Free-standing NT-films with variable regions of alignment and preparation method

Abstract

The invention relates to films of nanotubes (NT-films) with variable regions of alignment of the nanotubes on free-standing portions of the films. Therewith, the invention provides free-standing NT-films with a targeted pattern of different regions of alignment and / or random distribution. The invention further relates to methods for preparing such NT-films with free-standing portions comprising variable and targeted regions of alignment and the use thereof in optical applications like their use as pellicles in EUV lithography.

Description

[0001] Karlsruher Institut fur Technologie H70404WO 19.12.2025

[0002] FREE-STANDING NT-FILMS WITH VARIABLE REGIONS OF ALIGNMENT AND PREPARATION METHOD

[0003] INTRODUCTION

[0004] The invention relates to films of nanotubes (NT-films) with variable regions of alignment of the nanotubes within a film layer on free-standing portions of the films. Therewith, the invention provides free-standing NT-films with a targeted pattern of different regions of alignment within the plane region of the film. The invention further relates to methods for preparing such NT-films with free-standing portions comprising variable and targeted regions of alignment within a film layer and the use thereof in optical applications like their use as pellicles in EUV lithography.

[0005] BACKGROUND OF THE INVENTION AND PRIOR ART

[0006] Thin films with optical properties are widely used in the field of optics, including flexible electronics, biosensors, transistors, thermoelectrics, solar cells, photonics, terahertz spectroscopy, terahertz polarizers / limiters, polarized light emitters, heat management (the distribution of heat to and from certain locations) and electrical conductance.

[0007] Of particular interest in the optical field are nanomaterial based thin films, like nanotubebased thin films. The most common nanotubes are carbon nanotubes (CNTs). Carbon nanotubes are a quasi 1 D nanomaterial with structure dependent and therefore tailorable optical and electronic properties. Generally, a carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometer range (nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized, singlewalled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), which are nested single-wall carbon nanotubes having a nested tube-in-tube structure.

[0008] Carbon nanotubes can exhibit remarkable properties, such as exceptional tensile strength and thermal conductivity because of their nanostructure and strength of the bonds between carbon atoms. Some CNT structures exhibit high electrical conductivity while others are semiconductors. Namely, in this 1 D system, circumferential electron confinement results in CNTs that are either metallic (m) or semiconducting (s) and small changes in their diameter impart large changes in the spectral position of absorption maxima (bandgap) of the CNTs. For each chiral species (diameter) these maxima appear as sets of discrete excitonic transitions in the infrared, visible and ultra-violet. In addition, carbon nanotubes can be chemically modified. These properties and the ability to tune them with structure has made CNTs one of the most intensively studied nanomaterials of the past two decades. This, in particular, applies for the specific subaspect of pellicles for EUV lithography. CNTs have been described for this application.

[0009] Nanotubes, like in particular carbon nanotubes, are well suited for these applications and CNT pellicles are widely known and developed. Due to their beneficial properties, NT-films can be used in further optical fields. For example, terahertz spectroscopy is a novel field which has arisen in the past few years due to the development of suitable laser sources. Carbon nanotubes have been found to be able to rotate incident terahertz light by 30° rotation difference to the SWCNTs alignment direction, exactly by 90° as described by Baydin et al. [Baydin et al.: Giant terahertz polarization rotation in ultrathin films of aligned carbon nanotubes; Optica, 760 Vol. 8, No. 5, 2021; https: / / doi.org / 10.1364 / OPTICA.422826]. Currently, the polarizer used in this field are made of brittle, inflexible and bulky materials. The use of NT-films offers the possibility to provide significantly thinner polarizer. Limiters are important for high-power LASER applications and ensure the safety of operating personnel as well as sensible machine parts. The high resistance of carbon nanotubes to temperature and high radiation levels make SWCNT polarizers with a defined bar width an attractive alternative [Chen et al., Carbon Nanotube-Based Functional Materials for Optical Limiting, Journal of Nanoscience and Nanotechnology, Vol. 7, 1268-1283, 2007; doi: 10.1166 / jnn.2007.308]. Polarized light emission from SWCNTs has already been achieved but an additional gain in efficiency could be obtained by having the carbon nanotubes also free standing and aligned. The lack of a substrate will prevent quenching of electron-hole pairs [Mata no et al., Electrical Generation of Polarized Broadband Radiation from an On-Chip Aligned Carbon Nanotube Film, ACS Materials Lett. 2022, 4, 4, 626-633; https: / / doi.org / 10.1021 / acsma-terialslett.2c00058]. Further, SWCNTs are currently investigated by companies for heat dissipation within integrated electronics as higher power consumption also comes with higher heat build-up [https: / / www.fujitsu.com / global / about / resources / news / pressreleases / 2017 / 1130- 01.html]. Due to their high electron mobility, they might also function as electrical conductors.

[0010] Carbon nanotubes are also considered a 'material of choice' for next generation thermoelectrics because they have a high Seebeck coefficient, and this might be enhanced in an aligned film and the gradient between hot and cold end enhanced in a free-standing form. Finally, free-standing films may also be beneficial for liquid and gas separation applications.

[0011] Ultra-thin nanotube films for the use as pellicles in the EUV lithography are widely described. Such pellicle films are usually free-standing films comprising the carbon nanotubes either randomly distributed (unaligned), e.g. in the form of a plurality of randomly oriented carbon nanofibers forming an interconnected network structure, or with only one specific alignment within a planar film layer.

[0012] Jung et al. [Jung et al.: Ultrathin, Large-Area, and Multifunctional Polarizer Based on Highly Ordered Carbon Nanotubes Produced by Simple Shear Flow, Advanced Materials Technologies, Vol. 8, Issue 24, 2023; https: / / doi.Org / 10. 1002 / admt.202301176] describe a polarizer made from SWCNT ink, involving dispersing SWCNTs in chlorosulfonic acid (CSA) in liquid crystal concentrations of nanotubes and aligning these by mechanical shear.

[0013] Gallagher et al. [Gallagher et al.: CNTs in the context of EUV pellicle history, Proc, of SPIE Vol. 10583, 105831 E-1 ; doi: 10.1117 / 12.2297710] describe an image of a linear aligned carbon nanotube film, presumably of multi-walled carbon nanotubes without describing the preparation process itself. It is mentioned to use the films in the context of pellicles for the semiconductor industry.

[0014] Rust et al. describe a method of either radial or global alignment of carbon nanotubes via dead-end filtration in a process of filtering a carbon nanotube suspension through a porous filtration membrane in a wet-transfer process [Rust et al.: Global Alignment of Carbon Nanotubes via High Precision Microfluidic Dead-End Filtration; Adv. Fund. Mater., 2022, 32, 2107411; doi: 10.1002 / adfm.202107411 and Rust et al.: Radial Alignment of Carbon Nanotubes via Dead-End Filtration; Small, 2023, 19, 2207684; doi: 10. 1002 / smll.202207684],

[0015] US2023 / 259021 describes pellicles for EUV lithography masks and methods for manufacturing thereof and describes embodiments with two or more nanotube sheets, wherein the different nanotube sheets may differ from each other with respect to their alignment axes.

[0016] US2024 / 0337922 describes a method of storing EUV pellicles or pellicle films of carbon nanotube thin-films in a storage or transportation container. The carbon nanotubes forming the films may form bundles of same or different types of CNTs which may be aligned in parallel within the bundle.

[0017] US2004 / 0265489 describes composite carbon nanotube structures comprising a number of carbon nanotubes disposed in a matrix comprised of a metal or metal oxide and the use thereof as a thermal interface device in a packaged integrated circuit device.

[0018] US2015 / 0209761 describes multilayer substrates for the growth or support of CNT arrays to promote the growth of dense vertically aligned CNT arrays and the use thereof as thermal interface materials.

[0019] US2023 / 0305192 describes EUV transmissive membranes including a main layer with IR emissivity and a protective layer covering at least one side of the main layer. However, so far no free-standing CNT-films have been described comprising a selective and intentionally patterned alignment of the multiple CNTs within a film layer to provide a specific customised alignment pattern within a plane axis. Further, the methods described in the prior art involve the use of hazardous substances (CSA), which limits the handling and large-scale application significantly.

[0020] When the density and alignment of the carbon nanotubes in a film or layer is significantly low such films are less suitable as pellicles, as gaps might result in EUV light scattering leaving more space for undesired particles to be slipping through so that the desired protection cannot be achieved. Further, so far no process allowing for selective pattering of a layer by creating different regions with different alignments or different geometries within one planar axis has been nor the preparation of free-standing films with the possibility of applying different localized regions with different alignments on one film to provide a (planar) targeted pattern of different alignments.

[0021] DESCRIPTION OF THE INVENTION

[0022] Generally, there is a need of providing NT-films with considerable size of a free-standing area portion. For suitability in commercial use such free-standing NT-films need to provide good physical and mechanical stability and therewith long product lifetime. Generally, thin-films used as pellicles suffer from their potential of breaking, ripping and tearing of the membrane, in particular after being subjected to high doses of irradiation. Further, there is an ongoing need for providing thin-film membranes for optical applications with improved properties. Further, there is a need to provide suitable methods for preparing such free-standing thin NT films.

[0023] A pellicle is a super-thin film or membrane which is usually stretched over a frame or pellicle border which is used to attach the pellicle over the patterned part of photomasks. The pellicle films must be highly transparent to extreme ultraviolet (EUV) light (in the range of EUV wavelengths, usually 13.5 nm) whereas most substances absorb it, and that can withstand high temperatures in a high-vacuum environment. Further, an EUV pellicle must be able to satisfy a high transmittance to extreme UV light, which requires the use of a free-standing film that is very thin (in the nm range) and is able to provide a sufficiently large free-standing area size. The free-standing NT-films further need to be optically, mechanically and chemically homogenous closely packed films of a certain minimum density to ensure that no dust, debris, particles or contamination can pass through the membrane. The films must be able to withstand high doses of extreme UV radiation which leads to the development of high local temperatures in the pellicle and to withstand the corrosive chemical environments used within the lithographic system, which continuously etches and thins the pellicle during use until film failure. That means, pellicle films must withstand high temperatures in a high-vacuum environment. Furthermore, the films need to be mechanically stable and able to withstand the high G- forces within the lithographic system. These requirements show that pellicles are usually considered as consumable parts in the EUV lithographic systems. In principle, pellicles are used as a kind of protection or ‘dust cover’ that protect photomasks in the EUV device, however, such films can also be used to protect other components in the EUV lithographic system (like scanner or mirrors) as well as the EUV lithography products. For example, pellicles prevent dust, particles or other contaminants from falling onto the EUV pattern / mask and therewith avoids its subsequent unwanted replication by optical lithography. Without a pellicle, the results in the lithography process can be catastrophic. If a particle lands on a mask, the scanner could print repeating defects on the wafer, which negatively impacts yield. Further, it is also important to protect the photomask for cost reasons. The average price for a leading-edge optical mask is $100,000, while an EUV mask is approximately $300,000.

[0024] So, overall, for all of the applications mentioned herein the ability to provide free standing films of nanomaterials is particularly desirable for the reasons mentioned herein. Particularly, in optical applications the presence of a support layer or substrate would be detrimental to the optical properties of the film. Accordingly, a key aspect of this invention is related to the ability of providing free-standing films with improved stability and good optical properties despite the ultra-thin thickness of the films. Particularly in the lithographic system the EUV light passing the pellicle must be able to provide the optimum optical response across its entire surface and therefore needs to be highly homogenous. Wrinkles, defects or differences in the density of the film material over the pellicle need to be avoided.

[0025] The possibility to provide thin-films having localized regions, like a planar pattern, of aligned NTs offers a broader variability and flexibility in tailoring highly specified NT-films for individual applications with customised and improved properties. The ability to provide different regions, fields or areas of alignment on a single NT-film further offers the possibility to provide NT-films with a customised pattern.

[0026] It was the object of the present invention to provide thin-film membranes based on nanomaterials for use in optical applications having a considerable size of a freestanding area portion with sufficient stability making them suitable for the use as protective films and pellicles in EUV lithography. A further object of the invention relates to providing free-standing thin films based on nanomaterials, like nanotubes, with improved optical properties. Particularly, an object of the invention relates to providing free-standing thin nanotube films with high flexibility and variability regarding the optical characteristics and the alignment of nanotubes on the film. In a further aspect, it was desired to be able to provide free-standing ultra-thin nanotube films allowing to realize customized alignments and variable patterns of alignment on the film surface or over the planar film area (i.e in a x / y axis) to offer increased flexibility and variability for customized applications of the films. Further, it was an aim to develop methods allowing to control the location, the orientation, pitch and density of nanotubes on a free-standing thin-film surface. Accordingly, a particular aspect of the invention relates to providing ultra-thin NT-films with a large free-standing area, being optically, mechanically and chemically homogenous and providing closely packed films of high density allowing to ensure that no undesired particles can pass through the membrane. The NT-films should exhibit high stability against mechanical, physical and chemical impact as well as the ability to withstand high doses of extreme UV radiation, high local temperatures and the corrosive chemical environments used in lithographic systems. A further object of the invention relates to providing novel methods for preparing NT-films with free-standing area portions and high flexibility in the control of the optical characteristics.

[0027] For solving these objects, the inventors of the present invention developed novel ultrathin free-standing NT-based films with variable localized (planar) regions or areas of different (targeted) alignment of the NTs to provide an intentionally designed pattern over the planar area of a film. The inventors further developed a method for preparing such free-standing thin NT-films allowing to control and flexibly localize different regions of alignment of nanotubes on the surface or over the planar area of the free-standing films. Particularly, the inventors were able to control the alignment of NTs in different localized regions on the surface or over the planar area of the free-standing NT-films allowing to provide NT-films with the highest possible variability regarding realization of different localized regions and planar areas with different alignments on the same film as well as realizing combinations of regions with one or more different alignments and regions without alignment but random distribution of the nanotubes. Therewith, the invention for the first time offers the possibility to apply a targeted pattern of different areas of alignments, optionally with regions or areas of random distribution in a single layer film or sheet of nanotubes, i.e. in a planar x / y axis, which offers new options for customized NT-films with customized optical characteristics.

