Method for correcting a reflective optical element for grazing incidence and method for correcting an optical system

Abstract

The invention relates to a method for correcting an optical system having at least one reflective optical element for grazing incidence of radiation at an operating wavelength from the EUV wavelength range with a reflection layer on a substrate, wherein an actual value of an optical property of the optical system is determined and is compared to a target value of this optical property, and if the actual value deviates from the target value, a specification of the at least one reflective optical element is defined in such a way that given an appropriate correction of the reflective optical element, the optical system reaches this target value of the optical property, with the at least one reflective optical element being corrected in such a way that it reaches the target value, and / or the at least one reflective optical element is replaced with a further reflective optical element for grazing incidence of the defined specification.

Description

Method for correcting a reflective optical element for grazing incidence and method for correcting an optical system

[0001] The present invention relates to methods for correcting a reflective optical element for grazing incidence of radiation at an operating wavelength from the EUV wavelength range, comprising a reflection layer on a substrate, and to a method for correcting an optical system having at least one reflective optical element for grazing incidence of radiation at an operating wavelength from the EUV wavelength range, comprising a reflection layer on a substrate. This application claims priority from German patent application 10 2024 203 437.1 of April 15, 2025, which is incorporated herein by reference.

[0002] In EUV lithography devices, reflective optical elements for the extreme ultraviolet (EUV) wavelength range (e.g. wavelengths between approximately 5 nm and 20 nm) are used for the lithography of semiconductor components, such as photomasks or mirrors based on multilayer systems for quasi-normal incidence or mirrors with a metallic surface for grazing incidence.

[0003] Reflective optical elements for grazing radiation are based on the effect of total internal reflection. To achieve high reflectivity in the EUV wavelength range, a reflective layer is typically applied to a substrate, preferably containing one or more transition metals. Ruthenium, molybdenum, niobium, and palladium have proven particularly effective.

[0004] Reflective optical elements for grazing incidence exhibit a highly angle-dependent reflectivity. Since the angles of incidence across the surface of a reflective optical element for grazing incidence are generally not constant in an optical system, such as an illumination system or a projection system of an EUV lithography device, the reflectivity is inhomogeneous across the surface, which can lead to imaging errors, particularly telecentricity errors in EUV lithography devices due to a tilting effect in the optical pupil in the scanning direction.

[0005] Typically, attempts are made to correct telecentricity errors in optical systems by compensating for the tilt effect. Various approaches are described in DE 10 2012 202 675 A1. In particular, different reflective optical elements, especially for grazing incidence, are compensated for each other. However, this requires that very small tolerances are maintained for the reflective coatings. Furthermore, it can happen that, over time, a reflective optical element no longer falls within the required tolerance due to radiation exposure during operation.

[0006] It is an object of the present invention to show a possibility of correcting reflective optical elements for grazing incidence or an optical system with such an element.

[0007] In a first aspect, the problem is solved by a method for correcting a reflective optical element for grazing incidence of radiation at an operating wavelength from the EUV wavelength range, comprising a reflective layer on a substrate, in which the reflective layer is doped. By subsequently incorporating foreign atoms into the reflective layer, the complex refractive index at the operating wavelength is influenced with regard to both the refractive index and the absorption, and thus the reflectivity. The doping can be carried out selectively, over one or more partial areas, or over the entire reflective layer. Particularly in the case of larger-area reflective optical elements, the respective reflective optical element can be corrected in a particularly targeted manner with regard to an optical property of an optical system by selectively or partially doping the surface of the reflective layer.

[0008] The reflective layer is preferably doped by ion bombardment. When bombarding layers with ions, process parameters can be adjusted and controlled very precisely, and spatially resolved work can be carried out, allowing the change in reflection by doping to be influenced very precisely, even across a large area.

[0009] The reflective layer is advantageously doped with nitrogen or metal atoms. Reflective layers of reflective optical elements for grazing incidence in the EUV wavelength range preferably contain one or more transition metals. Doping such a reflective layer in particular allows for targeted and precise control of reflectivity. Furthermore, nitrogen and metals are particularly easy to ionize, allowing doping to be carried out by ion bombardment.

[0010] In a second aspect, the object is achieved by a method for correcting a reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range with a reflection layer on a substrate, in which the structure of the reflection layer is changed.

[0011] If the structure of the reflective layer is changed, its roughness usually also changes. For reflective optical elements designed for grazing incidence of EUV radiation, the root-mean-squared roughness (rms) is important, especially in the nanometer range. If the roughness increases in this range, more scattered radiation is generated upon irradiation and the reflectivity decreases. If the structure of the reflective layer is changed only locally, the reflectivity of the reflective optical element can be influenced locally.

