Optical element and EUV lithography system
By applying mechanical prestress to the reflective coating and doping the etchable material with non-etchable dopants, the problem of etching medium diffusion after the reflective coating is damaged is solved, thereby achieving control of the etching range and stability of the optical components.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CARL ZEISS SMT GMBH
- Filing Date
- 2024-09-30
- Publication Date
- 2026-05-29
AI Technical Summary
In the prior art, after the reflective coating is damaged, the etching medium can easily penetrate the damaged area and remove the etchable material, resulting in large-area defects on the surface of the reflective optical element. The prior art is difficult to effectively limit the spatial range of etching.
By placing the reflective coating under mechanical prestress, causing it to bend toward the substrate upon damage, the removal range of the etching medium is limited, and non-etchable dopants are added to the etchable material to form a passivation layer, preventing further etching.
It effectively limits the removal range of the etching medium, prevents under-etching around the reflective coating, and maintains the integrity and performance of the optical components.
Smart Images
Figure CN122122485A_ABST
Abstract
Description
[0001] Citation of relevant applications
[0002] This application claims priority to German patent application DE102023210486.5, dated October 24, 2023, the entire disclosure of which is incorporated herein by reference. Technical Field
[0003] This invention relates to an optical element for reflecting EUV radiation, comprising: a substrate, a reflective coating for reflecting EUV radiation, and an etchable material covered by the reflective coating and removable upon contact with an etching gas or removable upon contact with an etching gas. The invention also relates to an EUV lithography system comprising at least one such optical element. Background Technology
[0004] For the purposes of this application, "EUV lithography system" is understood to mean an optical system or optical arrangement used in EUV lithography, i.e., an optical system used in the field of EUV lithography. Besides EUV lithography equipment used to produce semiconductor components, an optical system can also be, for example, an inspection system for inspecting photomasks (hereinafter also referred to as master masks) used in EUV lithography equipment, an inspection system for inspecting semiconductor substrates to be constructed (hereinafter also referred to as wafers), or a metrology system for measuring EUV lithography equipment or its components (e.g., for measuring projection systems).
[0005] EUV radiation is understood to refer to radiation in the wavelength range of approximately 5 nm to approximately 30 nm (e.g., at 13.5 nm). Because EUV radiation is greatly absorbed by most known materials, it is typically guided through EUV lithography systems using reflective optical elements.
[0006] Optical elements used to reflect EUV radiation (hereinafter also referred to as EUV mirrors) are exposed to harsh conditions during operation in EUV lithography systems, particularly in EUV lithography equipment. For example, EUV radiation with high radiative power irradiates the reflective coating. Residual gases in the vacuum environment in which EUV mirrors typically operate (e.g., oxygen, nitrogen, hydrogen, water, and other residual gases typically found in ultra-high vacuum) can be converted into reactive substances such as ions or free radicals, for example, into hydrogen-containing plasma, by the action of EUV radiation.
[0007] Beneath the reflective coating, there may be etchable material that is etched away or removed upon contact with an etching medium (e.g., in the form of an etching gas, such as hydrogen radicals). In principle, this is not a problem if the reflective coating completely covers the etchable material without gaps. However, if the reflective coating is damaged and has localized damage sites, such as holes, the etching medium can penetrate to the etchable material beneath the reflective coating and remove it. This results in under-etching of the reflective coating at and around the location of the damage site during operation of the optical element. Under-etching can create very large defect areas on the optical surface of the reflective optics. The same applies if the surface of the reflective coating is cleaned by plasma or wet chemical methods, such as by an etching cleaning medium, such as an acid. In this case, the etching cleaning medium may also penetrate holes or other damage in the reflective coating to reach the underlying area and partially etch away the etchable material.
[0008] DE102019212736A1 describes an optical element for reflecting EUV radiation, comprising a coating that reflects EUV radiation and is covered with a capping layer. Distributed between the reflective coating and the capping layer is an intermediate layer comprising at least one reactive material that, together with an activated gas penetrating a gap in the capping layer, forms at least one reaction product sealing the gap.
