WO2020243113A1

WO2020243113A1 - Nanofiber pellicles and protective nanofiber release liners

Info

Publication number: WO2020243113A1
Application number: PCT/US2020/034591
Authority: WO
Inventors: Chi Huynh; Marcio D. LIMA
Original assignee: Lintec Of America, Inc.
Priority date: 2019-05-31
Filing date: 2020-05-26
Publication date: 2020-12-03
Also published as: TW202103917A

Abstract

Multilayer carbon nanofiber structures (e.g., multilayer structures comprising multiple stacked films and/or sheets) are described that are composites of multiwall carbon nanotubes and one or more of single wall and/or few walled carbon nanotubes. In some cases, the composites are stacks of one or more filtered nanofiber films and one or more drawn nanofiber sheets. Drawn nanofiber sheet elements can be partially densified and joined to a filtered film by brief exposure (1 second, 2 seconds, 3 seconds) to solvent steam. Major surfaces of the pellicle can be protected from contamination by removable nanofiber "release liners."

Description

NANOFIBER PELLICLES AND PROTECTIVE NANOFIBER RELEASE LINERS

RELATED APPLICATIONS

[0001] This application claims priority under 35 USC § 1 19(e) to U.S. Provisional Patent

Application No. 62/855,515 entitled“NANOFIBER PELLICLES AND PROTECTIVE

NANOFIBER RELEASE LINERS,” filed on May 31 , 2019, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates generally to carbon nanofibers. Specifically, the present disclosure relates to nanofiber pellicles and their protective nanofiber release liners.

BACKGROUND

[0003] 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. PCT Publication No. WO 2007/015710 is one example of an advance in developing commercially useful embodiments of nanofibers. This publication describes converting a nanofiber“forest” into a nanofiber sheet and/or yam. The nanofiber sheets and yams may then be applied in a variety of contexts.

SUMMARY

[0004] Example 1 is an apparatus comprising: a first layer comprising a first plurality of nanofibers, the first layer comprising a first major surface and a second major surface opposite the first major surface; and a second layer comprising a second plurality of nanofibers, the second layer comprising a third major surface, wherein the third major surface is in releasable contact with the first major surface.

[0005] Example 2 includes the subject matter of Example 1 , wherein: the first layer comprises a filtered nanofiber film and the first plurality of nanofibers comprises nanofibers randomly oriented within a plane of the first layer; and the second layer comprises a drawn nanofiber sheet and the second plurality of nanofibers comprises nanofibers aligned end to end in the plane of the second layer.

[0006] Example 3 includes the subject matter of either of Examples 1 or 2, wherein the drawn nanofiber sheet comprises an as-drawn nanofiber sheet.

[0007] Example 4 includes the subject matter of either of Examples 1 or 2, wherein the drawn nanofiber sheet comprises a partially densified nanofiber sheet.

[0008] Example 5 includes the subject matter of either of Examples 1 or 2, wherein the drawn nanofiber sheet comprises a densified nanofiber sheet.

[0009] Example 6 includes the subject matter of any of the preceding Examples, wherein the first layer comprises a stack of multiple nanofiber layers.

[0010] Example 7 includes the subject matter of any of the preceding Examples, wherein the stack of multiple nanofiber layers comprises at least one or more nanofiber films comprising nanofibers that are randomly oriented in a plane of the one or more nanofiber films.

[0011] Example 8 includes the subject matter of any of the preceding Examples, wherein the stack of multiple nanofiber layers further comprises one or more nanofiber films comprising nanofibers that are aligned end to end in a plane of the one or more nanofiber films.

[0012] Example 9 includes the subject matter of any of the preceding Examples, wherein the stack of multiple nanofiber layers lack an interface between layers.

[0013] Example 10 includes the subject matter of any of the preceding Examples, further comprising a transmissivity to radiation having a wavelength of 550 nm is greater than 73%.

[0014] Example 11 includes the subject matter of Examples 1 or 2, wherein the first layer comprises multiwall carbon nanofibers intermixed and randomly oriented with single wall and few wall carbon nanotubes.

[0015] Example 12 includes the subject matter of Example 1, wherein the first layer and the second layer ar e combined to form a carbon nanotube membrane and are configured to have transparency to wavelengths of radiation between 10 nm and 124 nm.

[0016] Example 13 includes the subject matter of Examples 6 through 8, wherein drawn sheets of multiwall carbon nanotubes form one or both exposed major surfaces at a top or a bottom of the apparatus.

[0017] Example 14 includes the subject matter of Example 2, wherein one or more exposed surfaces of the filtered nanofiber film are protected by one or more removable nanofiber release liners. [0018] Example 15 includes the subject matter of Example 2, wherein one or more exposed surfaces of the drawn nanofiber sheet are protected by one or more removable nanofiber filtered film release liners.

[0019] Example 16 includes the subject matter of Example 1, wherein the first plurality of nanofibers is aligned approximately end-to-end with the second plurality of nanofibers.

[0020] Example 17 is a method of forming a carbon nanotube membrane, comprising drawing a first layer of nanofibers into a first sheet, drawing a second layer of nanofibers into a second sheet, partially densifying at least one of the first layer and the second layer, forming a filtered film on a frame, forming the first layer and the second layer on the frame, and joining the first layer and the second layer to the filtered film by exposure to a solvent steam.

[0021] Example 18 includes the subject matter of Example 17, wherein the exposure to solvent steam occurs for three seconds or less.

[0022] Example 19 includes the subject matter of Example 17, and further includes the step of adding surfactant to the solvent steam prior to the step of joining the first layer and the second layer to a filtered film by exposure to the solvent steam.

[0023] Example 20 includes the subject matter of Example 17, and further includes the step of adding a nanofiber adhesion layer between the frame and the layers of nanofibers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 is a photomicrograph of an example forest of nanofibers on a substrate, in an embodiment.

[0001] FIG. 2 is a schematic illustration of an example reactor for nanofiber growth, in an embodiment.

[0002] 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.

[0003] 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.