[0028] The preparation method developed by the inventors allows to prepare NT-films with the required characteristics and stability and allows to provide high density aligned films to be produced which cannot be made with conventional methods known so far. The method described herein offers the additional benefit over known techniques by using a wet-transfer process wherein the nanotubes can be processed by suspending them in aqueous solutions, which allows safe and environmentally friendly handling. The use of aqueous suspensions of NTs allows safer handling and more accessible pretreatments like centrifugation, density ultra centrifugation, sorting, purification and filtering.

[0029] Additionally, the method developed by the inventors offers new opportunities for avoiding or repairing defects or wrinkles occurring during wet-transfer process techniques. The good mechanical, chemical and physical stability, observed by the inventors, is assumed to be supported by applying an alignment pattern on the planar area of the NT- films, which may affect improved structural stability and reduced optical scattering, which is expected to provide NT-films that may withstand EUV exposure longer than conventional films and therefore benefit from longer product lifetime. These properties make the free-standing thin films of the invention with the targeted alignment pattern particularly suitable for the use as protective films and pellicles in EUV lithography.

[0030] SPECIFIC ASPECTS

[0031] The present invention includes, without being limited thereto, the following aspects:

[0032] A first aspect relates to a film of nanotubes (NTs) comprising two or more different regions of NTs, wherein at least one region comprises NTs with a given alignment or orientation and at least one region comprises NTs with a different alignment or orientation or without alignment or orientation, and wherein at least one of the regions is located on a free-standing portion of the NT-film, particularly the two or more different regions of NTs with different alignment or orientation are located on a planar axis or surface area of the film, e.g. within a single layer of the film.

[0033] Said film preferably comprises more than two regions of NTs, wherein all regions differ from each other or wherein two or more regions among the multiple regions are identical while at least one region among the multiple regions is different from at least one other region, particularly the two or more different regions of NTs with different alignment or orientation are located on a planar axis or surface area of the film, e.g. within a single layer of the film.

[0034] Said film preferably comprises different regions of NTs which differ from each other

[0035] • by one or more regions having a first alignment or orientation of the NTs and one or more regions having a second or several different alignments or orientations of the NTs and which are different to the first alignment or orientation, or

[0036] • by one or more regions having the same or different alignment or orientation of the NTs and one or more regions comprising NTs without alignment or orientation (randomly distributed NTs), wherein the two or more different regions of NTs with different alignment or orientation are located on a planar axis or surface area of the film, e.g. within a single layer of the film.

[0037] In said film the NTs are preferably selected from carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs) and mixtures thereof. Therein, preferably

[0038] NTs from the group of carbon nanotubes (CNTs) are selected from single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs), multi-walled CNTs (MWCNTs) and coaxial nanotubes, and mixtures thereof, and

[0039] NTs from the group of boron nitride nanotubes (BN NTs) are selected from single-walled BNNTs (SWBNNTs), double-walled BNNTs (DWBNNTs), multi-walled BNNTs (MWBNNTs) and coaxial nanotubes, and mixtures thereof.

[0040] Therein the NTs are preferably carbon nanotubes (CNTs), preferably SWCNTs.

[0041] In said film the free-standing portion preferably has a free-standing area size of at least 1.0 cm2, preferably > 1.0 cm2, more preferably > 10.0 cm2, > 100.0 cm2.

[0042] In said film the free-standing portion preferably has a free-standing area size of up to 200 cm2, or up to 300 cm2, or up to 400 cm2, or up to 500 cm2.

[0043] Said film preferably has a maximum thickness of about 150 nm, preferably about 120 nm, more preferably about 100 nm, even more preferably about 80 nm, most preferred about 50 nm.

[0044] Said film preferably has a minimum thickness of about 1 nm, preferably about 2 nm, more preferably about 3 nm.

[0045] Said film preferably has an extreme ultraviolet light (EUV) transmittance at 13.5 nm of 80 % or more, preferably 85 % or more, more preferably 90 % or more, more preferably 92% or more, more preferably 93 % or more, more preferably 95 % or more.

[0046] Said film is preferably mounted onto a solid border with a defined aperture, wherein the free-standing portion of the NT-film covers the aperture, preferably the film is attached to the solid border stress-free by adhering the film to the solid border without fixation aids.

[0047] Preferably said solid border is a Si, SiC>2, Si N , a Si border with SiN coating, an aluminium or an aluminium alloy pellicle border or a stainless-steel pellicle border.

[0048] Preferably, in said film the aperture has a size corresponding to the area size of the freestanding portion of the NT-film.

[0049] A further aspect relates to a method of preparing a film of nanotubes (NTs) comprising two or more different regions of NTs with at least one region comprising NTs with a given alignment or orientation and at least one region comprising NT s with a different alignment or orientation or without alignment or orientation, and wherein at least one of the regions is located on a free-standing portion of the NT-film, particularly the two or more different regions of NTs are located on a planar axis or area of the film, e.g. within a single layer of the film, the method comprising the following steps i) preparing a liquid suspension of NTs in a solvent, preferably comprising an optional pre-filtration treatment of the liquid suspension of NTs; ii) providing a porous filtration membrane, which is insoluble in the solvent of the NT suspension and which comprises one or more imprinted, etched or hot embossed templates with the one or more regions for the desired alignment; iii) filtering the NT suspension through the porous filtration membrane to obtain a dry NT-film fixed on the porous filtration membrane; iv) applying onto the dry NT-film fixed on the porous filtration membrane of step iii) an intermediate sacrificial layer; v) removing the porous filtration membrane to obtain the NT-film with two or more different regions of NTs fixed on the sacrificial layer; vi) removing the intermediate sacrificial layer and vii) mounting the NT-film onto a solid border with a defined aperture, preferably by adhering to the solid border without fixation aids; viii) applying a film purification step to obtain the NT-film mounted onto the solid border with a free-standing portion of the NT-film covering the aperture.

[0050] Said method of preparing a film of nanotubes (NTs), wherein the steps iv), v), vi) and viii) further comprise the following: a) in step iv) an intermediate sacrificial layer is applied which is insoluble in solvents which are capable to dissolve the porous filtration membrane; b) in step v) the porous filtration membrane is removed by dissolving it with a solvent being capable to dissolve the porous filtration membrane or by mechanically peeling off; c) optionally prior to step vi) an additional supporting polymer layer, preferably an IR absorbing polymer layer, more preferably a polycarbonate (PC) layer, is applied onto the NT-film fixed on the intermediate sacrificial layer, e.g. by spincoating, with said supporting polymer layer facing the NT-film followed by transferring the NT-film onto a solid border with a defined aperture with the additional supporting polymer layer facing the border or being depart from the border with the NT-film contacting the border; d) in step vi) the intermediate sacrificial layer is removed by dissolving in a solvent which is capable to dissolve the intermediate sacrificial layer thereby fixing the NT-film onto the solid border to obtain the NT-film with the optional supporting polymer layer mounted onto the solid border optionally with the additional supporting polymer layer facing the border or depart from the border; and e) in step viii) the optional additional supporting polymer layer is removed to obtain the NT-film with the two or more different regions of NTs mounted onto the solid border, preferably stress-free by adhesion to the border without fixation aids, with a freestanding portion of the NT-film covering the aperture.

[0051] The method of preparing a film of nanotubes (NTs) described anywhere herein, wherein the steps iv), v), vi) and vii) further comprise the following: a) in step iv) an intermediate sacrificial layer is applied which is insoluble in solvents which are capable to dissolve the porous filtration membrane and b) in step v) the porous filtration membrane is removed by dissolving it with a solvent being capable to dissolve the porous filtration membrane; c) in step vi) the intermediate sacrificial layer is removed by dissolving in a solvent which is capable to dissolve the intermediate sacrificial layer to obtain a floating NT-film; d) in step vii) the floating NT-film is transferred to and mounted onto a solid border with a defined aperture to form a free-standing portion of the NT-film covering the aperture, preferably the NT-film is mounted onto the solid border stress-free by adhesion without fixation aids.

[0052] The method of preparing a film of nanotubes (NTs) described anywhere herein , wherein one or more of the following apply a) in step iii) the NT suspension is filtered through the porous filtration membrane in an amount to obtain a NT-film of the desired target thickness; and / or b) in step iii) the NT suspension is filtered through the porous filtration membrane in an amount to obtain a NT-film of high thickness of at least 100 nm, preferably at least 150 nm; and / or c) in step v) the porous filtration membrane is removed completely; and / or d) a step of applying an additional supporting polymer layer is omitted; and / or e) prior to step viii) the NT film is etched to the desired low thickness.

[0053] The method described anywhere herein , wherein the NTs are selected from carbon nanotubes (CNTs) and boron nitride nanotubes (BN NTs) and mixtures thereof, wherein NTs from the group of carbon nanotubes (CNTs) are selected from single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs), multi-walled CNTs (MWCNTs) and coaxial nanotubes, and mixtures thereof, and NTs from the group of boron nitride nanotubes (BN NTs) are selected from single-walled BN NTs (SWBNNTs), double-walled BN NTs (DWBNNTs), multi-walled BNNTs (MWBNNTs) and coaxial nanotubes, and mixtures thereof, preferably the NTs are carbon nanotubes (CNTs), more preferably SWCNTs.

[0054] The method described anywhere herein, wherein the liquid NT suspension of step i) is an aqueous suspension, preferably a suspension in water comprising one or more surfactants, more preferably a suspension purified and / or homogenized to be essentially free of aggregates with a size > 10 pm.

[0055] The method described anywhere herein, wherein the porous filtration membrane of step ii) is soluble in organic solvents and insoluble in aqueous solutions.

[0056] The method described anywhere herein, wherein the porous filtration membrane of step ii) is selected from porous membranes of polycarbonate, PTFE, nylon, polyimide, ceramics like AI2O3, cellulose esters, preferred is a porous polycarbonate or a polyimide membrane.

[0057] The method described anywhere herein, wherein in step ii) the one or more templates with the one or more regions for the desired alignment are imprinted or embossed onto the porous filtration membrane by hot-embossing technique.

[0058] The method described anywhere herein, wherein in step iii) the filtering of the NT suspension through the porous filtration membrane is carried out by vacuum filtration or by applying positive-pressure, including applying constant positive-pressure or a deadend filtration geometry with volume rate control, preferably in step iii) the filtering is carried out by applying constant pressure with pressure control.

[0059] The method described anywhere herein, wherein in step iii) the NT suspension is filtered through the porous filtration membrane in an amount sufficient to provide the desired film thickness.

[0060] The method described anywhere herein, wherein the intermediate sacrificial layer applied in step iv) is a film material which is insoluble in organic solvents and soluble in aqueous solutions.

[0061] The method described anywhere herein, wherein the intermediate sacrificial layer applied in step iv) is selected from the group comprising a polystyrene sulfonate (PSS) film, a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) film, a PVA film, a PVA film on a polyester carrier like Hydrokon® (company IKONICS), PMMA films or composites of the aforementioned film materials with further polymers. The method described anywhere herein, wherein the intermediate sacrificial layer applied in step iv) is selected from a PSS film or a PEDOT:PSS film, more preferably a PEDOT:PSS film.

[0062] The method described anywhere herein, wherein the intermediate sacrificial layer applied in step iv) is a PVA film, preferably a PVA film on a polyester carrier like Hydrokon® (company IKONICS).

[0063] The method described anywhere herein, wherein in step iv) the intermediate sacrificial layer is applied onto the NT-film by iv)-a) applying a solution of the sacrificial layer film material onto the inert intermediate substrate, or iv-b) applying a PVA-film, like Hydrokon® on an inert intermediate substrate, followed by transferring the dried NT-film fixed on the porous filtration membrane of step iii) onto the substrate with the NT-film facing the sacrificial layer film material and in step iv-a) drying the solution of the sacrificial layer film material or in step iv-b) laminating the PVA-film on the NT-film to obtain the NT-film deposited between the porous filtration membrane and the intermediate sacrificial layer on the inert intermediate substrate.

[0064] The method described anywhere herein comprising the step iv-b) followed by IR-heating the laminate at a temperature to soften the PVA and homogenize the lamination.

[0065] The method described anywhere herein, wherein in step v) the porous filtration membrane is removed by dissolving it with an organic solvent.

[0066] The method described anywhere herein, wherein the organic solvent for dissolving and removing the porous filtration membrane is chloroform.

[0067] The method described anywhere herein, wherein in step vi) the intermediate sacrificial layer is removed by dissolving in an aqueous solution, preferably in water.

[0068] The method described anywhere herein wherein the intermediate sacrificial layer is a PVA film on a polyester carrier like Hydrokon® (company IKONICS) and the polyester carrier is mechanically peeled off prior to transferring the NT-film onto the solid border and dissolving the intermediate sacrificial layer. The method described anywhere herein, wherein prior to transferring and mounting the layered film composition onto a solid border protruding edges are removed by cutting.

[0069] The method described anywhere herein, wherein the solid border with a defined aperture is a Si, SiC>2, SiN, a Si border with SiN coating, an aluminium or an aluminium alloy pellicle border or a stainless-steel pellicle border.

[0070] The method described anywhere herein, wherein in step v)

[0071] • the porous filtration membrane is removed completely by dissolving in a solvent; or

[0072] • the porous filtration membrane is removed by dissolving in a solvent until a residual thin film of the porous filtration membrane remains; or

[0073] • in case of completely removing the porous filtration membrane an additional supporting polymer layer, preferably an IR absorbing polymer layer, more preferably a polycarbonate (PC) layer, of defined thickness is applied onto the NT-film, e.g. by spin-coating.

[0074] The method described anywhere herein, wherein an IR-heat treatment step is applied after mounting the NT-film onto a Si, SiC>2, SiN border or a Si border with SiN coating with a defined aperture until wrinkles, strains, defects and inhomogeneities in the NT- film are reduced or eliminated.

[0075] The method described anywhere herein, wherein the porous filtration membrane and / or the additional supporting polymer layer is selected from an IR absorbing polymer, preferably polycarbonate.

[0076] The method described anywhere herein, wherein the IR-heat treatment is carried out using an IR lamp, IR laser or infrared tube emitter.

