Method for producing a reflective optical element

Abstract

The invention relates to a method for producing a reflective optical element for a working wavelength, in particular in the EUV wavelength range, comprising the following steps: applying a multilayer system (125) of alternately arranged layers of materials having different refractive indices at the working wavelength to a first substrate (121); removing the first substrate (121) from the multilayer system (125); applying the multilayer system (125) to a second substrate (129) in such a way that the layer applied first to the first substrate (121) is arranged furthest away from the second substrate (129). Alternatively, a second substrate can first be applied to the multilayer system and then the first substrate can be removed. In this way, a reflective optical element with increased real maximum reflectivity at the working wavelength can be obtained.

Description

Method for manufacturing a reflective optical element

[0001] The present invention relates to a method for manufacturing a reflective optical element for a working wavelength, particularly in the EUV wavelength range, and to a reflective optical element manufactured in this manner. The present application claims the priority of German application 10 2024 208 322.4 dated September 2, 2024, to which full reference is made.

[0002] Particularly at wavelengths in the EUV (ultraviolet) range, especially in the range of approximately 5 nm to 20 nm, reflective optical elements with a reflective coating based on a multilayer system of alternating layers of materials with different refractive indices at a specific wavelength are often used. These achieve theoretical maximum reflectivities of less than 76% at quasi-normal incidence. In practice, the maximum reflectivities achieved are significantly lower. In optical systems such as those used in EUV lithography devices, several reflective optical elements are typically arranged one behind the other. Therefore, even small increases in reflectivity have a greater impact on the overall reflectivity.

[0003] In the past, one approach to increasing reflectivity (Mandeep Singh et al., Proc SPIE, 3997, 412-419 (2000)) involved adding layers of another material with a low real part of the refractive index, such as rhodium, ruthenium, or strontium, to molybdenum-silicon or molybdenum-beryllium multilayer systems. The layer thicknesses were adjusted so that the additional layers partially replaced the adjacent molybdenum layers. However, it was found that when the layers were deposited in the necessary sequence, very thick interdiffusion layers formed between the additional layer and the adjacent layer of material with a high real part of the refractive index, significantly reducing reflectivity (Coloma Ribera et al., J. Appl. Phys. 120 (6), 065303 (2016)).

[0004] It is an object of the present invention to propose manufacturing methods with which reflective optical elements, in particular for the EUV wavelength range, can be produced with improved real reflectivity.

[0005] This problem is solved by a method for fabricating a reflective optical element for a working wavelength, particularly in the EUV wavelength range, comprising the steps of: - depositing a multilayer system of alternating layers of materials with different refractive indices at the working wavelength onto a first substrate; - removing the first substrate from the multilayer system; - depositing the multilayer system onto a second substrate such that the layer deposited first onto the first substrate is located furthest from the second substrate; and by a method for fabricating a reflective optical element for a working wavelength, particularly in the EUV wavelength range, comprising the steps of: - depositing a multilayer system of alternating layers of materials with different refractive indices at the working wavelength onto a first substrate;- Applying a second substrate to the multilayer system, such that the layer applied first to the first substrate is positioned furthest away from the second substrate; - Removing the first substrate from the multilayer system.

[0006] The methods proposed here are based on the approach that by applying the layers of the multilayer system in reverse order and subsequently inverting the multilayer system, reflective optical elements can be obtained, particularly for the EUV wavelength range. These elements exhibit thinner interdiffusion layers at the interfaces between layers with a lower real part of the refractive index and layers with a higher real part of the refractive index, and therefore show a smaller deviation of the real maximum reflectivity at the operating wavelength from the theoretical maximum reflectivity. The respective multilayer system functions as a reflective coating on the second substrate, thus forming part of the reflective optical element produced as proposed for a specific operating wavelength, particularly in the EUV wavelength range.

[0007] The proposed procedure involves first depositing the multilayer system onto a first substrate in the order in which its actual maximum reflectivity differs less from the theoretical maximum reflectivity than in other layer sequences. The multilayer system is then inverted and deposited onto a second substrate in reverse layer sequence. This inverted multilayer system on the second substrate yields the reflective optical element to be produced.

[0008] The proposed approach is particularly advantageous for multilayer systems whose theoretical maximum reflectivity is higher for a given layer sequence than for the corresponding reverse layer sequence, but whose real maximum reflectivity is lower when deposited in this reverse layer sequence than when deposited in the given layer sequence.

