Method for correcting a surface defect on a mirror

Abstract

Method for correcting a local surface defect (Δ(x,y)) on a mirror (Mi) which has a coating (26) for reflecting radiation, in particular for reflecting EUV radiation (16), comprising: determining the local surface defect (Δ(x,y)) of the mirror (Mi) and correcting the local surface defect (Δ(x,y)) by locally heating the coating (26), wherein the local heating of the coating (26) causes an irreversible local compaction of the coating (26), and wherein, before the application of the coating (26), a surface (25a) of the substrate (25), to which the coating (26) is subsequently applied, is processed, preferably with a particle beam, in particular with an ion beam (28), wherein, during processing, a layer stress effect on the surface (25a) of the substrate (25) to be expected due to the local heating of the coating (26) is at least partially compensated.

Description

Background of the invention

[0001] The invention relates to a method for correcting a local surface defect on a mirror having a coating for reflecting radiation, in particular for reflecting EUV radiation.

[0002] For the purposes of this application, a surface defect is defined as a deviation of the surface shape of the coated mirror, i.e., the optical surface, from a desired surface shape. The surface defect, often also referred to as a registration error, varies depending on the location across the surface of the coated mirror, which is why it is referred to as a local surface defect in this application.

[0003] The local surface defect typically arises from a deviation of the local coating thickness from a local target coating thickness. The deviation of the local thickness from the target coating thickness is typically due to coating errors during the application of the coating to the substrate.

[0004] It is known to correct surface defects of a coated mirror, for example, an EUV mirror, by targeted local compaction of the substrate (see, for example, WO 2020 / 011788 A1). Local compaction is generally associated with side effects, such as an aging effect on the mirror. This side effect can be particularly problematic for mirrors that are heated to high operating temperatures during operation.

[0005] US 6,844,272 B2 describes the correction of local errors in the shape of an optical surface by changing the local density of the surface or of layers near the surface, which results in a change in the height of the optical surface. The change in local density leads to a local expansion or contraction of a substrate or a coating applied to the substrate. The change in height can be caused, for example, by interdiffusion or by a chemical reaction between adjacent layers of the coating, which are generated by a local energy input. The energy input can be introduced, for example, with an ion beam, an electron beam, or a laser beam.

[0006] Furthermore, from DE 10 2021 213 679 A1 a method is known for generating a local change in the thickness of a coating for reflecting radiation, in particular for reflecting EUV radiation, which is applied to a substrate of a mirror, comprising: generating the local change in thickness by introducing a local energy input into the coating, which causes a compaction or expansion of the coating, in order to generate a local desired thickness of the coating which corresponds to a local desired reflectivity of the mirror. Object of the invention

[0007] The object of the invention is to provide a method for correcting a local surface defect on a mirror which influences the material of the substrate as little as possible. Subject of the invention

[0008] This object is achieved by a method of the type mentioned at the outset, comprising: determining the local surface defect of the mirror, and correcting the local surface defect by locally heating the coating, wherein the local heating of the coating causes an irreversible local compaction of the coating and wherein, before the coating is applied, a surface of the substrate to which the coating is subsequently applied is processed, preferably with a particle beam (e.g. with an atomic beam or molecular beam), in particular with an ion beam, wherein during processing a layer stress effect on the surface of the substrate to be expected due to the local heating of the coating is maintained or at least partially compensated.

[0009] Local heating introduces heat into the coating to locally increase its temperature. This local increase in temperature typically leads to irreversible local compaction or contraction (reduction in thickness) of the coating. Local heating can also lead to interdiffusion, or a chemical reaction, between materials in neighboring layers, forming new materials with a greater density than the materials before local heating.

[0010] It is understood that the method described here can only be applied to coatings that respond to local heating with compaction. This is typically the case for multilayer coatings comprising a plurality of layers of different materials, in particular a plurality of subsystems with layer pairs of identical thickness, such as those used to reflect EUV radiation under normal incidence.

[0011] Local heating allows for local compaction of the coating or coating layers without also compacting the substrate. This way, the side effects described above can be avoided. Ideally, the process step of compacting the substrate described above can be completely omitted.

