WO2025201829A1 - Pellicle membrane for an euv utilization apparatus and method of manufacturing thereof - Google Patents

Abstract

There is provided a membrane for use as pellicle in EUV utilization apparatus, the membrane comprising randomly oriented non-coated carbon nanotubes having one or both of: i) an average diameter less than 5 nm and a bundle (average) diameter of less than 30 nm, wherein the Young's modulus of the membrane is larger than 10MPa; and ii) surface features that provide roughness to the nanotubes, such that relative movement of the CNT tubes in the film is substantially blocked or inhibited. Also provided is a pellicle for an EUV utilization apparatus, the pellicle including such a membrane and a support frame for supporting the membrane, as well as an EUV utilization apparatus comprising such a membrane or pellicle. There is also provided a method of manufacturing a membrane for an EUV utilization apparatus.

Description

PELLICLE MEMBRANE FOR AN EUV UTILIZATION APPARATUS AND METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24167405.0 which was filed on March 28, 2024, EP application 24196466.7 which was filed on August 26, 2024, EP application 24217845.7 which was filed on December 05, 2024 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a carbon nanotube membrane, a pellicle comprising a carbon nanotube membrane, an exposure apparatus for semiconductor manufacturing processes, such as a lithographic apparatus, for example an EUV utilization apparatus, such as a lithographic apparatus comprising a pellicle or carbon nanotube membrane, and the use of a method, pellicle or carbon nanotube membrane in an EUV utilization method or apparatus. The present invention particularly relates to carbon nanotube membranes comprising carbon nanotubes having pre-selected characteristics such as average diameter, as well as carbon nanotubes having surface features that inhibit or prevent relative movement of carbon nanotubes along one another. The present invention has particular, but not exclusive, application to EUV lithography apparatuses and methods, but is also relevant to exposure apparatuses for semiconductor manufacturing processes, such as lithographic apparatuses, metrology apparatuses, inspection apparatuses.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).

[0005] A lithographic apparatus includes a patterning device (e.g. a mask or reticle). Radiation is provided through or reflected off the patterning device to form an image on a substrate. A membrane assembly, also referred to as a pellicle, may be provided to protect the patterning device from airborne particles and other forms of contamination. Contamination on the surface of the patterning device can cause manufacturing defects on the substrate. [0006] Pellicles may also be provided for protecting optical components other than patterning devices. Pellicles may also be used to provide a passage for lithographic radiation between regions of the lithography apparatus which are sealed from one another. Pellicles may also be used as filters, such as spectral purity filters or as part of a dynamic gas lock of a lithographic apparatus.

[0007] A mask assembly may include the pellicle which protects a patterning device (e.g. a mask) from particle contamination. The pellicle may be supported by a pellicle frame, forming a pellicle assembly. The pellicle may be attached to the frame, for example, by gluing or otherwise attaching a pellicle border region to the frame. The frame may be permanently or releasably attached to a patterning device.

[0008] Due to the presence of the pellicle in the optical path of the EUV radiation beam, it is necessary for the pellicle to have high EUV transmissivity. A high EUV transmissivity allows a greater proportion of the incident radiation through the pellicle. In addition, reducing the amount of EUV radiation absorbed by the pellicle may decrease the operating temperature of the pellicle. Since transmissivity is at least partially dependent on the thickness of the pellicle, it is desirable to provide a pellicle which is as thin as possible whilst remaining reliably strong enough to withstand the sometimes hostile environment within a lithography apparatus.

[0009] It is therefore desirable to provide a pellicle which is able to withstand the harsh environment of a lithographic apparatus, in particular an EUV lithography apparatus. It is particularly desirable to provide a pellicle which is able to withstand higher powers than previously.

[0010] Since pellicles are in the optical path of the lithography apparatus, if the transmissivity of the pellicle varies over time during use, it may fall outside the allowable tolerances of the lithography apparatus and require replacement. It is therefore desirable to provide a pellicle which has a consistent transmissivity during use or at least which has a reduced rate of drift of transmissivity than previously. [0011] A protective coating may be applied to a membrane material to protect the membrane material from being etched within the lithography apparatus. However, the protective coating may become damaged or separated from the membrane due to differences in the thermal expansion of the different materials.

[0012] During nominal use, the pressure within an EUV utilization apparatus changes. In addition, due to the pulsed nature of the EUV radiation generated, a pellicle membrane undergoes very rapid heating pulses which generate forces within the pellicle membrane.

[0013] The present invention has been devised in an attempt to address at least some of the problems identified above.

SUMMARY OF THE INVENTION

[0014] According to a first aspect of the present invention, there is provided a membrane for use as a pellicle in an exposure apparatus for a semiconductor manufacturing process, such as an EUV utilization apparatus, the membrane comprising randomly oriented non-coated carbon nanotubes having one, two, three, or each of: i) an average diameter less than 5 nm and a bundle (average) diameter of less than 30 nm, wherein the Young’s modulus of the membrane is larger than lOMPa; and ii) surface features that increase friction between the nanotubes, such that relative movement of the CNTs in the film is blocked or inhibited, iii) a hydrogen radical quencher or hydrogen radical scavenger material; and iv) a network of metal nanowires. The increase in friction may be due to, for example, increased roughness, steric effects, inter-CNT bonds, or morphology of the CNTs.

[0015] As such, the membrane may have one of features i), ii), iii), or iv) individually. The membrane may have any two of features i), ii), iii), or iv) together, such as i) and ii), i) and iii), i) and iv), ii) and iii), ii) and iv), or iii) and iv). The membrane may have any three of features i), ii), iii), or iv) together, such as i), ii), and iii), or i), ii) and iv), or i), iii), and iv), or ii), iii), and iv). The membrane may have each of i), ii), iii), and iv).

[0016] As mentioned, the environment within an exposure apparatus for a semiconductor manufacturing process, such as an EUV utilization apparatus, such as a lithographic apparatus or an inspection tool, is rather hostile with the membrane being subject to high temperatures, such as in excess of 600°C, EUV radiation, and hydrogen plasma. As the power of exposure apparatuses, such as EUV utilization apparatuses, increases, the environment will continue to be even more hostile and so it is useful to provide membranes which are able to withstand the environment within exposure apparatuses, such as an EUV utilization apparatus.

[0017] It has been found that a CNT membrane comprising carbon nanotubes having an average diameter of less than 5nm and a bundle average diameter of less than 30 nm forming a CNT membrane with a Young’s modulus of larger than 10 MPa is suitable for use in an EUV utilization apparatus. Without wishing to be bound by scientific theory, rigidity may negatively affect mechanical robustness to stressors. In addition, it has been found that providing increased friction, such as through increased roughness to the surface of the nanotubes, is also advantageous in stabilising the pellicle membrane. Since CNTs are very smooth structures that cross one another at effectively point-like, zero-dimensional contact-based connections, it is easy for them to slide along or past one another due to the low amount of friction. As such, this weakens the mechanical properties of a membrane which is formed from pristine CNTs. Such membranes gain their stability from entangled CNTs which are short in comparison to the entire mesh of CNTs forming the membrane. The entanglement between the comparatively short CNTs enables the mesh to stay intact when suspended over larger areas without additional support, other than the border which surrounds the area in which the CNT mesh is free-standing. As the CNT mesh is subjected to thermo-mechanical stress during nominal use, this can lead to the sliding of CNTs past each other, which can result in disentanglement of the CNTs and subsequent gapping, opening, or rupture of the mesh. Even if the failure is localised, this can lead to EUV transmissivity non-uniformity outside of the required specification as well as a loss in particle-stopping function of the pellicle locally, leading to increased defectivity counts at the reticle level. The present invention addresses these issues by providing surfaces features on the CNTs which increase the effective roughness of the CNTs to thereby prevent or reduce the amount by which CNTs can slide across one another in nominal use. By restricting the sliding of the CNTs relative to one another, the issue of EUV transmissivity nonuniformity is addressed and the issue of gapping is also addressed.

