Method for correcting local surface elevations on reflective surfaces

Abstract

The invention relates to a method for correcting local elevations (40) on the surface, having a reflective coating (27), of an optical element (25) for EUV microlithography. The method comprises the steps of: measuring the surface of an optical element (25) having the reflective coating (27); identifying local elevations (40) on the surface having the reflective coating (27) on the basis of the result of the surface measurement; evaluating the identified local elevations (40) with regard to their ability to be corrected by means of a specified machine for locally densifying the substrate (26) located beneath the reflective coating (27) by irradiation; determining processing specifications for the machine for locally densifying the substrate (26) located beneath the reflective coating (27) by irradiation, at least for the local elevations (40) evaluated as able to be corrected; and controlling the machine for locally densifying the substrate (26) located beneath the reflective coating (27) by irradiation, according to the processing specifications, in order to correct the identified local elevations (40) evaluated as able to be corrected.

Description

Method for correcting local surface elevations on reflective surfaces

[0001] This application claims priority from German patent application 10 2022 211 875.8, filed on November 9, 2022. The content of this German patent application is incorporated by reference into the present application text.

[0002] The invention relates to a method for correcting local surface elevations on the surface of an optical element for EUV microlithography, which surface is provided with a reflective coating.

[0003] Microlithography is used in the production of microstructured components, such as integrated circuits. The microlithography process is carried out in a so-called projection exposure system, which has an illumination device and a projection device. The image of a mask (also called a "reticle") illuminated by the illumination device is projected by the projection device onto a substrate coated with a light-sensitive layer (so-called "photoresist") and arranged in the image plane of the projection device, e.g., a silicon wafer, in order to transfer the mask structure to the light-sensitive coating of the substrate. In subsequent production steps, the transferred structure is implemented in the substrate, e.g., by etching.

[0004] Due to the progressive miniaturization in the semiconductor sector and the transition of the wavelength during exposure from DUV (e.g. 193 nm) to EUV (e.g. 13.5 nm) have a high image quality. For projection devices in the EUV range (5-20 nm), a mask in the object plane onto a substrate in the image plane, e.g. with a reduction factor of 8:1, using only reflective optical elements.

[0005] Corresponding reflective optical elements generally comprise a substrate on whose surface, at least in the areas intended as the actual mirror surface, a layer or multi-layer arrangement is applied, with which radiation in the wavelength intended for exposure (such as 13.5 nm) or a wavelength range is reflected.

[0006] Before the actual reflective coating is applied, the substrate of a reflective optical element is manufactured with very high accuracy and very tight tolerances, at least in the area of ​​the actual mirror surface. The application of the reflective coating is also carried out with high precision.

[0007] Due to the high quality of the individual optical elements required for the high imaging quality in microlithography, deviations from the ideal shape can occur despite the high-precision manufacturing of the reflective optical elements, which, despite their small size, can still lead to sometimes considerable reductions in the imaging quality of an optical element or a projection device composed of several optical elements.

[0008] To eliminate so-called wavefront errors, where the reflective surface of an already completely coated optical element deviates from the ideal shape over a large area - especially in an area with a lateral extension of more than 5 mm - it is known to partially irradiate the substrate below the reflective coating with suitable electrode radiation. compact, whereby regions of the reflective surface can be lowered in order to bring the surface of the optical element closer to the ideal shape or to give the surface of the optical element a shape which - possibly also taking into account the optical properties of other optical elements of a projection device - leads to a reduction in the imaging error of the optical element or of the entire projection device. A suitable method for this is disclosed, for example, in DE 10 2011 084 117 A1.

[0009] Corrections for local surface deviations with a lateral extent of, for example, less than 5 mm are currently very difficult to achieve, if at all, because the entire optics of a projection device must be comprehensively reworked. A coated optical element whose contour has already been adapted to a non-circular final geometry can often no longer be corrected.

[0010] The local surface deviations can be local elevations, local depressions, or holes in the coating and / or the substrate. If a correction option is created for one of these types of local surface deviations, with which at least some of the imaging errors caused by the local surface deviations can be reduced, the overall imaging quality of an individual optical element or of a projection device in which the optical element in question is used can generally be improved.

