Patent Description:
Recently, a technique of EUV lithography is being developed in finer semiconductor processing. A pellicle is used as a dust-proof protection film for attachment to a photomask in photolithography equipment with EUV.

As the pellicle, graphene having a film thickness of less than <NUM> is known (Patent Document <NUM>). Patent Document <NUM> describes that the graphene having a film thickness of less than <NUM> has a high transmittance of extreme ultraviolet.

Patent Document <NUM> describes a charge stripping film for an ion beam comprising a single layer body of a graphitic film having a carbon component of not less than <NUM> at% and a thermal conductivity in the film surface direction at <NUM>° C of not less than <NUM> W/mK, or a laminated body of the graphitic film with the thickness of <NUM> to <NUM>.

Patent Document <NUM> describes a formation method of a graphene-graphite film by bringing an alloy including a typical element metal and a transition metal into contact with a carbon source. Carbon on the surface of the carbon source is dissolved in the alloy by heating the alloy, and a graphene-graphite film or a composite film of nanocarbon and graphene-graphite are deposited on the surface of the object brought into contact with the alloy by cooling the alloy.

Patent Document <NUM> describes a pellicle which comprises a pellicle film that has a refractive index (n) of light having a wavelength of <NUM> of <NUM> to <NUM> and a pellicle frame to which the pellicle film is bonded. The pellicle film has a composition that contains <NUM>-<NUM>% by mole of carbon and <NUM>-<NUM>% by mole of hydrogen. The intensity ratio of the 2D-band to the G-band, namely (intensity in 2D-band)/(intensity in G-band) is <NUM> or less, or alternatively, the intensity in the 2D-band and the intensity in the G-band are <NUM> in the Raman spectrum of the pellicle film.

Conventional pellicles, however, have had a problem in achieving both a high EUV transmittance and a uniformity in EUV transmittance.

Thus, an object of the present invention is to provide a pellicle achieving both a high EUV transmittance and a uniformity in EUV transmittance.

This object has been achieved by a pellicle according to claim <NUM>, a pellicle composite according to claim <NUM> and a method for producing the pellicle according to claim <NUM>. Preferred embodiments are represented in the dependent claims.

A pellicle of the present invention can achieve both a high EUV transmittance and a uniformity in EUV transmittance.

In the present invention, the EUV refers to light having a wavelength of <NUM> to <NUM>. In the present specification, the phrase A to B means A or more and B or less.

The present invention relates to a pellicle that includes a graphite thin film having a film thickness of <NUM> to <NUM>, a ratio (I (D)/I (G)) of a D-band intensity (I (D)) to a G-band intensity (I (G)) in a Raman spectrum of <NUM> or more and <NUM> or less, and a surface roughness Sa of <NUM> to <NUM>.

According to Non-Patent Documents <NUM> and <NUM>, graphene that is a multilayer but has a thickness of <NUM> or more exhibits behaviors clearly different from those of monolayer graphene or multilayer graphene having a thickness of less than <NUM> in terms of carrier mobility and temperature dependence of electrical conductivity. The physical properties of the multilayer graphene having a thickness of <NUM> or more may be considered to be almost the same as the physical properties of a graphite crystal. In <FIG> of Example <NUM> described later, multilayer graphene in which each graphene is stacked one another in a state of streaks is observed. As the multilayer graphene also has a film thickness of <NUM> or more, the pellicle of the present invention can be determined to be formed of a graphite thin film having a graphite structure. Accordingly, the graphite thin film constituting the pellicle of the present invention is clearly distinguished from so-called graphene (that is, monolayer graphene or multilayer graphene having a thickness of less than <NUM>). The film thickness measured by TEM observation is shown in <FIG> for reference.

The graphite thin film forming the pellicle of the present invention has a film thickness of <NUM> to <NUM>, preferably <NUM> to <NUM>, particularly preferably <NUM> to <NUM>, from the viewpoints of EUV transmittance, uniformity in EUV transmittance, and film strength. The graphite thin film having a film thickness of <NUM> or more enables an excellent uniformity in EUV transmittance and a high film strength. The graphite thin film having a film thickness of <NUM> or less enables an excellent EUV transmittance and a uniformity in EUV transmittance.

The film thickness of the graphite thin film specified in the present invention is defined by a value obtained from the EUV transmittance. The EUV transmittance can be obtained by scanning a surface of the film, and a change of the film thickness according to the position can be obtained.

A relationship between the EUV transmittance (T) and the film thickness of the graphite thin film is represented by the following equation with use of <NUM>-nm transmittance (<NUM>) of monolayer graphene and the film thickness (<NUM>) of the monolayer graphene.