[0018] During use in an EUV utilization apparatus, carbon nanotube-based pellicle membranes are etched in the plasma radical environments. Such etching is undesirable as it decreases the operational lifetime of the pellicle membrane. It is possible to reduce the rate of etching in hydrogen plasma by heating the pellicle membrane, such as up to around 700°C, which causes adsorbed hydrogen to desorb. Without wishing to be bound by scientific theory, it is believed that hydrogen radicals preferentially react with other hydrogen radicals to form hydrogen over reacting with the carbon structure of sp2 graphene found in carbon nanotubes. Even though heating the pellicle membrane is a useful way of reducing the rate of etching of carbon from the membrane when in use, this provides an additional heatload on the reticle being protected by the pellicle membrane as well as to the reticle stage and other optical elements. Such additional thermal load can induce extra thermal induced overlay errors, which are undesirable. The present disclosure provides options for extending the operational lifetime of a carbon nanotube pellicle membrane. By providing a hydrogen radical quencher or hydrogen radical scavenger material, it is possible to protect the carbon nanotubes from being etched by the hydrogen plasma which is present when an EUV utilization apparatus is in operation. Such hydrogen radical quenchers or hydrogen radical scavengers serves to reduce the amount of hydrogen radicals within the hydrogen plasma. Such materials may be integral to the membrane, such as decorations on the carbon nanotubes, or may be temporarily associated with the membrane, such as in the form of a material, such as a gas, adsorbed on the membrane, which may be replenished during operation.

[0019] Additionally or alternatively, the membrane may be provided with a network of metal nanowires. As such, the membrane may be a metal-carbon nanotube composite including both metal wires and carbon nanotubes. The network of metal-carbon nanotube composite may therefore comprise metal wires intermixed with carbon nanotubes, forming a composite network in which metal wires and CNT are randomly mixed with each other. It will be appreciated that the wires do not necessarily have to be physically connected to other metal wires. Whilst it is possible to provide a coating to the carbon nanotubes, the coating would need to be sufficiently thick to make it functionally impenetrable to hydrogen, which would reduce the transmissivity of the pellicle membrane, but may also lead to spalling since the membrane undergoes temperature changes in nominal use and the coefficient of thermal expansion of the carbon nanotube and the coating may not be the same. By providing a network of metal nanowires, the benefits of hydrogen recombination assisted by the metal is realised without requiring the metal and the carbon nanotubes to have a compatible coefficient of thermal expansion, and also makes manufacture easier.

[0020] The Young’s modulus may be between around lOMPa to around 200 MPa. By around, it will be understood that this also includes the values of lOMPa and 200 MPa, with a variance of ±5% or less, ±4% or less, ±3% or less, ±2% or less, or ±1% or less. Since the membrane is subject to differential pressures which cause deflection in nominal use, by providing a membrane with a certain stiffness the amount of deflection can be controlled to within specification. Although it will be appreciated that since CNT membranes are formed of a mesh of carbon nanotubes and therefore comprise multiple pathways for gas to pass through, there may still be a pressure differential in nominal use that is not fully compensated for by the porous nature of the mesh.

[0021] The areal density of carbon in the membrane may be between 10 xlO¹⁵ and 500xl0¹⁵ C atoms/cm². It has been found that such an areal density is sufficient to provide a CNT membrane which is sufficiently strong to withstand the EUV environment and also have an EUV transmissivity of over 90%.

[0022] The carbon nanotubes may have a [8,3] chirality, [14,6] chirality, or a mixed chirality, optionally wherein the mixed chirality comprises at least 30% of [8,3] chiral CNTs.

[0023] The rate of etching by a hydrogen plasma of a CNT is at least partially dependent on the chirality of the carbon nanotube. Nanotubes with [8,3] chirality are etched more slowly than CNTs with the more prevalent [6,5] chirality. It is also possible to have nanotubes with [14,6] chirality that are etched in hydrogen plasma even more slowly than [8,3] nanotubes. As such providing a CNT membrane with a mixed chirality can be used to control the rate of etching of a CNT membrane within an EUV utilization apparatus. This may be useful as the EUV transmissivity will change during use and by being able to control the rate of etching, the rate at which EUV transmissivity drifts over time can consequently be controlled.

[0024] The CNT purity may at least 99at % C. The CNTs may be free from defectivity adders with size larger than 10 microns. When referring herein to CNT purity, it is understood that it excludes the added hydrogen scavengers and metal wires or any other deliberate additions to the CNT network or pellicle membrane. A defectivity adder may be a particle or particulate matter with EUV absorption greater than the CNTs. By having a high CNT purity of 99at% C or more, there is less risk of contamination caused by any additional atoms included in the CNTs and also the rate of etching is more predictable and uniform than would be the case when using contaminated CNTs.

[0025] The membrane may have an EUV transmissivity of greater than 90%. Since it is desirable to allow as much of the EUV radiation to pass through the pellicle membrane as possible to provide a decent throughput of the EUV utilization apparatus and also to control heating of the pellicle membrane, providing a membrane with an EUV transmissivity of greater than 90% means that the temperature of the pellicle membrane is controlled and the throughput is maintained at an acceptable level, whilst still providing an effective protective membrane.

[0026] The membrane may comprise singled-walled nanotubes, double-walled nanotubes, multiwalled nanotubes, or a combination of any of the aforesaid.

[0027] The carbon nanotubes may have a diameter of up to around 2 nm, up to around 2.5 nm, around 2.5 nm, around 2 nm, around 1.5 nm, or around 1 nm. With slightly thicker CNTs of around 1.5 nm in diameter or greater, it is possible to have [14,6] chirality, which is resistant to etching in hydrogen plasma.

[0028] The CNT surface features may comprise defects. The defects may comprises one or more of the following mechanisms: a) an ad-atom, b) chiral Stone -Wales effect, c) two missing C atoms (in a single defect), d) monovalency defect. Additionally or alternatively, the CNT surface features include functional groups.

[0029] Since pristine CNTs are smooth, there is a low degree of friction between CNTs, such that they are able to slide over and past one another easily. By providing surface defects, the effect of this is that there is a higher resistance to the CNTs passing over one another, akin to an increase in friction. Such surface defects can be introduced to increase the friction between touching CNTs.

[0030] Functional groups may serve a similar function of increasing the effective friction between CNTs by providing a steric effect, which means that protruding groups can physically make sliding of CNTs past one another more difficult.

[0031] The functional groups may include hydrophobic groups, hydrophilic groups, dipole-carrying groups, or two or more of hydrophobic groups, hydrophilic groups, and dipole-carrying groups. Such groups serve to increase the interactions between adjacent CNTs, thereby making it more difficult for the CNTs to move or slide past one another.

[0032] The hydrophobic groups may comprise one or more of phenyls, alkanes, alkenes, and alkynes, and the dipole-carrying groups may include a hydroxyl group.

[0033] The surface feature may include a covalently bonded crosslinking group that bonds adjacent CNTs together. By providing a covalently bonded crosslinking group, the CNTs are no longer able to slide over one another. Again, this has the effect of increasing the effective friction between CNTs, thereby making it more difficult for them to slide over or past one another.

[0034] The crosslinking group may be one or more of: 1,4-benzoquinones, 1,5-hexadiene, aryl diazonium salts, and poly(ethylene glycol) chains.

[0035] The crosslinking group may be a reaction product of radical polymerization of CNTs or ionbeam irradiation of CNTs, or a polymer coating.

[0036] The CNTs may have a non-linear morphology. In other words, the surface feature may be a non-linear morphology of the CNT. The CNTs may be helical CNTs. By providing helical CNTs, they are less able to slide over one another as compared to the usual substantially straight CNTs. This increases the effective friction between the CNTs and make the mesh of CNTs forming the membrane more resistant to thermo-mechanical forces. In addition, such helical CNTs have greater elastic properties than non-helical CNTs and so can spring back after being deformed. In this way, the entangled helical CNTs are effectively more difficult to slide past one another due to this structure and steric hindrance, which effectively increase the friction between the CNTs. This additional entanglement and restriction in ability for CNTs to pass one another is additional to the unorientated or random entangled nature of the bundle arrangement or mesh that forms the membrane. [0037] The surface feature may include amorphous carbon. Amorphous carbon may be deposited on the CNTs to form bumps which are more difficult to slide past one another than smooth, pristine CNTs. [0038] The membrane according to any preceding claim, wherein the network of metal nanowires is a random network. A random network is useful since it is less likely to adversely affect light, such as EUV radiation, passing therethrough. With an ordered network, there is a greater likelihood of interactions with light passing therethrough which can cause unwanted artifacts in the light. In addition, a random network is easier to manufacture.

[0039] The metal nanowires may be non-oxidised. Whilst an oxidized metal surface is able to recombine hydrogen radicals, a metallic surface which is not oxidized has a higher surface recombination efficiency. Therefore, a non-oxidised surface is more efficient at recombining hydrogen radicals, meaning that it is possible for fewer metal nanowires to be required and/or the rate of recombination is higher, thereby leading to a reduction in the amount of hydrogen radicals available to etch the carbon nanotubes.

[0040] The metal nanowires may comprise at least one metal with a high hydrogen recombination efficiency. The metal nanowires may comprise rhodium, platinum, or combinations thereof. By utilizing metals with a high hydrogen recombination efficiency, either less metal is required, which assists with manufacturing costs and difficulty, and/or leads to a high rate of hydrogen radical recombination.

