Processing an optical element for a projection printing plant

Abstract

The invention relates to a method for processing an optical element (1), the method comprising the steps of: determining a wavefront of an optical system (10); and irradiating the optical element with electrons depending on the determined wavefront of the optical system. The invention also relates to a method for processing an optical element (1), the method comprising: determining the shape of the surface (2) of the optical element; and irradiating the optical element with electrons depending on the determined shape of the surface of the optical element. The problem of providing methods for processing an optical element, which methods allow rapid processing of an optical element and using which methods larger amplitudes can also be converted to corrections without (substantially) modifying the coating, is solved by the irradiation being carried out at a first electron energy E1 and at least one additional electron energy E2. The invention also relates to an optical element (1) processed using one of the methods according to the invention; to an optical system (10), in particular a projection system, for microlithography having such an optical element; to a semiconductor technology plant, in particular a projection printing plant, having such an optical system, in particular a projection system; and to a device for electron irradiation (100), in particular for carrying out the methods according to the invention.

Description

[0001] Processing of an optical element for a projection exposure system

[0002] Technical area

[0003] The invention relates to a method for processing an optical element, the method comprising the steps of determining a wavefront of an optical system, and irradiating the optical element with electrons depending on the determined wavefront of the optical system. Furthermore, the invention relates to a method for processing an optical element, the method comprising the steps of determining the shape of a surface of the optical element, and irradiating the optical element with electrons depending on the determined shape of the surface of the optical element.Furthermore, the invention relates to an optical element which has been processed using one of the methods according to the invention, to an optical system, in particular a projection system, for microlithography comprising such an optical element, to a semiconductor technology system, in particular a projection exposure system, comprising such an optical system and to a device for electron irradiation, in particular for carrying out one of the methods according to the invention.

[0004] The subject matter of German patent application 102023208563.1 is hereby incorporated by reference.

[0005] background

[0006] 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 masks themselves. Surface defects that arise, for example, during manufacturing or through pre-processes, especially coatings, must therefore be corrected. Methods for processing the surface shape of optical elements are known from the prior art, in particular from the documents US 6,844,272 B2, US 6,849,859 B2, DE 102 39 859 A1, US 6,821,682 B1, US 2003 / 006214 A1, US 2003 / 0081722 A1, US 6,898,011 B2, US 7,083,290 B2, US 7,189,655 B2, US 2003 / 058986 A1, DE 10 2007 051 291 A1, EP 1 521 155 A2, US 4,298,247 A and DE 10 2018 211 596 A1.Some of the processing methods listed in the above-mentioned documents are based on locally compacting the substrate material of optical elements over a long period of time by irradiation (hereinafter referred to as “compacting”).

[0007] For example, DE 10 2018 211 596 A1 discloses a method for processing mirrors for lithography systems. In this method, the mirrors are irradiated with electrons to compact the mirror substrate beneath the coating. This allows changes to the surface shape of the optical elements at the level of the irradiated areas, so that, for example, surface defects left over from previous processes can be compensated. Individual optical elements can thus be subsequently reworked in a targeted manner.

[0008] However, processing is always a compromise between total irradiation time and component heating. To avoid local overheating, the beam current and accelerating voltage are kept constant throughout the entire irradiation process. This compacts the mirror substrate at a characteristic depth below the coating. However, the compaction that occurs after irradiation decreases with increasing operating temperature by approximately -1 pm / K for 1 nm compaction, since warmer glass has a higher elastic modulus, thus becoming stiffer and less compressive due to radiation-induced compressive stresses. This reversible, temperature-dependent decompaction increases with fluence, i.e., disproportionately with the achieved compaction, and contributes to undesirable so-called mirror heating.In addition, bonded joints on add-on components and coatings on optical elements, in particular, cannot tolerate high temperatures, which is why longer irradiation times are preferred, and individual irradiations are repeated multiple times until the desired dose has been accumulated. While the total irradiation time already reaches 24 hours for conventional optical elements, this increases immeasurably for larger surfaces.

[0009] Counteracting the extended processing time by increasing the local particle fluence also has disadvantages. When EUV mirrors are irradiated, for example, approximately 1% of the electron energy is deposited in the EUV coating, resulting in unavoidable compaction of the multilayer coatings of the EUV mirror. However, reflectivity already decreases when the reflection maximum is detuned by approximately 0.1% of the design wavelength of 13.5 nm. Furthermore, high-energy radiation promotes diffusion processes between individual layers in the coating, which also reduce reflectivity, and changes the distances through compaction. Thermal stress also accelerates aging between the layers through diffusion.

[0010] In addition, larger surface depressions may be necessary, for example in order to be able to specifically process individual optical elements based on system measurements. In the optical system of a microlithography system, deviations in the wavefront can, for example, lead to aberrations that can significantly impair the imaging performance of the system. The deviations can be caused, for example, by changes in the mechanical and / or optical properties of optical elements in the system. For example, thermal expansion can change the shape of the optical elements, such as the yoke and the thickness of a lens. The refractive index of the optical material can also change with temperature. In order to compensate for such deviations in the wavefront by processing individual optical elements, larger surface depressions of, for example, up to 10 nm are required, which cannot be achieved with known methods.Finally, software used in generic machining processes creates specifications for the beam movement during machining. For example, with an amplitude of 10 nm to be corrected, only steps of 40 pm are resolved. The granularity achieved with a single irradiation pass borders on the accuracy requirements.

[0011] The object of the present invention is to provide methods for processing an optical element which overcome the disadvantages of the prior art. In particular, methods are to be provided which allow rapid processing of an optical element and with which even larger amplitudes of corrections can be implemented without (significantly) altering the coating. A further object of the invention is to specify an advantageous optical element, an optical system, in particular a projection system, comprising such an optical element, a semiconductor technology system, in particular a projection exposure system, comprising such an optical system, and an advantageous device for electron irradiation, in particular for carrying out the method according to the invention.

[0012] Description of the invention

[0013] According to a first teaching, the above-mentioned object is achieved by a method for processing an optical element, the method comprising the steps of: determining the wavefront of an optical system, and irradiating the optical element with electrons depending on the determined wavefront of the optical system, in that the irradiation is carried out with a first electron energy Ei and at least one further electron energy E2.

[0014] The method is suitable for processing an optical element. The optical element can be an optical element for a semiconductor technology system, in particular for a projection exposure system. For example, the optical element is a reflective optical element, e.g. a mirror or a mask. The optical element can comprise a substrate with a substrate surface. The substrate can comprise, for example, SiSic, Zerodur® from Schott AG, ULE® from Corning Inc., quartz glass and / or any other type of glass. Furthermore, the optical element can comprise a coating. In particular, the optical element comprises a coating which is suitable for an EUV wavelength range. During the production of a coated optical element, an initially manufactured surface shape of the optical element can be negatively influenced, for example, by a subsequent coating.The process is therefore particularly advantageous for the processing of coated optical elements.

[0015] The coating of the optical element can, for example, have a spatially homogeneous coating thickness. It is also conceivable for the coating to have a spatially variable thickness. The coating can comprise one or more layers. For example, the coating can comprise at least one layer subsystem. The layer subsystem can comprise at least two layers comprising materials with different refractive indices. For example, combinations of more dispersing materials and more absorbing materials as layers are conceivable in order to reflect incident waves with a defined wavelength. A layer subsystem can, in particular, comprise molybdenum and silicon layers. Silicon is a more dispersing material, while molybdenum is a more absorbing material. The higher the dispersion to absorption ratio, the higher the reflectivity of the coating.The layer arrangement of the at least one layer subsystem can, for example, comprise a periodic sequence of at least two periods of individual layers. The periods can comprise individual layers made of different materials.