[0077] The method described anywhere herein, wherein in step viii) the film purification step is applied to remove process residuals and / or any residuals of the porous filtration membrane and / or any applied additional supporting polymer material to obtain the NT- film mounted onto the solid border with a purified free-standing portion of the NT-film covering the aperture.

[0078] The method described anywhere herein, wherein prior to removing the intermediate sacrificial layer by dissolution in step c) a frame of a rigid material with weak adhesion to the NT-film is applied onto the NT-film after removal of the porous filtration membrane material for ease of handling and for stretching the free-standing NT-film in the subsequent steps. The method described anywhere herein wherein the rigid material of the applied frame is selected from thermoplastic polymers, including an ethylene / methacrylic copolymer or a crystalline PET copolymer or from ionomers including neutralized ethylene acid copolymers (Surlyn™ or BYNEL™).

[0079] The method described anywhere herein, wherein the frame is applied onto the NT-film by 3D printing.

[0080] The method described anywhere herein, wherein the frame material exhibits a lower density than the solvent for dissolving the intermediate sacrificial layer.

[0081] The method described anywhere herein, wherein the frame is removed from the NT-film after mounting the NT-film onto the solid border with a defined aperture.

[0082] A further aspect of the invention relates to EUV pellicles comprising the NT-films described anywhere herein or obtainable by the method described anywhere herein.

[0083] A further aspect of the invention relates to the use of the NT-films described anywhere herein or obtainable by the method described anywhere herein in optical applications.

[0084] A further aspect of the invention relates to the use of the NT-films described anywhere herein or obtainable by the method described anywhere herein as pellicles in EUV lithography.

[0085] A further aspect of the invention relates to the use of the NT-films described anywhere herein or obtainable by the method described anywhere herein in flexible electronics, biosensors, transistors, thermoelectrics, solar cells, photonics, terahertz spectroscopy, as terahertz polarizers or limiters, as polarized light emitters, in heat management, in electrical conductance or in high-power laser applications or to protect elements of EUV radiation devices, like reticles (photo masks), mirrors or scanner.

[0086] A further aspect of the invention relates to the use of the NT-films described anywhere herein or obtainable by the method described anywhere herein in liquid or gas separation.

[0087] A further aspect of the invention relates to a method of annealing and / or improving the homogeneity of free-standing NT-films by subjecting the free-standing NT-film to IR-heat treatment, the method comprising a) providing a NT-film comprising residuals and / or an additional supporting layer of an IR absorbing polymer, preferably polycarbonate, b) mounting the NT-film onto a border being transparent to IR light and having a defined aperture so that a free-standing portion of the NT-film covers the aperture, c) heating the NT-film with the applied IR absorbing polymer material uniformly using IR light, d) conducting the IR-heat treatment until wrinkles, strains, defects and inhomogeneities in the NT-film are reduced or eliminated, e) removing the residuals and / or additional supporting layer of an IR absorbing polymer, preferably polycarbonate.

[0088] In said method the border being transparent to IR light is preferably a Si, SiC>2 or SiN border or a Si border with SiN coating.

[0089] Said method described above is preferably performed for annealing and / or improving the homogeneity of free-standing NT-films by subjecting a free-standing NT-film prepared in a wet transfer process to IR-heat treatment.

[0090] In said method described above in the IR-heat treatment the free-standing NT-films are preferably heated to a temperature above 150 °C, preferably a temperature between 150 and 200 °C.

[0091] A further aspect of the invention relates to a method of improving the handling and the homogeneity of thin free-standing NT-films prepared in a wet transfer process by applying a frame of a rigid material with weak adhesion onto the NT-film.

[0092] In said method described above the rigid material of the applied frame is preferably a thermoplastic polymer, preferably an ethylene / methacrylic copolymer or a crystalline PET copolymer or an ionomer, preferably an ionomer selected from neutralized ethylene acid copolymers (Surlyn™ or BYNEL™).

[0093] In said method described above the frame is preferably applied onto the NT-film by 3D printing.

[0094] In said method described above the frame material preferably exhibits a lower density than the solvent used in the wet transfer process.

[0095] In said method described above the frame is preferably removed from the NT-film after mounting the NT-film onto a solid border with a defined aperture. DETAILED DESCRIPTION OF THE INVENTION

[0096] The present invention is described in more detail as follows.

[0097] NT-Films with Variable Orientation and Alignment

[0098] A first aspect of the invention relates to films of nanotubes (NTs) comprising two or more regions with different alignment or orientation of the NTs, which means that at least one region comprises NTs with a given (first) alignment or orientation and at least one region comprises NTs with a different (second) alignment or orientation or no alignment or orientation. At least one of the aligned regions is located on a free-standing portion of the NT-film, particularly, at least two of the aligned regions is located on the free-standing portion of the NT-film. However, to benefit from the different alignments, it is most preferred that all the different aligned regions are present on the free-standing portion of the NT-film. It is further possible to provide additional regions without alignment / orientation, i.e. with random distribution of the NTs.

[0099] When referring to different regions with or without alignment, it is meant that the different regions are located on a planar axis or surface area of the film or sheet or within a single layer of the film or sheet. This means, a film or sheet may be defined by a planar area with a x and y direction, and the different regions are located on the planar axis (x / y axis) or the surface area of the film, e.g. within a single layer of the film or sheet.

[0100] The regions with or without alignment of the NT-films constitute defined, delimited areas on the film, layer or sheet and can be of any (two-dimensional) geometric and / or freestyle design, i.e. its appearance is freely selectable. Accordingly, a region with or without alignment can also be considered as an area with or without alignment. The alignment in accordance with the invention equips the NT-films with intentionally aligned regions and therewith an intentional pattern of different regions with or without alignment.

[0101] It is possible that all regions differ from each other, which means that no two or more regions with the same alignment and not more than one region without orientation are provided on the film. However, it is also possible that all regions with alignment or orientation provided on the film differ from each other, but more than one region without orientation is provided, e.g. like a boundary without orientation between two or more areas with same or different alignment.

[0102] It is, however, also possible to provide two or more regions among the multiple regions with identical alignment or orientation and further regions with one or more different alignment or orientation, with or without providing additionally one or more regions without orientation. Finally, it is also possible to provide multiple areas or regions each having the same alignment separated from each other by separating areas or boundaries without orientation. In fact, the novel films and methods for producing them offers a high flexibility to apply a targeted pattern of different regions or areas onto the NT-films, in accordance with the target application and needs.

[0103] Preferably, the films according to the invention comprise different regions of NTs, which differ from each other by one or more regions or areas having one (a first) alignment or orientation of the NTs and one or more regions or areas having a second or several different alignments or orientations of the NTs and which are different to the first alignment or orientation.

[0104] A further preferred embodiment relates to films according to the invention comprising different regions of NTs, which differ from each other by two or more regions having the same or different alignment or orientation of the NTs and comprising one or more regions of NTs without alignment or orientation.

[0105] In accordance with the general technical meaning the alignment of nanotubes in films refers to the process or result of arranging individual nanotubes (like CNTs and BN NTs) in a specific, controlled orientation within the thin film structure. Nanotubes, due to their high aspect ratio (length to diameter ratio), exhibit unique properties such as high strength, conductivity, and flexibility. Aligning these nanotubes in a particular direction within a film is crucial for optimizing the films’ performance in various applications, such as electronics, sensors, and energy storage devices. Allowing to provide different regions with different alignments on one film material even enhances the optimization potential.

[0106] Conventional techniques can be used to align nanotubes in films, including:

[0107] • Chemical Vapor Deposition (CVD): During the synthesis of carbon nanotubes, CVD can be used to control the alignment by adjusting the deposition conditions (such as temperature, substrate, and gas flow);

[0108] • Electric or Magnetic Field Alignment: In these techniques, nanotubes are dispersed in a solvent or polymer matrix and then exposed to an external electric or magnetic field. This causes the nanotubes to align with the direction of the field due to their anisotropic nature (i.e., they have different properties in different directions);

[0109] • Flow-assisted Alignment: In this method, nanotubes are dispersed in a fluid and subjected to shear forces, such as during spin-coating or extrusion, which cause the nanotubes to align along the flow direction;

[0110] • Surface Patterning or Templates: Nanotubes can also be aligned by using patterned substrates or templates with micro- or nanostructured grooves or ridges that guide the nanotubes into specific orientations as they deposit onto the surface; • Langmuir-Blodgett Technique: This method involves transferring a monolayer of nanotubes from the air-water interface onto a solid substrate, allowing for controlled alignment in a single layer.

[0111] The method used and described in more detail in the present application relates to a combination of flow-assisted alignment and surface patterning using templates, which turned out particularly suitable for providing the NT-films as described herein.

[0112] The importance of alignment can be seen from the following aspects:

[0113] • Enhanced Properties: Aligning nanotubes can significantly improve the mechanical, electrical, or thermal properties of the film. For example, aligned carbon nanotube films exhibit higher conductivity along the direction of alignment, making them valuable for applications in electronics, transistors, and flexible displays.

[0114] • Uniformity and Performance: Proper alignment ensures uniformity of the film’s properties across its surface and can improve the film's overall performance in devices. This is particularly important for high-performance applications, such as in conductive films or composite materials.

[0115] In accordance with the usual technical meaning regions without alignment or orientation means regions comprising the NTs randomly distributed.

[0116] Generally, regions without orientation can be applied as additional areas of the pattern of different regions or as boundaries for separating regions with alignments from each other. Accordingly, also the regions without alignment can be broadly and flexibly varied with respect to their design or appearance.

[0117] Within the general technical meaning a region or area with a certain alignment can also be considered as a domain. Areas without alignment are not considered as a domain but then rather constitute a space without alignment.

[0118] Nanotubes (NTs)

[0119] The nanotubes (NTs) can be selected from carbon nanotubes (CNTs) and boron nitride nanotubes (BN NTs) and mixtures thereof. NTs from the group of carbon nanotubes (CNTs) comprise from single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs), multi-walled CNTs (MWCNTs) and coaxial nanotubes, and mixtures thereof. NTs from the group of boron nitride nanotubes (BN NTs) can be selected from single-walled BN NTs (SWBNNTs), double-walled BNNTs (DWBNNTs), multi-walled BNNTs (MWBNNTs) and coaxial nanotubes, and mixtures thereof. An individual CNT or BN NT may be intersected with a few others to form a mesh-like microstructure thin film. SWCNTs have one (single) wall, DWCNTs have two walls, and MWCNTs have three or more walls. Different types of nanotubes can be mixed in any desired ratio, such as a mixture of two or more CNTs selected from SWCNTs, DWCNTs, and MWCNTs, as well as a mixture of two or more BN NTs selected from SWBNNTs, DWBNNTs, and MWBNNTs, as well as mixtures of CNTs and BNNTs selected from the above-mentioned groups thereof. If such mixtures of different NTs are applied, then the mixture is usually present in all regions of the film.

[0120] Preferably, the NTs are selected from carbon nanotubes (CNTs), more preferably the NTs are SWCNTs.

[0121] NT-Films

[0122] As mentioned above, it is an object of the invention to provide free-standing NT-films. In accordance with the general meaning a free-standing film refers to a thin, flexible, and self-supporting layer of material that is not attached to any substrate or underlying surface but nevertheless maintains its integrity and desired properties while remaining unsupported. A free-standing film according to the invention comprises completely freestanding films without any support, frame or carrier to which it is mounted. However, a film according to the invention can be mounted to a frame or border as a support, which then stabilizes the two-dimensional extension of the film material and helps to stretch the film. Such frames or borders (as they are usually referred to) are usually a solid and mechanically stable border or frame providing or circumventing a defined aperture or window-like opening, which is covered by the films according to the invention to provide the free-standing portion of the NT-film covering said aperture or window-like opening. It is also possible to mount the films of the invention onto a frame or border material comprising more than one opening or aperture, which are separated from each other by boundaries of the border material.

[0123] With the method for preparing the NT-films of the invention as described herein, it is possible to mount or attach the films on the solid border without the need for using any fixation aids like glue or mechanical fixation means. By adhering the film to the solid border, as possible with the method of the invention, the films can be mounted on the solid border stress-free without applying any stretching or tension forces to the film.

[0124] Such frame or solid border can be any material suitable to carry the NT-films according to the invention and not interfering the intended optical application. Usual frame materials can be selected from Si, SiC>2, SiN, a Si border with SiN coating, an aluminium or an aluminium alloy pellicle border or a stainless-steel pellicle border. The one or more apertures of such frames have a size corresponding to the desired area size of the free-standing portion of the NT-film.

[0125] In any case, the films according to the invention comprise at least one free-standing portion. Preferably, the one or more free-standing portions of the films of the invention have a free-standing area size of at least 1.0 cm2, preferably > 1.0 cm2, more preferably > 10.0 cm2, even more preferably > 100.0 cm2. The free-standing area size of the films according to the invention most preferably targets a size of up to 200 cm2, or up to 300 cm2, or up to 400 cm2, or up to 500 cm2.

[0126] If the film is mounted onto a frame comprising more than one aperture to provide more than one free-standing area portions, then these more than one free-standing area portions may have the same or different sizes and at least one of them, more preferably all of them have a free-standing area size within the above defined ranges.

[0127] Providing the films with free-standing area portions of the above defined ranges is desired with respect to the intended optical applications and particularly their use as pellicles. Such applications require sufficiently large free-standing films and generally, providing large free-standing thin films is difficult to achieve.

[0128] The free-standing NT-films according to the invention are preferably single-layer films, which means that no supporting layer or support membrane is deposited or applied onto or incorporated into the NT-film, like in nanotube films described in the prior art. E.g. the nanotube films according to US2023 / 0205073 comprise a two-dimensional material layer at least partially filling the voids of the nanotube film with mesh structures, or the pellicle membranes according to US2022 / 0365420 comprise a conformal coating.

[0129] Also US2023 / 259021 describes multilayer films in the form of a network membrane of nanotubes forming a mesh structure having voids partially filled with a two-dimensional material. Therein, it is further described to prepare nanotube networks formed over a support membrane or to prepare multilayer materials comprising a plurality of layered nanotube sheets.