[0004] FIG. 5 is a schematic illustration of a portion of a filtered nanotube film that includes larger and longer multiwall carbon nanofibers intermixed and randomly oriented with single wall and few wall carbon nanotubes, in an embodiment.

[0005] FIG. 6 is a cross-sectional side view of an example nanofiber pellicle of the present disclosure, the cross-section taken perpendicular to major surfaces of the pellicle in an embodiment. [0006] FIG. 7 is a cross-sectional side view of an example nanofiber pellicle of the present disclosure, the cross-section taken perpendicular to major surfaces of the pellicle, in an embodiment.

[0007] FIG. 8 illustrates the nanofiber pellicle of FIG. 7 on a frame, in an embodiment.

[0008] FIG. 9 illustrates the nanofiber pellicle of FIG. 7 on a frame with an intervening nanofiber adhesion layer, in an embodiment.

[0009] FIGS. 10A, 10B illustrate example nanofiber pellicles in which drawn sheets of multiwall carbon nanotubes form one or both of the exposed major surfaces, in embodiments.

[0010] FIGS. 11 A, 11B, and 1 1 C are schematic illustrations of drawn multiwall carbon nanofiber sheets, in embodiments.

[0011] FIG. 12A illustrates a multilayer nanofiber structure stack that includes an as-drawn, undensified multiwall carbon nanofiber sheet on a filtered nanofiber film, in an embodiment.

[0012] FIG. 12B illustrates a multilayer nanofiber structure stack that includes a densified multiwall carbon nanofiber sheet on a filtered nanofiber film, in an embodiment

[0013] FIGS. 13A, 13B, 13C illustrate various configurations of nanofiber filtered films whose major surfaces are protected by removable nanofiber“release liners,” in embodiments.

[0014] FIGS. 14A and 14B various configurations of a drawn nanofiber sheet whose major surfaces are protected by removable nanofiber filtered film“release liners,” in embodiments.

[0015] FIGS. 15 A, 15B, and 15C illustrate various composite configurations of nanofiber pellicles formed from a stack of one or more filtered films and one or more drawn nanofiber sheets that have been joined by exposure to a solvent steam, in embodiments.

[0016] The figures depict various embodiments of the present disclosure for purposes of illustration only. Numerous variations, configurations, and other embodiments will be apparent from the following detailed discussion.

DETAILED DESCRIPTION

OVERVIEW

[0017] Carbon nanofiber structures are generally formed from one of multiwall carbon nanotubes (MWCNT), few wall carbon nanotubes (FWCNT), or single wall carbon nanotubes (SWCNT), but generally not combinations thereof. In some cases this is because many development efforts to date have been focused on forming pure (e.g., greater than 90%) forms of one type of carbon nanotube so that the properties of that type of nanofiber can be understood and optimized. Furthermore, the processes used to forni pure forms of multiwall carbon nanotubes (e.g., carbon nanotubes having from 4 to 20 concentric walls and a diameter of from 4 nm to 100 nm), few wall carbon nanotubes (e.g., carbon nanotubes having two or three concentric walls and a diameter of from 2 nm to 6 nm), and single wall carbon nanotubes (e.g., 1 wall and a tube diameter of from 0.2 nm to 4 nm) can differ from one another. For example, while multi wall carbon nanotubes can be fabricated using a chemical vapor deposition process on a relatively thick layer of catalyst (e.g., from 10 nm to several microns thick) on a substrate, few and single wall carbon nanofibers are often formed using laser ablation, carbon arc processes, or chemical vapor deposition (using e.g., acetylene, ethane as precursor) on a layer of catalyst that is thin (e.g., 0.2 nm to 10 nm thick ) and which may be discontinuous across the substrate. Laser ablation generally produces shorter carbon nanotubes than those produced by chemical vapor deposition and may produce nanotubes with fewer crystallographic defects. For at least this reason, generally the processes used to produce one type of nanofiber do not produce measurable amounts of the other types of nanofibers.

[0018] Each of these three different types of carbon nanotubes has different properties. In one example, 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 film formed by removing the solvent from the suspended nanofibers. This configuration of nanofiber sheet is sometimes referred to as a“filtered film.'’ This physical uniformity (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).

[0019] The strength of van der Waals attraction between nanofibers also differs between single/few wall nanofibers and multiwall nanofibers. Generally, single/few wall nanofibers have a greater van der Waals attraction to each other than that observed for multi wall 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 multi wall carbon nanotubes. In some examples, 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 because of the larger diameter of multiwall carbon nanotubes. In some cases, 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.

[0020] Filtered films, particularly those made with single and/or few wall carbon nanotubes also generally have greater transpar ency to some wavelengths of radiation. In some examples, transmittance of incident radiation can be as high as 90% or 95%. In some cases, this transmittance is significantly higher than that observed in dr awn sheets of multi wall 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.

[0021] Despite the advantages of single wall carbon nanotubes and few wall carbon nanotubes described above, multiwall carbon nanotubes also have advantages not necessarily observed to the same degree in nanotube structures formed from single or few wall nanotubes. For examples, structures formed from multiwall 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. For example, raultiwall carbon nanotube structures (e.g., the nanotube forest, a nanotube sheet) have a greater thermal emissivity than nanotube structures formed from few/single wall nanotubes. In one comparative example, an emissivity of a nanofiber structure comprising multi wall carbon nanotubes is on the order of 0.275 (+/- 15%) whereas a nanofiber 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.

[0022] For example, nanofiber structures having transparency to certain wavelengths of radiation (e.g., extreme ultraviolet or“EUV” in the range of 10 nni to 124 nm) 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. [0023] Despite the high transparency in the EUV radiation wavelength range, challenges remain to adopting nanofiber EUV pellicles. For example, cooling a nanofiber 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. However, 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.

[0024] 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 are often too mechanically delicate to be used as a pellicle. In some cases, because of their relatively short lengths (e.g., less than 100 mm), 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.

[0025] Further complicating the use of nanofiber pellicles is that they are physically delicate. If a surface of a nanofiber pellicle is contaminated with particles that could unintentionally be printed on a photoactive surface via EUV lithography, the pellicle is likely useless because it cannot generally be cleaned without damage.

