The techniques of the present disclosure relate to a method of producing a reflective optical element for the extreme ultraviolet wavelength range, having a reflective coating in the form of a multilayer system on a substrate, wherein the multilayer system has mutually alternating layers of at least two different materials with different real parts of their refractive indexes at a wavelength in the extreme ultraviolet wavelength range, wherein a layer of one of the at least two materials forms a stack with the layer or layers arranged between the former and the closest layer of the same material with increasing distance from the substrate. The disclosed techniques also relate to a reflective optical element produced by the disclosed methods.
In EUV lithography apparatuses, reflective optical elements for the extreme ultraviolet (EUV) wavelength range (e.g., wavelengths between approximately 5 nm and 20 nm), such as photomasks or mirrors on the basis of multilayer systems, are used for the lithography of semiconductor devices. Since EUV lithography apparatuses generally have a plurality of reflective optical elements, they must have as high a reflectivity as possible to ensure sufficiently high overall reflectivity.
A. Kloidt et al., “Smoothing of interfaces in ultrathin Mo/Si multilayers by ion bombardment”, Thin Solid Films, 228 (1993) 154-157 discloses that ion-assisted polishing of layers of a periodic multilayer system in the soft x-ray wavelength range, i.e., between 0.1 nm and 5 nm, after the respective application thereof can lead to an increase in reflectivity. For this purpose, multilayer systems composed of molybdenum and silicon of 22 periods of thickness 2.6 nm were examined.
An object of the techniques of the present disclosure to provide a reflective optical element having good reflectivity.
This object may be achieved by a method of producing a reflective optical element for the extreme ultraviolet wavelength range, having a reflective coating in the form of a multilayer system on a substrate, wherein the multilayer system has mutually alternating layers of at least two different materials with different real parts of their refractive indexes at a wavelength in the extreme ultraviolet wavelength range, wherein a layer of one of the at least two materials forms a stack with the layer or layers arranged between the former and the closest layer of the same material with increasing distance from the substrate, wherein at least one layer is polished during or after deposition thereof, such that, in the resulting reflective optical element, roughness rises less significantly over all layers than in a corresponding reflective optical element with a reflective coating in the form of a multilayer system composed of unpolished layers, and more than 50 stacks are applied.
It has been found that the polishing of at least one layer and the provision of more than 50 stacks in the multilayer system that forms the reflective coating can achieve an increase in reflectivity compared to a corresponding reflective optical element having a reflective coating in the form of a multilayer system composed of unpolished layers having up to 50 stacks. The individual layers of the multilayer system having optical function may be applied by physical, chemical or physicochemical deposition.
According to some examples of the disclosed techniques, the layer thicknesses are chosen such that the thickness of at least one layer of one of the at least two materials in at least one stack differs by more than 10% from the thickness of the layer of that material in the adjacent stack(s). It has been found that, surprisingly, the increase in reflectivity achievable compared to reflective optical elements having corresponding multilayer systems composed of rough layers can be about one order of magnitude higher than in the case of reflective optical elements composed of layers, the thicknesses of which are constant from stack to stack over the entire multilayer system having optical function within the scope of manufacturing tolerances.
Accordingly, in some examples of the disclosed techniques, at least one layer in each stack is polished in order to obtain an elevated increase in reflectivity. According to other examples, polishing is performed on every single layer in order to obtain a particularly high increase in reflectivity in conjunction with a number of stacks of more than 50 stacks.
With regard to increasing reflectivity in comparison to reflective optical elements having a corresponding multilayer system composed of unpolished layers as a reflective coating with up to 50 stacks, it has been found to be advantageous when 55 to 70 stacks, preferably 60 to 70 stacks, are applied.