[0041] The metal nanowires may have a diameter less than the carbon nanotubes. The metal nanowires may have a diameter equal to that of the carbon nanotubes. The metal nanowires may have a diameter greater than that of the carbon nanotubes. Preferably, the diameter of the metal wires is as small as possible. The metal nanowires may have an average diameter less than the average diameter of the carbon nanotubes or carbon nanotube bundles. The metal nanowires may have an average diameter less than the thickness of the CNT pellicle membrane. A smaller diameter means that there is less absorption of EUV radiation by the metal nanowires. In addition, since hydrogen radical recombination on metals is primarily a surface phenomenon, thinner wires mean that there is a higher effective surface area for hydrogen radical recombination as compared to the case of having thicker wires, even with the same overall mass of metal present. The metal nanowires may have a diameter of 30 nm or less. The nanowires may have a diameter of from about 10 nm to about 20 nm.

[0042] The nano wires may be electrospun nano wires. Electrospinning of metal wires is able to provide wires with a very small diameter and is also a rapid and consistent process. Also, the metal nanowires are fully filled with metal material, i.e. they do not have a hollow core like present in the carbon nanotubes.

[0043] The nanowires may be interwoven with the carbon nanotubes. As such, rather than the membrane comprising a layer of metal nanowires and a layer of carbon nanotubes, having the metal nanowires interwoven with the carbon nanotubes allows the metal nanowires to more effectively protect the carbon nanotubes. The term “interwoven” is not intended to specify that there is a particular weaving pattern or that there is some weaving steps in the manufacturing process, rather simply that the metal nanowires and carbon nanotubes are combined in a composite mesh. For example, the metal nanowires and carbon nanotubes may be mixed or woven together in a wet or a dry mixing process before being formed into a pellicle membrane.

[0044] The hydrogen scavenger or hydrogen quencher material is an alkyne, optionally wherein the hydrogen scavenger or hydrogen quencher material is provided on the carbon nanotubes. An alkyne includes a triple bond between two carbon atoms and is able to readily react with hydrogen radicals. Alkynes are able to react with hydrogen radicals rapidly. Since the rate of reaction of alkynes with hydrogen radicals is shorter than the lifespan of chemi- or physi-sorbed hydrogen radical species, the reaction with alkynes is able to protect the carbon nanotubes by effectively outcompeting the carbon nanotubes for reaction with the hydrogen radicals. The alkynes may be provided as a layer, such as a monolayer, adsorbed to the membrane. As such, the alkynes may be associated by intermolecular forces, such as Van der Waals forces. The alkyne layer may be refreshed by the provision of further alkyne material during operation. The alkyne may be chemically bonded to the carbon nanotubes, which may be referred to as the carbon nanotubes being decorated with the alkyne. Alkynes, being formed of carbon and hydrogen, have good EUV transmissivity and so do not adversely affect the amount of EUV radiation which can pass therethrough. In addition, ethyne and propyne have low boiling points and so are gaseous at the operating temperature of an EUV utilization machine. As such, there is no risk that they will be deposited within the machine, such as on any optical elements such as mirrors. Furthermore, the reaction products of alkynes with hydrogen radicals have an even lower boiling point than the alkynes themselves, so there is similarly no risk of the reaction product being deposited. For example, ethyne has a boiling point of -84°C and its reaction product with hydrogen radicals, ethane, has a boiling point of -107°C.

[0045] According to an embodiment of the first aspect, there is provided a membrane for use as a pellicle in an exposure apparatus for semiconductor manufacturing, such as an EUV utilization apparatus, the membrane comprising a core comprising randomly orientated carbon nanotubes disposed between a first non-conformal top layer and a second non-conformal bottom layer. The top and/or bottom layers may be membranes. The core may be a film.

[0046] As such, the membrane is a multi-layered membrane comprising a carbon nanotube layer sandwiched between two non-conformal layers. The non-conformal layers are preferably substantially planar, such as planar membranes. It will be understood that the layers may not be perfectly flat due to intrinsic structure of the materials of the layers as well as due to manufacturing tolerances. It has been found that the provision of non-conformal layers on either face of a carbon nanotube core can raise the emissivity of the membrane as a whole up to around 72%, for specific values of the electrical conductivity of the non-conformal membranes. Without the non-conformal layers, the emissivity is less than 10%. A greater emissivity means that the membrane is able to withstand higher power loads than would otherwise be the case, or is able to operate at a lower temperature than a membrane without the two facial layers under the same power loads. [0047] In operation, a pellicle membrane is subject to etching by hydrogen plasma. Without wishing to be bound by scientific theory, it is believed that the etching is caused by the breaking of carboncarbon bonds within the carbon nanotubes followed by passivation of the dangling carbon bonds by hydrogen. Over time, carbon is released from the carbon nanotubes as volatile hydrocarbon species. Whilst it may be possible to reduce or eliminate etching by desorbing hydrogen from the membrane such that any dangling bonds are not passivated by hydrogen, the carbon-carbon bonds are still broken, which results in continuous mechanical pre-tension loss. Therefore, whilst etching may be addressed, the SP2 hexagonal crystalline CNT structure transforms into SP3 amorphous carbon over time. In use, it has been observed that the yield strength of a carbon nanotube membrane may be reduced by 50% after the equivalent of 1000 wafer exposures in a plasma of approximately 5 eV H ion energy, increasing to 90% by 2000 wafer exposure equivalents. By providing a coating on the CNTs themselves, it is possible to protect the CNTs, but this is at a cost of EUV transmissivity loss and an increase in flare, both of which are undesirable. The decrease in transmissivity means that a greater proportion of the energy falling upon such a membrane is absorbed, which results in higher operating temperatures. Whilst emissivity could be increased to combat this, the addition of a conformal metallic coating could lead to even lower EUV transmissivity and consequently even greater heating. In addition, due to the different in the coefficient of thermal expansion between carbon nanotubes and a conformal coating, the coating may become detached from the carbon nanotubes during nominal use.

[0048] The planar encapsulation of a carbon nanotube core according to the present disclosure both provides for increased emissivity, which significantly improves power capability, and also decreases or eliminates mechanical degradation. The planar encapsulation also decreases EUV flare as well as out of bound scattering as compared to uncoated carbon nanotubes, and also decreases or removes the geometrical EUV transmissivity loss caused by membrane porosity.

[0049] An increase in absorption results when electromagnetic waves of wavelengths greater than around 10 microns are incident upon an absorbing material, then, a certain reflectance, transmission, and absorption of the incident radiation occur. For carbon nanotubes, reflectance and absorbance are very low and transmission is very high. It has been shown that for wavelengths of greater than 10 microns, the solutions to Maxwell’s equations simplify and that as a function of parameter f, the reflectance, transmission, and absorption have a characteristic trend which encompasses any absorbing metallic material at such wavelength such that absorption reaches a maximum of 50% at f = 2. The parameter f represents the product of absorber thickness s and electrical conductivity o with a proportionality constant 120TI.

[0050] Placing a dielectric layer with refractive index squared (n²) adjacent an absorber changes the theoretical maximum absorption of radiation. Placing two infrared absorber layers separated by a vacuum results in a theoretical maximum absorption of radiation of greater than 10 microns, of 83%, which is much greater than the 50% for a single absorber layer, provided that the combined factor f of the layers attains a value of f=^2. When a dielectric is disposed between two absorber layers, the maximum absorption shifts from 83% to lower values, albeit still higher than 50%. For example, where the refractive index squared is 2.5, the maximum absorption drops to 70%.

[0051] For the case of two metal absorber layers separated by a vacuum, the refractive index squared is 1, f = A/2, and f = 120TIO(SI + S2) wherein si and S2 are the thicknesses of the two absorber layers. It is possible to rearrange this equation to:

V2

° ~ 120TT(S1 + s2)

[0052] It has been found that this trend shifts from wavelengths of greater than 10 microns to lower values, for specific values of the electrical conductivity and the number of electrons that contribute to conductivity per atom, such that the wavelength shifts into the region of maximal thermal emissivity provided by Wien’s displacement law. This is the case as Kirchoff’s law states that in thermal equilibrium absorptivity equals emissivity. Therefore if maximum absorptivity for shorter wavelengths is elevated, then so is emissivity. In this way not only is emissivity from the pellicle membrane of far infrared radiation increased, lower wavelengths are also efficiently radiated. It will be appreciated that the emissivity of all radiation of certain wavelengths will depend on the material of the top and/or bottom layers. For example, it has been found that when bi-layer graphene is used of electrical conductivity of 2.76 MJ-kg ’-m ¹ as top- and bottom-layer, the emissivity for all radiation of wavelength of 4.0 microns or greater is enhanced, such that pellicles according to the present disclosure heated to around 450° C can effectively radiatively cool over all its available wavelengths. For additional examples, a molybdenum layer would provide emissivity for all radiation of wavelength of 1.7 microns or greater, rhenium would provide emissivity for all radiation of wavelength of 4.4 microns or greater, and beryllium would provide emissivity for all radiation of wavelength of 8.8 microns or greater. The electrical conductivity of one or both of the top layer and the bottom layer may be from around 1 to around 6 MJ-kg ’-m ’.

