Pellicles for use in a lithographic apparatus, method for forming the pellicles, and lithographic apparatus

Abstract

This invention discloses a method for forming a surface film for use in a lithography apparatus. The method includes: providing a porous protective film formed of a first material; applying at least one two-dimensional material layer to at least one side of the porous protective film; and applying a capping layer to the at least one two-dimensional material layer on at least one side of the porous protective film, such that the at least one two-dimensional material layer is disposed between the capping layer and the porous protective film. The at least one two-dimensional material layer serves to seal adjacent sides of the porous protective film and form a smoother and flatter outer surface of the surface film. Advantageously, this allows protection of the porous protective film from etching while reducing EUV spots, regardless of the material used for the capping layer.

Description

Technical Field

[0001] This invention relates to a coating used in lithography equipment and an associated method for forming such a coating. The invention also relates to a lithography equipment comprising a protective film (for forming an image on a substrate) disposed in the path of a radiation beam of the lithography equipment. Prior Technology

[0002] A lithography apparatus is a machine configured to apply a desired pattern onto a substrate. Lithography apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). A lithography apparatus can project a pattern at a patterning device (e.g., a mask) onto a radiation-sensitive material (resist) layer provided on a substrate.

[0003] The wavelength of radiation used by a lithography apparatus to project a pattern onto a substrate determines the minimum size of a feature that can be formed on that substrate. Compared to conventional lithography apparatus (which may (e.g.) use electromagnetic radiation with a wavelength of 193 nm), a lithography apparatus using EUV radiation with wavelengths in the range of 4 nm to 20 nm can be used to form smaller features on a substrate.

[0004] A patterning device (e.g., a mask) for applying a pattern to a radiating beam in a lithography apparatus may form part of a mask assembly. The mask assembly may include a protective film that protects the patterning device from particle contamination. The film may be supported by a film frame.

[0005] It may be necessary to provide devices and / or methods to avoid or mitigate one or more of the problems associated with prior art. Summary of the Invention

[0006] According to a first aspect of the present invention, a method for forming a film for use in a lithography device is provided, the method comprising: providing a porous protective film formed of a first material; applying at least one two-dimensional material layer to at least one side of the porous protective film; and applying a cover layer to the at least one two-dimensional material layer on at least one side of the porous protective film, such that the at least one two-dimensional material layer is disposed between the cover layer and the porous protective film.

[0007] This film is suitable for use with a photomask adjacent to an EUV lithography apparatus. In use, this (reflective) photomask is irradiated with EUV radiation, for example, from an irradiation system. It should be understood that the photomask is configured to pattern the radiation beam in the cross-section of the beam received from the irradiation system to form a patterned radiation beam. A projection system collects the (reflected) patterned radiation beam and forms a (limited diffraction) image of the photomask on a substrate (e.g., a resist-coated silicon wafer). Any contamination on the photomask will typically alter the image formed on the substrate, resulting in printing errors.

[0008] To avoid particle contamination of the zoom mask, a thin protective film (called a surface film) is known to be used to protect it. The surface film is placed in front of the zoom mask and prevents particles from landing on it. The surface film is positioned so that it is not rapidly imaged by the projection system, and therefore particles on the surface film do not interfere with the imaging process. The surface film needs to be thick enough to prevent particles from colliding with the zoom mask, which will cause unacceptable printing errors, but it should be as thin as possible to reduce the absorption of EUV radiation by the surface film.

[0009] The method of forming a film based on the first state sample is particularly advantageous, as will be discussed below.

[0010] It should be understood that, as used herein, porous membrane is intended to mean a material having an open structure such as a nanotube membrane. It should be understood that, as used herein, two-dimensional material is intended to mean a material formed of one or more single-atom layers (such as graphene). At least one two-dimensional material layer is used to seal adjacent sides of the porous membrane.

[0011] It should be understood that a non-porous protective membrane may have two generally parallel surfaces defining two opposite sides of the membrane. The volume defined by the two generally parallel surfaces is substantially occupied by the material forming the non-porous protective membrane. It should be further understood that, in contrast, a porous protective membrane includes areas occupied by material, from which the porous membrane is formed, interspersed with voids not containing material. For this porous protective membrane, two generally parallel imaginary or non-solid surfaces may define the boundaries or sides of the membrane. The volume defined by the two generally parallel imaginary surfaces is only partially occupied by the material forming the porous protective membrane. Applying at least one two-dimensional material layer to at least one side of the porous protective membrane is intended to include applying at least one two-dimensional material layer to at least one imaginary or non-solid surface defining the boundaries or sides of the porous protective membrane.

[0012] The surface film is produced according to the method of the first state sample, wherein the main body of the surface film is formed of a porous material. Advantageously, this can produce a surface film with reduced density and therefore increased transmittance to extreme ultraviolet (EUV) radiation. This is particularly important for EUV lithography systems and improves system throughput.

[0013] A particularly promising material for use as a protective film in EUV lithography equipment is a fabric or protective film formed from carbon nanotubes (CNTs). This CNT film is a porous material and therefore provides extremely high EUV transmittance (>98%). Furthermore, the CNT film offers excellent mechanical stability, allowing it to be manufactured in thin layers while remaining robust against mechanical failures. However, low-pressure hydrogen is typically supplied within lithography equipment, where it forms a hydrogen plasma in the presence of EUV radiation (during exposure). It has been found that hydrogen ions and hydrogen radicals from the hydrogen plasma can etch the CNT-formed film, thereby limiting its potential lifetime and hindering the commercial implementation of CNT films.

[0014] To mitigate such etching of CNT films, it has been previously proposed to provide such CNT films with a protective capping layer. This capping layer can be formed from a material that is chemically stable in the environment of lithography equipment and has a low extinction coefficient against EUV radiation.

[0015] However, the difference between the refractive index of carbon and a suitable capping layer is generally greater than the difference between the refractive index of carbon and vacuum. Therefore, the inventors have realized that this capping layer will cause an increase in EUV spot size, which is undesirable. It is particularly advantageous to apply at least one two-dimensional material layer to the porous protective film and then subsequently apply the capping layer to at least one two-dimensional material layer (as specified by the method according to the first state sample), as discussed below.

[0016] It should be understood that porous materials will have a structure, and therefore, if there is a large contrast between the refractive indices of the porous material and the surrounding medium, the radiation (e.g., EUV radiation) will be scattered (e.g., via Mie scattering) as it propagates through the surface. This will result in undesirable diffusion or spotting of the radiation, thus again affecting the imaging performance of the lithography equipment. Because EUV radiation is so strongly absorbed by most materials, EUV lithography systems typically operate under high vacuum. Therefore, it is particularly desirable for the porous material to be formed of a material with a refractive index close to 1. It is also desirable for the porous material to be formed of a material with the lowest possible extinction coefficient for EUV radiation.

[0017] At least one two-dimensional material layer according to the method of the first state is used to seal adjacent sides of the porous protective film and form a smoother and flatter outer surface of the film. This allows the capping layer to be provided above this smoother and flatter outer surface. Advantageously, this allows protection of the porous protective film from etching while reducing EUV spots, regardless of the material used for the capping layer. Furthermore, in addition to being significantly smoother and flatter than the surface of the porous material, the surface of the two-dimensional material will have a smaller surface area. As a result, the volume of the capping layer is reduced when the (relatively thin) capping layer is provided on the two-dimensional material rather than directly on the porous material. Advantageously, this also results in higher EUV transmittance of the film for the same thickness of the capping layer.

[0018] Within the CNT protective film, carbon nanotubes can be discrete, or alternatively, they can be clustered together. Furthermore, the size of such clusters can vary. The inventors have discovered that when the capping layer is applied directly to the CNT protective film, the loss of EUV transmittance attributable to the capping layer largely depends on the degree of clustering within the CNT protective film. For example, for a fixed density of CNTs in the protective film, the smaller the number of CNTs per cluster, the greater the loss of EUV transmittance. Advantageously, by using the method according to the first state, since the capping layer is applied to at least one two-dimensional material layer (rather than a porous protective film), the loss of EUV transmittance is no longer dependent on the typical size of the structure within the porous protective film (e.g., the amount of clustering in the case of a CNT protective film). In fact, by applying the capping layer to a two-dimensional planar layer, the loss of EUV transmittance is minimized for a given thickness of the capping layer.

[0019] Finally, the porous layer structure is sealed using at least one two-dimensional material layer of the surface film formed according to the method of the first state sample. Advantageously, this produces a higher particle stopping power than CNT surface films without such two-dimensional material layers.

[0020] A wet transfer process can be used to apply at least one two-dimensional material layer to at least one side of a porous protective membrane.

[0021] This wet transfer process is known in the art. Typically, a wet transfer process involves growing a two-dimensional material (e.g., a graphene film) on a first substrate (e.g., a copper substrate). An adhesive layer is then formed on the other side of the two-dimensional material. The adhesive layer may, for example, comprise a polymer, such as polymethyl methacrylate (PMMA). The first substrate is then removed, for example, by selective etching. For instance, ammonium persulfate can be used to remove the copper-containing first substrate. The adhesive layer and the two-dimensional material may be rinsed (e.g., in water), depending on the situation. The two-dimensional material is then applied to one side of the porous protective film. Finally, the adhesive layer is removed, for example, by selective etching.

[0022] Applying at least one two-dimensional material layer to at least one side of a porous protective membrane may include: providing at least one two-dimensional material layer on a support substrate; pressing at least one two-dimensional material layer to one side of the porous protective membrane; and removing the support substrate.

[0023] A support substrate may be contained in a sacrificial layer on its surface. At least one two-dimensional material layer may be provided on the sacrificial layer. Removing the support substrate may involve etching the sacrificial layer to remove the support substrate.

[0024] Porous protective membranes may contain nanostructures.

[0025] The porous membrane may contain nanotubes.

[0026] For example, a porous protective membrane can be a fabric formed from CNTs. This can be referred to as a carbon nanotube protective membrane (CNTm).

[0027] The porous protective membrane is essentially self-supporting.

[0028] It should be understood that, in use, the membrane will be supported around its perimeter by a membrane frame that is mounted to a magnifying glass or mask. In this context, the porous membrane is essentially self-supporting, meaning that the porous membrane supports its own weight. That is, apart from at least one two-dimensional material layer and a cover layer, there is no additional membrane adjacent to the porous membrane that provides support for it.

[0029] In some embodiments, the porous protective membrane can be considered as forming most of the thickness of the surface film.

[0030] Each or at least one two-dimensional material layer may be applied as a substantially continuous layer adjacent to at least one side of the porous protective membrane.

[0031] Two-dimensional materials may include graphene.

[0032] In some embodiments, three graphene layers may be provided adjacent to one or both sides of the porous membrane.

[0033] In one embodiment, the porous protective film may be a carbon nanotube protective film and the two-dimensional material may include graphene. One advantage of using graphene as a two-dimensional material is that the film has previously been formed from carbon, and the properties of carbon in this environment are known. For example, by using another carbon-based material such as graphene, a significant increase in EUV reflection (which can be generated by other materials) can be avoided. Furthermore, other materials may have increased susceptibility to hydrogen etching within lithography equipment.

[0034] Two-dimensional materials may contain hexagonal boron nitride (h-BN).

[0035] Two-dimensional materials may contain molybdenum disulfide (MoS₂).

[0036] Advantageously, these materials (hBN and MoS2) are robust against hydrogen etching, and therefore for embodiments in which the two-dimensional material comprises hexagonal boron nitride (hBN) and / or molybdenum disulfide (MoS2), a capping layer with a smaller thickness can be applied.

[0037] In some embodiments, at least one two-dimensional material layer may be applied to both sides of the porous protective membrane, and a cover layer may be applied to each side of the surface membrane, such that at least one two-dimensional material layer is disposed between the cover layer and the porous protective membrane.

