Method for operating an optical system, and optical system

Abstract

This invention relates to a method of operating an optical system, particularly for the FUV / VUV wavelength range, wherein the optical system comprises at least one reflective optical element (1) having a metal surface (3a) to which a metal fluoride layer (4) is applied, wherein operating the optical system (10) involves irradiating the optical element (1) with radiation (6), particularly in the FUV / VUV wavelength range. In this method, the optical system is operated under multiple oxidizing environmental conditions (OB) of the optical element (1) during a first time interval and under multiple fluorinating environmental conditions (FB) of the optical element (1) during a second time interval, wherein the first time interval follows the second time interval, or the second time interval follows the first time interval. Preferably, the optical system (10) is operated alternately under the oxidizing environmental conditions (OB) and the fluorinating environmental conditions (FB) of the optical element (1). This invention also relates to an optical system.

Description

[Technical Field]

[0001] This invention relates to a method of operating an optical system, particularly for the FUV / VUV wavelength range, wherein the optical system includes at least one reflective optical element having a metal surface to which a metal fluoride layer is applied, and wherein operating the optical system involves irradiating the optical element with radiation, particularly in the FUV / VUV wavelength range. This invention also relates to an optical system, preferably for the FUV / VUV wavelength range, particularly for FUV / VUV lithography equipment or wafer inspection systems, comprising: at least one reflective optical element having a metal surface to which a metal fluoride layer is applied; and a light source that irradiates the reflective optical element with radiation, particularly in the FUV / VUV wavelength range. [Interactive Reference]

[0002] This application claims priority to German patent application No. 102024201798.1, filed on February 27, 2024, all of the disclosures of which are incorporated by reference or are considered part of the disclosures of this application. [Previous Technology]

[0003] Within the meaning of this invention, the VUV wavelength range is understood to be the wavelength range between 100 nm and 200 nm (VUV wavelength range according to Part 7 of DIN 5031). Within the meaning of this invention, the FUV wavelength range is understood to be the wavelength range between 200 nm and 280 nm. The optical systems described herein can be designed or configured to operate within the VUV wavelength range, within the FUV wavelength range, or within both wavelength ranges.

[0004] Specifically, in the VUV wavelength range, it is generally not possible to use only transmissive optical elements, and the resources of reflective optical elements are usually also required. Reflective optical elements with metallic surfaces have proven valuable herein, said surfaces typically being composed of aluminum or aluminum-containing materials because such materials have high reflectivity in the VUV wavelength range. The metallic surface can be formed on a substrate of an optical element made of a metallic material. Typically, the metallic surface is formed on a metal layer applied to the substrate of the optical element. An overlying protective layer may be applied to the metal layer to protect it from oxidation; this protective layer is typically a metal fluoride layer because metal fluorides have a large band gap.

[0005] Despite the protective coating of metal fluoride, it has been found that in optical systems operating under high radiation intensity, such as in lithography processes, especially when inspecting photomasks and wafers, reflective optics gradually degrade, resulting in significant reflectivity loss and consequently shortening their lifespan. This is because in such optical systems, the environment surrounding the reflective optics (which may be a vacuum or blowing gas) inevitably contains residual concentrations of oxygen-containing substances, such as water or oxygen, which combine with high-energy photons and oxidize the surface of the protective metal fluoride layer. In principle, by appropriately selecting the parameters around the reflective optics, the oxidation or passivation of the metal fluoride layer can be significantly slowed down (see below), although reflectivity will still be significantly lost over a relatively long operating time (e.g., a 20% reflectivity loss if a layer of approximately 3 nm MgO is applied to MgF2, and a 15% reflectivity loss if a layer of approximately 3 nm Al2O3 is applied to AlF3), which is generally unacceptable from the perspective of the overall transmission of the optical system.

[0006] Document DE 10 2018 211 499 A1 describes a reflective optical element configured as described above, wherein an oxide layer is additionally applied to a metal fluoride layer to protect the underlying layer and thus increase the lifetime of the reflective optical element. Applying an additional protective layer in the form of an oxide layer causes the problem further described above, namely that most oxides have high absorptivity at wavelengths less than 160 nm, and may therefore lead to a significant loss of reflectivity. Document DE 10 2018 211 499 A1 proposes to reduce the loss of reflectivity by forming an electric field standing wave with a minimum value in the oxide layer region during reflection.

[0007] Document DE 10 2021 200 490 A1 describes a method for forming a protective layer on a reflective optical element configured as further described above. To form the protective layer, an electromagnetic radiation with a wavelength less than 300 nm is used to irradiate a metal fluoride layer. The radiation causes the metal fluoride layer to passivate, thereby resisting degradation of the metal layer. The passivation protective layer is typically an oxide layer, which leads to the aforementioned disadvantages.

[0008] Document US 11,262,664 B2 describes a system and method for protecting optical elements from damage during VUV light irradiation. The system has a light source for generating VUV light and a cavity containing a fluorine-based compound with a defined partial pressure. The optical element is disposed within the cavity and exposed to the fluorine-based compound. The VUV light generated by the light source has sufficient energy to convert the fluorine-based compound in the cavity into primary products that may contain fluorine atoms. Thus, the aim is to prevent the loss of fluorine and the resulting oxidation of the optical element, for example, the optical element may be composed of a metal fluoride.

[0009] In the case of the method described in document US 11,262,664 B2, however, excess fluorine atoms adsorbed on the surface of the metal fluoride layer may diffuse and reach the interface with the underlying metal layer, causing it to fluorinate. This can also lead to a decrease in the reflectivity of the optical element. Theoretically, a balance between the oxide and fluoride substances around the optical element can be found. However, this is not feasible in practice, or is only possible with significant expenditure, because each operating mode of the optical system (different wavelength bands, different VUV radiation intensities, etc.) requires different concentrations of fluorine-containing gas, and each concentration needs to be known and accurately set. Therefore, the optical system described in document US 11,262,664 B2 may need to operate under excessively high concentrations of fluorine-containing gas, which will accelerate the degradation of the optical element and thus significantly reduce the transmittance of the optical system. [Summary of the Invention]

[0010] One object of the present invention is to provide a method and an optical system for operating an optical system, wherein the transmittance decreases only slightly even after long-term operation.