[0053] Hotter membranes also are able to radiate and whilst this may be slightly less efficient, they are still significantly more emissive than existing carbon nanotube pellicles. As such, it has been surprisingly realized that the addition of non-conformal top and bottom layers actually significantly increases emissivity. Whilst there is not a vacuum between the top and bottom layers, the presence of a carbon nanotube core, with its significant void space, still allows this advantage to be realised. If the porosity of the carbon nanotube layer is assumed to be 50% then the n² value, where n is the refractive index, lies somewhere between that of a vacuum (n²=l) and that of a closely packed dense CNT layer (n²=4). In particular, using the Maxwell-Garnett effective medium approach for a composite material like a porous CNT membrane in a vacuum, it is possible to estimate an n² value as follows: 2.2 wherein p = 0.5, assuming a porosity of 50% (50% vacuum and 50% CNTs). The effective refractive index squared is approximately 2.2, which leads to a maximum absorptivity of around 72%.

[0054] This effect is realized for a bi-layer system according to the present disclosure using the Hagen-Rubens equation, which relates electron relaxation time r and electrical conductivity o for a metal, wherein: om_e T ⁼ - n_e e 2² where m_e is the mass of an electron, e is the charge of an electron, and n is the electron density.

[0055] Where r is small, incident waves are in phase with electron density currents for higher frequencies and longer wavelengths. For most metallic materials, this entails the validity of the Hagen- Rubens relation holding for wavelengths of 10 microns or higher. The transition angular frequency for which the Hagen-Rubens relation holds is given by:

1 m = — — V3T

The relation between angular frequency and wavelength is given by:

2 TC = — wherein c is the speed of light in vacuum.

This leads to the equation:

Based on this equation, it is possible to determine which materials allow A_t to shift to lower values than 10 microns. For example, rhenium (Re), beryllium (Be), molybdenum (Mo), and graphene all provide a threshold of less than 10 microns. As such, these materials allow a membrane comprising sheets of such materials to thermally cool as they maximally radiate in the spectral region determined by Planck’s emission law and Wien’s displacement law.

[0056] For example, where the top and bottom layers comprise graphene membranes, this provides a A_t value of around 4.0 microns as well as a suitable value of f, such that maximum absorption is raised. Graphene has high electrical conductivity of around 2-200 x 10⁵ S.m ’. Where the top and bottom layers comprise graphene, it is possible to provide a value of f = 2 with a thickness of around 0.68 nm, which could be provided by way of a bilayer of graphene. This would result in a membrane with a maximum absorption of around 72% for light of 4.0 microns and greater. It is noteworthy that the separation between the two layers or dielectric spacer thickness between the two layers does not appear in the derived equations and therefore is not a limiting consideration in practice. Such a membrane would have a maximum emissivity at around 450°C.

[0057] The top and bottom layers do not conform to the CNT core morphology, but are substantially flat, planar layers.

[0058] The top layer and the bottom layer may be the same. The top layer and the bottom layer may be different.

[0059] One or both one or both of the top layer and the bottom layer may comprise a 2D material. The 2D material may be selected from one or more of carbon-based, silicon-based, and boron-based 2D materials. The 2D material may comprise graphene, silicene, TMDS (tetramethyldisiloxane), black phosphorus, carbon nitride, germanene, borophene, stanine, arsenene, aluminene, antimonene, or bismuthene.

[0060] One or both of the top layer and the bottom layer may comprise molybdenum, ruthenium, beryllium, or rhenium. The molybdenum, ruthenium, beryllium, or rhenium may be provided on top of the 2D material to improve emissivity. As discussed, it has been found that these materials have a low emission threshold when configured as two separate layers and are therefore able to efficiently radiate thermal radiation to provide cooling.

[0061] The carbon nanotubes in the core may be the carbon nanotubes according to the first aspect of the present disclosure.

[0062] The top layer and the bottom layer may each or both include a further conductive layer. It will be appreciated that the top and/or bottom layers may already be conductive, but it is possible to add an additional conductive layer. As such, the top and/or bottom layers may comprise two conductive layers or one none-conductive layer and one conductive layer. It will be appreciated that top layer and bottom layer do not necessarily require that they are the outermost layer since such top and bottom layers may have further layers provided thereon. Top and bottom is used to denote two opposing sides of the core film or membrane.

[0063] One or both of the top layer and the bottom layer may be continuous. One or both of the top layer and the bottom layer may be at least partially formed of patches. Optionally, the patches may be separated by gaps of 4 microns (pm) or less.

[0064] One or both of the top layer and the bottom layer may be bilayers.

[0065] One or both of the top layer and the bottom layer may be provided with a capping layer, optionally wherein the capping layer comprises one or more of aluminium oxide, yttrium oxide, and yttrium silicon oxide. Capping layers are selected to protect underlying layers from etching by hydrogen plasma and thereby extend the lifespan of the membrane.

[0066] The membrane may have an emissivity of greater than 50%. As discussed, the membrane of the present disclosure is structured such that it has an emissivity greater than the theoretical maximum emissivity of a single layered membrane, namely 50%. As such, single layer membranes are unable to achieve such theoretical emissivities. This is achieved by the provision of two layers which are set apart from one another.

[0067] There is also provided a method of manufacturing a membrane for an exposure apparatus for semiconductor manufacturing, such as an EUV utilization apparatus, the method including the steps of i) providing a core film comprising a plurality of randomly orientated carbon nanotubes, the core film having a top face and a bottom face, and ii) providing a substantially planar layer on the top face and on the bottom face.

[0068] The method may include depositing silicon oxide on the core film prior to step ii). In this way the silicon oxide may hold the carbon nanotubes in place whilst the membrane is undergoing manufacture.

[0069] The method may further include removing material from the top face and/or the bottom face to provide a planar top face and/or bottom face prior to step ii). It is desirable for the ultimate membrane to be planar and so by removing material from the top and bottom faces, it is possible to provide a planar surface onto which the top and bottom layer materials may be provided.

[0070] The method may include providing the substantially planar layer on the planar top face and providing the substantially planar layer on the planar bottom face.

[0071] The method may further include removing any support layers from the membrane. It will be appreciated that the top and bottom layers may be fragile due to their thinness and the material from which they are made. As such, the top and/or bottom layers may be provided on a support layer, which can be subsequently removed.

[0072] The method may further include providing a capping layer on one or both of the planar layer on the top face and/or the planar layer on the bottom face. As discussed, the capping layer serves to protect an underlying layer from hydrogen plasma in use.

[0073] The method may further include etching silicon oxide deposited on the core film. Since it is desirable to provide a membrane with high transmissivity, removal of the silicon oxide deposited on the core film is useful since the removal reduces the amount of material through which a laser beam must pass in nominal use and therefore increases the transmissivity of the membrane.

[0074] According to a second aspect of the present disclosure, there is provided a pellicle for an EUV utilization apparatus, the pellicle including the membrane according to any preceding claim and a support frame for supporting the membrane. [0075] According to a third aspect of the present disclosure, there is provided an apparatus for extending the operational lifetime of a pellicle membrane, the apparatus configured to provide a hydrogen radical quencher or hydrogen radical scavenger material to the pellicle membrane.

[0076] As described, the carbon nanotube -based pellicle membrane may include a hydrogen radical quencher or hydrogen radical scavenger material. This may be a material which is separate to the pellicle membrane, such as an alkyne gas, that is provided to the membrane in order to preferentially react with the hydrogen radicals which would otherwise etch carbon from the carbon nanotubes.

[0077] The apparatus may include a hydrogen radical quencher or hydrogen radical scavenger material doser configured to direct the hydrogen radical quencher or hydrogen radical scavenger material towards the pellicle membrane. The doser may be a pipe with an outlet that provides the quencher or scavenger material to the membrane.

[0078] The hydrogen radical quencher or hydrogen radical scavenger material may comprise an alkyne, optionally wherein the alkyne is ethyne, propyne, or combinations thereof.