[0130] NT-Film Thickness and Transmittance

[0131] The films according to the invention must be thin films or ultra-thin films to allow sufficient transmittance when used for the intended optical applications. Generally, the development aims at providing as thin as possible films. Apparently, the thickness of a film is crucial for its stability and self-standing properties. The thinner films are the higher the risk of instability and loss of integrity. Further, the inventors found that it is desirable to have thinner films, because the patterned alignment according to the invention was better on thinner films. It was therefore an object of the invention to provide the NT-films as thin as possible but still maintaining sufficient mechanical stability and self-standing properties without loss of integrity of the film material, especially upon handling the films in the optical applications. Surprisingly, the inventors of the invention were able to provide NT-films having a target maximum thickness of about 150 nm, preferably about 120 nm, more preferably about 100 nm, even more preferably about 80 nm, most preferred about 50 nm, which are still sufficiently stable.

[0132] The above defined thickness values, aiming at thin and ultra-thin films are particularly suitable for the intended use as EUV pellicles. However, for other applications thicker films may be more suitable. For example, NT-films with the herein described alignment may be provided as a kind of quasi-single crystal or block of NTs with the targeted alignment pattern. This may allow to transfer a NT material with anisotropic nanoscale dimensions into a material that is also anisotropic but on a macroscale. For the aligned NTs of the present invention, a low thickness is not necessarily the crucial point but may depend on the desired application or use of the material.

[0133] Usually, the film thickness can be determined by atomic force microscopy or by white light interferometry with conventional methods, using a piece of the film placed onto a solid substrate for the measurement. The accuracy and deviation of the values is in accordance with the technical measurement accuracy.

[0134] The films according to the invention may have a minimum thickness of 1 nm, which corresponds to a single layer of NTs, or a minimum thickness of 2 nm or 3 nm. With the methods according to the invention films having a final minimum thickness of about 50 nm have been prepared.

[0135] The prior art describes thin films of nanotubes, like CNT films, such as for example, WO2025 / 058829, US2024 / 0385508, US2024 / 0337922 A1 , US2024 / 0280893 and WO2021 / 080294. However, US2024 / 0337922 emphasizes the difficulties in providing ultra-thin films with the desired properties, explaining that providing a free-standing thin nanostructure film without supporting material or substrate cannot be guaranteed in every trial.

[0136] The film according to the invention should have an extreme ultraviolet light (EUV) transmittance at 13.5 nm of 80 % or more, preferably 85 % or more, more preferably 90 % or more, more preferably 92% or more, more preferably 93 % or more, more preferably 95 % or more. This is particularly desired for the preferred target application as pellicles for EUV lithography.

[0137] The invention further relates to NT-films which are obtainable by the methods described herein. Particularly, the invention further relates to NT-films comprising two or more different regions of NTs, wherein at least one region comprises NTs with a given alignment or orientation and at least one region comprises NTs with a different alignment or orientation or without alignment or orientation, and wherein at least one of the regions is located on a free-standing portion of the NT film, which are obtainable by the methods described herein. As explained above, the two or more different regions of NTs are located on a planar axis or area of the film, sheet or layer, i.e. on a x / y axis.

[0138] Method of Preparing the NT-Films

[0139] A further aspect of the invention relates to methods of preparing a film of nanotubes (NTs) comprising two or more different regions of NTs with at least one region comprising NTs with a given alignment or orientation and at least one region comprising NTs with a different alignment or orientation or without alignment or orientation, and wherein at least one of the regions is located on a free-standing portion of the NT-film, as described above. Such method comprises the following steps: i) preparing a liquid suspension of the NTs in a solvent; ii) providing a porous filtration membrane, which is insoluble in the solvent of the NT suspension and which comprises one or more imprinted, etched or hot embossed templates with the one or more regions for the desired alignment; iii) filtering the NT suspension through the porous filtration membrane to obtain a dry NT-film fixed on the porous filtration membrane; iv) applying onto the dry NT-film fixed on the porous filtration membrane of step iii) an intermediate sacrificial layer; v) removing the porous filtration membrane to obtain the NT-film with two or more different regions of NTs fixed on the sacrificial layer; vi) removing the intermediate sacrificial layer and vii) mounting the NT-film onto a solid border with a defined aperture; viii) applying a film purification step to obtain the NT-film mounted onto the solid border with a free-standing portion of the NT-film covering the aperture.

[0140] The NTs are selected from carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) and mixtures thereof, similar as defined above. Accordingly, NTs from the group of carbon nanotubes (CNTs) can be selected from single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs), multi-walled CNTs (MWCNTs) and coaxial nanotubes, and mixtures thereof, and NTs from the group of boron nitride nanotubes (BNNTs) can be selected from single-walled BNNTs (SWBNNTs), double-walled BNNTs (DWBNNTs), multi-walled BNNTs (MWBNNTs) and coaxial nanotubes, and mixtures thereof. Regarding the definition and the selection of preferred NTs, reference is made to the description above. NT-Filtration Step

[0141] In the method described herein the porous filtration membrane selected in step ii) must be one which is insoluble in the solvent used for preparing the NT suspension in step i). Accordingly, if the NTs are dissolved in an aqueous solution, then the porous membrane must be insoluble in the respective aqueous medium. It is, however, also possible to use an organic solvent for dispersing the NTs in step i), but then the porous filtration membrane used in step ii) must be insoluble in the respective organic solvents. In principle, the choice of solvent for the NTs in step i) is not critical as long as a suitable membrane is selected in step ii) which is insoluble in said solvent.

[0142] Preferably, in the method according to the invention the liquid NT suspension of step i) is an aqueous suspension, preferably a suspension in water comprising one or more surfactants. Carrying out the process using an aqueous NT suspension is beneficial under environmental and safety aspects, because hazardous solvents and the problems arising from their use can be avoided.

[0143] In order to obtain the high optical uniformity of the NT films, it is advantageous to use a pure and / or high-quality liquid NT suspension in step i). Such highly pure or high quality NT suspension can be obtained, e.g. by performing a pre-filtration or sieving of the NT solution before filtering it onto the a membrane in step iii). Such pre-filtration or sieving allows to remove large aggregates in the NT solution and allows to provide a film with increased uniformity, because the NT solution subjected to the filtering in step iii) is more uniform. Such pre-filtration can be performed with one or more pre-filtration steps using one or more filters of same or different pore size. Preferably two pre-filtration steps using two different filters are carried out, more preferably with a first filter membrane with a 50 pm pore size, followed by filtration through a second filter membrane with a 10 pm pore size. The filtering step of step iii) is then preferably carried out using a filter membrane with even smaller pore size, e.g. with a 100 nm pore size membrane, which allows to obtain thin films. The NTs in the (pre-filtered) solution i) cannot pass through the filtering membrane used in step iii) and the pre-filtration using membranes with larger pore sizes, e.g. 50 pm and 10 pm, acts like a sieving. However, such pre-filtration treatment is not mandatorily necessary when using directly a high quality NT suspension as the starting suspension.

[0144] Preferably the liquid suspension of step i) is a NT solution being essentially free of aggregates with a size > 10 pm.

[0145] When using an aqueous suspension in step i) the porous filtration membrane of step ii) is selected from those being soluble in organic solvents and insoluble in aqueous solutions. Preferably, such porous filtration membranes are selected from porous membranes of polycarbonate, PTFE, nylon, polyimide, ceramics like AI2O3, cellulose esters, among which porous polycarbonate and polyimide membranes are preferred, most preferred are porous polycarbonate membranes.

[0146] In a preferred aspect of the invention the porous filtration membrane is a polycarbonate membrane.

[0147] Such a porous polycarbonate membrane can be coated, e.g. by a thin layer of (PVP) polyvinylpyrrolidone. Such a PVP coating enhances the hydrophilicity of the membrane, which is beneficial for the filtration with the aqueous suspension.

[0148] A polycarbonate filtration membrane can be used in the two alternative methods described in more detail below. Alternatively, it is also possible to use porous polyimide membranes which are insoluble but can be removed by mechanically peeling off. Using such polyimide membranes which can be removed by peeling off have the advantage that no step of removing the filtration membrane with a solvent is necessary. Membranes which can be removed by peeling off exhibit a weak adhesion to the NT-films and are characterized by a flat surface.

[0149] The porosity of suitable filtration membranes can be defined by an average pore diameter between 50 to 200 nm. Usually, the pore sizes are provided by the manufacturer and part of the product specification. They can be determined using SEM (Scanning Electron Microscope) or AFM (Atomic Force Microscope). However, the porosity is not particularly critical for the process.

[0150] The one or more templates in the filtration membrane of step ii) correspond in size, shape and design to the desired one or more regions with the desired alignment or orientation. Such templates are imprinted or embossed onto the porous filtration membrane, preferably using hot-embossing technique.

[0151] A hot embossing can be carried out using the methods described in the prior art, e.g. using a custom-made nickel-plated shim under a force of 12-25 kN and 125 °C during moulding and active cooling to 40°C upon release from the shim. Such a shim can be prepared as described in the Examples below.

[0152] In step iii) the filtering of the NT suspension of step i) through the porous filtration membrane according to ii) can be carried out by vacuum filtration as well as by applying positive-pressure. Applying positive-pressure like applying a constant positive-pressure with pressure control or dead-end filtration geometry with volume rate control is possible. Preferably in step iii) the filtering is carried out by applying constant pressure with pressure control. In the filtration step iii) the NT suspension is filtered through the porous filtration membrane in an amount sufficient to provide the desired film thickness. Accordingly, via the filtration step the thickness of the NT-films can be controlled.

[0153] It is possible, and preferred, to control the filtered amount to result in the desired target thickness of the NT-films. Alternatively, it is also possible to prepare films with a thickness exceeding the target thickness and later, prior to step viii), reducing the thickness for example by RIE (reactive ion etching) or oxidation (oxygen plasma). Preparing thicker films in step iii) and reducing the thickness later in the process may be suitable for easier handling of the films in the further process steps and may ease to remove the filtration membrane in the step v) completely without remaining residues.

[0154] The Role of the Sacrificial Layer

[0155] In the methods described herein the intermediate sacrificial layer according to step iv) acts as an intermediate and only temporarily applied additional supporting layer for stabilizing the thin film during and after the removal of the porous filtration membrane. In principle, the stabilizing effect of the porous filtration membrane is replaced by the stabilizing effect of the intermediate sacrificial layer for the further process steps, to make non-destructive handling of the film easier. Therefore, such intermediate sacrificial layer is applied onto the NT-film on the surface site opposite to the surface site covered from the filtration membrane, so that after removal of the filtration membrane the film is still supported by the coating with the sacrificial layer. However, such intermediate sacrificial layer must later be removed again without deteriorating the NT-film.

[0156] In principle, all film forming materials are suitable, which do not interfere with the NT-film or deteriorate its properties. Particularly, the material for the intermediate sacrificial layer must be removable from the NT-film without leaving any residuals. Particularly, the material for the intermediate sacrificial layer should not penetrate or be soaked into the NT-film or otherwise interact or react with the NTs of the film. Examples of suitable intermediate sacrificial layer applied in step iv) / a) can be selected from the group comprising a polystyrene sulfonate (PSS) film, a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) film, and a PVA film, polymer films on a carrier, like preferably PVA films on a polyester carrier like the product Hydrokon® (company IKONICS), PMMA films or composites of the aforementioned film materials with further polymers.

[0157] Preferably a PSS film or a PEDOT:PSS film, more preferably a PEDOT:PSS film is selected when using the process described herein comprising mounting the NT film onto the solid border as a free-floating film in a solvent bath. In an optional process alternative, the NT film is deposited between the intermediate sacrificial layer and an additional (optional) supporting polymer layer and transferred onto the solid border followed by removing the intermediate sacrificial layer from the NT film already placed onto the solid border, and in such process alternative the intermediate sacrificial layer is preferably selected from PVA films like a PVA film on a polyester carrier (e.g. Hydrokon® of the company IKONICS).

[0158] Polymer films on a carrier like PVA films on polyester carrier (e.g. Hydrokon®) are also designated as „liner film", which means that the carrier material represents the so-called “liner”. In the PVA-films like Hydrokon® the PVA polymer is water soluble and has a melting point of 140°C. The liner is insoluble and peelable, which means that the liner (carrier) can be removed after applying the material onto the NT films by mechanical peeling leaving the PVA layer as the sacrificial layer fixed on the NT film.

[0159] In principle, in step iv / a) the intermediate sacrificial layer can be applied onto the NT- film by iv)-a) applying a solution of the sacrificial layer film material, like PSS or PEDOT:PSS, onto an inert intermediate substrate, or iv-b) applying a PVA-film, like Hydrokon® on an inert intermediate substrate, followed by transferring the dried NT-film fixed on the porous filtration membrane of step iii) onto the inert intermediate substrate with the NT-film facing the sacrificial layer film material and in the case of iv-a) drying the solution of the sacrificial layer film material and in the case of iv-b) laminating the PVA-film on the NT-film to obtain the NT-film deposited between the porous filtration membrane and the intermediate sacrificial layer on the inert intermediate substrate.

[0160] The removal of the sacrificial intermediate layer can be carried out by different methods depending on the selected sacrificial intermediate layer and the selected process and those are described below in more detail.

[0161] Under the aspect of scaling-up the process the use of PVA films, polymer films on a carrier, like PVA films on a polyester carrier like the product Hydrokon® (company IKONICS), PMMA films or composites of the aforementioned film materials with further polymers is preferred.

[0162] Inert Intermediate Substrate

[0163] If in the methods described herein the layered film compositions of any intermediate process step are applied onto an inert intermediate substrate, such inert substrate can be a conventional tray or plate, like a glass or metal plate. It is also possible to use a Si wafer. When using an intermediate sacrificial layer in the form of a PVA film on a carrier (liner), then the liner can be considered as and replace such inert intermediate substrate.

[0164] Solid Border / Pellicle Border

[0165] The frame or solid border referred to herein, onto which the NT films are mounted, can be any material suitable to carry the NT-films according to the invention and not interfering the intended optical application. Usual frame materials can be selected from Si, SiC>2, SiN, a Si border with SiN coating, an aluminium or an aluminium alloy pellicle border or a stainless-steel pellicle border.

