Method for manufacturing optical thin film

Abstract

To provide a method for manufacturing an optical thin film.SOLUTION: A method for manufacturing an optical thin film includes: preparing a first thin film forming material which is a solid containing at least one kind selected from a group consisting of indium oxide and zinc sulfide, and a second thin film forming material which is a solid containing silicon oxide; separately arranging the first thin film forming material and the second thin film forming material; and heating the first thin film forming material and the second thin film forming material by a vapor deposition method in a non-oxidative atmosphere to form a vapor deposition film on an object on which a film is formed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing an optical thin film. [Background technology]

[0002] The lens surfaces of cameras and telescopes are coated with thin films to reduce reflected light. When attempting to achieve a reflected light reduction effect with a single-layer film, it is effective to form an optical thin film on the outermost surface of the object to be coated, the optical thin film having a refractive index close to the square root of the refractive index of the object to be coated. To achieve an anti-reflection effect over a wide wavelength range, a multilayer film must be formed. Even with such a multilayer film, an optical thin film with a refractive index lower than the square root of the object to be coated is required to enhance the anti-reflection effect against obliquely incident light.

[0003] In order to obtain a thin film with a refractive index lower than the square root of the area to be coated, it is useful to include voids with a refractive index of 1.0 or higher within the thin film, and optical thin films containing air have been proposed using various methods, including the sol-gel method. For example, Patent Document 1 describes an anti-reflection coating comprising, in order from the glass substrate side, a first phase mainly composed of aluminum oxide formed by vacuum deposition, a second layer formed by vacuum deposition and made of at least one material selected from the group consisting of MgF2 and SiO2, and a third layer formed on the second layer and made of an aggregate of mesoporous silica nanoparticles. Patent Document 2 also describes an antireflection coating that includes, on a substrate, an inorganic underlayer made of an inorganic material, a surface modification layer containing an inorganic oxide such as SiO2, an adhesion layer containing a binder such as an acrylic resin laminated on the surface modification layer, and a low refractive index phase in which hollow silica particles are bound by the binder. Patent Document 3 describes a method for producing an optical thin film in which a mixed solution of a sol liquid in which magnesium fluoride (MgF2) particles are dispersed and an amorphous silicon oxide binder solution is mixed, the mixed solution being applied to a substrate and heat-treated, so that the substrate and the MgF2 particles are bound together by the amorphous silicon oxide binder and multiple voids are formed between the MgF2 particles. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-38948 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-222450 [Patent Document 3] International Publication No. 2006 / 030848 Summary of the Invention [Problem to be solved by the invention]

[0005] When forming an anti-reflection film or an optical thin film by a sol-gel method, foreign matter may easily get mixed into the film. If foreign matter gets mixed into the film or defects occur, the mixed foreign matter or defects may absorb light, and the light transmittance of the optical element on which the anti-reflection film or optical thin film is formed may decrease. Even if the light transmittance does not decrease, there are cases where it is better that the surface of the formed anti-reflection film or optical thin film is free of metal that was deposited during film formation. An object of one aspect of the present invention is to provide a method for producing an optical thin film that can reduce the presence of metal on the surface of the optical thin film. [Means for solving the problem]

[0006] A first aspect is a method for manufacturing an optical thin film, which includes preparing a first thin film forming material, which is a solid containing at least one selected from the group consisting of indium oxide and zinc sulfide, and a second thin film forming material, which is a solid containing silicon oxide; arranging the first thin film forming material and the second thin film forming material at a distance from each other; and heating the first thin film forming material and the second thin film forming material by a vapor deposition method in a non-oxidizing atmosphere to form a vapor-deposited film on an object to be formed. [Effects of the Invention]

[0007] An object of one aspect of the present invention is to provide a method for producing an optical thin film that can reduce the presence of metal on the surface of the optical thin film. [Brief explanation of the drawings]

[0008] [Figure 1] 1 shows arrangement form A of the first thin film forming material and the second thin film forming material in a plan view. [Figure 2] 1 shows arrangement form B of the first thin film forming material and the second thin film forming material in a plan view. [Figure 3] 1 shows arrangement form C of the first thin film forming material and the second thin film forming material in a plan view. [Figure 4] 10 is an SEM photograph of a cross section of an optical thin film according to Example 3. [Figure 5] 1 shows an arrangement form D of the first thin film forming material and the second thin film forming material in a plan view. DETAILED DESCRIPTION OF THE INVENTION

[0009] The following describes an embodiment of the method for producing an optical thin film according to the present disclosure. However, the embodiment described below is merely an example for embodying the technical concept of the present invention, and the present invention is not limited to the method for producing an optical thin film described below.

[0010] A method for manufacturing an optical thin film according to one embodiment includes preparing a first thin film forming material, which is a solid material containing at least one selected from the group consisting of indium oxide and zinc sulfide, and a second thin film forming material, which is a solid material containing silicon oxide; arranging the first thin film forming material and the second thin film forming material at a distance from each other; and heating the first thin film forming material and the second thin film forming material by vapor deposition in a non-oxidizing atmosphere to form a vapor-deposited film on an object to be formed.

[0011] The first thin film-forming material is a solid containing at least one selected from the group consisting of indium oxide and zinc sulfide. In this specification, the at least one selected from the group consisting of indium oxide and zinc sulfide contained in the first thin film-forming material may be referred to as a raw material contained in the first thin film-forming material. The first thin film-forming material preferably contains indium oxide or zinc sulfide. The first thin film-forming material more preferably contains indium oxide. The first thin film-forming material further preferably contains indium oxide without substantially containing zinc sulfide. The first thin film-forming material is preferably a solid formed from indium oxide powder or zinc sulfide powder.

[0012] The indium oxide contained in the first thin film-forming material is preferably indium(III) oxide (In2O3). Indium(III) oxide (In2O3) may contain unavoidable impurities. The content of indium(III) oxide (In2O3) in the indium(III) oxide (In2O3) contained in the first thin film-forming material is preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 99 mass% or more. The first thin film-forming material may be made of indium(III) oxide or may be made of indium(III) oxide and unavoidable impurities. The first thin film-forming material is preferably a solid formed from powder made of indium(III) oxide which may contain unavoidable impurities.

[0013] The zinc sulfide contained in the first thin film-forming material preferably has a zinc sulfide content of 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more. The first thin film-forming material is preferably a solid formed from sulfide powder. The first thin film-forming material may consist of zinc sulfide, or may consist of zinc sulfide and unavoidable impurities. The first thin film-forming material is preferably a solid formed from zinc sulfide powder that may contain unavoidable impurities.