[0009] DE102015225510A1 describes a mirror element comprising a substrate and a stack of layers including at least one reflective layer system. The curvature of the mirror element is generated by a non-vanishing bending force applied through the stack of layers based on a desired target curvature at a predetermined operating temperature. The resulting curvature varies by no more than 10% over a temperature range of at least 10 K.
[0010] DE102018204364A1 describes an optical configuration for EUV lithography comprising at least one component having a body having at least one surface region exposed to activated hydrogen during operation of the optical configuration. The body comprises at least one material that forms at least one volatile hydride upon contact with activated hydrogen in the surface region. Noble metal ions are implanted into the body at the surface region. The noble metal, particularly in the form of noble metal ions, typically has a strong catalytic effect on the recombination of activated hydrogen into molecular hydrogen and is intended to prevent the formation of volatile hydrides.
[0011] DE102020213639A1 describes a reflective optical element for reflecting EUV radiation, comprising a substrate and a reflective coating applied to the substrate. The reflective coating, a structured layer formed between the substrate and the reflective coating, and / or the substrate are doped with at least one noble metal. Summary of the Invention
[0012] The object of the present invention is to provide an optical element and an EUV lithography system in which the removal of etchable material is spatially limited in the event of damage to the reflective coating.
[0013] Invention Theme
[0014] According to the first aspect, this objective is achieved by an optical element of the type described at the beginning, wherein the reflective coating is under mechanical prestress so that, in the event of damage to the reflective coating, when the etchable material is removed by an etching medium, the reflective coating bends toward the substrate.
[0015] In this aspect of the invention, it is proposed to place the reflective coating under mechanical prestress to generate a bending force or bending moment. The bending force is so small that it has only a slight effect on the substrate or its surface, and even if it does, it only causes the substrate to bend negligibly. However, when the etchable material located beneath the reflective coating is removed, the bending force is sufficient to cause the reflective coating to bend toward the substrate. The prestress of the reflective coating may optionally be combined with another layer (see below) disposed between the reflective coating and the substrate to generate a force or bending moment toward the substrate.
[0016] As further described above, when the reflective coating has damaged areas (e.g., in the form of holes), the etchable material is removed. In this case, the etchable material is removed not only directly below the damaged area but also around it; that is, the reflective coating is under-etched around the damaged area. In this situation, due to bending forces, the under-etched portion of the reflective coating bends towards the substrate, and more specifically towards the cavity formed by the etchant material, thereby preventing the etching medium (e.g., in the form of plasma) from penetrating the damaged area from reaching the etching front on the removed material. If the etching medium, for example in the form of one or more etching gases, such as hydrogen plasma, no longer reaches the etching front, the lateral extent of the removal area is limited. The etchable material can in particular be a material that forms volatile hydrides with hydrogen.
[0017] In one development of this embodiment, the reflective coating is under compressive stress at least in the portion of the coating facing away from the substrate. This portion typically extends from the surface of the reflective coating away from the substrate to a predetermined thickness or depth within the reflective coating. The portion of the coating facing away from the substrate may have one or more layers under compressive stress. The compressive stress in the portion of the coating facing away from the substrate may be combined with tensile stress (see below) to bend the reflective coating toward the substrate in a bimetallic strip manner during the removal of the underlying etchable material. Instead of being generated by differential thermal expansion, the bending force or bending moment is typically generated by a combination of compressive and tensile stresses or differential compressive stresses in two or more layers, wherein layers disposed further from the substrate have greater compressive stress than layers disposed closer to the substrate.
[0018] In one development of this embodiment, the reflective coating is under tensile stress at least in the substrate-facing portion of the coating. The substrate-facing portion of the coating typically extends from the substrate-facing surface to a predetermined thickness within the reflective coating. In the substrate-facing portion of the coating, the reflective coating may have one or more layers under tensile stress. In this embodiment, the coating may consist of a portion facing away from the substrate and under compressive stress and a portion facing the substrate and under tensile stress. In this case, when the underlying etchable material is removed, the reflective coating bends towards the substrate due to mechanical prestress or bending force. In principle, additional coating portions not under tensile or compressive stress may also be located between the portions under compressive stress and the portions under tensile stress.
[0019] As further described above, at least in the coating portion facing the substrate, the reflective coating may alternatively be under compressive stress, which is less than the compressive stress in the coating portion facing away from the substrate. Similarly, in this case, the prestress of the reflective coating may cause it to bend towards the substrate when the etchable material is removed.