[0011] According to one form, this objective is achieved by a method described in the preamble, wherein the optical system is operated under multiple oxidizing environments of the optical element during a first time interval and under multiple fluorinating environments of the optical element during a second time interval, wherein the first time interval follows the second time interval, or the second time interval follows the first time interval.

[0012] This invention proposes operating the optical element of the optical system under multiple oxidizing environmental conditions during a first time interval and under multiple fluorinating environmental conditions during a second time interval. The inventors have discovered that when operating the optical element under static environmental conditions, as described in document DE102021200490A1 for oxidizing environmental conditions and document US 11,262,664 B2 for fluorinating environmental conditions, harmful interactions continue to occur during the operation of the optical system as the operating time increases. These interactions occur through surface oxidation of the metal fluoride layer in an oxidizing environment and fluorination of the metal surface or metal layer interface under fluorinating environmental conditions.

[0013] Preferably, the optical element is operated alternately under oxidizing and fluorinating environmental conditions. In this case, reversible surface oxidation of the metal fluoride layer can be limited by refluorination, while simultaneously preventing excessive addition of fluorine, thereby preventing irreversible fluorination at the interface between the metal fluoride layer and the metal layer or metal surface. Compared to fluorination of the metal surface that typically forms the interface between the metal layer and the metal fluoride layer, this utilizes the fact that oxidation and refluorination of the metal fluoride layer surface are reversible.

[0014] In one variation, an oxide layer has been formed or is formed on the side of the metal fluoride layer facing away from the metal surface under the oxidizing environment conditions of the optical element, wherein the thickness of the oxide layer increases with the duration of such oxidizing environment conditions. The oxide layer can be formed when the surface of the metal fluoride layer is irradiated, typically with radiation in the FUV / VUV wavelength range, under oxidizing environment conditions, i.e., in the presence of oxygen-containing gas around the reflective optical element. The formation of the oxide layer involves the conversion of a volumetric region of the metal fluoride layer near the surface into an oxide layer. Therefore, the oxide layer formed under oxidizing environment conditions does not extend to the entire thickness of the metal fluoride layer. For more detailed information on the ambient atmospheric parameters and radiation parameters that cause or promote the formation of the oxide layer, refer to document DE102021200490A1 cited in the introduction, the contents of which are incorporated herein by reference in their entirety. Alternatively, a thin oxide layer can be applied to the metal fluoride layer as early as possible during the formation process, or an oxide layer can be formed under previous oxidizing conditions without complete erosion under fluorinated conditions (see below).

[0015] As further described above, the thickness of the oxide layer on the metal fluoride layer increases with the duration of the oxidizing environment. The thin oxide layer acts as a diffusion barrier and prevents fluorine atoms present in the surrounding environment from reaching the metal surface or the interface between the metal layer and the metal fluoride layer. As the duration of the fluoridating environment increases, the thickness of the thin oxide layer on the surface of the metal fluoride layer decreases because, under the fluoridating environment conditions, as described in more detail below, it is wholly or partially converted into fluoride or refluorinated.

[0016] In one advantageous development, under the fluorinated environmental conditions of the optical element, the oxide layer remains on the side of the metal fluoride layer facing away from the metal surface. In this case, the fluorinated environmental conditions are switched to oxidizing environmental conditions before the oxide layer is completely removed. Thus, fluorination of the metal surface or the interface between the metal fluoride layer and the metal layer can be stopped or at least greatly slowed down, which increases the lifespan of the reflective optical element compared to the method described in document US 11,262,664 B2. Because the optical system operates under alternating environmental conditions of the reflective optical element, harmful but reversible reactions are confined to the surface of the protective metal fluoride layer, and irreparable deep damage due to fluorination of the metal surface or the interface between the metal fluoride layer and the metal layer is avoided.

[0017] In one development, the thickness of the metal fluoride layer is selected such that when the reflective optical element is irradiated by radiation of a wavelength used by the optical system, the electric field of the resulting standing wave has a minimum value in the region of the oxide layer. In this variant, the thickness of the metal fluoride layer and the thickness of other layers that may be present and applied to the metal surface are selected such that the electric field or its amplitude in the oxide layer is minimized as much as possible, and the absorption of the radiation used by the oxide layer is minimized as much as possible. For more detailed information regarding the design or appropriate selection of the thickness of the metal fluoride layer and the possibilities of applying other layers to the metal surface (e.g., in the form of an adhesion promoter layer), please refer to document DE102018211499A1, which is cited in the introduction and whose contents are incorporated herein by reference in their entirety.

[0018] In another variation, to switch between oxidizing and fluorinating environmental conditions, the concentration of at least one fluorinated gas and / or at least one oxygen-containing gas in the environment surrounding the reflective optical element is changed. Preferably, under fluorinating environmental conditions, at least one fluorinated gas is supplied to the environment surrounding the reflective optical element, while under oxidizing environmental conditions, no fluorinated gas is supplied. For switching between oxidizing and fluorinating environmental conditions, it is generally sufficient to change the concentration or metering of at least one fluorinated gas in the environment surrounding the reflective optical element. For example, to change the concentration, under fluorinating environmental conditions, at least one fluorinated gas can be supplied around the reflective optical element, while under oxidizing environmental conditions, a fluorine-free gas can be supplied. Generally, under oxidizing environmental conditions, it is not necessary to supply oxidizing gas around the optical element because the residual concentration of oxygen and / or water in the blowing gas is sufficient to create oxidizing environmental conditions. Therefore, the concentration of oxidizing gas around the optical element can remain constant. However, in principle, the concentration of oxygen-containing gas around the reflective optical element can also be changed instead or additionally. The concentration of fluorinated and / or oxidizing gases around the reflective optical element is chosen such that the surface of the optical element, which forms an interface with the surrounding environment, is oxidized under oxidizing conditions and refluorinated under fluorinated conditions. Under oxidizing conditions, a thin oxide layer is typically formed, or the thickness of an existing oxide layer is increased. Under fluorinated conditions, the thin oxide layer is converted back into fluoride or its thickness is reduced. Also under fluorinated conditions, the residual thickness of the oxide layer may remain on the metal fluoride layer (see above).

