Carbon nanotube pellicle membrane

Abstract

There is provided a carbon nanotube pellicle membrane for a lithography apparatus, the pellicle membrane including a sacrificial coating. Also provided is a pellicle for a lithographic apparatus, the pellicle including such a pellicle membrane and a support frame for supporting the pellicle membrane, as well as a lithographic apparatus comprising such a pellicle membrane or pellicle. Also described is a method and apparatus for manufacturing a pellicle membrane, as well as the use of such a pellicle membrane, pellicle, lithographic apparatus, method or manufacturing apparatus in a lithographic method or apparatus.

Description

CARBON NANOTUBE PELLICLE MEMBRANECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23194949.6 which was filed on September 1, 023 and EP application 24156760.1 which was filed on February 9, 2024 and which are incorporated herein in their entirety by reference.FIELD

[0002] The present disclosure relates to a carbon nanotube pellicle membrane for a lithographic apparatus. The present disclosure also relates to a pellicle comprising such a pellicle membrane and a support frame, as well as a lithographic apparatus comprising such a pellicle membrane or pellicle. The present disclosure also relates to a method of manufacturing a pellicle membrane, an apparatus for the manufacture of a pellicle membrane, and the use of such a pellicle membrane, pellicle, lithographic apparatus, method, or apparatus in lithographic method or apparatus. The present disclosure has particular, but not exclusive, application to EUV lithography and may be used in any EUV utilization apparatus.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. For example, inan EUV utilization apparatus, which is an apparatus that utilises EUV radiation, such as an inspection apparatus, a pellicle membrane may be used to similarly prevent contamination by providing a barrier which allows transmission of EUV radiation to block contaminants.

[0007] 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. 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. A pellicle membrane also needs to be sufficient strong to withstand the forces acting upon it when being transported as well as when in use. It is also necessary for a pellicle membrane to avoid distorting imaging, to avoid contaminating critical scanner components, and also to avoid deflecting outside predetermined specified limits. Since some of these requirements are conflicting, such as the need for strength but also high transmissivity, a balance of properties is required. For example, the pre-tension of a thin-film pellicle membrane can be increased in order to strengthen the pellicle membrane, but this can lead to a greater likelihood of mechanical failure of the pellicle membrane. Pellicle membranes can also degrade over time in use and it is desirable to mitigate the effects of degradation.

[0008] The present invention has been devised in an attempt to address at least some of the problems identified above.SUMMARY OF THE INVENTION

[0009] According to a first aspect of the present disclosure, there is provided a carbon nanotube pellicle membrane for a lithographic apparatus, the pellicle membrane including a sacrificial coating.

[0010] Within a lithographic apparatus, such as an EUV lithographic apparatus, the pellicle membrane is within is a low-pressure atmosphere of hydrogen. In operation, radiation used in the lithographic apparatus, such as EUV radiation, interacts with the low-pressure hydrogen to produce a plasma. Carbon nanotube (CNT) pellicle membranes are susceptible to etching by the hydrogen plasma and thereby face degradation. During exposure, carbon is gradually removed from the CNT pellicle and this causes damage to the structural integrity of the CNT pellicle and shortens the operational lifespan of the pellicle membrane as extended use may result in the breakage of the CNT pellicle membrane. The pellicle membrane is also heated during illumination, but only in the illuminated area. If the pellicle is heated sufficiently, the heated area of the CNT pellicle will not be degraded by the hydrogen plasma, but the area surrounding the heated area will still be degraded, which still causes the CNT pellicle to degrade and risks breakage of the pellicle membrane. To make the CNTs more resistant to this degradation-induced failure, the area surrounding the portion of the pellicle membrane which is heated by the radiation beam during illumination has been coated with other elements, such as Yttrium,so that the surrounding area is protected more from the degradation while the illuminated area will not be affected by a resulting reduction of EUV transmission of the pellicle. Even so, it has been found that this is insufficient to reduce the degradation to an acceptable level because such coating is not effective at blocking the etching of the CNTs. Without wishing to be bound by scientific theory, it is believed that degradation still occur since the species responsible for the degradation, such as hydrogen radicals and hydrogen ions, are not diffusion limited. As such, these reactive species will degrade the unprotected CNTs and such reactive species are not reduced by the protective coating. The addition of further CNT pellicle patches in selected regions of the pellicle membrane, such as around the border and outside an imaging area is subject to manufacturing, preparation and placement errors of the added CNTs pellicle patches and is not precise enough to follow the area around the imaging area. This can also introduce undesirable strain distributions across the pellicle membrane. The addition of a sacrificial coating has been found to address at least some of these issues. The provision of a sacrificial coating, which is one which is preferentially removed compared with the CNTs or which competes for etching agents, whether by susceptibility to etching or placement in an area of high etching, protects the CNTs of the main pellicle by degrading before the degradation of the CNTs of the main pellicle, thereby postponing the onset of structurally relevant degradation of the CNTs of the main pellicle and extending the lifetime of the main pellicle. As such, by serving as a more competitive sacrificial material than the CNTs, instead of being degraded in parallel with the CNTs, it shall be degraded first. Similarly, in contrast to cases where a non-sacrificial coating, such as yttrium is applied, the sacrificial material reacts with the reactive species to reduce the amount of reactive species which can diffuse towards the illuminated portion of the pellicle membrane.

[0011] The pellicle membrane may be a functionalised carbon nanotube membrane. A functionalised CNT membrane may provide advantageous mechanical performance.The sacrificial coating may comprise carbon. The carbon may be amorphous carbon, and / or crystalline carbon, and / or annealed carbon. The carbon may be crystalline carbon and / or annealed carbon and / or graphene flakes and / or carbon nanotubes and / or diamond particles. The sacrificial coating may comprise one or more different structural forms of carbon. The carbon may be amorphous diamondlike carbon, and / or amorphous graphene-like carbon. The sacrificial coating may be applied to a portion of the pellicle membrane. The sacrificial coating may be a continuous film, or comprise a dense layer of particles, fibres, or flakes. By using a sacrificial carbon material, the sacrificial carbon material is preferentially eroded by the hydrogen plasma over the CNTs forming the pellicle membrane, thereby protecting the CNTs. In addition, by controlling the placement of any sacrificial coating, it is possible to use CNTs as the sacrificial layer. The carbon may be in the form of graphene. The carbon may be in the form of graphene flakes. The flakes can comprise a single graphene sheet or multiple graphene sheets. The graphene flakes and / or diamond particles may be from around 0.1 to around 10 microns in diameter. The carbon may be in the form of carbon nanotubes. The carbon nanotubes may be singlewalled. The carbon nanotubes may be multi-walled. The carbon nanotubes may be from around 0.1 toaround 10 microns in length. The graphene flakes, nanotubes, and / or diamond particles can be functionalized.