[0079] The apparatus may include a controller configured to control the provision of the hydrogen radical quencher or hydrogen radical scavenger material. The controller may be configured to control the rate at which the quencher/scavenger material is provided. For example, in operation, the amount of hydrogen radicals in the atmosphere in which the pellicle membrane is located will vary depending on how the EUV utilization apparatus is operating. In cases where there is a greater amount of hydrogen radicals, the rate of provision of the quencher/scavenger material may be increased. Similarly, where there is a reduced amount of hydrogen radicals, the rate of provision of the quencher/scavenger may be reduced.

[0080] The apparatus may be configured to provide the hydrogen radical quencher or hydrogen radical scavenger material to one or both of a front face and a rear face of the pellicle membrane. Whilst the front face of the pellicle membrane is likely to be subject to the greatest amount of etching by hydrogen radicals, it is possible for the rear face to also be etched. As such, the apparatus may be configured to provide the protective quencher/scavenger material to either or both faces.

[0081] The apparatus may be configured to provide the hydrogen radical quencher or hydrogen radical scavenger material at a rate sufficient to provide a mono-layer to the pellicle membrane every 5 seconds or less, every 4 seconds or less, every 3 seconds or less, every 2 seconds or less, or every second or less. Since the quencher/scavenger material will react with hydrogen radicals or will otherwise desorb from the pellicle membrane in nominal use, it may be necessary to refresh the quencher/scavenger material in order to provide continued protection for the pellicle membrane.

[0082] The apparatus may be configured to provide the hydrogen radical quencher or hydrogen radical scavenger material at a pressure of 0.10 mPa or less, 0.09 mPa or less, 0.08 mPa or less, 0.07 mPa or less, 0.06 mPa or less, 0.05 mPa or less, 0.04 mPa or less, 0.03 mPa or less, 0.02 mPa or less, or 0.01 mPa or less. [0083] According to a fourth aspect of the present disclosure, there is provided an EUV utilization apparatus comprising the membrane, apparatus, or pellicle according to any preceding claim, optionally wherein the EUV utilization apparatus is an EUV lithography apparatus or an EUV inspection tool.

[0084] According to a fifth aspect of the present disclosure, there is provided a method of manufacturing a membrane for an EUV utilization apparatus, the method including the steps of i) providing surface features to a plurality of CNTs to prevent sliding of the CNTs over one another when formed into a CNT membrane, and ii) forming a CNT membrane of such CNTs.

[0085] According to a sixth aspect of the present disclosure, there is provided a method of extending the operational lifetime of a carbon nanotube based pellicle membrane, the method including one or both of: i) providing a hydrogen radical quencher or hydrogen radical scavenger material to the pellicle membrane; and ii) providing a network of metal nanowires associated with the pellicle membrane.

[0086] As described above, it is possible to extend the operational lifetime of a CNT pellicle membrane but providing one or both of a hydrogen radical quencher/scavenger material and a network of metal nanowires which serve to, either alone or in combination, reduce the amount of hydrogen radicals which are able to etch the carbon nanotubes, whether by reacting with the hydrogen radicals or by catalyzing the recombination of hydrogen radicals into hydrogen.

[0087] The method may include providing an alkyne as the hydrogen radical quencher or hydrogen radical scavenger material to the pellicle membrane, optionally wherein the alkyne comprises ethyne, propyne, or combinations thereof.

[0088] The method may include providing the hydrogen radical quencher or hydrogen radical scavenger material to the pellicle membrane to one or both of a front face and a rear face of the pellicle membrane.

[0089] The hydrogen radical quencher or hydrogen radical scavenger material is provided as a gas. By providing the quencher/scavenger as a gas, the gas is able to diffuse and provide protection for a larger area of the pellicle membrane. In addition, the gas is not liable to condense or otherwise be deposited and so contamination of surfaces is avoided.

[0090] Provision of the hydrogen radical quencher or hydrogen radical scavenger material may be controlled depending on the amount or concentration of hydrogen radicals generated in use. As such, more or less of the quencher/scavenger material may be provided depending on the amount of hydrogen radicals which need to be removed.

[0091] The method may include providing a network of metal nanowires interwoven with the carbon nanotubes. As described, the network of metal nanowires may serve to protect the carbon nanotubes by causing hydrogen radicals to recombine.

[0092] According to a seventh aspect of the present disclosure, there is provided the use of a membrane according to the first aspect, a pellicle according to the second aspect, an apparatus according to the third aspect, an EUV utilization apparatus according to the fourth aspect, a method according to the fifth aspect, or a method according to the sixth aspect in an EUV utilization method or apparatus. [0093] According to an eighth aspect of the present disclosure, there is provided the use of an alkyne to extend the operational lifetime of a carbon nanotube pellicle membrane.

[0094] As described, the provision of an alkyne, such as ethyne or propyne, to a carbon nanotube based pellicle membrane serves to extend the lifespan of the membrane via the alkyne reacting with hydrogen radicals to thereby serve as a hydrogen radical scavenger or quencher material.

[0095] According to a ninth aspect of the present disclosure, there is provided use of metal nanowires to extend the operational lifetime of a carbon nanotube pellicle membrane. As with the eighth aspect, the metal nanowires serve to protect the carbon nanotubes from etching by hydrogen radicals by catalyzing the recombination of the hydrogen radicals into hydrogen, such that the hydrogen radicals cannot etch carbon from the carbon nanotubes.

[0096] It will be appreciated that features described in respect of one embodiment may be combined with any features described in respect of another embodiment and all such combinations are expressly considered and disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0097] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawing in which corresponding reference symbols indicate corresponding parts, and in which:

[0098] Figure 1 depicts a lithographic apparatus according to an embodiment of the invention;

[0099] Figures 2A and 2B depict the deflection of a membrane according to the present disclosure during pump down at different pressure differentials and flow rates;

[0100] Figure 3 schematically depicts a pellicle assembly in a cross-sectional view;

[0101] Figure 4 schematically depicts a composite pellicle membrane according to the present disclosure;

[0102] Figure 5 schematically depicts an apparatus according to the present disclosure;

[0103] Figures 6A and 6B are schematic depictions of a pellicle comprising a carbon nanotube layer and a pellicle comprising a carbon nanotube layer comprising top and bottom layers, respectively;

[0104] Figures 7A and 7B are schematic depictions of a bilayer membrane comprising a vacuum and of a bilayer membrane comprising a carbon nanotube core, respectively;

[0105] Figures 8A to 8J are schematic depictions of a process for manufacturing; and

[0106] Figure 9 is a schematic depiction of step F depicted in Figure 8F.

[0107] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION [0108] Figure 1 shows a lithographic system including a pellicle 15 comprising a carbon nanotube membrane according to one aspect of the present disclosure. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W. In this embodiment, the pellicle 15 is depicted in the path of the radiation and protecting the patterning device MA. It will be appreciated that the pellicle 15 may be located in any required position and may be used to protect any of the mirrors in the lithographic apparatus. The pellicle 15 may be part of a spectral purity filter.

[0109] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

[0110] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source. A laser, which may for example be a CO2 laser, is arranged to deposit energy via a laser beam into a fuel, such as tin (Sn) which is provided from a fuel emitter. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may for example be in liquid form, and may for example be a metal or alloy. The fuel emitter may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region. The laser beam is incident upon the tin at the plasma formation region. The deposition of laser energy into the tin creates a plasma at the plasma formation region. Radiation, including EUV radiation, is emitted from the plasma during de-excitation and recombination of ions of the plasma.

[0111] The EUV radiation is collected and focused by a near normal incidence radiation collector (sometimes referred to more generally as a normal incidence radiation collector). The collector may have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region, and a second focal point may be at an intermediate focus, as discussed below.

[0112] The laser may be separated from the radiation source SO. Where this is the case, the laser beam may be passed from the laser to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser and the radiation source SO may together be considered to be a radiation system.

[0113] Radiation that is reflected by the collector forms a radiation beam B. The radiation beam B is focused at a point to form an image of the plasma formation region, which acts as a virtual radiation source for the illumination system IL. The point at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus is located at or near to an opening in an enclosing structure of the radiation source.

[0114] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 11.

[0115] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors 13, 14 which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors 13, 14 in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).

[0116] The radiation sources SO shown in Figure 1 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[0117] In an embodiment the membrane assembly 15 is a pellicle for the patterning device MA for EUV lithography. The membrane assembly 15 of the present invention can be used for a dynamic gas lock or for a pellicle or for another purpose. In an embodiment the membrane assembly 15 comprises a membrane formed from the at least one membrane layer configured to transmit at least 90% of incident EUV radiation. In order to ensure maximized EUV transmission and minimized impact on imaging performance it is preferred that the membrane is only supported at the border.