[0026] Thus, in accordance with some examples of the present disclosure, multilayer carbon nanofiber structures (e.g., multilayer structures comprising multiple stacked films andor sheets) are described that are composites of multiwall carbon nanotubes and one or more of single wall and/or few walled carbon nanotubes. In some cases, the composites are stacks of one or more filtered nanofiber films and one or more drawn nanofiber sheets. In some cases, the drawn nanofiber sheet elements can be partially densified and joined to a filtered film by brief exposure (1 second, 2 seconds, 3 seconds) to solvent steam. Major surfaces of the pellicle can be protected from contamination by removable nanofiber“release liners.” Prior to a description of example EUV lithography pellicles, a description of nanofiber forests and sheets follows.

NANOFIBER FORESTS

[0027] As used herein, the term“nano fiber” means a fiber having a diameter less than 1 mm. 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 densified using the techniques described below. As used herein, the terms “nanofiber” and“carbon nanotube” encompass both single walled carbon nanotubes and/or multi-walled carbon nanotubes in which carbon atoms are linked together to form a cylindrical structure. In some embodiments, carbon nanotubes as referenced herein have between 4 and 10 walls. As used herein, a“nanofiber sheet” or simply“sheet" refers to a sheet of nanofibers aligned via a drawing process (as described in PCT Publication No. WO 2007/015710, and incorporated by reference herein in its entirety) so that a longitudinal axis of a nanofiber of the sheet is parallel to a major surface of the sheet, rather than perpendicular to the major surface of the sheet (i.e., in the as-deposited form of the sheet, often referred to as a“forest”). This is illustrated and shown in FIGS. 3 and 4, respectively.

[0028] The dimensions of carbon nanotubes can vary greatly depending on production methods used. For example, the diameter of a carbon nanotube may be from 0.4 ran to 100 nm and its length may range from 10 mm 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 intriguing 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.

[0029] Due to their unique structure, carbon nanotubes possess particular mechanical, electrical, chemical, thermal and optical properties that make them well-suited for certain applications. In particular, carbon nanotubes exhibit superior electrical conductivity, high mechanical strength, good thermal stability and are also hydrophobic. In addition to these properties, carbon nanotubes may also exhibit useful optical properties. For example, carbon nanotubes may be used in light-emitting diodes (LEDs) and photo-detectors to emit or detect light at narrowly selected wavelengths. Carbon nanotubes may also prove useful for photon transport and/or phonon transport.

[0030] In accordance with various embodiments of the subject disclosure, nanofibers (including but not limited to carbon nanotubes) can be arranged in various configurations, including in a configuration referred to herein as a“forest.” As used herein, 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. As can be seen in FIG. 1, the nanofibers in the forest may be approximately equal in height and/or diameter. [0031] Nanofiber forests as disclosed herein may be relatively dense. Specifically, the disclosed nanofiber forests may have a density of at least 1 billion nanofibers/cm². In some specific embodiments, a nanofiber forest as described herein may have a density of between 10 billion/cm² and 30 billion/cm². In other examples, the nanofiber forest as described herein may have a density in the range of 90 billion nanofibers/cm². The forest may include areas of high density or low density and specific ar eas 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.

[0032] Methods of fabricating a nanofiber forest are described in, for example, PCT No. W02007/015710, which is incorporated herein by reference in its entirety.

[0033] Various methods can be used to produce nanofiber precursor forests. For example, in some embodiments nanofibers may be grown in a high-temperature furnace, schematically illustrated in FIG. 2. In some embodiments, 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, SiO2, glass ceramics). In examples where the nanofibers of the precursor forest are carbon nanotubes, carbon-based compounds, such as acetylene may be used as fuel compounds. After being introduced to the reactor, 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. Examples of 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.

[0034] In a process used to fabricate a multilayered nanofiber forest, one nanofiber forest is formed on a substrate followed by the growth of a second nanofiber forest in contact with the first nanofiber forest. Multi-layered nanofiber forests can be formed by numerous suitable methods, such as by forming a first nanofiber forest on the substrate, depositing catalyst on the first nanofiber forest and then introducing additional fuel compound to the reactor to encourage growth of a second nanofiber forest from the catalyst positioned on the first nanofiber forest. Depending on the growth methodology applied, the type of catalyst, and the location of the catalyst, the second nanofiber layer may either grow on top of the first nanofiber layer or, after refreshing the catalyst, for example with hydrogen gas, grow directly on the substrate thus growing under the first nanofiber layer. Regardless, the second nanofiber forest can be aligned approximately end-to-end with the nanofibers of the first nanofiber forest although there is a readily detectable interface between the first and second forest. Multi-layered nanofiber forests may include any number of forests. For example, a multi-layered precursor forest may include two, three, four, five or more forests.

NANOFIBER SHEETS

[0035] In addition to arrangement in a forest configuration, the nanofibers of the subject application may also be arranged in a sheet configuration. As used herein, 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. An illustration of an example nanofiber sheet is shown in FIG. 3 with labels of the dimensions. In some embodiments, the sheet has a length and/or width that is more than 100 times greater than the thickness of the sheet. In some embodiments, the length, width or both, are more than 10³, 10⁶ or 10⁹ 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 mm and any length and width that are suitable for the intended application. In some embodiments, 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.

[0036] As can be seen in FIG. 3, the axis in which the nanofibers are aligned end-to end is referred to as the direction of nanofiber alignment. In some embodiments, 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.

[0037] Nanofiber sheets may be assembled using any type of suitable process capable of producing the sheet. In some example embodiments, 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

[0038] As can be seen in FIG. 4, the nanofibers may be drawn laterally from the forest and then align end-to-end to form a nanofiber sheet. In embodiments where a nanofiber sheet is drawn from a nanofiber forest, the dimensions of the forest may be controlled to form a nanofiber sheet having particular dimensions. For example, the width of the nanofiber sheet may be approximately equal to the width of the nanofiber forest from which the sheet was drawn. Additionally, the length of the sheet can be controlled, for example, by concluding the draw process when the desired sheet length has been achieved.

[0039] 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.