[0020] Whether compressive or tensile stress occurs in the first and / or second coating portions, and the magnitude of such compressive or tensile stress, may be influenced by the corresponding production or coating method during the application of the respective layers and the layer material used. In principle, there are materials that tend to generate tensile forces and materials that tend to generate compressive stresses, wherein the formation of tensile or compressive stresses can be influenced by the production method.
[0021] Reflective coatings are typically produced using physical vapor deposition (PVD) methods, such as thermal evaporation or e-beam evaporation / electron beam evaporation, ion beam and magnetron sputtering or pulsed laser deposition, or chemical vapor deposition (CVD) based methods. Other methods, such as atomic layer deposition (ALD), are also possible.
[0022] One or more materials of the coating portion facing away from the substrate can be applied, for example, using methods that typically generate compressive stress. For instance, the coating portion facing away from the substrate can be applied by sputtering, as this coating method typically generates compressive stress. The coating portion facing the substrate can be produced, for example, by electron beam evaporation, which typically generates tensile stress rather than compressive stress.
[0023] In alternative developments, the entire reflective coating is under compressive stress, and at least one layer under tensile stress is disposed between the reflective coating and the etchable material. Even if the reflective coating is under tensile stress in the substrate-facing portion of the coating, at least one layer under tensile stress can be disposed between the reflective coating and the etchable material.
[0024] As further described above, there are, in principle, materials that tend to generate tensile stress and materials that tend to generate compressive stress. The material of the layer used for the reflective coating has optical functions and therefore cannot be arbitrarily chosen. This does not apply to the material of the layer under tensile stress, as this layer typically does not perform optical functions. The material of the layer under tensile stress can be, for example, Mo, MoSi, Mo / Si multilayers, MgF2, Ni, Cr, or Cr alloys. Ideally, the layer under tensile stress should be directly adjacent to the reflective coating under compressive stress to generate the maximum possible bending force, but this is not absolutely necessary.
[0025] As an alternative to a layer under tensile stress, at least one layer under compressive stress can be disposed between the reflective coating and the etchable material, wherein the compressive stress of this layer is lower than that of the reflective coating. In this case, a bending moment can also be generated, thus creating a curvature of the reflective coating toward the substrate. However, the smaller the difference between the two compressive stresses, the smaller the bending moment, and therefore the smaller the curvature of the reflective coating. Therefore, it is generally more advantageous to use a layer under tensile stress than a layer under compressive stress.
[0026] The second aspect of the invention relates to optical elements of the type described at the beginning, wherein the etchable material is doped with a dopant that is not or cannot be removed upon contact with the etching medium, wherein the dopant preferably accumulates at the etching front of the etchable material when the etchable material is removed by the etching medium in the event of damage to the reflective coating. Since the etching medium is typically atomic hydrogen or hydrogen plasma, a suitable dopant is, in principle, any material that does not form volatile hydrides. Preferably, the material used as the dopant is one that interferes as little as possible with further process steps (cleaning, polishing, measurement, structuring, plasma treatment, coating, etc.) and is stable relative to EUV radiation and thermal load during its lifetime. The inventors have discovered that when the etchable material is removed, the dopant typically accumulates at the etching front and forms, for example, a mesh structure there, resulting in the formation of a passivation layer on the initially etched material after a short time, which prevents further removal of the material, i.e., material removal stops by etching.
[0027] In one development of this embodiment, the dopant is selected from the group consisting of: metals, oxides, preferably SiO2. x GeO x BO x AlO x TiO x TaO xNitrides, carbides, and borides. Oxides, nitrides, carbides, or borides can be stoichiometric or non-stoichiometric. Non-etchable dopants can form oxides, nitrides, carbides, or borides of etchable materials—that is, materials removed upon contact with the etching medium, such as in the case of Si. In this case, the oxide could be SiO₂. x The nitride can be SiN x The carbide can be SiC X Borides can be SiB x The metal can be a noble metal or a base metal. The oxide, nitride, carbide, or boride can be a metal oxide, metal carbide, metal nitride, or metal boride, but it can also be an oxide, nitride, carbide, or boride of a non-metallic material. It should be understood that combinations of the mentioned materials can also be used as dopants.