[0011] The object of the present invention is to create a method with which local surface elevations on the surface provided with a reflective coating a mirror for EUV microlithography can be corrected.

[0012] This problem is solved by a method according to the main claim. Advantageous further developments are the subject of the dependent claims.

[0013] Accordingly, the invention relates to a method for correcting local elevations on the surface of an optical element (25) for EUV microlithography, provided with a reflective coating (27), comprising the steps of: - Measuring the surface of the surface of an optical element provided with the reflective coating; - Identifying local elevations on the surface coated with the reflective coating based on the result of the surface measurement; - Evaluating the identified local elevations with regard to their correctability using a specified machine for the local densification of the substrate arranged under the reflective coating by irradiation; - Determination of machining specifications for the machine for the local densification of the substrate arranged under the reflective coating by irradiation, at least for the local elevations assessed as correctable; and - Control of the machine for the local densification of the substrate arranged under the reflective coating by irradiation according to the processing specifications for Correction of the local surveys identified and assessed as correctable.

[0014] First, some terms used in connection with the invention are explained:

[0015] The “lateral extent” of an elevation refers to the characteristic size of a geometric figure that completely envelops the elevation in a lateral direction to the height of the elevation – in this case particularly in the plane of the reflective surface. If the geometric figure is a circle, the characteristic size is the diameter of the circle. To distinguish the elevation from the rest of the mirror surface, a threshold value for the deviation from the ideal shape of the reflective surface can be specified, above which a surface deviation is considered to be an elevation. A suitable and generally sufficient threshold value for a reflective surface for EUV applications can be 3 nm, for example.

[0016] A protrusion is considered "local" if its lateral extent is locally limited. The lateral extent up to which a protrusion is considered local can result from the technical conditions of the machine for local densification to be used in the course of the method according to the invention: Corresponding machines inevitably have a minimum resolution predetermined by the extent of the beam used for irradiation, e.g. electrode beam, when it hits the reflective surface; a protrusion can then, for example, be considered local if its lateral extent is in any case no greater than the minimum resolution of the machine for local densification, i.e. the minimum lateral extent of the beam of the device intended for densification. Radiation on the reflective surface. Alternatively, a Survey is considered local if it is within predefined absolute or relative limits. In such a case, the resolution of the machine to be used in the course of the method according to the invention for local compaction can be lower than a predetermined maximum extent for local elevations. The predetermined maximum extent can, for example, be based on the minimum extent of larger elevations which result in imaging errors, such as wavefront errors, which are corrected using other methods known from the prior art. For example, an elevation can be considered local if its extent is less than 5 mm.

[0017] The invention has recognized that local elevations on a reflective surface intended for the reflection of EUV radiation can generally be corrected by locally densifying the substrate arranged beneath the reflective coating. Since the method according to the invention is directed to the correction of already coated reflective surfaces, densifying the substrate arranged beneath the reflective coating by irradiation, as is known from the prior art, is generally suitable for this purpose.However, due to technical limitations, such compaction cannot usually be restricted with sufficient reliability to an arbitrarily small and / or arbitrarily shaped area on the reflective surface. Therefore, it is generally to be expected that an attempt to level a corresponding elevation by compaction via irradiation may also result in depressions or other deformations of the reflective surface immediately adjacent to the original elevation, which can also lead to imaging errors. Consequently, such compaction for correcting local elevations has not yet been considered in the prior art.

[0018] According to the invention, however, it is provided that - before an actual attempt is made to correct an identified local elevation by compacting the underlying substrate by irradiation - an assessment of the identified local elevation is first carried out with regard to its correctability using a machine for local compaction of the substrate arranged beneath the reflective coating by irradiation. The assessment of the correctability can be carried out in a variety of ways, e.g. on the basis of the geometry of the elevation with the aid of expert systems or artificial intelligence. Factors can also be taken into account which could prevent compaction to correct the local elevation, such as compaction of the substrate that has already been carried out or is to be carried out due to a wavefront error in the area of ​​a local elevation, as a result of which further compaction is excluded.The assessment of the correctability of local elevations must always be carried out with regard to the machine intended for local densification, since the correctability also depends on the technical parameters of this machine, such as the achievable accuracy and resolution of the irradiation. Therefore, the machine to be used for the subsequent correction must be specified during the assessment to ensure that a planned correction can actually be implemented accordingly.

