Induced Stress for Pulling EUV Films

Abstract

To develop a thin film that is transparent enough for protecting a photomask.SOLUTION: There is provided a method for protecting a photomask comprises the steps of: providing the photomask (210); providing a border (220); depositing at least two electrical contacts (230) on the border; mounting a film (240) comprising carbon nanotubes on the border (220) such that the film comprises a free-standing part, wherein after the mounting and depositing steps, the electrical contacts (230) are in contact with the film (240); inducing a current through the free-standing part of the film (240) by biasing at least one pair of the electrical contacts (230); and mounting the border (220) on at least one side of the photomask (210) with the free-standing part of the film above the photomask.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to the field of photolithography. In particular, it relates to a method for protecting a photomask by providing a free-standing pellicle on the photomask.

Background Art

[0002] In lithography, a pellicle is required to prevent particles from falling onto the patterned surface mask. Particles may fall onto the photomask during storage, handling, and lithography exposure. When the photomask is imaged in a lithography exposure tool, the pellicle mounted on the patterned photomask surface floats the particles outside the shop so that most particles do not form wafer pattern errors. The pellicle preferably comprises a transparent film that does not change the optical path and does not affect imaging.

[0003] State-of-the-art lithography tools expose photomasks using extreme ultraviolet (EUV) light centered at 13.5 nm. This wavelength is easily absorbed by all materials, so it is difficult to develop a thin film that is strong enough to stand on its own and transparent enough for pellicle applications.

[0004] Carbon nanotubes (CNTs) are considered a strong candidate because they protect the particles and cause minimal change to the exposure light. The CNT film can repeatedly achieve a transmittance of about 96% or more at an exposure wavelength of 13.5 nm, maintain the transmittance at other measurement wavelengths (190 - 500 nm), and is essentially robust. The concern about the thin film is the balance of film stress that is sufficient to avoid breaking the thin film and to avoid bending. In the CNT-based film, relaxation of the CNT-CNT bond is observed both after film formation and over time, resulting in film relaxation. This relaxation includes the mechanical integrity and optical consistency of the thin film. The thin film may come into contact with the scanner, handling system, or mask carrier configuration as a result of bending. Also, if there is a correlation between light and the thin film, since the light passes through the thin film at a predetermined angle, the correlation may change due to bending.

[0005] Therefore, there is a need for a thin film including a film having good transmittance for EUV and capable of preventing bending, and a method of forming such films in a lithography process. SUMMARY OF THE INVENTION

[0006] An object of a specific example of the present invention is to provide a method for obtaining a good thin film for protecting a photomask.

[0007] The above object is achieved by the method and apparatus according to the present invention.

[0008] A specific example of the present invention relates to a method for protecting a photomask. This method includes providing a photomask, providing a border portion, depositing at least two electrical contacts on the border portion, mounting a film including carbon nanotubes on the border portion such that the film includes a self-supporting portion, after the mounting step and the depositing step, the electrical contacts contact the film, Furthermore, a step of inducing a current through the self-supporting portion of the film by applying a bias to at least one set of electrical contacts. A step of mounting a border portion on at least one side of a photomask having a self-supporting portion of the film on the photomask. It includes.

[0009] That the thin film conditions can be achieved without additional materials, complex tools, or difficult procedures is an advantage of the specific examples of the present invention (the thin film is a structure including a film and a border portion). In a specific example of the present invention, the self-supporting CNTs forming the thin film change by passing an electric current, and a tensile CNT film placed on the border portion forming the thin film is produced. This procedure may be repeated a plurality of times as necessary, may be performed outside the scanner, or may be applied to a thin film already placed on the reticle.

[0010] Tension is induced on the CNT-based film by passing an electric current, if necessary. This may be achieved using available equipment. The application of the electric current can be performed in a clean room without the risk of adding particles.

[0011] The current generator may be connected using one set of electrical contacts to directly induce an electric current, or a potential difference may be applied to induce a current through the self-supporting portion of the film.