[0012] The sacrificial coating may be covalently bonded to the pellicle membrane. The sacrificial coating may be non-covalently bonded to the pellicle membrane. It will be appreciated that non- covalently bonded indicates that the primary bonding is not via chemical covalent bonds, but via other bonds, such as van der Waals or physical entanglement or overlap of the materials of the pellicle membrane and the sacrificial material. The sacrificial coating is preferably non-covalently bonded to the pellicle membrane. By avoiding covalently bonding the sacrificial material to the CNTs, there is less or no effect on the strain within the CNTs of the pellicle membrane caused by the sacrificial material. In addition, a covalent bond between the sacrificial material and the CNT membrane material may result in bond vacancies or other defects which may lead to reduced mechanical and / or chemical stability. The erosion of the sacrificial material during use will similarly not affect the strain within the CNTs over time. In addition, there is less thermal transfer between the membrane and the sacrificial coating. The effect of this is that there is less change in the location of where the highest rate of etching occurs in use, since this is at least partially dependent on the temperature of the pellicle membrane.

[0013] The sacrificial coating, which may be in the form of graphene flakes, CNTs or other carbonbased particles, can be additionally secured by trapping it between two CNT pellicle membranes, of which at least one pellicle membrane extends over the imaged area of the reticle in use. In this way, the sacrificial coating may be sandwiched between two pellicle membranes. The addition of the second pellicle membrane to sandwich the sacrificial coating serves to avoid the sacrificial coating being inadvertently removed during handling.

[0014] The pellicle membrane has a quality area through which a radiation beam passes in nominal use. The pellicle membrane also has a non-quality area though which a radiation beam does not pass in nominal use. The sacrificial coating is preferably provided in the non-quality area. By providing the sacrificial coating outside of the quality area, the sacrificial coating does not have any effect on the structure of the radiation beam passing through the quality area of the pellicle membrane.

[0015] The sacrificial coating may be provided around a perimeter of the pellicle membrane. By providing the sacrificial coating around the perimeter, the area through which a radiation beam passes in nominal use, the so-called quality area, is kept clear of additional materials and so transmissivity is not adversely effected. In some cases, when integrated circuit design demands so, the irradiated area on the reticle is smaller than the quality area, in that case the sacrificial coating may be provided around the perimeter of the irradiated area.

[0016] The thickness of the sacrificial material may be non-uniform. As described, etching of a CNT pellicle membrane is caused by exposure to hydrogen ions and radicals in a hydrogen plasma. Areas of a pellicle membrane which are illuminated in nominal use are somewhat protected from the etching since the temperature of the pellicle membrane caused adsorbed atomic hydrogen to desorb, which significantly reduces the rate at which etching occurs, meaning that the etch rate of illuminatedareas of the pellicle membrane is lower than surrounding areas. In addition, at the periphery of the pellicle membrane, the flux of the plasma is lower and the temperature is also lower, resulting in a lower etch rate. Between the illuminated areas and the periphery of the pellicle membrane, there is an area, where the rate of etching reaches a maximum value due to the combination of plasma flux and temperature that is insufficient for atomic hydrogen desorption. In use, there is a gradient in temperature from the portion of the pellicle membrane which is illuminated by a radiation beam, which is at a high temperature during irradiation (for example higher than T=+600 °C), towards the edge of the pellicle membrane, which is at a much lower temperature, such as room or ambient temperature. The rate of etching is at least partially dependent on temperature, and there exists a range of temperatures, at which the etch rate at a fixed hydrogen plasma parameters is highest. This range is estimated at from around T=+300 C to around +500 C. This being the case, there is a portion of the membrane between the edge of the membrane and the area illuminated by the radiation beam where etching is fastest. As such, by providing a sacrificial material which is thicker at the point of maximum rate of etching in nominal use, it is possible to extend the lifespan of the pellicle membrane through most efficient use of additional sacrificial material.

[0017] The sacrificial coating may be thicker and / or denser in an area of the pellicle membrane where etching is greatest in nominal use. A thicker and / or denser sacrificial coating can withstand etching for longer than a thinner or less dense coating under the same conditions, so the coating can be selectively provided as a thicker and / or denser coating in areas where the rate of etching is greatest.

[0018] The sacrificial coating may be formed in a gradient. The gradient may be a thickness gradient. Additionally or alternatively, the gradient may be a density gradient. Whilst it is possible to provide a sacrificial coating by laminating one or more additional layers of carbon nanotube membrane with a window cut out, such that they are in the form of a frame around the quality area, this has a number of disadvantages. Firstly, by stacking a number of layers, typically around ten, the transmissivity drops from 95% or more down to around 50%. It is very difficult to accurately place the cut-outs in the membrane, which should be placed to an accuracy of around 100 microns or less, which is exacerbated by the need to stack multiple layers. Furthermore, there is little to no flexibility with respect to the geometry of the sacrificial layers and partially exposed reticles will require a different pellicle each time, which is time consuming and would also require manufacturers to disclose productspecific exposure areas to suppliers, which is sensitive information. In addition, this option provides around ten times the amount of carbon, even in areas where etching is not critical and also cools down the adjacent CNT pellicle region, which is part of the exposed area. The high temperature of the exposed area serves to protect the area from etching, so cooling it down leads to an increase in the rate of etching, which is undesired, and so that addition of simply additional layers of pellicle membrane material with a window may serve to increase the etch rate of the quality area, which is the opposite effect to that which is desired.

[0019] The sacrificial coating may be formed in a gradient. The gradient may be a thickness gradient. Additionally or alternatively, the gradient may be a density gradient. The gradient may be provided on one or both of an inner edge and an outer edge of the sacrificial coating.

[0020] By providing a gradient, it is possible to avoid any sharp changes in thickness and / or density, and thereby better control the thermal characteristics of the membrane and control the location where the rate of etching is at its greatest and ensure that this coincides with the location of the sacrificial layer.

[0021] The sacrificial coating may be provided on one or both faces of the membrane. Preferably, the sacrificial coating is provided on one face of the membrane. Although there will be some degree of etching on both faces of the membrane, the majority of the etching will be observed on the face of the membrane facing away from the reticle since this is where the flux of hydrogen ions is highest.

[0022] The membrane, in the quality area, may have an EUV transmissivity of 90% or greater, an EUV transmissivity non-uniformity of 0.2% or less, and EUV scattering of 0.2% or less. In order to function as an effective pellicle membrane, the membrane needs to not only provide a barrier against contaminants falling on the reticle, but also needs to allow sufficient radiation through to provide a decent throughput of wafers being imaged, whilst also not interfering with the patterning of a radiation beam which passes through the membrane when in use.

[0023] The sacrificial coating may be provided by one or more of: electron-beam induced deposition (EBID), chemical vapour deposition (CVD), physical vapour deposition (PVD), and pulsed laser deposition (PLD). Other techniques for providing the sacrificial coating include evaporative techniques, sublimation techniques, and sputtering. Any technique which is able to provide the sacrificial coating may be used.

[0024] In EBID, a carbon coating can be selectively deposited onto a pellicle membrane by firing an electron beam at the pellicle membrane within an atmosphere that includes hydrocarbons. The electrons facilitate the deposition of carbon from the hydrocarbons onto the pellicle membrane. This provides a sacrificial coating of carbon on the pellicle membrane, and the structural form of the carbon is other than CNTs. Since the electron beam is steerable, the location control of the deposition of the sacrificial carbon coating is high and the sacrificial carbon can be deposited accurately. Chemical vapour deposition and physical vapour deposition may also be used to deposit sacrificial carbon. Pulsed laser deposition, similarly, can accurately control the disposition of sacrificial carbon on a pellicle membrane.

