Method for reducing a long-term change in a form, optical element and system for semiconductor technology

Abstract

The invention relates to a method for reducing a long-term change (∆P) in a form (P) of an optical surface (25a) of an optical element (Mi), preferably a mirror, in particular an EUV mirror. The method comprises processing the optical element (Mi), preferably an in particular transparent substrate (25) of the optical element (Mi), outside the optical surface (25a) in order to reduce surface damage, in particular in order to reduce a surface roughness Ra, of the optical element (Mi) outside the optical surface (25a). During processing, material is removed from the substrate (25) of the optical element (Mi) preferably by brushing by means of at least one brushing tool (27). The invention also relates to an optical element (Mi), comprising: an optical surface (25a), preferably for reflecting radiation, in particular for reflecting EUV radiation. The optical element (Mi), preferably an in particular transparent substrate (25) of the optical element (Mi), has a surface roughness Ra of 5 µm or less, preferably of 3 µm or less, particularly preferably of 1.5 µm or less, on a partial surface or on multiple partial surfaces outside the optical surface (25a) and / or the partial surface or the multiple partial surfaces are transparent. The invention also relates to an system for semiconductor lithography, which has at least one optical element (Mi) of this type, which is preferably arranged in a projection optical unit (10).

Description

[0001] Method for reducing long-term changes in a pass, optical element and system of semiconductor technology

[0002] Reference to related registration

[0003] This application claims priority over German patent application DE 102024206868.3 of July 22, 2024, the entire disclosure content of which is incorporated by reference into this application.

[0004] Background of the invention

[0005] The invention relates to a method for reducing long-term changes in the surface properties of an optical element, preferably a mirror, in particular an EUV mirror. The invention also relates to an optical element, preferably a mirror, in particular an EUV mirror, comprising: an optical surface, preferably for reflecting radiation, in particular for reflecting EUV radiation. The invention further relates to a semiconductor technology system, in particular an EUV lithography system.

[0006] Due to the steadily increasing demands on the precision of semiconductor technology systems, the requirements for the temporal stability of optical elements, particularly mirrors and / or lenses, in such systems, for example in EUV lithography systems, are also rising. Even the smallest changes in the optical elements, caused by external or internal influences, can have a destabilizing and thus detrimental effect on the optical performance of a semiconductor technology system. This applies particularly to temporal changes in the fit of the optical surface, i.e., the surface geometry of the optical surface. For the purposes of this application, fit error is understood as a deviation from the design shape of an optical component such as a lens or mirror. The fit error can be determined from a pixel-resolved measurement as the RMS value averaged over the entire optical surface.The long-term change in the pass should be as small as possible; in particular, the change in the pass over the operating life of the optical element should be as small as possible.

[0007] Some of the external influences that cause changes in the pass, among other things, occur instantaneously, while others exhibit a specific temporal behavior, with timescales ranging from seconds to years. Time-dependent processes include, for example, the heating of mirrors with useful radiation, particularly EUV radiation, during an exposure process in a lithography system. To understand the processes underlying the temporal changes in the pass, complex simulations and theoretical frameworks have been developed.

[0008] Since unwanted temporal changes in the pass negatively affect image quality, these changes should at least be reduced, if not completely eliminated. A detailed understanding of the cause of these temporal changes is not strictly necessary, as long as the temporal progression of the changes can be described and predicted sufficiently well to allow for appropriate countermeasures.

[0009] Long-term pass variation refers to a change in pass that occurs over a period of at least one day, usually at least one week, and typically persists throughout the entire service life of the optical element in a lithography system, which is generally several years. Long-term pass variation is defined as the difference between the pass at the beginning and end of a period: a long-term pass variation value of 100 pm / month therefore indicates a change in pass that occurs over a period of one month, a value of 100 pm / week indicates a change in pass that occurs over a period of one week, and so on.

[0010] A long-term change in the pass, also referred to as pass drift, which occurs in one or more optical elements, can be compensated for by a counter-drift. It is also possible to reduce the pass drift sufficiently by replacing the optical element exhibiting the pass drift or another optical element, such that the image errors or optical aberrations of the overall system, for example, a projection system of a lithography system, remain within the specified range.

[0011] Knowing the cause allows the drift process of the passe to be controlled by directly addressing the cause. However, even this does not guarantee that the cause can be completely eliminated. In the case of passe drift on the optical surfaces of mirrors, the exact origin of the passe drift is not fully understood.

[0012] Object of the invention

[0013] One object of the invention is to provide a method for reducing long-term changes in a pass on the optical surface of an optical element. A further object of the invention is to provide an optical element with reduced long-term changes in the pass on the optical surface, as well as a semiconductor lithography system with at least one such optical element.

[0014] Subject matter of the invention: This problem is solved according to a first aspect by a method of the type mentioned at the outset, comprising: Preferably mechanically abrasive or removing machining of the optical element, preferably of a particularly transparent substrate of the optical element, outside the optical surface to reduce surface damage, in particular to reduce surface roughness, of the optical element outside the optical surface.

[0015] Although the exact cause of Passe drift in optical elements, especially in EUV mirrors, is not fully understood, the inventors have recognized that Passe drift results from a change in mechanical stress, i.e., surface tension, both on the optical surface and on all other surfaces of the optical element, which in the case of a mirror are the back surface and the side surface(s). Since the origin of Passe drift lies in surface tension, one way to compensate for and reduce Passe drift is by influencing this surface tension.

[0016] Furthermore, it is suspected that Passe drift is caused by a chemical and / or physical interaction between the optical element material, typically a glass or glass-ceramic, and water originating from atmospheric humidity. However, the precise nature of this interaction remains a matter of speculation.

[0017] Possible explanations include a diffusion-driven process or swelling of the mirror material as a result of the restructuring of the glass bonds through water incorporation.