[0038] It should be understood that different types of two-dimensional materials can be provided on different sides of porous membranes.

[0039] Each of these capping layers can be a three-dimensional material.

[0040] Advantageously, three-dimensional materials are significantly easier to manufacture than two-dimensional materials. As discussed above, two-dimensional materials effectively seal the structure of porous protective films. This allows three-dimensional materials to be used as capping layers while enjoying the benefits of a second-state film, as discussed above.

[0041] The total EUV transmittance of each or every cover layer can be 96% or greater.

[0042] It should be understood that, unless otherwise stated, the total EUV transmittance of the capping layer herein means the percentage of EUV radiation transmitted after propagation through the membrane. For embodiments where the capping layer is provided on each side of the membrane, the total EUV transmittance of the capping layer means the total transmittance from both sides.

[0043] In some embodiments, the total EUV transmittance of at least one cover layer may be 96.5% or greater. In some embodiments, the total EUV transmittance of at least one cover layer may be 97% or greater. In some embodiments, the total EUV transmittance of at least one cover layer may be 97.5% or greater. In some embodiments, the total EUV transmittance of at least one cover layer is approximately 97.8%.

[0044] It should be understood that, generally speaking, the EUV transmittance of a capping layer depends on (a) the type of material forming the capping layer; and (b) the thickness of the capping layer. It should also be understood that, generally speaking, the EUV transmittance of a capping layer also depends on the density or porosity of the capping layer. Example materials are discussed below.

[0045] At least one capping layer may be suitable for protecting the porous layer and at least one two-dimensional material layer from hydrogen etching.

[0046] It should be understood that, in order to adequately protect the other two layers from hydrogen etching, the capping layer (a) may be formed of a suitable material that has not been subjected to strong hydrogen etching; and (b) may have a suitable thickness. Example materials are discussed below.

[0047] At least one cover layer may be formed of a material having an extinction coefficient of less than 0.02 nm⁻¹ for EUV radiation.

[0048] It is necessary to minimize the absorption of EUV radiation by the coating. Therefore, it is generally desirable to form a capping layer system from a material with the lowest extinction coefficient against EUV radiation.

[0049] In some embodiments, the cover layer is formed of a material having an extinction coefficient of less than 0.01 nm⁻¹ for EUV radiation. In some embodiments, at least one cover layer is formed of a material having an extinction coefficient of less than 0.005 nm⁻¹ for EUV radiation.

[0050] The capping layer can have a thickness of approximately 0.3 nm to 5 nm.

[0051] The capping layer may contain yttrium or yttrium oxide.

[0052] Yttrium has an extinction coefficient of approximately 0.0021 nm⁻¹ for EUV radiation. Yttrium oxide (Y₂O₃) has an extinction coefficient of approximately 0.01 nm⁻¹ for EUV radiation.

[0053] The capping layer may contain any of the following: aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), ruthenium (Ru), platinum (Pt), gold (Au), zirconium nitride (ZrN), aluminum (Al), or zirconium (Zr).

[0054] The method may further include attaching the membrane boundary to the periphery of the porous protective membrane.

[0055] Before applying at least one two-dimensional material layer to at least one side of the porous protective membrane, the membrane boundary may be attached to the periphery of the porous protective membrane.

[0056] According to a second aspect of the present invention, a film for use in a lithography device is provided, the film comprising: a porous protective film formed of a first material; at least one two-dimensional material layer adjacent to at least one side of the porous protective film; and at least one cover layer adjacent to the at least one two-dimensional material layer, such that the at least one two-dimensional material layer is disposed between the cover layer and the porous protective film.

[0057] The film of the second state sample according to the present invention can be formed using the method of the first state sample according to the present invention. The film of the second state sample according to the present invention can have any feature that can be produced by any feature of the method of the first state sample according to the present invention.

[0058] This film is suitable for use adjacent to a photomask within an EUV lithography apparatus. In use, this (reflective) photomask is irradiated with EUV radiation, for example, from an irradiation system. It should be understood that the photomask is configured to pattern the radiation beam in the cross-section of the beam received from the irradiation system to form a patterned radiation beam. A projection system collects the (reflected) patterned radiation beam and forms a (limited diffraction) image of the photomask on a substrate (e.g., a resist-coated silicon wafer). Any contamination on the photomask will typically alter the image formed on the substrate, resulting in printing errors.

[0059] To avoid particle contamination of the zoom mask, a thin protective film (called a surface film) is known to be used to protect it. The surface film is placed in front of the zoom mask and prevents particles from landing on it. The surface film is positioned so that it is not rapidly imaged by the projection system, and therefore particles on the surface film do not interfere with the imaging process. The surface film needs to be thick enough to prevent particles from colliding with the zoom mask, which will cause unacceptable printing errors, but it should be as thin as possible to reduce the absorption of EUV radiation by the surface film.

[0060] The second-state membrane system is particularly advantageous, as will be discussed below.

[0061] It should be understood that, as used herein, porous membrane is intended to mean a material having an open structure such as a nanotube membrane. It should be understood that, as used herein, two-dimensional material is intended to mean a material formed of one or more single-atom layers (such as graphene). At least one two-dimensional material layer is used to seal adjacent sides of the porous membrane.

[0062] The second-state surface film allows the main body of the surface film to be formed from a porous material. Advantageously, this can produce a surface film with reduced density and therefore increased transmittance to extreme ultraviolet (EUV) radiation. This is particularly important for EUV lithography systems and improves system throughput.

[0063] It should be understood that porous materials will have a structure, and therefore, if there is a large contrast between the refractive indices of the porous material and the surrounding medium, the radiation (e.g., EUV radiation) will be scattered (e.g., via Mie scattering) as it propagates through the surface. This will result in undesirable diffusion or spotting of the radiation, thus again affecting the imaging performance of the lithography equipment. Because EUV radiation is so strongly absorbed by most materials, EUV lithography systems typically operate under high vacuum. Therefore, it is particularly desirable for the porous material to be formed of a material with a refractive index close to 1. It is also desirable for the porous material to be formed of a material with the lowest possible extinction coefficient for EUV radiation.

[0064] A particularly promising material for use as a protective film in EUV lithography equipment is a fabric or protective film formed from carbon nanotubes (CNTs). This CNT film is a porous material and therefore provides extremely high EUV transmittance (>98%). Furthermore, the CNT film offers excellent mechanical stability, allowing it to be manufactured in thin layers while remaining robust against mechanical failures. However, low-pressure hydrogen is typically supplied within lithography equipment, where it forms a hydrogen plasma in the presence of EUV radiation (during exposure). It has been found that hydrogen ions and hydrogen radicals from the hydrogen plasma can etch the CNT-formed film, thereby limiting its potential lifetime and hindering the commercial implementation of CNT films.

[0065] To mitigate such etching of CNT coatings, it has been previously proposed to provide such CNT coatings with a protective capping layer. However, the difference between the refractive index of carbon and a suitable capping layer is generally greater than the difference between the refractive index of carbon and vacuum. Therefore, this capping layer will cause an increase in EUV spot, which is undesirable. According to the second state, at least one two-dimensional material layer of the coating is used to seal adjacent sides of the porous protective film and form a smoother and flatter outer surface of the coating. This allows the capping layer to be provided above this smoother and flatter outer surface. Advantageously, this allows protection of the porous protective film from etching while reducing EUV spot, regardless of the material used for the capping layer. Furthermore, in addition to being significantly smoother and flatter than the surface of the porous material, the surface of the two-dimensional material will have a smaller surface area. As a result, the volume of the capping layer is reduced when the (relatively thin) capping layer is provided on the two-dimensional material rather than directly on the porous material. Advantageously, this also results in higher EUV transmittance of the coating for the same thickness of the capping layer.

[0066] Within the CNT protective film, carbon nanotubes can be discrete, or alternatively, they can be clustered together. Furthermore, the size of such clusters can vary. The inventors have discovered that when the capping layer is applied directly to the CNT protective film, the loss of EUV transmittance attributable to the capping layer largely depends on the degree of clustering within the CNT protective film. For example, for a fixed density of CNTs in the protective film, the smaller the number of CNTs per cluster, the greater the loss of EUV transmittance. Advantageously, in the case of the film according to the second state, since the capping layer is applied to at least one two-dimensional material layer (rather than a porous protective film), the loss of EUV transmittance is no longer dependent on the typical size of the structure within the porous protective film (e.g., the amount of clustering in the case of the CNT protective film). In fact, by applying the capping layer to a two-dimensional planar layer, the loss of EUV transmittance is minimized for a given thickness of the capping layer.

[0067] Finally, the porous layer structure is sealed by at least one two-dimensional material layer of the surface film of the second state sample. Advantageously, this produces a higher particle stopping power than CNT surface films without such two-dimensional material layers.

[0068] Porous protective membranes may contain nanostructures.

[0069] The porous membrane may contain nanotubes.

[0070] For example, a porous protective membrane can be a fabric formed from CNTs. This can be called a carbon nanotube protective membrane.

[0071] The porous protective membrane is essentially self-supporting.

[0072] It should be understood that in use, the membrane will be supported around its perimeter by a membrane frame that is mounted to a magnifying glass or mask. In this context, the porous membrane is essentially self-supporting, meaning that the porous membrane supports its own weight. That is, apart from at least one two-dimensional material layer, there is no additional membrane adjacent to the porous membrane that provides support for it.

[0073] The porous protective film can be considered as the majority of the thickness of the surface film.

[0074] Each or at least one two-dimensional material layer may form a substantially continuous layer adjacent to at least one side of the porous protective membrane.

[0075] Two-dimensional materials may include graphene.

[0076] In some embodiments, three graphene layers may be provided adjacent to one or both sides of the porous membrane.

[0077] In one embodiment, the porous protective film may be a carbon nanotube protective film and the two-dimensional material may include graphene. One advantage of using graphene as a two-dimensional material is that the film has previously been formed from carbon, and the properties of carbon in this environment are known. For example, by using another carbon-based material such as graphene, a significant increase in EUV reflection (which can be generated by other materials) can be avoided. Furthermore, other materials may have increased susceptibility to hydrogen etching within lithography equipment.

[0078] Two-dimensional materials may contain hexagonal boron nitride (h-BN).

[0079] Two-dimensional materials may contain molybdenum disulfide (MoS₂).

[0080] Advantageously, these materials (hBN and MoS2) are robust against hydrogen etching, and therefore for embodiments in which the two-dimensional material comprises hexagonal boron nitride (hBN) and / or molybdenum disulfide (MoS2), a capping layer with a smaller thickness can be applied.

[0081] In some embodiments, at least one two-dimensional material layer may be provided adjacent to both sides of the porous protective membrane, and a cover layer may be provided on each side of the surface membrane such that at least one two-dimensional material layer is disposed between the cover layer and the porous protective membrane.

[0082] It should be understood that different types of two-dimensional materials can be provided on different sides of porous membranes.

[0083] Each of these capping layers can be a three-dimensional material.

[0084] Advantageously, three-dimensional materials are significantly easier to manufacture than two-dimensional materials. As discussed above, two-dimensional materials effectively seal the structure of porous protective films. This allows three-dimensional materials to be used as capping layers while enjoying the benefits of a second-state film, as discussed above.

[0085] The total EUV transmittance of each or every cover layer can be 96% or greater.

[0086] It should be understood that, unless otherwise stated, the total EUV transmittance of the capping layer herein means the percentage of EUV radiation transmitted after propagation through the membrane. For embodiments where the capping layer is provided on each side of the membrane, the total EUV transmittance of the capping layer means the total transmittance from both sides.

[0087] In some embodiments, the total EUV transmittance of at least one cover layer may be 96.5% or greater. In some embodiments, the total EUV transmittance of at least one cover layer may be 97% or greater. In some embodiments, the total EUV transmittance of at least one cover layer may be 97.5% or greater. In some embodiments, the total EUV transmittance of at least one cover layer is approximately 97.8%.