[0012] The current may also be induced by changing a magnetic field. An alternative method according to a specific example of the present invention is therefore the following steps: A step of providing a photomask. A step of providing a border portion. A step of mounting a film containing carbon nanotubes on the border portion so that the film includes a self-supporting portion. A step of inducing a current through the self-supporting portion of the film by changing a magnetic field within the self-supporting portion of the film. A step of mounting a border portion on at least one side of a photomask having a self-supporting portion of the film on the photomask. It includes.

[0013] This method provides an alternative solution to the same problem and aims to protect the photomask by providing a self-supporting portion of the film on the photomask.

[0014] Tension is induced on the CNT-based film by inducing a current through the self-supporting portion of the film.

[0015] The bending and wrinkling of the film can be reduced or completely removed by inducing a current through the self-supporting portion of the film.

[0016] In a specific example of the present invention, a current is induced by applying a bias to at least two sets of electrical contacts.

[0017] When a voltage is applied to at least two contact pairs, a better distribution of the current is obtained, which is an advantage of the specific example of the present invention. The more uniform the electric field applied to the self-supporting portion of the film, the more uniform the tensile strength of the film.

[0018] In a specific example of the present invention, one current pulse is induced through the self-supporting portion of the film. This may be achieved by applying a bias pulse to at least one set of electrical contacts to bias at least one set of electrical contacts, or by applying a magnetic field pulse to the self-supporting portion of the film.

[0019] In a specific example of the present invention, a train of current pulses with a predetermined cycle is induced through the self-supporting portion of the film. This may be achieved by applying a train of bias pulses to at least one set of contacts to bias at least one set of electrical contacts, or by applying a train of magnetic field pulses to the self-supporting portion of the film.

[0020] In a specific example of the present invention, the provided border is a silicon border.

[0021] In a specific example of the present invention, the bordering part is provided by providing a silicon-based material and partially removing the silicon-based material.

[0022] In a specific example of the present invention, an electrical contact is obtained by depositing a titanium seed layer and subsequently depositing palladium.

[0023] In a specific example of the present invention, the film is preferably prepared by dry transfer of carbon nanotubes.

[0024] The carbon nanotubes may be grown by a floating catalyst chemical vapor deposition method or extracted from a solution using vacuum filtration on a filter paper. For example, methods known to those skilled in the art such as a floating aerosol CVD reactor deposited on a filter paper, a CVD forest drawn into a film, vacuum filtration of a CNT solution, and spin coating onto an intermediate substrate. In a specific example of the present invention, the step of mounting the film includes the step of arranging carbon nanotubes on a filter paper or the step of pulling out a sheet from a carbon nanotube forest, and the film is mounted by pressing the filter paper against the bordering part or by pulling out the sheet.

[0025] The step of pulling out a CNT sheet from a vertically aligned CNT forest grown by fixed catalyst CVD to form a more aligned film is an advantage of a specific example of the present invention.

[0026] In a specific example of the present invention, this method includes the step of providing isopropyl alcohol on the film after deposition of the film.

[0027] The fact that increased densification and / or increased adhesion can be obtained by providing isopropyl alcohol (vapor, spray, or wet) on the film is an advantage of a specific example of the present invention.

[0028] In a specific example of the present invention, this method includes the step of providing a coating on the film.

[0029] The extension of the lifespan of the CNT film in an atmosphere of halogen radicals is an advantageous point of a specific example of the present invention. Even if it has a finite etching rate in halogen radicals, the CNT can have a thin film with a finite lifespan because it is not expensive to form.

[0030] In a specific example of the present invention, one or more layers may cover the CNT film. In a specific example of the present invention, the coating may be formed on both sides of the film.

[0031] The coating may be provided before or after supplying voltage or current.

[0032] In a specific example of the present invention, a partial coating may be provided on the film.

[0033] By reducing the amount of the coating material, the EUV transmission of the resulting CNT film is improved.

[0034] In a specific example of the present invention, the partial coating may be provided by deposition or by decorating defects in a film containing nanocarbon.