[0018] It is further assumed that the interaction between the optical element material and water is strongly influenced by the surface properties. Damage to the glass material due to deep defects, roughness, or destruction, e.g., as a result of cavitation damage to the surfaces during ultrasonic cleaning, intensifies this interaction process.

[0019] The inventors discovered that certain areas of the optical element exhibit a particularly high concentration of surface defects, such as erosion damage, while other areas are almost unaffected. An investigation revealed that roughly ground sections are especially prone to surface damage. It is therefore assumed that the interaction between the optical element material and water is intensified by the surface roughness.

[0020] The inventors therefore propose machining the optical element to reduce passe drift and thus minimize surface damage, particularly surface roughness. Since the optical surface, which in a mirror corresponds to the front surface, is polished and therefore typically exhibits only shallow, superficial damage, it is usually unnecessary to machine the optical surface itself after the mirror has been manufactured. This leaves the back and side surfaces of the optical element available for machining to reduce surface roughness and superficial damage.

[0021] In one variant of the method, a surface roughness R is achieved when processing a partial surface or several partial surfaces of the optical element outside the optical surface, the area of ​​which is preferably at least 10%, particularly preferably at least 20%, and especially at least 30% of the area of ​​the optical surface. a of 5 pm or less, preferably of 3 pm or less, particularly preferably of 1.5 pm or less, and / or the partial surface or surfaces become transparent through machining. The roughness R a The mean roughness, also called average roughness, indicates the average distance of a measuring point on the surface from the surface's centerline. The mean roughness thus corresponds to the arithmetic mean of the absolute value of the deviation from the centerline.

[0022] It has been shown that reducing the surface roughness of the optical element outside the optical surface to the values ​​specified above sufficiently reduces passe drift. For this purpose, it is not essential that the entire surface of the optical element or substrate outside the optical surface exhibits the surface roughness described above. Rather, it has been shown that different sub-areas of the optical element outside the optical surface can have varying effects on passe drift. Therefore, it may be sufficient to modify the optical element only on the sub-area or areas that have the greatest impact on passe drift (su).

[0023] At least on this sub-surface(s), the optical element should meet the surface roughness values ​​specified above. If the substrate of the optical element is transparent to light in the visible wavelength range, processing can create transparency of the substrate on at least one sub-surface. An observer can therefore see through the respective transparent sub-surface into the interior of the substrate. If no post-processing of the optical element is carried out outside the optical surface, the surface roughness outside the optical surface is significantly higher than the values ​​specified above and can, for example, be on the order of approximately 10 pm. The substrate is typically not transparent in the unprocessed surface areas.Even with a transparent substrate, it is therefore usually not possible to see into the interior of the substrate through the untreated, opaque surface areas.

[0024] To significantly reduce passe drift, the area of ​​the processed sub-surface or the area of ​​the processed sub-surfaces (i.e., the sum of the areas of all processed sub-surfaces) should not be too small and should typically be at least 10% of the area of ​​the optical surface.

[0025] In another variant, material is removed from the optical element, preferably from a substrate of the optical element, during the preferably mechanical processing of the optical element outside the optical surface. The substrate material is generally a glass material or a glass-ceramic (so). To reduce the depth of damage or surface roughness, it has proven advantageous to remove material in a specific sub-area to a depth in the micrometer range, for example, up to approximately 50–60 pm. For this purpose, material removal by mechanical processing has proven advantageous, but the material removal can also be carried out by other means, e.g., by laser processing or by an etching process.

[0026] Instead of a mirror, other optical elements, such as lenses or the like, can also be mechanically machined to correct the long-term change of the pass on their optical surfaces.

[0027] In another variant, material is removed by brushing with at least one brush tool, by grinding, and / or by polishing. Brushing with one or more brush tools has proven advantageous for removing a defined amount of material from the optical element or its substrate, thereby eliminating the damaged part of the surface. For this purpose, the optical element, or more precisely, one or more sections of it, can be brushed manually, but it is also possible to automate the process using a dedicated brush tool. Although some surface damage remains in the optical element's material after brushing, it is significantly shallower, thus considerably reducing the interaction between the optical element's glass and water.

[0028] Different brush tools can be used for brushing different surfaces. For example, cup brushes can be used to process the back surface, while peripheral brushes can be used to process the side or circumferential surface(s). However, it is generally not possible to reach all surfaces outside the optical surface using brush tools. In this case, in particular—but not exclusively—mechanical processing can also be carried out in other ways, for example by grinding, especially fine grinding, and / or by polishing or, if necessary, by other abrasive mechanical processing methods.

[0029] In another variant, the machining is performed on at least one partial surface of the optical element outside the optical surface, preferably on at least one partial surface on a back and / or side surface of the optical element. As described above, it is not absolutely necessary that the entire surface of the optical element be machined outside the optical surface to achieve a sufficiently large reduction in pass drift.

[0030] The question therefore arises as to which sub-surfaces should be preferentially processed in order to maximize the reduction of pass drift. These sub-surfaces are typically design-related contiguous areas of the optical element's surface, or more precisely, of the optical element's substrate; however, the sub-surfaces can also be defined based on their surface properties.

[0031] In a further development of this variant, the method comprises: selecting at least one partial surface from a plurality of partial surfaces of the optical element outside the optical surface depending on the proportion of the selected partial surface to the long-term change of the pass of the optical surface, wherein preferably at least the partial surface that generates a maximum proportion of the long-term change of the pass of the optical surface is selected for processing.

[0032] It is possible to identify and select for machining those sub-surfaces of the mirror that have the highest surface tension and whose surface removal therefore leads to the greatest reduction in passe drift (su). By machining a small number of appropriately selected sub-surfaces, it is possible to avoid more than 50% or more than 80% of the passe drift, i.e., the passe drift can be significantly reduced.

