the mirror during and after processing using a fluorine plasma jet for post-fluorination of the protective layer. [Figure 7] This is a schematic diagram of an optical device for the VUV wavelength range in the form of a VUV lithography apparatus. [Figure 8] This is a schematic diagram of an optical device for the VUV wavelength range in the form of a wafer inspection system. [Modes for carrying out the invention]

[0045] In the following description of the diagram, the same reference numeral is used for the same component or a component having the same function.

[0046] Figures 1a and 1b show schematic diagrams of a reflective optical element for the VUV wavelength range in the form of a mirror 1. The mirror 1 has an aluminum substrate 2, which has an aluminum surface 3 that acts as a mirror. As is clear from Figure 1a, during the operation of the optical device, VUV radiation 5 strikes the reflective optical element 1, and the VUV radiation 5 is reflected by the aluminum surface 3 of the mirror 1. Alternatively, the reflective aluminum surface 3 may be formed on an aluminum layer coated on a substrate, such as a quartz glass substrate or a substrate of another material. As is clear from Figure 1a, a native aluminum oxide layer 4 has already been formed on the aluminum surface 3.

[0047] Figure 1b shows the mirror 1 after irradiation with VUV radiation 5 over a defined period. As is clear from Figure 1b, irradiation with VUV radiation 5 increased the thickness of the aluminum oxide layer 4. To increase the reflectivity of the mirror 1 after irradiation with VUV radiation 5 to approximately 80%, which is required for use in, for example, a wafer inspection system, the mirror 1, after irradiation with VUV radiation 5 over a defined period, is introduced into a process chamber 6, as shown in Figure 2a, and subjected to processing with a fluorine plasma jet 7. Processing with the fluorine plasma jet 7 can be performed periodically whenever the irradiation with VUV radiation 5 in the optical device reaches a certain duration. Alternatively, processing with the fluorine plasma jet 7 can be performed whenever the reflectivity of the mirror 1 falls below a threshold.

[0048] In the processing of the mirror 1 using the fluorine plasma jet 7, the fluorine plasma jet 7 is guided to the aluminum surface 3 of the mirror 1 and moved across the surface 3 using 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 reactive fluorine species are present in an inert carrier gas, such as N2, He or Ar, Ne, Kr, etc. Within the plasma source 8, reactive fluorine species, such as reactive fluorine radicals, are generated from the reactive fluorine gas. The reactive fluorine gas may be a gas selected from the group including, for example, CF4, CHF3, C2F6, NF3, SF6, and F2. Fluorine-containing gases such as XeF2, XeF4, and XeF6 are also possible. It is also possible to add oxygen (O2) to the fluorine plasma jet 7.

[0049] The output of the plasma source 8 and other plasma parameters may be adjusted as described in the literature cited above or in F. Kazemi's doctoral dissertation, so that the aluminum oxide layer 4 is removed first by reactive (ionic) etching. After the removal of the aluminum oxide layer 4, the metallic aluminum on the aluminum surface 3 can be oxidized by a fluorine plasma jet 7 to form the aluminum fluoride layer 9 shown in Figure 2b.

[0050] Alternatively, the plasma parameters can be adjusted so that the following chemical reactions proceed while the aluminum oxide layer 4 is irradiated with the fluorine plasma jet 7. 2Al2O3+12F * →4AlF3+3O2

[0051] In the reaction equation above, F * This represents a reactive fluorine species.

[0052] In this case, the treatment using the fluorine plasma jet 7 converts the aluminum oxide layer 4 into the aluminum fluoride layer 9 shown in Figure 2b. The aluminum fluoride layer 9 forms a passivation protective layer and has significantly lower absorption to VUV radiation 5 than the aluminum oxide layer 4. The treated mirror 1 shown in Figure 2b has sufficient reflectivity to be introduced (and possibly reintroduced) into an optical device and to operate within the optical device.