[0118] If the patterning device MA is left unprotected, the contamination can require the patterning device MA to be cleaned or discarded. Cleaning the patterning device MA interrupts valuable manufacturing time and discarding the patterning device MA is costly. Replacing the patterning device MA also interrupts valuable manufacturing time. [0119] Figures 2a and 2b depict the sagging of a CNT pellicle membrane as a function of pretension and EUV transmissivity. In order to avoid being out of specification for sagging, the deflection needs to be less than just over 1000 microns, which is the light grey area depicted in the upper right portion of the graphs. The exact curves depend on the loadlock pumpdown characteristics, which nominally occurs at around 9 KPa (Figure 2a), but pellicle membranes are preferably able to withstand a 20KPa pressure differential. The graphs depict a pellicle membrane with a Young’s modulus of 100 MPa and a flow resistivity of 8 Pa/sccm.

[0120] Stone -Wales defects significantly increase the required separation force, with the greater percentage of Stone -Wales defects leading to an increase in the separation force. For example, the separation force of CNTs without defects is generally independent of CNT length, whereas the introduction of 5% vacancies leads to an increase in separation force as CNT length increases. Stone- Wales defects significantly increase the separation force between CNTs such that the separation force is higher than the separation force of pristine CNTs at all CNT lengths. The difference in separation force, when comparing CNTs with Stone-Wales defects and pristine CNTs, becomes increasingly great as CNT length increase. CNTs’ with 5% Stone -Wales defects require a larger separation force than CNTs with 2.5% Stone -Wales defects. At CNT lengths of around lOnm, a CNT with 5% Stone -Wales defect has around twice the separation force of a pristine CNT of the same length. AT a CNT length of around 50 nm, the separation force is around seven times greater for a CNT with 5% Stone-Wales defects compared to a pristine CNT. At a CNT length of 100 nm, the separation force is around ten times greater for a CNT with 5% Stone-Wales defects compared to a pristine CNT. In addition, the increasing length of the CNTs having such defects increases the forces, whereas simply increasing the length of pristine CNTs has little, if any, effect on the required separation force. As such, friction is largely independent of overlap length for pristine CNTs.

[0121] Figure 3 schematically illustrates the pellicle assembly 15 in situ on the patterning device MA. The pellicle assembly comprises a pellicle film 19 according to an embodiment of the present disclosure that is held in place by a pellicle frame 17. . The pellicle may be attached to the frame, for example, by gluing or otherwise attaching a pellicle border region (not shown) to the frame 17. The frame 17 may be permanently or releasably attached to the patterning device MA.

[0122] The pellicle film 19 is provided in the path of both the incident radiation beam B and the reflected patterned radiation beam B’. The radiation beam passes through the pellicle film 19 twice. The pellicle film 19 is substantially transmissive to EUV radiation (although it will absorb a small amount of EUV radiation). The pellicle film 19 acts to protect the patterning device MA from particle contamination. Whilst efforts may be made to maintain a clean environment inside the lithographic apparatus LA, particles may still be present inside the lithographic apparatus LA. In the absence of the pellicle film 19, particles may be deposited onto the patterning device MA. Particles on the patterning device MA in the focal plane of the lithographic apparatus may disadvantageously impact the pattern that is imparted to the radiation beam B and therefore the pattern that is transferred to the substrate W. The pellicle film 19 provides a barrier between the patterning device MA and the environment in the lithographic apparatus LA in order to prevent particles from being deposited on the patterning device MA.

[0123] In use, the pellicle film 19 of the pellicle assembly 15 is positioned at a distance from the patterning device MA by the pellicle frame 17. The distance is sufficient to separate particles (e.g. particle 22) incident upon the surface of the film from the focal plane of the radiation beam B. The distance between the pellicle film 19 and the patterning device MA, acts to reduce the extent to which any particles on the surface of the pellicle 19 impart a pattern to the radiation beam B. A particle present in the beam of radiation B, but not at a focal plane of the beam of radiation B (i.e., not at the surface of the patterning device MA), will not be in focus at the surface of the substrate W. In the absence of other considerations, it may be desirable to position the film a considerable distance away from the patterning device MA. However, in practice the space which is available in the lithographic apparatus LA to accommodate the pellicle assembly 15 is limited due to the presence of other components.

[0124] Figure 4 is a schematic depiction of a composite pellicle membrane 15 according to the present disclosure. The composite pellicle membrane 15 includes interwoven or intertwined carbon nanotubes 23 and metal nanowires 24. As depicted, the metal nanowires 24 are able to catalyse the recombination of hydrogen radicals into hydrogen to thereby protect the carbon nanotubes 23 from being etched.

[0125] Figure 4 is a schematic depiction of an apparatus according to the present disclosure. As depicted, there is a support structure MT, which may be a reticle stage, which supports patterning device MA, which may be a reticle. A pellicle membrane 15, which may be a pellicle membrane according to the present disclosure or which could be any other kind of carbon nanotube based pellicle membrane, is provided in the path of radiation beam B and is positioned to separate contaminants on the pellicle membrane 15 such that the contaminants do not impart a pattern on radiation beam B. A doser 25 is provided which is in fluid connection with a supply 26 via line 27. The doser 25 is configured to supply hydrogen radical scavenger or quencher material, such as an alkyne, such as ethyne, to the pellicle membrane 15. Such supply is depicted as a spray 28, but it will be appreciated that the gas may diffuse into the environment surrounding the pellicle membrane 15. Although only one doser 25 is depicted, it will be appreciated that there may be more than one doser. The positioning of the doser 25 is also schematic and the doser 25 may be positioned at any location where there is available space and where it can provide the quenching/scavenging material to the pellicle membrane. Since the rate of hydrogen etching is greatest just outside the spot of the pellicle membrane 15 through which the radiation beam B passes in nominal use, the doser 25 may be configured to provide the quenching/scavenging material to the area of the pellicle membrane 15 which is subject to the greatest rate of etching in nominal use.

[0126] In use, radiation beam B passes through pellicle membrane 15 causing the pellicle membrane to heat up. The radiation beam B also generates hydrogen plasma, which contains hydrogen ions and radicals, which can etch away carbon from the carbon nanotubes of the pellicle membrane 15. It is believed that it is necessary for hydrogen to be adsorbed to the surface of the pellicle to cause etching and the high temperatures observed in the area of the pellicle membrane 15 through which radiation beam B passes in nominal use cause any adsorbed hydrogen to desorb. Outside of this area, the temperature is not high enough to desorb the hydrogen but there is still a hydrogen plasma environment, so the rate of etching is at its greatest. The doser 25 provides an alkyne, such as ethyne, which rapidly reacts with hydrogen radicals and thereby prevents them from etching the carbon nanotubes. As mentioned, the ethyne may be provided to the areas where etching is most rapid in order to extend the operational lifespan of the pellicle membrane.

[0127] Figures 6a and 6b are schematic depictions of pellicles comprising carbon nanotube layers. Figure 6a depicts a pellicle 20A comprising a carbon nanotube core 21 A supported by a frame 22A. Pellicle 20A does not include top and bottom layers and so has a theoretical maximum IR absorption/emissivity of 50%. Figure 6b depicts a pellicle 20B comprising a carbon nanotube core having a top and bottom layers 23B with a capping layer 21B on both faces. As shown in Figure 6b, the pellicle membrane comprises of a layer of carbon nanotubes sandwiched between two planar layers. As described, the presence of the top and bottom planar layers with a space between comprising carbon nanotubes serves to improve the emissivity of the pellicle membrane and therefore provides improved thermal control.

[0128] Figures 7a and 7b are schematic depictions of a bilayer membrane comprising a vacuum and of a bilayer membrane comprising a carbon nanotube core, respectively. Turning firstly to Figure 7a, this depicts the case which would provide the theoretical highest absorption case in which there is a vacuum between the two layers. This would provide a maximum absorption of 83%. Figure 7b depicts an embodiment in which the core comprises carbon nanotubes. Whilst the presence of the carbon nanotubes decreases the theoretical maximum absorption to 72%, the carbon nanotubes serve to support the top and bottom layers and provide strength to the pellicle membrane. The embodiment of Figure 7b depicts graphene bilayers on the top and bottom of the CNT core.