Measuring the film thickness by cross-sectional TEM observation leads to evaluation of the film thickness only by one point of the film, and the measured value sometimes has an error of up to about <NUM>%.

The graphite thin film forming the pellicle of the present invention has a film-thickness error of preferably <NUM>% or less, more preferably <NUM>% or less, particularly preferably <NUM>% or less, from the viewpoint of uniformity in EUV transmittance. The graphite thin film having a film-thickness error of <NUM>% or less enables an excellent uniformity in EUV transmittance. The film-thickness error is represented by a ratio of standard deviation of the film thickness to the average film thickness, i.e., the following equation.

The graphite thin film forming the pellicle of the present invention can have an area of, for example, <NUM><NUM> or more and has an area of preferably <NUM><NUM> or more, more preferably <NUM><NUM> or more, further preferably <NUM><NUM> or more. The graphite thin film of the present invention can secure sufficient strength even with such a large area and is handled well. The graphite thin film has an area of preferably <NUM> × <NUM> or more that is larger than a photomask, more preferably <NUM> × <NUM> or more, particularly preferably <NUM> × <NUM> or more. The upper limit of the area of the graphite thin film is not particularly limited but is, for example, <NUM> × <NUM>.

The graphite thin film forming the pellicle of the present invention has a surface roughness (Sa) of <NUM> or more and <NUM> or less. The graphite thin film has a surface roughness of preferably <NUM> or more, more preferably <NUM> or more, further preferably <NUM> or more, and preferably <NUM> or less, more preferably <NUM> or less, further preferably <NUM> or less. The surface roughness Sa in the present invention means an arithmetic average height obtained on the basis of ISO <NUM>. When even having the same film thickness but having a larger surface roughness, the graphite thin film has a lower EUV transmittance. Further, as a uniformity in EUV transmittance becomes low, the graphite thin film preferably has a smaller surface roughness. The surface roughness (Sa) is preferably measured using a laser microscope. The measurement position of the surface roughness (Sa) is not particularly limited, but it is desirable to measure surface roughness at a plurality of locations including a central portion and an end portion and define the average of values at the locations as the surface roughness (Sa).

The graphite thin film forming the pellicle of the present invention has a density of preferably <NUM>/cm<NUM> to <NUM>/cm<NUM>, more preferably <NUM>/cm<NUM> to <NUM>/cm<NUM>, further preferably <NUM>/cm<NUM> to <NUM>/cm<NUM>, particularly preferably <NUM>/cm<NUM> to <NUM>/cm<NUM>, from the viewpoint of film strength.

A relationship between the number of graphene layers and the film thickness of the graphite thin film forming the pellicle of the present invention is represented by the following equation.

Number of graphene layers of graphite thin film = film thickness (nm)/film thickness (<NUM>) of monolayer graphene.

The graphite thin film forming the pellicle of the present invention includes preferably <NUM> to <NUM> graphene layers, more preferably <NUM> to <NUM> graphene layers, particularly preferably <NUM> to <NUM> graphene layers. The graphite thin film including <NUM> or more graphene layers enables an excellent uniformity in EUV transmittance and a high film strength. The graphite thin film including <NUM> or less graphene layers enables an excellent EUV transmittance and an excellent uniformity in EUV transmittance.

The graphite thin film forming the pellicle of the present invention can be evaluated by laser Raman measurement that allows determination of whether the film is of carbon or graphite. In the laser Raman spectroscopy, a G-band attributed to a graphite structure appears at around <NUM> to <NUM>-<NUM>, and a D-band attributed to an amorphous carbon structure appears at around <NUM> to <NUM>-<NUM>. A ratio ((I (D)/I (G)) between a G-band intensity (I (G)) and a D-band intensity (I (D)) in a Raman spectrum is <NUM> or more and <NUM> or less, more preferably <NUM> or more and <NUM> or less, particularly preferably <NUM> or more and <NUM> or less.

The Raman spectrum is preferably observed using a Raman microscope. A measurement position of the Raman spectrum is not particularly limited, but it is desirable to measure a part that has high film smoothness and is uniform in film thickness and a state of a surface irradiated with laser. A wavelength of the laser light used in the Raman spectroscopy is not particularly limited. Examples of a wavelength of general laser light include <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>.

The graphite thin film forming the pellicle of the present invention has a transmittance of <NUM>-nm-wavelength light (EUV) of preferably <NUM>% or more, more preferably <NUM>% or more, further preferably <NUM>% or more, particularly preferably <NUM>% or more. The graphite film with higher EUV transmittance is more preferable. The upper limit value achievable by the graphite thin film forming the pellicle of the present invention, however, is usually about <NUM>%.