[0019] In one development, the concentration of at least one fluorinated gas and / or at least one oxygen-containing gas in the surrounding environment of the optical element is controlled as a function of at least one control parameter, which forms a measure of the reflectivity of the reflective optical element. The concentration of the at least one fluorinated gas and / or at least one oxygen-containing gas, and therefore the switching time between fluorinated and oxidized environmental conditions, is preferably controlled according to the function of at least one control parameter based on a measured variable representing a measure of the reflectivity of the optical element. The controller can automatically respond to different operating modes of the optical system, such as changes in the electromagnetic spectrum of the radiation used, the intensity of the radiation used, etc. In contrast, in the case of the method described in US document US 11,262,664 B2, the ideal concentration of the fluorinated gas must be known according to the operating mode of the optical system; otherwise, excessive fluorinated gas may be introduced, thus shortening the lifespan of the reflective optical element.

[0020] In one development, if the control parameter is between a first lower threshold value and a second higher threshold value, the concentration of the at least one fluorinated gas and / or the at least one oxygen-containing gas remains constant. As further described above, the control parameter, or the variable derived therefrom, forms a measure of the reflectivity of the optical element. For optical systems with full transmission or optical elements with reflectivity between the higher and lower threshold values, the concentration of the fluorinated gas and / or oxygen-containing gas can be kept constant, i.e., there is no need to switch between fluorinated and oxidized environmental conditions. The lower threshold value may be, for example, on the order of about 95% of the optical system transmittance or the optical element reflectivity, and the higher threshold value may be about 99%. Of course, other values ​​are also possible depending on the optical system or control parameter.

[0021] In one development, if the control parameter is below the first critical value, the concentration of the fluorine-containing gas increases and / or the concentration of the oxygen-containing gas decreases, and / or if the control parameter exceeds the second critical value, the concentration of the fluorine-containing gas decreases and / or the concentration of the oxygen-containing gas increases. If it is below the first lower critical value, the thickness of the oxide layer will be too large, leading to a decrease in the transmittance of the optical system. By reducing the concentration of the oxygen-containing gas and / or increasing the concentration of the fluorine-containing gas, the transmittance of the optical system can be improved. Therefore, if it exceeds the second critical value, the thickness of the oxide layer is too small, threatening to cause degradation of the metal surface or the interface between the metal layer and the metal fluoride layer. This problem can be solved by reducing the concentration or partial pressure of the fluorine-containing gas and / or increasing the concentration or partial pressure of the oxygen-containing gas.

[0022] Preferably, after changing the concentration of at least one fluorine-containing gas and / or at least one oxygen-containing gas, the optical system operates under multiple constant environmental conditions for at least a predetermined duration. It is advantageous not to continuously run the control loop, but rather to set or predefine a duration in which the optical system operates under constant environmental conditions after the environmental conditions have been switched or changed. The duration is typically on the order of hours or days. Needless to say, even after the duration has elapsed, the environmental conditions do not change automatically, but rather change with the value of the control parameters. The duration can be specifically set according to the concentration of the fluoride gas, so that the thin oxide layer always acts as a diffusion barrier for fluorine atoms.

[0023] In one development, the control parameter is selected from the group consisting of: the total transmittance of the optical system, the reflectivity of the reflective optical element, the chemical composition of the surface of the optical element, and the temperature of the optical element. At least one control parameter is measured using a suitable measurement system. To measure the total transmittance of the optical system, the measurement system may be configured as an external component of the cavity, wherein the reflective optical element is introduced into the cavity during operation of the optical system. To measure the reflectivity of the optical element, the optical element may be selectively illuminated with light from a measurement light source, for example, in the VUV wavelength range, and the light reflected from the optical element can be detected by a detector. This utilizes the fact that oxides have strong absorption in the VUV wavelength range, while fluorides are transparent. For temperature measurement, a temperature sensor may be used, such as a thermocouple, radiation pyrometer, or similar device integrated into the mirror holder. Temperature is a measure of the absorptivity of the reflective optical element, and therefore also a measure of the degree of oxidation of the reflective optical element or its reflectivity.

[0024] Alternatively or additionally, the chemical composition of the surface of the optical element or a volumetric region near the surface can be measured, thereby measuring the chemical composition or stoichiometry of the reflective optical element or the metal fluoride layer. Measurement methods known to those skilled in the art, such as X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence spectroscopy (XRF), can be used for this purpose. In this case, the chemical composition or a measurement of the chemical composition, such as the concentration of fluorine and / or oxygen on the surface forming the interface with the environment surrounding the optical element, can be used as a control parameter. It goes without saying that the control parameters described herein can also be combined with other control parameters in the control of the optical system.

[0025] Another aspect of the present invention relates to the optical system mentioned in the preamble, configured to operate under multiple oxidizing environmental conditions of the optical element during a first time interval and under multiple fluorinating environmental conditions of the optical element during a second time interval, wherein the first time interval follows the second time interval, or the second time interval follows the first time interval. Preferably, the optical system is configured to operate alternately under the oxidizing environmental conditions and the fluorinating environmental conditions of the optical element. This optical system has the advantages described above in relation to the method.

[0026] In one specific embodiment, the reflective optical element is disposed in a cavity, and the optical system has a supply device for supplying at least one fluorine-containing gas and / or at least one oxygen-containing gas to the cavity, wherein the supply device is configured to set the concentration of the fluorine-containing gas and / or the concentration of the oxygen-containing gas in the cavity.

[0027] In the case of fluorinating agents or fluorinated gases, for example, the following fluorinated gases can be used: F2, HF, XeF2, NF3, CF4, SF6. When operating under fluorinated environmental conditions, the partial pressure of the fluorinating agent is typically between 10⁻⁹ mbar and 10⁻¹ mbar, or between 1 pptV and 100 ppmV for the blowing gas, particularly between 10⁻⁶ mbar and 10⁻³ mbar or between 1 ppbV and 1 ppmV. For example, the fluorinated gas can be set to the desired partial pressure via a needle valve and can be measured or controlled via a dedicated fluorine gas sensor (e.g., a residual gas analyzer). As further described above, the concentration of at least one oxygen-containing gas can be maintained constant or can be changed in a manner corresponding to the setting of the fluorinated gas concentration. For this purpose, the optical system can have a metering unit for oxygen-containing gas or oxidizing substances (such as H₂O or O₂). Furthermore, a corresponding sensor for determining the partial pressure or concentration of the oxide species or oxygen-containing gas can be integrated into the optical system.