[0016] 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 less than 193 nm, for example, 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.

[0017] Optionally, a protective layer can be applied between the substrate and the reflective layer of an optical element. A protective layer is intended, for example, to prevent the substrate from compacting, for example due to useful EUV radiation, i.e. the radiation used in an EUV projection exposure system. The layer arrangement of the protective layer can have a thickness of at least 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%. It is also conceivable that a protective layer made of a noble metal is provided, at least in part, the minimum layer thickness of which is selected such that penetration by reactive hydrogen particles, in particular hydrogen ions and / or hydrogen radicals, is prevented.A protective layer made of a noble metal impedes or prevents the formation of volatile hydrides, which can arise through so-called hydrogen-induced outgassing (HIO). In particular, the recombination of reactive hydrogen particles or the formation of water in areas close to the surface of the component is reduced or eliminated. The thickness of the protective layer is preferably selected such that the protective layer cannot be penetrated by hydrogen ions and / or hydrogen radicals. In particular, the layer thickness of the noble metal protective layer can be in the range of 50 to 1000 nm, in particular in the range of 100 to 500 nm. The layer can in particular comprise rhodium, ruthenium, iridium, palladium and platinum, in particular at least one element from the noble metal group which does not form hydrides with the reactive hydrogen particles.

[0018] It is also conceivable that an optical element could be a refractive element, such as a lens or prism, a diffractive element, such as a hologram or a diffuser, or combinations thereof, such as superlattices on lenses or mirrors. Electron irradiation also changes the refractive index in transmission optics. As with the irradiation of reflective optical elements, the electrons can not only eliminate defective optical effects but also introduce desired optical properties such as nanoaspheres or superlattices.

[0019] The method comprises determining the wavefront of an optical system. The optical system can be, for example, a projection system of a projection exposure apparatus. The optical system comprises at least one optical element. Preferably, the optical system comprises a plurality of optical elements. The optical system can comprise an arrangement of optical elements that serve to manipulate and / or shape light in order to generate, magnify, and / or correct images. The optical system can comprise various optical elements such as lenses, mirrors, prisms, diaphragms, filters, in particular spectral filters, and / or polarization-optical elements, such as retardation plates, rotators, or polarization filters, which are positioned in a specific arrangement to focus, deflect, scatter, diffract, and / or filter light beams.

[0020] The wavefront of the optical system can be understood in particular as the wavefront of radiation that is manipulated by the optical system, in particular that is at least partially guided and / or reflected by the optical system. A wavefront of radiation can be understood as the surface on which, during wave propagation, all points have the same travel time to the object point. Determining the wavefront of the optical system can comprise measuring the wavefront. The wavefront can be measured, for example, using an interferometer, in particular using a shear interferometer. Using the interferometer, interferograms can be generated, and wavefront derivatives can be determined from them, based on which a wavefront profile can be reconstructed.

[0021] The method comprises irradiating an optical element depending on the specific wavefront of the optical system. The irradiation can serve, in particular, to correct the optical element, in particular to correct the shape of the surface of the optical element. With the aid of electron irradiation, the substrate beneath the surface of the optical element can be locally compacted. In the process, the substrate contracts, which in turn leads to a deformation on the surface of the optical element. The correction of the optical element can, in turn, serve to correct the wavefront of the optical system. The optical element can, for example, be (re)introduced into the optical system after irradiation.

[0022] The irradiation is carried out with a first electron energy Ei and at least one further electron energy E2. The first electron energy Ei and the at least one further electron energy E2 differ from one another. In particular, the first electron energy Ei is greater than the at least one further electron energy E2. However, it is also conceivable that the at least one further electron energy E2 is greater than the first electron energy Ei. The penetration depth of the electrons into the substrate of the optical element can be influenced via the electron energy. In particular, the first electron energy Ei and the at least one further electron energy E2 are selected such that compactions are introduced into the substrate of the optical element at different penetration depths. The stopping power of the electrons decreases, to a first approximation, inversely proportional to the electron energy E.Most of the energy is deposited at the end of the trajectory, where the electrons have been slowed down to zero. Irradiating the optical element with different electron energies in turn results in compaction at different penetration depths. The irradiation is therefore effectively carried out with polychromatic electrons. The electron energy E depends on the acceleration voltage Ußschi of the electrons. With the help of the acceleration voltage U Besch i, the electron energy E and thus in particular the penetration depth of the electrons in the substrate of the optical element can be influenced. The irradiation is therefore carried out in particular with a first acceleration voltage U Besch n and at least one further acceleration voltage Ußeschiz.While in known methods entire irradiation processes were carried out with a constant acceleration voltage U Besch i, higher acceleration voltages can be used, for example, to access deeper layers that have not been irradiated before. In this way, the method can be used to introduce compaction at at least two different penetration depths. For example, it is conceivable that in addition to a penetration depth of, for example, 30 pm, compaction is introduced by irradiation with a further electron energy at a further penetration depth, in particular deeper than 30 pm, e.g. 300 pm. For this purpose, acceleration voltages between 20 keV and 150 keV, preferably 30 to 150 keV, in particular 60 to 80 keV, can be selected. For example, the first acceleration voltage Ußeschii is 60 keV and the further acceleration voltage Ußeschiz is more than 60 keV.At the same time, a stronger acceleration of electrons has the advantage that an existing coating on the optical element is less damaged.

[0023] It was discovered that the respective surface depressions multiply when the same dose is applied at different penetration depths. For example, the surface depression can double if the same dose is applied at two different penetration depths. In this way, larger deviations can be corrected with the same energy fluence, or the same surface deformation can be achieved with less energy fluence. This can shorten the irradiation time, while also reducing the stress on the irradiated coating on the surface of the optical element. Other side effects that scale with the dose, such as heating of the optical element, can also be mitigated accordingly.By distributing the dose across the depth of the substrate, for example, age-related decompaction, which affects the lifetime of irradiated optical elements, can be slowed down. Unlike X-rays, electrons decelerate completely after traveling a finite distance in matter. Therefore, radiation exposure does not decay exponentially, but ends abruptly at a defined depth. The smoothed gradients, especially at the abrupt end of the electron trajectories, relax more slowly due to diffusion or strain.

[0024] The method allows optical elements to be specifically reworked during production or subsequently. The method for processing the optical element can thus be used to correct the optical element, in particular to correct the surface shape of the optical element. Using the method for processing an optical element, surface depressions of, for example, more than 2 nm, preferably more than 5 nm, particularly preferably up to 10 nm, in particular more than 10 nm, can be achieved. This is particularly advantageous for targeted subsequent correction of optical elements based on a system measurement.

[0025] Furthermore, due to the irradiation with the first electron energy Ei and the at least one further electron energy E2, the surface shape and its change with heating can be adjusted independently of one another, at least within certain limits. The electrons change not only the density in the irradiated region of the optical element, but also the thermal expansion, the rigidity, and in the case of lenses, the refractive power with heating. The wavefront effect of the optical element behaves differently when heated than before. This can be exploited to counteract the imperfect thermal behavior of the optical element through targeted irradiation. According to a first advantageous embodiment of the method according to the first teaching, the first electron energy Ei and the at least one further electron energy E2 are selected depending on the specific wavefront of the optical system.In particular, the method for processing the optical element comprises irradiating the optical element with a first electron energy Ei and at least one further electron energy E2 depending on the specific wavefront of the optical system. The irradiation with the first electron energy Ei and the at least one further electron energy E2 can serve to correct the wavefront of the optical system, for example, a projection system.