[0088] It should be understood that, generally speaking, the EUV transmittance of a capping layer depends on (a) the type of material forming the capping layer; and (b) the thickness of the capping layer. It should also be understood that, generally speaking, the EUV transmittance of a capping layer also depends on the density or porosity of the capping layer. Example materials are discussed below.

[0089] At least one capping layer may be suitable for protecting the porous layer and at least one two-dimensional material layer from hydrogen etching.

[0090] It should be understood that, in order to adequately protect the other two layers from hydrogen etching, the capping layer (a) may be formed of a suitable material that has not been subjected to strong hydrogen etching; and (b) may have a suitable thickness. Example materials are discussed below.

[0091] At least one cover layer may be formed of a material having an extinction coefficient of less than 0.02 nm⁻¹ for EUV radiation.

[0092] It is necessary to minimize the absorption of EUV radiation by the coating. Therefore, it is generally desirable to form a capping layer system from a material with the lowest extinction coefficient against EUV radiation.

[0093] In some embodiments, the cover layer is formed of a material having an extinction coefficient of less than 0.01 nm⁻¹ for EUV radiation. In some embodiments, at least one cover layer is formed of a material having an extinction coefficient of less than 0.005 nm⁻¹ for EUV radiation.

[0094] The capping layer can have a thickness of approximately 0.3 nm to 5 nm.

[0095] The capping layer may contain yttrium or yttrium oxide.

[0096] Yttrium has an extinction coefficient of approximately 0.0021 nm⁻¹ for EUV radiation. Yttrium oxide (Y₂O₃) has an extinction coefficient of approximately 0.01 nm⁻¹ for EUV radiation.

[0097] The capping layer may contain any of the following: aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), ruthenium (Ru), platinum (Pt), gold (Au), zirconium nitride (ZrN), aluminum (Al), or zirconium (Zr).

[0098] The cover layer may contain multiple sublayers made of different materials.

[0099] The coating may be further included at the periphery of the porous protective membrane at the coating boundary.

[0100] According to a third aspect of the present invention, a lithography apparatus is provided, which can be used to form an image of a patterned device on a substrate using a radiation beam. The lithography apparatus includes a protective film disposed in a path of the radiation beam. The protective film includes: a porous protective film formed of a first material; at least one two-dimensional material layer adjacent to at least one side of the porous protective film; and at least one cover layer adjacent to the at least one two-dimensional material layer, such that the at least one two-dimensional material layer is disposed between the cover layer and the porous protective film.

[0101] It should be understood that the protective film according to the third state sample is substantially the same as the surface film according to the second state sample. Furthermore, since the protective film according to the third state sample also forms a transmission protective film within the lithography equipment, it is advantageous for the same reasons as the surface film according to the second state sample, as explained above.

[0102] A protective film disposed in the path of a radiation beam in a lithography apparatus according to a first aspect of the present invention can be formed using the method of the first aspect of the present invention. The protective film disposed in the path of a radiation beam in a lithography apparatus according to a third aspect of the present invention may have any feature that can be produced by any feature of the method of the first aspect of the present invention. Similarly, the protective film disposed in the path of a radiation beam in a lithography apparatus according to a third aspect of the present invention may have any feature of the film of the second aspect of the present invention.

[0103] The protective membrane can form part of a dynamic airlock.

[0104] This dynamic airlock can be formed, for example, by approaching an opening that allows the radiation beam to be transmitted from the projection system of a lithography device to a substrate supported on a substrate platform.

[0105] Alternatively, the protective film can form part of a spectral filter.

[0106] This spectral filter can be placed in any convenient or suitable location within a lithography apparatus. The spectral filter can be configured to avoid or at least reduce out-of-band radiation incident on the substrate supported on the substrate stage.

[0107] The protective film of the third state may have any of the characteristics of the film of the second state as described above, as discussed below.

[0108] Porous protective membranes may contain nanostructures.

[0109] The porous membrane may contain nanotubes.

[0110] For example, a porous protective membrane can be a fabric formed from CNTs. This can be called a carbon nanotube protective membrane.

[0111] The porous protective membrane is essentially self-supporting.

[0112] It should be understood that, in use, the membrane will be supported by a support frame surrounding its perimeter. In such a context, the porous membrane is essentially self-supporting, meaning that the porous membrane supports its own weight. That is, apart from at least one two-dimensional material layer, there is no additional membrane adjacent to the porous membrane that provides support for it.

[0113] The porous protective membrane can be considered as the majority of the thickness of the protective membrane.

[0114] Each or at least one two-dimensional material layer may form a substantially continuous layer adjacent to at least one side of the porous protective membrane.

[0115] Two-dimensional materials may include graphene.

[0116] In some embodiments, three graphene layers may be provided adjacent to one or both sides of the porous membrane.

[0117] In one embodiment, the porous protective film may be a carbon nanotube protective film and the two-dimensional material may include graphene. One advantage of using graphene as a two-dimensional material is that the film has previously been formed from carbon, and the properties of carbon in this environment are known. For example, by using another carbon-based material such as graphene, a significant increase in EUV reflection (which can be generated by other materials) can be avoided. Furthermore, other materials may have increased susceptibility to hydrogen etching within lithography equipment.

[0118] Two-dimensional materials may contain hexagonal boron nitride (h-BN).

[0119] Two-dimensional materials may contain molybdenum disulfide (MoS₂).

[0120] Advantageously, these materials (hBN and MoS2) are robust against hydrogen etching, and therefore for embodiments in which the two-dimensional material comprises hexagonal boron nitride (hBN) and / or molybdenum disulfide (MoS2), a capping layer with a smaller thickness can be applied.

[0121] In some embodiments, at least one two-dimensional material layer may be provided adjacent to both sides of the porous protective membrane, and a cover layer may be provided on each side of the protective membrane such that at least one two-dimensional material layer is disposed between the cover layer and the porous protective membrane.

[0122] It should be understood that different types of two-dimensional materials can be provided on different sides of porous membranes.

[0123] Each of these capping layers can be a three-dimensional material.

[0124] Advantageously, three-dimensional materials are significantly easier to manufacture than two-dimensional materials. As discussed above, two-dimensional materials effectively seal the structure of porous protective films. This allows three-dimensional materials to be used as capping layers while enjoying the benefits of a second-state film, as discussed above.

[0125] At least one capping layer may be suitable for protecting the porous layer and at least one two-dimensional material layer from hydrogen etching.

[0126] It should be understood that, in order to adequately protect the other two layers from hydrogen etching, the capping layer (a) may be formed of a suitable material that has not been subjected to strong hydrogen etching; and (b) may have a suitable thickness. Example materials are discussed below.

[0127] At least one cover layer may be formed of a material having an extinction coefficient of less than 0.02 nm⁻¹ for EUV radiation.

[0128] It is necessary to minimize the absorption of EUV radiation by the protective film. Therefore, it is generally desirable to form a cover layer system from a material with the lowest extinction coefficient against EUV radiation.

[0129] In some embodiments, the cover layer is formed of a material having an extinction coefficient of less than 0.01 nm⁻¹ for EUV radiation. In some embodiments, at least one cover layer is formed of a material having an extinction coefficient of less than 0.005 nm⁻¹ for EUV radiation.

[0130] The capping layer can have a thickness of approximately 0.3 nm to 5 nm.

[0131] The capping layer may contain yttrium or yttrium oxide.

[0132] The capping layer may contain any of the following: aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), ruthenium (Ru), platinum (Pt), gold (Au), zirconium nitride (ZrN), aluminum (Al), or zirconium (Zr).

[0133] The cover layer may contain multiple sublayers made of different materials.

[0134] The lithography equipment can be further included at the periphery of the porous protective membrane at the membrane boundary.

[0135] According to a fourth embodiment of the present invention, a film for use in a lithography device is provided, the film comprising: a protective film; a boundary at a periphery of the protective film and on a first side of the protective film; and a protective portion at a periphery of the protective film and on a second side of the protective film.

[0136] This coating is suitable for use with a 1x shrink mask adjacent to an EUV lithography apparatus. The coating system according to the fourth state is particularly advantageous, as discussed below.

[0137] As discussed above, the presence of low-pressure hydrogen gas within lithography equipment can etch the surface film, thereby limiting its potential lifespan. To mitigate this etching, surface films with protective capping layers have previously been proposed. However, minimizing the absorption of EUV radiation by the surface film is necessary, and therefore, the material and thickness of such capping layers are typically quite limited.

[0138] The inventors of this invention have realized that carbon etching by hydrogen ions and free radicals is temperature-dependent. Specifically, the inventors have realized that the carbon etching rate is higher at low and intermediate temperatures, but decreases to a negligible level at sufficiently high temperatures. The inventors have also realized that although the central portion of the film within an EUV lithography scanner can reach sufficiently high temperatures that hydrogen etching will be negligible (at least for a portion of the time), the periphery of the film will generally remain below this temperature and will therefore be more susceptible to hydrogen etching.

[0139] Advantageously, the surface coating according to the fourth state provides an additional protective portion on a portion (front side) of the protective film, which: (a) is most susceptible to risk from hydrogen etching; and (b) is not exposed to EUV radiation during use. This allows for an increase in the lifespan of the surface coating without affecting the performance of the lithography equipment.

[0140] The protective portion can be installed on a part of the protective membrane, which does not receive EUV radiation during use.

[0141] The protective portion can be placed on the part of the protective film that overlaps with the boundary.

[0142] That is, the protective part overlaps with the boundary (but is located on the opposite side of the surface film).

[0143] The protective portion may extend partially into the part of the protective film that does not coincide with the boundary.

[0144] That is, the protective portion can also extend inward to the area of ​​the protective film that is not attached to the boundary.

[0145] The protective portion can be formed from the same material as the main body of the protective film.

[0146] In such embodiments, the protective portion may be an increased thickness of the host material (e.g., a CNT protective film), which may serve as a sacrificial portion to provide the increased thickness to be etched by hydrogen.

[0147] Alternatively, the protective portion may contain a material suitable for protecting a portion of the protective film it is attached to from hydrogen etching.

[0148] In such embodiments, the protective portion includes a cover material. It should be understood that a thicker cover material may be provided in the protective portion (relative to the central portion of the protective film).

[0149] The protective film may contain nanotubes, graphene, and / or amorphous carbon.

[0150] For example, the protective film can be a fabric formed from CNTs. This can be called a carbon nanotube protective film. This is a particularly promising material for use as a protective film for the surface layer in EUV lithography equipment. This CNT surface layer is a porous material, and therefore can provide extremely high EUV transmittance (>98%). In addition, the CNT surface layer also provides excellent mechanical stability, and therefore can be manufactured in small thicknesses while remaining robust against mechanical failures.

[0151] The coating may further include a cover material on at least one surface of the coated protective film.

[0152] The capping material may comprise, alone or in combination, any of the following materials: yttrium (Y), yttrium oxide (YaOb), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), ruthenium (Ru), platinum (Pt), gold (Au), zirconium nitride (ZrN), aluminum (Al), or zirconium (Zr). The capping material may comprise multiple sublayers formed of different materials.

[0153] It should be understood that the second and fourth states disclosed herein can be combined.

[0154] Specifically, the protective film of the fourth state-type film may include: a porous protective film formed of a first material; at least one two-dimensional material layer adjacent to at least one side of the porous protective film; and at least one cover layer adjacent to the at least one two-dimensional material layer, such that the at least one two-dimensional material layer is disposed between the cover layer and the porous protective film.