[0053] Furthermore, as explained above, the processing using the fluorine plasma jet 7 is carried out in the process chamber 6 shown in Figure 2a. To minimize the risk of surface contamination, the processing is carried out under vacuum conditions in the example shown. Alternatively, the processing using the fluorine plasma jet 7 can be carried out in a process chamber 6 at a higher pressure, for example, in a process chamber 6 at atmospheric pressure.

[0054] The components present within the process chamber 6 must withstand attack by reactive fluorine species or fluorine gas. Generally, monitoring of the process chamber 6 for leaked fluorine gas, or corresponding safety checks, is also required. The temperature within the process chamber 6 should also be monitored and adjusted or regulated as necessary.

[0055] Figures 3a and 3c show alternative methods for treating mirror 1 to restore its reflectivity in Figure 1a. In this treatment, a hydrogen plasma jet 10 is irradiated onto the aluminum oxide layer 4 in the first step shown in Figure 3a to remove the aluminum oxide layer 4 from the aluminum surface 3. The hydrogen plasma jet 10 is a gas jet in which reactive hydrogen species are present in an inert carrier gas, such as N2, He or Ar, Ne, Kr, etc. The reactive hydrogen species are generated in an additional plasma source 11, for example, as molecular hydrogen moves through a heated filament. It will also be apparent that, where appropriate, the aluminum oxide layer 4 can be removed from the aluminum surface 3 in a manner different from that using the hydrogen plasma jet 10.

[0056] In the second step of the treatment of mirror 1, as shown in FIG. 3b, the exposed aluminum surface 3 is irradiated with a fluorine plasma jet 7 as described in connection with FIG. 2a. At this time, the metallic aluminum on the surface 3 is oxidized to aluminum fluoride, and in that case, for example, the following chemical reaction may proceed. Al + 3F * →AlF3

[0057] After the treatment described in FIGS. 3a and 3b, the reflective optical element 1 has a passivating aluminum fluoride layer 9 (see FIG. 3c), and the reflective optical element 1 can be used in an optical device. The treatment using this hydrogen plasma jet 10 and fluorine plasma jet 7 can be carried out in one and the same process chamber 6. However, it is also possible to carry out this treatment in two different process chambers or in two different plasma treatment systems.

[0058] By changing the duration for guiding the fluorine plasma jet 7 to a specific position on the exposed aluminum surface 3, a passivating aluminum fluoride layer 9 having a thickness profile dependent on the position can be obtained. For example, if the duration of irradiation of the fluorine plasma jet 7 is changed in a rotationally symmetric manner, a passivating aluminum fluoride layer 9 having a rotationally symmetric thickness profile can be generated. Such a rotationally symmetric thickness profile can be used, for example, as a gray filter to produce an apodization effect or to compensate for an apodization effect.

[0059] FIGS. 4a and 4b show volatile fluorine species M together with fluorine F a F bThis describes the processing of a mirror 1 for the VUV wavelength range, in which a special protective layer 12 made of at least one material M that forms a protective layer is applied to the aluminum surface 3. The protective layer 12 may be made of, for example, silicon or carbon. The protective layer 12 is deposited in a process chamber 6 by a conventional coating method in a previous step (not shown) immediately after depositing aluminum on the aluminum surface 3 in order to prevent the formation of a native aluminum oxide layer.

[0060] As shown in Figure 4a, the protective layer 12 is removed by reactive etching using a fluorine plasma jet 7, and simultaneously or afterward, the aluminum fluoride layer 9 shown in Figure 4b is formed. In the treatment of the silicon protective layer 12 shown in Figure 4a, for example, the following chemical reactions may occur: Si+4F * →SiF4 Al+3F * →AlF3

[0061] SiF4 is a volatile fluoride that does not remain on the aluminum surface 3, so that the silicon protective layer 12 is removed using the fluorine plasma jet 7 until the aluminum surface 3 is exposed. Correspondingly, in the case of a protective layer 12 made of carbon, irradiation with the fluorine plasma jet 7 also forms volatile CF4. Furthermore, oxygen may be added as a reactive gas to promote the formation of volatile CO2 and to increase the etching rate during this irradiation. The carbon is removed until the aluminum surface 3 is exposed. As shown in Figure 4b, the reaction between the metallic aluminum on the exposed aluminum surface 3 and the activated fluorine species in the fluorine plasma jet 7 forms a passivated aluminum fluoride layer 9 on the mirror 1.