[0009] In a particularly preferred embodiment, a multilayer system is applied, comprising stacks with three layers of different materials, wherein in a stack a first and a second adjacent layer of materials with a lower real part of the refractive index at the operating wavelength are arranged closer to the first substrate, and the second layer of material with a higher real part of the refractive index at the operating wavelength is arranged away from the first substrate, in order to minimize, for example, possible interdiffusion effects between the second layer with the lower real part of the refractive index at the operating wavelength, which is adjacent to the layer with the higher real part in the stack, and thus to obtain the highest possible real reflectivity.

[0010] The proposed approach is particularly advantageous when ruthenium with a lower real part of the refractive index is used as the material for the second layer. This is because known studies have shown that, on the one hand, using ruthenium as an additional layer can significantly increase the maximum reflectivity, and on the other hand, that depositing a layer with a higher real part of the refractive index onto ruthenium results in a thinner interdiffusion layer than the reverse process.

[0011] Molybdenum is particularly preferred as the first layer material with a lower real part of the refractive index, and silicon is preferred as the layer material with a higher refractive index at the operating wavelength. In combination with ruthenium, the formation of interdiffusion during the deposition of silicon onto ruthenium is so low that the reflectivity gain from placing an additional ruthenium layer between the molybdenum and silicon layers of a stack significantly outweighs the negative effects. The resulting reflective optical element not only exhibits higher reflectivity than with the reversed deposition direction, but also higher reflectivity than a corresponding reflective optical element without an additional ruthenium layer.

[0012] Advantageously, a barrier layer is deposited at the interface between the layer with a lower real part of the refractive index and the layer with a higher real part of the refractive index, in order to further suppress the interdiffusion effect even when depositing from a layer with a lower real part onto a layer with a higher real part.

[0013] In a preferred embodiment, the first substrate is destroyed to detach it from the multilayer system. This can be achieved, for example, by dissolving the first substrate in a liquid, or by grinding, polishing, or etching it away with ions; two or more of these steps can also be combined.

[0014] It is particularly advantageous to apply an etch stop layer to the first substrate before applying the multilayer system. This helps to better prevent damage to the multilayer system when the first substrate is removed.

[0015] In another preferred variant, a release layer is applied to the first substrate before the multilayer system is deposited. By essentially destroying only the release layer through a chemical reaction or by dissolving it in a liquid, the first substrate can be removed without damage and, if necessary, reused for further coatings. In yet another approach, the release layer can be used to control the adhesion to the first substrate on the one hand and to the multilayer system on the other. In particular, if the second substrate is bonded to the multilayer system before the first substrate is removed, and the adhesion between the multilayer system and the second substrate is stronger than that between the multilayer system and the release layer, the first substrate, along with the release layer, can be removed by peeling it off.

[0016] Advantageously, the first substrate is prepared to a specific finish and / or roughness before the multilayer system is applied. This ensures that the multilayer system also achieves the desired finish and / or roughness upon deposition.

[0017] Preferably, the second substrate is coated with an adhesion promoter layer or its surface is activated before being brought into contact with the multilayer system. This can also increase the bond strength between the second substrate and the multilayer system. This allows the substrate material to be selected with regard to, for example, dimensional stability and the lowest possible coefficient of thermal expansion, even if the adhesion between the substrate-side first layer of the multilayer system and the substrate would otherwise be low. The adhesion promoter layer can be a layer with one or more layers of one or more materials that exhibit sufficient adhesion to both the substrate material and the material of the substrate-side first layer of the multilayer system. Surface activation to improve adhesion properties can be achieved, among other methods, by chemical treatment or by applying a plasma to the substrate surface.

[0018] Advantageously, the second substrate is brought into contact with the multilayer system and pre-treated to a specific fit and / or roughness so that the resulting reflective optical element can meet the desired specifications. This approach is particularly advantageous if the first substrate has also been pre-treated to a specific fit and / or roughness before the multilayer system is applied, thus ensuring the multilayer system is appropriately shaped. When the fit and / or roughness of the multilayer system and the second substrate are congruent, not only can the optical properties of the resulting reflective optical element be better achieved, but the adhesion between the multilayer system and the second substrate can also be improved.