[0026] According to a further advantageous embodiment of the method according to the first teaching, determining the wavefront of the optical system comprises determining the initial and / or desired wavefront of the optical system. In particular, determining the wavefront of the optical system comprises comparing the initial and desired wavefront of the optical system. The optical element can be irradiated with electrons, for example, based on a determined deviation of the actual wavefront from the desired wavefront. With the aid of a wavefront measurement of the optical system, an actual wavefront, in particular an actual wavefront profile, can be determined with spatial resolution. The method can, in particular, comprise determining a required correction of the optical element in order to adjust the measured actual wavefront profile for the optical system to a predefinable desired wavefront profile.

[0027] According to a further advantageous embodiment of the method according to the first teaching, the optical system comprises at least one optical element, wherein the optical element of the optical system or an identically manufactured optical element is irradiated with electrons depending on the determined wavefront of the optical system. The method can, for example, comprise determining the wavefront of an optical system comprising at least one optical element and irradiating this optical element with electrons depending on the determined wavefront of the optical system. Instead of performing measurements on the optical element to be processed itself, the processing can be carried out based on the determined wavefront of the optical system.

[0028] Alternatively or additionally, the method can comprise determining the wavefront of an optical system comprising at least one optical element and irradiating an identically manufactured optical element with electrons depending on the determined wavefront of the optical system. An identically manufactured optical element can be understood, for example, as an optical element that at least substantially corresponds to the optical element of the optical system. For example, the identically manufactured optical element can have been manufactured based on the same optical design. The identically manufactured optical element is, in particular, an optical element that is separate from the optical system. Depending on the wavefront of the optical system, an optical element that is separate from the system can, in particular, be irradiated.

[0029] The separate optical element can be manufactured identically to the optical element present in the optical system. For example, it is conceivable that the wavefront of a customer's optical system is determined. To correct the wavefront, for example, an existing optical element of the optical system can be replaced with a separate, new optical element. The separate optical element preferably corresponds at least substantially to the optical element of the optical system. For example, the separate optical element can have been manufactured based on the same optical design. The separate optical element is, in particular, an optical element manufactured identically.The separate optical element can be machined using the process, for example, at the manufacturer's site, based on the specific wavefront of the optical system and can replace the previously existing optical element to correct the wavefront of the optical system. Here, too, machining the optical element can serve to correct the wavefront of the optical system.

[0030] According to a further advantageous embodiment of the method according to the first teaching, the optical system comprises at least one optical element, the method further comprising the steps of: determining an effect of the at least one optical element of the optical system on the wavefront of the optical system and irradiating the optical element of the optical system or an identically manufactured optical element with electrons depending on the determined effect of the optical element. The effect of the optical element on the wavefront can be derived, for example, by determining the wavefront of the optical system. The effect of the optical element can be determined, for example, by the shape of the surface of the optical element. It is also conceivable that the effect of the optical element can be caused by a refractive index of the optical element, for example caused by striae in the material of the optical element.The method may include determining the wavefront of the system based on the expected effect of irradiation with the first electron energy Ei and the at least one further electron energy E2. The wavefront of the optical system may be predicted, for example, using a computational method, such as beam propagation.

[0031] According to a further advantageous embodiment of the method according to the first teaching, the method further comprises the steps of determining a shape of a surface of the optical element, and irradiating the optical element with electrons depending on the determined shape of the surface of the optical element. The irradiation of the optical element with electrons can, for example, be determined both depending on the determined wavefront of the optical system and depending on the determined shape of the surface of the element. It is also conceivable that the shape of the surface is determined for control purposes after irradiation and / or subsequent annealing has already taken place. If the shape of the surface does not yet correspond to the desired shape, the optical element can be irradiated again. The renewed irradiation can take place with a different electron energy.

[0032] According to a second teaching, the above-mentioned object is achieved by a method for processing an optical element, the method comprising: determining the shape of a surface of the optical element, and irradiating the optical element with electrons depending on the determined shape of the surface of the optical element, in that the irradiation is carried out with a first electron energy Ei and at least one further electron energy E2.

[0033] The method for processing the optical element comprises determining the shape of a surface of the optical element. Determining the shape of a surface of the optical element can, for example, comprise determining the shape of at least a portion of the surface, in particular a portion of the surface of the useful optical area, of the optical element. It is also conceivable that the shape of the entire surface of the optical element, in particular the entire useful optical area of ​​the optical element, is determined. The shape of the surface of the optical element can be determined. For example, the surface of the optical element can be at least partially measured. The surface of the optical element means, for example, the surface of the substrate or the surface of the coating. In the case of coated optical elements, the surface shape of the coating can be determined.For example, it is conceivable that the shape of the surface of the optical element could be determined using an interferometer. It is also conceivable that the depth of the multilayer coating could be determined. For this purpose, an ellipsometer and / or a reflectometer could be used.

[0034] The optical element is irradiated with electrons, wherein the irradiation is carried out depending on the specific shape of the surface of the optical element. The irradiation can in particular serve to correct the optical element, in particular to correct the shape of the surface of the optical element. In particular, the irradiation of the optical element is carried out on the basis of a determined deviation of the specific shape of the surface from a desired shape of the surface. By irradiating the optical element with electrons, for example, a previously determined deviation from the desired shape of the surface can be corrected. After irradiation and / or subsequent annealing, the shape of the surface of the optical element can be determined again. If the shape of the surface still does not correspond to the desired shape, the optical element can be irradiated again.The re-irradiation can be carried out with a different electron energy.

[0035] In the method according to the second teaching, irradiation is also carried out with a first electron energy Ei and at least one further electron energy E2. Irradiating the optical element with different electron energies results in compaction at different penetration depths, and thus, larger deviations can be corrected with the same energy fluence, or the same surface deformation can be achieved with less energy fluence.

[0036] According to a first advantageous embodiment of the method according to the first or second teaching, determining the shape of the surface of the optical element comprises determining the initial and / or desired shape of the surface of the optical element. For example, determining the shape of the surface of the optical element can comprise determining the actual shape, i.e., the existing shape, of the surface of the optical element. The actual shape of the surface of the optical element can, in particular, deviate from a desired shape of the surface, the desired shape. The desired shape of the surface of the optical element can, for example, be determined taking into account other optical elements, in particular the surface shapes of other optical elements, of an optical system.If the actual shape of the surface of the optical element deviates from the desired shape of the surface, the deviation in the shape of the surface to be corrected can be determined based on the difference between the actual shape and the desired desired shape. In particular, a required local surface depression can be determined. The method comprises, for example, comparing the actual shape of the surface of the optical element with the desired shape of the surface. The method can further comprise determining a deviation between the actual shape of the surface and the desired shape of the surface. It is conceivable, for example, that the difference between the actual shape of the surface and the desired shape of the surface is determined mathematically. Depending on the determined deviation in the shape of the surface, a required correction can be determined.However, the surface of the optical element can also bend at locations far from the irradiation entry point, because the entire substrate can deform. Therefore, a mathematical determination can be quite complex.

[0037] The desired shape of the surface of the optical element can be determined, for example, based on a system measurement, in particular a determination of the wavefront of an optical system. It is conceivable, for example, that a wavefront aberration is determined for an optical system consisting of a large number of optical elements, for example a projection system. Based on the system measurement, in particular the determined wavefront aberration, the required processing, in particular a required surface correction, of the optical element can be determined. This does not necessarily have to involve correcting the optical element with the defective surface. A different optical element of the optical system can also be selected, for example because it can be removed more easily or modified more easily later. Surface depressions of up to 10 nm or more may be necessary.For example, the optical element can already be part of the system of optical elements, whereby the optical element can be subsequently corrected based on the system measurement. It is also conceivable that the optical element is manufactured based on a design for the system and then specifically modified for the optical system during production.