[0028] In one development of this embodiment, the metal is selected from the group consisting of: Ni, Ti, W, Ta, Fe, Mo, Cr, Al, Sc, V, Co, Y, Zr, Nb, Ru, Rh, Hf, Re, Os, Ir, Pt and lanthanides.
[0029] The dopant should be as insensitive to the etching medium as possible, allowing it to accumulate sufficiently on the surface at the etch front and not be removed when the etchable material is etched away. As further described above, the dopant should also be stable during EUV irradiation and interfere with subsequent process steps as little as possible.
[0030] In another embodiment, the doping concentration of the dopant is between 0.01 at% and 30 at%, preferably between 0.1 at% and 20 at%. Doping concentrations within the specified range have proven advantageous.
[0031] Depending on the material used as the dopant, higher or lower doping concentrations are possible. Typically, etchable materials can be doped with dopant at relatively high concentrations because the etchable material is located beneath the reflective coating, so the effect of the doping on the radiation absorption used does not need to be considered.
[0032] In one embodiment, the etchable material is selected from the group consisting of: amorphous silicon (aSi), silicon (Si), SiO2, Si-containing glass, Ge, GeO, B, BO, silicon-containing materials, and other materials that can typically be etched by an etching medium in the form of active hydrogen or hydrogen plasma to form volatile hydrides with hydrogen.
[0033] In another embodiment, the reflective coating has at least one damaged area where the etchable material is exposed to the surrounding environment. As further described above, the etching removal of the etchable material begins in or below the damaged area in the reflective coating and continues in the lateral direction, where the reflective coating is partially under-etched. To avoid under-etching, contact between the etching medium and the etching front is prevented, as further described above.
[0034] In another embodiment, the etchable material is contained in an intermediate layer between the reflective coating and the substrate. The intermediate layer is typically a functional layer that does not have optical functions. The intermediate layer can be, for example, an adhesion promoter layer or a structureable layer, which can be used to form, for example, grating structures.
[0035] In another embodiment, the etchable material is contained within the substrate. In this embodiment, the substrate itself has the etchable material, which is protected from the etchant or etch-removal by a reflective coating. In this case, the reflective coating is typically applied directly to the substrate. However, it is also feasible in principle for both the intermediate layer and the substrate to contain the etchable material.
[0036] In another embodiment, the reflective coating forms a multilayer coating for reflecting EUV radiation incident on a reflective optical element in a normal incidence manner, wherein the multilayer coating has alternating layers of a first material and a second material having different refractive indices.
[0037] Normal incidence of EUV radiation is generally understood as EUV radiation incident at an angle of incidence typically less than about 45° to the surface normal of the reflective optical element. Reflective multilayer coatings are typically optimized for reflection of EUV radiation at a predefined wavelength, which usually corresponds to the wavelength used in the EUV lithography system employing the optical element. For this purpose, the multilayer coating typically has multiple alternating layers composed of materials having a high real refractive index at the wavelength used and materials having a low real refractive index at the wavelength used. Materials can be, for example, silicon and molybdenum, but other material combinations are possible depending on the wavelength used.
[0038] In another embodiment, the reflective coating is configured to reflect EUV radiation incident on a reflective optical element at grazing incidence. Grazing incidence of EUV radiation is generally understood as EUV radiation incident at an angle of incidence typically greater than about 60° with respect to the surface normal of the reflective optical element. A reflective coating configured for grazing incidence typically has a maximum reflectivity at at least one angle of incidence greater than 60°. Such a reflective coating is typically formed of at least one material having a low refractive index and low absorption for EUV radiation incident at grazing incidence. In this case, the reflective coating may comprise or be formed of a metallic material, such as Mo, Ru, or Nb.
[0039] Another aspect of the invention relates to an EUV lithography system comprising: at least one optical element as described above, which is exposed to an etching medium during operation of the EUV lithography system. The EUV lithography system may be an EUV lithography apparatus for exposing wafers, or may be some other optical arrangement using EUV radiation, such as an EUV inspection system, for example for inspecting masks, wafers, etc., used in EUV lithography. The optical element may be, for example, an EUV reflector of a projection system or illumination system, such as a condenser reflector. At least during operation of the EUV lithography system, the optical element and thus the reflective coating are exposed to an etching medium, which is typically one or more etching gases present around the optical element. The etching medium may be, for example, activated hydrogen or hydrogen plasma, i.e., excited hydrogen and / or hydrogen radicals.