[0019] Preferably, the evaluation of the local elevations with regard to their correctability comprises a comparison of the local imaging performance with the existing elevation with the expected imaging performance after a possible compaction of the substrate arranged under the reflective coating by irradiation with the machine available for this purpose. In other words, it is checked whether the local imaging performance can be improved by a possible correction. For this purpose, the imaging performance In the area of ​​the elevation before a possible correction, directly based on the initial surface measurement, and in the same area after an assumed correction, e.g., comprising an estimate or simulation of the change in the surface due to a compaction of the substrate arranged beneath the reflective coating, the resulting imaging performance can then be compared to determine whether the imaging performance actually improves as a result of the considered correction.

[0020] When assessing the correctability, one or more strategies for correcting a local elevation can be examined. For example, one correction strategy may be to completely level off a local elevation. Another correction strategy may be to reduce or level off only parts of a local elevation, e.g. only the peaks of the local elevation, by compacting the underlying substrate. Another correction strategy may be to lower a larger area extending beyond the local elevation by correspondingly compacting the substrate in this area. All of the correction strategies mentioned above as examples, either individually or in any combination, can generally achieve improvements in local imaging performance for individual local elevations.If the correctability of a local elevation is assessed using several correction strategies, any correction is preferably carried out using the correction strategy with the greatest expected improvement in the local imaging performance or with the greatest resulting local imaging performance.

[0021] At the latest after a local elevation has been classified as correctable, a processing specification for the machine for the local compaction of the surface under the mirroring Coating arranged substrate is determined by irradiation for corresponding correctable local elevations. The processing specification is designed in such a way that - when it is subsequently processed by the machine for local compaction - it is used to control the machine in order to correct the identified local elevation that has been assessed as correctable by compacting the substrate lying under the reflective coating in the area of ​​the local elevation. The prior assessment ensures that, even if the correction could result in effects that could fundamentally impair the image quality, such as depressions, the image quality of the optical element and / or the projection device for which this element is intended is not impaired overall, but rather improved.

[0022] It is not mandatory that the processing specification be determined only after the identified local elevations have been evaluated for their correctability. Alternatively, it is also possible to determine processing specifications for all identified local elevations, which can then be used, for example, in their evaluation for correctability. The processing specifications can thus be used, for example, to estimate or simulate the local imaging performance after correction.

[0023] When evaluating the local elevations with regard to their correctability, determining processing specifications and / or controlling the machine, it is preferably taken into account whether the substrate in the area of ​​local elevations has already been compacted for other reasons or still needs to be compacted, e.g. to compensate for wavefront errors. Due to corresponding compactions required for other reasons, the possibility of correcting local elevations may be limited (e.g. because the substrate in the area a local elevation cannot be further compacted) or the assessment of the correctability leads to a different result. For example, if compaction in the area of ​​a local elevation is required for other reasons, it may no longer be possible to improve the imaging performance when correcting this elevation, even if an assessment of the correctability without taking into account the otherwise intended compaction of the substrate arranged under the reflective coating would indicate a correction due to an improvement in the imaging performance. If several correction strategies are checked when assessing the correctability of a local elevation, processing specifications can be determined for each of the correction strategies, which can then be used as the basis for assessing the correctability of local elevations in accordance with the individual correction strategies.Ultimately, however, only the processing specification that corresponds to the correction strategy that can achieve the greatest improvement in local imaging performance or the best imaging performance is actually implemented.

[0024] The processing specifications determined by the method according to the invention for correcting the identified local elevations assessed as correctable are ultimately used to control the machine for the local compaction of the substrate arranged beneath the reflective coating by irradiation. In this case, it is possible for a corresponding machine to implement the processing specifications determined according to the invention on its own. For this purpose, the processing specifications are regularly converted by the machine into a processing program which controls the machine in such a way that the elevations assessed as correctable are individually approached and the substrate located in this area beneath the reflective coating is specifically compacted. However, it is also possible for the Processing specifications can be processed together with processing specifications resulting from other correction methods, e.g. for reducing wavefront errors, into a common processing program so that the various processing specifications can be implemented in a single processing run.