[0038] According to a further advantageous embodiment of the method according to the first or second teaching, the first electron energy Ei and the at least one further electron energy E2 are selected depending on the specific shape of the surface of the optical element. In particular, the method for processing the optical element comprises irradiating the optical element with a first electron energy Ei and at least one further electron energy E2 depending on the specific shape of the surface or its change in the optical element. In particular, the first electron energy Ei and the at least one further electron energy E2 are determined depending on a detected deviation of the actual shape of the surface from a desired shape of the surface of the optical element.

[0039] Irradiation with the first electron energy Ei and the at least one further electron energy E2 can serve to correct the surface of the optical element, in particular to compact the substrate material of the optical element. Thus, irradiation with the first electron energy Ei and the at least one further electron energy E2 can be used to create depressions in the surface of the optical element.

[0040] It is conceivable that the method comprises additional irradiation of the optical element to produce, stabilize, and / or modify a protective layer. The first electron energy Ei and the at least one further electron energy E2 for correcting the optical element are, in particular, selected to be greater than a further electron energy Eprotect for producing the protective layer. To produce the protective layer, at least substantially the entire surface of the optical element is irradiated with fluences in the range of up to 4000 J / mm 2This leads to saturation of the compaction, i.e., a state in which the compaction of the substrate no longer increases or only increases negligibly upon further irradiation. The total absorption of the useful radiation, for example, EUV radiation, does not change as a result of the compaction. However, it can no longer cause damage if the compaction can no longer change within its penetration depth.

[0041] According to a further advantageous embodiment of the method according to the first or second teaching, a first dose profile is generated in the optical element using the first electron energy Ei and a further dose profile is generated using the at least one further electron energy E2, wherein the first electron energy Ei and the at least one further electron energy E2 are selected such that there is at least substantially no overlap of the dose profiles. A dose profile indicates how the radiation energy is deposited over the depth. This is measured, for example, in 1 / mm = parts per thousand per micrometer. Non-overlapping dose profiles therefore mean that the electron energy is absorbed by the material at different depths. It has been shown that compaction accumulates when the dose profiles at different penetration depths at least substantially do not overlap. As a consequence, the resulting surface depressions also accumulate.It was discovered that the respective surface subsidence doubles when the same dose is deposited at two different depths at one location. The dose describes how much radiation energy is deposited at one location. For example, if 0.5 J / mm is applied at any location x and y of the optical element at two different depths zi and z2, the dose will be doubled. 2 deposited, then the respective depressions double by 1.25 nm to a total of 2.5 nm. A monochromatic irradiation with the same total fluence of 1 J / mm 2 In contrast, the substrate would have been compacted in a single layer depth with an integral depth of 2.0 nm. With less energy fluence, a greater surface deformation can be achieved, thereby shortening the irradiation time. It was found that, for example, with eight energetically different partial irradiations of 0.5 J / mm 2 , in total 4 J / mm 2, so 10 nm compaction would be possible, for which 10 J / mm 2 with monoenergetic electrons. The energetic fluence indicates how much energy passes through the surface, regardless of the depth at which it is deposited.

[0042] According to a further advantageous embodiment of the method according to the first or second teaching, the electron energy E is changed during scanning with the electron beam. For example, irradiation is initially carried out with the first electron energy Ei and subsequently with at least one further electron energy E2. A change in the electron energy E during irradiation allows a more uniform distribution of the dose across the depth of the substrate of the optical element. Compactions at different depths can advantageously be added together so that greater surface depressions can be achieved. It is conceivable that the acceleration voltage Ußeschi is changed, for example increased, during irradiation. A step-by-step change is conceivable.

[0043] The current strength is, for example, 0.1 to 0.5 mA. A higher current strength can shorten the irradiation duration. A higher current strength can increase the local particle fluence. However, the current strength can be limited, for example, by the electron emitter, in particular the hot cathode, in the Wehnelt cylinder of the electron gun and the heat generated. Furthermore, current strengths above 0.5 mA, for example, should be avoided in order to prevent the optical element from being heated locally above 60 °C. Adhesive joints of the add-on components and the coating itself cannot tolerate higher temperatures. For this reason, it is better to irradiate for longer and play it safe than producing mirrors more quickly. Using higher acceleration voltages, for example, deeper layers of the substrate can be accessed. In this way, even with the same current strength, more energy fluence can be irradiated in the same time.The same energy fluence can be applied in a shorter time. Thanks to a decreasing interaction with increasing energy, a lower percentage of unwanted energy is deposited in the surface or sensitive coating.

[0044] Preferably, the electron energy E is swept during irradiation, wherein the swept is performed faster than a movement of the electron beam over the surface of the optical element. To process the optical element, the electron beam can be moved over the surface of the optical element during irradiation, in particular the part of the surface to be processed. The electron beam is, for example, swept the entire time with the same parameters, e.g., the boundaries Ei and E2. Alternatively, the optical element is moved beneath the beam. Sweeping can be understood as a rapid back-and-forth switching between the first electron energy Ei and the at least one further electron energy E2.During irradiation, switching back and forth between the first electron energy Ei and the at least one further electron energy E2 can be carried out faster than the movement of the electron beam over the surface of the optical element. The time required to switch between the electron energies Ei and E2 can be compared, for example, with the time it takes the electron beam to move by its diameter. In particular, the acceleration voltage is swept faster than the electron beam is moved over the optical element. Sweeping of the acceleration voltage can be understood as a rapid switching between the first acceleration voltage Ußeschii and the at least one further acceleration voltage UBeschi2.

[0045] According to a further advantageous embodiment of the method according to the first or second teaching, the irradiation comprises a plurality of partial irradiations, wherein the electron energy is changed between at least two partial irradiations. The irradiation can be divided into a plurality of partial irradiations to avoid local overheating. The partial irradiations are preferably introduced with the same spatial distribution. For example, the irradiation is divided into several thousand partial irradiations. The partial irradiations are carried out at least partially with the same electron energy. It is conceivable, for example, that the partial irradiations are initially carried out with a first electron energy Ei and subsequently with at least one further electron energy E2. The electron energy can, for example, be changed in a pause between two partial irradiations.For example, the accelerating voltage is changed during the pause between partial irradiations. Or a second electron gun with a fixed accelerating voltage is switched on while the first is switched off.

[0046] According to a further advantageous embodiment of the method according to the first or second teaching, during irradiation with the first electron energy Ei, irradiation is carried out with a first irradiation pattern and during irradiation with the further electron energy E2, irradiation is carried out with a further irradiation pattern. For example, during a first partial irradiation, irradiation is carried out with a first irradiation pattern and during at least one further partial irradiation, irradiation is carried out with a further irradiation pattern. Different irradiation patterns are thus distributed across at least two different partial irradiations. The irradiation patterns differ, for example, in their spatial frequencies and / or amplitudes. Preferably, during irradiation with the higher electron energy, lower spatial frequencies are given predominance with higher amplitudes.In particular, the lower spatial frequencies with their higher amplitudes are overweighted in partial irradiations with higher electron energy. Conversely, during irradiation with the lower electron energy, higher spatial frequencies with lower amplitudes can be overweighted. An irradiation pattern refers, in particular, to the areal distribution of the fluence, usually on the surface.

[0047] It was found that the lateral resolution decreases inversely with the deeper penetration of the electrons. For example, for a penetration depth of 30 pm, spatial frequencies above "5000 / m are hardly translated into surface deformations during compaction. If the penetration depth increases from 30 pm to 300 gm, even spatial frequencies of 500 / m are strongly damped. It was also found that, with the surface shape, higher amplitudes tend to occur at lower spatial frequencies, which is compatible with deep penetration depths. For example, strokes with 10 nm amplitudes are limited to low frequencies of around 20 / m. For finer structures with spatial frequencies around 500 / m, low amplitudes of around 0.4 nm can be expected. For medium frequencies up to 100 / m, the amplitudes can be as low as 2 nm.