[0033] In another variant, the method comprises: determining a deformation of the optical surface which is caused by a respective sub-surface on the optical surface at a uniform surface tension on the majority of sub-surfaces, wherein the deformation is preferably determined by finite element simulation.

[0034] In this variant, the mirror surface, usually only the surface outside the optical surface, is typically decomposed into design-related, interconnected sub-regions using a finite element simulation. A uniform parameter is then applied to each sub-region individually.

[0035] Surface tension is applied—or rather, the application of surface tension is simulated—and a location-dependent deformation of the optical surface is calculated using finite element simulation. In this way, a series of load cases, also called sensitivities, is defined, which can serve as a basis set for calculating deformations back to stresses on the sub-surfaces or to the contribution of the sub-surfaces to the long-term change of the pass (su). It should be noted that the sensitivities of this basis set are not completely linearly independent of each other, which can significantly complicate the back-calculation process. This method requires sufficiently good orthogonality of the load cases; otherwise, the back-calculation will not be unambiguous.

[0036] In a further development of the above-mentioned variant, the procedure additionally includes: spatially resolved measurement of the optical surface to determine the long-term change of the pass of the optical surface based on a location-dependent long-term change of the pass, as well as determining the contribution of a respective sub-area to the long-term change of the pass of the optical surface based on a deformation profile of the optical surface, which is composed of the deformations of the optical surface caused by the respective sub-areas.

[0037] Using the steps described above, the proportion of each sub-area to the long-term change in the pass of the optical surface can be determined, for example to select at least one sub-area for processing (so).

[0038] An investigation of several optical elements in the form of mirrors has shown that individual drifting mirrors exhibit different drift signatures. Only a bulge in the outer region of the mirror and—in the case of obscured mirrors—in an inner region appear to be present in all individual mirrors. For this reason, it is useful to perform a spatially resolved measurement of the optical surface of each individual mirror in order to determine the individual long-term change of the bulge on the optical surface. For this purpose, the optical surface is usually measured interferometrically. The measurement is taken at two points in time, with a time interval between them that is large enough to allow the long-term change of the bulge to be measured and determined by calculating the difference between the two measurements.The time interval between the two measurements is typically on the order of one or more days or weeks. An even more precise picture of the deformation change of the optical element can be obtained through a large number of such measurements.

[0039] The inventors discovered that the long-term change in the mirror's shape differs between storage in air or at ambient pressure and storage in a vacuum. In the first case, swelling occurs due to water diffusing into the optical element's material, while in the second, water diffuses out of the optical element into the vacuum environment. Therefore, it is advantageous to expose the mirror to the operating conditions during the interval between measurements. In the case of an EUV mirror, this could be achieved, for example, by placing it in a vacuum chamber during the interval.

[0040] Based on the difference between the two measurements of the optical surface, the long-term change in the pass of the optical surface can be determined, and those surface areas or sub-areas can be identified from which stresses originate that contribute the greatest share to the pass drift.

[0041] In a further development of this variant, to determine the proportion of the respective sub-area to the long-term change of the pass of the optical surface, the deformation profile, which is composed of the deformations of the optical surface caused by the respective sub-areas, is adapted to the location-dependent long-term change of the pass of the optical surface by means of an optimization algorithm, preferably by means of a quadratic optimization algorithm.

[0042] As described above, the load cases described above, or more precisely a deformation profile corresponding to a linear combination of the load cases, are fitted or adapted to a measured drift signature, i.e., to the location-dependent long-term change of the drift, in order to determine the sub-areas with the causative surface tension. Each individual drift signature of a mirror leads to a different result.

[0043] The type of optimization is crucial during the adaptation process. Different optimization methods and the presence of weighting and regularization parameters lead to different results. In principle, two fundamentally different optimization methods can be distinguished: the least-squares method and the quadratic program, abbreviated QuadProg. While the first method only allows weighting and regularization parameters to be included in the calculation to bring the target vector to a physically meaningful result, the second approach also considers constraints in the optimization.

[0044] The quadratic program method allows for the introduction of a global sign into the optimization, so that only compressive or only tensile stresses are considered in the back-calculation. This approach is due to the fact that the Passe drift must be attributable to a single physical cause, such as the diffusion of water from the optical element material in a vacuum or the swelling of the optical element due to water absorption, typically in air and at normal pressure.

[0045] Mathematically, these two optimization methods can be described as follows:

[0046] The merit function of a simple least-squares optimization is:

[0047] 4>(y) = W M • y - «II 2 min with the manipulator matrix M, the disturbance a and the correction recipe y.

[0048] The merit function of a QuadProg optimization

[0049] 1

[0050] < >(z) = -y T ■ H ■ y - f T ■ y min with H = M T • M, f = M T • Taking into account the constraints A y < b, a leads to the following form: with y = (x, t).

[0051] Here, u corresponds to the Lagrange multipliers, ö to the slack variables, and t represents the Gembicki coefficients. This expression can be solved iteratively using the gradient descent method.

[0052] In a further development of the variant described above, the reduction of the long-term change in the optical surface's pass due to machining at least one sub-area is estimated based on the contribution of at least one sub-area to the long-term change in the pass of the optical surface. This allows predictions as to whether machining the at least one sub-area will ensure that the optical element's pass drift specification is met over its lifetime. Since individual mirrors exhibit different contributions to pass drift from their respective sub-areas, a general value or estimate of the reduction in long-term pass drift due to machining a specific sub-area or all sub-areas can only be determined by averaging data from multiple mirrors.

[0053] A further aspect of the invention relates to an optical element of the type mentioned at the outset, wherein the optical element, preferably a particularly transparent substrate of the optical element, has a surface roughness R on a partial surface or on several partial surfaces outside the optical surface, the area of ​​which is preferably at least 10%, particularly preferably at least 20%, and in particular at least 30% of the area of ​​the optical surface. a of 5 pm or less, preferably of 3 pm or less, particularly preferably of 1.5 pm or less and / or the partial area or the multiple partial areas are transparent.