[0035] In a specific example of the present invention, a complete coating may be provided on the film.

[0036] For such a controlled complete coating in a specific example of the present invention, the extension of the lifespan of the CNT film (compared to a partial coating) is an advantageous point of a specific example of the present invention.

[0037] In a specific example of the present invention, a conformal coating is provided. In a specific example of the present invention, the complete coating may be deposited using physical vapor deposition, or atomic layer deposition, or chemical vapor deposition, or electrochemical deposition.

[0038] In a specific example of the present invention, this method includes applying lithography on a photomask protected by a mounted film containing carbon nanotubes.

[0039] Certain preferred forms of the present invention are described together with the independent claims and the dependent claims. The features of the dependent claims may, if necessary, be combined not only as simply stated in the claims but also with the features of the independent claims.

[0040] These and other forms of the present invention will be apparent from the specific examples described hereinafter and will be clear with reference to these specific examples.

Brief Description of the Drawings

[0041]

Figure 1

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Figure 8

[0042] The reference signs in the claims should not be construed as limiting their scope. In different drawings, the same reference signs indicate the same or similar elements.

Best Mode for Carrying Out the Invention

[0043] Although the present invention is described with reference to specific examples and specific drawings, the present invention is not limited thereto and should be limited only by the claims. The drawings described are merely schematic and not limiting. In the drawings, for the purpose of description, the size of elements may be exaggerated and not drawn to scale. Dimensions and relative dimensions do not correspond to the actual scale for the implementation of the invention.

[0044] The terms first, second, etc. in the description and claims are used to distinguish between similar elements and do not necessarily represent an order in terms of time, space, rank, or other means. Such terms are interchangeable under appropriate circumstances, and it should be understood that the specific examples of the invention described herein can be operated in a different order from that described and illustrated herein.

[0045] Also, the terms upper, lower, etc. in the description and claims are used for the purpose of description and do not indicate a relative position. Such terms are interchangeable under appropriate circumstances, and it should be understood that the specific examples of the invention described herein can be operated in a different position from that described and illustrated herein.

[0046] The term "comprising" used in the claims should not be construed as limiting only to the means listed thereafter and does not exclude other elements or steps. Thus, the recited features, numbers, steps, or components should be construed as such, and the presence or addition of one or more other features, numbers, steps, or components, or combinations thereof, should not be excluded. Thus, the scope of the expression "a device comprising means A and B" should not be limited to a device including only components A and B. In the present invention, it merely means that the components related to the device are A and B.

[0047] As used throughout this specification, the phrase "one embodiment" or "an embodiment" that is referenced in connection with a particular embodiment means that the particular advantage, structure, or feature described in connection with that embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases "one embodiment" or "an embodiment" in many places throughout this specification are not necessarily all referring to the same embodiment. Further, the particular advantages, structures, or features may be combined in any suitable manner in one or more embodiments as will be apparent to those skilled in the art from this disclosure.

[0048] Similarly, in the exemplary description of the invention, for the purpose of efficiently disclosing and assisting in the understanding of one or more of the many forms of the invention, it should be appreciated that many of the advantages of the invention are sometimes grouped together in one embodiment, drawing, or description thereof. However, this method of disclosure should not be construed as intending that the claimed invention requires more features than are recited in each claim. Rather, as the following claims represent, aspects of the invention may be less than all of the advantages of one described embodiment. Thus, the claims that follow the detailed description hereby incorporate the detailed description by reference, and each claim stands on its own as a separate embodiment of this invention.

[0049] Furthermore, some of the embodiments described herein include some features that are not included in other embodiments, and combinations of the advantages of different embodiments are meant to be within the scope of the invention and form different embodiments as will be understood by those skilled in the art. For example, in the following claims, some of the claimed embodiments can be used in other combinations as well.

[0050] In the description provided herein, many specific details are set forth. It should be understood, however, that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail so as not to obscure the understanding of this description.