[0040] As with nanofiber forests, 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.

[0041] 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.

NANOFIBER PELLICLE STRUCTURE AND FORMATION TECHNIQUES

[0042] As described above, examples described herein include nanofiber films formed from a combination of multiwalled carbon nanotubes and one or both of single wall and few' wall carbon nanotubes. These can be described as“composite films” due to the combination or mixture of different nanofiber types within a layer of a stack and/or layers composed of differently oriented nanofibers (e.g., randomly oriented filtered films, drawn sheets of aligned nanofibers). In some examples, the relative weight proportion in one type of filtered film layer is a maximum of 80 weight (wt.) % multiwalled carbon nanotubes and a minimum of 20 wt. % single and/or few wall nanotubes. Lengths of the multiwalled carbon nanotubes can be controlled by lengthening or shortening the growth process in the chemical vapor deposition reactor, as described above. But for examples herein, a multiwalled carbon nanotube length can have a median length of approximately 300 mm (+/-10%). As will be appreciated in light of the following description, multiwalled carbon nanotubes having a length of at least 250 mm 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. from 0.5 mm to 30 mm). Films that include either the longer multiwalled nanotubes or shorter few/single wall carbon nanotubes are generally not as durable as those that include a mixture of the multiwall and few/single wall nanotubes.

[0043] FIG. 5 is a schematic illustration of a composite nanotube filtered film 500, in an example of the present disclosure. As shown, the composite nanotube filtered film 500 includes single/few wall nanotubes 504 that are inter-dispersed with multiwall carbon nanotubes 508. In this example film 500, the single/few wall carbon nanotubes 508 can have at least two beneficial effects on the structure of the film 500 as a whole. For example, the single/few wall carbon nanotubes 508 can increase the number of indirect connections between proximate multiwalled carbon nanotubes 508 by bridging the gaps between them. There interconnections between the short and long nanofibers can improve the transfer and distribution of forces applied to the film and thus improve durability. In a second example of a beneficial effect, the single/few wall carbon nanotubes 504 can decrease a median or mean size of the gaps between adjacent and/or overlapping multiwall carbon nanotubes 508. Furthermore, 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.

[0044] FIG. 6 is a cross-sectional illustration of one example of a composite nanofiber pellicle 600, in an example of the present disclosure. As can be seen, the composite nanofiber pellicle 600 can be composite not only in terms of multiple different types of nanofibers within individual layers but also a composite of multiple layers, each of which includes different ratios of the different types of nanofibers. As will be presented below in Table 1, it will be appreciated that tailoring the composition of each layer individually in a multilayer structure and further tailoring the number and order of layers can affect the emissivity and mechanical durability of embodiments of the present disclosure.

[0045] The composite nanofiber pellicle 600 shown in FIG. 6 includes first and second layers 604A, 604B that are on opposing sides of third layer 608. The composition of first and second layers 604A, 604B comprises a majority (e.g., from 50 wt. % to 80 wt. %) of multiwall carbon nanotubes (i.e., nanotubes having from 4 to 20 walls). The composition of the third layer 608 is that of a majority (e.g., greater than 50 weight percent) of few wall (e.g., nanotubes having from 2 to 3 walls) and/or single wall carbon nanotubes. [0046] The composite nanofiber pellicle 600 can be formed in any of a variety of ways. For example, a dry mixture of the desired proportion of multiwalled carbon nanotubes and few/single walled carbon nanotubes can be mixed and then suspended in a solvent. In another example, separate suspensions of known concentrations are prepared of multiwalled carbon nanotubes and one or more of few wall carbon nanotubes and single wall nanotubes. The suspensions can then be mixed in proportions to arrive at the desired relative weights of the multiwall, and few/single wall nanotubes in the final filtered film.

[0047] When preparing the one or more suspensions, dry carbon nanotubes can be mixed with the solvent to uniformly distribute the nanotubes in the solvent as a suspension. Mixing can include mechanical mixing (e.g., using a magnetic stir bar and stirring plate), ultrasonic agitation (e.g., using an immersion ultrasonic probe) or other means. In some examples the solvent can be water, isopropyl alcohol (IP A), N -Methyl-2 -pyrroli done (NMP), dimethyl sulfide (DMS), and combinations thereof. In some examples 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 0.1 weight % to 10 wt. % of solvent. In one embodiment, a mixture of 50 wt. % multiwalled carbon nanotubes and 50 wt. % few/single wall carbon nanotubes can be prepared and suspended in water and SDS surfactant.

[0048] The solution can then be introduced into a structure that removes the solvent and causes the formation of a film of randomly oriented nanofibers on a substrate. Examples of this process include, but are not limited to, vacuum filtration onto a substrate of filter paper. Because this composite“filtered film” of nanotubes is hydrophobic, the filtered film can be separated from the filter paper (or other substrate) by immersing the substrate and film into water, thus causing the composite film to float on the surface of water. A frame can then be used to lift the film from the surface of the water, thus depositing the filtered film on the frame. If needed, the surface tension of the water (or other solvent) can be modified by adding surfactants or other solvents. The composite 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 nanotubes.

[0049] This example process can be repeated multiple times to produce multiple films of carbon nanotubes. In some examples, individual films (having the same or different proportions of multiwall and few/single walled carbon nanotubes in each film) are stacked on one another to form a multilayer composite film. Stacking two or more films can produce a more uniform stack with more uniform properties. For example if one film in the stack has a local defect (e.g., a hole or tear), adjacent films in the stack can provide physical continuity and uniformity of the properties that would otherwise be absent at the location of the defect. In some embodiments, a stack can include anywhere from 2 to 10 individual films, each of which can have a same or different composition (that is, a different relative proportion of multiwall to single/few wall carbon nanotubes) from other films in the stack.

[0050] In some examples, a stacked film can be exposed a densifying solvent that includes water, IPA, NMP, Dimethylformamide (DMF), toluene, or combinations thereof. Exposure to a densifying solvent can cause the films in a stack to adhere to one another. In some cases not only do the films in the stack adhere to one another, but they merge so as to become indistinguishable from one another, even when using microscopy techniques to examine a cross-section of the stack. In other words, the densified stack does not have visible or microscopically detectable interfaces between layers.