[0025] According to a further aspect of the present disclosure, there is provided a carbon nanotube pellicle membrane for a lithographic apparatus, the membrane comprising a carbon nanotube layer supported by a support mesh.

[0026] Carbon nanotube based pellicle membranes have a random network of carbon nanotubes. Such membranes have low stiffness and are at risk of sagging, which can cause them to touch other components within a lithography apparatus, cause imaging issues, or even rupture if the elastic limit isexceeded. As such, it is useful to support the CNT based pellicle membranes. The support mesh is configured to provide additional structural support to the CNT membrane whilst minimising impact on any of transmissivity, uniformity, and scattering. The support mesh can be provided locally, for example to reinforce a portion of the pellicle with a sacrificial coating.

[0027] One of the layers may comprise a mesh of carbon nanotubes and the other layer may comprise a patterned carbon nanotube layer. The function of the patterning of the carbon nanotube layer is to increase the stiffness of the layer in order to provide additional support for the other layer.

[0028] The patterned carbon nanotube layer may be embossed. In this way, the carbon nanotube layer is deformed in a pattern which provides additional stiffness. The embossing may cause the carbon nanotube layer to be corrugated or otherwise include a series of ridges and troughs. In other words, the cross-section of the patterned layer may be in a zig-zig or sinusoidal.

[0029] The embossed pattern may have a periodicity of less than a size of the smallest feature which can project a diffraction peak on a wafer. The smallest feature which can project a diffraction peak onto a wafer depends on the numerical aperture of the optics and the wavelength of light used. For EUV lithography using a wavelength of 13.5 nm and a numerical aperture of 0.33 and taking into account a four times demagnification between a reticle and a wafer, the smallest feature which can still project a diffraction peak, and thereby be imaged on a wafer, is around 164 microns. As such, by having the periodicity of the pattern as less than 164 microns, any pattern on the pellicle is too small to be imaged.

[0030] One of the layers may comprise a mesh of carbon nanotubes. These nanotubes may be randomly orientated. The other layer may comprise a grid having a periodicity of 165 microns or greater. At a standoff distance of 2mm, a single point source on a reticle diverges into a spot having a radius of around 165 microns. As such, an image, and therefore a pattern projected onto a wafer, is therefore masked by the pellicle membrane. If regular patterns occur within this area, a flare signal is produced, which may be projected onto the wafer. If no regular patterns occur within this spot, the pellicle does not affect imaging. As such, by having a grid having a periodicity of greater than this radius, there will be no regular patterns caused by the grid within any given spot and thus imaging will not be adversely affected by the presence of the grid.

[0031] A grid comprises grid bars. Grid bars may be in any configuration which can support a carbon nanotube layer. The grid bars may be in square, rectangular, hexagonal, or any other polygonal shape. The grid bars may have an effective width of around 0.5 microns or less. This is the calculated maximum width of the grid bars which would still allow for the avoidance of scattering from the bars in nominal use.

[0032] The grid bars may have a pitch of greater than 330 microns. By having a pitch of greater than 330 microns, which is double the calculated spot radius, the possibility of impact upon imaging is reduced or eliminated.

[0033] The grid may comprise a refractory metal. A refractory metal is one which retains its structural integrity at temperatures within a lithographic apparatus, such as 500°C or greater, 550°C or greater, 600 °C or greater, 650 °C or greater, or 700 °C or greater, and which is resistant to a hydrogen plasma environment. Tungsten and molybdenum are particularly suitable.

[0034] The pellicle membrane may further comprise any of the features of the first aspect of the present disclosure. For example, the pellicle membrane may include a sacrificial coating layer thereon.

[0035] According to a second aspect of the present disclosure, there is provided a pellicle for a lithographic apparatus, the pellicle including the pellicle membrane according to the first aspect of the present disclosure and a support frame for supporting the pellicle membrane.

[0036] According to a third aspect of the present disclosure, there is provided a lithographic apparatus comprising a pellicle membrane according to the first aspect of the present disclosure or pellicle according to the second aspect of the present disclosure. The lithographic apparatus may be an EUV utilization apparatus.

[0037] According to a fourth aspect of the present disclosure, there is provided a method of manufacturing a pellicle membrane, the method including the steps of providing a carbon nanotube pellicle membrane and providing a sacrificial coating thereon.

[0038] The method may include providing the carbon nanotube pellicle membrane in a vacuum chamber, providing a mask to cover a portion of the pellicle membrane to thereby provide a masked area and prevent deposition of a coating on the masked area, and depositing a sacrificial coating on an unmasked area of the pellicle membrane. A mask may be provided on an opposing face of the pellicle membrane to protect such an opposing face from inadvertent contamination.

[0039] Since it is desirable to keep the quality area clear of sacrificial coating, a mask may be provided to prevent deposition of the sacrificial material in areas where it is undesirable. This may be useful when the sacrificial material is provided by techniques which can deposit the sacrificial material over a wide area, such as electron-beam induced deposition (EBID), chemical vapour deposition (CVD), physical vapour deposition (PVD), and pulsed laser deposition (PLD), particularly CVD and PVD, which are less steerable than EBID and PLD.

[0040] The method may include providing an electron source configured to generate an electron beam, and moving the pellicle membrane and the electron beam relative to one another to selectively deposit the sacrificial coating on the pellicle membrane. As a beam of charged particles, it is possible to steer and / or focus an electron beam and cause it to scan across the pellicle membrane in order to selectively deposit the sacrificial material on the pellicle membrane. Additionally or alternatively, the pellicle membrane may be moved in order to selectively deposit the sacrificial material on the pellicle membrane.

[0041] The vacuum chamber may include volatile carbonaceous compounds to serve as the source of the sacrificial coating, optionally wherein the volatile carbonaceous compounds are C1-C5hydrocarbons. Heavier hydrocarbons may be used. Preferably, saturated hydrocarbons are used as they generally form better graphitic layers, but unsaturated hydrocarbons can also be used.

[0042] The method may include annealing the sacrificial coating. The structural form of the sacrificial carbon material may be adjusted by annealing. Carbon deposited by EBID is amorphous in nature and max be a mixture of different structural forms of carbon, ranging from polymeric compounds to amorphous diamond-like carbon. For the sacrificial purpose of the deposited carbon in the expected hydrogen plasma environment, the electronic configuration of the carbon may be relevant. In some models of carbon etching by hydrogen plasma, the carbon must transition through sp3 -hybridization states for the carbon-removing etching to occur. The heating of sp3 -hybridized carbon is believed to cause a transition into an sp2-hybridization state through a process called graphitization and also leads to desorption of etching hydrogen species from the carbon nanotubes, which suppresses etching. It has been observed that CNTs at elevated temperatures are etched at a rate less than would necessarily be expected. As such, sp3 -hybridized carbon, which the amorphous EBID deposited carbon comprises is favoured over sp2-hybridized CNT carbon during the hydrogen plasma etching process, thereby serving as a suitable sacrificial coating. In a transitional region from a hot region of the pellicle membrane to a cooler region, the sacrificial coating may be partially graphitized to sp2 hybridized carbon, simultaneously making it more etch resistant due to the elevated temperature and desorption of hydrogen. As such, areas which are insufficiently heated retain the sp3-hybridization state of the sacrificial carbon so that their sacrificial purpose is retained. Similarly, amorphous diamond like carbon, may be a mixture of amorphous carbon and diamond-like carbon. Such material may be deposited favourably in specific deposition steps, such as CVD or plasma discharge from a hydrocarbon gas, such as methane. Different types of DLC with different thermal degradation temperatures may be provided and they may have different temperatures above which they graphitize into sp2-hybridation states. In this way, specific types of sacrificial carbon can be used to optimise thermal etching behaviour of the sacrificial coating. For example, if etch rates for DLC are lower than for amorphous carbon, then deposition and / or annealing can be directed towards a greater proportion of DLC in the sacrificial coating in order to control the rate at which the sacrificial material is etched. Graphene-like carbon may have a similar sp2-hybridization as CNTs, and so in nominal use the additional sacrificial carbon would overlay the graphitic CNTs with additional graphitic carbon. Whilst the chemical stability of the graphitic carbon sacrificial material would then correspond to the CNTs, the additional graphitic layers are patchy and discontinuous with proportionally many more sp3 border states, which is in contrast to the more long-range graphitic sp2 structure of CNTs. Therefore, such additional border-bound carbon atoms of the fragmentary graphitic carbon sacrificial material are more readily attacked by the hydrogen plasma, thereby fulfilling the sacrificial purpose relative to the CNTs.