[0166] With the methods described hereinafter thin films with the target thickness can be prepared.

[0167] Optional IR-Heat Treatment / IR- Annealing

[0168] The inventors of the present invention surprisingly found that subjecting the free-standing NT-films to an IR-heat treatment has beneficial effects on improving their homogeneity and stability in the sense of an annealing of the film.

[0169] For such IR-annealing the NT-films preferably comprise residuals or an additionally applied layer of an IR absorbing polymer on their surface, which heats up and softens during the IR-heat treatment.

[0170] For providing such IR absorbing polymer material on the NT film surface, it can be beneficial that in step v) / c) the dissolving of the porous filtration membrane, which then needs to be an IR absorbing material, like preferably a polycarbonate filtration membrane, is not conducted until the filtration membrane material is completely removed but until a residual thin film of the porous filtration membrane material remains.

[0171] It is, however, also possible to apply the IR absorbing polymer, like a polycarbonate material, onto the NT film in an additional step, e.g. by spin-coating. Applying the IR absorbing polymer in a separate step has the advantage that its amount and thickness can easily be controlled to provide an IR absorbing polymer film of defined thickness on the NT-film. Such subsequently applied IR absorbing polymer film may further act as an additional (intermediate) supporting layer, which helps to mechanically stabilize the NT film for easier handling in the herein described process alternative.

[0172] Regarding the IR absorbing polymer, the key point is that it is a polymer that absorbs light in the IR, i.e. heats up and softens.

[0173] For applying such IR-heat treatment (IR-annealing), the material forming the solid border for the mounting of the NT-films must be selected from materials being transparent to IR light. Suitable examples comprise Si, SiC>2, a SiN border or a Si border with SiN coating. Silicon borders are particularly suitable, as they support the NT film during this step and, due to their transparency to IR radiation, allow a uniform heat distribution over the whole film, i.e. of the free-standing NT-film portions and of the film material mounted onto the border / frame material.

[0174] The method of the invention can therefore comprise the following additional step f) of heating the NT-film comprising the applied IR absorbing polymer material and being mounted onto the border or frame with the defined aperture uniformly using IR light and conducting such IR-heat treatment until wrinkles, strains, defects and inhomogeneities in the NT-film are reduced or eliminated.

[0175] Such IR-heat treatment can be carried out using an IR lamp, IR laser or infrared tube emitter or any other suitable IR-source. In such IR-heat treatment the free-standing NT- films are preferably heated to a temperature above 150 °C, more preferably to a temperature between 150 and 200 °C. Generally, a temperature is applied that it is above the glass transition temperature of the IR absorbing polymer. For example, when using polycarbonate (PC) with a glass transition temperature of -150 °C, a temperature > 150 °C should be applied. Further, the upper temperature limit should be selected to avoid disintegration or decomposing of the treated material. Usually, an upper limit of around 200 °C is suitable and sufficient to achieve the annealing effects and improvement of homogeneity.

[0176] In view of these surprising findings of the inventors, regarding the unexpected effect of such I R-heat treatment, a further aspect of the invention relates to a method of annealing and / or improving the homogeneity of free-standing NT-films by subjecting the freestanding NT-film to IR-heat treatment, the method comprising a) providing a NT-film comprising residuals and / or a layer of an IR absorbing polymer, like polycarbonate, b) mounting the NT-film onto a border being transparent to IR light and having a defined aperture so that a free-standing portion of the NT-film covers the aperture, c) heating the NT-film with the applied IR absorbing polymer material uniformly using IR light, d) conducting the IR-heat treatment until wrinkles, strains, defects and inhomogeneities in the NT-film are reduced or eliminated.

[0177] As explained above, the border or frame onto which the NT-films are mounted need to be transparent to IR light, like a Si, SiC>2 or SiN border or a Si border with SiN coating. This method for annealing and / or improving the homogeneity of free-standing NT-films is particularly suitable for subjecting a free-standing NT-film prepared in a process as described herein to the IR-heat treatment, wherein preferably the free-standing NT-films are heated to a temperature above 150 °C, more preferably to a temperature between 150 and 200 °C as explained above.

[0178] Film Purification Step

[0179] The NT films resulting from the herein described methods are finally subjected to a purification step prior to their use in optical applications.

[0180] Such purification step comprises the removal of any residual and / or additionally applied supporting polymer materials, like residual surfactant or other process chemicals, or residual contaminants from the preparation process, or removal of intentionally applied or spin-coated polycarbonate, to finally provide the NT films mounted onto a solid border with a purified free-standing portion of the NT film covering the aperture.

[0181] Suitable purification methods comprise conventional suitable methods for removing the residual and / or applied polymer material, including etching, oxidation, heating, dissolving with suitable solvents and combinations thereof.

[0182] Process Variant 1

[0183] In one aspect of the invention a process variant 1 is described herein, wherein the steps iv), v), vi) and viii) of the method described above further comprise the following: a) in step iv) an intermediate sacrificial layer is applied which is insoluble in solvents which are capable to dissolve the porous filtration membrane; b) in step v) the porous filtration membrane is removed by dissolving it with a solvent being capable to dissolve the porous filtration membrane or by mechanically peeling off; c) optionally prior to step vi) an additional supporting polymer layer, preferably an IR absorbing polymer layer, more preferably a polycarbonate (PC) layer, is applied onto the NT film fixed on the intermediate sacrificial layer, e.g. by spin-coating, with said additional supporting polymer layer facing the NT film, followed by transferring and positioning the NT film onto the solid border with the defined aperture with the applied additional supporting polymer layer facing the border or being depart from the border with the NT-film contacting the border; d) in step vi) the intermediate sacrificial layer is removed by dissolving in a solvent which is capable to dissolve the intermediate sacrificial layer to obtain a NT-film with the polymer coating layer mounted onto the solid border with the polymer coating layer facing the border; and e) in step viii) the optional additional supporting polymer layer is removed to obtain the NT film mounted onto the solid border with a free-standing portion of the NT film covering the aperture.

[0184] However, as mentioned above, it is possible to control the filtration of the NT to obtain a NT film with the desired target thickness or to obtain a NT-film with higher thickness than finally desired to enhance its mechanical stability during the process steps and then finally reduce the thickness to the target thickness, e.g. by etching. Generally, with the process of the invention as described herein it is not mandatory to apply such an additional supporting polymer layer, which is entirely optional.

[0185] The Sacrificial Layer in Process Variant 1

[0186] The intermediate sacrificial layer applied in step iv) / a) of process variant 1 described herein can be selected similarly as described below for the process variant 2, i.e. in accordance with the solvent and filtration membrane combination selected in the process variant 2 described below.

[0187] Alternatively, it is possible to use a filtration membrane which is insoluble and can be removed mechanically by separating from the NT film by peeling off, like porous polyimide membranes as referred to above. In principle, when using an insoluble porous polyimide filtration membrane, the selection of the sacrificial layer is not limited by the solubility of the filtration membrane which is removed mechanically before dissolving the sacrificial layer. Nevertheless, it is preferred to use a sacrificial layer being soluble in aqueous solutions, preferably in water.

[0188] As mentioned above, in the process variant 1 described herein it is particularly preferred to use a sacrificial intermediate layer selected from PVA films like a PVA film on a polyester carrier (e.g. Hydrokon® of the company IKONICS).

[0189] In the process variant 1 described herein, the intermediate sacrificial layer can be applied onto the NT-film by applying a PVA-film as the sacrificial layer, like Hydrokon®, on an inert intermediate substrate, followed by transferring the dried NT-film fixed on the porous filtration membrane of step iii) onto the inert intermediate substrate with the NT-film facing the sacrificial PVA-film and laminating the sacrificial PVA-film on the NT-film to obtain the NT-film deposited between the porous filtration membrane and the intermediate sacrificial layer on the inert intermediate substrate. Such lamination can be carried out using conventional lamination systems. After the lamination of the PVA-film with the NT film an optional IR-heating treatment can be applied, wherein the laminate is subjected to enhanced temperature which softens the PVA and improves the lamination by enhancing and homogenizing the bonding between the PVA-layer and the NT-film.

[0190] The Removal of the Porous Filtration Membrane in Process Variant 1

[0191] Generally, the removal of the porous filtration membrane depends on the material applied. In principle, if a soluble porous filtration membrane like the preferred polycarbonate is used, e.g. as described below for the process variant 2, then the removal thereof can be achieved similarly as described below for the process variant 2 with a suitable solvent and in this respect, reference is made to the description provided below.

[0192] If, alternatively, an insoluble filtration membrane is used, like the above-mentioned polyimide membranes, such membrane can simply be peeled off mechanically.

[0193] Mounting onto the Border

[0194] In the process variant 1 described herein, the NT-film fixed on the intermediate sacrificial layer is transferred and positioned on the solid border after removal of the porous filtration membrane in step c). As described above, , prior to positioning the NT film on the border an additional supporting polymer layer can be applied, which provides an additional mechanical support of the film, i.e. for enhancing the mechanical strength of the film and / or for ease of handling, and which may help to mount the NT film onto the solid border with the defined aperture directly and in a dry environment. Such additional supporting polymer is preferably an IR absorbing polymer, more preferably a polycarbonate layer, which is particularly beneficial if the above-described IR-annealing shall be carried out.

[0195] If applied, such additional supporting polymer layer is applied onto the NT film fixed on the intermediate sacrificial layer, with the supporting polymer layer facing the NT film. In the case where the optional polymer support layer is used, it can be applied facing the border or away from the border, i.e. with the NT film being in contact with the border or the polymer support layer being in contact with the border. Such additional supporting polymer layer can preferably be applied by spin-coating.

[0196] If applied, the additional supporting polymer layer mechanically stabilizes the NT-film, which is particularly beneficial if the filtration is controlled to provide the thin or ultrathin films in the target thickness, allowing to transfer and position the NT-films on the solid border with a defined aperture in dry environment. Therein, the layered film composition is positioned on the border with the additional supporting polymer layer facing (being in contact with) the border or being depart from the border with the NT-film contacting the border.

[0197] In this step of transferring and positioning the layered film composition, wherein the NT- film is still fixed to the intermediate sacrificial layer, on the solid border in dry environment, the film is not fixed or adhered to the border yet but just removably positioned thereon. This allows exact and precise adjustment of the film on the border.

[0198] Regarding suitable solid borders reference is made to those described above.

[0199] When using a sacrificial layer in the form of the above-mentioned preferred “liner films” wherein a soluble (sacrificial) polymer (e.g. PVA) is fixed on an insoluble carrier (liner), then the liner carrier can be removed by mechanically peeling off before transferring and positioning the NT film with the optional additional supporting polymer layer leaving the PVA layer as the sacrificial layer fixed on the NT film resulting in a layered film composition of sacrificial PVA layer / NT-film / optional supporting polymer (e.g. IR absorbing polymer, like preferably polycarbonate) layer. Such layered film composition exhibits sufficient mechanical stability to be transferred and positioned on the solid border.

[0200] As mentioned above, the additional supporting polymer is optional and can be omitted. Accordingly, three options are possible a) the porous filtration membrane ii) is remove entirely, b) the porous filtration membrane is removed partially to maintain residuals of the filtration membrane or c) an intentional supporting polymer layer is applied, e.g. by spin coating.

[0201] Particularly when an additional optional IR annealing step as described above is desired, then the options b) and c) are preferred.

[0202] However, finally, before use in optical applications, such residual porous filtration membrane material b) as well as any additionally applied supporting polymer material is removed from the NT film, e.g. in the final film purification step described above.

[0203] The Removal of the Sacrificial Layer in the Process Variant 1

[0204] In the process variant 1 as described herein, the intermediate sacrificial layer is removed after the NT film with the optional additional supporting polymer layer has been transferred and positioned on the solid border.

[0205] When using the above-mentioned preferred intermediate sacrificial layer materials these can easily be removed by dissolving in an aqueous solution, preferably in water. This step can be carried out by transferring the NT-film positioned on the solid border into the solvent for removing the intermediate sacrificial layer.

[0206] This can be achieved by using a suitable fixation device, like an assembly as shown in Figure 4, comprising (if present, the optional supporting polymer layer applied onto the NT-film, in Figure 4 not shown) the NT-film (1) applied onto the sacrificial layer (2) and said “optional supporting polymer layer / NT-film / sacrificial layer” composition is positioned between a first clamp (3a) and a second clamp (3b) and by clamping down the layered composition the clamped layers are positioned on the solid border with aperture (4). A clamp holder (5) can be used to fix the whole arrangement. Other means to fix the “optional supporting polymer layer / NT-film / sacrificial layer” composition are possible, e.g. by fixing the layered composition on one frame (3a) with adhesive tapes. Such “optional supporting polymer layer / NT-film / sacrificial layer” composition fixed with clamps (3a, 3b) or fixed on a frame (3a) and placed on the solid border (4) can then be transferred as a whole assembly and placed in the solvent bath, like in water. The assembly can be placed in the solvent bath vertically. The sacrificial layer is then dissolved by the solvent and the NT-film is left in close contact to the solid border. After draining the solvent the NT-film remains stuck on the solid border and is left to dry. The fixation device with the clamping or fixation frame system is then removed once dry and the NT-film adheres to the solid border without any fixation aids, i.e. stress free.

[0207] If the layered film composition of “optional supporting polymer layer / NT-film / sacrificial layer” prior to positioning on the border is significantly larger than the aperture plus frame, it is possible to remove protruding edges of the layered film composition by cutting before positioning on the solid border or after positioning but before dissolving the intermediate sacrificial layer. Preferably, for ease of handling, cutting protruding edges is carried out before the removal of the above-described liner (carrier), if present.

[0208] After dissolving and removing the sacrificial layer entirely the NT film with the optional applied supporting polymer layer is dried on the border and in this step the NT film is fixed and mounted onto the border.

[0209] Therein, the NT-film adheres to the border and can thus be fixed and mounted onto the border without the use of any fixation aids, i.e. no glue or mechanical fixation means are required. This offers the advantage that the films are not intentionally stretched but are rather stress-free attached to the border.