[0014] The second thin film forming material is a solid containing silicon oxide. The second thin film forming material is preferably a solid formed by molding silicon oxide powder. The silicon oxide contained in the second thin film forming material is preferably silicon monoxide (SiO) as a main component. In this specification, "mainly silicon monoxide (SiO)" means that the content of silicon monoxide (SiO) in the silicon oxide is 50 mass% or more. The content of silicon monoxide (SiO) in the silicon oxide is preferably 80 mass% or more, more preferably 90 mass% or more, and even more preferably 99 mass% or more. The second thin film forming material may be composed of silicon oxide containing silicon monoxide (SiO) as a main component, or may be composed of silicon oxide containing silicon monoxide (SiO) as a main component and unavoidable impurities. Silicon oxide containing silicon monoxide (SiO) as a main component may have the remainder other than silicon monoxide as silicon dioxide (SiO2). The second thin film forming material is preferably a solid formed from powder of silicon oxide containing silicon monoxide (SiO) as the main component.

[0015] In a method for manufacturing an optical thin film, a first solid thin film-forming material and a second solid thin film-forming material are placed at a distance from each other, and the first and second thin film-forming materials are heated by vapor deposition in a non-oxidizing atmosphere to evaporate the first and second thin film-forming materials, causing components contained in the gaseous first thin film-forming material and components contained in the gaseous second thin film-forming material to react in the gas phase, forming a deposited film on the substrate. By placing the first solid thin film-forming material and the second solid thin film-forming material at a distance from each other, the components contained in the first and second thin film-forming materials do not react in the solid or liquid phases during heating by vapor deposition. Therefore, the metals dissociated from the components contained in the first thin film-forming material in the gas phase are quite small, and even if the deposited film contains metals dissociated from the first thin film-forming material, the metals can be easily eluted from the deposited film by contacting the deposited film with a first acidic solution.

[0016] In disposing the first thin film material and the second thin film material, the distance separating the first thin film material and the second thin film material is preferably within the range of 1 mm to 4 mm. If the distance separating the first thin film material and the second thin film material is within the range of 1 mm to 4 mm, when heated by vapor deposition, the first thin film material and the second thin film material do not come into contact with each other, so they do not react in the solid or liquid phase, and the generation of metals that are easily dissociated from the first thin film material can be suppressed. In disposing the first thin film material and the second thin film material, the distance separating the first thin film material and the second thin film material is sufficient as long as the first thin film material and the second thin film material do not come into contact with each other and can be easily heated evenly, more preferably within the range of 1 mm to 3 mm, and even more preferably within the range of 1 mm to 2 mm.

[0017] When the first thin film-forming material contains indium (III) oxide (In2O3), the indium (III) oxide (In2O3) contained in the first thin film-forming material dissociates into indium (I) oxide (In2O), indium (In), and oxygen (O) upon heating. The oxygen (O) dissociated from the indium (III) oxide (In2O3) contained in the first thin film-forming material reacts with silicon monoxide (SiO) contained in the second thin film-forming material, as described below, to form silicon dioxide (SiO2), which, together with the dissociated indium (I) oxide (In2O), forms a vapor-deposited film and is deposited on the substrate. When indium (III) oxide (In2O3) is heated, minute indium (In) particles are also generated in the vapor phase, which, together with indium (I) oxide (In2O) and silicon dioxide (SiO2), forms a vapor-deposited film and is deposited on the substrate. Indium (In) dissociated in the gas phase is tiny, much smaller than metallic indium (In) dissociated in the solid phase. Therefore, when the deposited film is contacted with the first acidic solution, metallic indium (In) dissolves from the deposited film along with indium oxide (I) (In2O). For example, when indium oxide (III) (In2O3) powder is reacted with silicon oxide powder containing silicon monoxide (SiO) as the main component in the solid phase, metallic indium (In) with a diameter of approximately 10 μm may dissociate from the indium oxide (III) (In2O3). When indium oxide (III) (In2O3) is dissociated in the solid first thin film forming material by heating, metallic indium (In) dissociated in the gas phase has a diameter of less than 1 μm, much smaller than metallic indium (In) dissociated in the solid phase.

[0018] Silicon monoxide (SiO) contained in the second thin film forming material has a lower standard free energy of formation than indium(I) oxide (In2O), so it preferentially reacts with oxygen (O) in the gas phase to produce silicon dioxide (SiO2). When indium(III) oxide (In2O3) is contained in the first thin film forming material, the indium(III) oxide (In2O3) contained in the first thin film forming material and the silicon monoxide (SiO) contained in the second thin film forming material react in the gas phase when heated by vapor deposition, and a vapor deposition film containing silicon dioxide (SiO2) and indium(I) oxide (In2O) is deposited on the substrate. In a non-oxidizing atmosphere, oxygen (O) dissociated from indium (III) oxide (In2O3) contained in the first thin film-forming material reacts preferentially with silicon monoxide (SiO) to produce silicon dioxide (SiO2), so that almost no black silicon monoxide (SiO) remains in the vapor-deposited film. A thin film (optical thin film) obtained by contacting a vapor-deposited film with an acidic solution does not absorb visible light due to black silicon monoxide (SiO). In this specification, a thin film obtained by contacting a vapor-deposited film with an acidic solution is also referred to as an optical thin film. A thin film obtained by contacting a vapor-deposited film with an acidic solution is also referred to as a "thin film (optical thin film)." The acidic solution includes the first acidic solution or the second acidic solution described below. Oxygen (O) dissociated from indium (III) oxide (In2O3) contained in the first thin film-forming material preferentially reacts with silicon monoxide (SiO) to produce silicon dioxide (SiO2), thereby suppressing the further oxidation of dissociated indium (I) oxide (In2O) to produce indium (III) oxide (In2O3). Indium (I) oxide (In2O) contained in the vapor-deposited film is highly soluble in acidic substances. Therefore, by contacting the vapor-deposited film with an acidic solution containing an acidic substance, indium (I) oxide (In2O) and minute metallic indium (In) are preferentially dissolved, resulting in a thin film (optical thin film) with a void space satisfying the desired refractive index and a silicon dioxide (SiO2) skeleton.