[0051] As shown in FIG. 6, first and second layers 604A, 604B are on the exposed surfaces of the pellicle 600. As described above, the first and second layers 604A, 604B are composed of a majority (e.g., between 50 wt. % and 80 wt. %) multiwall carbon nanotubes. As also described above, films formed from multiwall carbon nanotubes have a higher thermal emissivity than those formed from few/single wall nanotubes. Thus configured, the exposed first and second layer 604A, 604B can improve the reliability of the pellicle 600 when used in an environment that includes EUV and/or a vacuum. By emitting thermal energy (formed in the pellicle by the incident radiation) more efficiently than a pellicle composed primarily of few/single wall nanotubes, the pellicle 600 can better withstand the operating environment in an EUV lithography device. This configuration further reduces the reabsorption of thermal radiation emitted by and/or conducted away from the pellicle 600.

[0052] FIG. 7 illustrates an alternative embodiment of a composite nanofiber pellicle 700 formed from a stack of filtered carbon nanotube films. Similar to the pellicle 600, the pellicle 700 includes first and second layers 704A, 704B that ar e formed primarily from multiwall carbon nanotubes. Third and fourth layers 708A, 708B are formed primarily from single/few wall carbon nanotubes.

[0053] FIG. 8 illustrates an assembly 800 that includes a pellicle frame 804 on which is disposed an example carbon nanotube pellicle (in the example shown, pellicle 700). It will be appreciated that any pellicle within the scope of the disclosure can be placed on a frame 804. Pellicle 700 is depicted in FIG. 8 as only one example embodiment. In some examples, the frame 804 can be fabricated from polymers such as polyethylene, polycarbonate, composite materials such as carbon fiber epoxy composites, and metals such as aluminum and stainless steel. In some examples, the frame 804 is dimensioned and configured to fit within an EUV lithography machine so that lithographically defined features can be exposed onto an underlying photoactive surface. In some examples the frame 804 is dimensioned and configured for convenient transportation from a pellicle manufacturing site to an EUV lithography site. In this example, the frame 804 is configured primarily to hold a freestanding carbon nanotube, as described herein, and conveniently release the freestanding pellicle for subsequent placement on a different frame that is configured for insertion into the EUV lithography machine. In some examples, having separate transportation and lithography frames enables the transportation frame to be fabricated according to design criteria that are easier to meet and with materials that are less expensive than those typically used when fabricating components of an EUV

lithography machine. Furthermore, frames configured specifically for the EUV lithography machine, which are likely to more expensive, can be maintained solely within the lithography manufacturing location (e.g., a cleanroom), reducing the rate of wear, breakage, and/or contamination.

[0054] FIG. 9 illustrates an alternative embodiment assembly 900 that includes the elements previously described, and an adhesion layer 904 disposed between the frame 804 and the nanofiber pellicle 700. Even though carbon nanotubes in the pellicle 700 will adhere to the frame 804 (whether made of a polymer or a metal or a composite) generally carbon nanotube adhesion is strongest with other carbon nanotubes. This can be particularly the case for carbon nanotubes having a smaller diameter, namely single wall and/or few wall carbon nanotubes. To combine the benefits of the frame 804 with the strong nanofiber to nanofiber adhesion, and adhesion layer 904 of carbon nanotubes can be deposited directly on the frame 804 prior to placing the pellicle 700 on the frame.

[0055] The adhesion layer 904 can be prepared by first preparing a suspension of carbon nanotubes and forming a“filtered film” as described above. The filtered film can be configured to match the exposed area of the frame 804 that will ultimately be in indirect contact with the pellicle 700. A portion of the filtered film spanning structures of the frame can be removed so that none of the film spans an opening (or openings) within and/or defined by the frame 804. Techniques to remove excess film from areas that are not directly overlying the frame 804 include using a laser, electrical discharge machine (EDM), mechanical techniques (cutting with the blade such as a surgical blade or a fracture surface of a silicon wafer). In some techniques, a solvent can be mechanically applied using an applicator such as a thin bar. For example acetone, IPA, NMP, DMF, toluene, or other solvent (and combinations thereof) can be applied to a bar which is then passed through the film to excise the desired portion of the filtered film.

[0056] The filtered film can be processed into the adhesion layer 904 by exposing the film on the frame 804 to a steam (i.e., vapor dr oplets at a temperatur e above boiling) of water, IPA, or a combination thereof. Exposure to the steam will cause the filtered film to adhere tightly to the frame 804, thus forming the adhesion layer 904. In some examples, a bottom layer of the pellicle can be formulated so as to include a greater percentage (e.g., greater than 50%, greater than 60%, greater than 70%) of few wall and/or single wall carbon nanotubes to further improve adhesion when in direct contact with the adhesion layer 904.

[0057] In some examples, a coating may be conformally deposited onto exposed surfaces of the pellicle. Example coatings include, but are not limited to one or more layers of metal (e.g., tungsten, iron, or other carbide forming metal, gold, silver, boron, ruthenium, silicon nitride, among others. Coatings can be from 1 nm to 10 nm thick. Thicker coatings are possible, but may decrease transparency to some wavelengths of radiation (depending on the properties of the coating and the wavelength of radiation). A coating may make it easier to removably adhere the structures together because a coating may reduce the van der Waals force between confronting layers. A coating applied after the various layers have been assembled and adhered to one another generally will not interfere with the transfer of heat from the single/few walled carbon nanotube filtered films (e.g., between exposed surfaces of the pellicle) to the more thermally emissive drawn sheets of multiwall carbon nanofiber tubes forming the exposed surfaces of the pellicle, in some examples. In some examples, a conformal layer on exposed surfaces of the nanofiber sheet can reduce degradation of the pellicle caused by hydrogen ions present in the lithography exposure chamber.

[0058] In some examples, such as those illustrated in FIGS. 10A and 10B, one or more major surface of the pellicle can be formed from a nanofiber sheet of multiwalled carbon nanotubes drawn from a nanofiber forest. Experimental results corresponding to some embodiments of this configuration appear in Table 1 in sample numbers 5-11.