[0129] Figures 8a to 8j are schematic depictions of a process for manufacturing a pellicle according to the present disclosure. In step A, a carbon nanotube core is provided. The carbon nanotube core is supported by a frame. In step B, silicon oxide is provided on the carbon nanotube core. The silicon oxide may be provided by any suitable means, such as, for example, atomic layer deposition. In step C, additional silicon oxide is provided. It will be appreciated that step C may not be required. It will also be appreciated that step B may be continued until a desired amount of silicon oxide has been provided. In step D, a top side may be etched to provide a planar face and in step E, a bottom side may be etched to provide a planar face. Steps D and E may be done in any order and may be done simultaneously. The etching may be effected by any suitable means, such as, for example, reactive ion etching. In step F, a top and a bottom layer are added to the now planar top and bottom faces. The top and bottom layers, may be, for example, graphene layers. The graphene layers may be graphene bilayers. In step G, a support layer which was used to support the planar top and bottom layers is removed. The support layer may be any suitable material, such as, for example, silicon or PMMA (polymethylmethacrylate). In step H, a protective capping layer may be applied. It will be appreciated that this is not strictly required and that it is possible to provide a pellicle membrane without such protective capping layer. Even so, a protective capping layer is useful for extending the operational lifespan of a pellicle membrane. The capping layer may comprise any material used as a capping layer, such as, for example, yttrium oxide. In step I, remaining silicon oxide may be etched away in an HF etch such that hydrofluoric acid selectively etches the silicon oxide and leaves the carbon nanotubes. Finally, step J depicts a final pellicle comprising two bilayers of graphene separated by a carbon nanotube core.

[0130] Figure 9 provides additional details regarding step F. In step I, a copper foil is provided onto which a graphene layer is grown in step II. In step III, a PMMA and/or naphthalene layer is provided on a face of the graphene layer opposite to the copper foil. In step IV, the copper foil is etched away leaving an exposed graphene layer with a PMMA support layer. The exposed graphene layer may then be applied to the planar CNT core layer to sandwich the CNT core layer between opposing graphene layers. The present invention provides means for improving the stability of carbon nanotube membranes within EUV utilization apparatuses, such as lithography apparatuses, and allows for the control of the degree of force required to allow the CNTs to slip past one another. By increasing the resistance to relative movement, which can be considered as the effective friction between CNTs, the resistance to gapping of the CNTs in the mesh forming the membrane can be increased. Additionally or alternatively, the present disclosure provides for ways of reducing the rate of etching by hydrogen plasma by catalysing the recombination of hydrogen radicals using a metal nanowire and/or by preferentially reacting the hydrogen radicals with a hydrogen radical quenching or scavenging material.

[0131] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0132] The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims and clauses set out below.

1. A membrane for use as a pellicle in EUV utilization apparatus, the membrane comprising randomly oriented non-coated carbon nanotubes having one, two, three, or each of: i) an average diameter less than 5 nm and a bundle (average) diameter of less than 30 nm, wherein the Young’s modulus of the membrane is larger than lOMPa; ii) surface features that provide roughness to the nanotubes, such that relative movement of the CNT tubes in the film is substantially blocked or inhibited; iii) a hydrogen radical quencher or hydrogen radical scavenger material; and iv) a network of metal nanowires.

2. The membrane of clause 1, wherein the Young’s modulus is between around 10 MPa to around 200 MPa. 3. The membrane of clause 1 or 2, wherein the areal density of carbon in the membrane is between 10 xlO¹⁵ and 500xl0¹⁵ C atoms/cm².

4. The membrane according to any one of clauses 1 to 3, wherein the carbon nanotubes have [8,3] chirality, [14,6] chirality, or a mixed chirality, optionally wherein the mixed chirality comprises at least 30% of [8,3] chiral CNT.

5. The membrane according to any one of clauses 1 to 4, wherein the CNT purity is at least 99at % C and/or is free from defectivity adders with size larger than 10 microns.

6. The membrane according to any preceding clause, wherein the membrane has an EUV transmissivity of greater than 90%.

7. The membrane according to any preceding clause, wherein the membrane comprises singled- walled nanotubes, double-walled nanotubes, multi-walled nanotubes, or a combination of any of the aforesaid.

8. The membrane according to any preceding clause, wherein the carbon nanotubes have a diameter of up to around 3 nm, up to around 2.5 nm, around 2 nm, around 1.5 nm, or around 1 nm.

9. The membrane according to any preceding clause, wherein CNT surface features comprise defects, optionally wherein the defect comprises one or more of the following mechanisms: a) an adatom, b) chiral Stone -Wales effect, c) two missing C atoms, d) monovalency defect, and/or wherein the CNT surface features include functional groups.

10. The membrane according to clause 9, wherein the functional groups include hydrophobic groups, hydrophilic groups, or dipole-carrying groups, or two or more of hydrophobic groups, hydrophilic groups, and dipole-carrying groups.

11. The membrane according to clause 10, wherein the hydrophobic groups comprises one or more of phenyls, alkanes, alkenes, and alkynes, and the dipole-carrying groups includes a hydroxyl group.

12. The membrane according to any preceding clause, wherein the CNT surface feature includes a covalently bonded crosslinking group that bonds adjacent CNTs together.

13. The membrane according to clause 12, wherein the crosslinking groups is one or more of: 1,4- benzoquinones, 1,5-hexadiene, aryl diazonium salts, and poly(ethylene glycol) chains.

14. The membrane according to clause 12, wherein the crosslinking group is a reaction product of radical polymerization of CNTs or ion-beam irradiation of CNTs, or a polymer coating.

15. The membrane according to any preceding clause, wherein the CNTs are helical CNTs.

16. The membrane according to any preceding clause, wherein the surface feature is amorphous carbon.

17. The membrane according to any preceding clause, wherein the network of metal nanowires is a random network.

18. The membrane according to any preceding clause, wherein the metal nano wires are nonoxidised. 19. The membrane according to any preceding clause, wherein the metal nanowires comprise at least one metal with a high hydrogen recombination efficiency.

20. The membrane according to any preceding clause, wherein the metal nanowires comprise rhodium, platinum, or combinations thereof.

21. The membrane according to any preceding clause, wherein the metal nano wires have a diameter less than, equal to, or greater than the average diameter of the carbon nanotubes, and/or bundle (average) diameter of the carbon nanotubes, and/or less than or equal to the thickness of the CNT membrane.

22. The membrane according to any preceding clause, wherein the nanowires are electrospun nanowires.

23. The membrane according to any preceding clause, wherein the nano wires are interwoven with the carbon nanotubes.

24. The membrane according to any preceding clause, wherein the hydrogen scavenger or hydrogen quencher material is an alkyne, optionally wherein the hydrogen scavenger or hydrogen quencher material is provided on the carbon nanotubes.

25. A membrane for use as a pellicle in an exposure apparatus for semiconductor manufacturing, the membrane comprising a core comprising randomly orientated carbon nanotubes disposed between a first substantially planar top layer and a second substantially planar bottom layer.

26. The membrane according to clause 25, wherein the top layer and the bottom layer are the same or are different.

27. The membrane according to clause 25 or clause 26, wherein one or both of the top layer and the bottom layer comprises a 2D material, optionally wherein the material is selected from one or more of carbon-based, silicon-based, and boron-based 2D materials, and/or graphene, silicene, TMDS (tetramethyldisiloxane), black phosphorus, carbon nitride, germanene, borophene, stanine, arsenene, aluminene, antimonene, or bismuthene.

28. The membrane according to any of clauses 25 to 27, wherein one or both of the top layer and the bottom layer comprise molybdenum, ruthenium, beryllium, or rhenium.

29. The membrane according to any of clauses 25 to 28, wherein the carbon nanotubes in the core are the carbon nanotubes of any of clauses 1 to 24.

30. The membrane according to any of clauses 25 to 29, wherein the top layer and the bottom layer include a further conductive layer.

31. The membrane according to any of clauses 25 to 30, wherein one or both of the top layer and the bottom layer are continuous or at least partially formed of patches, optionally wherein the patches are separated by gaps of 4 microns or less.

32. The membrane according to any of clauses 25 to 31 , wherein one or both of the top layer and the bottom layer are bilayers. 33. The membrane according to any of clauses 25 to 32, wherein one or both of the top layer and the bottom layer are provided with a capping layer, optionally wherein the capping layer comprises one or more of aluminium oxide, yttrium oxide, and yttrium silicon oxide.

34. The membrane according to any of clauses 25 to 33, wherein the membrane has an emissivity of greater than 50%, and/or wherein the conductivity of the top layer and/or the bottom layer is from about 1 to about 6 MJ-kg ’-m ’.

35. A pellicle for an EUV utilization apparatus, the pellicle including the membrane according to any preceding clause and a support frame for supporting the membrane.