[0040] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention based on the accompanying drawings, which illustrate essential details of the invention and are evident from the claims. In variations of the invention, individual features may be implemented individually or as a plurality of features in any desired combination. Attached Figure Description
[0041] Exemplary embodiments are illustrated in the schematic diagrams and explained in the following description. In the drawings:
[0042] Figure 1 A schematic diagram showing the meridional section of a projection exposure device passing through EUV projection lithography.
[0043] Figure 2a , 2b Show Figure 1 A schematic diagram of an EUV reflector in a projection exposure device. The EUV reflector has a substrate and a reflective coating, which is under-etched in the damaged area.
[0044] Figure 3a , 3b Display similar to Figure 2b A schematic diagram of an EUV reflector, in which the reflective coating bends towards the substrate around the damaged area, and
[0045] Figures 4a-4c Display similar to Figure 2b A schematic diagram of an EUV reflector, in which the etchable material beneath the reflective coating is doped with non-etchable dopants.
[0046] In the following description of the figures, the same reference numerals are used for the same or functionally identical parts. Detailed Implementation
[0047] The following is for reference. Figure 1 The main components of an optical apparatus used for EUV lithography, in the form of a microlithography projection exposure apparatus 1, are described by way of example. The description of the basic setup of the projection exposure apparatus 1 and its components should not be construed as limiting.
[0048] One embodiment of the illumination system 2 of the projection exposure apparatus 1, in addition to the light source or radiation source 3, also has an illumination optical unit 4 for illuminating the object field 5 in the object plane 6. In an alternative embodiment, the light source 3 may also be provided as a module separate from the rest of the illumination system. In this case, the illumination system does not include the light source 3.
[0049] The mask master 7, arranged in the object field 5, is illuminated. The mask master 7 is held by the mask master holder 8. The mask master holder 8 can be displaced by the mask master displacement driver 9, especially in the scanning direction.
[0050] Figure 1 The Cartesian xyz coordinate system is shown for illustrative purposes. The x-direction extends perpendicularly to the plane in the figure and into the figure itself. The y-direction extends horizontally, and the z-direction extends vertically. The scan direction is... Figure 1 It extends in the y-direction. It extends perpendicularly to the object plane 6 in the z-direction.
[0051] The projection exposure apparatus 1 includes a projection system 10. The projection system 10 is used to image the object field 5 onto the image field 11 in the image plane 12. The structure on the mask master 7 is imaged onto the photosensitive layer of the wafer 13, which is disposed in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be moved by a wafer displacement driver 15, particularly in the y-direction. The displacement of the mask master 7 first by the mask master displacement driver 9 and the displacement of the wafer 13 secondarily by the wafer displacement driver 15 can be synchronized with each other.
[0052] Radiation source 3 is an EUV radiation source. Radiation source 3 specifically emits EUV radiation 16, which is also referred to below as working radiation, illumination radiation, or illumination light. Working radiation particularly has wavelengths in the range of 5 nm to 30 nm. Radiation source 3 can be a plasma source, such as a laser-generated plasma (LPP) source or a gas discharge-generated plasma (GDPP) source. It can also be a synchrotron-based radiation source. Radiation source 3 can be a free-electron laser (FEL).
[0053] Illumination radiation 16 emitted from radiation source 3 is focused by a concentrator mirror 17. The concentrator mirror 17 may be a concentrator mirror having one or more elliptical and / or hyperboloidal reflective surfaces. The illumination radiation 16 may be incident on at least one reflective surface of the concentrator mirror 17 at either grazing incidence (GI) (i.e., at an angle of incidence greater than 45°) or normal incidence (NI) (i.e., at an angle of incidence less than 45°). The concentrator mirror 17 may be structured and / or coated, primarily to optimize its reflectivity to the radiation used, and secondarily to suppress intrusive light.