According to specific examples, polishing of at least one layer is conducted by ion-assisted polishing, reactive ion-assisted polishing, plasma-assisted polishing, reactive plasma-assisted polishing, bias plasma-assisted polishing, polishing via magnetron atomization with pulsed DC current, or atomic layer polishing. The polishing may be conducted either before, during or after the deposition of the at least one layer. Irrespective of the juncture at which the polishing is performed, any methods are usable, including, for example, ion-assisted polishing (see also U.S. Pat. No. 6,441,963 B2; A. Kloidt et al. (1993), “Smoothing of interfaces in ultrathin Mo/Si multilayers by ion bombardment”, Thin Solid Films 228 (1-2), 154 to 157; E. Chason et al. (1993), “Kinetics of Surface Roughening and Smoothing During Ion Sputtering”, MRS Proceedings, 317, 91), plasma-assisted polishing (see also DE 10 2015 119 325 A1), reactive ion-assisted polishing (see also Ping, Study of chemically assisted ion beam etching of GaN using HCl gas, Appl. Phys. Lett. 67 (9) 1995 1250), reactive plasma-assisted polishing (see also U.S. Pat. No. 6,858,537 B2), plasma immersion polishing (see also U.S. Pat. No. 9,190,239 B2), bias plasma-assisted polishing (see also S. Gerke et al. (2015), “Bias-plasma Assisted RF Magnetron Sputter Deposition of Hydrogen-less Amorphous Silicon”, Energy Procedia 84, 105 to 109), polishing via magnetron atomization with pulsed DC current (see also Y. Pei (2009), “Growth of nanocomposite films: From dynamic roughening to dynamic smoothening”, Acta Materialia, 57, 5156-5164), atomic layer polishing (see also U.S. Pat. No. 8,846,146 B2; Keren J. Kanarik, Samantha Tan, and Richard A. Gottscho, Atomic Layer Etching: Rethinking the Art of Etch, The Journal of Physical Chemistry Letters 2018 9 (16), 4814-4821, DOI: 10.1021/acs.jpclett.8b00997). It is optionally also possible to combine two or more polishing methods with one another and, for instance, to conduct them simultaneously or successively.
In other examples of the disclosed techniques, the object may be achieved by a reflective optical element produced by a method as described above.
It has been found that a reflective optical element for the EUV wavelength range produced in such a way may exhibit higher reflectivity compared to a corresponding reflective optical element having a multilayer system composed of unpolished layers as a reflective coating having up to 50 stacks.
In some specific examples, the reflective optical element, in at least one stack, has at least one layer of one of the at least two materials that has a thickness differing by more than 10% from the thickness of the layer of that material in the adjacent stack(s). It has been found that, surprisingly, the achievable increase in reflectivity compared with a corresponding reflective optical element composed of layers having thicknesses that are constant from stack to stack over the entire multilayer system having optical function within the scope of manufacturing tolerances can be about one order of magnitude higher compared to reflective optical elements having corresponding multilayer systems composed of rough layers.
Also in some specific examples, the reflective optical element has two stacks in which the thickness of the layer of one of the at least two materials differs by more than 10% from the thickness of the layer of that material in the respective adjacent stacks. This has the advantage of being producible with good average reflectivity with only slight changes in the coating parameters during the coating operation.
In certain examples, at least half of all stacks of the reflective optical element have at least one thickness of a layer of one of the at least two materials that differs by more than 10% from the thickness of the layer of the corresponding material in the respective adjacent stack(s). It is thus possible to provide reflective optical elements for a wide variety of different applications, especially of the optical type, in a very flexible manner.
According to still other examples, the layers of the multilayer system of the reflective optical element have a constant roughness or a roughness that decreases in the direction facing away from the substrate. It is thus possible to achieve particularly good increases in reflectivity compared to reflective optical elements having multilayer systems composed of unpolished layers and having numbers of stacks up to 50 as a reflective coating. Alternatively, the layers of the multilayer system of the reflective optical system have rising roughness in the direction facing away from the substrate, with a smaller rise in roughness than in the case of a corresponding reflective optical element composed of unpolished layers. This permits some degree of reduction in the demands on the polishing of individual layers, hence enabling reduction in the cost and inconvenience associated with the coating process, and nevertheless the finding of an increase in reflectivity. The rise may be, inter alia, linear, quadratic or exponential.
In certain examples, the reflective optical element has a roughness of not more than 0.2 nm. In the case of roughnesses of 0.2 nm or lower, the reflective optical element may have a significant increase in reflectivity compared to reflective optical elements having higher roughness and a number of stacks of 50 or lower.