[0025] The method according to the invention is particularly suitable for correcting local elevations on reflective surfaces which are due to elevations already present on the substrate. For the automated detection of such elevations, it is preferred if only those local elevations are identified on the basis of the result of the surface measurement of the surface provided with the reflective coating for which an elevation can be identified at the same position, preferably with a position tolerance of a maximum of ± 100 m, on the basis of a surface measurement of the substrate to be coated carried out before coating with the reflective coating. If a local elevation can therefore be identified with precise position both on the substrate itself - i.e. before coating - and after coating has taken place, it can be immediately assumed that the local elevation is due to an elevation in the substrate.

[0026] It is of course still possible to use the method according to the invention to correct local elevations that can only be detected on the coated reflective surface, but not on the underlying substrate. In such a case, however, it may be necessary to examine the local elevation more closely in order to determine its cause and subsequently assess its correctability.

[0027] It is preferred if the surface measurement of the surface provided with the reflective coating and / or the substrate arranged beneath the reflective coating, prior to coating thereof, comprises an interferometric method for surface measurement. Corresponding methods for high-precision surface measurement are known from the prior art and are considered reliable and efficient.

[0028] The radiation intended for densifying the substrate beneath the reflective surface preferably comprises electron beams. The use of electron beams is known and proven for densifying substrates with coatings designed to reflect EUV radiation. To perform such densification, the substrate must already be coated with the reflective coating. Furthermore, the densification must be carried out by irradiation in a vacuum.

[0029] If absolute values ​​are to be used for the identification of local elevations in the method according to the invention, it is preferred if the lateral extent of a local elevation is set to less than 5 mm and / or its amplitude is set to greater than 0.3 nm.

[0030] A suitable irradiation to densify the substrate can be, for example, irradiation with electrons with an energy between 5 keV and 80 eV at doses between 0.1 ] / mm 2 and 2,500 J / mm 2 and / or irradiation by a pulsed laser with wavelengths between 0.3 m and 3 m, repetition rates between 1 Hz and 100 MHz and pulse energies between 0.01 / z / and 10 m / .

[0031] The invention will now be described using advantageous embodiments with reference to the accompanying drawings. They show: Figure 1: a schematic representation of a projection exposure system for EUV microlithography with various optical elements; Figure 2a-e: a schematic representation of an embodiment of the method according to the invention for correcting local elevations on the surface of an optical element for EUV microlithography provided with a reflective coating; and Figure 3a-c : schematic representations of possible correction strategies for the method according to Figure 2a-e .

[0032] Figure 1 shows a projection exposure system 1 for microlithography in a schematic meridional section. The projection exposure system 1 comprises an illumination system 10 and a projection system 20.

[0033] With the help of the illumination system 10, an object field 11 in an object plane or reticle plane 12 is illuminated. The illumination system 10 for this purpose comprises an exposure radiation source 13 which, in the illustrated embodiment, emits illumination radiation at least comprising useful light in the EUV range, i.e. with a wavelength between 5 nm and 30 nm, in particular with a wavelength of 13.5 nm. The exposure radiation source 13 can be a plasma source, for example an LPP source (Laser Produced Plasma, plasma generated by a laser) or a DPP source (Gas Discharged Produced Plasma, plasma generated by means of a gas discharge). It can also be a synchrotron-based radiation source. The exposure radiation source 13 can also be a free-electron laser (FEL).

[0034] The illumination radiation emanating from the exposure radiation source 13 is first bundled in a collector 14. The collector 14 can be a collector with one or more ellipsoidal and / or hyperboloidal reflection surfaces. The at least one reflection surface of the collector 14 can be exposed to the illumination radiation 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 14 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.

[0035] After the collector 14, the illumination radiation propagates through an intermediate focus in an intermediate focal plane 15. If the illumination system 10 is constructed in a modular design, the intermediate focal plane 15 can in principle be used for the - also structural - separation of the illumination system 10 into a radiation source module, comprising the exposure radiation source 13 and the collector 14, and the illumination optics 16 described below. With such a separation, the radiation source module and illumination optics 16 then together form a modular illumination system 10.

