WO2021080699A1 - Régulation de porosité de films de nanofibres filtrés - Google Patents

Régulation de porosité de films de nanofibres filtrés Download PDF

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WO2021080699A1
WO2021080699A1 PCT/US2020/049875 US2020049875W WO2021080699A1 WO 2021080699 A1 WO2021080699 A1 WO 2021080699A1 US 2020049875 W US2020049875 W US 2020049875W WO 2021080699 A1 WO2021080699 A1 WO 2021080699A1
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nanofibers
nanofiber film
particles
nanofiber
film
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PCT/US2020/049875
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English (en)
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Marcio D. LIMA
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Lintec Of America, Inc.
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Publication of WO2021080699A1 publication Critical patent/WO2021080699A1/fr

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    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F1/00—Originals for photomechanical production of textured or patterned surfaces, e.g., masks, photo-masks, reticles; Mask blanks or pellicles therefor; Containers specially adapted therefor; Preparation thereof
    • G03F1/62—Pellicles, e.g. pellicle assemblies, e.g. having membrane on support frame; Preparation thereof
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/158—Carbon nanotubes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures

Definitions

  • the present disclosure relates generally to nanofiber films. Specifically, the present disclosure relates to porosity control of filtered nanofiber films.
  • Nanofibers are known to have unusual mechanical, optical, and electronic properties. However, devising configurations of nanofibers that can be integrated into commercial products has been challenging because of the nanoscale dimensions of the nanofibers.
  • One example of an advance in developing commercially useful embodiments of nanofibers is the fabrication of a nanofiber “forest.” This forest is an array of parallel nanofibers grown perpendicular to a substrate surface. The forest can be drawn from the substrate into a nanofiber sheet, in which the nanofibers are parallel to one another within the plane of the sheet. Nanofiber sheets can then optionally be formed into nanofiber yams.
  • Example 1 is a porous nanofiber film comprising a nanofiber film comprising a plurality of nanofibers oriented randomly relative to one another in a plane of the nanofiber film, and a plurality of pores defined by the nanofiber film, the pores having an average characteristic dimension from 100 nm to 10 pm.
  • Example 2 includes the elements of example 1 and at least some of the pores of the plurality have a circular cross-section.
  • Example 3 includes the elements of example 1, and at least some of the pores of the plurality have a polygonal perimeter.
  • Example 4 includes the elements of any of examples 1-3 further comprising a conformal oxide layer on surfaces of some of the nanofibers of the plurality of the nanofibers within the nanofiber film.
  • Example 5 includes the elements of example 4 wherein the conformal oxide layer comprises one or more of SiCh, AI2O3, TiCh, zinc sulfide, and yttrium oxide.
  • Example 6 includes the elements of any of examples 1-5, wherein the plurality of nanofibers comprises multiwall carbon nanofibers and one or more of few wall carbon nanofibers and single wall carbon nanofibers.
  • Example 7 is a nanofiber film comprising a plurality of nanofibers oriented randomly relative to one another in a plane of the nanofiber film, a plurality of non-nanofiber particles in the nanofiber film, each of the particles having a characteristic dimension from 100 nm to 10 pm.
  • Example 8 includes the elements of example 7 wherein the particles comprise a water-insoluble material.
  • Example 9 includes the elements of example 8 wherein the water-insoluble material comprises a polymer.
  • Example 10 includes the elements of example 8 wherein the water-insoluble material comprises a polyolefin.
  • Example 11 includes the elements of examples 7-10 wherein the particles comprise a material having a melting temperature or glass transition temperature of less than 400° Celsius.
  • Example 12 includes the elements of example 11 wherein the nanofibers comprise carbon nanotubes selected from one or more of single wall, few wall and multi-wall carbon nanotubes.
  • Example 13 includes the elements of example 7 wherein the particles comprise a material soluble in an organic solvent.
  • Example 14 includes the elements of example 7 wherein the particles comprise polystyrene microbeads.
  • Example 15 includes the elements of any of examples 7-14 wherein the particles have a characteristic dimension of from 100 nmto 10 pm.
  • Example 16 includes the elements any of examples 7-15, wherein the particles are spherical.
  • Example 17 includes the elements of any of examples 7-15 wherein the particles are cubic.
  • Example 18 includes the elements of any of examples 7-17 further comprising a conformal oxide layer on surfaces of at least some of the nanofibers of the plurality of the nanofibers within the nanofiber film.
  • Example 19 includes the elements of example 18 wherein the conformal oxide layer comprises one or more of Si02, A1203, Ti02, zinc sulfide, and yttrium oxide.
  • Example 20 is a method for producing a porous nanofiber film comprising suspending nanofibers and particles in a solvent, the nanofibers and particles not soluble in the solvent, separating the solvent from the nanofibers and the particles, the separating leaving a nanofiber film on a substrate, removing the nanofiber film from the substrate, and removing the particles from the nanofiber film, the removing leaving pores in the nanofiber film.
