Lifetime stabilization of coated optical systems by means of electron beam heating

Abstract

The invention relates to a method for stabilizing an optical element, the method comprising: providing an optical element having a substrate and a coating, and subjecting the optical element to heat treatment. The problem of providing a method and an optical element which overcome the disadvantages of the prior art is solved by virtue of the heat treatment of the optical element comprising irradiating the coating of the optical element with electrons. Furthermore, the invention relates to an optical element, to a microlithographic projection lens comprising such an optical element, and to a microlithographic projection exposure apparatus comprising such a projection lens, and also to a device for electron irradiation.

Description

[0001]June 13, 2024Lifetime stabilization of coated optics using electron beam heatingTechnical fieldThe invention relates to a method for stabilizing an optical element, the method comprising: providing an optical element with a substrate and a coating, and annealing the optical element. Furthermore, the invention relates to an optical element for a projection exposure system, to a projection objective for microlithography with such an optical element, to a projection exposure system for microlithography with such a projection objective, and to a device for electron irradiation.The subject matter of German patent application 102023205640.2 is hereby incorporated by reference.Background: Projection exposure systems for microlithography rely on the high precision of the optical elements used to project a mask into an image plane. Equally high demands are placed on the surface shape of the masks themselves. However, during system operation, the optical elements are exposed to high temperatures, which cause undesirable changes in the substrate and the coating of the optical elements, leading to undesirable imaging properties. können.To prevent undesirable changes in the optical elements due to thermal stress during operation, attempts have been made to stabilize the optical elements by prior annealing. However, due to permissible temperature limitations, the optical elements cannot be heated to the temperature necessary for complete stabilization of lifetime effects in a conventional annealing furnace. Instead, complex annealing processes are required, for example, to enable cooling of the attachments. The object of the present invention is to provide a method and an optical element that solve the above-described disadvantages of the prior art. A further object of the invention is to provide a projection lens and a projection exposure system with an improved optical element, as well as an advantageous device for electron irradiation.Description of the invention. According to a first aspect, the object is achieved for a method for stabilizing an optical element, the method comprising: providing an optical element with a substrate and a coating, and annealing the optical element, in that annealing the optical element comprises irradiating the coating of the optical element with electrons. The method comprises providing an optical element. The optical element can be a reflective optical element, for example, a mirror or a mask. It is also conceivable for the optical element to be a lens, a prism, a hologram, and / or a diffuser. The optical element comprises a substrate and a coating. The substrate can comprise, for example, SiSiC, Zerodur® from Schott AG, ULE® from Corning Inc., quartz glass, and / or any other type of glass. The coating can comprise one or more layers.The coating is preferably electrically conductive. In particular, it is a metallic coating. The coating can be a coating suitable for the EUV wavelength range. For example, it can be a MoSi coating. H / hl 220968WOJune 13, 2024 The coating may comprise at least one layer subsystem. The layer subsystem may comprise at least one layer formed or composed as a compound of at least one material from the group: nickel, carbon, boron carbide, cobalt, beryllium, silicon, and silicon oxides. These materials, on the one hand, have a sufficiently high absorption coefficient for EUV radiation and, on the other hand, do not change at least substantially under EUV radiation. The layer arrangement of the at least one layer subsystem may, for example, comprise a periodic sequence of at least two periods of individual layers. The periods may comprise individual layers made of different materials. The materials of the individual layers forming the periods may, for example, be nickel and silicon or cobalt and beryllium. sein.The coating can preferably comprise a reflective layer. The reflective layer can comprise at least one layer subsystem optimized for the reflection of EUV radiation, in particular radiation with a wavelength of 13 nm or 7 nm. The reflective layer can comprise a periodic sequence of at least one period of individual layers. The period can comprise individual layers with different refractive indices, e.g., in the EUV wavelength range. However, aperiodic layers or a reflective layer comprising only one layer are also conceivable. Optionally, a protective layer can be applied between the substrate and the reflective layer. The protective layer is intended to prevent the substrate from compacting, e.g., due to useful EUV radiation, i.e., the radiation used in an EUV projection exposure system.The layer arrangement of the protective layer can comprise at least a thickness of greater than 20 nm, in particular greater than 50 nm, so that the transmission of EUV