[0019] When the first thin film forming material contains indium oxide (III) (In2O3), the amount of indium (In) gas dissociated from indium oxide (III) (In2O3) in the atmosphere is approximately 3 to 5 volume % ("Thermodynamics of Oxides" by J.S. Kulikov, Nisso Tsushinsha, p. 146, 1987), and a very small amount may be present in the atmosphere when the vapor-deposited film is formed. The standard free energy of formation of indium (In) oxidizing to indium(III) oxide (In2O3) is even lower than the standard free energy of formation of silicon monoxide (SiO) oxidizing to silicon dioxide (SiO2). When the first thin-film-forming material and the second thin-film-forming material are arranged apart, the amount of indium (In) dissociated from indium(III) oxide (In2O3) in the gas phase is very small. Even if indium (In) is contained in the deposited film together with indium(I) oxide (In2O) and silicon dioxide (SiO2) dissociated from indium(III) oxide (In2O3), by contacting the deposited film with the first acidic solution, tiny metallic indium (In) can be eluted along with indium(I) oxide (In2O).

[0020] When the raw material for the first thin film or the second thin film is a powder, the first thin film or the second thin film may be a molded body obtained by molding the powder or a sintered body obtained by further firing the molded body. The sintered body is likely to be vaporized approximately uniformly when heated by vapor deposition, and a vapor-deposited film is likely to be deposited approximately uniformly on the surface of the substrate.

[0021] When the first thin film-forming material is a sintered body, the compact obtained by compacting the powder may be fired in an inert atmosphere, as described below, or in an atmosphere containing 20% ​​or more by volume of oxygen, such as air (oxygen concentration: approximately 20% by volume, pressure: 101.325 kPa). When the first thin film-forming material is a sintered body, the temperature at which the compact is fired to obtain the first thin film-forming material is preferably in the range of 1000°C to 1600°C, and more preferably in the range of 1100°C to 1500°C. In consideration of production efficiency, the time for firing the compact to obtain the first thin film-forming material is preferably in the range of 1 hour to 15 hours, and more preferably in the range of 3 hours to 12 hours. When the firing temperature and firing time are within the above-mentioned ranges, cracking and chipping of the sintered body are suppressed, and a sintered body that is easily vaporized approximately uniformly when heated by vapor deposition can be obtained.

[0022] When the second thin film-forming material is a sintered body, the compact formed from the powder is preferably fired in an inert atmosphere to prevent the inclusion of excess oxides. In this specification, an inert atmosphere refers to an atmosphere primarily composed of argon (Ar) and helium (He). The inert atmosphere may contain oxygen, and if oxygen is contained, the inert atmosphere has an oxygen concentration of 15% by volume or less. The oxygen concentration in the inert atmosphere may be 10% by volume or less, 5% by volume or less, 1% by volume or less, 0.1% by volume or less, 0.01% by volume or less, 0.001% by volume (10 ppm by volume) or less, or 0.0001% by volume (1 ppm by volume) or more. The temperature at which the molded body to obtain the second thin film forming material is fired is preferably in the range of 1000°C to 1600°C, more preferably in the range of 1100°C to 1500°C, and preferably in the range of 1100°C to 1200°C, so that the components contained in the second thin film forming material are easily dissociated by heating. Taking production efficiency into consideration, the time for firing the molded body to obtain the second thin film forming material is preferably in the range of 1 hour to 10 hours, more preferably in the range of 1 hour to 5 hours, and even more preferably in the range of 1 hour to 3 hours. When the firing temperature and firing time are within the aforementioned ranges, the reaction of the raw materials in the molded body is suppressed, cracking and chipping of the sintered body are suppressed, and a sintered body that is easily vaporized approximately uniformly when heated by vapor deposition is obtained.

[0023] The first thin film-forming material and the second thin film-forming material preferably have a shape that allows the raw material powder to be easily molded, such as a cylindrical, semi-cylindrical, or hollow cylindrical shape. The first thin film-forming material and the second thin film-forming material may also be in the form of cylindrical, semi-cylindrical, or hollow cylindrical pellets.

[0024] When arranging the first thin film forming material and the second thin film forming material, the first thin film forming material and the second thin film forming material may be arranged apart from each other if they have the same shape. For example, when the first thin film forming material and the second thin film forming material are semi-cylindrical, it is preferable to arrange the first thin film forming material and the second thin film forming material so that the straight portions of the semi-circles face each other in a planar view. By arranging the first thin film forming material and the second thin film forming material apart from each other, they do not come into contact with each other, and reaction in the solid or liquid phase is suppressed. Furthermore, when the first thin film forming material and the second thin film forming material are semi-cylindrical, by arranging them so that the straight portions of the semi-circles face each other in a planar view, components dissociated from the raw materials contained in the first thin film forming material by heating in a vapor deposition method and silicon monoxide (SiO), which is contained as a main component in the second thin film forming material, are more likely to react in the gas phase.

[0025] The method may include preparing two first thin film forming materials and one second thin film forming material, and disposing the second thin film forming material between the two first thin film forming materials. When one second thin film forming material is disposed between the two first thin film forming materials and spaced apart from each of the first thin film forming materials, heating by vapor deposition can supply a sufficient amount of oxygen (O) dissociated from the raw materials contained in the two first thin film forming materials in the gas phase to silicon monoxide (SiO) contained as a main component in the second thin film forming material, and a vapor-deposited film containing a sufficient amount of silicon dioxide (SiO2) as a skeleton can be formed approximately uniformly on the substrate.

[0026] It is preferable that the first thin film-forming material is hollow cylindrical, and the second thin film-forming material is cylindrical so that it can be placed in the hollow portion of the hollow cylindrical first thin film. When a cylindrical second thin film-forming material is placed in the hollow portion of the hollow cylindrical first thin film-forming material, spaced apart from the first thin film-forming material, heating by vapor deposition can supply a sufficient amount of oxygen (O) dissociated from the raw materials contained in the two first thin film-forming materials in the gas phase to silicon monoxide (SiO) contained as a main component in the second thin film-forming material, promoting the reaction of silicon monoxide (SiO) contained in the second thin film-forming material in the gas phase and forming a vapor-deposited film containing a sufficient amount of silicon dioxide (SiO2) as the skeleton, approximately uniformly distributed on the substrate.

[0027] In order for the first thin film-forming material to supply oxygen (O) dissociated from the raw materials contained in the first thin film-forming material sufficiently in the gas phase to silicon monoxide (SiO), which is contained as a main component in the second thin film-forming material, it is preferable that the volume of the first thin film-forming material is the same as the volume of the second thin film-forming material or that the volume of the first thin film-forming material is larger than the volume of the second thin film-forming material.

[0028] The volume of the first thin film-forming material is preferably in the range of 1 to 24 times the volume of the second thin film-forming material, more preferably in the range of 1 to 20 times, even more preferably in the range of 1 to 15 times, and particularly preferably in the range of 1 to 12 times. When the volume of the first thin film-forming material is in the range of 1 to 24 times the volume of the second thin film-forming material, oxygen (O) dissociated from the raw materials contained in the first thin film-forming material can be sufficiently supplied to silicon monoxide (SiO) contained as a main component in the second thin film-forming material in the vapor phase, promoting the reaction of silicon monoxide (SiO) contained in the second thin film-forming material in the vapor phase and forming a vapor-deposited film containing a sufficient amount of silicon dioxide (SiO2) as the skeleton, approximately uniformly distributed on the substrate.