  • Example 21 includes the elements of example 20 wherein the particles comprise a water insoluble material.
  • Example 22 includes the elements of example 20 wherein the particles comprise one or more of salt, starch, and sugar.
  • Example 23 includes the elements of example 20 wherein the removing comprises applying heat to the particles in the nanofiber film, the heat causing a conversion of the particles from a solid state to one or more of a liquid state or a gaseous state.
  • Example 24 includes the elements of example 20 wherein the removing comprises applying a particle solvent to the particles in the nanofiber film, the particle solvent dissolving the particles and leaving the nanofiber film undissolved.
  • Example 25 includes the elements of example 20 wherein separating the solvent from the nanofibers and the particles comprises providing a filter, the filter having a first side and a second side opposite the first side, placing the suspension of the solvent, the nanofibers and the particles on the first side of the filter, and allowing the solvent to pass through the filter from the first side to the second side, leaving the nanofiber film on the first side of the filter.
  • Example 26 includes the elements of and of examples 20 through 25 wherein the nanofibers comprise one or more of single wall, multi wall and few wall carbon nanotubes.
  • FIG. 1 is a photomicrograph of an example forest of nanofibers on a substrate, in an embodiment.
  • FIG. 2 is a schematic illustration of an example reactor for nanofiber growth, in an embodiment.
  • FIG. 3 is an illustration of a nanofiber sheet that identifies relative dimensions of the sheet and schematically illustrates nanofibers within the sheet aligned end-to-end in a plane parallel to a surface of the sheet, in an embodiment.
  • FIG. 4 is an SEM photomicrograph is an image of a nanofiber sheet being laterally drawn from a nanofiber forest, the nanofibers aligning from end-to-end as schematically, in an embodiment.
  • FIG. 5 is a schematic illustration of a portion of a filtered nanofiber film that includes larger and longer multi wall carbon nanofibers intermixed with single wall and/or few wall carbon nanofibers, all of which are randomly oriented within a plane of the film, in an embodiment.
  • FIG. 6A is a schematic plan view illustration of a porous nanofiber film, in an embodiment.
  • FIG. 6B is a schematic cross-sectional view illustration of the porous nanofiber film shown in the plan view of FIG. 6A, in an embodiment.
  • FIG. 7 is a method flow diagram of an example method for preparing a filtered nanofiber film having a controlled proportion of porosity, in an embodiment.
  • FIG. 8A is a plan view of a porous nanofiber film that has been processed to include a conformal protective layer on the nanofibers within the film, in an embodiment.
  • FIG. 8B is a magnified schematic view of a portion of the porous nanofiber film in which the conformal protective layer on the surfaces of individual nanofibers within the nanofiber film is shown, in an embodiment.
  • Nanofibers often have unusual and interesting properties there are not present in similarly composed bulk materials.
  • some nanofiber-based materials can be challenging to work with.
  • carbon nanofiber sheets while possessing may interesting properties, are physically delicate and can be tom, folded, or otherwise damaged during processing by even the most subtle forces. Air flows caused by air handling equipment or the breath of an operator can sometimes damage a nanofiber sheet. Because of this physically delicate nature, some development efforts are focused not only on exploring and applying the unusual properties of nanofiber materials, but also on improving the processability of these materials.
  • Techniques described herein include liquid-phase methods for the formation of nanofiber films in which nano-scale or micro-scale pores are formed in the film.
  • a suspension is prepared that includes both nanofibers and particles.
  • the particles can be removed from the nanofiber film using a solvent, heat, radiation, or a similar applied stimulus that converts the material of the particle from a solid phase to a liquid or gaseous phase.
  • the removed particle leaves a pore in the nanofiber film.
  • a porosity of the nanofiber film can thus be selected. Furthermore, a size and a shape of the particles can be selected, which can also influence the porosity (total area of voids/total area of the nanofiber film) of the nanofiber film after removal of the particle.
  • Forming filtered nanofiber films having a selected porosity can provide a number of technological benefits and increase the industrial applicability of filtered nanofiber films.
  • nanofiber sheets and films have been investigated for use as a protective layer (“pellicle”) for photolithographic masks, particularly for extreme ultra-violet (EUV) lithography.
  • pellicle protective layer
  • Forming nanofiber films with a selected percentage of porosity and with a desired pore size can have several positive effects for lithographic applications.
  • the presence of pores can improve mechanical resilience and a lifespan of a nanofiber pellicle by enabling gas to flow from one side of the nanofiber pellicle to the other.
  • This facilitation of gas flow helps to equilibrate pressure differences on opposing side of a nanofiber film, and thus reduces the stress and strain on the nanofiber pellicle as lithographic equipment is cycled between a vacuum and atmospheric pressure. Also, the presence of pores can improve the transmittance of a nanofiber film to certain wavelengths of radiation.