[0028] The light source used to illuminate reflective optical elements is typically the operating light source of the optical system, and its wavelength or wavelength range is usually in the FUV / VUV wavelength range. To set the fluorination environmental conditions, the light source must typically perform two tasks:

[0029] The light source is used to induce the photodissociation of at least one fluorine-containing gas to provide fluorine atoms for the fluorination of the oxide layer. The wavelength range of the light source can be tuned to the absorption profile of the fluorine-containing gas to utilize fluorine atoms as efficiently as possible. For this purpose, the light source used can provide, for example, light with a wavelength range between 115 nm and 1000 nm, preferably between 120 nm and 170 nm, and particularly preferably between 140 nm and 170 nm.

[0030] The light source also needs to provide activation energy for the fluorination process. For this purpose, the wavelength of the light provided by the light source can be appropriately adjusted or predefined. For this purpose, the light source used should produce light with a wavelength range of 115 nm to 1000 nm, preferably between 120 nm and 200 nm, and particularly preferably between 140 nm and 200 nm.

[0031] Alternatively or additionally, the photodissociation and / or activation energy of the fluorinated gas can be implemented by increasing the temperature or by other means, such as using plasma. For example, it is known to use NF3 for plasma-induced cleaning of PECVD coating equipment.

[0032] In another specific embodiment, the optical system further includes: a control device for controlling the concentration of at least one fluorine-containing gas and / or at least one oxygen-containing gas in the surrounding environment of the optical element as a function of at least one control parameter, which forms a measure of the reflectivity of the reflective optical element; and at least one measuring device for measuring the at least one control parameter. As further described above in conjunction with the method, it is advantageous to control the concentration of the fluorine-containing or oxygen-containing gas or to control the switching time between oxidizing and fluorinating environmental conditions. For this purpose, it is preferable to couple the fluorine-containing gas metering unit to the measuring device of the control parameter via a feedback loop, so that the desired concentration of the fluorine-containing gas and / or oxygen-containing gas is automatically set. For this purpose, the control device may have, for example, a mass flow controller.

[0033] In one development, the at least one control parameter is selected from the group consisting of: the total transmittance of the optical system, the reflectivity of the reflective optical element, the chemical composition of the surface of the optical element, and the temperature of the optical element. As further described above in the combined method, the control parameter can be measured using a suitable measuring device or a suitable sensor. The sensor or measuring device can be integrated into the cavity in which the reflective optical element is disposed. However, the measuring device can also be disposed outside the cavity or housing, for example, if the total transmittance of the optical system is measured as a control parameter.

[0034] The housing or cavity containing the reflective optical element, and all other components that come into contact with fluorine-containing gases (e.g., NF3, XeF2, SF6, CF4, HF, F2), their photodissociation substances (e.g., F, F2, F*), and / or their conversion products (e.g., HF), must be resistant to these substances. Resistance means, for example, forming a passivation layer on the inner side of the cavity. For the materials used, for example, the following materials and their alloys can be used: Ni, Fe, Cu, Co, Sc, Y, and Hf. An example of such materials is Monel metal, an alloy of nickel, copper, and iron. Specifically, volatile fluorine compounds must not form and deposit on the reflective optical element or other optical units. Therefore, the metal used must be free of Cr or Ti. Alternatively, a fluorine-resistant coating can be provided on the inner side or cavity wall to prevent corrosion. This coating can be applied, for example, in an electroplating process. Possible coating materials are, for example, NiP, Pt, or Ru / Rh mixtures.

[0035] In another specific embodiment, the cavity has an entrance window for allowing light from the light source to enter the cavity and / or an exit window for allowing light from the light source to exit the cavity. The entrance and / or exit windows can be used to separate the surrounding environment of the reflective optical element containing fluorine and / or oxygen-containing gas from the rest of the optical system. In principle, the cavity may also include the entire optical system, more precisely, all optical components of the optical system, if isolation from environmental conditions is not necessary, or if the components are resistant to fluorine or fluorides.

[0036] The metal fluoride layer of the reflective optical element may contain at least one material selected from the group consisting of: magnesium fluoride, aluminum fluoride, sodium fluoride, lithium fluoride, cryolite, cryolite, ytterbium fluoride, neodymium fluoride, thiodioxide fluoride, dysprosium fluoride, samarium fluoride, holmium fluoride, hafnium fluoride, lanthanum fluoride, europium fluoride, ruthenium fluoride, cerium fluoride, barium fluoride, and yttrium fluoride.

[0037] The metal surface of the reflective optical element may be formed on a metal layer containing at least one material selected from the group consisting of: aluminum, rhodium, ruthenium, palladium, osmium, iridium, platinum, magnesium, germanium or combinations thereof.

[0038] As further described above, optical systems operating under alternating oxidation and fluorination environments can replace reflective optical elements, typically mirrors, at a reduced frequency, thereby increasing the operating time of the optical system while reducing costs.

[0039] Further features and advantages of the invention will become apparent from the following description of working examples of the invention, with reference to the accompanying drawings illustrating important details of the invention, and the claims. Individual features may be understood individually on their own or as any of the desired combinations of multiple variations of the invention.

Implementation Method

[0049] In the following description of the figures, the same reference numerals are used for the same or having the same function.

[0050] Figures 1a to 1c schematically illustrate details of a reflective optical element 1 in the form of a mirror. The reflective optical element 1 has a substrate 2 on which a metal layer 3 is formed, the metal layer having a metal surface 3a on one side facing away from the substrate 2. In the example shown, the substrate 2 is made of silicon, but it goes without saying that the substrate 2 can also be formed of some other metallic or non-metallic materials, such as glass ceramics, ceramics, etc.