[0012] It has been shown that the method described above can correct surface defects or fitting errors on the order of approximately + / - 500 pm. This method is particularly suitable for mirrors with a low numerical aperture, for some mirrors with a high numerical aperture, and for future product generations of EUV mirrors.

[0013] In one variant, the local heating of the coating is achieved by electron beam heating or laser-induced annealing. In electron beam heating, an electron beam is directed at the substrate or coating to locally heat the coating. The electron energies are selected so that the electron beam does not penetrate the substrate at all, or only slightly, and the substrate is not compacted at all, or only very slightly. Another option for locally heating the coating is laser-induced annealing, in which a laser beam is directed at the coating to achieve local heating. In this case, the laser beam is focused into the coating to heat it and introduce as little heat into the substrate as possible. It is understood that other local heating methods can also be used, in particular those using electromagnetic radiation.For the purposes of this application, local heating is understood to mean a heating process that generates a location-dependent varying portion of the heat introduced into the coating and a portion of the heat introduced into the coating that is homogeneous over the surface of the substrate.

[0014] The substrate surface is typically machined or polished using an ion beam prior to coating application to ensure the reflective coating is applied to a surface that is as smooth as possible and has the desired surface shape. The typically significant change in layer stress caused by local heating or annealing generally requires that this change in layer stress be accounted for during ion beam processing of the substrate. The predicted or expected effect of the layer stress caused by local annealing on the substrate surface is accounted for or at least partially compensated for in advance during ion beam processing of the substrate.

[0015] In a further variant, an expected average change in the thickness of the coating due to compaction during local heating is taken into account or maintained during application of the coating. During application of the coating, the expected or predicted homogeneous, i.e. location-independent average change in the layer compaction that is generated during annealing is maintained. In other words, a thicker coating is applied to the substrate than would be the case if no local heating of the substrate were to take place. The layer thickness reserve is typically homogeneous, i.e. the thickness of the coating is increased by a constant value that is independent of the position on the surface. This is possible because the location-independent component of the compaction during local heating is usually known in advance, as described in more detail below.

[0016] In another variant, the mirror is measured with spatial resolution to determine the local surface defect. With spatially resolved measurement, for example, the reflectivity of the mirror can be measured with spatial resolution, and the local surface defect can be determined based on the spatially measured reflectivity. However, it is also possible to determine the local surface defect in other ways.

[0017] In another variant, a local compaction of the coating is determined from the determined local surface defects of the mirror, which corrects the local surface defects. The local compaction can be calculated from the surface defect at a specific position, which allows the local surface defect to be corrected at a specific position on the surface of the coating. Thus, by determining the local compaction, a layer compaction map is created.

[0018] In a further development, a local temperature-time curve for the local heating of the coating is determined based on the local compaction of the coating. Using a suitable model for layer compaction, the layer compaction map is converted into a temperature-time field or a local temperature-time curve for local heating of the coating.

[0019] In a further development of this variant, the temperature-time curve has a homogeneous component and a location-dependent variable component, whereby the location-dependent variable component of the temperature-time curve corresponds to the local compaction of the coating, which corrects the local surface defect.

[0020] The temperature-time curve or temperature field is transferred to a machine that enables local heating of the coating or mirror. This machine can be, for example, an electron beam heating machine—at low electron energies to avoid compaction of the substrate—or a laser-induced annealing machine.

[0021] The homogeneous portion of the temperature-time curve, for example, holding the coating at a temperature between approximately 100°C and 150°C for a period of several hours, produces a moderate, homogeneous compaction of the coating or coating layers. The laterally or locally varying portion of the temperature-time curve results in a locally variable compaction of the coating, which corrects the local surface defect. The magnitude of the local variation or the locally varying portion of the temperature-time curve is typically in the range of a few Kelvin.

[0022] The result of the process described above is a mirror in which 1) By the lead or the compensation of the layer stress effect when machining the surface of the substrate in combination with the homogeneous tempering, ie with the homogeneous part of the temperature-time curve, the correct layer stress state is set. 2) By taking into account the compaction effect or the average thickness change in combination with the homogeneous tempering, the correct layer compaction state is achieved on average. 3.) The laterally inhomogeneous surface or coating defects are corrected by the targeted local variation of the temperature as described above, i.e. by the locally varying portion of the temperature-time curve during tempering.