[0155] It should be understood that one or more of the states or features described or referenced above in the following description may be combined with one or more other states or features. Simple Explanation of the Diagram

[0156] Embodiments of the invention will now be described by way of example only, with reference to the accompanying illustrative drawings, in which: Figure 1 is a schematic illustration of a lithography system including lithography equipment and a radiation source; Figure 2 is a schematic illustration of a method for forming a film according to an embodiment of the present invention; - Figure 3A is a schematic illustration of a first embodiment of the method shown in Figure 2; - Figure 3B is a schematic illustration of a second embodiment of the method shown in Figure 2; Figure 4 is a schematic representation of a method for applying a graphene film to both sides of a CNT protective film; Figure 5 shows a schematic cross-section of the film according to an embodiment of the present invention; and Figure 6 shows the expected etching rate of carbon hydrogen etching as a function of temperature with a hydrogen ion flux of 1.5 · 10 19 m⁻²· s⁻¹ for four different ion energies: 5 eV, 10 eV, 20 eV and 30 eV; Figure 6 also shows the sp3 carbon concentration as a function of temperature. Implementation

[0157] Figure 1 illustrates a lithography system. The lithography system includes a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithography apparatus LA includes an illumination system IL, a support structure MT configured to support a magnifying mask assembly 15 (e.g., a magnifying mask or shield) including a patterning device MA, a projection system PS, and a substrate stage WT configured to support a substrate W. The illumination system IL is configured to adjust the radiation beam B before it is incident on the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the patterning device MA) onto the substrate W. The substrate W may include a previously formed pattern. In this case, the lithography apparatus aligns the patterned radiation beam B with the pattern previously formed on the substrate W.

[0158] The radiation source SO, the irradiation system IL, and the projection system PS can all be constructed and configured to isolate them from the external environment. A gas at a pressure below atmospheric pressure (e.g., hydrogen) can be provided in the radiation source SO. A vacuum can be provided in the irradiation system IL and / or the projection system PS. A small amount of gas at a pressure sufficiently below atmospheric pressure (e.g., hydrogen) can be provided in the irradiation system IL and / or the projection system PS.

[0159] The radiation source SO shown in Figure 1 belongs to the type that can be called a laser-generated plasma (LPP) source. A laser 1, which may be, for example, a CO2 laser, is configured to deposit energy via a laser beam 2 onto a fuel (such as tin (Sn)) supplied by a fuel emitter 3. Although tin is mentioned in the following description, any suitable fuel can be used. The fuel may be, for example, in liquid form and may be, for example, a metal or alloy. The fuel emitter 3 may include a nozzle configured to guide tin, for example, in the form of droplets, along a trajectory toward the plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. The deposition of laser energy into the tin generates plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during the de-excitation and recombination of ions in the plasma.

[0160] EUV radiation is collected and focused by a near-normal incident radiation collector 5 (sometimes more commonly referred to as a normal incident radiation collector). Collector 5 may have a multilayer structure configured to reflect EUV radiation (e.g., EUV radiation with a desired wavelength such as 13.5 nm). Collector 5 may have an elliptical configuration with two elliptical foci. The first focal point may be located at the plasma formation region 4, and the second focal point may be located at the intermediate focal point 6, as discussed below.

[0161] In other embodiments of the laser-generated plasma (LPP) source, collector 5 may be a so-called grazing incidence collector configured to receive EUV radiation at the grazing incidence angle and focus the EUV radiation at an intermediate focal point. For example, the grazing incidence collector may be a nested collector comprising a plurality of grazing incidence reflectors. The grazing incidence reflectors may be arranged axially symmetrical about the optical axis.

[0162] The radiation source SO may include one or more contaminant traps (not shown). For example, the contaminant trap may be located between the plasma forming region 4 and the radiation collector 5. The contaminant trap may be, for example, a rotating foil trap, or any other suitable form of contaminant trap.

[0163] Laser 1 can be separated from radiation source SO. In this case, laser beam 2 can be delivered from laser 1 to radiation source SO by means of a beam delivery system (not shown) including, for example, suitable guide mirrors and / or beam expanders and / or other optical devices. Laser 1 and radiation source SO can be considered together as a radiation system.

[0164] Radiation reflected by collector 5 forms a radiation beam B. Radiation beam B is focused at point 6 to form an image of plasma formation region 4, which serves as a virtual radiation source for lighting system IL. Point 6 where radiation beam B is focused can be called the intermediate focal point. Radiation source SO is configured such that intermediate focal point 6 is located at or near opening 8 in the enclosure structure 9 of radiation source SO.

[0165] A radiated beam B is transmitted from a radiation source SO to an illumination system IL, which is configured to adjust the radiated beam. The illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and the faceted pupil mirror device 11 together provide the desired cross-sectional shape and angular distribution for the radiated beam B. The radiated beam B is transmitted from the illumination system IL and incident on a magnifying mask assembly 15 held by a support structure MT. The magnifying mask assembly 15 includes a patterning device MA and a surface film 19. The surface film is mounted to the patterning device MA via a surface film frame 17. The magnifying mask assembly 15 may be referred to as the magnifying mask and surface film assembly 15. The patterning device MA reflects and patternes the radiated beam B. In addition to or in place of the faceted field mirror device 10 and the faceted pupil mirror device 11, the irradiation system IL may also include other mirrors or devices.

[0166] After reflection from the patterning device MA, the patterned radiating beam B enters the projection system PS. The projection system includes a plurality of mirrors 13 and 14 configured to project the radiating beam B onto the substrate W held by the substrate stage WT. The projection system PS can apply a reduction factor to the radiating beam to form an image with features smaller than the corresponding features on the patterning device MA. For example, a reduction factor of 4 can be applied. Although the projection system PS in FIG. 1 has two mirrors 13 and 14, the projection system PS can include any number of mirrors (e.g., six mirrors).

[0167] The lithography apparatus can be used, for example, in a scanning mode, where a pattern applied to a radiating beam is projected onto a substrate W, while simultaneously scanning a support structure (e.g., a mask stage) MT and a substrate stage WT (i.e., dynamic exposure). The speed and direction of the substrate stage WT relative to the support structure (e.g., the mask stage) can be determined by the reduction ratio and image inversion characteristics of the projection system PS. The patterned radiating beam incident on the substrate W may contain radiating bands. These radiating bands may be referred to as exposure slits. During scanning exposure, the movement of the substrate stage WT and the support structure MT allows the exposure slits to travel throughout the exposure field of the substrate W.

[0168] The radiation source SO and / or lithography equipment shown in Figure 1 may include components not described. For example, a spectral filter may be provided in the radiation source SO. The spectral filter may substantially transmit EUV radiation, but substantially block radiation of other wavelengths, such as infrared radiation.

[0169] In other embodiments of the lithography system, the radiation source SO may take other forms. For example, in an alternative embodiment, the radiation source SO may comprise one or more free-electron lasers. These one or more free-electron lasers may be configured to emit EUV radiation that can be provided to one or more lithography devices.

[0170] As briefly described above, the magnification mask assembly 15 includes a cover 19 disposed adjacent to the patterning device MA. The cover 19 is positioned in the path of the radiation beam B such that the radiation beam B passes through the cover 19 in both cases: when its self-illumination system IL approaches the patterning device MA and when it is reflected from the patterning device MA toward the projection system PS. The cover 19 comprises a thin film or protective film that is substantially transparent to EUV radiation (although it will absorb a small amount of EUV radiation). In this context, an EUV-transparent cover 19 or a substantially transparent film for EUV radiation means that the cover 19 transmits at least 65%, preferably at least 80%, and more preferably at least 90% of the EUV radiation. The cover 19 serves to protect the patterning device MA from particle contamination.

[0171] Although efforts can be made to maintain a clean environment inside the lithography apparatus LA, particles can still be present inside the lithography apparatus LA. In the absence of the surface film 19, particles can deposit onto the patterning apparatus MA. Particles on the patterning apparatus MA can adversely affect the pattern imparted to the radiation beam B and thus affect the pattern transferred to the substrate W. The surface film 19 advantageously provides a barrier between the patterning apparatus MA and the environment within the lithography apparatus LA to prevent particle deposition on the patterning apparatus MA.

[0172] The surface film 19 is positioned at a distance sufficient to prevent any particles incident on the surface of the surface film 19 from falling within the field plane of the lithography apparatus LA. This spacing between the surface film 19 and the patterning apparatus MA reduces the range at which any particles on the surface of the surface film 19 will pattern the radiation beam B imaged onto the substrate W. It should be understood that if a particle is present in the radiation beam B but not in the field plane of the radiation beam B (e.g., not on the surface of the patterning apparatus MA), then any image of that particle will not be focused on the surface of the substrate W. In the absence of other considerations, it may be necessary to position the surface film 19 at a considerable distance from the patterning apparatus MA. However, in practice, the space available in the lithography apparatus LA to accommodate the surface film is limited by the presence of other components. In some embodiments, the spacing between the surface film 19 and the patterning apparatus MA may be, for example, between approximately 1 mm and 10 mm, such as between 1 mm and 5 mm, such as between 2 mm and 2.5 mm.

[0173] The surface film may include a boundary portion and a protective film. The boundary portion of the surface film may be hollow and generally rectangular, and the protective film may be defined by the boundary portion. As known in the art, one type of surface film can be formed by depositing one or more thin material layers onto a generally rectangular silicon substrate. The silicon substrate supports one or more thin layers during this stage of surface film construction. Once a layer of desired or target thickness and composition has been applied, the central portion of the silicon substrate is removed by etching (this may be referred to as etch-back). The peripheral portion of the rectangular silicon substrate is not etched (or alternatively etched to a lesser extent than the central portion). This peripheral portion forms the boundary portion of the final surface film, while one or more thin layers form the protective film of the surface film (which is defined by the boundary portion). The boundary portion of the surface film may be formed of silicon.

[0174] Some embodiments of the present invention relate to a novel type of film and a method for forming the same film.

[0175] The surface layer (e.g., including the protective film and boundaries) may require some support from a more rigid surface layer frame. The surface layer frame provides two functions. First, it supports the surface layer and can also stretch the protective film. Second, it facilitates the connection of the surface layer to the patterning device (reducing mask). In a known configuration, the surface layer frame may include a main, generally rectangular body portion glued to the boundary portion of the surface layer and titanium attachment mechanisms glued to the sides of this body portion. An intermediate fixing member (referred to as a stud) is attached to the patterning device (reducing mask). The intermediate fixing member (stud) on the patterning device (reducing mask) can engage (e.g., releasably engage) with the attachment members of the surface layer frame.

[0176] Some embodiments of the present invention relate to a method for forming a film for use in a lithography device (such as the lithography device LA shown in FIG. 1). This method 100 is schematically illustrated in FIG. 2.

[0177] Method 100 includes step 102 of providing a porous membrane formed from a first material. It should be understood that, as used herein, a porous membrane is intended to mean a material having an open structure such as a nanotube membrane. In some embodiments, the porous membrane may comprise a nanostructure. In some embodiments, the porous membrane may comprise nanotubes. For example, the porous membrane may be a fabric formed from CNTs. This may be referred to as a carbon nanotube membrane.

[0178] Method 100 further includes step 104 of applying at least one two-dimensional material layer to at least one side of a porous protective membrane.

[0179] In some embodiments, the or at least one two-dimensional material layer is applied as a substantially continuous layer adjacent to at least one side of the porous protective membrane. This can be applied using a wet transfer method. Alternatively, the or at least one two-dimensional material layer can be transferred from a temporary or intermediate support substrate, as further discussed below with reference to FIG4.

[0180] It should be understood that, in this context, two-dimensional material is intended to mean a material formed from one or more single-atom layers (such as graphene). It should also be understood that a variety of different two-dimensional materials can be used.