[0059] As schematically shown in FIG. 10A, the assembly 1000 includes drawn multiwall carbon nanotube (MWCNT) sheets 1004A, 1004B between which is a filtered film that includes a majority of few/single wall carbon nanotubes (F/SWCNT). The sheets 1004A, 1004B have a greater thermal emissivity than the filtered film 1008, thus increasing cooling in a vacuum. The sheets 1004A, 1004B also provide the exposed major surfaces of the assembly 1000. In FIG. 10B, a second filtered film 1016 comprising a majority of F/SWNCT is at an opposite exposed major surface from the layer 1004A. In some examples, the layer 1004A can provide a higher rate of thermal emissivity whereas the layer 1016 can provide greater adhesion to a pellicle frame.

EXPERIMENTAL RESULTS

[0060] The following table reproduces results measured from a number of samples prepared according to some embodiments of the present disclosure. Samples measured include variations of stacks of“filtered films” (e.g., films produced from nanofibers suspended in a solution as described above), with or without one or two nanofiber sheets drawn from a nanofiber forest (as described above in the context of FIGS. 3, 4). The number and type of layers are identified are identified in the second and third columns from the left. The fourth column from the left labeled “CNT height” identifies a height of a nanofiber forest used to draw a nanofiber sheet. Thus, this column identifies a length of the multiwalled carbon nanofibers used to form the drawn sheet. The column labeled transmittance identifies the percentage of intensity of 550 nm wavelength of light transmitted through the stack of filtered films and/or drawn sheets.“Gap size” identifies an average size of gaps between bundles of nanofibers.

[0061] As can be observed, filtered films of nanotubes, as described above, with their random orientation of nanotubes within the film and their composite natur e (i.e., a mixture of multiwall carbon nanotubes and one or more of single wall and few wall nanotubes) have a higher transmittance. In some examples, for pellicles with two layers of filtered films, this transmittance is as high as 89% (for 550 nm wavelength radiation). Including one or more drawn sheets, with their multiwall carbon nanotubes aligned in the drawing direction, causes a drop in transmittance to between 72 and 79%. In some examples (e.g., sample numbers 5-11), the drawn sheet layers are on one or both of the major surfaces of a filtered film as illustrated in FIGS.

10A, 10B.

Table 1

[0062] In some examples, such as those illustrated in FIGS. 10A and 10B, one or more major surface of the pellicle can be formed from a nanofiber sheet of multiwalled carbon nanotubes drawn from a nanofiber forest. Experimental results corresponding to some embodiments of this configuration appear in Table 1 in sample numbers 5-11. NANOFIBER SURFACE PROTECTION

[00100] As described above, one application of the composite nanofiber stack is as a pellicle for EUV lithography. Embodiments described herein can help prevent particulates from contacting the photoactive surface being patterned or becoming unintentionally imaged onto the photoactive surface during EUV exposure. However, this goal can be frustrated by

contamination of the nanofiber pellicle itself prior to insertion into the EUV lithography machine. If the pellicle itself is contaminated by particles during transportation or removal from the packaging used during transportation, the risks of patterning defects on the photoactive surface increase. Furthermore, nanofiber pellicles can be expensive, difficult to produce, and physically delicate. Once contaminated, these expensive products may become useless because they are too delicate to clean without damaging them. For this reason, it would be beneficial to provide a removable layer on one or more of the exposed surfaces of the pellicle that could prevent particles from becoming disposed on and/or attached to surfaces of a pellicle itself. Instead, a sacrificial layer could collect any contaminants and then be removed thereby exposing the uncontaminated surfaces of the pellicle within a clean room and immediately prior to use. In some ways, this protective, removable layer could be analogous to a“release liner” used to prevent contamination of adhesive films.

[00101] However, complicating the use of a“release liner” on the pellicle is the highly adhesive and mechanically delicate nature of nanofiber sheets and films. Generally, nanofiber sheets and films will adhere to any structure that they come into contact with, even low surface energy structures (made from e.g., polytetrafluorethylene (PTFE)). Furthermore, nanofiber sheets and films are prone to wrinkling and tearing when brought into physical contact with other structures.

[00102] To accomplish the goal of protecting pellicle surfaces from contamination, embodiments described below include a pellicle composed of one or more filtered films whose exposed major surfaces are protected by releasably connected nanofiber“release liners.” The nanofiber“release liner” described herein can include in one example a filtered film of primarily single wall and few wall carbon nanofibers attached to a drawn muttiwall carbon nanofiber sheet, or the inverse structure (where the drawn multiwall carbon nanofiber sheet forms a removable structure to protect exposed pellicle major surfaced composed of filtered films). These filtered films can attach to, and later be removed from, a surface of a drawn multiwall carbon nanofiber sheet. The pairing of a filtered film to a multiwall drawn sheet in its un- densified state appears to be one of the few exceptions to the naturally strong adhesion exhibited by nanofiber structures. [00103] While not wishing to be bound by theory, it is believed that the topography of the drawn multiwall sheet facilitates removable connection with other nanofiber surfaces. For context, FIGS. 1 1 A, 11B, and 11C illustrate three different states of a multiwall, drawn carbon nanofiber sheet. FIG. 11 A schematically illustrates conformations of aligned multiwalled carbon nanotubes in a nanofiber sheet in its as-drawn state. The nanofibers are generally aligned end to end and within the plane of the sheet (perpendicular to the plane of the page) but exhibit nonlinear conformations and entanglements. The major surfaces of the sheet have a topographic surface of nanoscale peaks and valleys. FIG. 1 1 B illustrates a nanofiber sheet that has been partially densified by the provisioning and subsequent removal of a solvent. FIG. 11C illustrates a nanofiber sheet that has been densified by the provisioning and subsequent removal of a solvent. This“densification” can cause the aligned nanofibers in a sheet to draw closer together, thus becoming denser by reducing inter- fiber spacing, upon removal of the solvent. It will be noted that the greater the extent of densification, the more aligned the nanofibers are within the sheet of the plane and the less surface topography at the exposed surfaces of the drawn sheet.