[0036] The illumination optics 16 comprises a deflecting mirror 17. The deflecting mirror 17 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 15 can be designed as a spectral filter that separates a useful light wavelength of the illumination radiation from stray light of a different wavelength.

[0037] The deflection mirror 17 deflects the radiation originating from the exposure radiation source 13 onto a first facet mirror 18. If the first facet mirror 18 is arranged - as in the present case - in a plane of the illumination optics 16 which is optically conjugated to the reticle plane 12 as a field plane, it is also referred to as a field facet mirror.

[0038] The first facet mirror 18 comprises a plurality of micromirrors (not shown in detail) that can be individually pivoted about two mutually perpendicular axes for the controllable formation of facets. The first facet mirror 18 is thus a microelectromechanical system (MEMS system), as described, for example, in DE 10 2008 009 600 A1.

[0039] In the beam path of the illumination optics 16, a second facet mirror is arranged downstream of the first facet mirror 18 19, resulting in a double-faceted system, the basic principle of which is also referred to as a honeycomb condenser (fly's eye integrator). If the second facet mirror 19—as in the illustrated embodiment—is arranged in a pupil plane of the illumination optics 16, it is also referred to as a pupil facet mirror. The second facet mirror 19 can also be arranged at a distance from a pupil plane of the illumination optics 4, whereby the combination of the first and the second facet mirror 18, 19 results in a specular reflector, as is shown, for example, in the US 2006 / 0132747 A1, EP 1 614 008 B1 and US 6,573,978.

[0040] The second facet mirror 19 also comprises a plurality of micromirrors that can be individually pivoted about two mutually perpendicular axes. For further explanation, please refer to DE 10 2008 009 600 A1.

[0041] With the aid of the second facet mirror 19, the individual facets of the first facet mirror 18 are imaged into the object field 5, whereby this usually only involves an approximate image. The second facet mirror 19 is the last beam-forming mirror or actually the last mirror for the illumination radiation in the beam path before the object field 5.

[0042] Each facet of the second facet mirror 19 is assigned to exactly one of the facets of the first facet mirror 18 to form an illumination channel for illuminating the object field 5. This can, in particular, result in illumination according to the Köhler principle.

[0043] The facets of the first facet mirror 18 are each imaged by an associated facet of the second facet mirror 19, superimposed on one another, to illuminate the object field 5. The illumination of the object field 5 is as homogeneous as possible. It preferably has a uniformity error of less than 2%. Field uniformity can be achieved by superimposing different illumination channels.

[0044] By selecting the illumination channels ultimately used, which is easily possible by suitable adjustment of the micromirrors of the first facet mirror 18, the intensity distribution in the entrance pupil of the projection system 20 described below can also be adjusted. This intensity distribution is also referred to as illumination setting. In this case, it can also be advantageous if the second facet mirror 19 is not arranged exactly in a plane which is optically conjugated to a pupil plane of the projection system 20. In particular, the pupil facet mirror 19 can be arranged with respect to a pupil plane of the projection system 20 can be arranged tilted, as described for example in DE 10 2017 220 586 A1.

[0045] In the arrangement of the components of the illumination optics 16 shown in Figure 1, the second facet mirror 19 is arranged on a surface conjugated to the entrance pupil of the projection system 20. Deflecting mirror 17 and the two facet mirrors 18, 19 are tilted both relative to the object plane 6 and relative to each other.

[0046] In an alternative, not-shown embodiment of the illumination optics 16, a transmission optics comprising one or more mirrors can be provided in the beam path between the second facet mirror 19 and the object field 11. The transmission optics can, in particular, comprise one or two mirrors for normal incidence (NI mirrors, normal incidence mirrors) and / or one or two mirrors for grazing incidence (GL mirrors, gracing incidence mirrors). With an additional transmission optics, different positions of the entrance pupil for the tangential and sagittal beam path of the projection system 20 described below can be taken into account.

[0047] Alternatively, it is possible to dispense with the deflection mirror 17 shown in Figure 1, for which purpose the facet mirrors 18, 19 are then to be suitably arranged opposite the radiation source 13 and the collector 14.