[0051] When referring to a film in an embodiment of the invention, this reference is made with respect to a cover that protects a photomask. This film may also be referred to as a "dust cover".

[0052] When referring to a thin film in an embodiment of the invention, the reference may be made with respect to a mounted film and a border used in a scanner for exposure.

[0053] An embodiment of the invention relates to method 100 for protecting photomask 210. The method includes step 110 of providing photomask 210, step 120 of providing border 220, step 130 of depositing at least two electrical contacts 230 on border 220, step 140 of mounting a film 240 including carbon nanotubes on border 220 such that the film 240 includes a self-supporting portion, after the mounting step 140 and the deposition step 130, the electrical contacts 230 are in contact with the film 240, further, step 150 of obtaining a current through the self-supporting portion of the film by applying a bias to at least one set of electrical contacts 230, step 170 of mounting border 220 on at least one side of photomask 210 having the self-supporting portion of the film on the photomask 210, including.

[0054] Border 220 and film 240 are attached to photomask 210. This is done either directly or via the use of an additional spacer frame. The border may be mounted on both sides of the photomask.

[0055] This process does not necessarily need to be in the order as described above. The step 130 of depositing at least two electrical contacts 230 may be performed, for example, before or after the step 140 of mounting a film 240 including carbon nanotubes. The photomask may be provided, for example, immediately after the step 170 of mounting the border portion 120 on the photomask 210.

[0056] The current may be induced by changing the magnetic field within the self - standing portion of the film. In this case, electrical contacts are not strictly necessary. The change in the magnetic field may induce eddy currents in the film. The change in the magnetic field in the film may be formed, for example, by a transformer of an alternating current, or an electromagnet arranged such that a change in the magnetic field is formed in the self - standing portion of the film during operation.

[0057] The tension in the self - standing portion of the film is increased by applying a bias to one contact pair so that current can be obtained through the self - standing portion of the film. This results in a mesh of pulled CNTs that is adapted, for example, to scanner conditions, such as a pressure difference of 2 Pa and a maximum deviation of 0.7 mm. In an embodiment of the present invention, a bulge test measurement may be performed to qualify the tension of the film. The bulge tester can be used, for example, to qualify the maximum deviation of the film. The tension on the film may be tested using different types of equipment.

[0058] A flowchart of an exemplary method according to an embodiment of the present invention is shown in FIG. 1. FIG. 2 is a schematic diagram of a photomask protected using a film including carbon nanotubes obtained by using the method according to an embodiment of the present invention.

[0059] In the exemplary method, a border portion 120 is formed. This border portion 220 may be obtained, for example, from a Si wafer.

[0060] The edge portion 220 may be formed, for example, by providing a substrate material (e.g., a silicon wafer) having, for example, SiN deposited thereon as a mask material on a substrate and then partially removing the substrate material. For example, a mask material (e.g., SiN) between 30 nm and 50 nm may be deposited on the substrate. The partial removal may be achieved by laser cutting or a patterning / etching process (e.g., etching with 20% KOH at 70° C. for 8 hours and cleaning with DI water / IPA). Other edge portions 220 may be used. The edge portion may include, for example, a high-temperature ceramic.

[0061] In a specific example of the present invention, at least two electrical contacts 230 (also referred to as pads) are deposited on the edge portion 220. This may be performed before or after the mounting step 140 of the film 240 including carbon nanotubes. The pads 230 may have different dimensions, may be composed of different conductive materials, and may be formed separately according to the desired effect. In a specific example of the present invention, the electrical contact 230 may be obtained, for example, by forming a metal contact by performing a palladium deposition step after the deposition step 130 of a titanium adhesion layer. For example, a 50-nm palladium contact may be grown, and then a 0.3- to 2-nm titanium layer may be deposited (Ti / Pd). In a specific example of the present invention, the material of the electrical contact is preferably selected for many reasons such that a low resistance is obtained. For example, all of palladium (Pd), gold (Au), and platinum (Pt) have low resistance.