[00104] The nanoscale rough surface of an as-drawn, undensified sheet (shown in FIG. 11 A) can be used advantageously for the preparation of release liners formed by filtered films of nanofibers. This is illustrated in FIGS. 12A and 12B. FIG. 12A illustrates an assembly 1200 that includes a drawn multi wall carbon nanofiber sheet 1204 and filtered nano fiber film release liner 1208. As shown schematically, the filtered film 1208 has a much smoother and less

topographically rough surface than the as-drawn carbon nanofiber sheet 1204. When the two sheets 1204, 1208 are placed in contact with one another, the surface topography of the drawn nanofiber sheet 1204 prevents continuous contact between confronting surfaces of the two sheets 1204, 1208. The surface topography and roughness of the drawn nanofiber sheet 1204 thus enables the filtered film sheet 1208 to be removed after contact therebetween. That is, as-drawn, undensified multiwall carbon nanofiber sheets to not have a clearly defined surface, but rather a surface made diffuse by the contours of the nanofibers (sometime referred to as a“hairy”) surface. While not wishing to be bound by theory, it is believed that this“hairy” surface structure reduces the extent of contact with the filtered film. In some cases, the contact with the filtered film is at filamentous fibers at the exposed surfaces of the as-drawn, undensified multiwall nanofiber sheet. In this configuration, van der Waals forces are not strong enough to permanently adhere to the films together. This is unlike most other materials and/or structures, which generally adhere irreversibly to the as-drawn nanofiber sheet 1204 and-'or the filtered film 1208.

[00105] An example contrast to the depiction in FIG. 12A is schematically illustrated in FIG. 12B. The assembly 1212 depicted in this figure is that of a densified drawn sheet 1216 that has been densified by the application and subsequent removal of a solvent. The surface topography of the densified drawn sheet 1216 is much smoother and more uniform than that of the undensified sheet 1204. As a result, the interfacial contact between the densified drawn sheet 1216 and a filtered film of nanofibers 1220 is much greater than that at the interface between the as-drawn, un-modified sheet 1204 and the filtered film 1208. As a result, the adhesion between the layers 1216, 1220 of the assembly 1212 is more likely to be irreversible (or damage to one or both of these structures if removed).

[00106] FIGS. 13A, 13B, 13C illustrate various configurations of nanofiber structures protected by a differently configured nanofiber structure“release liners.” In these embodiments, the drawn multiwall nanofiber sheet is releasably applied to a nanofiber filtered film, the former of which can be removed to leave exposed surfaces of the filtered film. FIG. 13 A illustrates a first configuration 1300 in which a filtered film (according to any of the compositions described above) is disposed between drawn, undensified multiwall carbon nanofiber sheets 1308 A,

1308B. These drawn, undensified multiwall carbon nanofiber sheets 1308A, 1308B protect the major surfaces of the filtered film 1304 from contamination and can be removed when desired.

[00107] FIG. 13B illustrates a second configuration 1312 in which two filtered films 1320A, 1320B are placed in contact with one another (e.g., to act as a pellicle as described herein and characterized in the Experimental Examples section at sample numbers 1-4 of Table 1). The smooth, planar surfaces of the filtered films 1320A, 1320B likely make the contact between them irreversible. Furthermore, exposure of the filtered films 1320A, 1320B to steam of water, IPA, or a solution thereof can further cause the merging of the filtered films 1320A, 1320B. Drawn, undensified multiwall nanofiber sheets 1316A, 1316B, with their topographically rough and non-uniform surface are placed on the exposed surfaces of filtered films 1320A, 1320B to protect these surfaces from contamination. As indicated above the drawn, undensified multiwall nanofiber sheets 1316A, 1316B can be removed without damaging the filtered films 1320 A, 1320B.

[00108] The third configuration 1324 depicted in FIG. 13C is a variation of the second configuration 1312 except the third configuration 1324 includes a fully densified nanofiber sheet 1336 between filtered films 1332A, 1332B, all of which adhere irreversibly to one another (particularly after exposure to steam). As-drawn and densified multiwall nanofiber sheets 1328A, 1328B, with their topographically rough and non-uniform surface are placed on the exposed surfaces of filtered films 1332A, 1332B to protect these surfaces from contamination. Drawn, undensified multi wall nanofiber sheets 1328 A, 1328B can then be removed when desired without damage to the filtered films 1332 A, 1332B. [00109] FIGS. 14A and 14B illustrate embodiments in which the filtered film is used as a removable release liner for a drawn multiwall nanofiber sheet. FIG. 14A illustrates an assembly 1400 in which a drawn multiwall nanofiber sheet 1404 is disposed between filtered films 1408 A, 1408B. As described above, because of the nano-scale topography of the drawn multiwall nanofiber sheet 1404, the filtered films 1408A, 1408B can be removed. In this way, the filtered films 1408 A, 1408B protect the surfaces of the drawn multiwall nanofiber sheet 1404, thus acting as“release liners,” as described above. A multilayer assembly 1412 that is analogous to the assembly 1400, except rather than a single drawn multiwall nanofiber sheet, the assembly 1412 includes two drawn multiwall sheets 1420 A, 1420B separated by filtered film 1416B. The exposed surfaces of the assembly 1412 can be protected by removable“release liner” filtered films 1416A, 1416C.

[00110] One example technique for removing a nanofiber release liner from the pellicle without damaging the pellicle relies on static electricity. An acrylic sheet can be applied to the nanofiber release liner so that the electrical attraction from the acrylic sheet can lift the nanofiber release liner from the nanofiber pellicle. Any charge transferred to the pellicle itself can be removed by connecting the nanofiber pellicle (and an intervening conductive frame or holder) to ground.

PELLICLES FORMED FROM COMPOSITE FILM ASSEMBLIES

[00111] In some examples, stacks of one or more filtered films and one or more drawn nanofibers sheets can be used as pellicles. In these examples, the presence of a densified drawn multiwall nanofiber sheet can provide additional mechanical support for a filtered film, thus improving the ability of the pellicle as a whole to withstand physical stresses during operation of an EUV lithography printer (e.g., from pressure cycling).