[0043] The method may include controlling the structural form of the sacrificial coating. This may include relaxation of the sacrificial coating, such as by using an ion beam, electron beam, light, or heating to control crystallinity or amorphousness. In addition, the method may include a post-deposition treatment configured to remove unwanted contamination, such as, for example loose sacrificial coating that could become detached in use.

[0044] The method may include providing a sacrificial coating by spraying. Spraying the sacrificial coating allows for the controlled provision of the sacrificial coating and it is possible to carefully control where the sacrificial coating is provided.

[0045] The method may include providing a mixture of a carbon source in a carrier fluid. The concentration of carbon source in the carrier fluid may be less than 1%. The typical size of carbon particles, flakes, or fibres comprising carbon source may be less than 10 um, preferably less than 1 um. The carbon source may comprise one or more of: single or multi-wall CNT, single- or multi- graphene sheet flakes, diamond nano or micro particles, diamond-like carbon nano or micro particles. The carbon particles may comprise functionalized graphene flakes, functionalized carbon nanotubes, and / or functionalized diamond particles. The carrier fluid may be polar or non-polar. The carrier fluid may be aqueous. The carrier fluid may be water. The carrier fluid may comprise an alcohol. The carrier fluid may comprise N-methyl-2-pyrrolidone (NMP). The carrier fluid may comprise dimethyl sulphoxide (DMSO). Where the carrier fluid is polar, such as water or alcohol, the carbon source is preferably functionalised with polar groups. The functionalisation aids in suspending the carbon particles, flakes, or fibres in the carrier fluid. Functionalisation may include partial oxidation, which would incorporate carbonyl moieties and hydroxyl moieties into the carbon source. For non-polar solvents, pristine, i.e. non-functionalised, carbon sources may be used.

[0046] The method may include providing a second or an auxiliary CNT membrane to cover the sacrificial coating, comprising individual particles. This serves to prevent detachment of the sacrificial coating in handling by the air flow. The auxiliary CNT membrane can be larger than the sacrificial coating.

[0047] The method may include forming an aerosol and spraying the aerosol on the pellicle membrane. By forming an aerosol from the carrier fluid and the carbon source, this can then be sprayed onto the pellicle membrane. Aerosol droplets can be configured to be less than 100 um or less than 10 um in order to prevent damage to the pellicle by spraying and to provide a controlled, local deposition. The speed of aerosol droplets landing on the membrane can be limited to less than 10 m / s or less than 1 m / s to prevent damage to the pellicle. The method may include providing a mask to shield a portion of the pellicle membrane. The mask may be provided with a heater so that it may be heated to prevent build-up of carrier fluid. It is undesirable for the sacrificial coating to cover the entirety of the pellicle membrane, particularly the quality area and so the provision of a mask can prevent the sacrificial coating for being provided in areas where it is not required. The mask may be larger than the area of the pellicle membrane which will be exposed to EUV radiation in nominal use. This is to provide a margin for mounting of the pellicle. For example, the mask may be at least 0.1 mm larger than the area of the pellicle membrane which will be exposed to EUV radiation in nominal use. The mask may be thin at its edge, such as from around 0.1 mm up to around 1 mm where a steeper gradient is desired. The maskmay be thick at its edge, such as from 1mm to around 10 mm where a shallower gradient is desired. Since the sacrificial coating may only be desired to be provided in an area limited to a free-standing portion of the CNT pellicle, a second mask may be provided such that the sacrificial layer is only provided in a gap between the two masks, such as in the shape of a frame.

[0048] The method may include operating a piezo driver and / or a gas jet to form the aerosol. The piezo driver may be driven at from around 10 kHz to around 10 MHz, preferably between around 100 kHz and around 1 MHz. The average diameter of the droplets within the aerosol may be from around 1 micron to around 10 microns. At a frequency of around 100 kHz, a piezo driver will generate droplets of around 10 microns diameter. At a frequency of around 1 MHz, a piezo driver will generate droplets of around 1 micron in diameter. Such sizes are expected at least in the case of an aqueous carrier fluid with low carbon source concentration and no surfactants.

[0049] The method may include moving the spray relative to the pellicle membrane to selectively coat the pellicle membrane. One or both or the spray and the pellicle membrane may be moved in order to spray the pellicle membrane, although it is preferred to move the spray to avoid potential damage to the pellicle membrane.

[0050] The method may include spraying in an environment filled with a gas at atmospheric pressure. The method may include spraying a spray produced by a gas jet and using a gas to carry and direct aerosol particles. Since the sacrificial coating is being sprayed on, it is not necessary to use particularly increased or decreased pressures for any gas carrying the spray.

[0051] The method may include providing the sacrificial coating in a gradient. This may be achieved by, for example, altering the size of the aerosol droplets, altering the speed at which the spray is moved across the pellicle membrane, having more than one pass of the spraying over certain portions of the pellicle membrane, using a mask to intercept a portion of aerosol particles, prior to their landing on the pellicle, or altering the rate at which the spray is provided to the pellicle membrane. As previously described, having a gradient allows for better control of the location of the highest rate of etching in use.

[0052] The gradient may be one or both of a thickness gradient and a density gradient.

[0053] The method may include heating the pellicle membrane to evaporate carrier fluid.

[0054] The method may be conducted at atmospheric pressure or less than atmospheric pressure.

[0055] The method may include providing a substrate with a desired pattern, pressing the substrate and a carbon nanotube based membrane together to emboss the pattern into the carbon nanotube based membrane. The pattern of the substrate can be selected depending on the desired embossed pattern in the carbon nanotube based membrane.

[0056] The method may further include attaching the patterned carbon nanotube-based membrane with a mesh carbon nanotube based membrane. As such, the membrane will have a random mesh layer and a patterned layer which provides additional structural strength and stiffness.

[0057] The method may include masking off a portion of the pellicle membrane. As mentioned, it is desired to provide the sacrificial coating where the rate of etching is greatest and to avoid adding sacrificial coating to the quality area since this could adversely affect throughput or imaging quality.