[0054] Downstream of the light-collecting reflector 17, the illumination radiation 16 propagates through the intermediate focal point in the intermediate focal plane 18. The intermediate focal plane 18 can form a separation between the radiation source module, which includes the radiation source 3 and the light-collecting reflector 17, and the illumination optical unit 4.
[0055] The illumination optics unit 4 includes a deflecting mirror 19 and a first faceted mirror 20 disposed downstream therein in the beam path. The deflecting mirror 19 may be a planar deflecting mirror, or alternatively a mirror having a beam-affecting effect beyond a pure deflection effect. Alternatively or additionally, the deflecting mirror 19 may be in the form of a spectral filter that separates the wavelength of the illumination radiation 16 used from the incoming light having a wavelength deviated from it. The first faceted mirror 20 includes a plurality of individual first facets 21, which are also referred to below as field facets. Figure 1 Some of these facets 21 are shown only by way of example. In the beam path of the illumination optics unit 4, the second faceted mirror 22 is arranged downstream of the first faceted mirror 20. The second faceted mirror 22 includes a plurality of second facets 23.
[0056] The illumination optics unit 4 thus forms a two-sided facet system. This basic principle is also known as a fly-eye integrator. The second facet mirror 22 is used to image the individual first facet 21 into the object field 5. The second facet mirror 22 is the final beam-shaping mirror or, in effect, the final mirror reflecting the illumination radiation 16 in the beam path upstream of the object field 5.
[0057] The projection system 10 includes a plurality of reflectors Mi, which are numbered sequentially according to their arrangement in the beam path of the projection exposure device 1.
[0058] exist Figure 1In the example shown, the projection system 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors M1 are also possible. The penultimate mirror M5 and the last mirror M6 each have a channel opening for illumination radiation 16. The projection system 10 is a double-shielded optical unit. The projection optical unit 10 has an image-side numerical aperture greater than 0.4 or 0.5, and may also be greater than 0.6, and may be, for example, 0.7 or 0.75.
[0059] Just like the reflector of the illumination optics unit 4, the reflector Mi may have a highly reflective coating for illumination radiation 16.
[0060] Figure 2a , 2b Displayed in a greatly simplified way Figure 1 Details of the reflective optical element in the form of an EUV reflector 25 in the projection exposure apparatus 1, wherein the EUV reflector may be, for example, one of the reflectors Mi in the projection optics unit 10, the light-collecting reflector 17, or one of the reflectors 19, 20, 22 in the illumination optics unit 4. The reflector 25 has a substrate 26 and a highly reflective coating 27 for reflecting EUV radiation 16. An intermediate layer 28 covered by the reflective coating 27 is located between the reflective coating 27 and the substrate 25.
[0061] Intermediate layer 28 can be used as a structureable layer, for example, to create a grating structure (not shown), or it can perform another function. Intermediate layer 28 is formed of a material that is easily processed by etching. The material of intermediate layer 28 can be, for example, amorphous silicon, silicon, SiO2, Si-containing glass, Ge, GeO, B, BO, etc.
[0062] As in Figure 2a As can be seen in the plan view of the reflective coating 27, the reflective coating 27 has locally defined damaged areas 30 in the form of holes that extend through the entire thickness of the reflective coating 27. An etching medium 31, in the form of an etching gas surrounding the optical element 25, more specifically in the form of activated hydrogen, contacts the material 29 of the intermediate layer 28 via the damaged areas 30. The material 29 of the intermediate layer 28 is removed upon contact with the etching medium 31, i.e., it is an etchable material that reacts with the etching medium 31, and in doing so, a volatile material 32 is formed. If the etchable material 32 comprises silicon, the volatile material 32 can be, for example, a volatile hydride in the form of a SiH compound.
[0063] As in Figure 2bAs can be clearly seen, the removal of the etchable material 29 is not limited to the volume region of the intermediate layer 28 directly below the damaged area 30, but the etchable material 29 is also removed around the damaged area 30, with a lateral extent significantly larger than that of the damaged area 30. This creates an under-etched region in the form of a cavity 33 below and around the damaged area 30 of the reflective coating 27. In the example shown, the substrate 26 is made of a material that is not removed when in contact with the etching medium 31. The material of the substrate 26 can be, for example, one or more metals, such as Ni, Ti, W, Ta, Fe, Mo, Cr, Al, Sc, V, Co, Y, Zr, Nb, Ru, Rh, Hf, Re, Os, Ir, Pt, lanthanides, or one or more oxides, such as SiO2. x GeO x BO x AlO x TiO x TaO x Or one or more carbides, nitrides or borides.