[0054] As described above in connection with the process, reducing the surface roughness of the optical element outside the optical surface can reduce the long-term change in the optical surface's pass. Typically, the optical element or substrate outside the optical surface exhibits a significantly higher surface roughness than specified above. As also described above, it is not essential that the entire area outside the optical surface of the optical element has a surface roughness of the stated values; rather, it may be sufficient for individual sub-areas to have a correspondingly low surface roughness, the specific sub-areas depending on the particular mirror. As described above, the area of ​​the treated sub-area should be...The surface area of ​​the machined sub-areas must not be too small to produce a significant effect on the long-term change of the pass. On a substrate that is transparent at wavelengths in the visible range, the machined sub-area(s) can usually be distinguished from the unmachined surface areas with the naked eye, since the machined sub-area(s) are optically transparent, allowing a view into the interior or volume of the substrate, which is typically not possible with unmachined surface areas.

[0055] Another aspect of the invention relates to an optical element of the type mentioned above, wherein the optical surface exhibits a long-term change in pass under vacuum conditions, which is less than 20 pm / week, preferably less than 10 pm / week, particularly preferably less than 5 pm / week, and especially less than 2 pm / week. In this case, the long-term change is measured in the unit of one week, i.e., the temporal change in pass between the beginning and the end of the week is determined.

[0056] It has been shown that when the optical element is arranged or stored in a vacuum, i.e., under vacuum conditions, the long-term change in the passe is greatest at the beginning of storage, typically during the first week, so this value can be considered an upper limit. The long-term change in the passe, or passe drift, typically decreases with increasing duration of storage of the optical element under vacuum conditions, with an essentially exponential decay observed. Through the processing described above, the upper limit, or maximum value, of the passe drift at the beginning of storage in a vacuum can be reduced to the specified values.

[0057] If the optical element is stored in air, a long-term change in the pass will also occur, likely due to the absorption of water from the environment. This long-term change in the pass in air, without the treatment described above, is on the order of approximately 30 ppm / month or less.

[0058] Another aspect of the invention relates to an optical element of the type mentioned above, which can be combined in particular with the aspects described above and in which the optical surface under vacuum conditions exhibits a long-term change in the pass, which over an operating period of at least 10 years is at 100 pm or less, preferably at 80 pm or less, particularly preferably at 50 pm or less.

[0059] As described above, the long-term change in the pass is not constant over the operating lifetime of the optical element, but decreases with increasing duration of operation under vacuum conditions. This is likely due to the fact that the diffusion of water from the optical element material decreases with increasing duration of vacuum exposure. As also described above, it is advantageous for the optical performance of the semiconductor technology arrangement in which the optical element is located if the long-term change in the pass is as small as possible over the operating lifetime of the optical element. Another aspect of the invention relates to a semiconductor technology system comprising: at least one optical element as described above, wherein the optical element is preferably arranged in a projection optic, e.g.in a projection optic of an EUV lithography system. The semiconductor lithography system can be a lithography system for exposing a wafer or another optical arrangement used in semiconductor technology, for example, an inspection system, e.g., for inspecting masks, wafers, or the like used in EUV lithography. The semiconductor lithography system can also be a DUV lithography system.

[0060] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures in the drawing, which show details essential to the invention, and from the claims. The individual features can be implemented individually or in any combination in a variant of the invention.

[0061] drawing

[0062] Examples of implementation are shown in the schematic drawing and are explained in the following description. It shows

[0063] Fig. 1 schematically shows a projection exposure system for EUV projection lithography in meridional section.

[0064] Fig. 2 a schematic representation of an EUV mirror of a

[0065] Projection optics during processing with a brush tool, Fig. 3a-d schematic representations of four sub-surfaces on the back of the mirror of Fig. 2, which are subjected to a mechanical stress in a finite element simulation,

[0066] Fig. 4a-d schematic representations of deformations on an optical surface of the EUV mirror, which are produced by the mechanical stresses on the four sub-surfaces of Fig. 3a-d,

[0067] Fig. 5a-e shows a schematic representation of a location-dependent long-term change in the pass of the optical surface of the mirror, as well as

[0068] Fig. 6 shows a schematic representation of a long-term change in the pass on the optical surface over the operating time of the mirror.

[0069] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0070] The following section describes, with reference to Fig. 1, the essential components of an optical arrangement for EUV lithography in the form of a projection exposure system 1 for microlithography. The description of the basic structure of the projection exposure system 1 and its components is not intended to be restrictive.

[0071] One embodiment of a lighting system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, a lighting optic 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a separate module from the rest of the lighting system.

[0072] In this case, the lighting system does not include light source 3.

[0073] A reticule 7 located in the object field 5 is illuminated. The reticule 7 is held by a reticule holder 8. The reticule holder 8 can be moved, particularly in one scanning direction, via a reticule displacement drive 9.

[0074] Figure 1 shows a Cartesian xyz coordinate system for illustrative purposes. The x-direction runs perpendicular to the plane of the drawing. The y-direction runs horizontally, and the z-direction runs vertically. In Figure 1, the scan direction runs along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0075] The projection exposure system 1 comprises a projection system 10. The projection system 10 serves to image the object field 5 onto an image field 11 in an image plane 12. A structure on the reticulum 7 is imaged onto a light-sensitive layer of a wafer 13 located in the image plane 12 within the area of ​​the image field 11. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, particularly along the y-direction, via a wafer transfer drive 15. The displacement of the reticulum 7 via the reticulum transfer drive 9 and of the wafer 13 via the wafer transfer drive 15 can be synchronized with each other.