[0051] The metal fluoride layer 4 is coated using a conventional deposition method (evaporation). The metal fluoride layer 4 serves to protect the underlying metal layer 3 from oxidation. In the example shown, the metal layer 3 is made of aluminum, and the metal fluoride layer 4 is made of magnesium fluoride (MgF2), but other materials may also be used.

[0052] Research has found that the presence of the metal fluoride layer 4 alone is insufficient to protect the optical element 1 from degradation if it is exposed to high power or high irradiance during operation of the optical system (not shown in Figures 1a to 1c). Despite the presence of the metal fluoride layer 4, the Al material of the metal layer 3 will oxidize to form Al2O3 within a short period of several hours or days, resulting in a significant decrease in the reflectivity of the optical element 1. If the reflectivity loss is too great, the optical element 1 needs to be replaced.

[0053] To increase the lifespan of the reflective optical element 1, a thin passivation oxide layer 5 may be applied to the reflective optical element 1, as shown in FIG1a. For this purpose, the reflective optical element 1 is irradiated with at least one wavelength λ less than 300 nm, typically less than 200 nm. The radiation may be performed at one or more wavelengths λ, particularly between 115 nm and 200 nm. The radiation is performed under oxidizing environmental conditions OB, that is, in an environment where at least one oxygen-containing gas with a sufficient concentration is present.

[0054] The formation of oxide layer 5 involves the conversion of a volume region of the metal fluoride layer 4 near the surface into oxide layer 5. Therefore, the oxide layer 5 formed during irradiation under oxidizing environmental condition OB does not extend to the entire thickness of the metal fluoride layer 4. In the example shown, where the material of the metal fluoride layer is MgF2, the oxide layer 5 is composed of MgO. For more detailed information on oxidizing environmental condition OB that is conducive to the formation of thin oxide layer 5, please refer to reference DE102021200490A1. As can be seen from Figure 1a, the thickness d of oxide layer 5 increases with the increasing duration of exposure of the reflective optical element 1 to oxidizing environmental condition OB.

[0055] Figure 1b shows the optical element 1 of Figure 1a under the fluorinated environment condition FB. The fluorinated environment condition FB is designed to prevent the formation of an oxide layer on the metal fluoride layer 4, which would cause a decrease in the reflectivity of the reflective optical element 1 (see above). The fluorinated environment condition can be achieved, for example, in the manner described in document US 11,262,664 B2.

[0056] During the operation of the reflective optical element 1 under the fluorinated environment condition FB, fluorine atoms 8 may diffuse to the metal surface 3a, which results in the formation of a thin AlF3 intermediate layer between the metal layer 3 and the metal fluoride layer 4. The intermediate layer thickens as the duration of the fluorinated environment condition FB increases, which also leads to a decrease in the reflectivity of the reflective optical element 1.

[0057] Figure 1c shows a reflective optical element 1 operating alternately under oxidation environment condition OB and fluorination environment condition FB. As can be seen from Figure 1c, the thickness d of the oxide layer 5 increases under oxidation environment condition OB and decreases under fluorination environment condition FB. By selecting an appropriate switching time between oxidation environment condition OB and fluorination environment condition FB, the thin oxide layer 5 may remain permanently on the metal fluoride layer 4. That is, the oxide layer 5 formed or already formed under oxidation environment condition OB of the reflective optical element 1, or the oxide layer 5 applied to the metal fluoride layer 4 during the generation of the reflective optical element 1, remains on the side of the metal fluoride layer 4 away from the metal surface 3a under fluorination environment condition FB. Since oxidation environment condition OB does not last permanently, it can prevent the oxide layer 5 from growing to an excessively large thickness d, that is, limit the thickness d of the oxide layer 5 to prevent or limit the decrease in the reflectivity of the optical element 1.

[0058] In order to minimize the reduction in the reflectivity of the optical element, it is advantageous to choose a thickness D of the metal fluoride layer 4 such that the electric field of the standing wave formed when the reflective optical element 1 is irradiated by radiation 6 of the used wavelength λ has a minimum value in the region of the oxide layer 5. This can be achieved, for example, in the manner described in document DE102018211499A1.

[0059] It is advantageous to control the relative switching time between the oxidation environment condition OB and the fluorination environment condition FB. This control or such control process can be performed, for example, on the optical system 10 as described below in conjunction with Figures 2a and 2b, which is shown in Figure 3 and described in more detail below.

[0060] The optical system 10 shown in FIG3 includes a cavity 11 in which the reflective optical element 1 of FIG1c is disposed; a supply device 12; and an FUV / VUV radiation source 13. The reflective optical element 1 is disposed inside the cavity 11, which forms a periphery 14 of the reflective optical element 1. The reflective optical element 1 is mounted on an optical unit mounting base, in which a measuring device in the form of a temperature sensor 15 is embedded for measuring the temperature T of the reflective optical element 1.

[0061] In the illustrated example, the supply device 12 is used to supply protective gas in the form of inert gas IG to supply at least one oxygen-containing reactive gas OG and at least one fluorine-containing reactive gas FG into the cavity 11. The supply device 12 includes a first valve 16a for controlled supply of inert gas IG, a second valve 16b for controlled supply of at least one oxygen-containing gas OG, and a third valve 16c for controlled supply of at least one fluorine-containing gas FG. The second valve 16b and the third valve 16c are, in each case, controllable metering valves. The optical system 10 additionally includes a gas inlet 17a and a gas outlet 17b entering the cavity 11 in the region of the supply device 12.

[0062] In the example shown, the inert gas IR is argon, but other inert gas IRs may also be used, such as other light rare gases like helium or neon. Inert gas mixtures (especially the mentioned inert gases) may also be used as inert gas IRs. In the case of oxygen-containing gases, for example, water (H₂O) or molecular oxygen (O₂) may be used. For the use of fluorine-containing gases, for example, the following gases may be used: fluorine (F₂), hydrogen fluoride (HF), xenon fluoride (XeF₂), nitrogen fluoride (NF₃), carbon tetrafluoride (CF₄), and sulfur hexafluoride (SF₆).