In the present invention, the graphite thin film has a surface roughness Sa of <NUM> or more and <NUM> or less, and has, in the film, an excellent uniformity in EUV transmittance. The uniformity in EUV transmittance can be evaluated with a value of three times (3o) the standard deviation of the EUV transmittance obtained by scanning an upper surface of the film. The value is, for example, <NUM>% or less, preferably <NUM>% or less, more preferably <NUM>% or less, and the lower limit is not particularly limited but is, for example, about <NUM>%.

The graphite thin film forming the pellicle of the present invention has a thermal conductivity along a film plane direction of preferably <NUM> W/mK to <NUM> W/mK, more preferably <NUM> W/mK to <NUM> W/mK, particularly preferably <NUM> W/mK to <NUM> W/mK, from the viewpoint of releasing heat generated in the pellicle not to excessively raise the temperature. The graphite thin film having a thermal conductivity along the film plane direction of <NUM> W/mK or more can efficiently release heat generated in the pellicle to improve durability of the pellicle.

The graphite thin film forming the pellicle of the present invention preferably has an electrical conductivity of <NUM>,<NUM>/cm to <NUM>,<NUM>/cm.

The graphite thin film forming the pellicle of the present invention preferably has a carrier mobility of <NUM>,<NUM> cm2/Vs to <NUM>,<NUM> cm2/Vs.

The graphite thin film forming the pellicle of the present invention has a tensile strength of preferably <NUM> MPa to <NUM> MPa, more preferably <NUM> MPa to <NUM> MPa, particularly preferably <NUM> to <NUM> MPa. The graphite thin film having a tensile strength of <NUM> MPa or more enables handling such as tense stretching over a frame without damage.

The graphite thin film used in the pellicle of the present invention preferably has a refractive index at <NUM> of <NUM> to <NUM>.

The graphite thin film forming the pellicle of the present invention preferably contains <NUM> to <NUM>% by mole of carbon in entire composition, from the viewpoint of outgas and durability to EUV.

The graphite thin film forming the pellicle of the present invention preferably has an excellent crystal state. For example, smaller Raman spectrum intensity ratio I (D)/I (G) (<NUM> or less, more preferably <NUM>) means a smaller proportion of the amorphous structure and an excellent crystal state. The crystal state is desirably a monocrystal state or a polycrystalline state. The graphite thin film having a monocrystal structure is preferable in terms of its high film strength and high thermal conductivity. On the other hand, a polycrystalline graphite thin film is easily produced and preferable in terms of costs.

The pellicle of the present invention has excellent durability to EUV as evaluated in examples described later.

The method for producing the graphite thin film is not particularly limited, and examples of the method include a polymer film baking method, an arc-discharge method using a carbon solid as a raw material, a plasma CVD method using a hydrocarbon-based gas as a raw material, and a plasma jet method using a methane gas as a raw material and performed in vacuum.

The method for producing the graphite thin film forming the pellicle of the present invention and having a smooth surface is preferably a polymer baking method. For example, the graphite thin film having a smooth surface, i.e., having its surface roughness adjusted to <NUM> or more and <NUM> or less is obtained by subjecting a polymer film such as an aromatic polyimide to a carbonization heat treatment to give a carbonized film and subjecting the carbonized film to a graphitization heat treatment under tension. A specific method for applying a tension to the carbonized film is, for example, sandwiching the carbonized film between pieces of graphite or placing a weight on an end portion of the carbonized film, and these procedures may be performed in at least a part of the graphitization heat treatment (baking).

Further, it is preferable to subject the polymer film to the carbonization heat treatment under tension and thus obtain a carbonized film and to subject the carbonized film to the graphitization heat treatment under tension.

The carbonization heat treatment may be performed at about <NUM> to <NUM> for <NUM> to <NUM> minutes in an atmosphere of an inert gas such as nitrogen, argon, or a mixed gas of argon and nitrogen. A heating rate to the carbonization heat treatment temperature is not particularly limited but is, for example, <NUM>/min or more and <NUM>/min or less. After the carbonization heat treatment, the carbonized film may be cooled to room temperature by natural cooling or the like.