[0019] In a preferred embodiment, a second substrate is used, the material of which is transparent at a control wavelength, and the positioning of the second substrate relative to the multilayer system is interferometrically controlled at the control wavelength. In particular, if the multilayer system and / or the second substrate exhibit surface asymmetry and / or a curved face, the correct alignment of the multilayer system and the second substrate is crucial for the quality of the resulting reflective optical element. Interferometric monitoring during the positioning of the multilayer system and the second substrate relative to each other before joining can increase the positioning precision.

[0020] In another aspect, the problem is solved by a reflective optical element for a working wavelength, particularly in the EUV wavelength range, which is manufactured according to the methods proposed here and is configured as an EUV mask or as a faceted mirror or mirror facet. A preferred embodiment of a faceted mirror is that of a so-called micromirror array or MMA mirror (MMA for MicroMirrorArray), an actuated reflective optical element, as known, for example, from WO 2024 / 033320 A1, to whose contents reference is made in full.

[0021] Particularly in the case of reflective optical elements used in optical systems with more than one such element, the reflectivity gains achieved by applying the multilayer system in a sequence optimized for lower interdiffusion and subsequently inverting the multilayer system have a particularly positive effect. Optical systems with faceted mirrors or MMA mirrors are used, for example, in illumination systems for EUV lithography. The increased reflectivity also contributes to a more efficient lithography process in EUV masks.

[0022] The present invention will be explained in more detail with reference to preferred embodiments. To this end, we will show...

[0023] [Figure 1] schematically shows a first embodiment of a method for producing a reflective optical element;

[0024] [Figure 2] schematically shows a second embodiment of a method for manufacturing a reflective optical element;

[0025] [Figure 3] a schematic view of an EUV lithography device;

[0026] [Figure 4a] a schematic view of the theoretical structure of a first variant of a multilayer system;

[0027] [Figure 4b] a schematic view of the theoretical structure of a second variant of a multilayer system;

[0028] [Figure 5a] a schematic view of the real structure of the second variant of a multilayer system in a first coating sequence;

[0029] [Figure 5b] a schematic view of the actual structure of the second variant of a multilayer system with a second coating sequence; and

[0030] [Figure 6] the reflectivity as a function of wavelength at quasi-normal incidence for various reflective optical elements.

[0031] Figure 1 schematically and exemplarily illustrates a first embodiment of a method for manufacturing a reflective optical element 100 for a working wavelength, particularly in the EUV wavelength range. In the example shown in Figure 1, a first substrate 121 with a desired shape and roughness is provided in a first step 101. Since the first substrate 121 will not be part of the reflective optical element to be manufactured, its material can be selected primarily such that, if necessary, a complex shape such as a freeform can be formed, that the surface can be brought to the lowest possible roughness, and that the first substrate 121 can be removed from the multilayer system as easily as possible at a later time without damaging the multilayer system.

[0032] In the present example, a release layer 123 is applied to the first substrate 121 in a step 103. This layer serves to remove the first substrate 121 from the multilayer system at a later time with minimal damage, so that the first substrate 121 can be reused later. For this purpose, a release layer 123 is applied that can be removed by a chemical reaction or due to its particularly high solubility in a liquid, thereby breaking the bond between the multilayer system and the first substrate 121. Alternatively, the material for the release layer could be chosen such that the first substrate can be peeled off the multilayer system at a later time. In a variant not shown, an etch stop layer is applied to the first substrate 121. To remove the first substrate 121, it is then etched away or dissolved.The etch stop layer can then be removed, for example, by polishing and / or by ion bombardment. In another variant not shown, the multilayer system is applied directly to the first substrate 121, and the first substrate 121 is completely removed to detach the multilayer system, e.g., by grinding, polishing, and / or ion beam ablation.

[0033] In step 105, a multilayer system 125 consisting of alternating layers of materials with different refractive indices at the operating wavelength is deposited onto the first substrate 121 or onto the release layer 123 located thereon. For this purpose, any known chemical and / or physical deposition processes from the gas phase can be used. The desired fit is assumed by the resulting multilayer system 125. The roughness of the first substrate 121 is also preserved to a first approximation.