radiation through the layer arrangement is less than 10%, in particular less than 2%.S. H / hl 220968WOJune 13, 2024 The method comprises annealing the optical element. Annealing can be understood as a heat treatment in which the coating of the optical element is heated. In particular, the method comprises annealing the coating of the optical element. The coating of the optical element can, for example, be heated at least partially uniformly. By annealing the coating of the optical element, the coating of the optical element can be stabilized against lifetime effects. In particular, the coating can thereby be stabilized against heat input for later use. The annealing can, for example, take place upstream during the manufacturing process of the optical element. Undesirable changes in the coating, for example, the layer stress, can thus advantageously be avoided. In addition, the substrate can be at least partially annealed.In particular, at least near-surface regions of the substrate are annealed. This can mean, for example, regions that lie up to 5500 nm, preferably up to 3400 nm, particularly preferably up to 1700 nm, below the surface of the optical element. Annealing the optical element comprises irradiating the coating of the optical element with electrons. The introduction of the electron energy occurs locally in the coating, in particular in at least one layer. It has been recognized that the optical element, in particular the coating of the optical element, can be advantageously heated by means of electron irradiation. Annealing occurs in particular by depositing the energy of the electrons in the coating of the optical element through irradiation. When the coating is irradiated with electrons, the energy of the electrons can be converted into heat via a collision cascade.The coating can thus be specifically heated and pre-aged by irradiation with electrons. The heat input can, in turn, lead to relaxation processes that stabilize the coating of the optical element against heat input for later applications of the optical element. Irradiation of the coating of the optical element is carried out in particular in such a way that at least the coating S. H / hl 220968WOJune 13, 2024. The assignment of process parameters to a desired stabilization effect can be carried out according to a suitable calibration. The irradiation with electrons takes place in a vacuum, in particular in a high vacuum. For this purpose, the optical element can be placed in a vacuum chamber of a vacuum system. The vacuum system can contain an electron beam source, in particular an electron gun, which generates an electron beam. An electron gun can be an electrical arrangement for generating electron beams. The electron gun provides a focused and directed electron beam. The electron beam can, for example, have a diameter of 0.1 mm to 20 mm. Using the electron beam, the surface of the coating of the optical element can be scanned. The scanning is preferably carried out multiple times, for example, two to 10,000 times.Repeated thermal stress through multiple scanning leads to the stabilization of lifetime effects. Scanning can be performed point-by-point, for example. The field to be irradiated can be divided into discrete points. The electron beam can be directed at at least one discrete point on the surface and remain at this point for a specific time. For example, the dwell time Δt of the electron beam is 10 ns to 10 ms. During this time interval, a local temperature peak can occur at the discrete point. The temperature can rise suddenly due to the heat input. As soon as the beam reaches the next point, the temperature drops again. The electron beam radiation allows the optical element, especially the coating, to be heated locally to much higher temperatures than, for example, with an annealing furnace.Due to the strong temperature dependence of the relaxations, the temperature peaks can be used for stabilization. In addition to the temperature peaks, a net increase in the temperature ΔT of the coating of the optical element can also occur over time. SH / hl 220968WO June 13, 2024 Furthermore, other parts of the optical element, in particular the substrate of the optical element, can be influenced, in particular stabilized, by irradiating the coating of the optical element with electrons. The heat generated in the coating can, for example, also at least partially heat the substrate of the optical element. In particular, parts of the substrate close to the surface can also be heated by irradiating the coating with electrons. The heat can be transferred to other parts of the optical element. As a result, these parts of the optical element can also be stabilized.By irradiating the coating of the optical element with electrons, parts of the optical element that extend beyond the coating can also be advantageously stabilized against lifetime effects. In particular, compactions introduced into the substrate can be at least partially stabilized. Annealing can, for example, lead to a partial regression of a compaction previously introduced into the substrate, i.e., to decompaction. Decompaction is an effect that occurs over time when the substrate is used in a projection exposure system, such as an EUV projection exposure system, and can thus lead to a non-negligible change in the surface of the optical element. Annealing accelerates the decompaction process, whereby the change remaining over the lifetime of the substrate due to decompaction can advantageously be reduced to a negligible value.The process can thus be used to specifically optimize optical elements, for