[0062] A film including carbon nanotubes (CNTs) is assembled into a free-standing film 240 whose morphology depends on the manufacturing process.

[0063] When prepared by either wet transfer of CNTs grown by the floating catalyst chemical vapor deposition (CVD) method or vacuum filtration of CNTs dispersed in a solution followed by removal of the filter membrane, the CNTs in the film 240 are randomly arranged.

[0064] The step of pulling out a CNT sheet from a vertically arranged CNT forest grown by fixed catalyst CVD forms more aligned CNTs in a film arranged in one or multiple layers.

[0065] In a specific example, the CNT film forms a self-standing film 240 that is mounted by arranging them on the support edge portion 220. For example, filter paper (aerosol collection method) or a pulled-out sheet (vertical forest growth) may be pressed against the edge portion for adhesion.

[0066] Optionally, IPA (gas, spray, or wet) may be applied for densification and / or adhesion. The edge portion may have a thin layer (5 - 50 nm) of SiN, TiN, or other materials deposited for many reasons such as improving the chemical stability of, for example, CNTs or pads, balancing stress, or improving adhesion.

[0067] The self-standing film can be further processed, i.e., washed, coated, or stretched.

[0068] Due to the van der Waals forces between CNTs that are dominant at the nanoscale, the CNTs form a film. Over time, there are two concerns, and sagging is observed. That is, there are many optical effects that compromise between print quality and the risk of reflection during pressure fluctuations in the scanner.

[0069] Therefore, in a specific example of the present invention, a current is forced to pass through the film 240 mounted at the edge portion 220. This step of applying the current results in a change that appears in the macroscopic size vision and greatly reduces or even removes sagging and wrinkles.

[0070] In a specific example of the present invention, at least two, usually four or more electrical contacts 230 (also referred to as pads) are arranged on the border portion 220, and current is directly applied or a voltage difference is applied to induce current through the film. In a specific example of the present invention, the pads 230 may be distributed at different positions along the border portion. The pads 230 may be elongated contacts (for example, the length / width ratio is limited only by the dimensions of the border portion and the need to separate one pad from another). The pads 230 are formed to induce a desired tension across the film when current is supplied through the film by applying a bias to at least two pads 230.

[0071] The border portion 220 is mounted on the photomask 210, and the self-supporting portion of the film 240 is on the photomask. In the example of FIG. 2, a schematic drawing of the resulting stack is shown. Here, the photomask 210 with a pattern schematically drawn thereon is shown. On the photomask, there is a thin film having the border portion 220 attached to the photomask 210 and having the film 240 on the photomask 210. Although not drawn in this schematic drawing, the thin film may exist on both sides of the photomask. In this schematic drawing, the electrical contacts 230 are also shown. Those electrical contacts 230 are drawn on the border portion.

[0072] Examples of pads 230 on border portions of different sizes are shown in FIGS. 3 and 4, with their shapes respectively. FIG. 3 shows a border portion 220 with an outer dimension of 3×3 cm for a test sample having four pads 230 deposited on the edge of the border portion 220. 2 The border portion 220 is arranged between the outer square and the inner square. The CNT film 240 (indicated by the pattern) is self-supporting at the center 250 and supported on the border portion 220. FIG. 4 is a schematic diagram of a larger sample, having eight deposited pads, two for each edge of the silicon-based border portion 220, with an outer dimension of 120×150 cm 2is more compatible with the full-size thin film. The border portion 220 is disposed between the outer rectangle and the inner rectangle. The CNT film 240 stands on its own at the center 250 and rides on top of the border portion 220 at the edges.

[0073] In a specific example of the present invention, the border portion has an open volume. By providing a film on top of this border portion, the film over this volume becomes a self-standing film. The shape of the self-standing region of the film is defined by the border portion surrounding this region. In the examples of FIGS. 3 and 4, self-standing portions of a square or rectangular film are shown. However, the present invention is not limited thereto. Other shapes such as circular are also possible. They are defined by the border portion 220.