[0029] In order for the first thin film-forming material to supply oxygen (O) dissociated from the raw materials contained in the first thin film-forming material sufficiently in the gas phase to silicon monoxide (SiO), which is contained as a main component in the second thin film-forming material, it is preferable that the evaporation surface of the first thin film-forming material be the same as or larger than the evaporation surface of the second thin film-forming material. The evaporation surface of the first thin film-forming material and the evaporation surface of the second thin film-forming material may differ depending on the vapor deposition method. The evaporation surface of the first thin film-forming material and the evaporation surface of the second thin film-forming material may be the upper surface from which components of the first thin film-forming material and the second thin film-forming material are likely to evaporate when heated by a heat source. When the areas of the placement surface and the evaporation surface of the first thin film-forming material and the second thin film-forming material are the same, the area of ​​the placement surface may be used as the area of ​​the evaporation surface.

[0030] The evaporation surface area of ​​the first thin film-forming material is preferably in the range of 1 to 8 times the evaporation surface area of ​​the second thin film-forming material, more preferably in the range of 1 to 7 times, and even more preferably in the range of 1 to 6 times. When the evaporation surface area of ​​the first thin film-forming material is in the range of 1 to 8 times the evaporation surface area of ​​the second thin film-forming material, oxygen (O) dissociated from the raw materials contained in the first thin film-forming material can be sufficiently supplied to silicon monoxide (SiO) contained as a main component in the second thin film-forming material in the vapor phase, promoting the reaction of silicon monoxide (SiO) contained in the second thin film-forming material in the vapor phase and forming a vapor-deposited film containing a sufficient amount of silicon dioxide (SiO2) as the skeleton, approximately uniformly distributed on the substrate.

[0031] The height of the first thin film-forming material may be shorter or longer than the height of the second thin film-forming material. The height of the first thin film-forming material is preferably within a range of 0.5 to 3 times the height of the second thin film-forming material, in order to adequately supply oxygen (O) dissociated from the raw materials contained in the first thin film-forming material to silicon monoxide (SiO), the main component of the second thin film-forming material, in the vapor phase. The height of the first thin film-forming material is more preferably within a range of 0.5 to 2.5 times the height of the second thin film-forming material, and even more preferably within a range of 1 to 2 times the height of the second thin film-forming material. When the area of ​​the evaporation surface of the first thin film-forming material is within the range of 1 to 8 times the area of ​​the evaporation surface of the second thin film-forming material, it is preferable that the height of the first thin film-forming material is within the range of 0.5 to 3 times the height of the second thin film-forming material in order to sufficiently supply oxygen (O) dissociated from the raw materials contained in the first thin film-forming material in the gas phase to silicon monoxide (SiO), which is contained as a main component in the second thin film-forming material.

[0032] The method for manufacturing an optical thin film includes heating a first thin film forming material and a second thin film forming material by vapor deposition in a non-oxidizing atmosphere to form a vapor-deposited film on an object to be film-formed.

[0033] Examples of vapor deposition methods include physical vapor deposition methods such as electron beam evaporation, resistance heating evaporation, ion plating, and sputtering. Electron beam evaporation or resistance heating evaporation is preferred, with electron beam evaporation being more preferred. Electron beam evaporation or resistance heating evaporation can form a uniformly deposited film even on a large surface or a curved surface with a small radius of curvature. Furthermore, electron beam evaporation has good thermal efficiency because it heats the first and second thin film-forming materials by directly irradiating them with an electron beam. Even oxide thin film-forming materials with high melting points and low thermal conductivity can be efficiently vaporized, and the components of the first and second thin film-forming materials can react with the components of the second thin film-forming material in the gas phase and deposit the reactant on the substrate in a relatively short time to form a deposited film. Even when an electron beam is directly irradiated onto the first and second thin film forming materials in the electron beam evaporation method, the first and second thin film forming materials are disposed at a distance from each other, and the components of the first and second thin film forming materials react in the gas phase rather than in the solid or liquid phase. This allows a deposited film to be formed without metals being generated from metal ions contained in the first thin film forming material and adhering to the surface of the deposited film. In the electron beam evaporation method, the deposited film may be formed using ion assistance, or ion-beam assisted deposition (IAD). The ion source for ion beam assistance is preferably an inert gas ion, and examples of the inert gas ion include Ar ions and He ions.

[0034] Since the silicon monoxide (SiO) contained in the second thin film-forming material is a black oxide, if a vapor-deposited film is formed in a non-oxidizing atmosphere, the black silicon monoxide (SiO) will be contained in the vapor-deposited film, and the thin film using this vapor-deposited film may not be usable as an optical thin film. Nevertheless, the reason for forming the vapor-deposited film in a non-oxidizing atmosphere is that silicon monoxide (SiO) reacts preferentially with oxygen to produce silicon dioxide (SiO2), and when the first thin film-forming material contains indium oxide (III) (In2O3), further oxidation of indium oxide (I) (In2O) dissociated from the first thin film-forming material (In2O3) can be suppressed to produce indium oxide (III) (In2O3), which has low solubility in acidic solutions.