example, during production or subsequently. It has been shown that optical elements can be advantageously stabilized using this process, particularly against lifetime effects. The process allows for targeted heat input, which induces local relaxation processes. By irradiating the coating of the optical element with electrons, the coating of the optical element and optionally near-surface S can be stabilized. H / hl 220968WOJune 13, 2024 Areas of the substrate can be specifically annealed. This avoids, in particular, undesired heating of temperature-sensitive components, such as bonded attachments. Thermally induced compaction phenomena of the substrate material of an optical element during use can be avoided by stabilizing it according to the present method. Compared to annealing based on alternative energy sources such as laser radiation, annealing by means of electron irradiation also offers the advantage, among other things, that electron absorption is largely independent of the optical properties of the optical element. The method can therefore be used particularly for different metallic coatings without increased adaptation effort. Furthermore, the backscattered electrons are largely non-directional. In particular, the coating of the optical element can backscatter the electrons impinging on it.The fact that the backscattered electrons are largely undirected has the advantage that there are no reflection zones in the vacuum chamber that require cooling. In contrast, laser radiation, in particular, is directed and may require a cooled absorber. According to a first advantageous embodiment of the method, the electron energy ETemp for annealing is selected depending on the thickness of the coating of the optical element. For example, the thickness of the coating is up to 500 nm, preferably up to 450 nm, particularly preferably up to 400 nm. In particular, the electron energy ETemp for annealing is selected depending on the thickness of at least one layer of the coating of the optical element. The coating can thus advantageously be heated locally. It has been recognized that the penetration depth of the electrons is influenced by the electron energy. The greater the electron energy, the deeper the penetration depth of the electrons.To a first approximation, the stopping power decreases inversely proportional to the electron energy. H / hl 220968WOJune 13, 2024 Most of the energy is deposited at the end of the trajectory, where the electrons have been decelerated to zero. In this way, targeted local heat input can be ensured. The heating depth can be adjusted via the electron energy. The electron energy ETemp can, for example, be selected such that the energy is introduced locally in the coating, in particular at least one layer. The electron energy ETemp can be adapted to different coatings. Preferably, the electron energy ETemp is selected such that the electron energy ETemp is at least essentially completely absorbed in the coating of the optical element. The electron energy ETemp is, for example, selected such that the energy is absorbed exclusively in the coating, in particular at least one layer. Penetration of electrons into underlying layers orIngress into the substrate of the optical element should preferably be avoided to prevent undesirable effects such as unwanted compaction. Therefore, a low electron energy ETemp is preferred for annealing. gewählt.In particular, the acceleration voltage UTemp for annealing is selected depending on the thickness of the coating of the optical element, in particular at least one layer. The electron energy ETemp is particularly dependent on the voltage for accelerating the electrons, the acceleration voltage UTemp. With the aid of the acceleration voltage UTemp, the electron energy ETemp and thus in particular the penetration depth of the electrons can be influenced. According to a further advantageous embodiment of the method, the acceleration voltage UTemp for annealing is 1 to 30 keV, preferably 1 to 10 keV, more preferably 11 to 15 keV, more preferably 16 to 20 keV, more preferably 21 to 30 keV. The acceleration voltage UTemp is preferably less than 30 keV. With correspondingly low acceleration voltages, energy can be deposited in a targeted manner in the coating of the optical element.In particular, energy can be introduced locally into at least one cover layer of the optical element. SoS. H / hl 220968WOJune 13, 2024, for example, depending on the thickness of the at least one layer to be stabilized, an acceleration voltage UTemp in the range of 1 to 30 keV can be selected. The acceleration voltage UTemp can be, for example, 5 keV. The acceleration voltage UTemp can be calculated depending on the layer system. According to a further advantageous embodiment of the method, the current ITemp for annealing is 0.1 to 5 mA. With an acceleration voltage UTemp in the range of 1 to 20 keV and a current ITemp in the range of 0.1 to 5 mA, power levels in the range of 0.1 to 100 W can be generated. The current ITemp for annealing is preferably 0.5 to 5 mA, particularly preferably more than 0.5 mA. While the penetration depth of the electrons is adjusted in particular via the acceleration voltage UTemp, the power can additionally be adjusted via the current ITemp. This in turn can influence the temperature.A higher current ITemp can be selected, for example, to achieve higher power and thus higher temperatures. This allows, for example, the irradiation duration to be shortened. According to a further advantageous embodiment of the method, the temperature during annealing is ≥ 60 °C. The temperature