  • nanofiber means a fiber having a diameter less than lpm. While the embodiments herein are primarily described as fabricated from carbon nanotubes, it will be appreciated that other carbon allotropes, whether graphene, micron or nano-scale graphite fibers and/or plates, and even other compositions of nano-scale fibers such as boron nitride may be processed using the techniques described below. As used herein, the terms “nanofiber” and “nanotube” are used interchangeably and encompass both single wall nanotubes, few wall nanotubes and/or multi wall nanotubes in which atoms are linked together to form a cylindrical structure. In some embodiments, multiwall nanotubes as referenced herein have between 6 and 20 walls.
  • the dimensions of nanotubes can vary greatly depending on production methods used.
  • the diameter of a carbon nanotube may be from 0.4 nm to 100 nm and its length may range from 10 pm to greater than 55.5 cm.
  • Carbon nanotubes are also capable of having very high aspect ratios (ratio of length to diameter) with some as high as 132,000,000:1 or more. Given the wide range of dimensional possibilities, the properties of carbon nanotubes are highly adjustable, or “tunable.” While many interesting properties of carbon nanotubes have been identified, harnessing the properties of carbon nanotubes in practical applications requires scalable and controllable production methods that allow the features of the carbon nanotubes to be maintained or enhanced.
  • nanotubes possess particular mechanical, electrical, chemical, thermal and optical properties that make them well-suited for certain applications.
  • carbon nanotubes exhibit superior electrical conductivity, high mechanical strength, good thermal stability and are also hydrophobic.
  • carbon nanotubes may also exhibit useful optical properties.
  • carbon nanotubes may be used in light-emitting diodes (LEDs) and photo-detectors to emit or detect light at narrowly selected wavelengths.
  • LEDs light-emitting diodes
  • Carbon nanotubes may also prove useful for photon transport and/or phonon transport.
  • nanofibers can be arranged in various configurations, including in a configuration referred to herein as a “forest.”
  • a “forest” of nanofibers or carbon nanotubes refers to an array of nanofibers having approximately equivalent dimensions that are arranged substantially parallel to one another on a substrate.
  • FIG. 1 shows an example forest of nanofibers on a substrate.
  • the substrate may be any shape but in some embodiments the substrate has a planar surface on which the forest is assembled.
  • the nanofibers in the forest may be approximately equal in height and/or diameter.
  • Nanofiber forests as disclosed herein may be relatively dense.
  • the disclosed nanofiber forests may have a density of at least 1 billion nanofibers/cm 2 .
  • a nanofiber forest as described herein may have a density of between 10 billion/cm 2 and 30 billion/cm 2 .
  • the nanofiber forest as described herein may have a density in the range of 90 billion nanofibers/cm 2 .
  • the forest may include areas of high density or low density and specific areas may be void of nanofibers.
  • the nanofibers within a forest may also exhibit inter-fiber connectivity. For example, neighboring nanofibers within a nanofiber forest may be attracted to one another by van der Waals forces. Regardless, a density of nanofibers within a forest can be increased by applying techniques described herein.
  • nanofibers may be grown in a high-temperature furnace, schematically illustrated in FIG. 2.
  • catalyst may be deposited on a substrate, placed in a reactor and then may be exposed to a fuel compound that is supplied to the reactor.
  • Substrates can withstand temperatures of greater than 800°C or even 1000°C and may be inert materials.
  • the substrate may comprise stainless steel or aluminum disposed on an underlying silicon (Si) wafer, although other ceramic substrates may be used in place of the Si wafer (e.g., alumina, zirconia, SiC , glass ceramics).
  • the nanofibers of the precursor forest are carbon nanotubes
  • carbon-based compounds, such as acetylene may be used as fuel compounds.
  • the fuel compound(s) may then begin to accumulate on the catalyst and may assemble by growing upward from the substrate to form a forest of nanofibers.
  • the reactor also may include a gas inlet where fuel compound(s) and carrier gasses may be supplied to the reactor and a gas outlet where expended fuel compounds and carrier gases may be released from the reactor.
  • carrier gases include hydrogen, argon, and helium. These gases, in particular hydrogen, may also be introduced to the reactor to facilitate growth of the nanofiber forest. Additionally, dopants to be incorporated in the nanofibers may be added to the gas stream.
  • the nanofibers of the present application may also be arranged in a sheet configuration.
  • the term “nanofiber sheet,” “nanotube sheet,” or simply “sheet” refers to an arrangement of nanofibers where the nanofibers are aligned end to end in a plane of the drawn sheet. This is in contrast to forests in which the nanofibers are arranged with co-planar ends adjacent to the growth substrate and longitudinal axes perpendicular to a surface of the growth substrate.