[0210] For clarification, when referring to mounting the NT-film on a solid border stress-free by adhering without fixation aids, the above described fixation device or assembly (like one as illustrated in Figure 4) shall not be understood as such an excluded fixation aid. Such described fixation device is just temporarily used to hold the assembly with the optional supporting polymer layer / NT-film / sacrificial layer” arrangement together during the step of removing the sacrificial layer. After removing said fixation device, the NT-film adheres to the solid border without fixation aids as described anywhere herein.

[0211] The obtained mounted NT film can then be subjected to the above-described IR- annealing (if it comprises residual and / or applied IR absorbing polymer material).

[0212] Finally, potentially present residual and / or applied polymer material is removed in the film purification step mentioned above.

[0213] Process Variant 2

[0214] In a further aspect of the invention an alternative process variant 2 is described herein, wherein the above-described steps iv), v), vi) and vii) further comprise the following: a) in step iv) an intermediate sacrificial layer is applied which is insoluble in solvents which are capable to dissolve the porous filtration membrane and b) in step v) the porous filtration membrane is removed by dissolving it with a solvent being capable to dissolve the porous filtration membrane; c) in step v) the porous filtration membrane is removed by dissolving it with a solvent being capable to dissolve the porous filtration membrane; d) in step vi) the intermediate sacrificial layer is removed by dissolving in a solvent which is capable to dissolve the intermediate sacrificial layer to obtain a floating NT-film; e) in step vii) the floating NT-film is transferred to and mounted onto a solid border with a defined aperture to form a free-standing portion of the NT-film covering the aperture.

[0215] The Sacrificial Layer in the Process Variant 2

[0216] The intermediate sacrificial layer applied in step iv) / a) of such process variant 2 is selected in accordance with the solvent and filtration membrane combination selected in the process described above. That means, if the combination of aqueous NT suspension and filtration membrane with solubility in organic solvents is selected, then the intermediate sacrificial layer applied in step iv) / a) of such process variant 2 is a material (e.g. a film material) which is insoluble in organic solvents and soluble in aqueous solutions. Vice versa, if the combination of NT suspensions in organic solvents and filtration membrane with solubility in aqueous media is selected, then the intermediate sacrificial layer applied in step iv) / a) of such process variant 2 is a material (e.g. a film material) which is soluble in organic solvents and insoluble in aqueous solutions. It is preferred to use a combination of aqueous NT suspension and filtration membrane with solubility in organic solvents and intermediate sacrificial layer being insoluble in organic solvents and soluble in aqueous solutions. When using the process variant 2 described herein, the intermediate sacrificial layer can be applied onto the NT-film by applying a solution of the sacrificial layer film material onto an inert intermediate substrate, e.g. a tray or plate, and then the dried NT-film fixed on the porous filtration membrane of step iii) is transferred onto the inert substrate carrying the applied sacrificial layer material with the NT-film facing the sacrificial layer film material. After drying the NT-film is thereby deposited between the porous filtration membrane and the formed intermediate sacrificial layer on the inert intermediate substrate. Preferably, a PSS film or a PEDOT:PSS film is used herein.

[0217] Alternatively, also in process variant 2 described herein the above-mentioned PVA on a carrier (PVA / liner) can be used as the intermediate sacrificial layer. Then, the NT film is applied onto the PVA / liner facing the PVA layer. It is then not necessary to use an inert (glass) substrate as the liner acts like and replaces the inert substrate. It is then advantageous to apply the below-described (optional) handling aid in the form of a frame of a rigid material with weak adhesion to the NT-film. This option is preferred under the aspect of scaling-up the process.

[0218] The Removal of the Porous Filtration Membrane in the Process Variant 2

[0219] In the next step of process variant 2, the porous filtration membrane is removed, which can be achieved by dissolving the filtration membrane in a suitable solvent. Due to the above-described combination of materials with opposing solubilities, the solvent for removing the filtration membrane does not affect the NT-film and the intermediate sacrificial layer. In the preferred combination of aqueous NT suspension and filtration membrane with solubility in organic solvents and intermediate sacrificial layer being insoluble in organic solvents and soluble in aqueous solutions, the solvent used in step c) for dissolving and removing the filtration membrane is an organic solvent.

[0220] In principle, the choice of organic solvent is not limited provided that it does not negatively affect the other insoluble film layers. A suitable and preferred organic solvent for dissolving and removing the porous filtration membrane is chloroform.

[0221] The Removal of the Sacrificial Layer in the Process Variant 2

[0222] In the next step or said process variant 2, also the intermediate sacrificial layer is removed. When using the above-mentioned preferred combination of materials with opposing solubilities, then in step vi) / c) the intermediate sacrificial layer is preferably removed by dissolving in an aqueous solution, preferably in water. This step can be carried out by transferring the NT-film with the intermediate sacrificial layer into the solvent for removing the intermediate sacrificial layer, which then provides the pure NT- film free-floating in said solvent. When using the above-described alternative intermediate sacrificial layer of a PVA film on a carrier (PVA / liner) with the applied handling aid, then the PVA film is dissolved in the water bath, thereby releasing the liner, providing the NT-film floating in the solvent.

[0223] Mounting onto the Border

[0224] For further handling, this free-floating NT-film is usually transferred and mounted onto a peripheral support in the solvent bath in step d) by adhering the floating NT-film onto a solid border or frame with a defined aperture (like a window) having the desired size of the free-standing portion of the NT-films. This can for example be achieved by contacting the floating NT-film with one corner or edge of the border / frame and releasing the solvent, causing the film to adhere to the solid border / frame. Therewith, the NT film can be attached to the border without intentional stretching but rather stress-free After drying the film is fixed on the solid border. No further fixation aids are necessary. However, other methods are also possible.

[0225] The size of the aperture of the border / frame must certainly be smaller than the free- floating film so that the adhered film completely covers the aperture to form the freestanding portion of the NT-film which covers the aperture.

[0226] If the free-floating film prior to mounting to the border / frame is significantly larger than the aperture plus frame, it is possible to remove protruding edges of the NT-film by cutting before mounting the film onto the solid border. Preferably, for ease of handling, cutting protruding edges is carried out before the removal of the porous filtration membrane. Further preferably, also protruding edges of the porous filtration membrane without deposited NT-film can be removed by cutting before subjecting the layered film composition to the step v) / b) of dissolving the filtration membrane.

[0227] As mentioned above, such solid border can be any material suitable to carry the NT-films according to the invention and not interfering the intended optical application. Usual border materials can be selected from Si, SiC>2, SiN, a Si border with SiN coating, an aluminium or an aluminium alloy pellicle border or a stainless-steel pellicle border.

[0228] The Handling Aid / Frame

[0229] The inventors of the present invention further developed a method of further improving the handling of the NT-films in the wet transfer process according to this process variant 2 by applying a frame (or ring) of a rigid material with weak adhesion to the NT-film onto the NT-film after (complete or partial) removal of the porous filtration membrane material and prior to removing the intermediate sacrificial layer by dissolution in step vi) / c). Such applied frame or ring is applied for ease of handling and for stretching the free-standing NT-films in the subsequent steps before mounting the free-floating film onto the solid border. Such frame or ring can finally be removed from the NT-film, preferably after mounting the NT-film onto the solid border with a defined aperture. Depending on the target application said frame needs not necessarily to be removed, as it is positioned on the part covered from the border. Its presence usually does not harm the application. In view thereof, weak adhesion can be defined in the sense of being reversibly removable without damaging the NT-film. Applying such frame or ring is particularly suitable in case of preparing BNNT-films, because such frame or ring supports the BNNT film to float on the solvent. Such a frame or ring is also particularly suitable in case of using as an intermediate sacrificial layer the above mentioned PVA / liner material.

[0230] Suitable rigid materials for such applied frames or rings can be selected from thermoplastic polymers, including an ethylene / methacrylic copolymer or a crystalline PET copolymer or from ionomers including neutralized ethylene acid copolymers (Surlyn™ or BYNEL™ of the company DuPont). Such frames or rings can be applied onto the NT-film by 3D printing.

[0231] The use of the printable material is not particularly critical, preferably the printable polymer is a thermoplastic that can be heated / melted to adhere onto the nanotube film but having only weak adhesion to allow its reversable removal after use. Such printable polymers can be purchased in sheets and cut to size, placed onto the nanotube films and heated. The inventors found that 3D printing allows high accuracy in positioning of the ring and good reproducibility and flexibility in the ring design.

[0232] To support the above-mentioned floating of the NT-films, the frame or ring material should exhibit a lower density than the solvent used in the wet transfer process, like the solvent for dissolving the intermediate sacrificial layer.

[0233] In view of these surprising findings of the inventors, regarding the development of a handling aid for a wet transfer process, a further aspect of the invention relates to a method of improving the handling and the homogeneity of thin free-standing NT-films prepared in a wet transfer process by applying a frame of a rigid material with weak adhesion onto the NT-film. In such method the rigid material of the applied frame is preferably a thermoplastic polymer, preferably an ethylene / methacrylic copolymer or a crystalline PET copolymer or an ionomer, preferably an ionomer selected from neutralized ethylene acid copolymers (Surlyn™ or BYNEL™). In such method the frame can be applied onto the NT-film by 3D printing. Further, the frame material should exhibit a lower density than the solvent used in the wet transfer process. In such method the frame is reversibly removed from the NT-film after mounting the NT-film onto a solid border with a defined aperture without deteriorating the NT-film material.

[0234] In principle, in this process variant 2 it is also possible to remove the sacrificial layer using a fixation device like the assembly as described above and as illustrated in Figure 4. Then, applying the handling aid or frame (e.g. the ring) to make the NT-film floating, is not required as the transfer onto the solid border is not achieved by floating.

[0235] The Use of the NT Films

[0236] The NT-films according to the invention and as described herein are particularly suitable for the use in optical applications. The use of the NT-films according to the invention and as described herein includes their use in flexible electronics, biosensors, transistors, thermoelectrics, solar cells, photonics, terahertz spectroscopy, as terahertz polarizers or limiters, as polarized light emitters, in heat management, in electrical conductance, in high-power laser applications and to protect elements of EUV radiation devices, like reticules (photo masks), mirrors or scanner.

[0237] Also, the use of the NT-films according to the invention and as described herein in liquid or gas separation is possible and comprised from the invention.

[0238] A particularly preferred application of the NT-films according to the invention relates to their use as pellicles in EUV lithography, including their use as protecting elements of EUV radiation devices, like reticules (photo masks), mirrors or scanner.

[0239] Accordingly, a further aspect of the invention relates to EUV pellicles comprising the NT- films according to the invention and as described herein. Such EUV pellicles can therefore be characterized and further defined by the same characteristics, technical features and properties described above in detail.

[0240] DESCRIPTION OF THE FIGURES

[0241] Fig. 1 Schematic illustration of the hot embossing setup with all its components and the demonstrated shims used in the method according to the Examples. The stripe shim has thereby four regions with different bar widths of 150, 300, 450 and 600 nm. The KIT-logo is mirrored to show up correctly in the imprinted membrane and consists of parallel 300 nm bars.

[0242] Fig. 2 Cross-polarized images in bright (45°) and dark (0°) orientation to the incident polarization direction. The aligned regions becoming brighter at 45° and getting darker at 0°, while the untemplated regions remain unchanged, showing that patterning can be used selectively.

[0243] A: Lab sample with KIT logo

[0244] B: Sample with full scale alignment in a border

[0245] Fig. 3 Examples of free-standing CNT films mounted onto SiN borders before IR-annealing and after IR-annealing.

[0246] Fig. 4 Exploded view drawing of an assembly for mounting the NT-film onto a solid border with aperture comprising: the NT-film (1) applied onto the sacrificial layer (2), positioned between a first clamp (3a) and a second clamp (3b), applied onto the solid border with aperture (4) and fixed with a clamp holder (5).

[0247] EXAMPLES

[0248] The invention is described further by the following examples, without being limited thereto.

[0249] Example 1 Preparation of CNT EUV Pellicles according to Process Variant 2

[0250] To create EUV pellicles a wet chemical process involving the filtration of a dilute suspension of nanomaterials dispersed in water with surfactants followed by a lift off and transfer process was used. Therein, a suspension of carbon nanotubes in water was prepared, a filter membrane was imprinted with the desired pattern, the carbon nanotubes were filtered onto this membrane and aligned to the structure provided to them, the filter membrane was removed and the carbon nanotube films transferred to a frame as a support.

[0251] 1. Preparation of Suspensions of Single Wall Carbon Nanotubes (SWCNTs)

[0252] A suspension of SWCNTs has been prepared as described in [Liet al.: Endohedral Filling Effects in Sorted and Polymer-Wrapped Single- Wall Carbon Nanotubes’’, The Journal of Physical Chemistry C, Vol. 125, Issue 13, 2021; doi.org / 10.1021 / acs.jpcc.1c01390\.

[0253] First, 40 mg of EA-P2 (lot no. 02-A011 , Carbon Solutions) SWCNTs are dispersed in a 40 mL aqueous solution of 2 % sodium deoxycholate (DOC) (20 gL, BioXtra 98%) by tip sonication for 45 min (0.9 WmL-1) in an ice bath followed by centrifugation (45,560 g, Beckman Optima L-80 XP, SW 40 Ti rotor) for 1 h. The top eighty percent of the supernatant are collected and used further. All dispersions are prepared with deionized water (18.2 MOhmcm, pH 6.93) from an Arium pro UV (Sartorius).

[0254] After the suspension is prepared, the concentration is increased by filtering it through a 300 kDa NMWL, Biomax polyethersulfon, Merck Milipore membrane, repeatedly. The SWCNTs form a loose filter cake at the membrane, which can be re-dispersed using 0.5 wt % DOC solutions. Upon doing this multiple times, a concentration of -800 ug / ml and 0.5 wt% DOC can be reached, which was found necessary to obtain low DOC concentrations after dilution. This concentrated suspension was then diluted to 8 ug / ml SWCNTs in order to reach a low concentration of DOC (-0.04 wt% DOC), which can be determined by measuring the optical density (OD) of the high plasmon peak located at 274 nm and comparing it to a previously measured standard. Cleaning the solution of dust particles before filtering it through three steel meshs in series with an opening / pore size of around 1 urn is advantageous for the process. 2. Hot-Embossing of the Filtration Membrane

[0255] All filtration membranes were obtained from it4ip with a diameter of 47 mm, a pore density of 6 x108cm-2and a thickness of 25 pm and being made of polycarbonate. The pore size was chosen to be 80 nm, but also other pore sizes have been considered (50, 100 and 200 nm). Prior to their usage, the desired template (pattern) has to be imprinted using the hot-embossing technique described for example by Rust et al. (2022, doi: 10. 1002 / adfm.202107411 cited above). A schematic illustration of the hot-embossing setup is shown in Figure 1.