[0034] In the example shown in Figure 1, in step 107 the first substrate 121 is removed non-destructively from the multilayer system 125 by dissolving the release layer 123. In a subsequent step 109, a second substrate 129 is provided in this example, which, like the first substrate 121, has a desired fit and roughness that is complementary to the fit and roughness of the multilayer system 125. In the example shown here, the second substrate 129 is provided with an adhesion promoter layer 127, for example, a material or a combination of two or more materials adapted to the material of the second substrate 129 and the adjacent layer of the multilayer system 125, or for example, an adhesive, in order to enable long-lasting adhesion of the multilayer system 125 to the second substrate 129.By selecting an adhesive with a suitable viscosity, it can be evenly distributed over the surface of the second substrate 129, which is to be bonded to the multilayer system 125. UV-curable adhesives can be used for substrate materials that are transparent to UV radiation. In other variations, cleaning the substrate surface and / or chemical and / or plasma-assisted surface activation of the second substrate 129 is sufficient, or, with suitable material selection and extremely smooth surfaces, no additional aids are necessary, as sufficient adhesion develops when the multilayer system 125 and the second substrate 129 are pressed together via van der Waals forces. Typical substrate materials for reflective optical elements for an operating wavelength in the EUV wavelength range are silicon, silicon carbide, silicon-infiltrated silicon carbide, fused silica, titanium-doped fused silica, glass, and glass-ceramics.Furthermore, a substrate can also be made of copper, aluminum, a copper alloy, an aluminum alloy, or a copper-aluminum alloy.

[0035] When positioning the multilayer system 125 and the second substrate 129 relative to each other, it can be helpful if the second substrate 129 and, if applicable, an adhesion promoter layer 127 are transparent for a control wavelength and the positioning is controlled interferometrically through the second substrate 129, so that, especially in the case of a curved pass as in the present example, the positioning can be carried out as accurately as possible.

[0036] In step 111, the multilayer system 125 is applied to the second substrate 129 such that the layer applied first to the first substrate is positioned furthest from the second substrate. The resulting reflective optical element 100, consisting of the multilayer system 125 on the second substrate 129, can now be inspected and, if necessary, cleaned or corrected in the usual manner before or after it is incorporated into an optical system.

[0037] Figure 2 schematically and by way of example shows a second embodiment of a method for producing a reflective optical element 200 for a working wavelength, particularly in the EUV wavelength range. Steps 201 to 205, in which a first substrate 221 is first provided with a release layer 123 and then with a multilayer system 225, correspond to steps 101 to 105 from Figure 1. The second embodiment shown in Figure 2 differs from the first embodiment shown in Figure 1 in that the sequence of the steps is different.In the first embodiment, the first substrate 121 is first removed from the multilayer system 125 and then the second substrate 129 is applied. In the second embodiment, the second substrate 229 with adhesion promoter layer 127 is first applied to the multilayer system 225 in steps 207 and 209, before the first substrate 221 is removed in step 211. The advantage of the second embodiment over the first is that the multilayer system 225 is always stabilized by a substrate and is therefore better protected against damage, and that it is ensured that the multilayer system is always applied to the second substrate 229 inverted relative to the coating direction.

[0038] Figure 3 schematically depicts an embodiment of an illumination system 20, which can be part of a projection exposure device for EUV lithography. A collector mirror 23 is arranged around a light source formed by a plasma droplet 22, which is excited by an infrared laser 21. To obtain high radiation intensities in the EUV wavelength range, for example around 13.5 nm, tin can be excited to a plasma using an infrared laser operating at a wavelength of 10.6 μm. In addition to radiation in the EUV wavelength range, the plasma also emits longer-wavelength radiation, for example in the UV wavelength range, particularly in the DUV wavelength range. Following the collector 23 are a field facet mirror 24 with individual field facets 12 and a pupil facet mirror 25 with individual pupil facets 13.Both the field facet mirror 23 and the pupil facet mirror 25 can be designed as MMA mirrors. The field facets 12 and the pupil facets 13 can have MEMS actuators (MEMS: Microelectromechanical System) or PMN actuators (PMN: Lead-Magnesium Niobate), as known, for example, from WO 2024 / 033320 A1. Before the radiation strikes an EUV mask 28 with a structure to be projected onto a wafer, it is deflected by a folded mirror 27. The beam 26 in the illumination system 20 strikes the individual field facets 12 of the field facet mirror 24, which directs the beam to the optical pupil of the illumination system 20.In the illustrated example 1 of the illumination system 20, the pupil facet mirror 25 is arranged at this distance, its pupil facets 13 being dimensioned and arranged such that the EUV radiation 8 reflected by the respective field facets 12 strikes the corresponding pupil facet 13. At the pupil facet mirror 25, the EUV beam 8 is directed to the EUV mask 28. Both the individual facets 12, 13 of the field and pupil facet mirrors 24, 25, and the EUV mask 28 are made of reflective optical elements, which, as proposed here, are formed by coating a multilayer system onto a first substrate and applying the multilayer system to a second substrate in the opposite direction to the coating direction.