In more specific examples, especially examples used in EUV lithography or in wafer or mask inspection systems, the reflective optical element includes molybdenum and silicon as the at least two materials having different real parts of their refractive indexes at a wavelength in the extreme ultraviolet wavelength range.
The disclosed techniques will be elucidated in detail with reference to working examples. The figures show:
Example of the techniques disclosed herein include techniques for producing reflective optical elements for the extreme ultraviolet wavelength range that include a reflective coating in the form of a multilayer system on a substrate. The multilayer system has mutually alternating layers of at least two different materials with different real parts of their refractive indexes at a wavelength in the extreme ultraviolet wavelength range. A layer of one of the at least two materials forms a stack with the layer or layers arranged between the former and the closest layer of the same material with increasing distance from the substrate. In addition to forming the above-described layers and layer stacks, the example method may include the following aspects:
In certain preferred embodiments, the layer thicknesses are chosen such that the thickness of the layer of one of the at least two materials in at least one stack differs by more than 10% from the thickness of the layers of that material in the adjacent stack(s).
The thicknesses of the individual layers 56, 57 and also of the repeating stacks 55 may, in the simplest case, be constant over the entire multilayer system 54 or vary over the area or the total thickness of the multilayer system 54, depending on what spectral or angle-dependent reflection profile or what maximum reflectivity at the operating wavelength is to be achieved. When the layer thicknesses over the entire multilayer system 54 are essentially constant, i.e., constant within the scope of the manufacturing tolerances, reference is also made to a period 55 rather than a stack 55. In certain examples discussed here, the layer thicknesses are chosen such that the thickness of the layer of one of the at least two materials in at least one stack 55′ differs by more than 10% from the thickness of the layers of that material in the adjacent stack(s) 55. In the example shown in
The reflection profile of the optical element 50 can also be influenced in a controlled manner by supplementing the basic structure composed of absorber 56 and spacer 57 with further more and less absorbent materials in order to increase the possible maximum reflectivity at the respective operating wavelength. To that end, absorber and/or spacer materials in some stacks can be mutually interchanged, or the stacks can be constructed from more than one absorber and/or spacer material. Furthermore, it is also possible to provide additional layers as diffusion barriers between spacer and absorber layers 57, 56. A material combination that is customary for an operating wavelength of 13.5 nm, for example, is molybdenum as the absorber material and silicon as the spacer material. A period 55 here often has a thickness of approximately 6.7 nm, with the spacer layer 57 usually being thicker than the absorber layer 56. Further customary material combinations include ruthenium/silicon or molybdenum/beryllium. Any diffusion barriers present for protection from interdiffusion may consist of, for example, carbon, boron carbide, silicon nitride, silicon carbide, or of a composition comprising one of these materials. In addition, it is also possible to provide, atop the multilayer system 54, a protective layer 53 that may also have multiple layers, in order to protect the multilayer system 54 from contamination or damage.
Typical substrate materials for reflective optical elements for EUV lithography are silicon, silicon carbide, silicon-infiltrated silicon carbide, quartz glass, titanium-doped quartz glass, glass and glass ceramic. Especially in the case of such substrate materials, it is additionally possible to provide a layer between multilayer system 54 and substrate 59 which is composed of a material having high absorption for radiation in the EUV wavelength range which is used in the operation of the reflective optical element 50 in order to protect the substrate 59 from radiation damage, such as unwanted compaction. Furthermore, the substrate can also be composed of copper, aluminum, a copper alloy, an aluminum alloy or a copper-aluminum alloy. Between substrate 59 and multilayer system 54 having optical function, there may also be one or more layers or layer systems that assume functions other than optical functions, for example compensation or reduction of layer stresses induced in the multilayer system 54 that forms a reflective coating.