[0256] Thereby a gold-plated nickel shim is created by means of e-beam lithography of a polymer mask, which in turn is then used as a negative template for the galvanic etching process. The resulting shim is then used as a stamp, which is heated close below the melting temperature of polycarbonate (-120 °C) and then imprinted into the filtration membrane at 12-18kN for a round imprinting field of 10 to 22 mm diameter. The membrane is covered with a sheet of polyimide, which allows for easier removal of the membrane from the structure field of the shim after hot-embossing. Additionally, a round PTFE piece with approximately the size of the structure field is placed onto the polyimide to ensure a homogenous load transfer to the membrane. After the load and temperature setpoint have been realized, the shim is cooled down under water cooling until it reaches ~40°C, which takes approximately 10 minutes. The imprinted structure should be barely visible when tilting the membrane in the light and the membrane should not be transparent. A transparent membrane would indicate that the pores might be fully deformed and thus not suitable for filtration.

[0257] The first structure field (first aligned region) displayed in this Example is the KIT logo comprising parallel aligned 300 nm equidistant stripes, which have a structure depth of around 100 nm. The second structure (second aligned region) consists of four quadrants separated by a cross, containing bar widths of 150 nm, 300 nm, 450 nm and 600 nm, respectively.

[0258] 3. Filtration of Carbon Nanotubes

[0259] The NT dispersion can be subjected to one or more pre-filtration steps. For example, in a first pre-filtration step the NT dispersion can be filtered through a 50 pm filter membrane and in a second pre-filtration step the NT dispersion can be filtered through a 10 pm filter membrane. This allows to remove large aggregates to provide a NT solution with increased uniformity.

[0260] For preparing the NT film, a 80 nm or 100 nm pore size membrane that has been templated / patterned as described above is used for the filtration process. 2 ml of an 8 ug / ml SWCNT (0.04 wt% DOC) suspension (optionally pre-filtered) are filtered onto the membrane with the trialed and tested flow profile. The filtration is carried out using a positive-pressure dead-end filtration geometry, which uses microfluidic flow and pressure sensors obtained from Elveflow in order to obtain precise control of the volume rate. In the present Example a flow profile comprising a slow filtration regime in the beginning, starting at 150 pl / min and ramped up to 700 pl / min after 1250 pl have passed the membrane was used, however this flow profile is not critical and other filtration conditions can also be applied. It is also possible to use constant pressure with pressure control. After the dispersion is completely filtered, the filtration setup ramps up to the maximum pressure (2 bar) until no flow can be detected anymore. This ensures that the film is dry and fixed on the membrane. Otherwise the filter cake will be still loose and won’t adhere to the membrane.

[0261] After the filtration is finished, the membrane is dried outside the filtration cell until all moisture is gone.

[0262] The filtration step determines the final film thickness, which can be calculated using a concentration curve to determine how much volume of the NT is needed to filter in order to achieve a certain thickness.

[0263] 4. Removal of the Filtration Membrane

[0264] In the following step the filtration membrane is removed, and the film is prepared for a transfer onto the solid border in water.

[0265] Prior to dissolving the filtration membrane, most of the membrane not having a SWCNT film deposited on it is removed by cutting with a cutting tool. A glass slide is provided as an inert support and cleaned using acetone and isopropanol to remove hydrocarbons and dust. Then, approximately 25 pl of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is applied onto the glass slide. Subsequently, the filtration membrane is applied with the SWCNT film side facing the glass slide, followed by drying on a hot-plate being angled by ca. 5° by carefully moving the glass slide bit by bit onto the hot plate. Thereby gas bubbles should be avoided, which could cause voids of PEDOT:PSS not homogenously interfacing the glass slide and the film. Another preventive measure to ensure a flat nanomaterial layer is to filter the PEDOT:PSS with a one way filter (0.22 urn PES, Merck Milipore) in order to remove aggregates of the solution, which may form over time. The PEDOT:PSS is used as an intermediate sacrificial layer, as it will rapidly dissolve in water, but not in organic solvents like chloroform. Chloroform is then used to dissolve the polycarbonate filtration membrane by transferring the glass slide into a chloroform bath with the SWCNTs facing down. A stainless-steel holder is used in order to prevent the SWCNTs touching the beaker. After ~3 minutes the glass slide can be picked up and transferred into a second bath with fresh chloroform, this time facing upwards. Here the slide will remain another 5 minutes until it will be lifted carefully out of the chloroform and reemerged into it. This routine thins out the remaining polycarbonate on the SWCNT film. It should be noted that if the SWCNT film is very thin it is not advisable to repeat this step too often, as the polycarbonate also acts as a stabilizing agent during transfer. For thick SWCNT films the residual polycarbonate can be removed as the film of CNTs themselves have enough inherent stability. If needed, the residual polycarbonate for thin SWCNTs can be removed after the transfer in the film purification step.

[0266] At the end of this process the SWCNT thin film has been transferred to the glass substrate and is held there by a water-soluble adhesion / sacrificial layer between the glass slide and the nanomaterial thin film.

[0267] 5. Transfer onto the Border in Water

[0268] Finally, the prepared films on the glass slides are transferred to a solid border. A water bath is used that lifts the film and the optional polycarbonate residues / spin-coating by means of dissolving the sacrificial PEDOT:PSS layer. In order to prevent rips, tears and wrinkles this process has to be done slowly by placing the glass slide at an angle of approximately 10°. This results in a gradual detachment of the film from the microscope slide. Once the film is freely floating on the water surface, it can be manipulated by PTFE tweezers or other suitable devices in order to be pushed towards the border. There it will be attached to the top corner by simply pushing it on to it, the film will adhere to the frame / border as the water recedes. SiN borders, aluminium or aluminium alloy pellicle borders and polished stainless-steel borders have been used herein. As soon as the films edge is adhered, water can be slowly released from the bowl in order to lower the water surface level. Thereby the film attaches subsequently to the border also covers the orifice in the middle of the border. This affords the final free-standing and patterned film.

[0269] In the present example, the films were placed on the border manually, but mechanically assisted transfers are likewise applicable and provide higher accuracy and more precise placing of the patterned area.

[0270] Filtration and hot-embossing can be upscaled to meet the dimensions of a certain application.

[0271] 6. Evaluation of Alignment Pattern

[0272] To demonstrate the variability of this method, Figure 2A shows the application of a pattern in the form of the KIT logo, that was made on the free-standing portion of the film and evaluated by means of cross polarized light microscopy, which is commonly used in this research field to show the alignment qualitatively. Therein, applying the KIT logo merely serves as an example to demonstrate that with the new method of the invention arbitrary shapes of aligned regions surrounded by disordered areas of CNTs to provide any desired target pattern can be realized.

[0273] Cross polarized microscopy shows the effects of the pattern in the form of the KIT logo at 45° (bright) and 0° (dark) angles. Aligned film regions appear bright at 45° and dark at 0° as apparent from Figure 2A. Figure 2B shows a sample with full scale alignment in a border.

[0274] Furthermore, the obtained SWCNT films as shown in Figure 2A were free-standing over a free-standing portion having an area size of 1 cm2.

[0275] The obtained SWCNT films as shown in Figure 2B were free-standing over a freestanding portion having an area size of 200 x 200 pm.

[0276] With the process described herein it becomes possible to prepare films with even larger free-standing portion, e.g. 10 x 10 cm.

[0277] With this templating approach using hot-embossing technique, the method according to the invention also allows for different patterns all over the filtration membrane and finally on the resulting NT-film.

[0278] Example 2 Preparation of a Hot Embossing Shim

[0279] The preparation of the shim is carried out as described by Rust et al. (2022, doi: 10.1002 / adfm.202107411 ; cited above).

[0280] The pattern of the shim used for the hot embossing method of Example 1 is programmed with Klayouter, Open Source and converted into a machine-readable format by LayoutBEAMER Software from GenISYS. Following a substrate baking step (180 °C for 300 s) a PMMA resist (AR-P 672.045; Allresist GmbH), spin-coated at 3000 rpm for 60s onto a non-oxidized standard silicon wafer to a height of 100 nm was used for electron beam lithography. An optimal dose of 380 pC cm-2yielded the best structure quality. After exposure, the nanostructures in PMMA were developed in a solution of methyl isobutyl ketone (MIBK) and isopropyl alcohol (IPA) in a concentration ratio 1 :3 by spray development. Chromium (7 nm) and gold (25-30 nm) layers were evaporated on top of the wafer and PMMA structures. The chromium layer serves as adhesive layer and the gold layer as conductive plating base for the subsequent deposition of nickel. During metallization the substrate was tilted at 30°. For the nickel deposition the metallized nanostructured wafer was immersed in an electrolytic bath. Electroplating was carried out in a boric acid containing nickel sulphamate electrolyte (pH 3.4 to 3.6 at 52 °C) for 43 h. To ensure a slow growth of the nickel layer and to achieve a defect-free filling of the irradiated nanostructured areas the current density was adjusted to 0.25 A dm-2 (corresponding a growth speed of approximately 0.05 pm min-1) at the beginning of the plating process. After 30 minutes the current density was increased to 0.5 A dm-2(approx. 0.1 pm min-1) and in further steps up to 1.0 A dm-2(approx. 0.2 pm min-1). A shim thickness of at least 500 pm was required. The nickel shim was separated using a simple lift-off process and subsequently the resist was stripped with acetone (60 s) and the shim cleaned with I PA (60 s, shaker). Metallization layers (gold and chromium) were not etched and remain on the surface of the nickel shim.

[0281] Example 3 Annealing by IR-Heat Treatment

[0282] A film material according to Example 1 was subjected to IR-heat treatment.

[0283] Therefore, at the step 4 of Example 1 of dissolving and removing the polycarbonate filtration membrane a very thin layer of polycarbonate was left on the nanomaterial film, which is still present after transfer to the frame or support for the EUV pellicle.

[0284] These borders (as they are commonly referred to) can essentially be made of any material and typically polished stainless steel borders or borders of silicon with a silicon nitride coating are used, which were tested in the present Example.

[0285] The borders with the film are placed onto a glass slide, which is held in place by a metal frame. The IR light from a spiral-wound filament light source is then directed from below to heat the film. Heating is continued until a temperature of 175 °C is reached and this occurs because of the strong IR absorption of the polycarbonate. 175 °C was chosen because it is slightly above the glass transition temperature of polycarbonate (147 °C) and the polymer starts to soften after reaching 155 °C. At this temperature the thin polycarbonate film becomes mobile and surface tension in the film is released. The local temperature of the film is measured with an IR Camera, and it is observed that the temperature of the film on the border and in the free-standing area portion is the same when a silicon border is used. Such homogenous and uniform heat distribution over the whole film (free-standing and mounted portion) is important to ensure that the process does not induce any thermal strain at the border edge.

[0286] Such uniform heat distribution was not achievable with a stainless-steel border.

[0287] Accordingly, the inventors found that IR annealing needs a border (or frame) onto which the NT-film is mounted, which is transparent to IR light such that the heating on and offside the border material is equivalent.

[0288] Additionally, the interface between the nanotubes and the silicon acts as an additional thermal barrier, which inhibits the heatflux from the film to the border. For a stainless- steel border there is a temperature difference between the free-standing area and the border and this leads to ripping of the film, as iron (the main component of stainless- steel), is absorbing the IR radiation. This also happens, if the film is irradiated from the top of the film.

[0289] After turning the IR light off the pellicle is cooled in air and thereafter a highly taught, wrinkle free free-standing film is obtained. The effect is shown in Figure 3 showing heavily wrinkled carbon nanotubes before IR-annealing compared to smoothened films thereafter.

[0290] Accordingly, with the method of IR-annealing as described herein free-standing NT films can be improved with respect to smoothening and tightening a free-standing NT film. The ability to heal or repair a wrinkled film is a huge advantage in pellicle manufacture and ensures that the optical properties of the film are uniform across a large area. The removal of wrinkles at the edges and corners also reduces strain in the film and this leads to a longer product lifetime when in use.

[0291] In the present Example a material carrying polycarbonate residues from an incomplete filtration membrane removal step on the thin NT-film was subjected to the IR-annealing. Using such materials are beneficial, if for the reasons mentioned herein the dissolution of the polycarbonate membrane shall not be proceeded to often. However, instead it is also possible to completely remove the filtration membrane and then controllably spin coat a known thickness of polycarbonate or any other IR absorbing polymer onto the film. This allows to further improve the results to higher homogeneity of heating with IR light.

[0292] Example 4 3D Printing of a Polymer Ring

[0293] In the wet transfer process according to Example 1 it is also possible to apply a frame (or a ring) of a rigid material with weak adhesion onto the NT-film as a handling aid.

[0294] Such a handling aid can be applied after the step of removing the filtration membrane, where the nanomaterial thin film has been transferred to the inert support and is supported and held there by the sacrificial layer deposited between the substrate and the nanomaterial thin film.

[0295] Such handling aid is particularly suitable for transferring BNNT-films to support their floating. For transferring CNT-films applying such handling aid is possible but not necessary.

[0296] In the present Example such a handling aid in the form of a polymer ring made of Surlyn™ has been applied by 3D printing. Therefore, a Ultimaker S3 3D printer was used to print the polymer ring onto the glass slide with the transferred nanotube film. Thereby the glass printing bed was exchanged to a custom carbon fiber-built plate, which features recesses made to fit the glass slides, so the films are level with the rest of the carbon built plate. The stl.-file was drawn using Inventor Pro 2024 and the model was sliced for Ultimaker printers using Cura 5.3.1. The polymer ring consists of two rectangles. While the larger one is meant to stretch the freestanding film, the smaller rectangle allows to be used as a handle and helps to align the film to the outer edge of the border.