[0039] Figure 4a shows a schematic view of the theoretical structure of a first variant of a multilayer system for a reflective optical element, specifically for a working wavelength in the EUV wavelength range and quasi-normal incidence. This multilayer system consists of numerous alternating layers of a material with a higher real part of the refractive index at the working wavelength, at which, for example, lithographic exposure is performed (also called spacer 410), and a material with a lower real part of the refractive index at the working wavelength (also called absorber 420), with each absorber-spacer pair forming a stack. This effectively simulates a crystal whose lattice planes correspond to the absorber layers at which Bragg reflection occurs.The thicknesses of the individual layers 410, 420, as well as the repeating stacks, can be constant across the entire multilayer system or vary depending on the desired spectral or angle-dependent reflection profile. For example, a common material combination for a working wavelength of 13.5 nm is molybdenum as the absorber and silicon as the spacer material. Optionally, the multilayer system can also include a protective layer, which may consist of more than one layer. For example, in a molybdenum-silicon multilayer system, the protective layer could consist of a layer of silicon nitride and a layer of ruthenium, titanium dioxide, or zirconium dioxide as a vacuum seal.

[0040] To improve the theoretically achievable maximum reflectivity at the operating wavelength, the multilayer system—as schematically illustrated in Figure 4b—includes two layers 420, 421 made of material with a lower real part of the refractive index at the operating wavelength. For molybdenum-silicon multilayer systems, for example, the theoretical maximum reflectivity can be increased (R2 > R1) by adding a ruthenium layer.

[0041] If one attempts to fabricate the multilayer system according to Figure 4b in the usual manner, it turns out that, with the necessary coating direction on a permanent substrate (indicated by arrow P1 in Figure 5a), where material from layer 521 with a lower real part of the refractive index is deposited onto layer 510 made of material with a higher real part, a significantly thicker interdiffusion layer 512 is formed than if material from layer 510 with a higher real part of the refractive index is deposited onto layer 521 made of material with a lower real part of the refractive index, as indicated by arrow P2 in Figure 5b. The reflectivity loss due to the interdiffusion layer can be considerably higher than the theoretical reflectivity gain from modifying the multilayer system by adding an additional layer made of another material with a lower real part of the refractive index.This effect is particularly pronounced in reflective optical elements for a working wavelength of around 13.5 nm with a multilayer system based on molybdenum / ruthenium / silicon layers.

[0042] In the proposed procedure, however, a multilayer system is applied to a first substrate, which has stacks with three layers of different materials, wherein in a stack a first and a second adjacent layer of materials with a lower real part of the refractive index at the operating wavelength are arranged closer to the first substrate and the second layer of material with a higher real part of the refractive index at the operating wavelength is arranged away from the first substrate, i.e. in the coating direction indicated in Figure 5b.

[0043] To limit interdiffusion at the interface between layers with a lower real part of the refractive index and layers with a higher real part of the refractive index, a barrier layer can be deposited there, for example, made of carbon or boron carbide. For example, in a reflective optical element for an operating wavelength of around 13.5 nm with a multilayer system based on molybdenum / ruthenium / silicon layers, barrier layers would advantageously be deposited at the interface between the molybdenum and silicon layers.