  • An illustration of an example nanofiber sheet is shown in FIG. 3 with labels of the dimensions.
  • the sheet has a length and/or width that is more than 100 times greater than the thickness of the sheet.
  • the length, width or both are more than 10 3 , 10 6 or 10 9 times greater than the average thickness of the sheet.
  • a nanofiber sheet can have a thickness of, for example, between approximately 5 nm and 30 pm and any length and width that are suitable for the intended application.
  • a nanofiber sheet may have a length of between 1 cm and 10 meters and a width between 1 cm and 1 meter. These lengths are provided merely for illustration.
  • the length and width of a nanofiber sheet are constrained by the configuration of the manufacturing equipment and not by the physical or chemical properties of any of the nanotubes, forest, or nanofiber sheet. For example, continuous processes can produce sheets of any length. These sheets can be wound onto a roll as they are produced.
  • the axis in which the nanofibers are aligned end-to end is referred to as the direction of nanofiber alignment.
  • the direction of nanofiber alignment may be continuous throughout an entire nanofiber sheet. Nanofibers are not necessarily perfectly parallel to each other and it is understood that the direction of nanofiber alignment is an average or general measure of the direction of alignment of the nanofibers.
  • Nanofiber sheets may be assembled using any type of suitable process capable of producing the sheet.
  • nanofiber sheets may be drawn from a nanofiber forest.
  • An example of a nanofiber sheet being drawn from a nanofiber forest is shown in FIG. 4.
  • the nanofibers may be drawn laterally from the forest and then align end-to-end to form a nanofiber sheet.
  • the dimensions of the forest may be controlled to form a nanofiber sheet having particular dimensions.
  • the width of the nanofiber sheet may be approximately equal to the width of the nanofiber forest from which the sheet was drawn.
  • the length of the sheet can be controlled, for example, by concluding the draw process when the desired sheet length has been achieved.
  • Nanofiber sheets have many properties that can be exploited for various applications. For example, nanofiber sheets may have tunable opacity, high mechanical strength and flexibility, thermal and electrical conductivity, and may also exhibit hydrophobicity. Given the high degree of alignment of the nanofibers within a sheet, a nanofiber sheet may be extremely thin. In some examples, a nanofiber sheet is on the order of approximately 10 nm thick (as measured within normal measurement tolerances), rendering it nearly two-dimensional. In other examples, the thickness of a nanofiber sheet can be as high as 200 nm or 300 nm. As such, nanofiber sheets may add minimal additional thickness to a component.
  • the nanofibers in a nanofibers sheet may be functionalized by a treatment agent by adding chemical groups or elements to a surface of the nanofibers of the sheet and that provide a different chemical activity than the nanofibers alone.
  • Functionalization of a nanofiber sheet can be performed on previously functionalized nanofibers or can be performed on previously unfunctionalized nanofibers. Functionalization can be performed using any of the techniques described herein including, but not limited to CVD, and various doping techniques.
  • Nanofiber sheets as drawn from a nanofiber forest, may also have high purity, wherein more than 90%, more than 95% or more than 99% of the weight percent of the nanofiber sheet is attributable to nanofibers, in some instances. Similarly, the nanofiber sheet may comprise more than 90%, more than 95%, more than 99% or more than 99.9% by weight of carbon.
  • Another planar form of assembled nanofibers is a “filtered film,” in which one or more of multiwall nanotubes, few wall nanotubes, and/or single wall nanotubes are dispersed in a solvent as a suspension (the nanofibers being insoluble in the solvent).
  • This dispersion can subsequently be formed into a solid-state film of carbon nanotubes that are randomly oriented relative to one another in the plane of the film. In some cases, the dispersion is such that a majority of nanotubes are suspended individually and not adsorbed onto other nanotubes.
  • the greater the degree of dispersion e.g., the few nanotubes are adsorbed on to one another in the solvent, the more uniform (i.e., uniform thickness) a subsequently formed nanofiber film can be.
  • This physical uniformity in some examples, further improved by stacking multiple filtered films on one another) can also improve the uniformity of the properties across the film (e.g., transparency to radiation).
  • multiwall nanotubes are considered to have from 4 to 20 concentric walls and a diameter of from 4 nm to 100 nm; few wall nanotubes are considered to have two or three concentric walls and a diameter of from 2 nm to 6 nm; and single wall carbon nanotubes are considered to have 1 wall and a tube diameter of from 0.2 nm to 4 nm.
  • Each of these three different types of nanotubes can have different properties.
  • few wall carbon nanotubes and single wall carbon nanotubes can be more conveniently dispersed in a solvent (i.e., with the majority of nanotubes suspended individually and not adsorbed onto other nanotubes) for subsequent formation into a sheet of randomly oriented carbon nanotubes.
  • This ability of individual nanotubes to be uniformly dispersed in a solvent can in turn produce a dimensionally uniform nanotube filtered film formed by removing the solvent from the suspended nanofibers.