[0064] In order to reduce the lateral extent of the under-etched region 33, it is necessary to prevent or reduce the contact between the etching medium 31 and the etchable material 29.
[0065] exist Figure 3a , 3b In the example shown, because the reflective coating 27 has mechanical prestress, the lateral extension of the under-etched region 33 is reduced. In the event of damage to the reflective coating 27, this mechanical prestress causes the reflective coating 27 to bend towards the substrate 26 when the etchable material 29 is removed by the etching medium 31. Figure 3a As indicated by the arrows, the mechanical prestress of the reflective coating 27 applies a bending force or bending moment to the portion of the reflective coating 27 located above the under-etched region 33. Due to the bending force or bending moment, the reflective coating 27 bends towards the substrate 26 around the damaged portion 30 and protrudes into the cavity 33 or the volume of the under-etched region. Figure 3a , 3b As can be seen, this restricts or prevents the etching medium 31 from approaching the etchable material 29.
[0066] exist Figure 3a In the example shown, the mechanical prestress is generated by a reflective coating 27 having a first coating portion 27a and a second coating portion 27b. The first coating portion 27a faces away from the substrate 26 and is under compressive stress, while the second coating portion 27b faces the substrate 26, is under tensile stress, and is directly adjacent to the first coating portion 27a in the thickness direction. The tensile and compressive stresses are generated by... Figure 3a , 3b The arrow in the text indicates this.
[0067] In the example shown, the reflective coating 27 is a multilayer coating with alternating silicon and molybdenum layers 34a, 34b for reflecting normally incident EUV radiation 16. For example, both the first coating portion 27a and the second coating portion 27b each have a pair of alternating silicon and molybdenum layers 34a, 34b, but typically the number of layer pairs is significantly greater. The reflective coating 27 may also have additional layers, such as a capping layer formed of, for example, Ru, or functional layers, such as a barrier layer, to prevent interdiffusion between the respective Si layer 34a and the adjacent Mo layer 34b.
[0068] exist Figure 3a In the example shown, the first coating portion 27a, and more specifically, layers 34a and 34b of the first coating portion 27a, is applied by means of a coating method that generates compressive stress, such as by magnetron sputtering. The second coating portion 27b, and more specifically, layers 34a and 34b thereof, is applied by means of a coating method that generates tensile stress, specifically by means of electron beam evaporation in the example shown. Alternatively, the second coating portion 27b may be subjected to a compressive stress lower than that of the first coating portion 27a.
[0069] Figure 3b The optical element 25 shown is with Figure 3a The difference in the optical element 25 shown is that the etchable material 29 is not contained in the intermediate layer, but rather within the substrate 26 itself. Figure 3b In the example shown, to combine Figure 2a , 2b and Figure 3a The described method forms the under-etched region 33. In Figure 3b In the case of the illustrated optical element 25, the reflective coating 27 is configured to reflect grazing-incident EUV radiation 16 and has only a single layer made of a metallic material (more specifically, Ru). The reflective coating 27 in the form of a Ru layer is under compressive stress overall. Below the reflective coating 27 is a layer 35 under tensile stress, which can be formed, for example, of Mo, MoSi, Mo / Si multilayers, MgF2, Ni, Cr, or Cr alloys. When the etchable material 29 is removed around the damaged area 30, the compressive stress of the reflective coating 27, together with the tensile stress of the layer 35 under tensile stress, causes the reflective coating 27 to bend toward the substrate 26 or toward the cavity 33. Alternatively, instead of the layer 35 under tensile stress, a layer under compressive stress with a lower compressive stress than that of the reflective coating 27 can be used.