[0012] Preferably, the reflective layer is heated or energy is otherwise introduced into the reflective layer. If the reflective layer contains crystallites, these typically grow upon energy input. This can lead to roughening and thus reduced reflectivity due to changes in grain size. With more complex heat treatments, the structure of the reflective layer can also become more amorphous and typically exhibit slightly less roughness, which can lead to less scattered radiation and thus higher reflectivity.

[0013] Advantageously, the surface of the reflective layer is smoothed. Where the reflective layer surface is smoothed, less scattered radiation occurs, thus increasing reflectivity. Smoothing is preferably carried out using ion- or plasma-assisted methods, or by atomic layer smoothing.

[0014] In a third aspect, the problem is solved by a method for correcting a reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range, comprising a reflective layer on a substrate, wherein the reflective optical element has a metastable layer, wherein the metastable layer is converted into a more stable state. By converting a metastable layer into a more stable state, the complex refractive index can change, thus subsequently influencing the reflectivity of the optical element with this layer.

[0015] There are various types of metastable layers, which can be converted into a more stable state, particularly through the introduction of energy, such as through irradiation or heat treatment. For example, a metastable layer can be a porous layer with additional material in its pores that, for example, becomes volatile upon heating and escapes from the porous layer. Likewise, a metastable layer can be a layer with a columnar or porous structure that can be densified locally through the introduction of energy, which can also change the complex refractive index.

[0016] In preferred embodiments, the metastable layer is formed as a mixed material system, and the materials of the metastable layer are mixed, separated, or undergo a phase transition. Through targeted irradiation, the complex refractive index can be precisely influenced, even at individual points or on partial surfaces. Due to the different material distribution in the metastable and more stable states, the complex refractive index of the reflective optical element changes, and thus also its reflectivity. Depending on the materials of the mixed material system, a volume change may also occur during the transition to a more stable state, which can also contribute to the change in the complex refractive index.

[0017] In a fourth aspect, the object is achieved by a method for correcting a reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range, comprising a reflective layer on a substrate, in which method a coating is applied to the reflective layer, wherein two layers of the same material with different coating parameters are applied. The coating can be single-layered or multi-layered. By targeted local or complete coating of the reflective layer with one or more materials and constant or varying coating or layer thicknesses, the reflectivity of a reflective optical element for grazing incidence of EUV radiation can be very precisely influenced and corrected subsequently, in particular by a specific roughness distribution over the surface of the reflective layer.By applying two layers of the same material with different coating parameters, they exhibit different roughnesses inherently and / or after a smoothing process. Furthermore, by applying the two layers with complementary thickness gradients, one can obtain an overall layer with a homogeneous thickness but inhomogeneous roughness across the surface of the reflective layer.

[0018] Preferably, a protective coating system or a multilayer system is applied as the coating. If the additional coating comprises one or more materials that are inert, particularly those that act as oxidation or reduction protection or as hydrogen barriers, the lifetime of the reflective optical element after its correction can be significantly increased. In a multilayer system coating, the coating comprises alternating layers of at least two different materials with different real parts of the refractive index at a wavelength in the extreme ultraviolet wavelength range.

[0019] In a fifth aspect, the object is achieved by a method for correcting an optical system with at least one reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range with a reflection layer on a substrate, wherein an actual value of an optical property of the optical system is determined and compared with a target value of this optical property and, if the actual value deviates from the target value, a specification of the at least one reflective optical element is determined such that, with appropriate correction of the reflective optical element, the optical system achieves this target value of the optical property, wherein the at least one reflective optical element is corrected according to one of the previously explained methods in such a way thatthat it reaches the target value and / or the at least one reflective optical element is replaced by another reflective optical element of the specified specification.

[0020] This approach makes it possible to correct an optical system that exhibits a change in an optical property, particularly after a certain period of use, caused by degradation of one or more optical elements, and thus to avoid having to provide a new optical system with the desired optical property.Particular flexibility can be achieved in that the provision can be carried out either by a previously described correction of a reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range or by producing a suitable reflective optical element of this type or by stocking suitable reflective optical elements for grazing incidence, so that by exchanging a reflective optical element for grazing incidence of the optical system, this can be corrected such that the optical property again corresponds to a predetermined specification.