[0023] The process described above is typically carried out under vacuum conditions. Instead of the irreversible compaction described above, it is also possible for the reflective coating to respond to local heating with expansion, i.e., an increase in thickness. The process described above can also be applied in this case. It should be understood that in both cases, the thickness changes should not negatively affect the reflection properties for the incident radiation.

[0024] 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 of the drawing, which illustrate details essential to the invention, and from the claims. The individual features can be implemented individually or in combination in a variant of the invention. drawing

[0025] Exemplary embodiments are shown in the schematic drawing and are explained in the following description. It shows Fig. 1 schematically shows in meridional section a projection exposure system for EUV projection lithography, Fig. 2a a schematic representation of the ion beam processing of a surface of a substrate in the production of an EUV mirror of the projection exposure system of Fig. 1, Fig. 2b a schematic representation of the substrate of Fig. 2a, to which a reflective coating was applied with a thickness lead, during local heating with a laser beam, and Fig. 2c shows a schematic representation of the EUV mirror after completion of the manufacturing process.

[0026] In the following description of the drawings, identical reference symbols are used for identical or functionally identical components.

[0027] In the following, with reference to Fig. 1 describes, by way of example, 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 to be understood as limiting.

[0028] One embodiment of an illumination system 2 of the projection exposure system 1 has, in addition to a light or radiation source 3, an illumination optics 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 illumination system. In this case, the illumination system does not include the light source 3.

[0029] A reticle 7 arranged in the object field 5 is illuminated. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be displaced, in particular in a scanning direction, via a reticle displacement drive 9.

[0030] In Fig. For illustrative purposes, a Cartesian xyz coordinate system is shown in Figure 1. The x-direction is perpendicular to the plane of the drawing. The y-direction is horizontal, and the z-direction is vertical. The scanning direction is in the Fig. 1 along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0031] The projection exposure system 1 comprises a projection system 10. The projection system 10 serves to image the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced, in particular along the y-direction, via a wafer displacement drive 15. The displacement of the reticle 7, on the one hand, via the reticle displacement drive 9, and the wafer 13, on the other hand, via the wafer displacement drive 15, can be synchronized with one another.

[0032] Radiation source 3 is an EUV radiation source. Radiation source 3 emits, in particular, EUV radiation 16, which is also referred to below as useful radiation, illumination radiation, or illumination light. The useful radiation has, in particular, 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).

[0033] 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 hyperboloidal reflection surfaces. The at least one reflection surface of the collector mirror 17 can be exposed to the illumination radiation 16 at grazing incidence (Gl), i.e., at angles of incidence greater than 45°, or at normal incidence (NI), i.e., at angles of incidence less than 45°. The collector mirror 17 can be structured and / or coated, on the one hand, to optimize its reflectivity for the useful radiation and, on the other hand, to suppress stray light.

[0034] 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.

[0035] The illumination optics 4 comprises a deflecting mirror 19 and, downstream of this in the beam path, a first facet mirror 20. The deflecting mirror 19 can be a flat deflecting mirror or, alternatively, a mirror with a beam-influencing effect beyond the pure deflection effect. Alternatively or additionally, the deflecting mirror 19 can be designed as a spectral filter that separates a useful light wavelength of the illumination radiation 16 from stray light of a different wavelength. The first facet mirror 20 comprises a plurality of individual first facets 21, which are also referred to below as field facets. Of these facets 21, Fig. 1 shows only a few examples. A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optics 4. The second facet mirror 22 comprises a plurality of second facets 23.

[0036] The illumination optics 4 thus form a double-faceted system. This basic principle is also referred to as a fly's-eye integrator. With the help of the second facet mirror 22, the individual first facets 21 are imaged into the object field 5. The second facet mirror 22 is the last beam-forming mirror, or actually the last mirror for the illumination radiation 16 in the beam path before the object field 5.

[0037] 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.

[0038] In the Fig. In the example shown in Figure 1, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or a different number of mirrors M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection system 10 is a doubly obscured optical system. The projection optical system 10 has an image-side numerical aperture that is greater than 0.4 or 0.5 and can also be greater than 0.6, for example, 0.7 or 0.75.

[0039] The mirrors Mi, just like the mirrors of the illumination optics 4, can have a highly reflective coating for the illumination radiation 16.