[0062] Figures 5a and 5b show, in contrast to the mirror 1 shown in Figures 1a and 1b, a mirror 1 coated with a protective layer 13 in the form of a metal fluoride layer before irradiation with VUV radiation 5. The metal fluoride may be, for example, LiF, MgF2, or AlF3. If the metal fluoride is AlF3, the protective layer 13 may also be formed by the treatment of the mirror 1 described above and may correspond to the aluminum fluoride layer 9.

[0063] When the protective layer 13 is irradiated with VUV radiation 5, the metal fluoride in the mirror 1 absorbs oxidizing gases in the environment, such as O2, or possibly O3, H2O, N2O, O * , OH * NO * , O( 1 It reacted with D) and other substances to form a metal oxide. In addition, as shown in Figure 5b, aluminum was also converted to aluminum oxide (Al2O3) at the surface 3 that forms the interface between the protective layer 13 and the aluminum substrate 2. As similarly shown in Figure 5b, when irradiated with VUV radiation 5, a color center 14 is formed within the protective layer 13.

[0064] The conversion of metal fluorides in the protective layer 13 to metal oxides and the formation of color centers 14, along with the oxidation of metallic aluminum on the surface 3 to Al2O3, all result in a clear reduction in the reflectivity of the mirror 1.

[0065] Therefore, the mirror 1 shown in Figure 5b is subjected to treatment with a fluorine plasma jet 7, as shown in Figure 6a. Irradiation of the mirror 1, or more specifically the protective layer 13, is carried out in a process chamber not shown in Figure 6a. The process chamber or plasma coating system may be designed as further described above in relation to Figure 2a.

[0066] The output of the plasma source 8 and other plasma parameters are adjusted so that the following chemical reactions proceed within the protective layer 13 or on the surface 3. 2MO+F * →2MF+O2 2Al2O3+12F * →4AlF3+3O2

[0067] In the above reaction equations, MO represents a metal oxide and MF represents a metal fluoride. As is clear from these reaction equations, the formed metal oxide MO is converted to metal fluoride MF, and therefore the fluorine species F * As a result of this reaction, the protective layer 13 is refluorinated. In addition, the treatment using the fluorine plasma jet 7 also converts the aluminum oxide formed on the aluminum surface 3 into aluminum fluoride, which similarly leads to an increase in reflectivity.

[0068] As is clear from Figure 6b, the treatment using the fluorine plasma jet 7 also eliminates the color center 14. These processes described above increase the VUV reflectivity of the mirror 1 to 80%, extending the lifespan of the mirror 1.

[0069] Mirror 1, processed in the manner described above, can be used in different optical devices for the VUV wavelength range.

[0070] Figure 7 shows an optical system for the VUV wavelength range in the form of a VUV lithography apparatus 21. The VUV lithography apparatus 21 comprises two optical systems, namely an illumination system 22 and a projection system 23. The VUV lithography apparatus 21 further has a radiation source 24, which can be, for example, an excimer laser.

[0071] The radiation 25 emitted by the radiation source 24 is adjusted using an illumination system 22 so that a mask 26, also called a reticle, is illuminated by the radiation 25. In the example shown, the illumination system 22 has a housing 32, which contains both transmitted and reflected optical elements. As a representative example, this figure shows a transmitted optical element 27 that focuses the radiation 25 and a reflected optical element 28 that deflects the radiation.

[0072] The mask 26 has a structure on its surface, which is transferred to an optical element 29 to be exposed, such as a wafer, using a projection system 23, for the purpose of manufacturing a semiconductor component. In the example shown, the mask 26 is designed as a transmission optical element. In alternative embodiments, the mask 26 may also be designed as a reflection optical element.