[0044] Figure 6 shows the reflectivity as a function of wavelength at quasi-normal incidence for various reflective optical elements at an operating wavelength of approximately 13.5 nm. The solid line represents the reflectivity of a reflective optical element with a multilayer system of alternating molybdenum and silicon layers. A molybdenum-silicon layer pair has a thickness of approximately 6.9 nm, and the multilayer system comprises 50 molybdenum-silicon layer pairs. The thickness of the interdiffusion layers between molybdenum and silicon, and silicon and molybdenum, ranges from 0.7 nm to 1.0 nm.The dashed line, on the other hand, represents the reflectivity of a reflective optical element fabricated as proposed here. This element comprises a multilayer system with 50 molybdenum-ruthenium-silicon layer triplets, each 6.9 nm thick. Silicon was deposited onto ruthenium on a first substrate, resulting in an interdiffusion layer between the silicon and ruthenium with a thickness of only about 0.5 nm. This layer was then deposited in reverse order onto a second, final substrate, and the first substrate, necessary only for deposition, was removed. This process increases the maximum reflectivity at the operating wavelength from 0.68 for the molybdenum-silicon multilayer system to 0.7 for the inverted-coated molybdenum-ruthenium-silicon multilayer system. For practical applications, e.g.,Using an EUV mask, faceted mirror, or MMA mirror also offers the advantage of increasing the spectral width of the reflection profile. The value of the full width at half maximum (FWHM) can thus be increased by approximately 12% using the method proposed here.

[0045] For comparison, the dotted line also shows the reflectivity of a conventionally manufactured reflective optical element with a multilayer system consisting of 50 molybdenum-ruthenium-silicon layer triplets, also 6.9 nm thick. In this case, ruthenium was deposited onto silicon via a substrate, resulting in an interdiffusion layer between the ruthenium and silicon with a thickness of approximately 2.3 nm. Consequently, the maximum reflectivity is only 0.55. The full width at half maximum (FWHM) is reduced by 31.5% compared to the inverted-coated molybdenum-ruthenium-silicon multilayer system.

Claims

Method for manufacturing a reflective optical element for a working wavelength, particularly in the EUV wavelength range, comprising the steps of: - depositing a multilayer system of alternating layers of materials with different refractive indices at the working wavelength onto a first substrate; - removing the first substrate from the multilayer system; - depositing the multilayer system onto a second substrate such that the layer deposited first onto the first substrate is positioned furthest away from the second substrate. Method for manufacturing a reflective optical element for a working wavelength, particularly in the EUV wavelength range, comprising the steps of: - depositing a multilayer system of alternating layers of materials with different refractive indices at the working wavelength onto a first substrate; - depositing a second substrate onto the multilayer system such that the layer deposited first onto the first substrate is located furthest away from the second substrate; - removing the first substrate from the multilayer system. Method according to claim 1 or 2, characterized in that a multilayer system is applied, comprising stacks with three layers of each different material, wherein in a stack a first and a second adjacent layer of materials with a lower real part of the refractive index at the working wavelength are arranged closer to the first substrate and the second layer of material with a higher real part of the refractive index at the working wavelength is arranged away from the first substrate. Method according to claim 3, characterized in that ruthenium is used as a material for the second layer with a lower real part of the refractive index. Method according to claim 4, characterized in that molybdenum is used as a further layer material with a lower real part of the refractive index and silicon is used as a layer material with a higher refractive index at the working wavelength. Method according to one of claims 1 to 5, characterized in that a barrier layer is deposited at the interface between the layer with a lower real part of the refractive index and the layer with a higher real part of the refractive index. Method according to one of claims 1 to 6, characterized in that the first substrate is destroyed in order to detach the first substrate from the multilayer system. Method according to claim 7, characterized in that an etch stop layer is applied to the first substrate before the multilayer system is applied. Method according to one of claims 1 to 6, characterized in that a release layer is applied to the first substrate before the multilayer system is applied. Method according to one of claims 1 to 9, characterized in that the first substrate is brought to a specific fit and / or roughness before the application of the multilayer system. Method according to one of claims 1 to 10, characterized in that the second substrate is provided with an adhesion promoter layer or its surface is activated before it is brought into contact with the multilayer system. Method according to one of claims 1 to 11, characterized in that the second substrate is brought to a specific fit and / or roughness before it is brought into contact with the multilayer system. Method according to one of claims 1 to 12, characterized in that a second substrate is applied, the material of which is transparent for a control wavelength, and that the positioning of the second substrate relative to the multilayer system is controlled interferometrically at the control wavelength. Reflective optical element for a working wavelength, particularly in the EUV wavelength range, manufactured according to one of claims 1 to 13 and designed as an EUV mask or as a faceted mirror.

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