[0181] In some embodiments, the two-dimensional material may comprise graphene. In some embodiments, three graphene layers may be provided adjacent to one or both sides of the porous membrane. In one embodiment, the porous membrane may be a carbon nanotube membrane and the two-dimensional material comprises graphene. The monolayer graphene may have a thickness of approximately 0.35 nm. The distance between two stacked graphene layers may be approximately 0.14 nm. Therefore, the thickness of the three graphene layers may be approximately 1.3 nm. []

[0182] One advantage of using graphene as a two-dimensional material is that the surface layer has previously been formed from carbon, and the properties of carbon in this environment are known. For example, by using another carbon-based material such as graphene, a significant increase in EUV reflection (which can be produced by other materials) can be avoided. Furthermore, other materials may have increased susceptibility to hydrogen etching within lithography equipment. []

[0183] Alternatively or additionally, in some embodiments, the two-dimensional material may comprise hexagonal boron nitride (h-BN). Alternatively or additionally, in some embodiments, the two-dimensional material may comprise molybdenum disulfide (MoS₂). Advantageously, these materials (hBN and MoS₂) are robust against hydrogen etching, and therefore for embodiments in which the two-dimensional material comprises hexagonal boron nitride (hBN) and / or molybdenum disulfide (MoS₂), a capping layer of relatively small thickness can be applied in subsequent steps. A single layer of hexagonal boron nitride (h-BN) or molybdenum disulfide (MoS₂) may have a thickness of approximately 0.65 nm.

[0184] In some embodiments, the or at least one two-dimensional material layer has a thickness of about 0.5 nm to 5 nm. In some embodiments, the or at least one two-dimensional material layer has a thickness of about 0.5 nm to 2 nm.

[0185] Method 100 further includes step 106 of applying a capping layer to at least one two-dimensional material layer on at least one side of a porous protective membrane such that at least one two-dimensional material layer is disposed between the capping layer and the porous protective membrane.

[0186] In some embodiments, the capping layer may be a three-dimensional material. Advantageously, three-dimensional materials are significantly easier to manufacture than two-dimensional materials. As discussed below, two-dimensional materials (provided at step 104) effectively seal the structure of the porous protective membrane. This allows for the use of three-dimensional materials in the capping layer while enjoying the benefits of method 100 of FIG. 2, as discussed below.

[0187] It should be understood that, unless otherwise stated, the total EUV transmittance of the capping layer herein means the percentage of EUV radiation transmitted after propagation through the membrane. For embodiments where the capping layer is provided on each side of the membrane, the total EUV transmittance of the capping layer means the total transmittance from both sides.

[0188] In some embodiments, the total EUV transmittance of the capping layer is 96% or greater. In some embodiments, the total EUV transmittance of at least one capping layer may be 96.5% or greater. In some embodiments, the total EUV transmittance of at least one capping layer may be 97% or greater. In some embodiments, the total EUV transmittance of at least one capping layer may be 97.5% or greater. In some embodiments, the total EUV transmittance of at least one capping layer is approximately 97.8%.

[0189] It should be understood that, generally speaking, the EUV transmittance of a capping layer depends on (a) the type of material forming the capping layer; and (b) the thickness of the capping layer. It should also be understood that, generally speaking, the EUV transmittance of a capping layer also depends on the density or porosity of the capping layer. Example materials are discussed below.

[0190] In some embodiments, at least one capping layer is formed of a material suitable for protecting the porous layer and at least one two-dimensional material layer from hydrogen etching. It should be understood that, in order to protect the other two layers from hydrogen etching, the capping layer (a) may be formed of a suitable material that has not been subjected to strong hydrogen etching; and (b) may have a suitable thickness. Example materials are discussed below.

[0191] It is necessary to minimize the absorption of EUV radiation by the coating. Therefore, it is generally desirable to form a capping layer system from a material with the lowest extinction coefficient against EUV radiation.

[0192] In some embodiments, at least one cover layer may be formed of a material having an extinction coefficient of less than 0.02 nm⁻¹ for EUV radiation. In some embodiments, the cover layer is formed of a material having an extinction coefficient of less than 0.01 nm⁻¹ for EUV radiation. In some embodiments, at least one cover layer is formed of a material having an extinction coefficient of less than 0.005 nm⁻¹ for EUV radiation.

[0193] In some embodiments, the capping layer may have a thickness of approximately 0.3 nm to 5 nm. That is, the capping layer may have a thickness as small as one atomic monolayer (i.e., a thickness of 0 or 1 atom and an interatomic distance of, for example, approximately 0.3 nm). In other embodiments, the capping layer may have a greater thickness to provide better protection for the underlying two-dimensional material and the porous protective film. In some embodiments, the capping layer may have a thickness greater than 1 nm. In some embodiments, the capping layer may have a thickness greater than 1.5 nm. In some embodiments, the capping layer may have a thickness greater than 2 nm. In some embodiments, the capping layer may have a thickness greater than 5 nm.

[0194] In some embodiments, the capping layer comprises yttrium (Y) or yttrium oxide (YaOb). Yttrium has an extinction coefficient of approximately 0.0021 nm⁻¹ for EUV radiation. Yttrium oxide (Y₂O₃) has an extinction coefficient of approximately 0.01 nm⁻¹ for EUV radiation. Therefore, in an embodiment where a 1.5 nm thick capping layer of yttrium oxide (Y₂O₃) is present on each side of the protective film, the total EUV transmittance of at least one capping layer is approximately 97%.

[0195] In some embodiments, step 106 of applying a capping layer to at least one two-dimensional material layer on at least one side of a porous protective membrane may include applying a plurality of sublayers, each sublayer comprising a different material. For example, in some embodiments, step 106 of applying a capping layer to at least one two-dimensional material layer on at least one side of a porous protective membrane includes: applying a first sublayer of a first material to at least one two-dimensional material layer on at least one side of the porous protective membrane; and applying a second sublayer of a second material to the first sublayer. The first sublayer may have a smaller extinction coefficient against EUV radiation compared to the second sublayer. The second (outermost) sublayer may have improved chemical stability compared to the first sublayer. In some embodiments, the first sublayer may comprise a metal, and the second sublayer may comprise a metal oxide.

[0196] In some embodiments, the porous sheath can be substantially self-supporting. It should be understood that in use, the sheath is supported around its periphery by a sheath frame mounted to a magnifying glass or mask MA. In this context, "substantially self-supporting" means that the porous sheath supports its own weight. That is, apart from at least one two-dimensional material layer and a cover layer, there is no additional sheath adjacent to the porous sheath to provide support.

[0197] It should be understood that a non-porous protective membrane may have two generally parallel surfaces defining two opposite sides of the membrane. The volume defined by the two generally parallel surfaces is substantially occupied by the material forming the non-porous protective membrane. It should be further understood that, in contrast, a porous protective membrane includes areas occupied by material, from which the porous membrane is formed, interspersed with voids not containing material. For this porous protective membrane, two generally parallel imaginary or non-solid surfaces may define the boundaries or sides of the membrane. The volume defined by the two generally parallel imaginary surfaces is only partially occupied by the material forming the porous protective membrane. Applying at least one two-dimensional material layer to at least one side of the porous protective membrane is intended to include applying at least one two-dimensional material layer to at least one imaginary or non-solid surface defining the boundaries or sides of the porous protective membrane.

[0198] The thickness of a porous protective film can be defined as the distance between two generally parallel imaginary or non-physical surfaces that define the boundary or sides of the film. A porous protective film can have a thickness of approximately 1 nm to 100 nm. A porous protective film can have a thickness of approximately 10 nm to 100 nm. A porous protective film can have a thickness of approximately 50 nm to 100 nm. In some embodiments, the porous protective film can be considered as forming most of the thickness of the surface film. []

[0199] In some embodiments, the method 100 shown in FIG2 may further include attaching a film boundary to the periphery of the porous protective membrane. In such embodiments, the film boundary may be attached to the periphery of the porous protective membrane before at least one two-dimensional material layer is applied to at least one side of the porous protective membrane (at step 104).

[0200] The method 100 shown in Figure 2 is particularly advantageous, as discussed below. At least one two-dimensional material layer is used to seal the adjacent sides of the porous membrane.

[0201] The method 100 shown in Figure 2 produces a surface film, the bulk of which is formed of a porous material. Advantageously, this produces a surface film with reduced density and, therefore, increased transmittance against extreme ultraviolet (EUV) radiation. This is particularly important for EUV lithography systems and improves system throughput. A particularly promising material for use as a surface film protector in EUV lithography equipment is a fabric or protector formed of carbon nanotubes (CNTs). This CNT surface film is a porous material and therefore provides extremely high EUV transmittance (>98%). Furthermore, the CNT surface film also provides excellent mechanical stability and can therefore be manufactured in small thicknesses while remaining robust against mechanical failures. However, low-pressure hydrogen is typically supplied within lithography equipment, which forms a hydrogen plasma in the presence of EUV radiation (during exposure). It has been found that hydrogen ions and hydrogen radicals from the hydrogen plasma can etch the surface film formed by CNTs, thereby limiting the potential lifetime of the surface film and hindering the commercial implementation of CNT surface films.

[0202] To mitigate such etching of CNT films, it has been previously proposed to provide such CNT films with a protective capping layer. This capping layer can be formed from a material that is chemically stable in the environment of lithography equipment and has a low extinction coefficient against EUV radiation.

[0203] However, the difference between the refractive index of carbon and a suitable capping layer is usually greater than the difference between the refractive index of carbon and vacuum. Therefore, the inventors have realized that this capping layer will cause an increase in EUV spot size, which is undesirable. It is particularly advantageous to apply at least one two-dimensional material layer to the porous protective film (at step 104) and then apply the capping layer to at least one two-dimensional material layer (at step 106), as will be discussed below.

[0204] It should be understood that porous materials will have a structure, and therefore, if there is a large contrast between the refractive indices of the porous material and the surrounding medium, the radiation (e.g., EUV radiation) will be scattered (e.g., via Mie scattering) as it propagates through the surface. This will result in undesirable diffusion or spotting of the radiation, thus again affecting the imaging performance of the LA (Lamination Atmosphere) lithography equipment. Because EUV radiation is so strongly absorbed by most materials, EUV lithography systems typically operate under high vacuum. Therefore, it is particularly desirable for the porous material to be formed of a material with a refractive index close to 1. It is also desirable for the porous material to be formed of a material with the lowest possible extinction coefficient for EUV radiation.

[0205] At least one two-dimensional material layer (applied at step 104 of method 100 shown in Figure 2) is used to seal adjacent sides of the porous protective film and form a smoother and flatter outer surface of the film. This allows the capping layer to be provided above this smoother and flatter outer surface. Advantageously, this allows protection of the porous protective film from etching while reducing EUV spots, regardless of the material used for the capping layer. Furthermore, in addition to being significantly smoother and flatter than the surface of the porous material, the surface of the two-dimensional material will have a smaller surface area. As a result, the volume of the capping layer is reduced when the (relatively thin) capping layer is provided on the two-dimensional material rather than directly on the porous material. Advantageously, this also results in higher EUV transmittance of the film for the same thickness of the capping layer.

[0206] Within the CNT protective film, carbon nanotubes can be discrete, or alternatively, they can be clustered together. Furthermore, the size of such clusters can vary. The inventors have discovered that when the capping layer is applied directly to the CNT protective film, the loss of EUV transmittance attributable to the capping layer largely depends on the degree of clustering within the CNT protective film. For example, for a fixed density of CNTs in the protective film, the smaller the number of CNTs per cluster, the greater the loss of EUV transmittance. Advantageously, by means of method 100 shown in FIG2, since the capping layer is applied (in step 106) to at least one two-dimensional material layer (rather than a porous protective film), the loss of EUV transmittance is no longer dependent on the typical size of the structure within the porous protective film (e.g., the amount of clustering in the case of a CNT protective film). In fact, by applying the capping layer to a two-dimensional planar layer, the loss of EUV transmittance is minimized for a given thickness of the capping layer.