  • the strength of van der Waals attraction between nanofibers also differs between single/few wall nanofibers and multiwall nanofibers.
  • single/few wall nanofibers have a greater van der Waals attraction to each other than that observed for multiwall nanofibers.
  • This increased attraction between single/few wall nanofibers can improve the ability of few/single wall carbon nanotubes to adhere to one another to form a coherent nanofiber structure, such as a filtered film.
  • the sheets or films formed from single wall carbon nanotubes and few wall carbon nanotubes are able to conform to a topography of an underlying surface at smaller dimensions than sheets or films formed from multiwall carbon nanotubes.
  • sheets or films formed from single wall carbon nanotubes and/or few wall carbon nanotubes can conform to a topography of an underlying substrate as small as 10 nm, which is at least 50% smaller than the feature size a multiwall carbon nanotube film can conform to.
  • the multiwall carbon nanotubes are more likely than single/few wall nanotubes to agglomerate together and thereby produce a structurally non-uniform film that is less likely to conform and/or adhere to an underlying surface.
  • Preparation of a filtered film can begin by preparing a dry mixture of the desired proportion of one or more of multiwall nanotubes, few wall nanotubes, and/or single wall nanotubes. This mixture of one or more of the different types of nanotubes can be then suspended in a solvent. In another example, separate suspensions of known concentrations of nanotubes in a solvent are prepared. For example, separate suspensions of multi walled carbon nanotubes, few wall carbon nanotubes, and single wall nanotubes can be prepared. The suspensions can then be mixed in a desired proportion to arrive at the desired relative proportions of the multiwall, and few/single wall nanotubes in the combined suspension and ultimately the final filtered film.
  • the liquid phase of the suspension can be, for example, polar compounds such as polar protic or polar aprotic compounds.
  • the solvent used to prepare nanotube suspensions can include water, isopropyl alcohol (IP A), N-Methyl-2 ⁇ pyrrolidone (NMP), dimethyl sulfide (DMS), and combinations thereof.
  • a surfactant can also be included to aid the uniform dispersion of carbon nanofibers in the solvent.
  • Example surfactants include, but are not limited to, sodium cholate, sodium dodecyl sulfate (SDS), and sodium dodecyl benzene sulphonate (SDBS).
  • Weight percentage of surfactant in the solvent can be anywhere between 0.01 weight % to 10 wt. % of solvent.
  • a mixture of 50 wt. % multi walled carbon nanotubes and 50 wt. % few/single wall carbon nanotubes can be prepared and suspended in water and SDS surfactant.
  • Dispersion of the nanotubes in the solvent can include mechanical mixing (e.g., using a magnetic stir bar and stirring plate), mechanical shaking, ultrasonic agitation (e.g., using an immersion ultrasonic probe) or other means.
  • examples described herein include nanofiber films that can be formed from one type of nanofiber (e.g., single wall, few wall, multiwall) or a combination of these different types of nanofibers. Examples that are composed of more than one type of nanofiber can be described as “composite films” due to the combination or mixture of different nanofiber types.
  • a multi walled carbon nanotube length can have a median length of approximately 300 pm (+/-10%).
  • multi walled carbon nanotubes having a length of at least 250 pm or longer can be included in a filtered film to improve the mechanical stability of filtered films that also include single wall and/or few wall carbon nanotubes, which generally are shorter (e.g.
  • Films that are formed exclusively from either the longer multiwalled nanotubes or shorter few/single wall carbon nanotubes are generally not as durable (i.e., resistant to mechanical failure such as cracking or disintegrating) as those that include a mixture of the multiwall and few/single wall nanotubes.
  • FIG. 5 is a schematic illustration of a composite nanotube filtered film 500, in an example of the present disclosure.
  • the composite nanotube filtered film 500 includes single/few wall nanotubes 504 that are inter-dispersed with multiwall carbon nanotubes 508.
  • the single/few wall carbon nanotubes 504 can have at least two beneficial effects on the structure of the film 500 as a whole.
  • the single/few wall carbon nanotubes 504 can increase the number of indirect connections between proximate multiwalled carbon nanotubes 508 by bridging the gaps between proximate multiwalled carbon nanotubes 508.
  • the interconnections between the short and long nanofibers can improve the transfer and distribution of forces applied to the film and thus improve durability.
  • the single/few wall carbon nanotubes 504 can decrease a median and/or mean size of the gaps between adjacent and/or overlapping multiwall carbon nanotubes 508, which can be advantageous for some embodiments.
  • too many longer multiwalled carbon nanotubes can, when dispersed in a solvent, agglomerate. This can result in a non-uniform film. Shorter nanotubes are more easily dispersed in a solvent and thus are more likely to form a dimensionally uniform film having a uniform density of nanotubes per unit volume.