of ≥ 60 °C can be achieved by irradiating the coating with electrons locally in the coating and optionally near the surface of the substrate. For example, the temperature is 60 °C. However, significantly higher temperatures are also conceivable. The temperature is thus disproportionately high compared, for example, to storage over a longer period at room temperature, which can ensure stabilization.During annealing, temperature gradients can arise at interfaces, whereby the coating can be heated disproportionately and lifetime effects of the coating as well as near-surface areas can be at least almost completely stabilized. According to a further advantageous embodiment of the method, the method further comprises: at least partially compacting the substrate of the optical device. H / hl 220968WOJune 13, 2024 elements, wherein the compacting comprises irradiating the optical element with electrons. The method can thus comprise compacting a substrate of an optical element by means of electron irradiation and annealing the optical element by means of electron irradiation. The processing flexibility can be increased by the independent electron irradiations. The substrate can be locally compacted by irradiation with electrons. The substrate material can be locally densified over the long term by the irradiation. This can achieve a change, in particular a correction, of the surface shape of the optical element near the irradiated areas. Surface defects left behind by pre-processes, in particular coatings, can thus be compensated.By irradiating the coating of the optical element, compaction, particularly compaction in near-surface regions of the substrate, can preferably be stabilized. Preferably, the electron energy EComp used for compaction is greater than the electron energy ETemp used for annealing. This allows a deeper penetration depth to be achieved. In particular, the electron energy EComp can be deposited in the substrate beneath the coating of the optical element to achieve local compaction. The effective zones for correction and annealing are thus different.The acceleration voltage UKomp for compacting is preferably more than 30 keV, particularly preferably 31 to 40 keV, further particularly preferably 41 to 50 keV, further particularly preferably 51 to 60 keV, further particularly preferably 61 to 70 keV, further particularly preferably 71 to 80 keV, further particularly preferably 81 to 90 keV, further particularly preferably 91 to 100 keV. The compaction is preferably carried out in a range between 1 µm and 100 µm, particularly preferably between 1 µm and 10 µm, further particularly preferably between 11 µm and 20 µm, further particularly preferably between 21 µm and 30 µm, further particularly preferably between 31 µm and 40 µm, further particularly preferably between 41 µm and 50 µm, further particularly preferably between 51 µm. H / hl 220968WOJune 13, 2024 µm and 60 µm, more particularly preferably between 61 µm and 70 µm, more particularly preferably between 71 µm and 80 µm, more particularly preferably between 81 µm and 90 µm, more particularly preferably between 91 µm and 100 µm below the coating of the optical element. The compaction of the substrate of the optical element preferably takes place before the annealing of the optical element or substantially simultaneously therewith. In particular, the irradiation of the substrate of the optical element takes place before the irradiation of the coating of the optical element or substantially simultaneously therewith. In this way, both the coating and the introduced compaction can advantageously be stabilized. Furthermore, the annealing can be carried out in situ. The compaction and annealing can be carried out in parallel or sequentially in situ. The acceleration voltages UKomp and UTemp of the respective processes can be coordinated with each other.While annealing takes place on the surface of the optical element's coating, compaction is created in an underlying layer in the substrate. The two processes are coordinated so that heating is efficient and compaction is not negatively affected by annealing. For this purpose, the acceleration voltage Ucomp for compaction is preferably selected to be slightly higher than the acceleration voltage Utemp for annealing. This ensures that the introduced compaction is heated as efficiently as possible. The exact value of the acceleration voltages can be selected depending on the coating thickness. werden.If the substrate is compacted before the optical element is annealed, compaction and stabilization of the optical element can advantageously be performed with the same electron gun. For example, the electron gun can first be operated with an accelerating voltage UComp for compaction and then with an accelerating voltage UTemp for annealing. H / hl 220968WOJune 13, 2024. The compaction of the substrate and the annealing of the optical element can also occur essentially simultaneously. Essentially simultaneous means, in particular, at least essentially simultaneous compaction of the substrate and annealing of the optical element. Essentially simultaneous compaction of the substrate and annealing of the optical element means, in particular, that the compaction of the substrate and the annealing of the optical element can occur in parallel. For this purpose, different electron guns can be used, for example. The electron guns can be operated with different acceleration voltages Ucomp for compaction and Utemp for annealing. The electron guns can be provided in a system for electron irradiation. This eliminates the need for an additional