[0048] With the help of the projection system 20, the object field 11 in the reticle plane 12 is transferred to the image field 21 in the image plane 22.

[0049] The projection system 20 comprises a plurality of mirrors Mi and 25, which, according to their arrangement in the Beam path of the projection exposure system 1 are numbered.

[0050] In the example shown in Figure 1, the projection system 20 comprises six mirrors 25, M1 to M6. Alternatives with four, eight, ten, twelve, or a different number of mirrors 25, M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation, thus making the projection system 20 shown a doubly obscured optics. The projection system 20 has an image-side numerical aperture that is greater than 0.3 and can also be greater than 0.6, for example, 0.7 or 0.75.

[0051] The reflection surfaces of the mirrors 25, Mi can be designed as freeform surfaces without a rotational symmetry axis. Alternatively, the reflection surfaces of the mirrors 25, Mi can also be designed as aspherical surfaces with exactly one rotational symmetry axis of the reflection surface shape. The mirrors 25, Mi, like the mirrors of the illumination optics 16, can have highly reflective coatings for the illumination radiation. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

[0052] The projection system 20 has a large object-image offset in the y-direction between a y-coordinate of a center of the object field 11 and a y-coordinate of the center of the image field 21. This object-image offset in the y-direction can be approximately as large as a z-distance between the object plane 21 and the image plane 22.

[0053] The projection system 20 can in particular be anamorphic, ie it has in particular different image scales ß x , ß y in the x- and y-direction. The two magnifications ß x , ß y of the projection system 20 are preferably (ß x , ß y ) = (+ / - 0.25, / + - 0.125). A magnification ß of 0.25 corresponds to a reduction in the ratio 4:1, while a magnification ß of 0.125 results in a reduction in the ratio 8:1. A positive sign for the magnification ß means an image without image inversion, a negative sign an image with image inversion.

[0054] Other magnifications are also possible, including identical and absolutely identical magnifications ß x , ß y in x and y directions are possible.

[0055] The number of intermediate image planes in the x- and y-directions in the beam path between the object field 11 and the image field 21 can be the same or different, depending on the design of the projection system 20. Examples of projection systems 20 with different numbers of such intermediate images in the x- and y-directions are known from US 2018 / 0074303 A1.

[0056] The projection system 20 can, in particular, have a homocentric entrance pupil. This can be accessible. But it can also be inaccessible.

[0057] A reticle 30 (also called a mask) arranged in the object field 11 is illuminated by the illumination system 10 and transferred to the image plane 21 by the projection system 20. The reticle 30 is held by a reticle holder 31. The reticle holder 31 can be displaced, in particular in a scanning direction, via a reticle displacement drive 32. In the illustrated embodiment, the scanning direction runs in the x-direction.

[0058] A structure on the reticle 30 is imaged onto a light-sensitive layer of a wafer 35 arranged in the region of the image field 21 in the image plane 22. The wafer 35 is held by a wafer holder 36. The wafer holder 36 can be displaced, in particular along the x-direction, via a wafer displacement drive 37. The displacement of the reticle 30, on the one hand, via the reticle displacement drive 32 and, on the other hand, of the wafer 35, via the wafer displacement drive 37, can be synchronized with one another.

[0059] The projection exposure system 1 shown in Figure 1 according to the above description essentially represents known prior art.

[0060] To ensure high quality of the semiconductors to be produced using the projection exposure system 1, it is necessary for the extremely high-resolution projection system 20 to have high imaging performance and, if possible, low imaging errors. In the prior art, for example, DE 10 2011 084 117 A1, methods are known with which various imaging errors that reduce imaging performance, namely wavefront errors, can be corrected by irradiating the mirror with electron radiation. For imaging errors resulting from local elevations, the correction method according to the invention is provided, which is explained below by way of example with reference to Figures 2a-e.

[0061] The mirrors 25, Mi provided for the projection system 20 (see Figure 1) are reflective optical elements 25, in which a coating 27 reflecting EUV radiation is applied to a substrate 26. The coating 27 can be a multilayer coating, although not shown in detail.