  • Filtered films particularly those made with single and/or few wall carbon nanotubes also generally have greater transparency to some wavelengths of radiation.
  • transmittance of incident radiation can be as high as 90% or 95%. In some cases, this transmittance is significantly higher than drawn sheets of multiwall carbon nanotubes (such as those drawn from a carbon nanotube forest, described below). While not wishing to be bound by theory, it is believed that the aligned orientation of nanotubes in a drawn sheet increases scattering of the radiation relative to a filtered film. In part, the greater transparency of filtered films (with their randomly oriented nanotubes) has prompted interest in forming transparent filters and pellicles from filtered carbon nanotube films in a variety of applications.
  • multiwall carbon nanotubes also have advantages not necessarily observed to the same degree in nanotube structures formed from single or few wall nanotubes.
  • structures formed from multi wall carbon nanotubes are generally observed to have greater emissivity than those formed from few/single wall carbon nanotubes. While not wishing to be bound by theory, it is believed that the greater number of walls and greater diameter of multiwall carbon nanotubes are factors in the increased emissivity.
  • multi wall carbon nanotube structures e.g., the nanotube forest, a nanotube sheet
  • an emissivity of a nanofiber structure comprising multiwall carbon nanotubes is on the order of 0.275 (+/- 15%) whereas ananofiber structure comprising single wall carbon nanotubes can have a significantly lower emissivity of 0.05 (+/- 15%).
  • High emissivity can be particularly advantageous in technological applications in which processes can cause heating within the nanofiber structure, but mechanisms of conductive or convective cooling of the nanofiber structure are limited or not technically feasible.
  • nanofiber structures having transparency to certain wavelengths of radiation have promise for use as a filter (also referred to as a “pellicle”) in EUV lithography devices.
  • the pellicle can act as a particle filter that prevents foreign particles from landing on a surface of the material being patterned and/or from landing on a surface of the lithography mask being used to pattern a photoactive surface. This reduces the rate of lithographically introduced defects, thus improving manufacturing yields of the patterned devices.
  • cooling ananofiber pellicle may be important for preventing overheating of the pellicle due to absorption of EUV energy during lithographic patterning. Elevated temperatures in the pellicle can degrade nanofiber structure integrity.
  • the opportunities for convective and or conductive cooling of the nanofiber structure in this environment are low given that EUV lithography is performed in a vacuum and the pellicle is mostly suspended (with peripheral edges being attached to a frame). For this reason, thermal emission is the primary mechanism of cooling of a nanofiber pellicle used for EUV application.
  • multiwall carbon nanotube structures generally have a higher emissivity, which would address the problem of cooling in EUV pellicle
  • multiwall carbon nanotubes when aligned in a drawn sheet also are less transmissive than randomly oriented single/few wall carbon nanofibers in a filtered film.
  • the more transparent (but less emissive) few wall/single wall nanofiber films are often too mechanically delicate to be used as a pellicle.
  • films and sheets made from few wall/single wall nanofibers are fragile and will disintegrate when subjected to pressure cycles (e.g., changes in pressure of +/- 1 atmosphere to 2 atmospheres (from atmospheric pressure to vacuum)) commonly used in EUV lithography machines.
  • introducing pores into a nanofiber film can have a number of benefits. These include increased transmittance to desired wavelengths of radiation (i.e., the proportion of transmitted radiation to incident radiation) and an increased permeability to gas, which in turn improves the ability of a film to equilibrate to a pressure differential between opposing sides of the film without breaking.
  • FIGS. 6A and 6B illustrate one example (in a plan view and a cross-sectional view, respectively) of a porous nanofiber film 600.
  • the nanofiber film 600 includes a plurality of pores 604 formed by techniques described below.
  • the filtered nanofiber film 600 is rectangular, having a length L, a width W, and a thickness T. Dimensions L and W can be from millimeters to meters in size.
  • the thickness T can be, for example, from 100 nanometers to 100 microns. It will be appreciated that nanofiber films can be formed to have any perimeter shape, whether a regular polygon, irregular polygon, circle, ellipse, or a customized perimeter shape not falling into any single category.
  • Each of the pores 604 has a circular cross-section that (as is shown in FIG. 6B) passes through the entire thickness T of the nanofiber film 600 and has a diameter D that may vary with the thickness of the film and the diameter of the particle forming the pore 604.
  • the pore 604 will have a maximum diameter that is approximately equal to the diameter of the particle used to produce the void.
  • the shape of the pores 604 can be selected according to the shape of the removable particle selected.
  • spherical particles will produce pores having a circular cross-section.
  • cylindrical microwire or nanowire particles will produce pores having a rectangular or circular (depending on the orientation of the microwire or nanowire within the filtered film).
  • Particles used in the films can be homogeneous, having essentially the same shape and size, or may be heterogeneous, having different morphologies, shapes or sizes. Other possible particle shapes and corresponding pores shapes will be appreciated in light of the present disclosure.