machine, such as an annealing furnace, for a downstream annealing process.The process thus enables significant savings in production time and space. Furthermore, the layers can be heated to much higher temperatures using electron beams than with a furnace. Temperatures of ≥ 60 °C, especially higher than 60 °C, can be achieved. werden.It is also conceivable that a counterfield, in particular a dynamic counterfield, is applied to the optical element, in particular the coating of the optical element. Thus, the optical element can be irradiated with the same, higher electron energy EComp for both compaction and tempering, whereby the counterfield can specifically slow down the electrons for irradiating the coating. This also allows tempering of the coating without requiring a change in the voltage source of the electron radiation. According to a second teaching, the above-mentioned object for an optical element, in particular a reflective optical element, is achieved by stabilizing the optical element using a method according to the first aspect. Lifetime effects of the optical element, in particular the coating of the H / hl 220968WOJune 13, 2024 optical element, have been stabilized by annealing. The optical element is particularly suitable for use in a projection exposure system. For example, the optical element is a coated mirror. The coating of the substrate of the mirror can comprise at least one layer subsystem optimized for the reflection of EUV radiation, i.e., radiation with a wavelength of 13 nm or 7 nm. This reflection layer can comprise a periodic sequence of at least one period of individual layers, wherein the period can comprise two individual layers with different refractive indices in the EUV wavelength range. However, aperiodic layers or coatings comprising only one layer are also possible. möglich.According to a third teaching, the above-mentioned object for a projection lens for microlithography is achieved in that the projection lens comprises an optical element according to the second aspect. Projection lenses for microlithography are exposed to high loads from the useful radiation, whose wavelength is preferably 13.5 nm, and radiation of other wavelengths, as a result of which they are heated during operation. By using an optical element according to the second aspect, which has thus been stabilized using a method according to the first aspect, a change in the surface over time, e.g., by decompaction of the material, can be advantageously minimized. According to a fourth teaching, the above-mentioned object for a projection exposure system for microlithography is achieved in that the projection exposure system comprises a projection lens according to the third aspect.A projection exposure system can be provided for generating an image of an object arranged in an object plane in an image plane using a projection light-emitting light source. The projection exposure system according to the fourth aspect is a projection exposure system for microlithography. Projection exposure systems for microlithography. H / hl 220968WOJune 13, 2024 are used to manufacture microstructured or nanostructured components in microelectronics or microsystems technology. With the help of projection exposure systems, structures formed on a photomask are imaged in a reduced manner onto wafers or the like in order to create the corresponding structures on the wafer using microlithographic processes. In particular, a projection exposure system can be an EUV projection exposure system or a DUV projection exposure system. Due to the increasing miniaturization and reduction of structure widths, projection exposure systems are operated with working light with ever shorter wavelengths, for example, wavelengths in the extreme ultraviolet (EUV) range. Projection exposure systems are operated around the clock, and any malfunction affects the performance of the projection exposure system.By using long-term stable projection optics, the maintenance times of the projection exposure system can be advantageously reduced. According to a fifth teaching, the above-mentioned object is achieved for an apparatus for electron irradiation comprising a vacuum system with a vacuum chamber, wherein the vacuum system contains an electron beam source, in particular an electron gun, for generating an electron beam, in that the apparatus is configured to generate a counterfield, in particular a dynamic counterfield, on the optical element, in particular the coating of the optical element. As a result, in particular, the optical element can be irradiated with the same electron energy both for compacting and for annealing, wherein the counterfield specifically decelerates the electrons for irradiating the coating. The present disclosure also encompasses the subject matter of the following clauses: SH / hl 220968 WO, June 13, 2024.Method for stabilizing an optical element, the method comprising: - providing an optical element with a substrate and a coating, and - annealing the optical element, characterized in that the annealing of the optical element comprises irradiating the coating of the optical element with electrons. 