The carbonized film obtained by the carbonization heat treatment is subjected to a graphitization heat treatment such that the carbonized film is set in a high-temperature furnace and heat-treated (baked) at <NUM> or more (preferably <NUM> or more) for <NUM> to <NUM> minutes in an inert gas such as an argon gas or a mixed gas of an argon gas and a helium gas. The heat treatment temperature in the graphitization is more preferably <NUM> or more, further preferably <NUM> or more. The upper limit is not particularly limited but is, for example, <NUM> or less. In the graphitization, it is important to heat-treat the carbonized film with applying tension, for example, by placing a weight on an end portion of the carbonized film. These procedures can obtain the graphite thin film having its surface roughness adjusted. A heating rate to the graphitization heat treatment temperature is not particularly limited but is, for example, <NUM>/min or more and <NUM>/min or less.

The polymer preferably used as a raw material for the graphite thin film is an aromatic polymer, and the aromatic polymer is preferably at least one selected from polyamide, polyimide, polyquinoxaline, polyparaphenylene vinylene, polyoxadiazole, polybenzimidazole, polybenzoxazole, polybenzthiazole, polyquinazolinedione, polybenzoxazinone, polyquinazolone, a benzimidazobenzophenanthroline ladder polymer, or derivatives of these polymers. The films formed of these polymers may be produced by known production methods. As a particularly preferable polymer, an aromatic polyimide, polyparaphenylene vinylene, and polyparaphenylene oxadiazole can be exemplified. Particularly, an aromatic polyimide is preferable. Especially, an aromatic polyimide is particularly preferable that is prepared through polyamic acid obtained from a dianhydride (particularly an aromatic dianhydride) and a diamine (particularly an aromatic diamine), which are described below.

Examples of the dianhydride that can be used for synthesis of the aromatic polyimide include pyromellitic dianhydride, <NUM>,<NUM>,<NUM>,<NUM>-naphthalenetetracarboxylic dianhydride, <NUM>,<NUM>',<NUM>,<NUM>'-biphenyltetracarboxylic dianhydride, <NUM>,<NUM>,<NUM>,<NUM>-naphthalenetetracarboxylic dianhydride, <NUM>,<NUM>',<NUM>,<NUM>'-biphenyltetracarboxylic dianhydride, <NUM>,<NUM>',<NUM>,<NUM>'-benzophenonetetracarboxylic dianhydride, <NUM>,<NUM>-bis(<NUM>,<NUM>-dicarboxyphenyl)propane dianhydride, <NUM>,<NUM>,<NUM>,<NUM>-perylenetetracarboxylic dianhydride, bis(<NUM>,<NUM>-dicarboxyphenyl)propane dianhydride, <NUM>,<NUM>-bis(<NUM>,<NUM>-dicarboxyphenyl)ethane dianhydride, <NUM>,<NUM>-bis(<NUM>,<NUM>-dicarboxyphenyl)ethane dianhydride, bis(<NUM>,<NUM>-dicarboxyphenyl)methane dianhydride, bis(<NUM>,<NUM>-dicarboxyphenyl)ethane dianhydride, oxydiphthalic dianhydride, bis(<NUM>,<NUM>-dicarboxyphenyl)sulfone dianhydride, p-phenylene bis(trimellitic monoester anhydride), ethylene bis(trimellitic monoester anhydride), bisphenol A bis(trimellitic monoester anhydride), and analogs thereof. The compounds can be used singly or as a mixture in any ratio. Pyromellitic dianhydride and <NUM>,<NUM>'<NUM>,<NUM>'-biphenyltetracarboxylic dianhydride are particularly preferable, particularly from the viewpoint of the fact that as the polyimide film has a very stiffer polymer structure, the polyimide film is more highly oriented and from the viewpoint of availability.

Examples of the diamine that can be used for synthesis of the aromatic polyimide include <NUM>,<NUM>'-diaminodiphenylpropane, <NUM>,<NUM>'-diaminodiphenylmethane, benzidine, <NUM>,<NUM>'-dichlorobenzidine, <NUM>,<NUM>'-diaminodiphenyl sulfide, <NUM>,<NUM>'-diaminodiphenyl sulfone, <NUM>,<NUM>'-diaminodiphenyl sulfone, <NUM>,<NUM>'-diaminodiphenyl ether, <NUM>,<NUM>'-diaminodiphenyl ether, <NUM>,<NUM>'-diaminodiphenyl ether, <NUM>,<NUM>-diaminonaphthalene, <NUM>,<NUM>'-diaminodiphenyldiethylsilane, <NUM>,<NUM>'-diaminodiphenylsilane, <NUM>,<NUM>'-diaminodiphenylethylphosphineoxide, <NUM>,<NUM>'-diaminodiphenyl N-methylamine, <NUM>,<NUM>'-diaminodiphenyl N-phenylamine, <NUM>,<NUM>-diaminobenzene (p-phenylenediamine), <NUM>,<NUM>-diaminobenzene, <NUM>,<NUM>-diaminobenzene, and analogs thereof. The compounds can be used singly or as a mixture in any ratio. The aromatic polyimide is particularly preferably synthesized using <NUM>,<NUM>'-diaminodiphenyl ether or p-phenylenediamine as a raw material, from the viewpoint of further increasing the orientation of the polyimide film, and availability.