[0040] Fig. 2a,b show two exemplary process steps in the production of a mirror Mi (i = 1, ..., 6) of the projection system 10 of the EUV lithography system 1 of Fig. 1, Fig. Figure 2c shows the mirror Mi after completion of the manufacturing process. The mirror Mi comprises a substrate 25 and a multilayer coating 26 highly reflective for EUV radiation 16, which is Fig. 2b and in Fig. 2c. In the example shown, the substrate 25 is made of quartz glass doped with titanium dioxide. The substrate 25 may also be made of another material having a low coefficient of thermal expansion, which should typically be less than 100 ppb / K at 22°C.

[0041] For the sake of simplicity, the following considerations assume that the surface 25a of the substrate 25 is to be flat and that the coating 26 applied to the flat surface 25a of the substrate 25 is to have a predetermined thickness d that is constant over the entire surface 25a, as shown in Fig. 2c is shown.

[0042] Fig. 2a shows a processing step in the production of the mirror Mi of Fig. 2c, which is carried out before the application of the multi-layer coating 26. In the Fig. In the processing step shown in Figure 2a, a surface 25a of the substrate 25, to which the coating 26 is subsequently applied, is processed with an ion beam 28 in order to smooth the surface 25a. The ion beam 28 is generated by an ion gun 29. However, during ion beam processing, the desired, in the present example, flat surface shape of the surface 25a of the substrate 25 is not generated; rather, during ion beam processing, a surface shape that varies depending on location is generated, as shown in Fig. 2a can be seen.

[0043] The deviation of the surface shape of the surface 25a of the substrate 25 from the desired flat surface shape is selected such that the effect of a layer stress on the surface 25a generated in a subsequent local heating or tempering step in the coating 26 is at least partially, ideally completely, compensated, so that after the tempering step the desired flat surface shape of the surface 25a is established, which in Fig. 2c is shown.

[0044] In a step not shown in the picture, after the Fig. 2a, the highly reflective multilayer coating 26 is applied to the machined surface 25a of the substrate 25. As shown in Fig. 2b,c, the multilayer coating 26 comprises a plurality of alternating layers 27a, 27b made of a material with a low refractive index and a material with a high refractive index. In the example shown, in which the useful wavelength of the EUV lithography system 1 is approximately 13.5 nm, the layers 27a, 27b are silicon and molybdenum, which are applied in pairs one above the other on a surface 25a of the substrate 25. The representation of additional functional layers of the coating 26 was omitted in Fig. 2b,c is waived.

[0045] The coating 26 is applied to the substrate 25 using a conventional method for depositing thin layers 27a, 27b, e.g., by physical or chemical vapor deposition. The coating 26 is applied with a thickness allowance dv, i.e., the coating 26 is applied with a greater thickness than is required for the function of the coating 26. The thickness allowance dv takes into account the homogeneous component of the compaction that is generated during the subsequent local heating of the coating 26, which is described in more detail below. To generate the thickness allowance dv, each individual layer 27a, 27b of the coating 26 is applied with a slightly greater thickness than actually intended by the design of the coating 26.

[0046] When applying the coating 26 to the surface 25a of the substrate 25, coating defects usually occur, which lead to a coating with an - unwanted - local variation of the thickness d(x,y) + dv, as shown in Fig. 2b. The deviation of the thickness d(x,y) from the uniform target thickness d forms the local surface error Δ(x,y), for which the following applies: Δ(x,y) = d(x,y) - d. In the example described here, the local surface error Δ(x,y) thus corresponds to the local coating error during the application of coating 26.

[0047] To correct the local surface defect Δ(x,y), the coating 26 is locally heated or tempered by means of a pulsed laser beam 30 generated by a laser source 31, as shown in Fig. 2b. In order to correct the local surface error Δ(x,y), it is first necessary to determine it. To determine the surface error Δ(x,y), the Fig. The coated substrate 25 shown in Figure 2b is measured with spatial resolution. The measurement to determine the local surface error Δ(x,y) can, for example, be a reflectivity measurement in which the spatially dependent reflectivity of the coating 26 on the surface 25a is measured with spatial resolution. The local surface error Δ(x,y) can be determined from the spatially dependent reflectivity.