[0058] The method may further include any of the steps according to the fourth aspect of the present disclosure.

[0059] According to a fifth aspect of the present disclosure, there is provided an apparatus for the manufacture of a pellicle membrane, the apparatus including a vacuum chamber, a support for a pellicle membrane, an electron beam generator configured to generate an electron beam and direct the electron beam at the support, a volatile carbonaceous compound source configured to provide volatile carbonaceous compound to the vacuum chamber, the electron beam being configured to deposit a sacrificial carbon coating on the pellicle membrane, wherein the apparatus includes one or both of i) a mask configured to selectively cover a portion of the pellicle membrane to prevent deposition of the sacrificial coating on the portion covered by the mask, and ii) means for moving the electron beam and pellicle membrane relative to one another to control where the sacrificial carbon coating is provided. The vacuum chamber may be part of a lithographic apparatus. It will be appreciated that manufacture also covers refurbishment or reconditioning of existing pellicle membranes.

[0060] An electron beam allows for the controlled deposition of carbon sacrificial material on a pellicle membrane. In some embodiments, the electron beam may be sufficiently large to illuminate the majority or all of the pellicle membrane. In such embodiments, the sacrificial carbon would be deposited on the entire illuminated area unless a mask is provided. As such, a mask may be provided to prevent sacrificial material being deposited on certain predetermined areas of the pellicle membrane, particularly the quality area through which radiation passes in nominal use. Alternatively or additionally, the electron beam and the pellicle membrane may be moved relative to one another to control where the sacrificial coating is provided. This may be useful for large pellicle membranes where the electron beam is not sufficiently wide to cover the whole pellicle membrane. It will be appreciated that both a mask and relative movement of the electron beam and pellicle membrane may be employed.

[0061] The apparatus may include a heater configured to heat the pellicle membrane to anneal the sacrificial carbon layer. Any suitable heater may be used, for example a laser heater or an electrical heater. A heater is suitable if it is able to heat the sacrificial carbon up to a temperature sufficient to at least partially anneal the carbon. By annealing the sacrificial carbon and / or by controlling the deposition of the sacrificial carbon, it is possible to control the structural type of the carbon and thereby control the relative rate at which it is etched relative to the CNTs.

[0062] According to a sixth aspect of the present disclosure, there is provided the use of a pellicle membrane according to the first aspect of the present disclosure, a pellicle according to the second aspect of the present disclosure, a lithographic apparatus according to the third aspect of the present disclosure, a method according to a fourth aspect of the present disclosure, or the apparatus according to a fifth aspect of the present disclosure in an lithographic method or apparatus.

[0063] 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. Reference to a lithographic apparatus includes reference to an EUV utilization apparatus, which may be an EUV lithographic apparatus used to image integrated circuits or an inspection apparatus, such as a mask inspection apparatus.BRIEF DESCRIPTION OF THE DRAWINGS

[0064] 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:

[0065] Figure 1 depicts a lithographic apparatus according to an embodiment of the disclosure;

[0066] Figure 2 depicts an embodiment according to the present disclosure of a method for manufacturing a pel liclc membrane according to the present disclosure;

[0067] Figure 3 depicts another embodiment of a method for manufacturing a pellicle membrane according to the present disclosure;

[0068] Figure 4 depicts a pellicle assembly according to the present disclosure;

[0069] Figure 5 is a schematic depiction of a pellicle with the corresponding temperature profile of the pellicle, ion flux to the pellicle, and effective etch rate of the pellicle in the charts underneath (in order from top to bottom);

[0070] Figures 6a to 6c depict a pellicle after exposure showing weaker or thinner regions around the area which is irradiated in nominal use;

[0071] Figures 7a to 7c depict a pellicle membrane in accordance with the present disclosure;

[0072] Figures 8a to 8b depict a close up of a pile of CNTs or graphene sheets forming a sacrificial layer;

[0073] Figure 9 is a schematic depiction of a pellicle according to the present disclosure with the corresponding temperature profile of the pellicle, ion flux to the pellicle, and effective etch rate of the pellicle in the charts underneath (in order from top to bottom);

[0074] Figure 10 depicts a chamber housing a pellicle and a sprayer for providing the sacrificial coating;

[0075] Figure 11 is a schematic depiction of a method of providing a sacrificial layer by moving a sprayer;

[0076] Figure 12 is a schematic depiction of a mask provided over a pellicle membrane;

[0077] Figure 13 is a schematic depiction of a square grid showing the highest and lowest transmission pointes of a subaperture spot; and

[0078] Figure 14 is a schematic depiction of one way in which a pellicle membrane may be embossed.

[0079] 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

[0080] Figure 1 shows a lithographic system according to the present invention. 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 RB before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam RB (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, a 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 patterning device MA may be referred to as the reticle. The support structure MT may be referred to as the reticle stage. It will be appreciated that the present disclosure is also applicable to other EUV utilization apparatuses, such as, for example, an inspection apparatus, such as a mask inspection apparatus.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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. A heater may be positioned and configured to provide thermal energy which is reflected by one or more of the mirror devices to the substrate W.

[0087] 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). Preferably, a heater is positioned and configured to provide thermal energy which is reflected by one or more of the mirror devices to the substrate W.

[0088] 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 filtermay be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[0089] 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.

[0090] Figure 2 depicts an embodiment of the apparatus and method for manufacturing a pellicle15 according to the present disclosure. An electron beam 16 is provided and is configured to direct an electron beam onto the pellicle 15. The pellicle 15 includes a pellicle support 17 which supports the pellicle membrane 18. The pellicle support 17 may comprise silicon. The pellicle membrane 18 includes a non-quality area 19 which is not illuminated by EUV in nominal use. The pellicle membrane 18 also includes a quality area (not shown in Figure 2 as it is covered by mask 20). The quality area is the portion of the pellicle membrane through which EUV radiation passes in nominal use. The mask 20 may have an arm 21 to hold it in position, although other means for locating the mask 20 in the desired position may be provided. The arm 21 may be configured such that it does not block the electron beam, thereby forming a shadow which would result in the shaded area not having a sacrificial coating provided. For example, the arm 21 may extend perpendicular to a plane of the mask 20 such that the arm does not extend outside the edge of the mask 20. One or more other masks (not shown) may be provided to protect other portions of the pellicle membrane from contamination during deposition of the sacrificial coating.

[0091] When the pellicle membrane 18 is being manufactured or re-manufactured, such as after it has been used for a predetermined period of time or a predetermined amount of sacrificial material has been etched away through use, it is located within a vacuum chamber (not shown). The vacuum chamber may be a standalone vacuum chamber or may be part of an EUV utilization apparatus, such as an EUV lithographic apparatus or an inspection apparatus. An electron beam 16 is directed towards the pellicle membrane and interacts with an atmosphere comprising hydrocarbons, such as C1-C5 hydrocarbons, which results in a sacrificial layer of carbon being provided on the unmasked area of the pellicle membrane 18, such as the non-quality area 19. In the depiction of Figure 2, the electron beam16 is wide enough to illuminate the whole of the pellicle membrane with the beam of electrons and so a mask 20 is used to control which areas are free of the sacrificial material.