2. Method according to clause 1, characterized in that the electron energy ETemp, in particular the acceleration voltage UTemp, for annealing is selected depending on the thickness of the coating, in particular at least one layer of the coating, of the optical element. wird.3. Method according to clause 1 or 2, characterized in that the acceleration voltage UTemp for annealing is 1 to 30 keV, preferably 1 to 10 keV, more preferably 11 to 15 keV, more preferably 16 to 20 keV, more preferably 21 to 30 keV. 4. Method according to one of clauses 1 to 3, characterized in that the current intensity ITemp for annealing is 0.1 to 25 mA, in particular 0.1 to 5 mA, more particularly 5.1 to 10 mA, more particularly 10.1 to 15 mA, more particularly 15.1 to 20 mA, more particularly 20.1 to 25 mA. 5. Method according to one of clauses 1 to 4, characterized in that the temperature during annealing is > 60 °C. H / hl 220968WOJune 13, 2024 Method according to one of clauses 1 to 5, characterized in that the method further comprises: - at least partially compacting the substrate of the optical element, wherein the compacting comprises irradiating the optical element with electrons. Method according to clause 6, characterized in that the electron energy EComp used for the compacting is greater than the electron energy ETemp used for the annealing. Method according to clause 6 or 7, characterized in that the acceleration voltage UComp for compacting is more than 30 keV, particularly preferably 31 to 40 keV, further particularly preferably 41 to 50 keV, further particularly preferably 51 to 60 keV, further particularly preferably 61 to 70 keV, further particularly preferably 71 to 80 keV, further particularly preferably 81 to 90 keV, further particularly preferably 91 up to 100 keV.Method according to one of clauses 6 to 8, characterized by the compacting of the substrate of the optical element before annealing the coating of the optical element or substantially simultaneously therewith. erfolgt. Optical element, in particular reflective optical element, stabilized by a method according to one of clauses 1 to 9.S H / hl 220968WOJune 13, 2024 11. Projection objective for microlithography comprising an optical element according to clause 10.12. Projection exposure system for microlithography comprising a projection objective according to clause 11. Further refinements and advantages of the invention are explained in the following detailed description of some exemplary embodiments of the present invention in conjunction with the drawings. Brief description of the drawings. Exemplary embodiments and variants of the invention are explained in more detail below with reference to the drawings. The aspects of the disclosure can best be understood from the following detailed description in conjunction with the accompanying figures. The figures are schematic and simplified, showing only details to improve the understanding of the claims, while other details are omitted. The same reference numerals are used throughout for identical or corresponding parts.The individual features of each aspect can be combined with any or all features of the other aspects. These and other aspects, features, and / or technical effects are evident from and illustrated by the figures described below: Fig. 1 shows a schematic representation of an embodiment of a projection exposure apparatus according to the fourth aspect; Fig. 2 shows a diagram of the variation of the maximum penetration depth of electrons into an MoSi coating of an optical element as a function of the electron energy; H / hl 220968WOJune 13, 2024 Fig. 3a-c simulations of the trajectories of electrons introduced into an optical element by an embodiment of a method according to the first aspect. Fig. 4a-b schematic representations of embodiments of a method according to the first aspect for stabilizing an optical element. Fig. 5a-b shows an irradiation field for carrying out an embodiment of a method according to the first aspect for stabilizing an optical element as well as a temperature development for discrete points of the irradiation field. Fig. 6 shows a schematic representation of a system for electron irradiation for carrying out an embodiment of a method according to the first aspect for stabilizing an optical element. Detailed Description of the Drawing Fig. 1 shows, by way of example, the basic structure of a projection exposure system 10 for microlithography, in which the invention can be applied.An illumination system of the projection exposure system 10 comprises, in addition to a light source 12, illumination optics 13 for illuminating an object field 14 in an object plane 15. EUV radiation 23 generated by the light source 12, as useful optical radiation, is directed by a collector integrated in the light source 12 such that it passes through an intermediate focus in the region of an intermediate focal plane 24 before impinging on a field facet mirror 11. After the field facet mirror 11, the EUV radiation 23 is reflected by a pupil facet mirror 25. With the aid of the pupil facet mirror 25 and an optical assembly 26 with mirrors 26a, 26b, and 26c, field facets of the field facet mirror 11 are imaged into the object field 14. H / hl 220968WOJune 13, 2024 A reticle 16 arranged in the object field 14 is illuminated and held by a schematically illustrated reticle holder 17. A projection optics 18, shown only schematically, serves to image the object field 14 into an image field 19 in an image plane 20. A structure on the reticle 16 is imaged onto a light-sensitive layer of a wafer 21 arranged in the region of the image field 19 in the image plane 20, which is held by a wafer holder 22, also shown in detail. The light source 12 can emit useful radiation, in particular in a wavelength range between 5 nm and 30 nm. The invention can also be used in a DUV projection system, which is not shown.A DUV system is fundamentally constructed like the EUV projection system 10 described above, whereby mirrors and lenses can be used as optical elements in a DUV system, and the