[0048] From the determined local surface error Δ(x,y), a local compaction or a compaction map of the coating 26 is determined in order to correct the local surface error Δ(x,y). The local compaction has a homogeneous component that is constant across the surface 25a of the substrate 25, as well as a location-dependent, laterally varying component that can have a positive or negative sign. By using the homogeneous component, local surface errors Δ(x,y) with a positive or negative sign can be compensated, although local heating can only produce compaction, i.e., a reduction in the thickness d(x,y) + dv. The amount of the homogeneous component is typically significantly greater than the amount of the laterally varying component of the local compaction.

[0049] Based on the local compaction of the coating 26 determined in the manner described above, which corrects the local surface defect Δ(x,y), a local temperature-time curve is determined for the local heating of the coating 26. Corresponding to the local compaction, the temperature-time curve also has a homogeneous component and a location-dependent variable component. The homogeneous temperature-time curve, for example, maintaining the temperature of the coating 26 at a temperature between approximately 100°C and 150°C for a period of several hours, produces a mean, homogeneous compaction of the coating 26 or of the layers 27a, 27b of the coating 26. The laterally or locally varying component of the temperature-time curve results in a locally variable compaction of the coating 26.The varying part of the temperature-time curve corresponds to the local compaction of the coating 26, which corrects the local, location-dependently varying surface error Δ(x,y).

[0050] Due to the homogeneous part of the temperature-time curve in combination with the lead or the compensation of the layer stress effect when machining the surface 25a of the substrate 25 in the Fig. In the step shown in Figure 2a, the averagely correct layer stress state is set on the mirror Mi, which in the example shown produces the desired planar geometry of the surface 25a of the substrate 25 of the mirror Mi.

[0051] In addition, the thickness adjustment dv in combination with the homogeneous part of the compaction or the temperature-time curve on average sets the correct layer compaction state, ie the Fig.The constant thickness d shown in Figure 2c corresponds to the constant target thickness of the coating 26, which is specified by the design. The optical surface 26a of the mirror Mi, which forms the interface between the coating 26 and the environment, is also flat, i.e., it corresponds to the desired surface shape.

[0052] It is understood that the method described above for compensating the local surface error Δ(x,y) can also be applied to mirrors that have a curved surface, for example, a concave or convex curvature. Using this method, surface errors Δ(x,y), i.e., local thickness deviations of the coating 26, on the order of several hundred picometers, can be corrected.

Claims

[1] Method for correcting a local surface defect (Δ(x,y)) on a mirror (Mi) which has a coating (26) for reflecting radiation, in particular for reflecting EUV radiation (16), comprising: determining the local surface defect (Δ(x,y)) of the mirror (Mi) and correcting the local surface defect (Δ(x,y)) by locally heating the coating (26), wherein the local heating of the coating (26) causes an irreversible local compaction of the coating (26), and wherein, before the application of the coating (26), a surface (25a) of the substrate (25), to which the coating (26) is subsequently applied, is preferably processed with a particle beam, in particular with an ion beam (28), wherein, during processing, a layer stress effect on the surface (25a) of the substrate (25) to be expected due to the local heating of the coating (26) is at least partially compensated. [2] Method according to claim 1, wherein the local heating of the coating (26) is carried out by electron beam heating or by laser-induced annealing. [3] Method according to one of the preceding claims, in which, during application of the coating (26), an average change in thickness (d v ) of the coating (26). [4] Method according to one of the preceding claims, in which the mirror (Mi) is measured in a spatially resolved manner to determine the local surface defect (Δ(x,y)). [5] Method according to one of the preceding claims, in which a local compaction of the coating (26) is determined from the determined local surface defect (Δ(x,y)) of the mirror (Mi), which corrects the local surface defects (Δ(x,y)). [6] Method according to claim 5, in which, starting from the local compaction of the coating (26), which corrects the local surface defects (Δ(x,y)), a local temperature-time curve for the local heating of the coating (26) is determined. [7] Method according to claim 6, wherein the temperature-time curve has a homogeneous portion and a location-dependent variable portion, wherein the location-dependent variable portion of the temperature-time curve corresponds to the local compaction of the coating (26) which corrects the local surface defect (Δ(x,y)).

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