[0073] In the example shown, the projection system 22 has at least one transmission optical element. The example shown, as a representative example, shows two transmission optical elements 30, 31, which, for example, reduce the structure on the mask 26 to the size desired for exposure of the wafer 29.

[0074] Both the illumination system 22 and the projection system 23 can combine a wide variety of transmitted optical elements, reflected optical elements, or other optical elements as desired, including in more complex combinations. Furthermore, optical devices that do not include transmitted optical elements can also be used for VUV lithography.

[0075] Figure 8 shows an optical device for the VUV wavelength range in the form of a wafer inspection system 41. However, the optical device may also be a mask inspection system. The wafer inspection system 41 has an optical system 42 including a radiation source 54, and radiation 55 is directed from the radiation source 54 onto the wafer 49 by the optical system 42. For this purpose, the radiation 55 is reflected onto the wafer 49 by a concave mirror 46. In the case of a mask inspection system, it would be possible to inspect a mask instead of a wafer 49. The radiation reflected, diffracted and / or refracted by the wafer 49 is directed onto a detector 50 by an additional concave mirror 48 for additional evaluation. The additional concave mirror 48 is also associated with the optical system 42 via a transmission optical element 47. The wafer inspection system 41 further has a housing 52, within which two mirrors 46, 48 and the transmission optical element 47 are arranged. The radiation source 54 may be, for example, exactly one radiation source, or a combination of multiple individual radiation sources to provide a substantially continuous radiation spectrum. In the modification, one or more narrowband radiation sources 54 may also be used.

[0076] At least one reflective optical element 28 of the VUV lithography apparatus 21 shown in Figure 7, and at least one of the reflective optical elements 46, 48 of the wafer inspection system 41 shown in Figure 8 may be further processed in the manner described above and irradiated with a fluorine plasma jet 7. In particular, 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 that has been post-fluorinated by the method described in relation to Figures 6a and 6b.

Claims

1. A method for treating a reflective optical element (1) for the VUV wavelength range having an aluminum surface (3), wherein the treatment of the reflective optical element (1) is To remove the aluminum oxide layer (4) formed on the aluminum surface (3), the reflective optical element (1) is irradiated with a hydrogen plasma jet (10). Methods that include...

2. The hydrogen plasma jet (10) is N 2 The method according to claim 1, comprising at least one carrier gas selected from the group including and Ar.

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

4. The method according to claim 3, wherein the irradiation of the fluorine plasma jet (7) forms an aluminum fluoride layer (9) on the aluminum surface (3).

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

6. The protective layer (12) contains fluorine (F) along with volatile fluorine species (M a F b The method according to claim 5, wherein the material is formed from at least one material (M) that forms the )

7. The method according to claim 5 or 6, wherein the protective layer (12) is formed from silicon or carbon.

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

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

10. The method according to any one of claims 1 to 9, wherein the reflective optical element (1) is irradiated with VUV radiation (5) before the above-mentioned process.

11. The method according to any one of claims 1 to 10, wherein the hydrogen plasma jet (10) and / or the fluorine plasma jet (7) are moved across the aluminum surface (3) while the reflective optical element (1) is being processed.

12. The method according to any one of claims 1 to 11, wherein the processing of the reflective optical element (1) is performed under vacuum conditions.

13. The fluorine plasma jet (7) contains at least one reactive fluorine gas selected from the group consisting of CF 4 , CHF 3 , C 2 F 6 , NF 3 , SF 6 and F 2 The method according to any one of claims 1 to 12, comprising.

14. The fluorine plasma jet (7) contains at least one carrier gas, preferably N 2 The method according to any one of claims 1 to 13, comprising at least one carrier gas selected from the group including He and Ar.

15. An optical element (1) for use in the VUV wavelength range, wherein the optical element (1) is treated by the method described in any one of claims 1 to 14.

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

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