[0062] Even before the coating 27 is applied, the surface of the substrate 26 is measured using known interferometric methods in order to determine, among other things, local elevations 40' of the substrate with an amplitude of more than 0.3 nm and a lateral extent of less than 5 mm, as shown by way of example in Figure 2a. In this process, not only is the presence of a corresponding local elevation 40' determined, but also the position of the local elevation 40' on the substrate 26 is precisely recorded.

[0063] After the surface of the substrate 26 has been coated, the surface of the optical element 25 provided with the reflective coating 27 is measured again using an interferometric method, among other things, to determine local elevations 40 (see Figure 2b). For this method step, too, only elevations with a diameter of more than 0.3 nm and a lateral extent of less than 5 mm are considered as local elevations 40 and their position is recorded.

[0064] In the illustrated embodiment, only those elevations are ultimately identified as local elevations 40 for which an elevation that is fundamentally suitable as a local elevation 40, 40' is detected at exactly the same position both during the surface measurement of the substrate 26 without coating 27 (cf. Figure 2a) and during the surface measurement of the optical element 25 after coating (cf. Figure 2b). For the question of identical position, a tolerance of + 100 / zm can generally be accepted. Elevations that are only detected in one of the two surface measurements must, if necessary, be examined more closely to determine whether they can fundamentally be corrected using the present method. If this is the case, they can be manually identified as local elevations 40. identi fi ed and treated as such in the further proceedings.

[0065] In a next step, an assessment is made as to whether the identified local elevations 40 can actually be corrected by the machine provided for this purpose (not shown) for locally densifying the substrate 26 arranged beneath the reflective coating 27 by irradiation, in particular whether a correction possible with the machine results in an improvement in the imaging performance of the optical element 25. Corresponding machines, in particular for densification using electrode beams are already known in the art, for example for correcting wavefront errors, and do not require a detailed explanation. Nevertheless, it should be briefly summarized that local irradiation (e.g. with electron beams) of the substrate 26 through the reflective coating 27, carried out in a vacuum, results in local densification ora local compaction of the substrate 26 is achieved, which is also reflected as a depression on the surface of the substrate 26 and the coating 27 located thereon. Of course, the irradiation and the substrate 26 must be coordinated in such a way that the densification or compaction can actually be achieved by irradiation; however, a variety of suitable irradiation-substrate combinations are known from the prior art.

[0066] To evaluate the correctability of a single local elevation 40, it is intended to first determine the imaging performance of the optical element 25 in the area of ​​the local elevation 40. Subsequently, the imaging performance of the optical element 25 is determined in the same area, but on the basis of an assumed correction result after a possible correction. For this purpose, it is intended to define a machining specification for the machine for the local compression of the to determine the density of the substrate 26 arranged beneath the reflective coating 27 by irradiation and to simulate the result. When simulating the irradiation process, any densification of the substrate by irradiation that has already been carried out or is still to be carried out in the region of the local elevation 40, e.g., to compensate for wavefront errors, is taken into account in order to obtain a more realistic simulation result. The simulation result thus determined is indicated in Figure 2c by the dashed lines.

[0067] If, in view of the simulation, as in the present case, an increase in the imaging performance of the optical element 25 is to be expected through the evaluated correction strategy, the local elevation 40 is considered to be correctable.

[0068] If this has not already been done in the course of the evaluation of a local elevation 40, a processing specification for the machine for the local compaction of the substrate 26 arranged under the reflective coating 27 by irradiation is determined, with which the evaluated correction can actually be implemented by the machine provided for this purpose.

[0069] This very processing specification is finally used to control the machine for the local densification of the substrate 26 arranged under the reflective coating 27 by irradiation suitable for correcting the identified local elevations 40 assessed as correctable. Based on the processing specifications, the machine will suitably irradiate the areas of local elevations 40 assessed as correctable (cf. Figure 2d) so that the substrate in this area is densified in such a way that the local elevation 40 is leveled or at least reduced (cf. Figure 2e). This results in more highly densified areas 26' in the substrate 26; however, these are irrelevant for the optical properties of the optical element 25.

[0070] Figure 2 assumes optimal correctability of the local elevation 40 shown there as an example. However, due to limitations of the machine intended for local compaction and / or other factors, it may not always be guaranteed that a complete leveling of a local elevation 40, as shown in Figure 2, is possible.