[0074] In a specific example of the present invention, a bias is applied on one set or more sets of electrical contacts 230. In the example of FIG. 3, if current is supplied from contact 2 to contact 4, the current supply process will deform the film such that the film is more pulled in the 2-4 direction. If current is supplied from 2-1, only the limited portion between contact 2 and contact 1 will be more pulled as a result of supplying the current. One current may be induced along one path, or multiple currents may be induced between multiple contact pads. For example, a first voltage may be supplied over contact 1 and contact 3, and simultaneously, a second voltage may be supplied over contact 2 and contact 4. By doing so, a first current is obtained between node 1 and node 3, and a second current is obtained between node 2 and node 4. The first voltage may be different from the second voltage, and additionally the first current may be different from the second current.

[0075] In a specific example of the present invention, to measure the change in the film, the electrical resistance of the film 240 is measured using the van der Pauw method. To determine the amount of the process, the resistance is measured before stretching, during stretching, and after stretching. The sample procedure may include a step of measuring the resistance and subsequently a step of applying a current or voltage between two contacts (I24 / V24 in FIG. 5). While doing so, the resistance is measured and the film is visually inspected for a flat and wrinkle-free appearance. The stretching may be measured (e.g., indirectly in an inflation test that measures the value of the deviation while applying pressure to the film). If additional stretching is required, an additional set of contacts may be used to apply the current or voltage. For example, I13 / V13. This procedure may be repeated on different sets of contacts (I12 / V12, I34 / V34, I41 / V41, I23 / V23, I24 / V24 or I13 / V13) while measuring the resistance and checking for flat and wrinkle-free stretching on the film. The application of bias to different sets of electrical contacts 230 may be repeated until a uniform strain is achieved, the film is flat and wrinkle-free, and the resistance saturates at a constant value. The goal is to use the current to strategically induce a uniform tensile stress across the film.

[0076] As described above, in a specific example of the present invention, the change in the resistance of the film may be detected using the van der Pauw method. This is the reference resistance. The current or voltage may be applied simultaneously to a plurality of electrical contacts. An example of such a configuration is schematically shown in FIG. 6 showing a voltage source or current source on sets of contacts 1 and 3 (I13 or V13) and sets of contacts 2 and 4 (I24 or V24). For example, I24 or V24, and I13 or V13 are in contact simultaneously to form a dual-channel configuration. In a specific example of the present invention, for this multi-channel approach where a plurality of sets of electrical contacts are biased, it may be extended to complex contacts and current / voltage sets for the configuration shown in FIG. 6 or for configurations with more contact pads.

[0077] In a specific example of the present invention, this method is suitable for a thin film composed of an uncoated CNT film or a thin film formed from a CNT film coated with a different material. In a specific example of the present invention, the CNT film coating 160 is performed after disposing the contact pad 130 on the edge portion and mounting the uncoated CNT film.

[0078] In a specific example of the present invention, the amount of time for applying a bias to a set of electrical contacts so that current can be obtained through the self-standing portion of the film may range, for example, from a few milliseconds to several minutes. The given current may range, for example, from a few milliamperes to 200 mA or more, the given voltage may vary, for example, from zero to 40 V, and the given voltage may be, for example, greater than 20 V or greater than 40 V, but these are not limitations required for this procedure.

[0079] In a specific example of the present invention, the application of a pulse train (current or voltage) may be used to induce such stress. The on / off ratio of such a pulse train may be adjustable, for example. The on / off ratio may be, for example, greater than 20%, 40%, 60%, 80%. However, this is not strictly required.

[0080] In a specific example of the present invention, current may be provided so that the heating of the self-standing portion of the film is limited. During application, the temperature of the film may be, for example, lower than 200 °C or lower than 30 °C. This may be achieved, for example, by controlling the convective gas flow and the duty ratio of the applied current or voltage. Therefore, the current may be limited to a maximum value, and / or the pulse duration may be limited to a maximum value, and / or in the case of a pulse train, the on / off ratio may be limited to a maximum value.