[0076] Radiation source 3 is an EUV radiation source. Specifically, radiation source 3 emits EUV radiation 16, which is also referred to below as useful radiation, illumination radiation, or illumination light. The useful radiation has a wavelength in the range between 5 nm and 30 nm. Radiation source 3 can be a plasma source, for example, an LPP source (laser-produced plasma) or a DPP source (gas-discharged produced plasma). It can also be a synchrotron-based radiation source. Radiation source 3 can be a free-electron laser (FEL).

[0077] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector mirror 17. The collector mirror 17 can be a collector mirror with one or more ellipsoidal and / or hyperboloid reflective surfaces. The at least one reflective surface of the collector mirror 17 can be illuminated by the illumination radiation 16 at grazing incidence (Gl), i.e., with angles of incidence greater than 45°, or at normal incidence (NI), i.e., with angles of incidence less than 45°. The collector mirror 17 can be structured and / or coated to optimize its reflectivity for the useful radiation and to suppress stray light.

[0078] After the collector mirror 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector mirror 17, and the illumination optics 4.

[0079] The illumination optics 4 comprise a deflecting mirror 19 and, downstream of this in the beam path, a first faceted mirror 20. The deflecting mirror 19 can be a planar deflecting mirror or, alternatively, a mirror with an effect that influences the beam beyond the mere deflection effect. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter that separates a useful wavelength of the illumination radiation 16 from stray light of a different wavelength. The first faceted mirror 20 comprises a plurality of individual first facets 21, which are also referred to as field facets in the following. Only a few of these facets 21 are shown by way of example in Fig. 1. Downstream of the first faceted mirror 20 in the beam path of the illumination optics 4 is a second faceted mirror 22. The second faceted mirror 22 comprises a plurality of second facets 23.

[0080] The illumination optics 4 thus form a double-faceted system. This basic principle is also known as a honeycomb condenser (fly's eye integrator). With the aid of the second faceted mirror 22, the individual first facets 21 are imaged into the object field 5. The second faceted mirror 22 is the last beam-shaping, or indeed the last, mirror for the illumination radiation 16 in the beam path before the object field 5.

[0081] The projection system 10 comprises a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the projection exposure system 1.

[0082] In the example shown in Fig. 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have an aperture for the illumination radiation 16. The projection system 10 is a double-obscured optical system. The projection optics 10 have an image-side numerical aperture greater than 0.4 or 0.5, and which can also be greater than 0.6, for example, 0.7 or 0.75.

[0083] The mirrors Mi, like the mirrors of the illumination optics 4, can have a highly reflective coating for the illumination radiation 16. Fig. 2 shows one of the EUV mirrors Mi of the projection optics 10 from Fig. 1. The mirror Mi has a substrate 25, which in the example shown consists of titanium-doped quartz glass transparent to visible light, but which can also be made of another material with a low coefficient of thermal expansion, for example, a glass-ceramic. The substrate 25 of the mirror Mi has an optical surface 25a, which corresponds to the front of the substrate 25 and onto which a reflective multilayer coating 26 is applied. The multilayer coating 26 has a plurality of alternating layers of materials with a high and a low refractive index to reflect the illumination radiation 16.In the example shown, where the illumination radiation 16 has a wavelength of 13.5 nm, the materials of the layers are silicon and molybdenum.

[0084] For the purposes of the following considerations, the optical surface 25a is understood to be that portion of the total surface of the substrate 25 which is polished to optical quality and therefore has a very low surface roughness and is provided with the reflective coating 26.

[0085] As can also be seen in Fig. 2, the substrate 25 has an annular circumferential side surface 25b and a back surface 25c outside the optical surface 25a. For the sake of simplicity, both the front surface (optical surface 25a) and the back surface 25c of the substrate 25 are shown as planar, but they generally deviate from a planar geometry. It has been observed that during operation of the mirror Mi in the projection exposure system 1, the surface shape P, i.e., the surface form, of the optical surface 25a changes over a longer period. It is assumed that this long-term change AP of the surface shape P is due to mechanical stresses resulting from an interaction between the material of the substrate 25 and water. More precisely, it is assumed that water diffuses out of the material of the substrate 25 into the vacuum environment in which the mirror Mi is operated.The strength of the interaction between the glass material of substrate 25 and water depends strongly on the properties of the respective surfaces 25a, 25b, 25c of substrate 25, in particular on surface damage or surface roughness. Since the optical surface 25a itself is polished to a high optical quality, no or only very slight mechanical stresses typically occur on the optical surface 25a itself, resulting in a negligible long-term change AP of the passe P. However, this does not apply to the side surface 25b and the back surface 25c of substrate 25. To reduce the long-term change AP of the passe P, it is therefore advantageous to process the substrate 25 to reduce surface damage or surface roughness on the side surface 25b and / or the back surface 25c of substrate 25.

[0086] For this purpose, in the example shown, the substrate 25 is mechanically processed on its back side 25c using a cup-shaped brush tool 27 to remove material from the back side 25c of the substrate 25. Processing with the brush tool 27 allows material to be removed to a depth of approximately 50 µm, thereby significantly reducing depth damage. In addition, the surface roughness R can be reduced. aThe surface roughness of the back surface 25c of the substrate 25 is reduced by brushing to a value of 5 pm or less, 3 pm or less, and in particular 1.5 pm or less. For this purpose, the brush tool 27 has filaments 28 coated with a material of high hardness. The brush tool 27 is moved and rotated along a predetermined path at a defined distance from the back surface 25c of the substrate 25. Processing with the brush tool 27 makes the back surface 25c of the substrate 25 optically transparent, i.e., it is possible to see through the back surface 25c of the substrate 25 into the interior of the substrate 25. The side surface 25b can be processed accordingly using a suitable brush tool in the form of a circumferential brush to reduce the surface roughness R. aTo achieve a significant reduction in the long-term change AP of the pass P of the mirror Mi, it is not absolutely necessary to mechanically machine the entire back surface 25c of the substrate 25 and the entire side surface 25b. Rather, it may be sufficient to machine one or more sub-surfaces of the optical element Mi outside the optical surface 25a that contribute significantly to the long-term change AP of the pass P of the optical surface 25a. The problem here is that the contribution of each sub-surface of the mirror Mi to the long-term change AP of the pass P differs for each instance of the mirror Mi. Therefore, it is not possible to predefine specific sub-surfaces for all mirrors Mi that are preferentially machined with the brush tool 27.