[0021] It should be noted that in optical systems which have more than one reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range, more than one such optical element can also be corrected and / or replaced if necessary in order for the respective optical property to again correspond to the specification to be met. In particular, in this way an optical system can be corrected in which the target value of an optical property is not achieved due to the degradation or an error of more than one optical element or one or more optical elements other than the at least one reflective optical element for grazing incidence. Advantageously, the optical element or elements, in particular reflective optical elements for grazing incidence, can be selected whose correction or replacement is required.Change can lead to a sufficient approximation to the target value of the optical property in question of the optical system with the least effort.

[0022] The replacement element can either be manufactured after the new specification for the respective reflective optical element for grazing EUV incidence has been determined, or various potential replacement elements can be manufactured in advance. To this end, existing reflective optical elements for grazing incidence can be corrected as described above. Likewise, a replacement or replacement element can be manufactured in advance to specifically adjust a specific reflectivity, possibly even spatially resolved.

[0023] In the event that the at least one reflective optical element is replaced by another reflective optical element of the specified specification, it is preferably replaced by another reflective optical element which has a different substrate roughness than the at least one reflective element and / or which has an additional layer of a specific roughness between the substrate and the reflective layer in order to specifically set a specific reflectivity, possibly also spatially resolved.

[0024] In preferred embodiments, the at least one reflective optical element for grazing incidence is one that has a reflective layer that has a reflectivity less than the maximum reflectivity within a tolerance range with a minimum and a maximum reflectivity at the operating wavelength. This makes it possible, particularly when correcting a reflective optical element for grazing incidence, to both improve and deteriorate its reflectivity, thus allowing for more flexible influence on the optical property of the optical system to be improved.

[0025] Advantageously, the optical system is corrected to achieve a desired telecentricity value. Particularly in EUV lithography of semiconductor components, a telecentricity error in the scanning direction of the lithography device is undesirable. This can be caused, among other things, by inhomogeneous reflectivity across the reflection surface of the at least one reflective optical element for grazing incidence, which can lead to a tilting effect in the optical pupil of the optical system of an EUV lithography device in the scanning direction. Other causes can be defects or lifetime problems in other optical elements of the optical system. The telecentricity value can be corrected by replacing or correcting the at least one reflective optical element. Other imaging errors that can be corrected as described include wavefront errors, image field curvatures and distortions, uniformity errors, etc.

[0026] The present invention will be explained in more detail with reference to preferred embodiments.

[0027] schematically shows the sequence of a first embodiment of a correction of an optical system, including the correction of a reflective optical element for grazing EUV incidence and

[0028] schematically shows the sequence of a second embodiment of a correction of an optical system, including the replacement of a reflective optical element for grazing EUV incidence.

[0029] In particular, optical systems for use in an EUV lithography device can have at least one reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range, wherein this reflective optical element has a reflection layer on a substrate. Due to faulty optical elements or due to changes to optical elements over the operating period, both of which can manifest themselves, for example, in changed reflectivities, changes in optical properties such as telecentricity can occur. In a first step 101, the actual value of an optical property of the optical system is first determined and, in a further step 103, compared with a target value of this optical property. These steps can be repeated as long as the actual value agrees with the target value within a predetermined tolerance.Correction is only necessary if this is no longer the case.

[0030] However, if the actual value deviates from the target value beyond the tolerance limit, a new specification is defined for the at least one reflective optical element for grazing EUV incidence (step 105), so that after a correction of this element, the actual value of the optical property again corresponds to the target value within the tolerance. In variants, new specifications can be defined, particularly for reflective optical elements that did not causally contribute to the deviation of the actual value from the target value of a specific optical property of the optical system. For optical systems that have more than one reflective optical element for grazing incidence, a new specification can also be defined for more than one of these reflective optical elements in order to correct or replace the reflective optical elements accordingly (see below).

[0031] The correction of the at least one reflective optical element for grazing incidence can be carried out in various ways (step 107: doping the reflection layer, changing the structure of the reflection layer, coating the reflection layer or converting a possibly present metastable layer into a more stable state), which can also be combined with each other.

[0032] In a first variant, the reflective layer on the substrate can be doped, for example, to influence the reflectivity with regard to telecentricity by modifying the complex refractive index accordingly, so that the new specification is met and the actual value matches the target value again. Advantageously, the reflective layer is doped by ion bombardment, which can also be used to dope the reflective layer in a targeted and precise manner, for example, with nitrogen or metal atoms, even with spatial resolution.

[0033] In a second variant, the structure of the reflective layer can be modified. A change in the layer structure usually also changes the roughness of the reflective layer. If the square roughness increases in spatial domain in the range of approximately 1 nm to 100 nm, more scattered radiation is generated upon irradiation and the reflectivity decreases. If the structure of the reflective layer is only changed locally, the reflectivity of the reflective optical element can be influenced there.