[0092] Figure 3 is similar to Figure 2, but depicts a case in which the electron beam 16 is not wide enough to illuminate the whole of the pellicle membrane 18. As the electron beam 16 can be accurately focused, a mask 20 is not necessary and the quality area 21 can be free of a mask, although it will be appreciated that a mask may still be provided. The electron beam 16 is moved relative to the pellicle membrane 18 along path 22. It will be appreciated that it may be one or both of the electron beam 16 and the pellicle membrane 18 which are moved in order to scan the electron beam 16 over the area where the sacrificial coating is to be applied. The path 22 depicted in Figure 3 is exemplary and anypath which results in a sacrificial layer being deposited in the required region is acceptable. The path 22 may make one or more passes to deposit the sacrificial material.

[0093] Figure 4 depicts a pellicle 15 including a support frame 17, a non-quality area 18 and a quality area 21. A sacrificial coating is provided in the non-quality area 18 and the quality area 21 is free from the sacrificial coating. In use, an EUV radiation beam passes through the quality area 21, causing the quality area 21 to heat up. This is believed to cause desorption of atomic hydrogen from the quality area 21, which reduces the rate at which carbon is etched from the carbon nanotubes comprising the pellicle membrane 18. The portion of the pellicle membrane 18 surrounding the quality area does not reach as high a temperature as the quality area and so atomic hydrogen is not desorbed and therefore the rate of etching is higher. Further outwardly from the quality area, the temperature of the pellicle membrane during nominal use further reduces, which reduces the rate of etching. As such, the area of the pellicle membrane which experiences the highest rate of etching is the area just outside of the quality area and so the sacrificial material is provided in such an area.

[0094] Figure 5 depicts a pellicle 120a, 15, mounted on a reticle with a quality area 101 that is illuminated by EUV radiation 110. The reticle includes an ultra low expansion layer 100 and a back side 102. The pellicle includes a frame 121 and a CNT film 122. The temperature profile of the pellicle 201, the ion flux to the pellicle 202, and the effective etch rate 203 illustrate the effect of accelerated chemical sputtering in the area around the EUV beam 124. The medium temperature between T1 and T2, T_etch, corresponds to around a one hundred times greater yield of chemical sputtering or etching than at the maximum irradiated area and around a ten times higher yield of chemical sputtering or etching than at ambient temperatures, such as where the film in in contact with the frame which acts as a heat sink. As such, as shown in the lowermost chart 203, the rate of etching is highest just outside the area of the pellicle membrane which is illuminated with EUV radiation. Therefore, it is desirable to provide the sacrificial material where etching is most prevalent in nominal use.

[0095] Figures 6a to 6c depict the weaker area 124 where etching is fastest. As shown in Figure 6a, there is a thinner region 124 of the pellicle membrane 120a where etching has occurred. This is for illustrative purposes only and is not a scaled representation or depiction of the damage caused by etching. In some cases, a layer which is being imaged only requires illumination of a portion of the reticle, with the rest being shadowed by reticle masking blades, meaning that the region which is etched 124 is altered, as shown by Figures 6b and 6c.

[0096] Figure 7a is a cross-section of a pellicle including a pellicle membrane in accordance with the present disclosure. In this embodiment, the sacrificial layer 141 is provided with a shallow thickness gradient 142 on its outer edge. In other embodiments, the gradient is provided additionally or alternatively on the inner edge.

[0097] Figure 7b is similar to Figure 7a, but the gradient of the sacrificial layer 151 is steeper. In both cases, the sacrificial layer 141, 151 is not in good thermal contact with the frame which supports the pellicle membrane.

[0098] Figure 7c is a top-down view of the pellicle membrane of Figure 7a or 7b showing the location of the sacrificial layer.