[0087] The following describes a method that makes it possible to select suitable sub-surfaces for mechanical processing of a specific example of the mirror Mi. In particular, at least the sub-surface that generates a maximum contribution to the long-term change AP of the pass P on the optical surface 25a can be selected for processing.

[0088] Figures 3a-d show, by way of example, four hatched partial surfaces 29a-d on the back side 25c of the mirror Mi, which are formed in the form of recesses or depressions. A central obscuration, i.e., a hole in the middle of the mirror Mi, is also visible. The four partial surfaces 29a-d shown in Figures 3a-d are readily accessible to the brush tools 27. However, it is understood that the mirror Mi, more precisely the back side 25c of the substrate 25 and also the side surface 25b of the substrate 25, has further partial surfaces that can also be processed using the brush tool 27 or other suitable brush tools, which have been omitted from the illustration for the sake of clarity. The three sub-areas 29a-c shown in Fig. 3a-c together have an area that is essentially equal to the area of ​​the fourth sub-area 29d, which is more than 10% of the area of ​​the optical surface 25a.Machining the three sub-surfaces 29a-c with the brush tool 27 therefore takes approximately the same amount of time as machining the fourth sub-surface 29d. To decide whether it is more advantageous to machine the three sub-surfaces 29a-c or the fourth sub-surface 29d by brushing in order to minimize the long-term change AP of the passe P, the following procedure can be used:

[0089] In a first step, a finite element simulation is performed in which the same mechanical surface tension is specified for each of the four sub-surfaces 29a-d, as well as for all other sub-surfaces that together form the back surface 25c and the side surface 25b of the substrate 25. For each sub-surface 29a-d, or for each load case, the finite element simulation determines a location-dependent deformation 30a-d of the optical surface 25a, which is generated at the optical surface 25a at the uniformly specified surface tension on each sub-surface 29a-d. The deformations 30a-d, more precisely the contour lines of the deformations 30a-d generated by the four sub-surfaces 29a-d on the optical surface 25a, are shown in Fig. 4a-d. As can be seen in Fig. 4a-d, the four sub-surfaces 29a-d produce characteristic, each different, deformations 30a-d on the optical surface 25a.

[0090] To select a suitable sub-area 25a-d, a spatially resolved measurement, for example interferometric, of the optical surface 25a of the mirror Mi is performed. The measurement is carried out at a first and a second time point, which in the example shown are approximately three weeks apart. By calculating the difference between the two measurements, a spatially dependent long-term change AP(x,y) of the pass P of the optical surface 25a is determined, the contour lines of which are shown in Fig. 5a. The long-term change AP of the pass P is obtained by averaging the spatially dependent long-term change AP(x,y) of the pass P.

[0091] Fig. 5b shows a deformation profile 31 on the optical surface 25a, which was fitted by an optimization algorithm in the form of a quadratic program to the spatially dependent long-term change AP(x,y) of the pass P of the optical surface 25a shown in Fig. 5a. The deformation profile consists of fit parameters or weights of the deformations 30a-d attributable to the four sub-surfaces 29a-d, as well as fit parameters or weights of deformations attributable to further sub-surfaces. Fig. 5c shows the deformation profile 31 in which three of the fit parameters corresponding to the three sub-surfaces 29a-c of Fig. 3a-c were set to zero. A comparison with Fig. 5b shows that omitting the deformations 30a-c caused by the three sub-surfaces 29a-c has practically no effect on the deformation profile 31. Fig.Figure 5d shows the case where the deformation 30d, generated by the fourth sub-surface 29d, is additionally omitted when determining the deformation profile 31. It is clearly evident that the deformation profile 31 shown in Figure 5d differs from the deformation profiles 31 shown in Figures 5b and 5c. In the deformation profile 31 shown in Figure 5e, only the deformation 30d, generated by the fourth sub-surface 29d, was omitted. As expected, the deformation profile 31 in Figure 5e differs only slightly from the deformation profile 31 in Figure 5d, but significantly from the deformation profiles 31 in Figures 5b and 5c. The fourth sub-surface 29d of Fig. 3d therefore has a significantly larger proportion of the deformation profile 31, which was adapted to the location-dependent long-term change AP(x,y) of the pass P, than the three sub-surfaces 29a-c of Fig. 3a-c. Accordingly, the fourth sub-surface 29d is selected for machining with the brush tool 27.The effect of brushing the fourth sub-surface 29d on the long-term change AP of the passe P can be estimated by determining the long-term change AP of the passe P at the optical surface 25a for the deformation profile 31 shown in Fig. 5e and comparing it with the long-term change AP of the passe P resulting from the spatially dependent passe change AP(x,y) of Fig. 5a. In this way, the percentage by which the long-term change AP of the passe P is reduced by brushing the fourth sub-surface 29d can be estimated. Although this estimate is subject to a certain degree of uncertainty, measurements after brushing in the example described here have shown that the long-term change AP of the passe P was reduced by approximately the amount corresponding to the calculated effect of brushing the fourth sub-surface 29d.

[0092] Starting from the time of brushing, the long-term change AP of passe P, which would occur without brushing, is 20 pm / month, as determined by a measurement immediately before brushing. The long-term change AP of passe P calculated in the manner described above predicts a reduction of 50%, i.e., to 10 pm / month. The actual long-term change AP of passe P, determined by measurements after brushing the fourth sub-area 29d, is approximately 7 pm / month. The estimated long-term change or drift rate AP of 10 pm / month agrees well with the measured value of 7 pm / month, within the accuracy expected of such an estimate.