36. An apparatus for extending the operational lifetime of a pellicle membrane, the apparatus configured to provide a hydrogen radical quencher or hydrogen radical scavenger material to the pellicle membrane.

37. The apparatus according to clause 36, wherein the apparatus include a hydrogen radical quencher or hydrogen radical scavenger material doser configured to direct the hydrogen radical quencher or hydrogen radical scavenger material towards the pellicle membrane.

38. The apparatus according to clause 36 or 37, wherein the hydrogen radical quencher or hydrogen radical scavenger material comprises an alkyne, optionally wherein the alkyne is ethyne, propyne, or combinations thereof.

39. The apparatus according to any of clauses 36 to 38, wherein the apparatus includes a controller configured to control the provision of the hydrogen radical quencher or hydrogen radical scavenger material.

40. The apparatus according to any of clauses 36 to 39, wherein the apparatus is configured to provide the hydrogen radical quencher or hydrogen radical scavenger material to one or both of a front face and a rear face of the pellicle membrane.

41. The apparatus according to any of clauses 36 to 40, wherein the apparatus is configured to provide the hydrogen radical quencher or hydrogen radical scavenger material at a rate sufficient to provide a mono-layer to the pellicle membrane every 5 seconds or less, every 4 seconds or less, every 3 seconds or less, every 2 seconds or less, or every second or less.

42. The apparatus according to any of clauses 36 to 41, wherein the apparatus is configured to provide the hydrogen radical quencher or hydrogen radical scavenger material at a pressure of 0.10 mPa or less, 0.09 mPa or less, 0.08 mPa or less, 0.07 mPa or less, 0.06 mPa or less, 0.05 mPa or less, 0.04 mPa or less, 0.03 mPa or less, 0.02 mPa or less, or 0.01 mPa or less.

43. An EUV utilization apparatus comprising the membrane, pellicle, or apparatus according to any preceding clause, optionally wherein the EUV utilization apparatus is an EUV lithography apparatus or an EUV inspection tool.

44. A method of manufacturing a membrane for an EUV utilization apparatus, the method including the steps of i) providing surface features to a plurality of CNTs to prevent sliding of the CNTs over one another when formed into a CNT membrane, and ii) forming a CNT membrane of such CNTs. 45. A method of extending the operational lifetime of a carbon nanotube based pellicle membrane, the method including one or both of: i) providing a hydrogen radical quencher or hydrogen radical scavenger material to the pellicle membrane; and ii) providing a network of metal nanowires associated with the pellicle membrane.

46. The method according to clause 45, wherein the method includes providing an alkyne as the hydrogen radical quencher or hydrogen radical scavenger material to the pellicle membrane, optionally wherein the alkyne comprises ethyne, propyne, or combinations thereof.

47. The method according to clause 45 or 46, wherein the method includes providing the hydrogen radical quencher or hydrogen radical scavenger material to the pellicle membrane to one or both of a front face and a rear face of the pellicle membrane.

48. The method according to any of clauses 45 to 47, wherein the hydrogen radical quencher or hydrogen radical scavenger material is provided as a gas.

49. The method according to any of clauses 45 to 48, wherein provision of the hydrogen radical quencher or hydrogen radical scavenger material is controlled depending on the amount or concentration of hydrogen radicals generated in use.

50. The method according to any of clauses 45 to 49, wherein the method includes providing a network of metal nanowires interwoven with the carbon nanotubes.

51. A method of manufacturing a membrane for an exposure apparatus for semiconductor manufacturing, the method including the steps of i) providing a core film comprising a plurality of randomly orientated carbon nanotubes, the core film having a top face and a bottom face, and ii) providing a substantially planar layer on the top face and on the bottom face.

52. The method according to clause 51, wherein the method further includes depositing silicon oxide on the core film prior to step ii).

53. The method according to clause 51 or clause 52, wherein the method further includes removing material from the top face and/or the bottom face to provide a planar top face and/or bottom face prior to step ii).

54. The method according to clause 53, wherein the method further includes providing the substantially planar layer on the planar top face and providing the substantially planar layer on the planar bottom face.

55. The method according to clause 54, wherein the method further includes removing any support layers from the membrane.

56. The method according to any of clauses 51 to 54, wherein the method further includes providing a capping layer on one or both of the planar layer on the top face and the planar layer on the bottom face.

57. The method according to clause 52 or any of clauses 53 to 56 when dependent on clause 52, wherein the method further includes etching the silicon oxide deposited on the core film. 58. The use of a membrane according to any of clauses 1 to 34, a pellicle according to clause 35, an apparatus according to any of clauses 36 to 42, an EUV utilization apparatus according to clause 43, a method according to clause 44, or a method according to any of clauses 45 to 57 in an EUV utilization method or apparatus. 59. The use of an alkyne to extend the operational lifetime of a carbon nanotube pellicle membrane.

60. The use of metal nanowires to extend the operational lifetime of a carbon nanotube pellicle membrane.

Claims

1. A membrane for use as a pellicle in EUV utilization apparatus, the membrane comprising randomly oriented non-coated carbon nanotubes having one, two, three, or each of: i) an average diameter less than 5 nm and a bundle (average) diameter of less than 30 nm, wherein the Young’s modulus of the membrane is larger than lOMPa; ii) surface features that provide roughness to the nanotubes, such that relative movement of the CNT tubes in the film is substantially blocked or inhibited; iii) a hydrogen radical quencher or hydrogen radical scavenger material; and iv) a network of metal nanowires.

2. The membrane of claim 1, wherein the Young’s modulus is between around 10 MPa to around 200 MPa.

3. The membrane of claim 1 or 2, wherein the areal density of carbon in the membrane is between 10 xlO¹⁵ and 500xl0¹⁵ C atoms/cm².

4. The membrane according to any one of claims 1 to 3, wherein the carbon nanotubes have [8,3] chirality, [14,6] chirality, or a mixed chirality, optionally wherein the mixed chirality comprises at least 30% of [8,3] chiral CNT.

5. The membrane according to any one of claims 1 to 4, wherein the CNT purity is at least 99at % C and/or is free from defectivity adders with size larger than 10 microns.

6. The membrane according to any preceding claim, wherein the membrane has an EUV transmissivity of greater than 90%.

7. The membrane according to any preceding claim, wherein the membrane comprises singlewalled nanotubes, double-walled nanotubes, multi-walled nanotubes, or a combination of any of the aforesaid.

8. The membrane according to any preceding claim, wherein the carbon nanotubes have a diameter of up to around 3 nm, up to around 2.5 nm, around 2 nm, around 1.5 nm, or around 1 nm.

9. The membrane according to any preceding claim, wherein CNT surface features comprise defects, optionally wherein the defect comprises one or more of the following mechanisms: a) an ad- atom, b) chiral Stone -Wales effect, c) two missing C atoms, d) monovalency defect, and/or wherein the CNT surface features include functional groups.

10. The membrane according to claim 9, wherein the functional groups include hydrophobic groups, hydrophilic groups, or dipole-carrying groups, or two or more of hydrophobic groups, hydrophilic groups, and dipole-carrying groups.

11. The membrane according to claim 10, wherein the hydrophobic groups comprises one or more of phenyls, alkanes, alkenes, and alkynes, and the dipole-carrying groups includes a hydroxyl group.

12. The membrane according to any preceding claim, wherein the CNT surface feature includes a covalently bonded crosslinking group that bonds adjacent CNTs together.

13. The membrane according to claim 12, wherein the crosslinking groups is one or more of: 1,4- benzoquinones, 1,5-hexadiene, aryl diazonium salts, and poly(ethylene glycol) chains.

14. The membrane according to claim 12, wherein the crosslinking group is a reaction product of radical polymerization of CNTs or ion-beam irradiation of CNTs, or a polymer coating.

15. The membrane according to any preceding claim, wherein the CNTs are helical CNTs.

16. The membrane according to any preceding claim, wherein the surface feature is amorphous carbon.

17. A pellicle for an EUV utilization apparatus, the pellicle including the membrane according to any preceding claim and a support frame for supporting the membrane.

18. An apparatus for extending the operational lifetime of a pellicle membrane, the apparatus configured to provide a hydrogen radical quencher or hydrogen radical scavenger material to the pellicle membrane.

19. An EUV utilization apparatus comprising the membrane, pellicle, or apparatus according to any preceding claim, optionally wherein the EUV utilization apparatus is an EUV lithography apparatus or an EUV inspection tool.

20. A method of manufacturing a membrane for an EUV utilization apparatus, the method including the steps of i) providing surface features to a plurality of CNTs to prevent sliding of the CNTs over one another when formed into a CNT membrane, and ii) forming a CNT membrane of such CNTs.