[0063] The FUV / VUV radiation source 13 is used to irradiate the reflective optical element 1, more precisely its surface 1a, with FUV / VUV radiation 6. Therefore, the wavelength λ of the radiation 6 is within the FUV / VUV wavelength range. For example, in the illustrated embodiment, the FUV / VUV radiation 6 enters the cavity 11 through an incident window 20a made of MgF2 and exits the cavity 11 through an exit window 20b made of MgF2. The cavity 11 is hermetically sealed by the two windows 20a and 20b.

[0064] The inner side 11a of cavity 11 is resistant to fluorinated gas FG and its conversion products. Therefore, in the illustrated example, cavity 11 is made of a metal in the form of Monel steel, at least on its inner side 11a, which forms a passivation layer to prevent corrosion. In principle, cavity 11 can also be made of other corrosion-resistant metals, such as those that do not contain Cr and Ti. Alternatively, a corrosion-resistant coating (e.g., composed of NiP, Pt, or a Ru / Rh mixture) can be applied to the inner side 11a of cavity 11. The corrosion-resistant coating can be applied to the inner side 11a of cavity 11, for example, through an electroplating process. Components disposed in cavity 11 and in contact with fluorinated gas FG are also resistant to fluorinated gas FG and its conversion products.

[0065] Furthermore, the optical system 10 includes, for example, a first sensor 18a for measuring the concentration cOG of oxygen-containing gas OG in cavity 11 and a second sensor 18b for measuring the concentration cFG of fluorine-containing gas FG in cavity 61. The sensors 18a and 18b can be configured to measure the concentrations of different oxygen-containing or fluorine-containing gases in cavity 11. For example, the first sensor 18a can be configured to measure the concentration cOG or partial pressure of water H2O and oxygen O2 in cavity 11.

[0066] The optical system 10 also includes a control device 19 for controlling a portion of the pressure or concentration cFG of the fluorinated gas FG within the cavity 11 to a target value. This control is implemented through actual measurement by a second sensor 18b for measuring the concentration cFG of the fluorinated gas FG in the cavity 11 and through control by a third valve 16c. The second sensor 18b can be configured to measure only a portion of the pressure cFG of the fluorinated gas FG, but a residual gas analyzer can also be used, which can also determine the portion of the pressure or concentration of other gases contained in the cavity 11. Such a residual gas analyzer can perform the functions of the two sensors 18a and b shown in FIG. 3. In the case where the third valve 16c is a metering valve, such as a mass flow controller, the use of the second sensor 18b to measure the concentration cFG of the fluorinated gas FG in the cavity 11 can be selectively omitted.

[0067] The control device 19 is also used to switch or convert between the oxidation environment condition OB described in relation to FIG1c and the fluorination environment condition FB in the surrounding environment 14 of the cavity 11 or the reflective optical element 1, based on at least one control parameter, which is a measure of the reflectivity R of the optical element 1.

[0068] The control parameter may be, for example, the total transmittance T(t) of the optical system 10 or a control variable derived therefrom. As shown in Figure 2b, the total transmittance T(t) of the optical system 10 at time t is defined as the quotient of the light intensity Itransmitted(t) measured at the output of the optical system 10 and the light intensity Ilight source(t) produced by the light source 13 at time t: T(t) = Itransmitted(t) / Ilight source(t). The relative light intensity and the total transmittance T(t) can be determined using a measuring device not shown, for example, in the form of an optical sensor.

[0069] In the control parameter P controlling the operation of the optical system 10, the ratio of the total transmittance T(t) measured at time t to the total transmittance T0(t) attributed to long-term changes in the optical system 10, which is not attributed to changes in the reflectivity R of the reflective optical element 1, is used. A prerequisite for using the control parameter P is that the relative short-term changes or fluctuations in the total transmittance T(t) of the optical system 10 per hour or per day can be attributed to changes in the reflectivity R of the reflective optical element 1, while long-term changes, for example, caused by other optical components, are known or can be calculated for control purposes. Therefore, the total transmittance T(t) decreases over time, or the control parameter P is a direct measure of the degree of oxidation of the surface 1a or the reflectivity R of the reflective optical element 1.

[0070] In the example shown, three cases are distinguished for control, as shown in Figure 2a: In the first case, the total transmittance or control parameter P is small and below a lower critical value P1, for example, it may be about 95%. In this case, the concentration of fluorinated gas FG, cFG, is increased to reverse the surface oxidation of the reflective optical element 1, or to refluorinate the latter, and to increase the total transmittance or control parameter P of the optical system 10. In principle, the concentration of oxidizing gas OG, cOG, in cavity 11 can also be reduced here, but this is not absolutely necessary.

[0071] In the second case, where the control parameter P is located between the first lower critical value P1 and the second higher critical value P2, for example, approximately 99%, the concentration cFG of the fluorine-containing gas FG remains constant.

[0072] In the third case where the control parameter P exceeds the second critical value P2, the oxide layer 5 is almost completely fluorinated. Therefore, the metering or concentration cFG of the fluorinated gas FG is reduced, causing oxidation conditions to reappear. In this case, specifically, the concentration cOG of the oxidizing gas OG in cavity 11 can be increased to accelerate the oxidation process.

[0073] As can also be seen from Figure 2a, the control loop in the control device 19 is not continuous, but rather operates only after the concentrations cFG and cOG of at least one fluorine-containing gas FG and / or at least one oxygen-containing gas OG change below or above relative critical values ​​P1 and P2. The optical system 10 operates under constant environmental conditions for at least a predetermined duration tH, and then is checked again to determine whether it is necessary to switch between the oxidizing environmental condition OB and the fluorinating environmental condition FB. The duration tH is typically on the order of several hours or days.

[0074] In addition to control via the overall transmission or control parameter P, the switching time between the oxidation environment condition OB and the fluorination environment condition FB can be adjusted by the control device 19 using other control parameters determined by a suitable measuring device. For example, the reflectivity R of the reflective optical element 1 can be measured by measuring devices 15a and 15b, which have a VUV measuring light source 15a for illuminating the reflective optical element 1 with measuring light, and a detector 15b for detecting the measuring light reflected from the optical element 1. In addition to or alternative to the reflectivity R of the optical element 1, the absorptivity of the optical element 1 can also be determined by measuring the temperature T of the optical element 1 using the temperature sensor 15 described further above. The chemical composition of the surface 1a of the reflective optical element 1 can also be determined by means of a suitable measuring device or measuring method, such as using XPS or XRF. To control the environmental conditions OB and FB of the optical element 1, one of the control parameters P, R, T, etc., can be measured, but control can also be based on two or more of the control parameters P, R, T, etc. The control device 19 can be configured in the form of appropriate hardware and / or software and connected via wires to the corresponding measuring devices 15, 15a, 15b.