[0093] It has been shown that in the days following brushing, the long-term change AP of Passe P initially oscillates; that is, a back-and-forth between different passe signatures of the location-dependent long-term change AP(x,y) of Passe P was observed. After a period on the order of weeks, however, the oscillations subside. The measurements described above refer to the first stable measurement of the location-dependent long-term change AP(x,y) of Passe P after brushing and yield an approximately constant value for the long-term change AP of Passe P of 7 pm / month.

[0094] For all measurements described above, the mirror Mi was stored in a vacuum. This is advantageous because the mirror Mi is operated in a vacuum in the projection exposure system 1, and the long-term change AP of the pass P under operating conditions of the mirror Mi is to be investigated. The above considerations show that brushing the fourth sub-surface 29d reduces the long-term change AP of the pass P of the mirror Mi by approximately 50%. The calculation described above was also performed by brushing all sub-surfaces of the mirror Mi that are accessible for processing with a brush tool 27. This revealed that brushing all accessible sub-surfaces reduces the long-term change AP of the pass P by approximately 80%.

[0095] Since the proportion of the long-term change AP of the passe P of the mirror Mi produced by the respective sub-surfaces 29a-d generally varies among different individuals of the same mirror type Mi, a general value applicable to all mirror individuals can only be determined as an average of the calculations or measurements of several mirror individuals. The above-mentioned value for the reduction of the long-term change AP of the passe P of approximately 80% is such an average value calculated over several mirror individuals.

[0096] The remaining long-term change AP of the passe P of approximately 20% is due to the areas of the mirror Mi that cannot be machined with a brush tool 27, as well as to the still minor interaction between water and mirror material via the significantly finer surface after brushing. It is possible to further reduce the long-term change AP of the passe P by fine grinding or other mechanical processes on those areas of the mirror Mi that cannot be machined, or can only be machined with difficulty, with a brush tool 27. This will reduce surface damage and surface roughness, thereby further decreasing the long-term change AP of the passe P.

[0097] The long-term change AP of the passe P is not constant over the entire operating life of the mirror Mi, which is on the order of approximately 10 years. Instead, it is greatest immediately after the mirror Mi is placed in a vacuum and decreases with increasing time that the mirror Mi remains in the vacuum. A simulation of the diffusion process in a vacuum, where the relevant diffusion parameters such as the diffusion rate and the total amount of water were calculated by fitting them to the available measurement data, predicts a total long-term change AP of the passe P of approximately 200–250 pm over the entire operating life of approximately 10 years. This is represented by a solid curve in Fig. 6. It should be noted that diffusion is only one possible interaction mechanism between water and the mirror material.In diffusion, water is absorbed by the glass through a physical process and can also escape again. In hydrolysis, however, the water molecules form a chemical bond with the glass substrate. The scale on the left side of Fig. 6 shows an estimate of the CH2O concentration of water in the material of mirror Mi, where 100% represents the CH2O concentration before the mirror Mi was placed in a vacuum.

[0098] Due to the fact that the measurements on mirror Mi were only carried out in a suitable measuring environment during the first few weeks after being placed in a vacuum, the curve shown in Fig. 6 is subject to considerable uncertainty over long periods. The long-term change AP of passe P, shown as a dashed line in Fig. 6, is due to the approximately 80% reduction caused by brushing, as described above. Because of the brushing, the long-term change AP of passe P over the operating life of mirror Mi, approximately 10 years, is about 50 pm.

[0099] A long-term change AP of the pass P of this magnitude is tolerable, so no additional mitigation measures are required to reduce the long-term change AP of the pass P. Such mitigation measures, which compensate for an unwanted, time-progressive change in the pass P on a mirror Mi or another optically active element such as a lens, can, for example, consist of replacing one or more optical elements to maintain a nearly stable optical system. The crucial factor here is whether the overall system meets the specification as a result of this mitigation measure.

[0100] The long-term change AP of the pass P over the operating lifetime of the mirror Mi depends significantly on the long-term change AP of the pass P in the first few weeks after the mirror Mi is placed in a vacuum, due to the essentially exponential decay. The maximum long-term change AP of the pass P typically occurs in the first week after placement in a vacuum and should be less than 20 pm / week, less than 10 pm / week, less than 5 pm / week, and especially less than 2 pm / week to ensure the desired low long-term change AP of the pass P of the mirror Mi over its entire operating lifetime. Based on the diffusion models, a drift of 5 pm / week in the pass P occurring in the first week can be expected to result in a total drift, or long-term change AP of the pass P, of approximately 50 pm over the entire lifetime of the mirror Mi. Such a total drift, i.e.,Such a long-term change AP of the mirror's pass P over the entire operating period is just about tolerable without further mitigation measures. The more the mirror drift exceeds this value, the more complicated the mitigation measures would become. The value of the drift occurring in the first week, for example, determines how frequently a mirror replacement would have to be carried out.

[0101] In summary, the physical cause of the drift mechanism of the pass P of a given mirror Mi is directly addressed in the manner described above. Since surface tension on the back surface 25c and the side surface 25b of the substrate 25 of the mirror Mi can be considered the cause of the long-term change AP of the pass P, which most likely arises from an interaction between water and the mirror material and is intensified by deep damage or surface damage, particularly cavitation damage, attenuating this effect by brushing and thus removing the deep and cavitation damage is advantageous. For this purpose, those sub-surfaces that contribute significantly to the long-term change AP of the pass P are identified.