In the example reflective optical element 50 of
Some embodiments with different roughness progressions will be described hereinafter by way of example, first with reference to some reflective optical elements having a purely periodic structure, i.e., consisting solely of bilayers. The examples discussed here by way of example are reflective optical elements optimized for a wavelength of 13.5 nm, as used in EUV lithography for instance, and for quasi-normal incidence, i.e., an angle of incidence of roughly 0° to the surface normal. On a substrate composed of silicon, they have bilayers of silicon as a spacer layer and molybdenum as absorber layer, with all bilayers of the respective reflective optical element being identical within the scope of manufacturing accuracy.
The example shown in
What is specifically being compared here with one another are reflective optical elements respectively having 40 to 70 bilayers, the respective layer thicknesses of which have been optimized for maximum reflectivity. The corresponding layer thicknesses are plotted in
Correspondingly, reflective optical elements having a multilayer system that forms a reflective coating have also been examined, said multilayer system having polished layers of a roughness rising in a linear manner, but with lower slope than in the case of the reflective optical comparative elements just described that have a multilayer system having rough layers that forms a reflective coating. Both roughness progressions (dotted for reflective optical elements having unpolished layers, solid for reflective optical elements having polished layers) as a function of the number of layers are shown in
In addition, reflective optical elements having roughness rising in a quadratic manner over the number of layers have also been examined, both for reflective optical comparative elements having multilayer systems composed of rough layers and for reflective optical elements having multilayer systems composed of polished layers as a reflective coating. As apparent in
As apparent from
As well as the narrowband reflective optical elements having periodic multilayer systems that have just been discussed, broadband reflective optical elements having aperiodic multilayer systems have also been examined, i.e., with multilayer systems that depart in at least one stack from periodicity that is otherwise observed.
The examples shown hereinafter are reflective optical elements in which the layers of the multilayer system have a roughness that rises in a quadratic manner in the direction facing away from the substrate, with the rise in roughness being smaller than in the case of a corresponding reflective optical element composed of unpolished layers, as in the narrowband optical elements last discussed (see also
The examples illustrated in
Corresponding reflective optical elements having 50, 55, 60 and 65 layers were also examined, but are not shown here. The broadband capacity δ thereof, defined as the quotient of the difference between maximum and minimum reflectivity on the one hand, and arithmetic average reflectivity over the entire angle range, called average reflectivity, on the other hand, is shown in
In addition, broadband reflective optical elements with a quadratic rise in roughness have also been examined, in which at least half of all stacks have at least one thickness of a layer of one of the at least two materials that differs by more than 10% from the thickness of the layer of the corresponding material in the respective adjacent stack(s). In the examples considered hereinafter, it was possible to choose the layer thicknesses completely freely. Thus, by contrast with the examples considered in connection with
It is thus surprisingly possible in the case of broadband reflective optical elements with high numbers of stacks of especially 55 to 70 stacks, preferably 60 to 70 stacks, by the polishing of layers, preferably all layers, in the application of the respective multilayer system, to achieve a greater than proportional increase in average reflectivity which is about one order of magnitude higher than in the case of narrowband reflective optical elements based on purely periodic multilayer systems.
A comparable result was also achieved in the case of broadband reflective optical elements in which the multilayer system was found to have fewer degrees of freedom than in the most recent examples from
Increases in reflectivity by layer polishing and increasing the number of layers were also observed in the case of reflective optical elements with multilayer systems based on ruthenium/silicon or on molybdenum/beryllium. It was also possible to detect the effect irrespective of whether layers were additionally provided in order to reduce interdiffusion between absorber and spacer layers or as protection on the vacuum-facing side of the respective multilayer system having optical function that forms a reflective coating.
The above description is intended by way of example only. Although the techniques are illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made within the scope and range of equivalents of the claims.
Number | Date | Country | Kind |
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10 2021 202 483.1 | Mar 2021 | DE | national |
This is a Continuation of International Application PCT/EP2022/056074, which has an international filing date of Mar. 9, 2022, and which claims the priority of German Patent Application 10 2021 202 483.1, filed Mar. 15, 2021. The disclosures of both applications are incorporated in their respective entireties into the present Continuation by reference.
Number | Date | Country | |
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Parent | PCT/EP2022/056074 | Mar 2022 | US |
Child | 18467095 | US |