[0075] The concentration cFG of the fluorinated gas FG in the cavity 11, where a fluorinated environment FB exists around the optical element 14, depends on the type of fluorinated gas FG. When the cavity 11 is operated under vacuum conditions, i.e., without an inert gas IG, the partial pressure or concentration cFG of the fluorinated gas FG is typically between 10⁻⁹ mbar and 10⁻¹ mbar, particularly between 10⁻⁶ mbar and 10⁻³ mbar. During operation of the cavity 11 using a blown gas or an inert gas IG, the concentration cFG in the blown gas or inert gas IG is typically between 1 pptV and 100 ppmV, particularly between 1 ppbV and 1 ppmV.

[0076] The light source 13 generates light in the FUV / VUV wavelength range, both of which cause photodissociation of the fluorine-containing gas FG to form fluorine substances, such as fluorine atoms F or fluorine radicals, and provide the activation energy required to convert the oxide of the oxide layer 5 into fluorides.

[0077] Under oxidizing environmental conditions OG, the concentration of fluorine-containing gas FG, cFG, is lower than the value specified above, typically zero, meaning that no fluorine-containing gas FG is supplied to the surrounding environment 14 or cavity 11 under oxidizing environmental conditions OG. The concentration of oxygen-containing gas, present in the form of oxygen O2, cOG, is typically between approximately 1 pptV and 100 ppmV. The concentration of water in cavity 11 as oxygen-containing gas OG, cOG, should be low and typically should not exceed approximately 100 ppbV. If a purge gas is present, the residual gas concentrations of oxygen O2 and water in cavity 11 may be sufficient to generate oxidizing environmental conditions OG. In this case, it is not necessary to supply additional oxygen-containing gas to cavity 11 to establish oxidizing environmental conditions OG.

[0078] For example, in the case of the optical system 10 of FIG3, the cavity 11 is separated from the other components of the optical system 10 through windows 20a and 20b, but this is not absolutely necessary. The cavity 11 may also include all the optical components of the optical system 10, as long as it is resistant to fluorine gas FG or its reaction products. The optical system 10 in FIG3 can be configured in a variety of ways. Two examples of such an optical system 10 are described below.

[0079] Figure 4 illustrates an optical system for the VUV wavelength range in the form of a VUV lithography device 21. The VUV lithography device 21 includes two optical configurations, namely an illumination system 22 and a projection system 23. The VUV lithography device 21 is additionally equipped with a light source 24, which may be, for example, an excimer laser.

[0080] The illumination system 22 adjusts the radiation 25 emitted by the light source 24, thereby illuminating the light mask 26 (also called a magnifying glass). In the example shown, the illumination system 22 has a housing 32 in which two transmissive and reflective optical elements are disposed. A transmissive optical element 27 is shown in the figure, which focuses the radiation 25; and a reflective optical element 28, which deflects the radiation.

[0081] The photomask 26 has a structure on its surface that is transferred using the projection system 23 to the optical element 29 to be exposed, such as a wafer, for the formation of a semiconductor element. In the example shown, the photomask 26 is configured as a transmissive optical element. In an alternative embodiment, the photomask 26 may also be configured as a reflective optical element.

[0082] The projection system 22 in the illustrated example has at least one transmissive optical element. The illustrated example shows two transmissive optical elements 30, 31 in a representative manner, for example, to reduce the structure on the photomask 26 to the required size for exposing the wafer 29.

[0083] In both the lighting 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 configurations without transmissive optical elements can also be used for VUV lithography.

[0084] Figure 5 illustrates an optical system for the VUV wavelength range in the form of a wafer inspection system 41, but a photomask inspection system may also be involved. The wafer inspection system 41 has an optical configuration 42 of a light source 54, from which radiation 55 is guided to a wafer 49 through the optical configuration 42. For this purpose, the radiation 55 is reflected onto the wafer 49 by a concave mirror 46. In the case of a photomask inspection system, the photomask to be inspected can be used instead of the wafer 49. The radiation reflected, diffracted, and / or refracted by the wafer 49 is guided to a detector 50 via a transmission optical element 47 for further evaluation by another concave mirror 48 also associated with the optical configuration 42. The wafer inspection system 41 is additionally equipped with a housing 52 in which two mirrors 46, 48 and the transmission optical element 47 are disposed. For example, the light source 54 may be a single light source or a combination of multiple individual light sources to provide a substantially continuous radiation spectrum. In variations, one or more narrowband light sources 54 may also be used.

[0085] The VUV lithography equipment 21 shown in Figure 4 and the wafer inspection system 41 shown in Figure 5 are configured to operate alternately under the oxidation environment condition OB and the fluorination environment condition FB of the relative mirrors 28 and 46, 48. [Simplified Explanation of the Diagram]

[0040] Exemplary embodiments are schematically illustrated in the diagrams and will be explained in the following embodiments, in which:

[0041] Figure 1a shows a radiation schematic diagram of a reflective optical element under oxidizing conditions;

[0042] Figure 1b shows a schematic diagram of the radiation of a reflective optical element under fluorinated environment conditions;

[0043] Figure 1a shows a schematic diagram of the radiation of a reflective optical element under alternating oxidation and fluorination conditions;

[0044] Figure 2a shows a schematic diagram of a control circuit for alternately controlling oxidation or fluorination environmental conditions;

[0045] Figure 2b shows a schematic diagram of the change in the total transmittance of the optical system over time;

[0046] Figure 3 shows a schematic diagram of an optical system with a reflective optical element configured to alternately control oxidation and fluorination conditions around the reflective optical element;

[0047] Figure 4 schematically illustrates an optical system for the VUV wavelength range in the form of a VUV lithography device; and

[0048] Figure 5 schematically illustrates an optical system for the VUV wavelength range in the form of a wafer inspection system.