[00112] For this use, the different configurations of nanofiber sheets and films can be irreversibly joined together by exposing a stack to a steam of solvent(s) so that the layers are not releasable from one another. Exposure to steam also densities the drawn nanofiber sheet, improving mechanical properties and transmittance of radiation. In some examples, it can be beneficial to densify a drawn nanofiber sheet with a brief exposure to a steam of solvent(s).

Brief exposure to steam can produce some densification of the nanofiber sheet while preserving a surface topography of the drawn nanofiber sheet that can receive a remov able nanofiber “release liner,” as described above.

[00113] In some examples, this fixed connection between layers can be described as a “merged” configuration in which the composite structure lacks defined interfaces between the joined layers. In other examples, the multiple layers can be all filtered films, all drawn multiwall sheets, or combinations of filtered films and drawn sheets, depending on the intended application and desired construction. FIGS. 15A, 15B, and 15C illustrate processes by which multiple sheets can be merged together so as to form a composite structure. It will be appreciated that the number of films and the combinations of different types of films is not limited by the example embodiments depicted in these figures.

[00114] FIG. 15A illustrates an example in which a stack of multiple filtered films are merged together into a single layer. In this example, multiple filtered films are stacked together and exposed to a steam. The application of the steam causes a non-releasable (or irreversible) connection to form between the layers. In the example shown filtered film layers 1504A, 1504B are stacked on one another and exposed to the steam to form a merged film 1508.

[00115] FIG. 15B illustrates a different configuration in which the filtered film 1516 is placed in contact with drawn multi wall sheet 1512. This stack of the filtered film 1516 and the drawn multiwall sheet 1512 are exposed to a steam, as described above in the context of FIG. 15 A, to produce merged composite structure 1520. In this case, the drawn multiwall sheet 1512 has been densified into densified sheet 1512’ which has become irreversibly attached to the filtered film 1516 to form composite structure 1520.

[00116] FIG. 15C illustrates another example of producing a merged composite. As shown, a filtered film 1524 is placed between undensified drawn multiwall sheets 1528A, 1528 B and exposed to steam. This produces composite structure 1536 in which densified drawn multiwall sheets 1532A, 1532B are permanently attached to opposing major surfaces of filtered film 1524.

EXPERIMENTAL EXAMPLES

Table 2

[0063]

Table 3

FURTHER CONSIDERATIONS

[0064] The foregoing description of the embodiments of the disclosure has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the claims to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many

modifications and variations are possible in light of the above disclosure.

[0065] The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.

Claims

What is claimed is:

1. An apparatus comprising:

a first layer comprising a first plurality of nanofibers, the first layer comprising a first major surface and a second major surface opposite the first major surface; and a second layer comprising a second plurality of nanofibers, the second layer comprising a third major surface, wherein

the third major surface is in releasable contact with the first major surface.

2. The apparatus of claim 1, wherein:

the first layer comprises a filtered nanofiber film and the first plurality of nanofibers comprises nanofibers randomly oriented within a plane of the first layer; and the second layer comprises a drawn nanofiber sheet and the second plurality of

nanofibers comprises nanofibers aligned end to end in the plane of the second layer.

3. The apparatus of claim 2, wherein the drawn nanofiber sheet comprises an as- drawn nanofiber sheet.

4. The apparatus of claim 2, wherein the drawn nanofiber sheet comprises a partially densified nanofiber sheet.

5. The apparatus of claim 2, wherein the drawn nanofiber sheet comprises a fully densified nanofiber sheet.

6. The apparatus of claim 1 , wherein the first layer comprises a stack of multiple nanofiber layers.

7. The apparatus of claim 6, wherein the stack of multiple nanofiber layers comprises at least one or more nanofiber films comprising nanofibers that are randomly oriented in a plane of the one or more nanofiber films.

8. The apparatus of claim 7, wherein the stack of multiple nanofiber layers further comprises one or more nanofiber films comprising nanofibers that are aligned end to end in a plane of the one or more nanofiber films.

9. The apparatus of claim 6, wherein the stack of multiple nanofiber layers lack an interface between layers.

10. The apparatus of any of the preceding claims, further comprising a transmissivity to radiation having a wavelength of 550 nm is greater than 73%.

11. The apparatus of any of claims 1 or 2, wherein the first layer comprises multiwall carbon nanofibers intermixed and randomly oriented with single wall and few wall carbon nanotubes.

12. The apparatus of claim 1 , wherein the first layer and the second layer are combined to form a carbon nanotube membrane and are configured to have transparency to wavelengths of radiation between 10 nm and 124 nm.

13. The apparatus of any of claims 6-8, wherein drawn sheets of multiwall carbon nanotubes form one or both exposed major surfaces at a top or a bottom of the apparatus.

14. The apparatus of claim 2, wherein one or more exposed surfaces of the filtered nanofiber film are protected by one or more removable nanofiber release liners.

15. The apparatus of claim 2, wherein one or more exposed surfaces of the drawn nanofiber sheet are protected by one or more removable nanofiber filtered film release liners.

16. The apparatus of claim 1, wherein the first plurality of nanofibers is aligned approximately end-to-end with the second plurality of nanofibers.

17. A method of forming a carbon nanotube membrane, comprising:

drawing a first layer of nanofibers into a first sheet;

drawing a second layer of nanofibers into a second sheet;

partially densifying at least one of the first layer and the second layer;

forming a filtered film on a frame;

forming the first layer and the second layer on the frame; and

joining the first layer and the second layer to the filtered film by exposure to a solvent steam.

18. The method of forming a carbon nanotube membrane of claim 17, wherein the exposure to solvent steam occurs for three seconds or less.

19. The method of forming a carbon nanotube membrane of claim 17, further comprising the step of adding surfactant to the solvent steam prior to the step of joining the first layer and the second layer to a filtered film by exposure to the solvent steam.

20. The method of forming a carbon nanotube membrane of claim 17, further comprising the step of adding a nanofiber adhesion layer between the frame and the layers of nanofibers.