[0034] To change the structure of the reflective layer, it can be heated or energy can be introduced into it in some other way. Existing crystallites can then grow, which can lead to a change in grain size and roughening, thus reducing reflectivity. With more complex heat treatments, the structure of the reflective layer can also become more amorphous and typically exhibit slightly less roughness, which can lead to less scattered radiation and thus higher reflectivity.

[0035] The structure of the reflective layer can also be modified primarily in the surface area by smoothing the surface. Where the reflective layer surface is smoothed, less scattered radiation occurs, thus increasing reflectivity. Smoothing is preferably performed using ion- or plasma-assisted methods, or by means of atomic layer smoothing, in order to specifically and precisely influence the reflectivity of the reflective optical element with spatial resolution.

[0036] In a third variant, a metastable layer can be provided from the outset in the reflective optical element for grazing incidence, and this metastable layer can be converted into a more stable state. There are many different types of metastable layers, which can be converted into a more stable state, in particular by the introduction of energy, such as through irradiation or heat treatment. For example, a porous layer can be provided as the metastable layer, which has additional material in its pores that, for example, becomes volatile upon heating and escapes from the porous layer. Likewise, a layer with a columnar or porous structure can be provided as the metastable layer, which can also be only locally densified by the introduction of energy, whereby the complex refractive index can also be changed.

[0037] In preferred embodiments, the metastable layer is formed as a mixed material system, and the materials of the metastable layer are mixed or separated, or undergo a phase transition. Through targeted irradiation, the complex refractive index can be precisely influenced, even at individual points or on partial areas. Due to the different material distribution in the metastable and more stable states, the complex refractive index of the reflective optical element changes, and thus also its reflectivity. Depending on the materials of the mixed material system, a volume change may also occur during the transition to a more stable state, which can also contribute to the change in the complex refractive index. For example, a sequence of alternating ruthenium and silicon layers can be applied as a metastable layer.If activation energy is introduced into this multilayer film, for example by irradiation with electrons or ions or with a laser, a stable ruthenium-silicon compound is formed locally at the site of irradiation, which leads locally to a change in volume and an associated change in the complex refractive index at the working wavelength and thus to a change in reflectivity.

[0038] In a fourth variant, a coating is applied to the reflection layer to correct the at least one reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range. The coating can be single-layer or multi-layer. By targeted local or complete coating of the reflection layer with one or more materials and constant or varying coating or layer thicknesses, the reflectivity of a reflective optical element for grazing incidence of EUV radiation can be very precisely influenced and subsequently corrected. For example, two layers of the same material can be applied with different coating parameters so that they have different roughnesses per se and / or after a smoothing process.Furthermore, if these two layers are applied with a complementary thickness gradient, an overall layer with homogeneous thickness but inhomogeneous roughness over the surface of the reflective layer can be obtained.

[0039] The coating can also be applied as a protective coating system or a multilayer system. If the additional coating contains one or more inert materials, particularly those that act as oxidation or reduction protection or as hydrogen barriers, the lifetime of the reflective optical element after correction can be significantly increased. At the same time, the coating should also provide the highest possible transmission for radiation at the operating wavelength. For operating wavelengths in the EUV wavelength range, a two-layer coating consisting of a carbon layer on the reflective layer and a ruthenium layer on top, or a single-layer zirconium oxide coating, have proven effective.

[0040] In a multilayer coating system, the coating comprises alternating layers of at least two different materials with different real parts of the refractive index at a wavelength in the extreme ultraviolet wavelength range. Multilayer systems have the advantage that they can be optimized for high reflectivity specifically for narrow operating wavelength ranges, allowing the reflectivity of a reflective optical element to be precisely corrected in this way. For operating wavelengths in the EUV wavelength range, for example, multilayer systems consisting of alternating layers of molybdenum and ruthenium or of alternating ruthenium layers, each applied with different coating parameters, can be produced using any desired vapor deposition process.For example, the coating parameters can be influenced by different energies of the particles impacting the substrate or by different stoichiometries of the material to be coated, which is provided as a so-called sputtering target, for example, during sputtering. Due to the different coating parameters, the multilayer systems exhibit different densities and thus also different refractive indices.

[0041] A second exemplary embodiment for the correction of an optical system with at least one reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range, for example for use in an EUV lithography device, is shown schematically in Figure 2, wherein steps 201, 203, 205 correspond to steps 101, 103, 105 from Figure 1. To correct the optical system, in the example shown in Figure 2, the at least one reflective optical element for grazing EUV incidence is exchanged for another reflective optical element for grazing incidence of the specified specification (step 207) in order to meet the target value, for example with regard to telecentricity.