[0207] Finally, the porous layer structure is sealed by at least one two-dimensional material layer of the film formed using method 100 shown in Figure 2. Advantageously, this produces a higher particle stopping power than CNT films without such two-dimensional material layers.

[0208] Two example embodiments of the general method 100 schematically shown in Figure 2 are now described with reference to Figures 3A and 3B.

[0209] Figure 3A schematically illustrates a first embodiment of the method 100 shown in Figure 2. The method includes providing a porous protective membrane 200 formed of CNTs. This may be referred to as a carbon nanotube protective membrane or CNTm. The porous protective membrane 200 is mounted on a membrane boundary 210 at the periphery of the porous protective membrane. The membrane boundary 210 may comprise a generally rectangular frame. The membrane boundary 210 may be formed, for example, of silicon, which is used in conventional non-porous protective membranes. Alternatively, the membrane boundary 210 may be formed, for example, of carbon nanotubes, quartz, or steel, which may provide additional benefits. The porous protective membrane 200 may previously be attached to the membrane boundary 210 using known techniques.

[0210] The porous CNT protective film 210 has an EUV transmittance of approximately 97.5%. The porous CNT protective film 210 can have a thickness of approximately 100 nm. The following examples are based on the porous CNT protective film 210 having an EUV transmittance of approximately 97.5% and a thickness of approximately 100 nm.

[0211] As explained above, the thickness of a porous protective film can be defined as the distance between two generally parallel imaginary or non-solid surfaces that define the boundary or sides of the film. Also as explained above, the volume defined by the two generally parallel imaginary surfaces is only partially occupied by the material forming the porous protective film (the porous protective film includes regions occupied by the material interspersed with voids not containing the material). To achieve the porosity of the example porous CNT protective film 210, based on the extinction coefficient of carbon to EUV radiation, to achieve a conventional non-porous protective film formed from carbon with approximately 97.5% EUV transmittance, the non-porous protective film would have a thickness of approximately 4 nm. In contrast, the example porous CNT protective film 210 has a thickness of approximately 100 nm.

[0212] The method includes providing a graphene film 220. For example, while the graphene film 220 may comprise a single graphene layer or three graphene layers (3GL), it should be understood that the graphene film 220 may comprise any number of graphene layers. Generally, the graphene film 220 comprises at least one two-dimensional material layer. The EUV transmittance of the graphene film 220 for a single graphene layer may be approximately 99.8%. The EUV transmittance of the graphene film 220 (for three graphene layers) may be approximately 99.5%.

[0213] The method further includes step 104 of applying a graphene film 220 to one side of the porous protective film 200. In this embodiment, the graphene film 220 is applied to a side opposite to or away from the side to which the surface film boundary 210 is attached. In this embodiment, the graphene film 220 is applied as a substantially continuous layer adjacent to one side of the porous protective film 200. A wet transfer method can be used to apply the graphene film 220. Alternatively, the graphene film 220 can be transferred from a temporary or intermediate support substrate, as further discussed below with reference to FIG4. The combination of the porous CNT protective film 200 and the graphene film 220 can be referred to as G-CNTm. The EUV transmittance of the porous CNT protective film 200 having the graphene film 220 (for a single graphene layer) can be approximately 97.3%.

[0214] The method further includes step 106 of applying a capping layer 230 to the graphene film 220 such that the graphene film 220 is disposed between the capping layer 230 and the porous protective film 200. Although schematically shown as a material layer being formed and then applied to the graphene film 220, it should be understood that in practice, the capping layer 230 may be formed on the graphene film 220 (i.e., in situ formation).

[0215] In this embodiment, the method further includes applying a cover layer 230 to a second side of the porous protective film 200. Specifically, the cover layer 230 is applied to the same side to which the film boundary 210 is attached.

[0216] In some embodiments, the capping layer 230 comprises yttrium oxide (Y₂O₃). Therefore, in some embodiments, step 106 of applying the capping layer 230 to the graphene film 220 comprises: directly applying the yttrium oxide (Y₂O₃) layer to the graphene film 220 or an intermediate sublayer. For example, in one embodiment, the capping layer comprises a yttrium oxide (Y₂O₃) layer having a thickness of 1.5 nm.

[0217] The graphene film 220 is used to seal the adjacent sides of the porous protective film 200 and form a smoother and flatter outer surface of the film. This can be seen from the schematic magnified portion of the interface compared with the two capping layers 230. This allows the capping layer 230 applied to the graphene film 220 to be provided above the smoother and flatter outer surface. Advantageously, this allows the porous protective film 200 to be protected from etching while reducing EUV spots, regardless of the material used for the capping layer 230.

[0218] Furthermore, besides being significantly smoother and flatter than the surface of the porous material 200, the graphene film 220 will have a smaller surface area. Therefore, when the (relatively thin) capping layer 230 is provided on the graphene film 220 rather than directly on the porous protective film 200, the volume of the capping layer 230 is reduced. Again, this can be seen from the schematic enlarged portion of the interface and the comparison of the two capping layers 230. Advantageously, this also results in higher EUV transmittance of the surface film for the same thickness of the capping layer 230.

[0219] The graphene film 220 encloses the structure of the porous protective film 200, which advantageously produces a higher particle stopping power than a CNT surface film without this graphene film 220.

[0220] In this embodiment shown in Figure 3A, the capping layer 230 on the cavity side of the surface film (i.e., on the same side as the surface film boundary 210) has corrugations because the capping layer is deposited directly on the porous protective film 200. Although this may be detrimental to EUV transmission non-uniformity and spot reduction, it may be beneficial to EUV reflection.

[0221] In some embodiments of method 100 shown in FIG2, in step 104, at least one two-dimensional material layer may be applied to both sides of the porous protective film, and a capping layer may be applied to each side of the surface film, such that at least one two-dimensional material layer is disposed between the capping layer and the porous protective film. An example of this embodiment of method 100 for forming the surface film is now described with reference to FIG3B.

[0222] Figure 3B schematically illustrates a second embodiment of the method 100 shown in Figure 2. The embodiment of the method shown in Figure 3B is very similar to the embodiment of the method shown in Figure 3A. Therefore, in the following text, only the differences will be explained in detail.

[0223] An embodiment of the method shown in Figure 3B includes providing two graphene films 220. The graphene films can substantially serve as the single graphene film described above with reference to Figure 3B.

[0224] In an embodiment of the method shown in Figure 3B, the method includes step 104 of applying two graphene films 220 to opposite sides of a porous protective film 200. That is, in this embodiment, the graphene films 220 are applied to each of the following: the side to which the film boundary 210 is attached and the opposite side. Again, in this embodiment, each of the graphene films 220 is applied as a substantially continuous layer adjacent to one side of the porous protective film 200. The porous CNT protective film 200 and the combination of the two graphene films 220 can be referred to as G-CNTm-G. The EUV transmittance of the porous CNT protective film 200 having two graphene films 220 (each film comprising a single graphene layer) can be approximately 97.1%.

[0225] An embodiment of the method shown in Figure 3B includes step 106, in which a capping layer 230 is applied to each of two graphene films 220 such that each graphene film 220 is positioned between a capping layer 230 and a porous protective film 200.

[0226] In some embodiments, step 106 of applying the capping layer 230 to each of the graphene films 220 includes applying a yttrium oxide (Y₂O₃) layer directly to the graphene film 220 or an intermediate sublayer. For example, in one embodiment, the capping layer comprises a yttrium oxide (Y₂O₃) layer having a thickness of 1.5 nm.

[0227] In an embodiment of the method shown in Figure 3B, the graphene film 220 is used to seal both sides of the porous protective film 200 and form a smoother and flatter outer surface of the film. Advantageously, this allows the porous protective film 200 to be protected from etching, while further reducing EUV spots, regardless of the material used for the capping layer 230.

[0228] Furthermore, in addition to being significantly smoother and flatter than the surface of the porous material 200, the graphene film 220 will have a smaller surface area. Since the two capping layers 230 are provided on the graphene film 220 rather than directly on the porous protective film 200, the volume of the capping layer 230 is reduced. Advantageously, this also results in higher EUV transmittance of the surface film for the same thickness of the capping layer 230.

[0229] The EUV absorptivity of the two single-graphene layers is approximately 0.4%. However, because the capping layer 230 is applied to the sealed surface of the graphene film 220, rather than the porous CNT protective film 200, the EUV absorptivity of the two capping layers is reduced. The exact reduction depends on the amount of bundled material within the CNT protective film 200; however, on average, the EUV absorptivity of the two capping layers is reduced by approximately 1.5%. Therefore, adding a single-graphene layer 220 to each side of the CNTm results in a net gain of approximately 1.1% in EUV transmittance.

[0230] The combination of a porous CNT protective film 200, two graphene films 220, and two capping layers 230 can be referred to as CG-CNTm-GC. The EUV transmittance of the CG-CNTm-GC surface film (which consists of single graphene layers on both sides, each capped with a 1.5 nm Y₂O₃ layer) is approximately 94.1%. In contrast, if the same CNTm protective film is capped with 1.5 nm Y₂O₃ layers on both sides (without the graphene film 220), the EUV transmittance of the (C-CNTm-C) surface film will be approximately 93% (this value depends on the number of CNTs bundled).

[0231] The above estimates do not take into account the scattering of EUV radiation by the surface film, which can result in a loss of several percent of EUV radiation. By using the structure of the sealed CNT protective film 200, such losses can be avoided or at least reduced. Therefore, in addition to the estimated 1.1% gain in EUV transmittance mentioned above, it is also expected that the surface film formed using method 100 shown in Figure 2 will further reduce the loss of EUV radiation.

[0232] In the embodiment of the method shown in Figure 3B, the fact that the graphene film 220 seals both sides of the porous protective film 200 has an additional advantage, as discussed below.

[0233] Within the CNT sheath 200, the carbon nanotubes can be separate, or alternatively, they can be clustered together. Furthermore, the size of such clusters can vary. The inventors have discovered that when a capping layer is applied directly to the CNT sheath, the loss of EUV transmittance attributable to the capping layer largely depends on the degree of clustering within the CNT sheath. For example, for a fixed density of CNTs in the sheath, the smaller the number of CNTs per cluster, the greater the loss of EUV transmittance. Advantageously, since two capping layers 230 are applied to the graphene film 220 (rather than the porous sheath 200), the loss of EUV transmittance is no longer dependent on the typical size of the structure within the porous sheath 200 (e.g., the amount of clustering in the case of the CNT sheath). In fact, by applying the capping layer 230 to the planar graphene film 220, the loss of EUV transmittance is minimized for a given thickness of the capping layer 230.

[0234] A wet transfer process can be used to apply graphene film 220 to one or more sides of porous protective film 200. This wet transfer process is known in the art. Typically, a wet transfer process involves growing a two-dimensional material (e.g., graphene film 220) on a first substrate (e.g., a copper substrate). Subsequently, an adhesive layer is formed on the other side of the two-dimensional material. The adhesive layer may, for example, contain a polymer, such as polymethyl methacrylate (PMMA). Subsequently, the first substrate is removed, for example, by selective etching. For example, ammonium persulfate can be used to remove the first substrate containing copper. Depending on the situation, the adhesive layer and the two-dimensional material may be rinsed (e.g., in water). Subsequently, the two-dimensional material is applied to one side of porous protective film 200. Finally, the adhesive layer is removed, for example, by selective etching.

[0235] An alternative method for applying graphene film 220 to one or more sides of porous protective film 200 is now discussed with reference to FIG4. FIG4 is a schematic representation of method 300 for applying graphene film 220 to both sides of CNT protective film 200. Method 300 can be used in step 104 of the embodiment of the method for forming the film shown in FIG3B.