[0070] Figures 4a-4c Optical element 25 is shown, which is related to Figure 3a , 3bThe optical element 25 shown differs in that the intermediate layer 28, comprising the etchable material 29, is doped with a dopant 36, which is not removed upon contact with the etching medium 31. In the example shown, if the reflective coating 27 is damaged, the dopant 36 accumulates at the etch leading edge 37 of the etchable material 29 when it is removed through the etching medium 31. Figure 4b As shown, with increasing etch removal, the thickness of the passivation layer 38 composed of dopant 36 above or below the etch front 37 increases until etch removal completely stops, as... Figure 4c As shown.
[0071] Dopant 36 can be, for example, one or more metals, particularly Ni, Ti, W, Ta, Fe, Mo, Cr, Al, Sc, V, Co, Y, Zr, Nb, Ru, Rh, Hf, Re, Os, Ir, Pt, lanthanides, or one or more oxides, particularly SiO. x GeO x BO x AlO x TiO x TaO x Or one or more nitrides, carbides or borides. The concentration of dopant 36 in the etchable material 29 is typically between 0.01 at% and 30 at% for example, between 0.1 at% and 20 at%
[0072] It should be understood that when substrate 26 has an etchable material 29, substrate 26 may also be doped with a non-etchable dopant 36. In this case, it can also be combined with... Figures 4a-4c The relevant method stops etching to remove the etchable material 29 from the substrate 26.
Claims
1. An optical element (25) for reflecting EUV radiation (16), comprising: base plate (26), A reflective coating (27) is used to reflect the EUV radiation (16). Etchable material (29), which is covered by the reflective coating (27) and removed upon contact with the etching medium (31), Its features The reflective coating (27) is under mechanical prestress so that, in the event of damage to the reflective coating (27), when the etchable material (29) is removed by the etching medium (31), the reflective coating (27) bends toward the substrate (26), wherein the reflective coating (27) is under compressive stress at least in the coating portion (27a) facing away from the substrate (26) and under tensile stress at least in the coating portion (27a) facing the substrate (26).
2. The optical element according to claim 1, wherein, At least one layer (35) under tensile stress is disposed between the reflective coating (27) and the etchable material (29).
3. The optical element according to the preamble of claim 1, Its features The etchable material (29) is doped with a dopant (36) that is not removed upon contact with the etching medium (31), wherein the dopant (36) is selected from the group consisting of base metals, oxides, preferably SiO2. x GeO x BO x AlO x TiO x TaO x Nitrides, carbides and borides, wherein, in the event of damage to the reflective coating (27), when the etchable material (29) is removed by the etching medium (31), the dopant (36) preferably accumulates at the etching front (37) of the etchable material.
4. The optical element according to claim 3, wherein, The base metal is selected from the group consisting of: Ni, Ti, W, Ta, Fe, Mo, Cr, Al, Sc, V, Co, Y, Zr, Nb, Hf, Re, and the lanthanides.
5. The optical element according to claim 3 or 4, wherein, The concentration of the dopant (36) in the etchable material (29) is between 0.01 at% and 30 at%, preferably between 0.1 at% and 20 at%.
6. The optical element according to any one of the preceding claims, wherein, The etchable material (29) is selected from the group consisting of: amorphous silicon, silicon, SiO2, Si-containing glass, Ge, GeO, B and BO.
7. The optical element according to any one of the preceding claims, wherein, The reflective coating (27) has at least one damaged area (30), and the etchable material (29) is exposed to the surrounding environment at the damaged area.
8. The optical element according to any one of the preceding claims, wherein, The etchable material (29) is contained in an intermediate layer (28) between the reflective coating (27) and the substrate (26).
9. The optical element according to any one of the preceding claims, wherein, The etchable material (29) is contained in the substrate (26).
10. The optical element according to any one of the preceding claims, wherein, The reflective coating (27) forms a multilayer coating for reflecting EUV radiation (16) incident normally onto the reflective optical element (25), wherein the multilayer coating has alternating layers (34a, 34b) composed of a first material and a second material having different refractive indices.
11. The optical element according to any one of claims 1 to 9, wherein, The reflective coating (27) is configured to reflect EUV radiation (16) that is incident on the reflective optical element (25) at grazing incidence.
12. An EUV lithography system (1), comprising: At least one optical element (25) as described in any of the preceding claims is exposed to an etching medium (31) during operation of the EUV lithography system (1).