[0048] According to a further advantageous embodiment of the method according to the first or second teaching, the acceleration voltage U Besch i is modulated, wherein the modulated voltage range is simultaneously slowly changed as the electron beam is directed over the surface of the optical element. In particular, a wobble can be slowly changed as the optical element is scanned. The wobble parameters, in particular the energy limits, can be continuously changed. The wobble frequency can also be varied. It was recognized that as a result the depth profile varies continuously over the location. The spatial frequencies effective at the surface are attenuated differently from location to location, the energy fluence changes with the average acceleration voltage from location to location, and the non-linear dependence of the surface subsidence on the energy fluence also varies from location to location.

[0049] According to a further advantageous embodiment of the method according to the first or second teaching, the irradiation with the first electron energy Ei and the irradiation with the at least one further electron energy E2 take place at least substantially simultaneously. It is conceivable, for example, that at least two electron guns with different acceleration voltages are used for the irradiation. For example, the irradiation takes place with a first electron gun with a first acceleration voltage Ui and at least one further electron gun with a further acceleration voltage U2. An electron gun is understood to be an electrical arrangement for generating electron beams. The electron gun provides a bundled and directed electron beam. The electron beam can, for example, have a diameter of 0.1 mm to 20 mm.An electron irradiation device may, for example, comprise two electron guns operated with a first acceleration voltage Ui and a second acceleration voltage lh. The at least two electron guns are preferably operated at least substantially simultaneously. This allows a shortened irradiation time to be achieved. In particular, the at least two electron guns can be integrated into a single electron irradiation device.

[0050] Alternatively, it is conceivable that the irradiation with the first electron energy Ei and the irradiation with at least one further electron energy E2 occur consecutively. For example, the irradiation is carried out with the same electron gun, which is initially operated with a first acceleration voltage Ui and subsequently with at least one further acceleration voltage U2.

[0051] It is also conceivable to use two different electron irradiation devices that are operated with permanently different acceleration voltages. The optical element is first irradiated in a first electron irradiation device, which is operated, for example, with a first acceleration voltage Ui, and then irradiated in at least one further electron irradiation device, which is operated, for example, with a further acceleration voltage U2. For example, switching between two electron guns, each operated with a fixed acceleration voltage, is possible. In this form of flow production, however, the additional setup times increase the process times.Preferably, the irradiation with the first electron energy Ei takes place via a first entry point xi, and the irradiation with the at least one further electron energy E2 takes place via a further entry point X2 on the surface of the optical element. In particular, the at least two electron guns aim at different points on the surface of the optical element. This advantageously ensures a locally moderate thermal load on the surface of the optical element, in particular the substrate surface. The entry points xi(t) and X2(t) can be time-dependent. It is conceivable that the entry points xi(t) and X2(t) cover the entire surface to be irradiated over time t.

[0052] According to a further advantageous embodiment, the method according to the first or second teaching further comprises annealing the optical element, wherein the annealing of the optical element preferably takes place after irradiation. In particular, the method comprises annealing the substrate of the optical element. The annealing leads to a partial reduction in the previously introduced compaction of the substrate of the optical element, i.e., to decompaction. Decompaction is an effect that occurs over time when the optical element is used in a semiconductor technology system, for example, 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, which advantageously reduces the residual decompaction change over the substrate's service life to a negligible value or adequately compensates for any existing drift in the optical element. During annealing, decompaction can be accelerated so that it progresses more slowly afterward. For example, the optical element is annealed for several days to delay the inevitable residual decompaction for many years. Annealing preferably takes place at a temperature of 60 °C. The previously introduced compactions can be oversized to prevent effects in the subsequent process and thus achieve the desired target shape of the optical element's surface after annealing.

[0053] According to a third teaching, the above-mentioned object is achieved for an optical element, in particular a reflective optical element, in that the optical element has been processed using a method according to the first or second teaching. It is conceivable, for example, that the optical element is used in an inspection system, for example a mask inspection system or a wafer inspection system. In particular, the optical element is suitable for use in a projection exposure system, for example in a projection system. Optical elements for projection exposure systems are subject to high accuracy requirements. Using the method according to the first or second teaching, an optical element can be processed in order to meet corresponding accuracy requirements and predetermined specifications.For example, the optical element is a coated optical element, in particular a coated mirror. The coating of the mirror substrate can comprise at least one layer subsystem optimized for the reflection of EUV radiation, i.e., radiation with a wavelength in the range between 5 nm and 15 nm, in particular 13.5 nm. This reflective 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.

[0054] According to an advantageous embodiment of the optical element, the optical element has a coating with a spatially homogeneous thickness.

[0055] According to a further advantageous embodiment of the optical element, the optical element has a coating with a spatially variable thickness. According to a fourth teaching, the above-mentioned object for an optical system, in particular a projection system, for microlithography is achieved in that the optical system comprises an optical element according to the third teaching. The optical system can, for example, comprise a projection objective. Projection objectives for microlithography are subject to high accuracy requirements. In addition, they are exposed to high loads from the useful radiation, whose wavelength is, for example, 7 nm or 13 nm, preferably 13.5 nm, and radiation of other wavelengths, as a result of which they heat up during operation.

[0056] It is conceivable, for example, that a previously existing, for example defective, optical element is removed from the optical system and replaced with the optical element according to the third teaching. Alternatively, it is conceivable that the optical element is removed from the optical system in order to repair the optical system, is processed after removal according to the method of the first or second teaching, for example in order to correct wave aberrations, and is reinserted into the optical system. It is conceivable, for example, that an optical system is initially assembled with at least partially provisional, for example at least partially less expensive, optical elements, wherein subsequently at least one optical element is replaced with a specifically adapted optical element according to the third teaching or is processed using a method according to the first or second teaching.These measures have the advantage that the costs for repairing the optical system are further reduced. In view of the fact that the optical elements of the optical system are very expensive to manufacture, replacing or reusing the first optical element after removal from the optical system and subsequent processing, e.g. to eliminate material defects in the optical element or its coating, can significantly contribute to reducing the costs of repairing the optical system. A further advantage, for example, is that the optical system can be tested under real operating conditions, e.g. on-site at the customer's premises, and not just under ideal conditions at the manufacturer's premises, as is usually the case. According to a fifth teaching, the above-mentioned object is achieved for a semiconductor technology system in that the system comprises an optical system, in particular a projection system, according to the fourth teaching.The semiconductor technology system can, for example, be a projection exposure system. A projection exposure system can be designed to generate an image of an object arranged in an object plane using a radiation source emitting projection light. Projection exposure systems for microlithography 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 onto wafers or the like in order to generate 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 increasing miniaturization and shrinking feature sizes, projection exposure systems are being operated with working light at increasingly shorter wavelengths, for example, wavelengths in the extreme ultraviolet (EUV) range. It is also conceivable that the semiconductor technology system could be a mask inspection system or a wafer inspection system.

[0057] Semiconductor technology systems, especially projection exposure systems, operate around the clock, and any malfunction affects the system's performance. Malfunctions can manifest themselves, for example, as deviations in the wavefront, which can be remedied with an optical element according to the third teaching or by processing at least one optical element using a method according to the first or second teaching.