[0236] The method 300 of applying graphene film 220 to CNT protective film 200 includes: providing graphene film 220 on support substrate 310; pressing graphene film 220 to one side of porous protective film 200; and removing support substrate 310.

[0237] The support substrate 310 includes a base substrate 312 and a sacrificial layer 314 disposed on the surface of the base substrate 312. A graphene film 220 is disposed on the sacrificial layer 314.

[0238] Although schematically shown as a graphene film 220 formed and then applied to a sacrificial layer 314 on a support substrate 310, it should be understood that in practice, the graphene film 220 can be formed on the sacrificial layer 314 (i.e., in situ formation).

[0239] The graphene film 220 disposed on the support substrate 310 may be referred to as a manufacturing intermediate. Method 300 may include providing two such manufacturing intermediates.

[0240] As shown in the middle right part of Figure 4, method 300 includes pressing each of the graphene films 220 onto one side of the porous protective film 200 by applying pressure via the support substrate 310.

[0241] The support substrate 310 can be removed by etching the sacrificial layer 314 to release the base substrate 312 from the graphene film 220.

[0242] Some embodiments of the present invention relate to a coating used in lithography equipment (such as the lithography equipment LA shown in FIG. 1). This coating can be formed, for example, using the method 100 schematically shown in FIG. 2.

[0243] Some embodiments of the present invention relate to a lithography apparatus that can be used to form images of patterned devices on a substrate using a radiation beam. The lithography apparatus may generally have the form of the lithography apparatus LA shown in FIG. 1. The lithography apparatus LA according to such embodiments further includes a protective film disposed in the path of the radiation beam B. The protective film can be formed using the method 100 schematically shown in FIG. 2.

[0244] In some embodiments, the protective film may form part of a dynamic airlock. This dynamic airlock may be formed, for example, close to an opening through which a radiation beam B is transmitted from the projection system PS of the lithography device LA to the substrate W supported on the substrate stage WT.

[0245] In some embodiments, the protective film may form part of a spectral filter. This spectral filter may be provided in any convenient or suitable location within the lithography apparatus. The spectral filter may be configured to avoid or at least reduce out-of-band radiation incident on the substrate supported on the substrate stage.

[0246] Some embodiments of the present invention relate to a film used in a lithography device (such as the lithography device LA shown in FIG1), as described now with reference to FIG5, which shows a schematic cross-section of a film 400 according to an embodiment of the present invention.

[0247] The film 400 includes: a protective film 410; a boundary 420; and a protective portion 430. The boundary 420 is provided at the periphery of the protective film 410 and on a first side 412 of the protective film 410. The protective portion 430 is provided at the periphery of the protective film 410 and on a second side 414 of the protective film 410.

[0248] The 400 series of coatings shown in Figure 5 are particularly advantageous, as discussed below. As discussed above, the presence of low-pressure hydrogen within the lithography equipment (LA) can etch the coating, thus limiting its potential lifetime. To mitigate such etching of CNT coatings, it has previously been proposed to provide coatings with protective capping layers. However, it is necessary to minimize the absorption of EUV radiation by the coating, and therefore, the material and thickness of such capping layers are typically quite limited.

[0249] The interaction between hydrogen ions and carbon materials is quantitatively described in the following two published papers, the contents of which are hereby incorporated by reference: (1) J. Roth, C. García-Rosales, "Analytic description of the chemical erosion of graphite by hydrogen ions", Nucl. Fusion 1996, 36 / 12, 1647-1659; and (2) J. Roth, C. García-Rosales, "Corrigendum - Analytic description of the chemical erosion of graphite by hydrogen ions", Nucl. Fusion 1997, 37, 897. This quantitative description of the interaction between hydrogen ions and carbon materials can be called the Roth-García-Rosales (RGR) model. The RGR model can be used to predict the etching yield of carbon materials as a function of temperature, which varies with the typical hydrogen ion energies encountered in lithography equipment (such as the energies of ions forming from 1 eV to 30 eV). In EUV lithography, the typical hydrogen ion flux incident on the surface film can be approximately 1.10¹⁹ m⁻² s⁻¹. In EUV lithography, the typical hydrogen ion flux incident on the surface film can be on the order of 1.10¹⁹ m⁻² s⁻¹ (e.g., from 10¹⁸ m⁻² s⁻¹ to 10²⁰ m⁻² s⁻¹).

[0250] Figure 6 shows the expected etching rate of carbon hydrogen etching as a function of temperature with a hydrogen ion flux of 1.5 × 10¹⁹ m⁻²·s⁻¹ for four different ion energies: 5 eV, 10 eV, 20 eV, and 30 eV. Figure 6 also shows the sp³ carbon concentration as a function of temperature. As can be seen from Figure 6, for these typical environmental conditions in lithography equipment LA, the hydrogen etching rate of the surface film formed solely by CNTs is expected to decrease to a negligible level at a temperature of approximately 1050 K. However, those skilled in this art will understand that different minimum temperatures may be required under different conditions.

[0251] The inventors of this invention have realized that carbon etching by hydrogen ions and free radicals is temperature-dependent. Specifically, the inventors have realized that the carbon etching rate is higher at low and intermediate temperatures, but decreases to a negligible level at sufficiently high temperatures. The inventors have also realized that although the central portion of the film 19 within the EUV lithography scanner LA can reach sufficiently high temperatures that hydrogen etching will be negligible (at least for a portion of the time), the periphery of the film will generally remain below this temperature and will therefore be more susceptible to hydrogen etching.

[0252] Advantageously, the surface film 400 shown in Figure 5 provides an additional protective portion 430 on a portion (front side 414) of the protective film 410, which: (a) is most susceptible to risk from hydrogen etching; and (b) is not exposed to EUV radiation during use. This allows for an increase in the lifespan of the surface film without affecting the performance of the lithography equipment (LA).

[0253] In some embodiments, the protective portion 430 is disposed on a portion of the protective membrane 410, which does not receive EUV radiation during use.

[0254] The protective portion 430 may be disposed on a portion of the protective film 410 that overlaps with the boundary 420. That is, the protective portion 430 may overlap with the boundary 420 (but is disposed on the opposite side 414 of the outer film 400). The protective portion 430 may extend partially into a portion 416 of the protective film 410 that does not overlap with the boundary 420. That is, the protective portion 430 may also extend partially inward into an area of ​​the protective film 410 that is not attached to the boundary 420.

[0255] In some embodiments, the protective portion 430 may be formed of the same material as the body of the protective film 410. In such embodiments, the protective portion 430 may be an increased thickness of the body material (e.g., a CNT protective film), which may serve as a sacrificial portion to provide the increased thickness to be hydrogen etched.

[0256] In some embodiments, the protective portion 430 may be formed of a material suitable for protecting a portion of the protective film 410 attached thereto from hydrogen etching. For such embodiments, the protective portion 430 may comprise a capping material. The capping material may comprise, alone or in combination, any of the following materials: yttrium (Y), yttrium oxide (YaOb), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), ruthenium (Ru), platinum (Pt), gold (Au), zirconium nitride (ZrN), aluminum (Al), or zirconium (Zr). The capping material may comprise a plurality of sublayers formed of different materials.

[0257] It should be understood that a thicker version of this cover material can be provided in the protective portion 430 (relative to the central portion of the protective film 410).

[0258] In some embodiments, the protective film 410 comprises nanotubes. For example, the protective film 410 may be a fabric formed of CNTs. This may be referred to as a carbon nanotube protective film. This is a particularly promising material for use as a surface protective film in EUV lithography equipment. This CNT surface film is a porous material and therefore provides extremely high EUV transmittance (>98%). In addition, the CNT surface film also provides excellent mechanical stability and therefore can be manufactured in small thicknesses while remaining robust against mechanical failures. In other embodiments, the protective film 410 may comprise graphene and / or amorphous carbon.

[0259] In some embodiments, the film 400 further comprises a cover material on at least one surface of the protective film 410.

[0260] It should be understood that the features of the film 400 shown in Figure 5 and described above can be combined with the features of the film formed using the method shown in Figure 2 and described above.

[0261] For example, the protective film 410 may include: a porous protective film formed of a first material (e.g., a CNT protective film); at least one two-dimensional material layer (e.g., graphene) adjacent to at least one side of the porous protective film; and at least one cover layer adjacent to the at least one two-dimensional material layer, such that the at least one two-dimensional material layer is disposed between the cover layer and the porous protective film.

[0262] In this document, a reference mask or magnifying mask can be interpreted as a reference patterning device (a mask or magnifying mask is an example of a patterning device) and the terms are used interchangeably. Specifically, the term mask assembly is synonymous with magnifying mask assembly and patterning device assembly.

[0263] Although embodiments of the invention may be specifically referred to herein in the context of lithography apparatus, embodiments of the invention may be used in other apparatuses. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing objects such as wafers (or another substrate) or masks (or other patterning devices). Such apparatus may generally be referred to as lithography tools. These lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0264] The term "EUV radiation" can be considered to encompass electromagnetic radiation with wavelengths in the range of 4 nm to 20 nm, for example, in the range of 13 nm to 14 nm. EUV radiation can have wavelengths less than 10 nm, for example, wavelengths in the range of 4 nm to 10 nm, such as 6.7 nm or 6.8 nm.