[0042] The replacement element can either be manufactured after the new specification for the respective reflective optical element for grazing EUV incidence has been determined, or various potential replacement elements can be manufactured in advance. To this end, existing reflective optical elements for grazing incidence can be corrected as described above. Likewise, a replacement or replacement element can be manufactured in advance to specifically adjust a specific reflectivity, possibly with spatial resolution.

[0043] Preferably, the at least one reflective optical element for grazing incidence is replaced with another reflective optical element of the specified specification, which has a different substrate roughness than the at least one reflective element and / or which has an additional layer of a specific roughness between the substrate and the reflective layer in order to specifically adjust a specific reflectivity, possibly also spatially resolved. This ensures that the actual value, for example, of the telecentricity, again matches the target value.

[0044] The approach illustrated here using two examples in conjunction with Figures 1 and 2 makes it possible to correct a change in an optical property in an optical system, particularly one caused by the degradation of one or more optical elements after a certain period of use, and thus avoid having to provide a new optical system with the desired optical property. In the case of optical systems that have more than one reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range, more than one such optical element can also be corrected and / or replaced if necessary. This can be particularly advantageous if the deviation from the target value of the optical property is due to the degradation or an error of more than one optical element of the optical system or not just the at least one reflective optical element for grazing incidence.

[0045] Particularly for methods for correcting an optical system in which the at least one reflective optical element for grazing incidence is corrected, the at least one reflective optical element for grazing incidence is preferably one that has a reflective layer that has a reflectivity less than the maximum reflectivity within a tolerance range with a minimum and a maximum reflectivity at the operating wavelength. In this way, particularly when correcting a reflective optical element for grazing incidence, it is possible to both improve and deteriorate its reflectivity in order to be able to influence the optical property of the optical system to be improved, such as telecentricity, more flexibly.If one or more reflective optical elements for grazing incidence are kept ready for replacement, it may also be advantageous to provide specimens with a less than optimal reflection layer, which can be changed to better or worse reflectivity by an additional correction step, such as explained in connection with step 107 of Figure 1.

Claims

Method for correcting a reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range with a reflection layer on a substrate, characterized in that the reflection layer is doped. Method according to claim 1, characterized in that the reflection layer is doped by ion bombardment. Method according to claim 1 or 2, characterized in that the reflection layer is doped with nitrogen or metal atoms. Method for correcting a reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range with a reflection layer on a substrate, characterized in that the structure of the reflection layer is changed. Method according to claim 4, characterized in that the reflection layer is heated. Method according to claim 4 or 5, characterized in that the surface of the reflection layer is smoothed. Method for correcting a reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range with a reflection layer on a substrate, wherein the reflective optical element has a metastable layer, characterized in that the metastable layer is converted into a more stable state. Method according to claim 7, wherein the metastable layer is formed as a material mixing system, characterized in that materials of the metastable layer are mixed or separated or are subjected to a phase transition. Method for correcting a reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range with a reflection layer on a substrate, in which a coating is applied to the reflection layer, characterized in that two layers of the same material with different coating parameters are applied. Method according to claim 9, characterized in that a protective layer system or a multi-layer system is applied as the coating. Method for correcting an optical system with at least one reflective optical element for grazing incidence of radiation of an operating wavelength from the EUV wavelength range with a reflection layer on a substrate, wherein an actual value of an optical property of the optical system is determined and compared with a target value of this optical property and, if the actual value deviates from the target value, a specification of the at least one reflective optical element for grazing incidence is determined such that, with appropriate correction of the reflective optical element, the optical system achieves this target value of the optical property, wherein the at least one reflective optical element is corrected according to one of claims 1 to 11 in such a way thatthat it reaches the target value and / or the at least one reflective optical element is replaced by another reflective optical element for grazing incidence of the specified specification. Method according to claim 11, wherein the at least one reflective optical element is exchanged for a further reflective optical element for grazing incidence of the specified specification and characterized in that the at least one reflective optical element is exchanged for a further reflective optical element which has a different substrate roughness than the at least one reflective element and / or which has an additional layer of a specific roughness between the substrate and the reflective layer. Method according to claim 11 or 12, characterized in that as at least one reflective optical element for grazing incidence one is used which has a reflection layer which has a reflectivity less than the maximum reflectivity within a tolerance range with a minimum and a maximum reflectivity at the working wavelength.

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