  • porous nanofiber films may include internal pores that are separated from the external environment by a layer of filtered film material. Such films can be produced by forming a nanofiber filtered film to have a thickness that this greater than the diameter (or other characteristic dimension) of the particle that is subsequently removed.
  • the average diameter D (or other characteristic dimension, such as a length of a side for a rectangle) of a pore 604 can be selected according to the corresponding size of the particle that is ultimately removed from the filtered film 600 to produce the pore 604.
  • the average diameter D can be within any of the following ranges: less than 100 microns; less than 10 microns; less than 1 micron; less than 100 nanometers; less than 10 nanometers; greater than 10 nanometers; greater than 100 nanometers; from 100 nm to 100 microns; from 100 nm to 500 nm; from 100 nm to 250 nm; from 250 nm to 750 nm; from 500 nm to 1 micron; from 1 micron to 10 microns; from 1 micron to 5 microns; from 5 microns to 10 microns; from 2.5 microns to 5 microns; from 3 microns to 7 microns; from 1 micron to 25 microns; from 25 microns to 75 microns; from 75 microns to 100 microns.
  • Example particles include those formed of materials that are not soluble in the solvent used to form the suspension (i.e., the suspension of nanofibers and/or the suspension of both nanofibers and particles). Furthermore, example materials are those that can be removed by liquids, gases or radiation that will not damage the integrity of the nanofiber sheet or by temperatures that will not damage the integrity of the nanofiber sheet. In one example, polystyrene particles (also sometimes referred to as “polystyrene microspheres”) can be used and subsequently removed by exposure to heating at 300°C to 400°C.
  • Embodiments of the present disclosure quantify porosity according to the following equation.
  • the total area of the rectangular nanofiber sheet is calculated by multiplying the length L by the width W.
  • the total area of a nanofiber film having a different shaped perimeter can be calculated according to an appropriate dimensional relationship.
  • the area of each pore 604 is calculated according to the area of a circle, which is P multiplied by (D/2) 2 .
  • the relationship used to determine the area of each pore can be selected based on the shape of the particle used to produce the pore.
  • FIG. 7 An example method 700 for the preparation of porous nanofiber films is illustrated in FIG. 7.
  • Method 700 can begin with the preparation 704 of a suspension of nanofibers, as described above.
  • a suspension of non-nanofiber particles can also be prepared 708.
  • the preparation 708 of the suspension of particles can in some examples be accomplished simply by the addition of the particles to the nanofiber suspension. In other examples, preparation 708 of the suspension of particles is accomplished by preparing a suspension that is separate from the nanofiber suspension in a solvent that is miscible with the solvent used to suspend the nanofibers.
  • one or both of the characteristic size of the particles e.g., diameter, length of a side
  • concentration of the particles relative to the concentration of nanofibers in the suspension can be varied.
  • Increasing one or both of the particle characteristic size and concentration will increase the porosity of the final filtered film (after removal of the particles).
  • the morphology of the filtered film will differ, however, depending on which of these parameters if varied.
  • Increasing the concentration of particles will increase porosity of the filtered film by increasing the number of pores.
  • Increasing the characteristic size of each of the particles will increase the porosity by increasing the individual pore size.
  • One or both of the characteristic dimensions of the particles and/or the concentration of particles in the suspension of nanofibers can be varied depending on the extent of porosity and/or the morphology desired.
  • the concentration of particles employed can be selected based on the desired pore to pore distance. For example, an upper limit on the concentration of particles can be set by instituting a limit of an average of at least one pore diameter between pores in the film. Pore diameter itself can be limited by the maximum diameter of the particles themselves [0084]
  • the nanofibers and particles can be combined 712 in a combined suspension that includes dispersed nanofibers as well as dispersed micro particles.
  • An anti-flocculant or surfactant can be used to keep the solids suspended, and ultrasound or physical mixing can also be used.
  • the combined suspension is applied 716 to a filter that is permeable to the solvent but that is not permeable to the nanofibers or particles.
  • a filter is that of common laboratory filter paper. Other types of filters may be used as long as they are permeable to solvent and not permeable to nanofibers and particles.
  • the solvent is then separated 720 from the suspended nanofibers and particles by allowing the solvent to pass through the filter, leaving the filtered film with embedded particles on a top surface of the filter.
  • the solvent can be separated from the nanofibers and particles (i.e., the solid phase components of the suspension) by simply allowing the solvent to flow through the filter under the force of gravity.
  • the solvent can be forcibly drawn or pushed through the filter by selective application of a pressure differential, for instance, negative pressure (i.e., vacuum) to a side of the filter opposite the applied combined suspension or positive pressure to a side of the filter with the applied combined suspension.
  • a pressure differential for instance, negative pressure (i.e., vacuum) to a side of the filter opposite the applied combined suspension or positive pressure to a side of the filter with the applied combined suspension.