When the polymer is an aromatic polyimide, the production method may be either a thermal curing method or a chemical curing method. The thermal curing method is a method in which polyamic acid as a precursor is converted into an imide by heating. The chemical curing method is a method in which polyamic acid is converted into an imide using a dehydrating agent represented by an anhydride such as acetic anhydride or using a tertiary amine such as picoline, quinoline, isoquinoline, or pyridine, as an imidization accelerator. The chemical curing method is preferable, because the resultant film easily have a small linear coefficient of expansion, a high elasticity, and a large birefringence index, and the film is not damaged and have a good-quality even when a tension is applied to the film during baking. The chemical curing method is excellent also in terms of improving the degree of thermal conductance of the graphite film.

The polyamic acid is usually produced by dissolving at least one dianhydride and at least one diamine in an organic solvent, and stirring the resultant organic solvent solution of polyamic acid under controlled temperature conditions until the polymerization of the dianhydride and the diamine is completed. These solutions of polyamic acid are obtained at a concentration of usually <NUM>% by mass or more (preferably <NUM>% by mass or more) and <NUM>% by mass or less, preferably <NUM>% by mass or more and <NUM>% by mass or less. The solution of polyamic acid at a concentration of this range can give an appropriate molecular weight and appropriate viscosity of the solution. It is preferable to set the dianhydride and the diamine in the raw-material solution at substantially equimolar amounts, and the mole ratio of the dianhydride to the diamine (dianhydride/diamine) is, for example, <NUM>/<NUM> or less and <NUM>/<NUM> or more, preferably <NUM>/<NUM> or less and <NUM>/<NUM> or more, more preferably <NUM>/<NUM> or less and <NUM>/<NUM> or more.

The polyimide film is produced by casting the polyimide precursor, the organic solvent solution of polyamic acid onto a support such as an endless belt or a stainless-steel drum, and drying and imidizing the solution. Specifically, the method for producing the film by chemical curing is as follows. First, a dehydrating agent of a stoichiometric amount or more and an imidization accelerator of a catalytic amount are added to the solution of polyamic acid, and the mixture is cast or applied onto a supporting plate, an organic film (e.g., PET), or a support (e.g., a drum or an endless belt) to form a film shape and the organic solvent is evaporated to give a film having self-supportability. Next, the film is further imidized while heated and dried, to give a polyimide film. The temperature in the heating is preferably from <NUM> to <NUM>. The polymer film preferably has a thickness of, for example <NUM> to <NUM>.

Further, the pellicle of the present invention may be formed of a stacked body in which the graphite thin film and another film are stacked. The pellicle formed of the stacked body can give heat resistance and hydrogen radical resistance.

The other film is not particularly limited but is, for example, a film formed of SiOx (x ≤ <NUM>), SixNy (x/y is <NUM> to <NUM>), SiO, SiC, N, Y<NUM>O<NUM>, YN, Mo, Ru, Rb, Sr, Y, Zr, Nb, or Rh, from the viewpoint of a stable material with respect to EUV.

The pellicle of the present invention may be a pellicle formed of a composite including the graphite thin film and a pellicle frame. A method for tensely stretching the pellicle over the pellicle frame is not particularly limited. For example, the method may be a method using a film adhesive material layer or a method for tensely stretching the pellicle by mechanically sandwiching the pellicle from above and below.

A shape of the pellicle frame does not matter as long as an area of the pellicle enough for covering a photomask can be secured. The shape may be a circle or an oval, or may be a polygonal shape such as a tetragon, or another different shape. The pellicle frame that is polygonal may have roundish corners. The pellicle frame may also have a ventilation hole for equalizing its atmospheric pressure with the atmospheric pressure in an EUV exposure device.

A material for the pellicle frame is not limited as long as the frame allows tense stretching of the pellicle. Examples of the material include single metals such as silicon, aluminum, and stainless steel, alloys, graphite, and ceramic.

The film adhesive is, for example, an acrylic resin adhesive, an epoxy resin adhesive, a polyimide resin adhesive, or a silicone resin adhesive. The film adhesive preferably causes less outgassing, from the viewpoint of retaining the degree of vacuum during EUV exposure. As a method for evaluating the outgassing, for example, a thermal desorption spectrometer can be used. As the pellicle and the pellicle frame are heated by irradiation with EUV, the film adhesive preferably has high heat resistance.