light source of a DUV system emits useful radiation in a wavelength range from 100 nm to 300 nm. Fig. 2 shows the variation of the maximum penetration depth of electrons in a layer system of a MoSi coating of an optical element for different electron energies. It has been recognized that when an optical element is irradiated with electrons, the penetration depth of the electrons is influenced by the electron energy. For example, the heating depth can be adjusted via the electron energy. The greater the electron energy, the deeper the penetration depth of the electrons. This allows the coating of the optical element to be irradiated in a targeted manner, for example, by adapting the electron energy to the thickness of the coating.In this way, it can be ensured that the electron energy ETemp for annealing is at least essentially completely deposited in the coating of the optical element, thus locally annealing the coating. For compacting the substrate of the optical element, however, a higher electron energy EComp can be selected in order to specifically compact the substrate locally. H / hl 220968WOJune 13, 2024. Trajectories of simulated electron beams are shown in Figs. 3a to 3c. The bright lines represent the simulated trajectories of the penetrating electrons. The trajectories depicted by dark lines represent electrons that leave the material again during the scattering process. The respective material depths are indicated on the right side of the figures. Fig. 3a shows a simulation of the trajectories at an electron energy of 5 keV. At an electron energy of 5 keV, the electrons do not reach the substrate of the optical element, which begins below the lowest dashed line. Figs. 3b and 3c analogously show the trajectories for electron energies of 15 keV and 55 keV, respectively. In Fig. 3b, the electrons also penetrate into near-surface regions of the substrate of the optical element located below the coating.The electrons reach penetration depths of approximately up to 5500 nm below the surface of the optical element. At an electron energy of 55 keV, the electrons penetrate significantly deeper into the substrate, as shown in Fig. 3c. Figs. 4a and 4b show the compaction of a substrate of an optical element by means of electron irradiation and the subsequent annealing of the optical element by means of electron irradiation. Fig. 4a shows the irradiation of an optical element 1 with an electron energy EComp1 (left) for compacting the substrate 2 of the optical element 1. Following compaction, the coating 3 of the optical element 1 is irradiated to anneal it, as shown in Fig. 4b (left). Annealing stabilizes both the coating 3 and the resulting compaction.If the heat introduced into the depth by annealing is insufficient, it is possible to enable effective annealing of coating 3 and compaction of the optical element 1 by adjusting the processing parameters during compaction, in particular the electron energy EComp2 and thus the depth of the compaction zone. The penetration depth of the electrons depends on the electron energy. The electron energy EComp2 can be selected to be lower than the electron energy EComp1, whereby the electrons penetrate less deeply into the S. H / hl 220968WO June 13, 2024 Penetrate substrate 2 to compact the substrate material there, as shown in Fig. 4a (right). The electron energy ETemp1 during annealing in Fig. 4b (right) is not changed, as it is adapted to the layer system 3 of the optical element 1. ist.Fig. 5a shows the division of an irradiation field for irradiating the coating of an optical element into discrete points i,j. Using an electron beam, the surface of the coating of the optical element can be scanned. The scanning is preferably performed multiple times, for example, two to 10,000 times. The electron beam can be directed at at least one discrete point i,j on the surface and remain at this point for a specific time. For example, the residence time Δt of the electron beam is 10 ns to 10 ms. During this time interval, a local temperature peak can occur at the discrete point, as shown in Fig. 5b. The temperature can suddenly rise due to the heat input. Fig. 5b outlines the temperature development at each point i,j; i,j+1; etc.If the electron beam rests at one point, the temperature T rises suddenly due to heat input and drops again as soon as the beam reaches the next point. In addition to the temperature peaks, there is also a net increase in temperature ΔT in the overall mirror over time. Due to the strong temperature dependence of the relaxations, the temperature peaks can be used for stabilization. Fig. 6 shows a system for electron irradiation with two electron guns A, B. Electron gun A is used to compact the substrate 2, while electron gun B irradiates the coating 3 of the optical element 1. The compaction of the substrate 2 and the annealing of the optical element 1 occur essentially simultaneously. For this purpose, the electron guns A, B are operated with different acceleration voltages Ucomp for compacting the substrate 2 and Utemp for annealing the optical element 1.The electron gun A is operated with an accelerating voltage UComp of 60 keV, while the electron gun B with an accelerating voltage S. H / hl 220968WO June 13, 2024 UTemp of 10 keV. The current IComp for compaction is 0.1 mA, achieving a power PComp of 6 W. The current ITemp for annealing is 2 mA, achieving a power PComp of 20 W. In this way, temperatures of ≥ 60 °C can be achieved in the coating 3 of the optical element 1, thus stabilizing the coating 3 and near-surface areas of the substrate 2. H / hl 220968WO June 13, 2024