[0071] It is therefore preferable, when evaluating the correctability of the local elevation 40, not to pursue only a single strategy for correcting a local elevation 40, but to consider the results of various correction strategies and then to actually implement the correction strategy that promises the best correction result.

[0072] Figure 3 shows three possible correction strategies as examples, with the respective correction strategy for a local elevation 40 being indicated as a dashed line on the left-hand side, while the right-hand side shows a possible result after application of the correction strategy.

[0073] The correction strategy according to Figure 3a basically corresponds to the correction strategy also used in Figure 2, namely to compact the substrate 26 over the entire area of ​​the local elevation 40, possibly to varying degrees, so that basically the entire elevation 40 is leveled (cf. dashed line in Figure 3a, left-hand side). Even if the latter may be successful in individual cases (cf. Figure 2), for other local elevations 40 there may be a risk that previously raised areas of the local elevation 40 are depressed to such an extent by compacting the substrate that they remain as a depression on the surface of the optical element 25.

[0074] In the correction strategy according to Figure 3b, an area larger than the actual elevation is lowered by suitable irradiation and the associated larger-area densification of the substrate 26. A possible result of such a correction strategy is shown on the right side of Figure 3b.

[0075] The correction strategy according to Figure 3c proposes to densify the substrate 26 only in those areas where the elevation deviates from the actually desired surface shape by more than a predetermined threshold value. The result of such a correction strategy is shown as an example on the right side of Figure 3c.

[0076] Depending on the location of an optical element 25 where a local elevation 40 is present and / or the actual shape of the individual local elevation 40, each of the correction strategies shown as an example in Figure 3 can, in principle, result in increased local imaging performance. The correction strategy that promises the highest resulting imaging performance is then implemented.

Claims

Patent claims Method for correcting local elevations on the surface of an optical element (25) for EUV microlithography, provided with a reflective coating (27), comprising the steps: - measuring the surface of the surface of an optical element (25) provided with the reflective coating (27); - identifying local elevations (40) on the surface provided with the reflective coating (27) based on the result of the surface measurement; - evaluating the identified local elevations (40) with regard to their correctability with a predetermined machine for the local densification of the substrate (26) arranged under the reflective coating (27) by irradiation; - Determination of processing specifications for the machine for the local densification of the substrate (26) arranged under the reflective coating (27) by irradiation, at least for the local elevations (40) assessed as correctable; and - Controlling the machine for the local densification of the substrate (26) arranged under the reflective coating (27) by irradiation according to the processing specifications for the correction of the identified local elevations (40) assessed as correctable. Method according to claim 1, characterized in that comprising evaluating the local elevations (40) with regard to their correctability based on the local imaging performance with the existing elevation (40) and / or after correction possible with the machine for the local densification of the substrate (26) arranged beneath the reflective coating (27) by irradiation. Method according to one of the preceding claims, characterized in that evaluating the local elevations (40) with regard to their correctability comprises checking several different strategies for correcting a local elevation (40). Method according to one of the preceding claims, characterized in that evaluating the local elevations (40) with regard to their correctability, determining processing specifications and / or controlling the machine takes place taking into account densifications of the substrate (26) arranged beneath the reflective coating (27) by irradiation that have already taken place or are still to be carried out.Method according to one of the preceding claims, characterized in that only those local elevations (40) are identified on the basis of the result of the surface measurement of the surface provided with the reflective coating (27) for which an elevation (40') can be determined at the same position, preferably with a position tolerance of maximum + 100 µm, on the basis of a surface measurement of the substrate (26) to be coated carried out before coating with the reflective coating (27). Method according to one of the preceding claims, characterized in that the surface measurement of the surface provided with the reflective coating (27) and / or of the substrate (26) arranged beneath the reflective coating (27) prior to its coating comprises an interferometric method for surface measurement. Method according to one of the preceding claims, characterized in that the radiation provided for compacting the substrate (26) lying beneath the reflective surface comprises electron beams. Method according to one of the preceding claims, characterized in that an elevation (40) is considered local if its lateral extent is less than 5 mm and / or its amplitude is greater than 0.3 nm.

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