[0035] The non-oxidizing atmosphere includes an inert atmosphere, a reducing atmosphere, and a vacuum, and may be one or more of these. An inert atmosphere is an inert atmosphere if the concentration of oxygen contained in the atmosphere is 15% by volume or less, similar to the inert atmosphere used to sinter a molded product obtained by pressing a raw material mixture. A reducing atmosphere is an atmosphere whose main component is a mixed gas containing hydrogen, carbon monoxide, etc. A vacuum is an atmosphere with a pressure of 1.0 x 10 -5 Pa or more 1.0×10 -2 In this specification, a vacuum refers to an atmosphere in which the pressure is 1.0×10 Pa or less without introducing an inert gas such as argon (Ar) gas or helium (He) gas, which is the main component of the inert atmosphere, or a mixed gas containing hydrogen, carbon monoxide, etc., into the atmosphere. -5 Pa or more 1.0×10 -2This refers to an atmosphere in which the pressure is below 100 Pa and the oxygen concentration is below 15% by volume. When the non-oxidizing atmosphere is a vacuum, the majority of the gas components in the atmosphere are water vapor. When the non-oxidizing atmosphere during deposition of a vapor-deposited film is a reducing atmosphere containing a mixed gas containing hydrogen and carbon monoxide, or a vacuum, and the first thin-film-forming material contains indium(III) oxide, even if the molar ratio of indium(III) oxide to silicon monoxide (In2O3 / SiO) becomes relatively high and the amount of oxygen in the vapor generated from the first thin-film-forming material increases, the hydrogen or carbon monoxide contained in the mixed gas in the atmosphere, or the hydrogen contained in the vacuum, oxidizes faster than the indium(I) oxide (In2O) generated from the first thin-film-forming material, thereby suppressing the generation of indium(III) oxide (In2O3) in the vapor-deposited film. If the oxygen concentration in the non-oxidizing atmosphere during deposition of the film is low, the indium oxide (I) (In2O) generated from the first thin film-forming material can be prevented from being re-oxidized by oxygen in the atmosphere due to the low oxygen content in the atmosphere, and the generation of indium oxide (III) in the deposited film can be prevented. The pressure of the non-oxidizing atmosphere during deposition of the film varies depending on the type of deposition method used. When electron beam deposition is used as the deposition method, the atmospheric pressure during deposition of the deposited film on the substrate should be 1.0 x 10 -5 Pa or more 5.0×10 -2 Pa or less, 1.0 × 10 -5 Pa or more 1.0×10 -2 Pa or less is acceptable, 5.0 × 10 -5 Pa or more 1.0×10 -2 The atmospheric pressure during deposition of the deposited film may be 0.1 Pa or less. When the first thin film-forming material contains indium oxide (III) (In2O3), if the atmospheric pressure during deposition of the deposited film is within the above range, it is possible to prevent indium oxide (I) (In2O) produced from the first thin film-forming material from being re-oxidized by oxygen in the atmosphere, thereby preventing indium oxide (III) (In2O3) from being produced in the deposited film. The pressure of the non-oxidizing atmosphere during deposition of the deposited film can be controlled, for example, by introducing an inert gas such as argon or a mixed gas into the deposition apparatus.

[0036] The substrate on which the vapor-deposited film is formed may be made of glass or plastic. Examples of glass include optical glass. Examples of plastic include polyester, acrylic, polycarbonate, polyamide, polyimide, polyethersulfone, polysulfone, and polyolefin polymers. The substrate may be in the form of, for example, a flat or curved lens-shaped substrate, or a flexible sheet.

[0037] A method for producing an optical thin film includes contacting a vapor-deposited film with a first acidic solution having a pH in the range of 1.0 to 3.0 to obtain a thin film having voids. By contacting the vapor-deposited film with the first acidic solution having a pH in the range of 1.0 to 3.0, indium oxide (I) (In2O) and minute indium (In), which are components that are easily eluted from the vapor-deposited film, are preferentially eluted, and the eluted portions become voids, resulting in a porous thin film (optical thin film) having a silicon dioxide (SiO2) skeleton. When the first acidic solution has a pH in the range of 1.0 to 3.0, the indium oxide (I) (In2O) and minute indium (In) contained in the vapor-deposited film are eluted from the vapor-deposited film too quickly, which prevents the silicon dioxide (SiO2) that forms the thin film's skeleton from detaching from the substrate. This results in a thin film (optical thin film) containing the silicon dioxide (SiO2) skeleton with the desired strength.

[0038] The first acidic solution preferably contains at least one selected from the group consisting of oxalic acid and nitric acid. The acidic substance contained in the first acidic solution is preferably an acid with a buffering effect and multiple acid dissociation constants, more preferably oxalic acid, which is a weak acidic substance. Acids with a weak buffering effect tend to increase the pH, so a long acid treatment time is likely to be required.

[0039] The pH of the first acidic solution is in the range of pH 1.0 to pH 3.0, preferably pH 1.5 to pH 2.5. When the pH of the first acidic solution to be brought into contact with the vapor-deposited film is in the range of pH 1.0 to pH 3.0, indium oxide (I) (In2O) and minute indium (In) contained in the vapor-deposited film are preferentially eluted at a rate that does not destroy the silicon dioxide (SiO2) skeleton in the vapor-deposited film, thereby maintaining the silicon dioxide (SiO2) skeleton and enabling the production of a thin film (optical thin film) with voids that achieves the desired refractive index. Whether the acidic substance contained in the first acidic solution is a strong acidic substance or a weak acidic substance, the first acidic solution is a relatively weak acid as long as the pH is in the range of pH 1.0 to pH 3.0.

[0040] If the pH of the first acidic solution is below 1.0, the acidity is too strong, and the dissolution rate of indium oxide (I) (In2O) and minute indium (In) particles contained in the vapor-deposited film becomes too fast, making it difficult to maintain the silicon dioxide (SiO2) skeleton, and the thin film may not be strong enough. Also, if the pH of the first acidic solution is below 1.0, the acidity is too strong, and the adhesion between the resulting thin film and the substrate is poor, and the thin film may peel off from the substrate after contact with the first acidic solution. If the pH of the first acidic solution is above 3.0, the dissolution rate of indium oxide (I) (In2O) and minute indium (In) particles contained in the vapor-deposited film becomes slow, and it takes a long time to completely dissolve the indium oxide (I) (In2O) and minute indium (In), reducing production efficiency, which is undesirable.

[0041] The temperature at which the vapor-deposited film is brought into contact with the first acidic solution may be room temperature, which is in the range of 15°C to 28°C, and preferably in the range of 15°C to 25°C. If the temperature at which the vapor-deposited film is brought into contact with the first acidic solution is too high, the solvent in the first acidic solution evaporates, lowering the pH and causing the pH of the first acidic solution to fall below 1.0, which is undesirable. If the temperature at which the vapor-deposited film is brought into contact with the first acidic solution is below or above room temperature (15°C to 28°C), equipment such as a cooling device is required to constantly monitor and adjust the pH in order to maintain the pH of the first acidic solution in the range of 1.0 to 3.0, which may increase production costs.

[0042] The time for which the vapor-deposited film is exposed to the first acidic solution is sufficient as long as the entire thin film becomes transparent when the temperature is room temperature, 15°C to 28°C, and varies depending on the thickness and size of the vapor-deposited film. When the temperature is room temperature, 15°C to 28°C, the time for which the vapor-deposited film is exposed to the first acidic solution is preferably 30 to 90 minutes, more preferably 30 to 80 minutes. If the contact time is less than 30 minutes, the entire thin film may not become transparent, and the indium (I) oxide (InO) and minute indium (In) particles contained in the vapor-deposited film may not be sufficiently eluted to form voids with the desired refractive index. If the vapor-deposited film is exposed to the first acidic solution for more than 90 minutes, all of the indium (I) oxide (InO) and minute indium (I) particles will be eluted from the vapor-deposited film, resulting in unnecessary time and reduced production efficiency.