Hereinafter, the present invention is more specifically described with reference to examples.

The graphite thin films and the pellicles (pellicle composites) obtained in the examples below were measured by the following methods.

A film thickness of the film was calculated from the EUV transmittance measured by the following method. The EUV transmittance is obtained by scanning a surface of the film according to the following measurement method. A relationship between the EUV transmittance (T) and the film thickness of the graphite thin film is represented by the following equation with use of <NUM>-nm-wavelength light transmittance (<NUM>) of monolayer graphene and the film thickness (<NUM>) of the monolayer graphene.

In the present invention, the surface roughness (Sa) of the film was obtained by measurement with a laser microscope and calculation based on ISO <NUM>. The laser microscope was set at a magnification of <NUM> times and a cutoff value (λc) of <NUM> pm. The measurement position of the surface roughness (Sa) is not particularly limited, but it is desirable to measure surface roughness at a plurality of locations including one central portion and four end portions and define the average of values at the locations as the surface roughness (Sa).

The measurement of EUV transmittance was performed as follows. The pellicle was irradiated with <NUM>-nm-wavelength light (EUV) using EUV irradiation equipment (NewSUBARU (facility name) BL-<NUM>, University of Hyogo). EUV was irradiated so as to scan a surface of the film with setting a light source intensity at several dozen µW/cm<NUM> and setting an irradiation direction of EUV vertical to the film surface, and the EUV transmittance was measured.

The uniformity in EUV transmittance is represented by a value of three times (3o) the standard deviation of the EUV transmittance obtained by scanning a surface of the film. Smaller value of 3σ means better uniformity in EUV transmittance.

The pellicle was irradiated with EUV having a wavelength of <NUM> and a light source intensity of approximately <NUM> mW/cm<NUM> for <NUM> hours using EUV irradiation equipment (NewSUBARU (facility name) BL-<NUM>, University of Hyogo). The durability to EUV was evaluated by comparing (<NUM>) the appearance, (<NUM>) the XAFS at the C-K edge, or (<NUM>) the Raman spectrum, between before and after the durability test.

The dimensions and the film thickness of the graphite thin film were measured, and the volume (cm<NUM>) of the graphite thin film was calculated. Separately, the mass (g) of the graphite thin film was measured, and the density of the graphite thin film was calculated by the equation density (g/cm<NUM>) = mass (g)/volume (cm<NUM>).

The electrical conductivity of the graphite thin film was measured by the van der Pauw method. This method is the most appropriate method for measuring the electrical conductivity of a thin film-shaped sample. The details of the measurement method is described in <NPL>). This approach is characterized by a point that electrodes are arranged at any four points of end portions in a thin film sample having any shape, to allow the measurement of the electrical conductivity and that accurate measurement can be performed when the sample has a uniform thickness. In the present invention, the measurement was performed by using a <NUM> × <NUM> sample and attaching a silver paste electrode to each of four corners of the sample. The measurement was performed using Resistivity/DC & AC Hall measurement system, ResiTest <NUM> manufactured by TOYO Corporation.

The graphite thin film was cut out into a size of <NUM> × <NUM>, and the cut-out film was reinforced at both ends with a <NUM>-pm-thick polyimide tape. The measurement sample prepared was set in a vertical motorized test stand (EMX-1000N manufactured by IMADA CO. The tensile speed was set at <NUM>/min, and the tensile strength was measured by a digital force gauge (ZTA-5N manufactured by IMADA CO.

The proportion of carbon in the graphite thin film prepared was measured using a scanning electron microscope (SEM) SU4600 manufactured by Hitachi High-Tech Corporation and a large-diameter silicon drift detector (SDD) (EDX-XMax) manufactured by HORIBA, Ltd. The proportion of carbon was determined by carbon-atom concentration (%) calculated after assaying the graphite thin film at an accelerating voltage of <NUM> kV and analyzing the assay with accompanying software.

The crystal state of the film was evaluated by the ratio (I (D)/I (G)) between the G-band intensity (I (G)) and the D-band intensity (I (D)) in a Raman spectrum. The film having a ratio I (D)/I (G) of <NUM> or less is evaluated as having an excellent graphite crystal with less defects. The ratio I (D)/I (G) is preferably <NUM>.

The Raman intensity was measured with a laser Raman microscope. The measurement position is not particularly limited, but it is desirable to measure values at a plurality of locations including one central portion and four end portions and use the average of respective values of the G-band intensity (I (G)) and the D-band intensity (I (D)) at the locations.