[0081] The inventors specifically note that the resistance of the CNT film increases with the applied current. While not wishing to be bound by a particular theory, this relationship between resistance and current would seem to suggest nanoscale changes in the film itself. Figure 7 shows a scanning electron microscope (SEM) image of the film before applying a current through the free-standing portion of the film. Figure 8 shows the SEM image after applying a current to obtain increased tension. As can be seen from the figures, there is no difference in the SEM. Therefore, the inventors speculate that the change is either in the nanomorphology (healing bonds) or that contaminants that volatilize must be further processed.

[0082] In a specific example of the present invention, a coating is applied on the film 160. This is an optional step and may be performed, for example, before applying a bias to at least one set of electrical contacts 150, thereby forcing a current through the free-standing portion of the film. However, the present invention is not limited thereto, and the coating may be performed after biasing the contact pair.

[0083] The CNT film 240 may be partially or fully coated. A partial coating may be obtained by a very thin (e.g., film thickness less than 3 nm) deposition or decoration of CNT defects. A full coating may be performed, for example, by physical vapor deposition (PVD), i.e., electron beam or thermal evaporation, remote plasma sputtering, atomic layer deposition (ALD), chemical vapor deposition (CVD), electrochemical deposition (ECD), and sol-gel technology, or a combination of multiple methods. The coating may include, for example, the following materials: Zr, Y, B, Ti, Ru, Mo, and related compounds, nitrides or oxides. However, this list is not limiting to those materials.

[0084] The process flow shown in Figure 1 is an exemplary process flow according to a specific example of the present invention. However, the present invention is not limited thereto. Steps may be skipped if not necessary (e.g., the coating step). It is also possible to change the order of the steps (e.g., coat the film after applying a current).

[0085] The coating may include a multi-layer coating of a self-standing carbon nanotube (CNT) film (preferably two layers, but may also be multi-layer). This coating protects the film from an aggressive environment such as hydrogen radicals when used as an EUV thin film, for example. The coating can increase the emissivity for radiative cooling.

[0086] When used as an EUV thin film, the self-standing CNT film is introduced into the halogen radical (H * ) atmosphere inside the scanner. The CNT structure in the film (also referred to as the film texture) is attacked by H * , and the thin film is damaged by the removal of CNTs due to the vaporization of carbon into hydrocarbon molecules. To avoid film degradation, the CNT film texture must be protected to reduce or remove H * damage. This is done by coating the CNT film texture with a thin film consisting of one or more layers (e.g., on both sides). Protection may also be performed by deposition targeted on the weak portions along the CNT tubes in the film texture.

[0087] Atomic layer deposition (ALD) may be used to deposit a conformal thin film on a complex structure. However, due to the inactivity of the CNT surface having a small number of active sites for ALD nucleation at the initial stage of film growth, it is difficult to form a conformal and pinhole-free coating on CNTs.

[0088] In a specific example of the present invention, the first coating may be a partial coating layer. This may be achieved by selective coating of defects by atomic layer deposition (ALD) or chemical vapor deposition (CVD). H * etching may start at the defect sites of CNTs and then proceed along the tube length. It is known that ALD / CVD can start nucleation on the defect sites and selectively protect CNT defects from H * . The partial coating of CNT defects increases the lifetime of the coated CNT thin film compared to the case where it is not coated.

[0089] Optionally, a second coating layer may be applied on one or both sides of the CNT film on top of the partial ALD / CVD coating to improve radioactivity. The coating may be provided by various thin film deposition techniques already mentioned above, such as thermal or electron beam evaporation, sputtering, etc.

[0090] In a specific example of the present invention, a complete coating may be provided on the CNT film 240 for protection. This layer may be achieved by a conformal coating process.

[0091] Conformal coating may be achieved by providing a seed (adhesion) layer deposited by thermal or electron beam evaporation before the ALD coating. This deposition approach of the seed layer is gentle on the CNT structure and can retain the initial CNT characteristics. For the uniformity of the seed layer, evaporation is performed, for example, at an angle of 15°, but this angle may be from 1° to 89°, and the substrate is rotated during deposition. The ALD on top of the seed CNT structure is uniform and conformal.