[0058] According to a sixth teaching, the above-mentioned object is achieved by a device for electron irradiation, in particular for carrying out a method according to the first and / or second teaching, the device comprising: an electron emitter and an electron accelerator for accelerating the electrons emitted by the electron emitter, in that the device is configured to irradiate with a first electron energy Ei and at least one further electron energy E2. The device for electron irradiation is used in particular for processing optical elements, in particular optical elements for microlithography. For example, the device for electron irradiation can comprise at least one electron gun. It is also conceivable that the device in particular comprises means for carrying out a method according to the first and / or second teaching.It is also conceivable that the device comprises at least two electron guns with different acceleration voltages. The device is particularly configured to irradiate with a first acceleration voltage U accelerator and at least one further acceleration voltage U accelerator. It is conceivable that the device for electron irradiation may also comprise, for example, a Wehnelt cylinder, electron optics, and / or a deflection unit, in addition to at least one electron emitter and at least one electron accelerator, as well as an optional energy filter.

[0059] According to a further advantageous embodiment of the device for electron irradiation, the device comprises at least one energy filter, wherein the energy filter selects the electrons of an electron energy E. It is conceivable, for example, that the energy filter selects electrons of the first electron energy Ei and / or the further electron energy E2. Selecting can be understood to mean that the energy filter isolates the electrons of electron energy E, while electrons of other electron energies are blocked. The energy filter is preferably arranged behind the electron accelerator in the beam path. The energy filter is, for example, a Wien filter. It has been recognized that the energy spectrum of an electron beam broadens when devices for electron irradiation are operated outside a specified acceleration voltage.The additional energy filter allows electrons with a specific, yet adjustable energy to be selected. This also allows the penetration depth of the electrons to be specifically influenced. The non-uniform energy spectrum also influences the fluence, which can be controlled via the filter's bandwidth. In addition to a Wien filter, other monochromatizing elements can also be used as energy filters.

[0060] Further embodiments and advantages of the invention are explained in the following detailed description of some exemplary embodiments of the present invention in conjunction with the drawing.

[0061] Short description of the drawing

[0062] In the following, embodiments and variants of the invention are explained in more detail with reference to the drawings. The teachings 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 teaching can each be combined with any or all features of the other teachings. These and other aspects, features and / or technical effects are apparent from and are clarified by the figures described below. It shows

[0063] Fig. 1 is a schematic representation of a basic structure of a projection exposure system in which the invention can be used,

[0064] Fig. 2a, 2b, 2c show schematic representations of the physical effects of electron irradiation which may be taken into account for the invention, Fig. 3 shows a dose profile of the electron beam energy absorbed in quartz glass simulated by means of a Monte Carlo simulation,

[0065] Fig. 4a, 4b a schematic representation of an embodiment of a method for processing an optical element according to the second teaching,

[0066] Fig. 5 is a schematic representation of an embodiment of an electron irradiation system according to the sixth teaching, which can be used to carry out a method according to the first teaching or the second teaching,

[0067] Fig. 6a, 6b a schematic representation of a further embodiment of an electron irradiation system according to the sixth teaching, which can be used to carry out a method according to the first teaching or the second teaching, and

[0068] Fig. 7 is a schematic representation of the material law in silicate glasses with exponent y «0.7 under irradiation with monoenergetic electrons.

[0069] Detailed description of the drawing

[0070] Fig. 1 shows, by way of example, the basic structure of a semiconductor technology system in which the invention can be applied. The semiconductor technology system is designed as a projection exposure system 10. An illumination system of the projection exposure system 10 has, in addition to a light source 12, an optical system serving as 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 aligned by means of 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.

[0071] A reticle 16 arranged in the object field 14 is illuminated and held by a schematically illustrated reticle holder 17. An optical system, illustrated only schematically, which is designed as projection optics 18, 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.

[0072] The invention can also be used in a DUV projection system, which is not shown. A DUV system is essentially 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 of 100 nm to 300 nm or in the UV range, which extends up to the visible range with a wavelength of 400 nm, for example, a useful radiation of 365 nm.

[0073] An optical system of a projection exposure system, for example, the projection optics 18 of the projection exposure system 10, comprises at least one optical element, which is designed, for example, as a mirror. The substrate of a mirror comprises, in particular, glass. Glass consists essentially of one-third silicon fSij - and two-thirds oxygen atoms (O). The innermost electrons from the K shell are at approximately 1.8 keV fSij or approximately 0.5 keV (O) compared to an exemplary particle energy E o of 60 keV of an electron beam used to process the mirror. Thus, incident electrons essentially encounter a sea of ​​free electrons in the glass. In this regime, the stopping power decreases, according to Bethe, in a first approximation inversely proportional to the electron energy E e The differential energy loss dE 1 along the trajectory s behaves as oc — -E e . dE 1

[0074] In Fig. 2a, a simplified Bethe formula oc ~~E e integrated history

[0075] E e (s) = E Q / (s Q — s) / s0 of the electron energy E e along the trajectory s. The slower the electron moves, the faster it decelerates. After the range s0, the initial energy E o completely degraded. Conversely, the energy loss is deposited in the glass as radiation damage. Firstly, the more strongly the electrons are accelerated, the less damage the sensitive EUV coating suffers. Secondly, most of the energy is deposited at the end of the trajectory, where the electrons have decelerated to zero. This is also the regime where the irradiated electrons interact strongly in resonance with the bound shell electrons, which determine the chemical structure.

[0076] Fig. 2b illustrates the dependencies and shows the trajectory dE

[0077] E e s) the relative energy loss of the electron p derived from Fig. 2a e (s) = — = Initially, only half of the mean dose is deposited, while at the end of the trajectory the dose increases increasingly. Overall, the distribution is normalized to 1 J 0 p e (s)ds = 1.

[0078] The length of the trajectory can be estimated by integrating the rule of thumb for s0oc EQ, as shown in Fig. 2c. The simplified Bethe formula results in a quadratic relationship between the initial energy E oof the electron and its range s0oc EQ . The quadratic relationship allows the penetration depth to be manipulated within wide limits by the acceleration voltage U of the electron beam. For example, by doubling the acceleration voltage, the range values ​​can be quadrupled. The proportionality factor S Q / EQ depends on the irradiated material. As shown in Fig. 2c, for electrons with an initial energy E o = 60 keV the braking distance in quartz glass (SiCh) can be given as s0= 30 pm.

[0079] Except for the light electrons in the wide atomic shell, small

[0080] Probability of irradiated electrons also directly contact the tiny heavy

[0081] atomic nuclei. In the worst case, an incident electron is bounced off the solid wall like a football. The light electron with mass m e«lu / 2000 almost doubles its momentum p0« 2^2m e E e to the heavier atom mo, which has the kinetic energy « 4m e / m o ■ E e records: in

[0082] In the case of the oxygen atom (mo = 16u) this is 8 eV or in the case of the silicon atom (msi = 28u) 5 eV in frontal collisions with 60 keV electrons.

[0083] The atoms are bound to their positions more strongly than the 5 to 8 eV collision energy. They are only displaced from their positions above approximately 20 eV. For example, a threshold of 20 eV for displacement processes can only be exceeded above 160 keV of electron energy. Below this threshold, the glass structure is not destroyed; rather, the excitation of the bonding electrons into the metastable state merely changes the (average) bond angle in the oxygen bridge between the silicon atoms. In this regime of ionization processes, the resulting compaction does not depend on the particle flux density or the dose rate. Instead, the compaction depends on the accumulated dose, not on the rate at which it is introduced.

[0084] With higher accelerating voltages, increasing amounts of X-rays would be generated, which would penetrate the glass, thus escaping at least largely without a trace. A small amount of absorbed X-rays could release secondary electrons and electron-electron collisions anywhere in the substrate, even outside the immediate range of the electron beam. At even higher electron energies, for example, more than 10 MeV, the atomic nuclei can be radioactively converted—either through electron capture or through absorption of the released gamma radiation. Such high accelerating voltages are to be avoided due to the enormous requirements of radiation protection.