[0043] The method of bringing the vapor-deposited film into contact with the first acidic solution includes immersing the object on which the vapor-deposited film has been formed in the first acidic solution, or immersing only the vapor-deposited film formed on the object in the first acidic solution. The object on which the vapor-deposited film has been formed may be held in a holder, and the holder holding the object on which the vapor-deposited film has been formed may be immersed in the first acidic solution.

[0044] The method for producing an optical thin film preferably includes contacting the vapor-deposited film with a first acidic solution and then drying the thin film having voids and the object on which the film is to be formed. In the method for producing an optical thin film, drying the resulting thin film preferably involves drying the thin film having voids before contacting it with a second acidic solution, which will be described later. It is believed that drying the thin film having voids after contacting the vapor-deposited film with the first acidic solution and before contacting it with the second acidic solution, will shrink and strengthen the thin film, thereby increasing the adhesion between the thin film and the object on which the film is to be formed. Drying is preferably performed by blowing air.

[0045] The method for producing an optical thin film includes contacting the thin film obtained after contact with the first acidic solution with a second acidic solution having a pH of less than 1.0.

[0046] The second acidic solution contacted with the resulting thin film has a pH of less than 1.0, and may be pH 0.9 or less, pH 0.5 or less, or pH -1.14 or more. Oxygen (O) dissociated from the first thin film-forming material reacts with silicon monoxide (SiO) in the gas phase, and oxygen (O) that does not react with silicon monoxide (SiO) may react with indium oxide (I) (In2O) dissociated from the first thin film-forming material. When oxygen (O) dissociated from the first thin film-forming material reacts with indium oxide (I) (In2O), it becomes indium oxide (III) (In2O3), which may be contained in the deposited film. Even when the deposited film is contacted with a first acidic solution with a pH of 1.0 to 3.0, the resulting thin film may contain indium oxide (III) (In2O3). Even if the thin film contains indium(III) oxide (In2O3), the thin film can be contacted with a second acidic solution having a pH of less than 1.0 after contact with the first acidic solution to dissolve the indium(III) oxide (In2O3) in the thin film and reduce the refractive index of the resulting thin film (optical thin film). Even if the thin film contains metallic indium (In) after contact with the first acidic solution, the metallic indium (In) can be dissolved in the second acidic solution having a pH of less than 1.0. The thin film obtained after contacting the vapor-deposited film with the first acidic solution is preferably contacted with the second acidic solution for 20 minutes or more without applying ultrasonic vibrations to prevent the thin film from peeling off from the substrate. It is more preferable to contact the thin film obtained after contacting the vapor-deposited film with the first acidic solution with the second acidic solution without applying ultrasonic vibrations.

[0047] The second acidic solution preferably contains nitric acid. By including nitric acid, the second acidic solution can be made a strongly acidic solution with a pH of less than 1.0. By contacting the deposited film in advance with a first acidic solution, which is a relatively weak acid with a pH of 1.0 or more and a pH of 3.0 or less, the thin film is preferentially eluted with indium (I) oxide (In2O) at a relatively slow rate while suppressing the elution of silicon dioxide (SiO2), which forms the skeleton of the thin film. Therefore, by contacting the thin film with a second acidic solution with a strong acidity of less than 1.0, which is a pH of less than 1.0, the indium (III) oxide (In2O3) remaining in the thin film can be eluted without the silicon dioxide (SiO2), which forms the skeleton of the thin film, peeling off from the substrate.

[0048] The second acidic solution preferably contains nitric acid, and the concentration of nitric acid is 0.63 mass% or more. If the concentration of nitric acid is 0.63 mass% or more, the second acidic solution can be used as a second acidic solution with a pH of less than 1.0. The concentration of nitric acid contained in the second acidic solution may be 9.0 mass% or less, and is preferably 1.0 mass% or less. When the second acidic solution contains nitric acid, if the concentration of nitric acid contained in the second acidic solution is in the range of 0.63 mass% or more and 9.0 mass% or less, the second acidic solution can be adjusted to a pH of less than 1.0, and indium(III) oxide (In2O3) remaining in the thin film can be eluted while maintaining the skeleton of the thin film, thereby lowering the refractive index of the resulting thin film.

[0049] The time for which the thin film is contacted with the second acidic solution is preferably from 30 to 240 minutes, more preferably from 60 to 210 minutes, and even more preferably from 90 to 180 minutes. If the time for which the thin film is contacted with the second acidic solution is from 30 to 240 minutes, the skeleton of the thin film can be maintained while indium(III) oxide (In2O3) remaining in the thin film is eluted, thereby lowering the refractive index of the resulting thin film.

[0050] The temperature of the second acidic solution when contacting the thin film with the second acidic solution is preferably in the range of 50°C or higher and 80°C or lower. If the temperature of the second acidic solution when contacting the thin film is in the range of 50°C or higher and 80°C or lower, the indium(III) oxide (In2O3) remaining in the thin film can be eluted while maintaining the skeleton of the thin film, thereby lowering the refractive index of the resulting thin film. The temperature of the second acidic solution when contacting the thin film may be 55°C or higher, 60°C or higher, or 75°C or lower.

[0051] The method of bringing the thin film into contact with the second acidic solution includes immersing the object on which the thin film has been formed in the second acidic solution, or immersing only the thin film formed on the object on which the film has been formed in the second acidic solution. The object on which the thin film has been formed may be held in a holder, and the holder holding the object on which the thin film has been formed may be immersed in the second acidic solution.

[0052] The method for producing an optical member may include drying the obtained thin film, which may be done by removing the thin film from the first acidic solution or the second acidic solution and drying it naturally or by drying with an air blower.

[0053] The method for producing an optical thin film includes obtaining an optical thin film having an optical thin film containing silicon oxide and having a refractive index of 1.38 or less, and an object to be filmed after contact with a first acidic solution or the second acidic solution, the optical thin film having a plurality of columnar structures extending from the surface of the object to the surface of the optical thin film and gaps between the columns of the columnar structures, the optical thin film being free of bright spots due to metal reflection when visually inspected by irradiating the optical thin film with light having an illuminance of 1150 lx to 1250 lx. The optical thin film is free of bright spots due to metal reflection when irradiated with light having an illuminance of 1150 lx to 1250 lx, and no metal is present on the surface of the optical thin film. In this specification, bright spots refer to small spots of light due to metal reflection. The diameter of the bright spots is in the range of 5 μm to 15 μm, typically around 10 μm. When the optical thin film is irradiated with light having an illuminance of reflected light in the range of 1150 lx to 1250 lx, if even one bright spot due to metal reflection is found by visual inspection, it can be confirmed that metal remains on the surface of the optical thin film. The optical thin film preferably has a thin film obtained by the above-mentioned manufacturing method, with the metal adhering to the surface of the thin film dissolved therein.