Claims

1. A method of operating an optical system (10), particularly for the FUV / VUV wavelength range, wherein the optical system (10) comprises at least one reflective optical element (1) having a metal surface (3a) to which a metal fluoride layer (4) is applied, wherein operating the optical system (10) involves irradiating the optical element (1) with radiation (6), particularly in the FUV / VUV wavelength range, characterized in that the optical system (10) is operated under a plurality of oxidizing environmental conditions (OB) of the optical element (1) in a first time interval and under a plurality of fluorinating environmental conditions (FB) of the optical element (1) in a second time interval, wherein the first time interval follows the second time interval, or the second time interval follows the first time interval.

2. The method of claim 1, wherein the optical system (10) operates alternately under the oxidizing environment (OB) and the fluorinating environment (FB) of the optical element (1).

3. The method as claimed in claim 1 or 2, wherein an oxide layer (5) has been formed or formed on the side of the metal fluoride layer (4) facing away from the metal surface (3a) under the oxidation environment (OB) of the optical element (1), wherein the thickness (d) of the oxide layer (5) increases with the length of time of such oxidation environment (OB).

4. The method as claimed in claim 3, wherein under the fluorinated environmental conditions (FB) of the optical element (1), the oxide layer (5) remains on the side of the metal fluoride layer (4) away from the metal surface (3a).

5. The method of claim 3, wherein the thickness (D) of the metal fluoride layer (4) is selected such that when the reflective optical element (1) is irradiated by radiation (6) of a wavelength (λ) used by the optical system (10), the electric field of the resulting standing wave has a minimum value in the region of the oxide layer (5).

6. The method as claimed in claim 1 or 2, wherein, in order to switch between the oxidizing environment (OB) and the fluorinated environment (FB), the concentration (cFG, cOG) of at least one fluorinated gas (FG) and / or at least one oxygen-containing gas (OG) in the surrounding environment (14) of the reflective optical element (1) is changed, wherein preferably, under the fluorinated environment (FB), at least one fluorinated gas (FG) is supplied to the surrounding environment (14) of the reflective optical element (1), while under the oxidizing environment (OB), no fluorinated gas (FG) is supplied.

7. The method of claim 6, wherein the concentration (cFG, cOG) of the at least one fluorine-containing gas (FG) and / or the at least one oxygen-containing gas (OG) in the surrounding environment (14) of the optical element (1) is controlled as a function of at least one control parameter (P, R, T), which forms a measure of the reflectivity (R) of the reflective optical element (1).

8. The method of claim 7, wherein if the control parameter (P) is between a first lower threshold value (P1) and a second higher threshold value (P2), the concentration (cFG, cOG) of the at least one fluorine-containing gas (FG) and / or the at least one oxygen-containing gas (OG) remains constant.

9. The method of claim 8, wherein if the control parameter (P, R, T) is lower than the first lower threshold value (P1), the concentration (cFG) of the fluorine-containing gas (FG) increases and / or the concentration (cOG) of the oxygen-containing gas (OG) decreases, and / or if the control parameter (P, R, T) exceeds the second higher threshold value (P2), the concentration (cFG) of the fluorine-containing gas (FG) decreases and / or the concentration (cOG) of the oxygen-containing gas (OG) increases.

10. The method of claim 6, wherein after changing the concentration (cFG, cOG) of at least one fluorine-containing gas (FG) and / or at least one oxygen-containing gas (OG), the optical system (10) operates under multiple constant environmental conditions for at least a predetermined duration (tH).

11. The method of claim 6, wherein the control parameter is selected from the group consisting of: the total transmittance (T(t)) of the optical system (10), the reflectivity (R) of the reflective optical element (1), the chemical composition of the surface (1a) of the optical element (1), and the temperature (T) of the optical element (1).

12. An optical system, preferably used in the FUV / VUV wavelength range, particularly in an FUV / VUV lithography device (21) or a wafer inspection system (41), comprising: at least one reflective optical element (1) having a metal surface (3a) to which a metal fluoride layer (4) is applied; a light source (13) irradiating the reflective optical element (1) with radiation, particularly in the FUV / VUV wavelength range, characterized in that the optical system (10) is configured to operate under a plurality of oxidation environment conditions (OB) of the optical element (1) in a first time interval and under a plurality of fluorination environment conditions (FB) of the optical element (1) in a second time interval, wherein the first time interval succeeds the second time interval, or the second time interval succeeds the first time interval.

13. The optical system as claimed in claim 12, wherein the optical system (10) is configured to operate alternately under the oxidizing environment (OB) and the fluorinating environment (FB) of the optical element (1).

14. The optical system as claimed in claim 12 or 13, wherein the reflective optical element (1) is disposed in a cavity (11), and the optical system (10) has a supply device (12) for supplying at least one fluorine-containing gas (FG) and / or at least one oxygen-containing gas (OG) to the cavity (11), wherein the supply device is configured to set the concentration (cFG) of the fluorine-containing gas (FG) and / or the concentration (cOG) of the oxygen-containing gas (OG) in the cavity (11).

15. The optical system of any one of claims 12 to 13 further comprises: a control device (19) for controlling the concentration (cFG, cOG) of at least one fluorine-containing gas (FG) and / or at least one oxygen-containing gas (OG) in the surrounding environment (14) of the optical element (1) as a function of at least one control parameter (P, R, T), which forms a measure of the reflectivity (R) of the reflective optical element (1); and at least one measuring device (15, 15a, 15b) for measuring the at least one control parameter (P, R, T).

16. The optical system as claimed in claim 15, wherein the at least one control parameter is selected from the group consisting of: the total transmittance (T(t)) of the optical system (10), the reflectivity (R) of the reflective optical element (1), the chemical composition of the surface (1a) of the optical element (1), and the temperature (T) of the optical element (1).

17. The optical system of any one of claims 12 to 13, wherein a cavity (11) has an entrance window (20a) for allowing light (6) from the light source (13) to enter the cavity (11) and / or an exit window (20b) for allowing light (6) from the light source (13) to exit the cavity (11).

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