Claims

June 13, 2024 Patent claims 1. A method for stabilizing an optical element (1), the method comprising: - providing an optical element (1) with a substrate (2) and a coating (3), and - annealing the optical element (1), characterized in that the annealing of the optical element (1) comprises irradiating the coating (3) of the optical element (1) with electrons, wherein the method further comprises: - at least partially compacting the substrate (2) of the optical element (1), wherein the compacting comprises irradiating the optical element (1) with electrons, wherein the compacting of the substrate (2) of the optical element (1) takes place before the annealing of the coating (3) of the optical element (1) or substantially simultaneously therewith. 2.Method according to claim 1, characterized in that the electron energy ETemp, in particular the acceleration voltage UTemp, for tempering is selected depending on the thickness of the coating (3), in particular at least one layer of the coating (3), of the optical element (1).

3. Method according to claim 1 or 2, characterized in that s. - 2 - the acceleration voltage UTemp for annealing is 1 to 30 keV, preferably 1 to 10 keV, more preferably 11 to 15 keV, more preferably 16 to 20 keV, more preferably 21 to 30 keV.

4. Method according to one of claims 1 to 3, characterized in that the current intensity ITemp for annealing is 0.1 to 25 mA, in particular 0.1 to 5 mA, more particularly 5.1 to 10 mA, more particularly 10.1 to 15 mA, more particularly 15.1 to 20 mA, more particularly 20.1 to 25 mA.

5. Method according to one of claims 1 to 4, characterized in that the temperature during annealing is > 60 °C.

6. Method according to one of claims 1 to 5, characterized in that the electron energy EKomp used for compacting is greater than the electron energy ETemp used for annealing.7.Method according to one of claims 1 to 6, characterized in that the acceleration voltage UKomp for compacting is more than 30 keV, particularly preferably 31 to 40 keV, further particularly preferably 41 to 50 keV, further particularly preferably 51 to 60 keV, further particularly preferably 61 to 70 keV, further particularly preferably 71 to 80 keV, further particularly preferably 81 to 90 keV, further particularly preferably 91 to 100 keV.

8. Method according to one of claims 1 to 7, characterized in that S. H / hl 220968WO June 13, 2024 - 3 - a counter field, in particular a dynamic counter field, is applied to the optical element (1), in particular the coating (3) of the optical element (1) wird.

9. Method according to claim 8, characterized in that the optical element (1) is irradiated with the same electron energy both for compacting and for annealing, the electrons for irradiating the coating (3) being specifically slowed down by the opposing field.

10. Optical element (1), in particular a reflective optical element, stabilized according to a method according to one of claims 1 to 9.

11. Projection objective for microlithography comprising an optical element according to claim 10.

12. Projection exposure system (10) for microlithography comprising a projection objective according to claim 11.13.Device for electron irradiation, in particular for carrying out a method according to claim 8 or 9, the device comprising: a vacuum system with a vacuum chamber, wherein the vacuum system contains at least one electron beam source, in particular an electron gun, for generating an electron beam, characterized in that the device is set up to generate a counterfield, in particular a dynamic counterfield, on the optical element (1), in particular the coating (3) of the optical element (1). H / hl 220968WO June 13, 2024

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