[0054] A method for confirming the presence of bright spots on the surface of the optical thin film can be, for example, by using a lighting fixture capable of irradiating light with a reflected light illuminance in the range of 1150 lx to 1250 lx, placing the optical element within the range of the light from the lighting fixture approximately 10 to 20 cm away from the light source of the lighting fixture, and visually checking the surface of the optical thin film of the optical element. The optical thin film of the optical element irradiated with light with a reflected light illuminance in the range of 1150 lx to 1250 lx may be photographed with a digital camera to confirm the presence of bright spots. When photographing the optical thin film of the optical element with a digital camera, the surface of the optical thin film may be photographed from a position approximately 5 to 15 cm away from the surface of the optical thin film of the optical element. To make it easier to confirm the presence of bright spots, the lighting fixture and the optical element may be placed in a darkroom, and the optical thin film may be irradiated with light with a reflected light illuminance in the range of 1150 lx to 1250 lx, and the bright spots may be visually confirmed, or the optical thin film of the optical element may be photographed with a digital camera. The lighting fixture may use a fluorescent lamp as a light source, or may use a light-emitting device equipped with a light-emitting element such as a light-emitting diode.

[0055] The optical thin film preferably has a thin film obtained by dissolving metal adhering to the surface of the thin film obtained by the above-mentioned manufacturing method. The optical thin film preferably has a columnar structure extending from the surface of the substrate to the surface of the optical thin film, the columnar structure including silicon dioxide (SiO2). The optical thin film preferably has a skeleton formed by the columnar structure extending from the surface of the substrate to the surface of the optical thin film. The optical thin film has a plurality of columnar structures. For example, in a cross-sectional SEM photograph of the optical thin film taken with a scanning electron microscope (SEM), there may be five or more connections between the columnar structures and the substrate (e.g., an undercoat film) within a 500 nm width range. Furthermore, in a cross-sectional SEM photograph, there may be 20 or fewer connections between the columnar structures and the substrate within a 500 nm width range.

[0056] The refractive index of the optical thin film is preferably lower than that of silicon dioxide (SiO2), and is preferably 1.38 or less. Since the optical thin film has a refractive index of 1.38 or less, it can enhance the anti-reflection effect across the entire visible range. The refractive index of the optical thin film can be calculated by measuring the reflection spectrum with a spectrophotometer, measuring the minimum value of the reflected light intensity when the incident light intensity is 100%, and using the Fresnel coefficient from this minimum value of the measured reflectance. The refractive index of the optical thin film is preferably 1.30 or less, more preferably 1.27 or less, even more preferably 1.25 or less, and particularly preferably 1.20 or less.

[0057] The porosity of the thin film that is an optical thin film is preferably in the range of 30% or more and 90% or less. If the porosity of the thin film is 30% or more, the refractive index of the thin film can be reduced, and if it is 90% or less, the refractive index of the thin film can be reduced while maintaining the strength to maintain the thin film formed on the substrate. The porosity of the thin film that is an optical thin film is more preferably in the range of 40% or more and 90% or less, even more preferably in the range of 50% or more and 90% or less, and even more preferably in the range of 60% or more and 85% or less. The porosity (total porosity Vp) of the thin film that is an optical thin film can be calculated using the Lorentz-Lorenz equation based on the examples described below.

[0058] The optical thin film obtained by the optical thin film manufacturing method and the object to be coated can be used as an optical component. Furthermore, by applying the optical thin film to the light extraction portion of a light emitting device, it is possible to promote the emission of light from the light emitting device to the outside, thereby improving the light extraction efficiency and reducing heat generation. [Example]

[0059] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0060] Example 1: Preparing a First Film-Forming Material 10.85 g of indium(III) oxide (In2O3) powder (purity of indium(III) oxide: 99.99% by mass) was weighed and press-molded to obtain a semi-cylindrical pellet (solid). This pellet (solid) was sintered in an air atmosphere (oxygen concentration: approximately 20% by volume, pressure: 101.325 kPa) at a temperature between 1100°C and 1500°C for 12 hours to obtain a sintered first thin film-forming material, and two first thin film-forming materials of the same size were prepared. In Examples 1 and 2, the first thin film-forming material was made of indium(III) oxide (In2O3). Indium(III) oxide (In2O3) may contain unavoidable impurities. The first thin film forming material has a semi-cylindrical lower surface (placement surface) and upper surface (evaporation surface) that are semi-circular in plan view, and the side surface of the semi-cylindrical shape has a flat surface that forms a straight portion of the semi-circle in plan view and a curved surface that forms a semi-circular portion of the semi-circle. The total volume of the two first thin film forming materials is 2470 mm 3 The total area of ​​the evaporation surfaces of the two first thin film forming materials is 247 mm 2 The height of each of the two first thin film forming materials is 10 mm.

[0061] Example 1: Preparing a second film-forming material 0.65 g of silicon monoxide (SiO) powder (silicon monoxide purity: 99.9% by mass) is weighed and press-molded to obtain a semi-cylindrical pellet (solid). This pellet (solid) is fired in an inert atmosphere (argon (Ar): 99.99% by volume) at a temperature ranging from 1100°C to 1200°C for 2 hours to prepare a sintered second thin film-forming material. In Examples 1 and 2, the second thin film-forming material is made of silicon monoxide (SiO). Silicon monoxide (SiO) may contain unavoidable impurities. The second thin film-forming material has a semi-cylindrical lower surface (placement surface) and upper surface (evaporation surface) that are semi-circular in plan view, and the side surface of the semi-cylindrical shape has flat surfaces that form linear portions of the semi-circle and curved surfaces that form semi-circular portions of the semi-circle in plan view. The volume of the second thin film-forming material is 420 mm. 3 The evaporation surface area of ​​the second thin film forming material is 42 mm 2 The height of the second thin film forming material is 10 mm.