[0099] Figures 8a and 8b are zoomed in views of highlighted section 160 of Figures 7a and 7b. Figure 8a depicts a carbon-nanotube 170 sacrificial layer on the nominal CNT film 122 forming the pellicle membrane and Figure 8b depicts a graphene flake 180 sacrificial layer on the nominal CNT film 122. This is not a limiting example and other forms of carbon forming a sacrificial layer are possible, including masked deposition by e-beam and / or PVD and / or CVD and masked particle deposition, for example via aerosol.[000100] Figure 9 is similar to Figure 5, except that there is now a sacrificial layer 151. By this localised placement of the sacrificial carbon, the thermal profile of the CNT film can be designed and controlled such that the sacrificial layer falls in the region of highest etching during nominal use, without adversely affecting throughput or imaging quality.[000101] Figure 10 is a schematic depiction of a chamber in which the pellicle membrane can be provided with a sacrificial coating using aerosol particles, carrying CNTs, graphene flakes or other forms of carbon nano or micro-particles. A chamber 200 into which a pellicle 120 on a frame on a holder 230 is provided. The pellicle is optionally heated to aid in carrier fluid evaporation. A mask 220 is provided which defines a patterned deposition 221 of droplets 212 which carry the sacrificial carbon material. A sprayer 210 provides a spray 211a of droplets which are then caught by the mask 220 to define the pattern of the ultimate sacrificial coating. An optional supply 251 and 252 may serve to provide a consistent environment within the chamber 200 and to exhaust evaporated carrier fluid. The mask offset from the pellicle H, thickness W, and opening size A and offset from the pellicle frame edge B together define the profile of the resulting deposited sacrificial layer. The deposition chamber and the mask can be cleaned afterwards to remove any excess carbon sacrificial material.[000102] Figure 11 is similar to Figure 10 except that the spray 211b is narrower than in Figure 10. In this embodiment, the sprayer 210 is moveable relative to the pellicle membrane in order to spray the pellicle membrane. The spray 211b may be tilted 230 or shifted 240 to allow controlled thickness of the deposited layer at different pellicle locations.[000103] Figure 12 depicts one way in which the mask 222 may be positioned. Supports to hold an inner part of the mask 222a, 222b, that are offset from the mask plane 222 allow for some droplets to deposit on the partially blocked (line of sight) area due to diffusion, Brownian motion, or flow. The mask 220a, 220b, can accommodate any of the exposed areas and can be configured depending on the desired pattern of the ultimate sacrificial layer.[000104] Figure 13 is a schematic depiction of a grid having grid lines. Although the grid is shown as a square grid, other grid shapes, such as rectangular or hexagonal are equally suitable. In use, a pellicle does not sit in the imaging place of the lithographic apparatus, but is set at a standoff. This is so that any sharp point in the imaging plane, for example the reticle, has blurred into a disc of finite size at the pellicle plane. Uniformity of EUV transmission across the pellicle, taking into account smoothingby this disc or spot, is required for decent imaging performance of the scanner. At a standoff distance of 2 mm, a single point source on the reticle diverges into a spot of radius of around 165 microns. As such, if a regular pattern occurs within this radius, a flare signal is produced which will be projected onto the wafer. If no regular pattern occurs within the disc or spot, the pellicle does not affect imaging. Figure 13 depicts this disc or spot relative to the grid bars. Where the spot does not overlap with a grid bar, there is maximum transmission and no impact upon imaging. In the worst case scenario where the spot overlaps with a junction of two grid lines, this is where there is the highest risk of an impact upon imaging. However, since there is not a regular, repeating pattern, even in this worst case scenario, there is no effect upon imaging.[000105] Figure 14 depicts one way in which a patterned CNT membrane may be manufactured. In a first step, a pattern is provided to a fil, such as a thiol-ene film. As depicted this is achieved by pressing a stamp into the film, although other ways of producing the pattern, such as etching, such as ion beam etching, are also appropriate. This is accompanied by the addition of pressure and heat, followed by cooling, and release of the patterned film. A CNT film is then placed onto the pattered film and pressure is applied to emboss the pattern into the CNT film. This pattern provides the CNT film with additional strength.[000106] In summary, the present disclosure provides for the provision of a sacrificial material on a carbon nanotube pellicle membrane that preferentially etches during nominal use in order to protect the carbon nanotubes, particularly those nanotubes comprising the vicinity of the quality area of the pellicle membrane. By sacrificially etching a sacrificial material, the rate of etching of the carbon nanotubes is reduced, thereby extending the lifespan of the pellicle membrane. EBID can be used to provide a sacrificial carbon coating in a highly controlled way, which allows the sacrificial coating to be provided on portions of the pellicle membrane which are not illuminated by EUV radiation in nominal use, thereby retaining the transmissivity of the pellicle membrane. Alternatively, the sacrificial coating may be applied by aerosol, with suspended CNTs, graphene flakes or other forms of carbon-based nano, or micro particles.[000107] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.[000108] 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 carbon nanotube pellicle membrane for a lithography apparatus, the pellicle membrane including a sacrificial coating.2. The pellicle membrane according to clause 1, wherein the pellicle membrane is a functionalised carbon nanotube membrane.3. The pellicle membrane according to any preceding clause, wherein the sacrificial coating comprises carbon.4. The pellicle membrane according to clause 3, wherein the carbon of the sacrificial coating is amorphous carbon, and / or crystalline carbon and / or annealed carbon and / or graphene flakes and / or carbon nanotubes or diamond particles.5. The pellicle membrane according to clause 4, wherein the graphene flakes are single sheet or multiple sheet flakes, and / or wherein the graphene flakes are from around 0.1 microns to around 10 microns in diameter.6. The pellicle membrane according to clause 4 or clause 5, wherein the carbon nanotubes are singlewalled or multi-walled, and / or wherein the carbon nanotubes are from around 0.1 to around 10 microns in length.7. The pellicle membrane according to any of clauses 4 to 6, wherein the diamond particles are from about 0.1 microns to about 10 microns in diameter.8. The pellicle membrane according to any of clauses 4 to 7, wherein the graphene flakes, carbon nanotubes, and / or diamond particles are functionalized.9. The pellicle membrane according to any preceding clause, wherein the sacrificial coating is non- covalently bonded to the pellicle membrane, or wherein the sacrificial coating is covalently bonding to the pellicle membrane.10. The pellicle membrane according to any preceding clause, wherein the sacrificial coating is disposed between two carbon nanotube membranes, of which at least one extends over the quality area of the reticle in use.11. The pellicle membrane according to any preceding clause, the pellicle membrane having a quality area through which a radiation beam passes in nominal use and a non-quality area through which a radiation beam does not pass in nominal use.12. The pellicle membrane according to clause 11, wherein the sacrificial coating is provided in the non-quality area, or at least in a portion of the non-quality area.13. The pellicle membrane according to any preceding clause, wherein the sacrificial coating is provided around a perimeter of the pellicle membrane.14. The pellicle membrane according to any preceding clause, wherein the thickness of the sacrificial coating is non-uniform.15. The pellicle membrane according to any preceding clause, wherein the sacrificial coating is thicker and / or denser in an area of the pellicle membrane where etching is greatest in nominal use.16. The pellicle membrane according to any preceding clause, wherein the sacrificial coating is formed in a gradient, optionally wherein the gradient is one or both of a thickness gradient and a density gradient.17. The pellicle membrane according to clause 16, wherein the gradient is provided on one or both of an inner edge and an outer edge of the sacrificial coating.18. The pellicle membrane according to any preceding clause, wherein the sacrificial coating is provided on two or more discrete areas of the pellicle membrane.19. The pellicle membrane according to any preceding clause, wherein the sacrificial coating is provided on one or both faces of the pellicle membrane, preferably on one face.20. The pellicle membrane according to any preceding clause, wherein the membrane has, in a quality area, an EUV transmissivity of 90% or greater, an EUV Transmissivity Non-uniformity of 0.2% or less, and EUV scattering of 0.2% or less.21. The pellicle membrane according to any preceding clause, wherein the sacrificial coating is provided by one or more of electron-beam induced deposition, chemical vapour deposition, physical vapour deposition, pulsed laser deposition, aerosol deposition, any of which may include a physical mask or other methods to enable local deposition.22. A carbon nanotube pellicle membrane for a lithographic apparatus, the membrane comprising a carbon nanotube layer supported by a support mesh.23. The pellicle membrane according to clause 22, wherein one of the layers comprises a mesh of carbon nanotubes and the other layer comprises a patterned carbon nanotube layer.24. The pellicle membrane according to clause 23, wherein the patterned carbon nanotube layer is embossed.25. The pellicle membrane according to clause 24, wherein the embossed pattern has a periodicity of 164 microns or less.26. The pellicle membrane according to clause 22, wherein one of the layers comprises a mesh of carbon nanotubes and the other layer comprises a grid having a periodicity of 165 microns or greater.27. The pellicle membrane according to clause 26, wherein the grid comprises grid bars having an effective width of around 0.5 microns or less, and / or a pitch of greater than 330 microns.28. The pellicle membrane according to clause 26 or clause 27, wherein the grid comprises a refractory metal, optionally wherein the refractory metal is tungsten or molybdenum.29. The pellicle membrane according to any of clauses 22 to 28, wherein the pellicle membrane further comprises any of the features of any of clauses 1 to 21.30. A pellicle for a lithographic apparatus, the pellicle including the pellicle membrane according to any preceding clause and a support frame for supporting the pellicle membrane.31. A lithographic apparatus comprising the pellicle membrane or pellicle according to any preceding clause.32. A method of manufacturing a pellicle membrane, the method including the steps of providing a carbon nanotube pellicle membrane and providing a sacrificial coating thereon.33. The method according to clause 32, the method further including providing the carbon nanotube pellicle membrane in a vacuum chamber, providing a mask to cover a portion of the pellicle membrane to thereby provide a masked area and prevent deposition of a coating on the masked area, and depositing a sacrificial coating on an unmasked area of the pellicle membrane.34. The method according to clause 32 or clause 33, the method further including providing an electron source configured to generate an electron beam, and moving the pellicle membrane and the electron beam relative to one another to selectively deposit the sacrificial coating on the pellicle membrane.35. The method according to clause 33 or clause 34, wherein the vacuum chamber includes volatile carbonaceous compounds to serve as the source of the sacrificial coating, optionally wherein the volatile carbonaceous compounds are C1-C5 hydrocarbons.36. The method according to any of clauses 32 to 35, wherein the method further includes annealing the sacrificial coating.37. The method according to any of clauses 32 to 36, wherein the method includes controlling the structural form of the sacrificial coating, optionally wherein the method additionally or alternatively includes a step of removing contaminants from the sacrificial coating and / or the pellicle membrane, preferably post-deposition of the sacrificial coating.38. The method according to any of clauses 32 to 37, wherein the method includes providing the sacrificial coating by spraying the pellicle membrane with aerosol and providing a mask to limit the pellicle membrane area on which a sacrificial coating is formed.39. The method according to any of clauses 32 to 38, wherein the method includes providing a mixture of a carbon source in a carrier fluid for spraying aerosol onto the pellicle membrane, optionally wherein the carrier liquid is polar or non-polar, optionally wherein the carbon particles comprise functionalized graphene flakes and / or carbon nanotubes and / or diamond particles.40. The method according to any of clauses 32 to 39, wherein the method includes forming an aerosol and spraying the aerosol on the pellicle membrane.41. The method according to clause 40, wherein the step of forming the aerosol includes operating a piezo driver and / or a gas jet to form the aerosol, optionally wherein the piezo driver is driven at from around 10 kHz to around 10 MHz, preferably from 100 kHz to 1 MHz, optionally wherein the average diameter of droplets within the aerosol is from around 1 micron to around 10 microns.42. The method according to any of clauses 38 to 41, wherein the method includes moving the spray relative to the pellicle membrane to selectively coat the pellicle membrane.43. The method according to any of clauses 38 to 42, wherein the spraying is carried by a gas at atmospheric pressure.44. The method according to any of clauses 38 to 43, wherein the method includes providing the sacrificial coating in a gradient.45. The method according to clause 44, wherein the gradient is one or both of a thickness gradient and a density gradient.46. The method according to any of clauses 38 to 45, wherein the method includes heating the pellicle membrane to evaporate carrier fluid.47. The method according to any of clauses 38 to 46, wherein the method is conducted at atmospheric pressure or less than atmospheric pressure.48. The method according to any of clauses 32 to 47, wherein the method further includes providing a second pellicle membrane to sandwich the sacrificial layer between the two pellicle membranes.49. A method of manufacturing a carbon nanotube pellicle membrane, the method including providing a substrate with a desired pattern, pressing the substrate and a carbon nanotube based membrane together to emboss the pattern into the carbon nanotube based membrane.50. The method according to clause 49, wherein the method further includes attaching the patterned carbon nanotube-based membrane with a mesh carbon nanotube membrane. 1. The method according to clause 50, wherein the method further includes any of the steps of any of clauses 32 to 41.52. An apparatus for the manufacture of a pellicle membrane, the apparatus including a vacuum chamber, a support for a pellicle membrane, an electron beam generator configured to generate an electron beam and direct the electron beam at the support, a volatile carbonaceous compound source configured to provide volatile carbonaceous compound to the vacuum chamber, the electron beam being configured to deposit a sacrificial carbon coating on the pellicle membrane, wherein the apparatus includes one or both of i) a mask configured to selectively cover a portion of the pellicle membrane to prevent deposition of the sacrificial coating on the portion covered by the mask, and ii) means for moving the electron beam and pellicle membrane relative to one another to control where the sacrificial carbon coating is provided.53. The apparatus according to clause 52, the apparatus further including a controller configured to control the electron beam generator and / or the relative position of the electron beam and pellicle membrane.54. The apparatus according to clause 52 or clause 53, wherein the apparatus further includes a heater configured to heat the pellicle membrane to anneal the sacrificial carbon layer and / or a heater to heat the mask.55. The use of a pellicle membrane according to any of clauses 1 to 29, a pellicle according to clause 30, a lithographic apparatus according to clause 31, a method according to any of clauses 32 to 51, or the apparatus according to any of clauses 51 to 54 in lithographic method or apparatus.