[0265] While references may be specifically made herein to the use of lithography equipment in IC manufacturing, it should be understood that the lithography equipment described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, and so on. While specific embodiments of the invention have been described above, it should be understood that the invention may be practiced in other ways different from those described. The above description is intended to be illustrative and not restrictive. Therefore, it will be apparent to those skilled in the art that modifications can be made to the described invention without departing from the scope and terms of the claims set forth below. 1. A method for forming a film for use in a lithography device, the method comprising: Provide a porous protective membrane formed from the first material; At least one two-dimensional material layer is applied to at least one side of the porous protective membrane; and A cover layer is applied to at least one two-dimensional material layer on at least one side of a porous protective membrane, such that at least one two-dimensional material layer is disposed between the cover layer and the porous protective membrane. 2. The method of Clause 1, wherein at least one two-dimensional material layer is applied to at least one side of the porous protective membrane by a wet transfer procedure. 3. The method of claim 1, wherein applying at least one two-dimensional material layer to at least one side of the porous protective membrane comprises: At least one two-dimensional material layer is provided on the support substrate; Press at least one two-dimensional material layer onto one side of the porous protective membrane; and Remove the support substrate. 4. As in item 3, wherein: The supporting substrate includes a sacrificial layer on its surface; At least one two-dimensional material layer is provided on the sacrificial layer; and Removing the support substrate involves etching the sacrificial layer to remove the support substrate. 5. The method of any of the preceding clauses, wherein the porous protective membrane comprises a nanostructure. 6. The method of paragraph 5, wherein the porous protective membrane comprises nanotubes. 7. The method of any of the preceding clauses, wherein the porous protective membrane is substantially self-supporting. 8. The method of any of the preceding clauses, wherein the or at least one two-dimensional material layer is applied as a substantially continuous layer adjacent to at least one side of the porous protective membrane. 9. The method of any of the preceding clauses, wherein the two-dimensional material comprises graphene. [] 10. The method of any of the preceding clauses, wherein the two-dimensional material comprises hexagonal boron nitride (h-BN). 11. The method of any of the preceding clauses, wherein the two-dimensional material comprises molybdenum disulfide (MoS2). 12. The method of any of the preceding clauses, wherein at least one two-dimensional material layer is applied to both sides of the porous protective membrane, and wherein a capping layer is applied to each side of the surface membrane such that at least one two-dimensional material layer is disposed between the capping layer and the porous protective membrane. 13. The method of any of the preceding clauses, wherein the or each cover layer is a three-dimensional material. 14. The method of any of the preceding clauses, wherein the total EUV transmittance of the or each cover layer is 96% or greater. 15. The method of any of the preceding clauses, wherein at least one capping layer is adapted to protect the porous layer and at least one two-dimensional material layer from hydrogen etching. 16. The method of any of the preceding clauses, wherein at least one cover layer is formed of a material having an extinction coefficient of less than 0.02 nm⁻¹ for EUV radiation. 17. The method of any of the preceding clauses, wherein the capping layer has a thickness of about 0.3 nm to 5 nm. 18. The method of any of the preceding clauses, wherein the capping layer comprises yttrium or yttrium oxide. 19. The method of any of the preceding clauses, wherein the capping layer comprises any of the following: aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), ruthenium (Ru), platinum (Pt), gold (Au), zirconium nitride (ZrN), aluminum (Al) or zirconium (Zr). 20. The method of any of the preceding clauses further comprises attaching the membrane boundary to the periphery of the porous protective membrane. 21. The method of clause 20, wherein the surface membrane boundary is attached to the periphery of the porous membrane before at least one two-dimensional material layer is applied to at least one side of the porous membrane. 22. A coating for use in a lithography device, the coating comprising: A porous protective membrane, which is formed from a first material; At least one two-dimensional material layer adjacent to at least one side of the porous protective membrane; and at least one cover layer adjacent to at least one two-dimensional material layer, such that at least one two-dimensional material layer is disposed between the cover layer and the porous protective membrane. 23. The protective film as described in clause 22, wherein the porous protective film comprises a nanostructure. 24. The protective film as described in clause 23, wherein the porous protective film comprises nanotubes. 25. The protective film of any one of clauses 22 to 24, wherein the porous protective film is substantially self-supporting. 26. The film of any one of clauses 22 to 25, wherein the or at least one two-dimensional material layer forms a substantially continuous layer adjacent to at least one side of the porous protective film. 27. A film as described in any of clauses 22 to 26, wherein the two-dimensional material comprises graphene. [] 28. A film as described in any of clauses 22 to 27, wherein the two-dimensional material comprises hexagonal boron nitride (h-BN). 29. A film as described in any of clauses 22 to 28, wherein the two-dimensional material comprises molybdenum disulfide (MoS2). 30. The film of any one of clauses 22 to 29, wherein at least one two-dimensional material layer is provided adjacent to both sides of the porous protective film, and wherein a cover layer is provided on each side of the film such that at least one two-dimensional material layer is disposed between the cover layer and the porous protective film. 31. The film of any one of clauses 22 to 30, wherein the or each cover layer is a three-dimensional material. 32. The coating of any one of clauses 22 to 31, wherein the total EUV transmittance of the coating or each cover layer is 96% or greater. 33. The film of any one of clauses 22 to 32, wherein at least one capping layer is adapted to protect the porous layer and at least one two-dimensional material layer from hydrogen etching. 34. The film of any one of clauses 22 to 33, wherein at least one cover layer is formed of a material having an extinction coefficient of less than 0.02 nm⁻¹ for EUV radiation. 35. The film of any one of clauses 22 to 34, wherein the capping layer has a thickness of about 0.3 nm to 5 nm. 36. A coating as described in any of clauses 22 to 35, wherein the capping layer comprises yttrium or yttrium oxide. 37. The coating of any one of clauses 22 to 36, wherein the capping layer comprises any one of the following: aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), ruthenium (Ru), platinum (Pt), gold (Au), zirconium nitride (ZrN), aluminum (Al) or zirconium (Zr). 38. The film of any of the preceding clauses, wherein the cover layer comprises a plurality of sublayers formed of different materials. 39. The film as described in any of clauses 22 to 38, further comprising the film boundary at the periphery of the porous protective film. 40. A lithography apparatus for forming an image of a patterned device on a substrate using a radiation beam, the lithography apparatus comprising a protective film disposed in the path of the radiation beam, the protective film comprising: A porous protective membrane, which is formed from a first material; At least one two-dimensional material layer adjacent to at least one side of the porous protective membrane; and At least one cover layer adjacent to at least one two-dimensional material layer, such that the at least one two-dimensional material layer is disposed between the cover layer and the porous protective membrane. 41. The lithography apparatus of item 40, wherein the protective film forms a dynamic airlock. 42. The lithography apparatus of item 40, wherein the protective film forms part of a spectral filter. 43. A lithography apparatus as described in any of items 40 to 42, wherein the porous protective membrane comprises a nanostructure. 44. The lithography apparatus of section 43, wherein the porous protective membrane comprises nanotubes. 45. A lithography apparatus as described in any of clauses 40 to 44, wherein the porous protective membrane is substantially self-supporting. 46. ​​A lithography apparatus of any one of clauses 40 to 45, wherein the or at least one two-dimensional material layer forms a substantially continuous layer adjacent to at least one side of a porous protective membrane. 47. A lithography apparatus as described in any of items 40 to 46, wherein the two-dimensional material comprises graphene. [] 48. A lithography apparatus as described in any of clauses 40 to 47, wherein the two-dimensional material comprises hexagonal boron nitride (h-BN). 49. A lithography apparatus as described in any of clauses 40 to 48, wherein the two-dimensional material comprises molybdenum disulfide (MoS₂). 50. A lithography apparatus of any one of claims 40 to 49, wherein at least one two-dimensional material layer is provided adjacent to both sides of a porous protective membrane, and wherein a cover layer is provided on each side of the protective membrane such that at least one two-dimensional material layer is disposed between the cover layer and the porous protective membrane. 51. A lithography apparatus as described in any of clauses 40 to 50, wherein the or each cover layer is a three-dimensional material. 52. A lithography apparatus of any one of clauses 40 to 51, wherein at least one cover layer is adapted to protect the porous layer and at least one two-dimensional material layer from hydrogen etching. 53. A lithography apparatus of any one of clauses 40 to 52, wherein at least one cover layer is formed of a material having an extinction coefficient of less than 0.02 nm⁻¹ for EUV radiation. 54. A lithography apparatus as described in any of clauses 50 to 53, wherein the cover layer has a thickness of approximately 0.3 nm to 5 nm. 55. A lithography apparatus as described in any of clauses 50 to 54, wherein the cover layer comprises yttrium or yttrium oxide. 56. A lithography apparatus as described in any of clauses 50 to 55, wherein the cover layer comprises any one of the following: aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), ruthenium (Ru), platinum (Pt), gold (Au), zirconium nitride (ZrN), aluminum (Al), or zirconium (Zr). 57. A lithography apparatus as described in any of clauses 50 to 56, wherein the cover layer comprises a plurality of sublayers formed of different materials. 58. The lithography apparatus of any one of clauses 40 to 57 further includes a membrane boundary at the periphery of the porous membrane. 59. A coating for use in a lithography device, the coating comprising: Protective film; The boundary, which is located at the periphery of the protective film and on the first side of the protective film; and The protective portion is located around the periphery of the protective film and on the second side of the protective film. 60. The protective film as described in clause 59, wherein the protective portion is provided on a portion of the protective film that does not receive EUV radiation during use. 61. The protective portion of the film as described in paragraph 59 or 60 is provided on the portion of the protective film that overlaps with the boundary. 62. The protective film of clause 61, wherein the protective portion extends partially into a portion of the protective film that does not coincide with the boundary. 63. The coating of any one of clauses 59 to 62, wherein the protective portion is formed of the same material as the main body of the coating. 64. The coating of any one of clauses 59 to 63, wherein the protective portion comprises a material suitable for protecting a portion of the protective coating to which it is attached from hydrogen etching. 65. A protective film as described in any of clauses 59 to 64, wherein the protective film comprises nanotubes, graphene, and / or amorphous carbon. 66. The film of any one of clauses 59 to 65 further comprises a cover material on at least one surface of which a protective film is coated. 67. The protective film of any one of clauses 59 to 66, wherein the protective film comprises: A porous protective membrane, which is formed from a first material; At least one two-dimensional material layer adjacent to at least one side of the porous protective membrane; and at least one cover layer adjacent to at least one two-dimensional material layer, such that at least one two-dimensional material layer is disposed between the cover layer and the porous protective membrane.

[0266] 1: Laser 2: Laser Beam 3: Fuel launcher 4: Plasma formation zone 5: Near-normal incidence radiation collector 6: Central focus 7: Plasma 8: Opening 9: Enclosure Structure 10: Faceted Field Mirror Device 11: Faceted pupil mirror device 13: Mirror 14: Mirror 15: Reduction Mask Assembly 17: Surface Frame 19: Surface film 100: Method 102: Steps 104: Steps 106: Steps 200: Porous protective membrane 210: Surface boundary 220: Graphene film 230: Cover layer 300: Method 310: Support substrate 312: Substrate 314: Sacrifice Layer 400: Surface film 410: Protective Film 412: First side 414: Second side 416: Part 420: Boundary 430: Protected Part B: Extreme ultraviolet radiation beam IL: Irradiation system LA: Microfilm Equipment MA: Patterning device MT: Support structure PS: Projection system SO: Radiation source W: substrate WT: substrate table

Claims

1. A film for use in a lithography device, the film comprising: a porous protective film formed of a first material; a two-dimensional material layer adjacent to each side of the porous protective film; and a cover layer adjacent to the two-dimensional material layer on each side of the porous protective film, such that each two-dimensional material layer is disposed between a cover layer and the porous protective film.

2. The protective film as claimed in claim 1, wherein the porous protective film comprises a nanostructure.

3. The membrane as requested in item 1 or 2, wherein the porous membrane is substantially self-supporting.

4. The film as claimed in claim 1 or 2, wherein the or at least one two-dimensional material layer forms a substantially continuous layer adjacent to at least one side of the porous protective film.

5. The film as claimed in claim 1 or 2, wherein the two-dimensional material comprises graphene or nanotubes.

6. The film as claimed in claim 1 or 2, wherein the two-dimensional material comprises hexagonal boron nitride (h-BN) or molybdenum disulfide (MoS2).

7. The film as requested in item 1 or 2, wherein the film or each cover layer is a three-dimensional material.

8. The film as requested in item 1 or 2, wherein the total EUV transmittance of the film or each of the cover layers is 96% or greater.

9. The film of claim 1 or 2, wherein the at least one capping layer is adapted to protect the porous protective film and the at least one two-dimensional material layer from hydrogen etching.

10. The film of claim 1 or 2, wherein the at least one cover layer is formed of a material having an extinction coefficient of less than 0.02 nm⁻¹ for EUV radiation.

11. The film as claimed in claim 1 or 2, wherein the cover layer has a thickness of approximately 0.3 nm to 5 nm.

12. The film as claimed in claim 1 or 2, wherein the cover layer comprises yttrium or yttrium oxide.

13. The coating as claimed in claim 1 or 2, wherein the cover layer comprises any one of the following: aluminum oxide (Al2O3), hafnium oxide (HfO2), zirconium oxide (ZrO2), ruthenium (Ru), platinum (Pt), gold (Au), zirconium nitride (ZrN), aluminum (Al) or zirconium (Zr).

14. The cover film of claim 1 or 2, wherein the cover layer comprises a plurality of sublayers formed of different materials, and / or further comprises a cover film boundary at the periphery of one of the porous protective films.

15. A method for forming a film for use in a lithography apparatus, the method comprising: providing a porous protective film formed of a first material; applying a two-dimensional material layer to each side of the porous protective film; and applying a capping layer to the two-dimensional material layer on each side of the porous protective film, such that each two-dimensional material layer is disposed between the capping layer and the porous protective film.

16. A lithography apparatus for forming an image of a patterned device on a substrate using a radiation beam, the lithography apparatus comprising a protective film disposed in a path of the radiation beam, the protective film comprising: a porous protective film formed of a first material; a two-dimensional material layer adjacent to each side of the porous protective film; and a capping layer adjacent to each side of the two-dimensional material layer, such that each two-dimensional material layer is disposed between the capping layer and the porous protective film.

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