Hereinafter, polyimide films (A-<NUM>) to (A-<NUM>), which were raw materials for the graphite thin film, were produced in Production Examples <NUM>-<NUM> to <NUM>-<NUM>.

A <NUM>% by mass DMF (dimethylformamide) solution of polyamic acid synthesized from pyromellitic dianhydride, <NUM>,<NUM>'-diaminodiphenyl ether, and p-phenylenediamine in the mole ratio of <NUM> : <NUM> :<NUM> was synthesized, and the solution was applied onto a metal substrate with a spin coater. The stacked body including the metal foil and the solution of polyamic acid was heated at <NUM>, <NUM>, <NUM> for <NUM> seconds each, and then the metal foil was peeled to prepare a polyimide film (A-<NUM>) that was an <NUM>-cm-diameter circle and had a film thickness of <NUM>.

A polyimide film (A-<NUM>) that was an <NUM>-cm-diameter circle and had a film thickness of <NUM> was prepared in the same manner as in Production Example <NUM>-<NUM> except that the concentration of the polyamic acid was set at <NUM>% by mass.

A polyimide film (A-<NUM>) that was an <NUM>-cm-diameter circle and had a film thickness of <NUM> was prepared in the same manner as in o Production Example <NUM>-<NUM> except that the concentration of the polyamic acid was set at <NUM>% by mass.

Next, a graphite thin film was prepared using any one of the polyimide films (A-<NUM>) to (A-<NUM>) in Production Examples <NUM>-<NUM> to <NUM>-<NUM>.

The polyimide film (A-<NUM>) obtained in Production Example <NUM>-<NUM> was sandwiched between graphite sheets and heated to <NUM> at a rate of <NUM>/min using an electric furnace in a nitrogen gas atmosphere, kept at <NUM> for <NUM> minutes, and then naturally cooled to give a carbonized film. A weight was placed on an end portion of the resultant carbonized film, and the carbonized film was heated under tension to <NUM> at a rate of <NUM>/min in an argon gas atmosphere, kept at <NUM> for <NUM> minutes, and then naturally cooled to give a graphite thin film (B-<NUM>) having an about <NUM> to <NUM>-cm-diameter circular shape.

The film thickness of the graphite thin film (B-<NUM>) measured by the EUV transmittance was <NUM>, and the surface roughness (Sa) of the graphite thin film (B-<NUM>) was <NUM>. The surface state of the graphite thin film (B-<NUM>) was a mirrored surface. The electrical conductivity of the graphite thin film (B-<NUM>) was <NUM>,<NUM>/cm.

<FIG> shows a photograph obtained by observing a section of the graphite thin film (B-<NUM>) with a TEM. Based on <FIG>, multilayer graphene, in which each graphene is stacked one another in a state of streaks can be confirmed, having a film thickness of <NUM> or more, so that the graphite thin film (B-<NUM>) is found to have a graphite structure.

A graphite thin film (B-<NUM>) was obtained in the same manner as in Production Example <NUM>-<NUM> except that the polyimide film (A-<NUM>) obtained in Production Example <NUM>-<NUM> was used replacing the polyimide film (A-<NUM>) obtained in Production Example <NUM>-<NUM>.

The film thickness of the graphite thin film (B-<NUM>) measured by the EUV transmittance was <NUM>. The surface state of the graphite thin film (B-<NUM>) was a mirrored surface.

The film thickness of the graphite thin film (B-<NUM>) measured by the EUV transmittance was more than <NUM>. The surface state of the graphite thin film (B-<NUM>) was a mirrored surface.

The polyimide film (A-<NUM>) obtained in Production Example <NUM>-<NUM> was sandwiched between graphite sheets and heated to <NUM> at a rate of <NUM>/min using an electric furnace in a nitrogen gas atmosphere, kept at <NUM> for <NUM> minutes, and then naturally cooled to give a carbonized film. The resultant carbonized film was heated to <NUM> at a rate of <NUM>/min in an argon gas atmosphere, kept at <NUM> for <NUM> minutes, and then naturally cooled to give a graphite thin film (B-<NUM>).

The film thickness of the graphite thin film (B-<NUM>) measured by the EUV transmittance was <NUM>. A surface of the graphite thin film (B-<NUM>) had wrinkles.

The film thickness of the graphite thin film (B-<NUM>) measured by the EUV transmittance was <NUM>, and the surface roughness (Sa) of the graphite thin film (B-<NUM>) was <NUM>. A surface of the graphite thin film (B-<NUM>) had wrinkles.