[0297] The filament (Taulman T-Lyne, DuPont) was used with a rather low printing temperature (185 °C, fan speed 50%) and no print bed heating in order to prevent the Surlyn™ infiltrating the nanotube film and contacting the glass. The print speed was also reduced to only 20 mm / s to allow for higher precision, while avoiding stringing. The geometry was reproduced by two parallel lines with a layer height of 0.2 mm, resulting in a very stable yet flexible film.

[0298] T-Lyne (DuPont) is a crystalline polyethylene (PET) copolymer specifically developed for high durability, flexibility, unique viscosity and a wide temperature range of printing. To achieve the crystalline finish, it is necessary to print with high layer sizes and with a low speed at temperatures between the range 185-242 °C.

[0299] Such frame or ring used as a handling aid can be seen in the Figure 2A.

[0300] Example 5 Preparation of CNT EUV Pellicles according to Process Variant 1

[0301] The preparation of CNT-films according to process variant 1 according to the invention can in principle be carried out similarly as described above in Example 1 with the following differences:

[0302] As the porous filtration membrane either a porous polycarbonate membrane or a porous polyimide membrane is used and the pattern is applied by hot embossing technique as described in Example 1. After filtering the SWCNT-film on the filtration membrane it is blown with nitrogen / compressed air to remove contamination. This is carried out with the membrane deposited on a glass support to avoid it flapping around.

[0303] As the sacrificial layer Hydrokon® is used, i.e. a PVA / liner film.

[0304] The SWCNT-film can be laminated onto the PVA sacrificial layer using a Si wafer as an inert support, with the liner placed wafer side down and PVA layer facing up. The SWCNT-film is then placed onto the PVA film facing down (i.e. in contact with the PVA) to provide a layered film composition of Si support / liner / PVA / SWCNT / filtration membrane. Therein, the liner does not adhere to the Si support. This arrangement can then be wrapped in Teflon by folding a sheet of Teflon around the layered film composition of Si support / liner / PVA / SWCNT / filtration membrane and passed twice through a laminator rotating the arrangement by 90 degrees between passes.

[0305] However, it is also possible to simply laminate the SWCNT-film onto the PVA sacrificial layer (PVA liner) directly without using a Si support and Teflon wrapping. This is preferred for simplifying the process and under the aspect of scaling-up the process.

[0306] Once laminated the liner / PVA / SWCNT / filtration membrane is evacuated.

[0307] The evacuated layered film composition can optionally be IR-heat treated for 5 minutes at 450 W power to optimize the lamination of the composition.

[0308] The vacuum is released slowly to provide the laminated layered film composition of Si support / liner / PVA / SWCNT / filtration membrane.

[0309] When using insoluble porous polyimide as filtration membrane the same can be removed by peeling off leaving the SWCNT-film adhered to the PVA / liner.

[0310] When using polycarbonate as filtration membrane the same can be removed by dissolving in a chloroform bath. It is then preferred to use a broadly protruding piece of PVA / liner sacrificial layer to ensure that edges of the PVA are not lifted up in the chloroform and to avoid wrinkles in the film. The liner / PVA / SWCNT-film should be fixed in the chloroform bath to avoid its floating on the chloroform surface. The chloroform treatment can be repeated one or more times.

[0311] After removing the filtration membrane, the resulting protruding liner / PVA / SWCNT-film is cut into suitable size. an additional supporting layer shall be applied, then, a polycarbonate overlayer can be spin coated as described in Example 6.

[0312] The spin-coated liner / PVA / SWCNT-film can be further cut to the size of the border with 0.5 cm extra on all sides.

[0313] The layered film composition of PVA / SWCNT / (optional) spin-coated PC is peeled off the liner and transferred onto the border starting with the opposite sides and with the PC facing down (i.e. in contact with the border). After transferring and positioning the layered film composition on the border the arrangement is then transferred into a water bath and immersed to dissolve the PVA layer. After approximately 45 minutes the PVA is dissolved and removed entirely. It is important that the PVA is removed entirely, including PVA on the backside of the border. Excess PVA that is only partially dissolved, and which still remains on the back may cause defects of the NT / PC film when the water is removed.

[0314] This transfer can be achieved by placing and fixing the the “optional supporting polymer layer / SWCNT-film / sacrificial layer” composition in a fixation device as shown in Figure 4 and as described above in more detail. This whole assembly of “optional supporting polymer layer / SWCNT-film / sacrificial layer” in the fixation aid is placed in the water bath, e.g. vertically, wherein the sacrificial layer is dissolved in the water and the SWCNT-film is left in close contact to the solid border. After draining the water the SWCNT-film remains stuck on the solid border and is left to dry. The fixation device with the clamping system is then removed once dry and the SWCNT-film adheres to the solid border without any fixation aids, i.e. stress free.

[0315] Optionally, the resulting SWCNT-film with the optional supporting polymer layer (like a spin-coated PC layer) can then be subjected to IR-annealing as described above to improve the film quality and remove defects and wrinkles.

[0316] Finally, the optional spin-coated polycarbonate layer is removed in the final film purification step using suitable measures for removing the polycarbonate.

[0317] Example 6 Spin-Coating

[0318] Spin-coating with a polycarbonate (PC) layer can be carried out as follows:

[0319] 8 wt % Polycarbonate in cyclohexanone is prepared as a stock solution by boiling it once at 170 °C and then stirring overnight at 150°C.

[0320] 5 mL of PC / cyclohexanone + 3.5 mL chloroform are mixed and vortexed to create a homogenous liquid.

[0321] 0.3mL of the solution are placed on the NT-film using a prespin of 500 rpm for 15 seconds followed by 2000 rpm for 60 seconds. This process can be repeated multiple times (usually twice).

Claims

CLAIMS[1] A film of nanotubes (NTs) comprising two or more different regions of NTs, wherein at least one region comprises NTs with a given alignment or orientation and at least one region comprises NTs with a different alignment or orientation or without alignment or orientation, and wherein at least one of the regions is located on a free-standing portion of the NT-film, wherein the two or more different regions of NTs are located on a planar axis or surface area of the film.[2] The film according to claim [1], wherein the two or more different regions of NTs are located within a single layer of the film.[3] The film according to claim [1] or [2], comprising more than two regions of NTs, wherein• all regions differ from each other or• two or more regions among the multiple regions are identical while at least one region among the multiple regions is different from at least one other region, or• different regions of NTs differ from each other by one or more regions having a first alignment or orientation of the NTs and one or more regions having a second or several different alignments or orientations of the NTs and which are different to the first alignment or orientation, or• by comprising one or more regions having the same or different alignment or orientation of the NTs and one or more regions comprising NTs without alignment or orientation (randomly distributed NTs), wherein the two or more different regions of NTs are located on a planar axis or surface area of the film, e.g. within a single layer of the film.[4] The film according to any one of the preceding claims, having a maximum thickness of 150 nm, preferably 120 nm, more preferably 100 nm, even more preferably 80 nm, most preferred 50 nm.[5] The film according to any one of the preceding claims, wherein the NTs are selected from carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) and mixtures thereof; wherein preferably NTs from the group of carbon nanotubes (CNTs) are selected from single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs), multi-walled CNTs (MWCNTs) and coaxial nanotubes, and mixtures thereof, and NTs from the group of boron nitride nanotubes (BNNTs) are selected from single-walled BNNTs (SWBNNTs), double-walled BNNTs (DWBNNTs), multi-walled BNNTs (MWBNNTs) and coaxial nanotubes, and mixtures thereof; more preferably the NTs are carbon nanotubes (CNTs), preferably SWCNTs.[6] The film according to any one of the preceding claims, wherein the free-standing portion has a free-standing area size of at least 1.0 cm2, preferably > 1.0 cm2, more preferably > 10.0 cm2, > 100.0 cm2.[7] A method of preparing a film of nanotubes (NTs) comprising two or more different regions of NTs with at least one region comprising NTs with a given alignment or orientation and at least one region comprising NTs with a different alignment or orientation or without alignment or orientation, and wherein at least one of the regions is located on a freestanding portion of the NT-film, as defined in any one of the preceding claims, the method comprising the following steps i) preparing a liquid suspension of NTs in a solvent; ii) providing a porous filtration membrane, which is insoluble in the solvent of the NT suspension and which comprises one or more imprinted, etched or hot embossed templates with the one or more regions for the desired alignment; iii) filtering the NT suspension through the porous filtration membrane to obtain a dry NT-film fixed on the porous filtration membrane; iv) applying onto the dry NT-film fixed on the porous filtration membrane of step iii) an intermediate sacrificial layer; v) removing the porous filtration membrane to obtain the NT-film with two or more different regions of NTs fixed on the sacrificial layer; vi) removing the intermediate sacrificial layer and vii) mounting the NT-film onto a solid border with a defined aperture; viii) applying a film purification step to obtain the NT-film mounted onto the solid border with a free-standing portion of the NT-film covering the aperture.[8] The method according to claim [7], wherein the steps iv), v), vi) and viii) comprise one or more of the following: a) in step i) the liquid suspension of the NTs is subjected to one or more prefiltration treatments; and / or b) in step iv) an intermediate sacrificial layer is applied which is insoluble in solvents which are capable to dissolve the porous filtration membrane; and / or c) in step v) the porous filtration membrane is removed by dissolving it with a solvent being capable to dissolve the porous filtration membrane or by mechanically peeling off; and / or d) prior to step vi) an additional supporting polymer layer, preferably an IR absorbing polymer layer, more preferably a polycarbonate (PC) layer, is applied onto the NT-film fixed on the intermediate sacrificial layer, e.g. by spin-coating, with said supporting polymer layer facing the NT-film followed by transferring theNT-film onto a solid border with a defined aperture with the additional supporting polymer layer facing the border; e) in step vi) the intermediate sacrificial layer is removed by dissolving in a solvent which is capable to dissolve the intermediate sacrificial layer thereby fixing the NT-film onto the solid border to obtain the NT-film with the supporting polymer layer mounted onto the solid border with the additional supporting polymer layer facing the border or being depart from the border with the NT-film contacting the border; and / or f) in step viii) the additional supporting polymer layer is removed to obtain the NT-film with the two or more different regions of NTs mounted onto the solid border with a free-standing portion of the NT-film covering the aperture as defined in any one of the preceding claims.[9] The method according to claim [7], wherein the steps iv), v), vi) and vii) comprise one or more of the following: a) in step i) the liquid suspension of the NTs is subjected to one or more pre-filtration treatments; and / or b) in step iv) an intermediate sacrificial layer is applied which is insoluble in solvents which are capable to dissolve the porous filtration membrane; and / or c) in step v) the porous filtration membrane is removed by dissolving it with a solvent being capable to dissolve the porous filtration membrane; and / or d) in step vi) the intermediate sacrificial layer is removed by dissolving in a solvent which is capable to dissolve the intermediate sacrificial layer to obtain a floating NT-film; and / or e) in step vii) the floating NT-film is transferred to and mounted onto a solid border with a defined aperture to form a free-standing portion of the NT-film covering the aperture.[10] The method according to claims [7] to [9], wherein• the liquid NT suspension of step i) is an aqueous suspension, preferably a suspension in water comprising one or more surfactants; and• the porous filtration membrane of step ii) is soluble in organic solvents and insoluble in aqueous solutions, preferably selected from porous membranes of polycarbonate, PTFE, nylon, polyimide, ceramics like AI2O3, cellulose esters, more preferably the porous filtration membrane of step ii) is a porous polycarbonate membrane; and• the intermediate sacrificial layer applied in step a) is a film material which is insoluble in organic solvents and soluble in aqueous solutions, preferably selected from the group comprising a polystyrene sulfonate (PSS) film, a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) film, and a PVA film; and• in step c) the solvent for dissolving and removing the porous filtration membrane is an organic solvent, preferably chloroform; and• in step d) the intermediate sacrificial layer is removed by dissolving in an aqueous solution, preferably in water.[11] The method according to claims [7] to [10], wherein in step ii) the one or more templates with the one or more regions for the desired alignment are imprinted or embossed onto the porous filtration membrane by hot-embossing technique.[12] The method according to claims [7] to [11], wherein in step iii) the NT suspension is filtered through the porous filtration membrane in an amount sufficient to provide the desired film thickness.[13] The method according to claims [7] to [12], wherein in step c) dissolving the porous filtration membrane is conducted until a residual thin film of the porous filtration membrane remains or the porous filtration membrane is removed completely and a supporting polymer layer, preferably polycarbonate, of defined thickness is applied onto the NT-film, preferably by spin-coating.[14] The method according to claims [7] to [13], wherein the NT-film comprises a residual thin film of the porous filtration and / or an additional supporting layer of an I R absorbing polymer, preferably polycarbonate, and is subjected to an I R- heat treatment step after mounting the NT-film onto a Si, SiC>2, SiN border or a Si border with SiN coating with a defined aperture to reduce or eliminate wrinkles, strains, defects and inhomogeneities in the NT-film.[15] EUV pellicles comprising the NT-films according to claims [1] to [6] or obtainable by the method according to claims [7] to [14],[16] The use of the NT-films according to claims [1] to [6] or obtainable by the method according to claims [7] to [14] in• optical applications,• as pellicles in EUV lithography,• in flexible electronics, biosensors, transistors, thermoelectrics, solar cells, photonics, terahertz spectroscopy, as terahertz polarizers or limiters, as polarized light emitters, in heat management, in electrical conductance or in high-power laser applications or to protect elements of EUV radiation devices, like reticles (photo masks), mirrors or scanner, and / or• in liquid or gas separation.[17] A method of annealing and / or improving the homogeneity of free-standing NT-films by subjecting the free-standing NT-film to IR-heat treatment, the method comprising a) providing a NT-film comprising an IR absorbing polymer, preferably polycarbonate, b) mounting the NT-film onto a border being transparent to IR light and having a defined aperture so that a free-standing portion of the NT-film covers the aperture, c) heating the NT-film material film with the applied IR absorbing polymer material uniformly using IR light, d) conducting the IR-heat treatment to reduce or eliminate wrinkles, strains, defects and inhomogeneities in the NT-film.

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