[0085] For the sake of completeness, it should be mentioned that polychromatic electron emitters also occur in nature, such as the carbon radionuclide 14C with a maximum of 155 keV, which is difficult to use for this technical application. With a half-life of 5730 years, 1 gram generates currents of only nanoamperes. The antineutrino emitted at the same time escapes without a trace. Radium 228Ra, a member of the thorium decay chain and formerly found in luminous digits, emits more actively. However, the artificial short-lived cobalt isotope 60Co, which is used in medicine for radiotherapy, emits unwanted hard gamma radiation in addition to beta radiation.

[0086] Simulations confirm the general principles. Fig. 3 shows a dose profile p(z) of the electron beam energy absorbed in quartz glass, simulated using a Monte Carlo simulation, in per mille of the incident electron energy of 30 keV, 60 keV, 80 keV, or 150 keV per micrometer layer depth [1 / mm]. In particular, the simulation takes into account the inelastic scattering of electrons in matter according to Mott, which causes the fine electron beam to fan out into a lobe. Elastic scattering by atoms is calculated according to the Mott model with a stochastic deflection angle. The mean free path between two elastic collisions is cubed from the cross sections of the atoms and their density according to their chemical composition. Inelastic scattering by electrons is approximated according to Bethe as continuous deceleration without a change in direction between two atomic collisions.

[0087] The zigzag trajectories fan out the electron beam into a cone-shaped beam. The energy emitted into each cuboid volume element is summed from numerous trajectories, corrected for the loss due to characteristic X-rays from collisions with the atoms and bremsstrahlung from scattering by the electrons. A further portion of the incident energy is lost through backscattering of the electrons, which leave the substrate again through the surface. Summing the absorbed energy of all volume elements of a layer yields the dose profile p(z) in Fig. 3, normalized to the incident energy. The integral of the dose profile over the layer depth z does not equal exactly one, because in the relevant energy range of 30 to 150 keV, 10 to 15% of the energy is lost through X-rays and backscattering.Multiplied by the energy fluence, i.e. the irradiated energy per surface element, the profile reflects the dose distribution in the irradiated mirror, since the electron beam spreads out significantly less than the beam diameter (full width ~ 1 mm at half maximum) and, above all, the lateral distribution varies more slowly (typical spot lengths > 1 cm).

[0088] Multiplied by the energy fluence [J / mm 2 ] the dose profile would yield the point irradiated dose [J / mm 3The Monte Carlo simulations confirm the trend from Bethe's simplified approximation formula. It is evident that the more the electrons are accelerated, the lower the percentage of stress on the sensitive surface coating is, and the more disproportionately deeper the electrons penetrate into the substrate. With increasing electron energy, the flatter dose maximum shifts ever deeper into the substrate—according to an approximately quadratic relationship—as outlined in the invention disclosure. The compaction, which is sublinearly dependent on the dose, thus saturates more slowly the more the electrons are accelerated. As a result, the achieved integral compaction across the depth, which manifests itself as the desired surface depression, increases—at the same energy fluence.

[0089] 4a and 4b schematically show an embodiment of a method according to the second teaching for processing an optical element 1, such as a mirror of an optical system, for example the projection lens 18, or the reticle 16 from Fig. 1. In Fig. 4a, an optical element 1 is first provided, which comprises a substrate 5 with a layer subsystem 6, which is suitable for reflection in the EUV wavelength range, and a protective layer 7. The layer subsystem 6 can consist of a periodic sequence of at least one period of individual layers, wherein the period comprises two individual layers with different refractive indices in the EUV wavelength range. The layer subsystem 6 can also consist of only one layer or comprise a layer system with an aperiodic sequence of layers. The protective layer 7 arranged beneath the reflection layer 6 is intended to prevent the penetration of EUV radiation into the substrate 5.The actual shape of the surface 2 of the optical element can be determined using an interferometer. This determines that the optical element 1 exhibits an undesirable surface shape deviation 8 from the desired target shape of the surface 2.

[0090] In Fig. 4b, this surface shape deviation 8 is corrected by irradiation with a first electron energy Ei and at least one further electron energy E2, and by the resulting compaction of the substrate regions 5a, 5b. The irradiation is provided by electrons from an electron irradiation device 100. By compacting the substrate regions 5a, 5b, a desired surface depression 9 can be achieved to achieve the desired shape of the surface 2 of the optical element 1. The processed optical element 1 can then be used as a mirror or as a reticle 16 in a projection exposure system.

[0091] Fig. 5 shows a device 100 for electron irradiation of an optical element. The optical element 1 can, for example, have been removed from an optical system (not shown) with a wavefront deviation in order to be subsequently processed using the device 100 for electron irradiation. The device 100 is configured to irradiate with a first electron energy Ei and at least one further electron energy E2. The device 100 comprises a first irradiation unit A with an electron emitter 101, which is designed as a hot cathode, and an electron accelerator 102 for accelerating the emitted electrons, which is designed as a hole anode. The electrons emitted by an electron emitter 101 are bundled in a Wehnelt cylinder 104 and accelerated by the electron accelerator 102. An energy filter 103 is arranged in the beam path behind the electron accelerator 102.With the help of the energy filter 103, electrons of a first electron energy Ei are selected. An electron optics system 105 focuses the narrowed beam onto a first location xi on the surface 2 of the optical element 1, where the electrons are decelerated to a first depth zi. The electron beam 3 is moved over the optical element 1 by a magnetic deflection unit 106. The optical element 1, in particular the surface of the optical element 1, and the electron accelerator 102, in particular the anode of the electron accelerator 102, are usually grounded. The entire device 100 is usually evacuated so that the electrons are not scattered by air molecules. With the help of the first irradiation unit A, an irradiation pattern 4a is introduced at a depth zi. The irradiation pattern 4a corresponds in particular to the substrate region 5a from Figure 4b.

[0092] The device 100 for electron irradiation of the optical element 1 also comprises a second electron gun A'. The second irradiation unit A' comprises an energy filter 103', with the aid of which electrons with a further electron energy E2 are selected. With the aid of the second irradiation unit A', a second irradiation pattern 4b is simultaneously or sequentially introduced into the optical element 1 at a shallower depth Z2, which is set by a lower acceleration voltage UBeschi2. The second Wehnelt cylinder 104' can be used to set a weaker beam current, which allows finer deflections in the fluence, which are indicated as the amplitude of the wavy curve 4b. By varying the travel speed of the second deflection unit 106', it is possible to generate a finer irradiation pattern, i.e., with higher spatial frequencies.It is also conceivable that an optical element 1' which is merely identical in manufacture to the optical element 1 of the optical system is irradiated by means of the device 100. It is conceivable, for example, that a customer's optical system has a wavefront aberration caused by a surface deviation of a defective optical element (not shown). However, since removing the defective optical element would entail a great deal of effort, the optical element 1 of the optical system is alternatively removed, for example. The separate optical element 1', which is essentially identical in manufacture to the optical element 1, can be processed by the manufacturer, for example, and then made available to the customer for installation in the optical system.

[0093] The irradiation of the optical element 1' can, for example, occur depending on the initially determined wavefront of the optical system. Depending on the determined wavefront, the optical element 1' is irradiated by means of the device 100 with a first electron energy Ei and a further electron energy E2. The electron energies Ei and E2 are selected, in particular, depending on the wavefront of the optical system.