  • heat may be applied to the filtered film to help remove solvent by drying.
  • air or dry nitrogen can be drawn through the film to aid in drying.
  • the filtered film (which includes the particles embedded therein) can be removed 724 from the filter by immersion in deionized water. Because the nanofiber film is hydrophobic, can be less dense than water, and is merely on (but not bonded to) the filter, the filtered nanofiber film will naturally lift from the filter to float on a surface of the water. A frame or other substrate can then be used to lift the film from the surface of the water, thus depositing the filtered film on the frame or substrate. If needed, the surface tension of the water (or other solvent) can be modified by adding surfactants or other solvents. The filtered film can then be dried (e.g., using a low humidity environment, heat, vacuum). This process can be repeated to form different films of, optionally, differently composed mixtures of multiwall, few wall, and/or single wall nanofibers and with different concentrations of particles.
  • the filtered film (or stack of filtered films) can be dried by exposure to a low relative humidity environment, heat, or other drying technique.
  • sol gel precursors can be infiltrated 728 into the film itself (i.e., beyond exposed surfaces of the film and into inter-fiber spaces within an interior of the film) and reacted 728.
  • This reaction 728 can form a protective coating (equivalently referred to herein as a conformal layer) around the surfaces of individual nanofibers within the film itself and at the exposed surfaces of the film.
  • Example coatings include, but are not limited to SiC , AI2O3,
  • T1O2 zinc sulfide, yttrium oxide, combinations thereof, among others.
  • FIGS. 8A and 8B Schematic illustrations of a porous nanofiber film that includes protective layers on individual nanofibers produced by a sol gel reaction appear in FIGS. 8A and 8B.
  • the porous nanofiber film 800 that defines pores 804 is shown in FIG. 8A.
  • the nanofiber film 800 and the pores 804 defined therein, as illustrated in FIG. 8A, can be produced according to any of the methods described above.
  • sol gel precursors can be infiltrated 728 into the nanofiber film itself.
  • the precursors can be provided in a gas, vapor, or liquid phase so as to enter the inter-fiber gaps within the sheet and between the nanofibers themselves.
  • the precursor molecules can thus deposit on exposed surfaces of individual nanofibers within the film and not merely on the exposed surfaces of the film.
  • FIG. 8B schematically illustrates a magnified portion of the nanofiber film 800 showing conformally coated nanofibers 808 that include nanofibers 812 and corresponding conformal protective layers 816.
  • the conformal protective layers 816 coat exposed surfaces of individual nanofibers 812.
  • sol gel precursor molecules can pass beyond a surface of the nanofiber film 800 via inter-fiber gaps 820, thereby depositing on the surfaces of the individual nanofibers 812 themselves.
  • the precursor molecules can then be reacted in situ on the surfaces of the individual nanofibers 812 (e.g., by heating) to form the conformal protective layer 816.
  • the particles can then be removed 732 from the nanofiber film, thus leaving pores in the filtered nanofiber film.
  • the technique used to remove the particles is varied according to the composition of the particle itself.
  • Water-soluble particles e.g., NaCl, CaCl, sugar, starch
  • the appropriate solvent e.g., an organic or aprotic solvent for polymeric particles or an acidic solution for dissolution of metallic particles or other particles soluble at low pH.
  • the particles can be removed 732 by decomposition such as by exposure to heat.
  • polystyrene particles can be removed by heating the filtered film in air (or oxygen) at from 300°C to 400°C, which combusts the polymer molecules.
  • Other polymer particles can be removed by a similar heating technique.
  • a vacuum or a pressurized gas can be used along with heat to remove the particle.

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Abstract

La présente invention concerne des techniques de préparation de films de nanofibres qui comprennent une proportion pouvant être régulée de pores à l'échelle nanométrique ou à l'échelle micrométrique. Ces techniques comprennent la préparation d'une suspension de nanofibres et de particules éliminables. Lors de la formation d'un film à partir de la suspension, les particules peuvent être éliminées du film de nanofibres à l'aide d'un solvant, de la chaleur, d'un rayonnement ou d'un stimulus similaire qui convertit le matériau de la particule d'une phase solide en une phase liquide ou gazeuse. Cette conversion élimine la particule du film de nanofibres et laisse un pore dans le film de nanofibres.
PCT/US2020/049875 2019-10-24 2020-09-09 Régulation de porosité de films de nanofibres filtrés WO2021080699A1 (fr)

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US20170312697A1 (en) * 2015-10-21 2017-11-02 King Fahd University Of Petroleum And Minerals Process for forming a sintered iron oxide impregnated carbon nanotube membrane
WO2019021277A1 (fr) * 2017-07-23 2019-01-31 Technion Research & Development Foundation Limited Membrane de nanotubes de carbone
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