[0092] The provision of the above-mentioned seed layer as protection of the CNT film structure during the deposition of the subsequent layer, including exposure of the CNT to severe reaction conditions (such as plasma-based sputtering) or strong oxidants (such as a given ALD process) known to damage the CNT during the process, is an advantageous point of the specific example of the present invention.

[0093] In a specific example of the present invention, an intermediate thin and deformable film coating can be added, for example, between hard coating films such as carbon, to reduce the stress of the multilayer coating.

[0094] In a specific example, two layers may be provided using conformal coating. Since the total stress is a combination of compressive or tensile stresses induced by each layer, providing two layers may balance the stress. This results in a more uniform tensile film and a film with fewer wrinkles or a wrinkle-free film.

[0095] In a specific example of the present invention, a CNT structure preferable for conformal coating may be provided. The film texture of the CNT film may include, for example, N-doped CNTs. The doping process of the CNT film using other atoms makes the CNT film more preferable for ALD or CVD nucleation by introducing reactive surface species. イ The CNT film may include multiple layers of CNTs in order to be more preferable for conformal coating. In that case, one of the walls may be used as a sacrificial CNT wall that functions to enable ALD seeding.

[0096] The number of ALD cycles required to obtain a continuous conformal ALD coating may be reduced by coating on the seeded CNTs in a specific example of the present invention. For example, a very thin layer (for example, a ruthenium coating) may be formed by ALD.

[0097] In a specific example of the present invention, the dry approach may be applied to conformal coating. In this case, no solution is used to deposit the coating.

[0098] In a specific example, it is an advantage of the specific example of the present invention that the inherent, structural, mechanical, and electrical CNT properties can be retained by avoiding damage to the CNTs during seed layer growth.

[0099] For example, as long as the CNTs are protected by the seed layer from ion collisions during the coating process, the fact that various coating approaches are possible is an advantage of the specific example of the present invention.

[0100] The fact that it can be applied to a highly transparent free-standing film of single-walled CNTs, which is known to have fewer defects than multi-layer CNTs, is an advantage of the specific example of the present invention.

[0101]

[0102] ​The ability to engineer defects on the CNT surface for controlled coating by ALD while retaining the free-standing CNT film structure is an advantage of embodiments of the present invention because the CNT structure is favorable for conformal coating.

[0103] The method according to an embodiment of the present invention may include a lithography process on a photomask protected by a mounted film including carbon nanotubes.

[0104] In an embodiment of the present invention, a CNT thin film may be formed by mounting a CNT film between raised borders and providing an electric current to the free-standing portion of the film. The CNT thin film is a solution for reticle protection in lithography.

[0105] In an embodiment of the present invention, the lithography may be extreme ultraviolet lithography using a wavelength of 13.5 nm. However, the present invention is not limited to this wavelength. The wavelength may be in the range between, for example, 3 nm and 248 nm. When using carbon nanotubes, the film can be made such that EUV light passes through the film without being affected because the film is very thin. Further, by the method according to an embodiment of the present invention, the tensile stress increases, and the fact that this tensile stress is maintained for a long time is also an advantage. The tensile stress is, for example, permanently and stably maintained.

Claims

[Claim 1] A method (100) for protecting a photomask (210), the method comprising: Providing (110) a photomask (210); Providing (120) a border (220); Depositing (130) at least two electrical contacts (230) on the border (220); and mounting (140) a film (240) comprising carbon nanotubes onto the border (220) such that the film (240) comprises a free-standing portion; After the mounting step (140) and the depositing step (130), electrical contacts (230) contact the membrane (240); Further, inducing (150) a current through the free-standing portion of the membrane (240) by applying a bias to at least one set of electrical contacts (230); Mounting (170) a border (220) on at least one side of the photomask (210) with a free-standing portion of the membrane; The method includes:

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