A graphite thin film (B-<NUM>) was obtained in the same manner as in Production Example <NUM>-<NUM> except that the polyimide film (A-<NUM>) obtained in Production Example <NUM>-<NUM> was used replacing the polyimide film (A-<NUM>) obtained in Production Example <NUM>-<NUM>. The tensile strength of the graphite thin film (B-<NUM>) was <NUM> MPa. The graphite thin films (B-<NUM>) to (B-<NUM>) that were prepared in the same manner using the same raw material as the graphite thin film (B-<NUM>) are considered to also have tensile strength equivalent to the tensile strength of the graphite thin film (B-<NUM>).

Hereinafter, pellicles formed of a graphite thin film and a pellicle frame were prepared using the graphite thin films (B-<NUM>) to (B-<NUM>).

The graphite thin film (B-<NUM>) whose surface state was a mirrored surface and which had a film thickness of <NUM> was tensely stretched over a bottomless square jig to form a pellicle. The jig had an outside dimension of <NUM> square, an inside dimension of <NUM> square, and a height of <NUM>, and an A5052 alloy that was less likely to cause outgassing was used as the material for the jig. The pellicle was fixed using an epoxy resin adhesive.

The pellicle including the graphite thin film (B-<NUM>) had an EUV transmittance of <NUM>%, which was measured by the method described above, and had a uniformity in EUV transmittance of <NUM>%.

A pellicle was obtained in the same manner as in Example <NUM> except that the graphite thin film (B-<NUM>) was used replacing the graphite thin film (B-<NUM>).

The pellicle including the graphite thin film (B-<NUM>) had an EUV transmittance of <NUM>% and a uniformity in EUV transmittance of <NUM>%.

The pellicle including the graphite thin film (B-<NUM>) had an EUV transmittance of <NUM>%.

Tables <NUM> and <NUM> show the measurement results of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM>.

*The value in the parenthesis is an intensity ratio of Raman spectrum after EUV irradiation.

The measured surface roughness Sa of the graphite thin film in Example <NUM> was <NUM>. The graphite thin films in Examples <NUM> and <NUM> had a mirrored surface state very similar to the surface state in Example <NUM>, so that the graphite thin films in Examples <NUM> and <NUM> are considered to have the surface roughness Sa equivalent to the value in Example <NUM>. Thus, Examples <NUM> to <NUM> satisfying the requirement of the present invention could achieve both an excellent EUV transmittance (<NUM>% or more) and an excellent uniformity in EUV transmittance (<NUM>% or less).

The pellicles in Examples <NUM> to <NUM> all had excellent durability to EUV. Further, <FIG> shows the result of evaluating the durability to EUV of the pellicle in Comparative Example <NUM> by the C-K edge XAFS measurement. According to <FIG>, a change is hardly observed in the spectrum between before and after the irradiation with EUV, and the pellicle had excellent durability. The graphite thin films (B-<NUM>) to (B-<NUM>) that are thinner than the graphite thin film (B-<NUM>) have small absorption of EUV and are thus less likely to be affected by EUV. Accordingly, the graphite thin films (B-<NUM>) to (B-<NUM>) are said to have durability equivalent to or greater than the durability of the graphite thin film (B-<NUM>).

On the other hand, the graphite thin films in Comparative Examples <NUM> and <NUM> having a large film thickness had an excellent uniformity in EUV transmittance but had a low EUV transmittance. As the graphite thin films in Comparative Examples <NUM> and <NUM> have an excessively large film thickness, EUV didn't transmit through the films. Further, Comparative Examples <NUM> to <NUM> produced without application of a tension to the carbonized film during the graphitization heat treatment generated wrinkles on the surfaces of the graphite thin films. Because of the facts that the surface roughness Sa measured in Comparative Example <NUM> was <NUM>, which did not satisfy the requirement of the present invention and that the surface states in Comparative Examples <NUM>, <NUM>, and <NUM> were very similar to the surface state in Comparative Example <NUM>, the surface roughness in Comparative Examples <NUM>, <NUM>, and <NUM> are considered to be equivalent to the value in Comparative Example <NUM>, and Comparative examples <NUM> to <NUM> all had a poor uniformity in EUV transmittance.

Claim 1:
A pellicle comprising a graphite thin film,
wherein the graphite thin film has a film thickness of <NUM> or more and <NUM> or less,
characterized in that the graphite thin film has a ratio (I (D)/I (G)) of a D-band intensity (I (D)) to a G-band intensity (I (G)) in a Raman spectrum of <NUM> or more and <NUM> or less, and
the graphite thin film has a surface roughness Sa of <NUM> or more and <NUM> or less, wherein surface roughness Sa is the arithmetic average height obtained on the basis of ISO <NUM>.