[0094] 6a and 6b show an alternative device 100 for irradiating an optical element 1, which consists of an irradiation unit. The optical element 1 is irradiated, for example, depending on a specific wavefront of an optical system (not shown). Alternatively, it is conceivable that, for example, an identically manufactured optical element 1' is irradiated depending on a specific wavefront of the optical system in which an existing optical element 1 is to be replaced. The irradiation of the optical element 1 of the optical system or alternatively of the optical element 1' takes place, for example, depending on the wavefront of the optical system. Furthermore, it is conceivable that the irradiation takes place at least partially based on a measured surface 2, 2' of the optical element 1 or 1'. In the embodiment shown in Fig.In the first partial irradiation B1 shown in Fig. 6a, a first irradiation pattern 4a is irradiated into the optical element 1 at a first depth Z1 using a first acceleration voltage Ußschii. In a further step B2 shown in Fig. 6b, the acceleration voltage Ußeschiz and, if applicable, the beam current in the Wehnelt cylinder 104 are adjusted in order to subsequently irradiate a further irradiation pattern 4b at a changed depth Z2. The further irradiation pattern 4b can differ in terms of the dose but also in terms of the spatial frequencies.

[0095] It would also be conceivable to irradiate the second irradiation pattern 4b in a second device (not shown) for electron irradiation, which would be permanently operated at the second acceleration voltage Ußeschiz.

[0096] Even the simplest embodiment with two partial irradiations of different energies allows for correction of larger amplitudes compared to known methods. Thus, the compaction of the two partial irradiations adds up when the dose profiles at different penetration depths barely overlap. As a consequence, the resulting surface subsidence also adds up, as shown in Fig. 7 using a simple numerical example with highlighted data points. If at any location x, y at two different depths z, 0.5 J / mm 2 deposited, then the respective depressions double by 1.25 nm to a total of 2.5 nm. A monochromatic irradiation with the same 1 J / mm 2 The total fluence would have compacted the substrate in a single layer depth with an integral depth of 2.0 nm. With 8 such energetically different partial irradiations of 0.5 J / mm 2 (total 4 J / mm 2} 10 nm compaction would be possible, what would otherwise be 10 J / mm 2 with monoenergetic electrons. The surface depression u as depth-integral compaction follows sublinearly (y < 1] the energetic fluence F as depth-integral volume dose according to a power law u oc F . The data points illustrate the numerical examples mentioned. The advantages are even more pronounced when the accelerating voltage is varied to higher values. This is because, with the same beam current, which is limited in particular by the hot cathode of a Wehnelt cylinder, more energy fluence is radiated in the same time, which is distributed over deeper layers, or conversely, the same energy fluence can be introduced in a shorter time. Thanks to decreasing interaction with increasing energy, a percentage less unwanted energy is deposited in the surface - as shown in Fig. 2b.

Claims

Patent claims 1. A method for processing an optical element (1, 1'), the method comprising the steps: Determining the wavefront of an optical system (10), and Irradiating the optical element (1, 1') with electrons depending on the determined wavefront of the optical system (10), characterized in that the irradiation is carried out with a first electron energy Ei and at least one further electron energy E2.

2. Method according to claim 1, characterized in that the first electron energy Ei and the at least one further electron energy E2 are selected depending on the specific wavefront of the optical system (10).

3. Method according to claim 1 or 2, characterized in that determining the wavefront of the optical system (10) comprises determining the 1st and / or the desired wavefront of the optical system (10).

4. Method according to one of claims 1 to 3, characterized in that the optical system (10) comprises at least one optical element (1), wherein the optical element (1) of the optical system (10) or an optical element (1') of identical manufacture is irradiated with electrons depending on the determined wavefront of the optical system (10).

5. Method according to one of claims 1 to 4, characterized in that the optical system (10) comprises at least one optical element (1), the method further comprising the steps of: Determining an effect of the at least one optical element (1) of the optical system (10) on the wavefront of the optical system (10), and Irradiating the optical element (1) of the optical system (10) or an identically manufactured optical element (1') with electrons depending on the specific effect of the optical element (1).

6. Method according to one of claims 1 to 5, characterized in that the method further comprises the steps: Determining a shape of a surface (2, 2') of the optical element (1, 1'), and Irradiating the optical element (1, 1') with electrons depending on the specific shape of the surface (2, 2') of the optical element (1, 1').

7. A method for processing an optical element (1, 1'), the method comprising: Determining a shape of a surface (2, 2') of the optical element (1, 1'), and Irradiating the optical element (1, 1') with electrons depending on the specific shape of the surface (2, 2') of the optical element (1, 1'), characterized in that the irradiation is carried out with a first electron energy Ei and at least one further electron energy E2.

8. Method according to claim 6 or 7, characterized in that determining the shape of the surface (2, 2') of the optical element (1, 1') comprises determining the initial and / or desired shape of the surface (2, 2') of the optical element (1, 1').

9. Method according to one of claims 6 to 8, characterized in that the first electron energy Ei and the at least one further electron energy E2 are selected depending on the specific shape of the surface (2, 2') of the optical element (1, 1').

10. Method according to one of claims 1 to 9, characterized in that a first dose profile is generated with the first electron energy Ei and a further dose profile is generated with the at least one further electron energy E2 in the optical element (1, 1'), wherein the first electron energy Ei and the at least one further electron energy E2 are selected such that at least substantially no overlap of the dose profiles occurs.

11. Method according to one of claims 1 to 10, characterized in that the electron energy E is changed during scanning with the electron beam.

12. Method according to claim 11, characterized in that the electron energy E is swept during the irradiation, the wobbling being carried out faster than a movement of the electron beam (3) over the surface (2, 2') of the optical element (1, 1').

13. Method according to one of claims 1 to 12, characterized in that the irradiation comprises a plurality of partial irradiations, wherein the electron energy E is changed between at least two partial irradiations.

14. Method according to one of claims 1 to 13, characterized in that during the irradiation with the first electron energy Ei is irradiated with a first irradiation pattern (4a) and during the irradiation with the further electron energy E2 is irradiated with a further irradiation pattern (4b).

15. Method according to one of claims 1 to 14, characterized in that the acceleration voltage U Besch 1 is modulated, the modulated voltage range being slowly changed as the electron beam (3) is directed over the surface (2, 2') of the optical element (1, 1').

16. Method according to one of claims 1 to 15, characterized in that the irradiation with the first electron energy Ei and the irradiation with the at least one further electron energy E2 take place at least substantially simultaneously.

17. The method according to claim 16, characterized in that the irradiation with the first electron energy Ei takes place via a first entry point xi and the irradiation with the at least one further electron energy E2 takes place via a further entry point X2 of the surface (2, 2') of the optical element (1, 1').

18. Method according to one of claims 1 to 17, characterized in that the method further comprises: Tempering the optical element (1, 1'), wherein the tempering of the optical element (1, 1') preferably takes place after irradiation.

19. Optical element (1, 1'), in particular a reflective optical element, which has been processed by a method according to one of claims 1 to 18.

20. Optical element (1, 1') according to claim 19, characterized in that the optical element (1, 1') has a coating with a spatially homogeneous thickness.

21. Optical element (1, 1') according to claim 19, characterized in that the optical element (1, 1') has a coating with a spatially variable thickness.

22. Optical system, in particular projection system, for microlithography comprising an optical element (1, 1') according to one of claims 19 to 21.

23. Semiconductor technology system, in particular projection exposure system (10), comprising an optical system, in particular a projection system, according to claim 22.

24. Device (100) for electron irradiation, in particular for carrying out a method according to one of claims 1 to 18, the device (100) comprising: an electron emitter (101) and an electron accelerator (102) for accelerating the electrons emitted by the electron emitter (101), characterized in that the device (100) is configured to irradiate with a first electron energy Ei and at least one further electron energy E2.

25. Device (100) according to claim 22, characterized in that the device (100) further comprises: at least one energy filter (103), wherein the energy filter (103) selects the electrons of an electron energy E.

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