[0102] Using stress load cases that translate unit stresses into deformations for individual surface areas or sub-areas 29a-d of the mirror Mi, it is possible to deduce, by means of a back-calculation, which sub-area(s) 29d are responsible for the mirror drift. In particular, an optimization of a quadratic program, in which a uniform sign for the stress, corresponding to a tensile or compressive stress, can be defined, serves to accurately describe the cause of this surface stress. This type of optimization can be particularly advantageous for identifying those sub-areas that exhibit a particularly high proportion of the long-term change AP of the passe P. It is generally advantageous if the brush tool 27 or...Several brush tools should be used to reach as many partial surfaces 29a-d of the mirror Mi affected by surface damage as possible; however, the time required for brushing can be reduced if only those partial surfaces 29a-d accessible for brushing are processed by brushing which have a significant proportion of the long-term change AP of the passe P.

Claims

Patent claims 1. Method for reducing long-term change (AP) of a pass (P) of an optical surface (25a) of an optical element (Mi), preferably a mirror, in particular an EUV mirror, characterized by mechanically abrasive machining of the optical element (Mi), preferably of a particularly transparent substrate (25) of the optical element (Mi), outside the optical surface (25a) to reduce surface damage, in particular to reduce surface roughness R a , of the optical element (Mi) outside the optical surface (25a).

2. Method according to claim 1, wherein a surface roughness R is obtained by machining a partial surface (29d) or several partial surfaces (29a-d) of the optical element (Mi) outside the optical surface (25a), the area of ​​which is preferably at least 10%, particularly preferably at least 20%, and especially at least 30% of the area of ​​the optical surface (25a). a of 5 pm or less, preferably of 3 pm or less, particularly preferably of 1.5 pm or less is produced and / or the sub-area (29d) or the multiple sub-areas (29a-d) become transparent through processing.

3. Method according to claim 1 or 2, wherein, during mechanical processing of the optical element (Mi), material is removed from the optical element (Mi), preferably from a substrate (25) of the optical element (Mi), outside the optical surface (25a).

4. Method according to claim 3, wherein the removal of material is carried out by brushing using at least one brush tool (27), by grinding and / or by polishing.

5. Method according to one of the preceding claims, wherein the processing is carried out on at least one partial surface (29d) of the optical element (Mi) outside the optical surface (25a), preferably on at least one partial surface (29d) on a rear (25c) and / or on a side surface (25b) of the optical element (Mi).

6. Method according to claim 4, further comprising: selecting the at least one partial surface (29d) from a plurality of partial surfaces (29a-d) of the optical element (Mi) outside the optical surface (25a) depending on the proportion of the respective partial surfaces (29d) to the long-term change (AP) of the pass (P) of the optical surface (25b), wherein preferably at least the partial surface (29d) that generates a maximum proportion of the long-term change (AP) of the pass (P) on the optical surface (25a) is selected for processing.

7. Method according to one of the preceding claims, further comprising: Determining a deformation (30a-d) of the optical surface (25a) which is caused by a respective subsurface (29a-d) on the optical surface (25a) at a uniform surface tension on the plurality of subsurfaces (29a-d), wherein the deformation (30a-d) is preferably determined by a finite element simulation.

8. Method according to claim 7, further comprising: spatially resolved measurement of the optical surface (25a) to determine the long-term change (AP) of the pass (P) of the optical surface (25a) based on a spatially dependent long-term change (AP(x,y)) of the pass (P), and determining the contribution of a respective sub-area (29a-d) to the long-term change (AP) of the pass (P) of the optical surface (25a) based on a deformation profile (31) derived from the deformations (30a-d) of the optical surface (25a) which is composed of the respective sub-surfaces (29a-d).

9. Method according to claim 8, in which, to determine the proportion of the respective sub-area (29a-d) to the long-term change (AP) of the pass (P) of the optical surface (25a), the deformation profile (31), which is composed of the deformations (30a-d) of the optical surface (25a) caused by the respective sub-areas (29a-d), is adapted to the location-dependent long-term change (AP(x,y)) of the pass (P) of the optical surface (25a) by means of an optimization algorithm, preferably by means of a quadratic optimization algorithm.

10. Method according to claim 8 or 9, wherein the reduction of the long-term change (AP) of the pass (P) of the optical surface (25a) by processing the at least one partial surface (29d) is estimated based on the proportion of at least one partial surface (29d) to the long-term change (AP) of the pass (P) of the optical surface (25a).

11. Optical element (Mi), preferably a mirror, in particular an EUV mirror, comprising: an optical surface (25a), preferably for reflecting radiation, in particular for reflecting EUV radiation (16), characterized in that the optical element (Mi), preferably a particularly transparent substrate (25) of the optical element (Mi), has a surface roughness R on a partial surface (29d) or on several partial surfaces (29a-d) outside the optical surface (25a), the area of ​​which is preferably at least 10%, particularly preferably at least 20%, in particular at least 30% of the area of ​​the optical surface (25a). a from 5 pm or less, preferably from 3 pm or less, particularly preferably from has a diameter of 1.5 m or less and / or the sub-area (29d) or the multiple sub-areas (29a-d) are transparent.

12. Optical element according to the preamble of claim 11, in particular according to claim 11, characterized in that the optical surface (25a) has a maximum long-term change (AP) of the pass (P) under vacuum conditions which is less than 20 pm / week, preferably less than 10 pm / week, particularly preferably less than 5 pm / week, and in particular less than 2 pm / week.

13. Optical element according to the preamble of claim 11, in particular according to claim 11 or 12, characterized in that the optical surface (25a) has a long-term change (AP) of the pass (P) under vacuum conditions, which is at 100 pm or less, preferably at 80 pm or less, particularly preferably at 50 pm or less over a period of at least 10 years.

14. Semiconductor lithography system, in particular EUV lithography system (1) comprising: at least one optical element (Mi) according to one of claims 11 to 13, which is preferably arranged in a projection optic (10).

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