Claims

CLAIMS1. A carbon nanotube pellicle membrane for a lithography apparatus, the pellicle membrane having a quality area through which a radiation beam passes in nominal use and a non-quality area through which a radiation beam does not pass in nominal use, wherein the pellicle membrane includes a sacrificial coating provided on at least a portion of a non-quality area of the pellicle membrane.

2. The pellicle membrane according to Claim 1, wherein the pellicle membrane is a functionalised carbon nanotube membrane.

3. The pellicle membrane according to any preceding claim, wherein the sacrificial coating comprises carbon.

4. The pellicle membrane according to claim 3, wherein the carbon of the sacrificial coating is amorphous carbon, and / or crystalline carbon and / or annealed carbon and / or graphene flakes and / or carbon nanotubes or diamond particles.

5. The pellicle membrane according to claim 4, wherein the graphene flakes, carbon nanotubes, and / or diamond particles are functionalized.

6. The pellicle membrane according to any preceding claim, wherein the sacrificial coating is non- covalently bonded to the pellicle membrane, or wherein the sacrificial coating is covalently bonding to the pellicle membrane.

7. The pellicle membrane according to any preceding claim, wherein the sacrificial coating is disposed between two carbon nanotube membranes, of which at least one extends over the quality area of the reticle in use.

8. The pellicle membrane according to claim 7, wherein the sacrificial coating is provided in the non- quality area.

9. The pellicle membrane according to any preceding claim, wherein the sacrificial coating is provided around a perimeter of the pellicle membrane.

10. The pellicle membrane according to any preceding claim, wherein the sacrificial coating is thicker and / or denser in an area of the pellicle membrane where etching is greatest in nominal use.

11. The pellicle membrane according to any preceding claim, wherein the sacrificial coating is formed in a gradient, optionally wherein the gradient is one or both of a thickness gradient and a density gradient.

12. The pellicle membrane according to claim 11, wherein the gradient is provided on one or both of an inner edge and an outer edge of the sacrificial coating.

13. The pellicle membrane according to any preceding claim, wherein the sacrificial coating is provided on two or more discrete areas of the pellicle membrane.

14. The pellicle membrane according to any preceding claim, wherein the sacrificial coating is provided on one or both faces of the pellicle membrane, preferably on one face.

15. The pellicle membrane according to any preceding claim, wherein the membrane has, in a quality area, an EUV transmissivity of 90% or greater, an EUV Transmissivity Non-uniformity of 0.2% or less, and EUV scattering of 0.2% or less.

16. A pellicle for a lithographic apparatus, the pellicle including the pellicle membrane according to any preceding claim and a support frame for supporting the pellicle membrane.

17. A method of manufacturing a pellicle membrane, the method including the steps of providing a carbon nanotube pellicle membrane and providing a sacrificial coating thereon on at least a portion of a non-quality area of the pellicle membrane area through which a radiation beam does not pass in nominal use.

18. An apparatus for the manufacture of a pellicle membrane, the apparatus including a vacuum chamber, a support for a pellicle membrane, an electron beam generator configured to generate an electron beam and direct the electron beam at the support, a volatile carbonaceous compound source configured to provide volatile carbonaceous compound to the vacuum chamber, the electron beam being configured to deposit a sacrificial carbon coating on the pellicle membrane, wherein the apparatus includes one or both of i) a mask configured to selectively cover a portion of the pellicle membrane to prevent deposition of the sacrificial coating on the portion covered by the mask, and ii) means for moving the electron beam and pellicle membrane relative to one another to control where the sacrificial carbon coating is provided.

19. The apparatus according to claim 18, the apparatus further including a controller configured to control the electron beam generator and / or the relative position of the electron beam and pellicle membrane.

20. The apparatus according to claim 18 or claim 19, wherein the apparatus further includes a heater configured to heat the pellicle membrane to anneal the sacrificial carbon layer and / or a heater to heat the mask.

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