[0062] Example 1: Forming a vapor-deposited film A 30 mm diameter circular double-polished glass plate (manufactured by SCHOOT AG, BK=7) was used as the substrate. The substrate was placed in a vapor deposition apparatus, and a second thin film material was placed below the substrate, spaced apart in a crucible, between two first thin film forming materials. The first and second thin film forming materials were spaced apart by 1 mm. As shown in Figure 1, a second thin film forming material 2 was placed between the two first thin film forming materials 1, 1. The two first thin film forming materials 1 and the second thin film forming material 2 were arranged so that the surface constituting the semicircular linear portion 1s in a planar view of the first thin film forming material 1 faces the surface constituting the semicircular linear portion 2s in a planar view of the second thin film forming material 2, and so that the surface constituting the semicircular linear portion 1s in a planar view of the other first thin film forming material 1 faces the surface constituting the semicircular curved portion 2c of the second thin film forming material 2. In FIG. 1, the gap (distance) G between the first thin film forming material 1 and the second thin film forming material represents the shortest distance separating the first thin film forming material 1 and the second thin film forming material. The shortest distance separating the first thin film forming material and the second thin film forming material is 1 mm. In this specification, the arrangement of two first thin film forming materials and one second thin film forming material in Example 1 is also referred to as "arrangement arrangement A." The pressure inside the vapor deposition device was set to 2.0×10 -4 Under reduced pressure to 100 Pa, the first and second thin film forming materials were irradiated with an electron beam (G-12100, manufactured by Plasmatech Co., Ltd.) at 170 mA to form a vapor-deposited film containing indium oxide (In2O) and silicon dioxide (SiO2) on at least one surface of the substrate-like substrate. The temperature of the substrate during deposition was set to 100°C, and ion-beam assisted deposition (IAD) (acceleration voltage and current values ​​of 800 V and 800 mA) was used using Ar ions emitted from an ion gun (NIS-150, manufactured by Shincron Co., Ltd.). A neutralizer (RFN-2, manufactured by Shincron Co., Ltd., bias current value of 1000 mA) was also used to prevent charging of the substrate.

[0063] Example 1: Contacting a Deposited Film with a First Acidic Solution A first acidic solution containing oxalic acid and nitric acid, with an oxalic acid concentration of 180 ppm by mass, a nitric acid concentration of 40 ppm by mass, and a pH of 2.0, is prepared. An object on which a vapor-deposited film has been formed is immersed in the first acidic solution at room temperature (20°C to 25°C) to bring the vapor-deposited film into contact with the first acidic solution. Indium (I) oxide (InO) is preferentially eluted from the vapor-deposited film that has been contacted with the first acidic solution, thereby obtaining a thin film having voids. The contact time (immersion time) between the vapor-deposited film and the first acidic solution is 45 minutes, resulting in a thin film with low absorption of visible light. The refractive index of the resulting thin film having voids is measured using the method described below. A thin film having voids and a refractive index of 1.246 according to Example 1 is obtained. The porosity of the thin film according to Example 1 is 50%.

[0064] The first thin film forming material and the second thin film forming material may be arranged as shown in FIG. 2. As shown in FIG. 2, in a vapor deposition apparatus, the first thin film forming material 1 and the second thin film forming material 2 are arranged separately below the substrate so that the plane constituting the semicircular linear portion 1s in a plan view of the first thin film forming material 1 and the plane constituting the semicircular linear portion 2s in a plan view of the second thin film forming material 2 face each other. The distance separating the first thin film forming material and the second thin film forming material is 1 mm. In FIG. 2, the distance (distance) G separating the first thin film forming material 1 and the second thin film forming material represents the shortest distance separating the first thin film forming material 1 and the second thin film forming material. In this specification, the arrangement of the first thin film forming material and the second thin film forming material in Example 2 is also referred to as "arrangement B."

[0065] Example 2: Preparing a First Film-Forming Material 7.51 g of indium (III) oxide (In2O3) powder (purity of indium (III) oxide: 99.99% by mass) was weighed and press-molded to obtain a semi-cylindrical pellet (solid). This pellet (solid) was fired in an air atmosphere (oxygen concentration: approximately 20% by volume, pressure: 101.325 kPa) at a temperature between 1100°C and 1500°C for 12 hours to obtain a sintered body, and two sintered bodies of the same size were prepared as first thin film-forming materials. The first thin film-forming material had a semi-cylindrical bottom surface (placement surface) and top surface (evaporation surface) that were semi-circular in plan view, and the side surfaces of the semi-cylindrical shape had flat surfaces that constituted the straight portions of the semi-circle and curved surfaces that constituted the semi-circular portions of the semi-circle in plan view. The total volume of the two first thin film-forming materials was 1710 mm 3 The total area of ​​the placement surface and evaporation surface of the two first thin film forming materials is 171 mm 2 The height of each of the two first thin film forming materials is 10 mm.

[0066] Example 2: Preparing a second film-forming material 0.22 g of silicon monoxide (SiO) powder (silicon monoxide purity: 99.9% by mass) is weighed and press-molded to obtain a semi-cylindrical pellet (solid). This pellet (solid) is fired in an inert atmosphere (argon (Ar): 99.99% by volume) at a temperature between 1100°C and 1200°C for 2 hours to prepare a sintered body, a second thin film forming material. The second thin film forming material has a semi-cylindrical bottom surface (placement surface) and top surface (evaporation surface) that are semi-circular in plan view, and the side surface of the semi-cylindrical shape has a flat surface that forms a straight portion of the semi-circle in plan view and a curved surface that forms a semi-circular portion of the semi-circle. The volume of the second thin film forming material is 145 mm. 3 The evaporation surface area of ​​the second thin film forming material is 29 mm 2 The height of the second thin film forming material is 5 mm.

[0067] Example 2: Forming a Vapor-Deposited Film and Contacting the Vapor-Deposited Film with a First Acidic Solution In a deposition apparatus, two first thin film forming materials and a second thin film material spaced apart between the two first thin film forming materials are arranged below the object to be film-formed, to form the same arrangement (arrangement A) as in Example 1. The distance separating the first thin film forming material and the second thin film forming material is 1 mm. In the same manner as in Example 1, a thin film according to Example 2 having voids with a refractive index of 1.281 is obtained. The porosity of the thin film according to Example 3 is 57%.

[0068] Comparative Example 1: Preparing a first thin film-forming material and preparing a second thin film-forming material A first thin film forming material, which is a sintered body, and a second thin film forming material, which is a sintered body, are prepared in the same manner as in Example 2. The volume of the first thin film forming material is 1970 mm 3 The deposition surface and evaporation surface of the first thin film forming material are each semicircular in plan view. The total area of ​​the deposition surface or evaporation surface of the first thin film forming material is 197 mm 2 The volume of the second thin film forming material is 1970 mm 3 The placement surface and evaporation surface of the second thin film forming material are semicircular in plan view, and the area of ​​the placement surface and evaporation surface of the second thin film forming material is 197 mm 2 The height of the second thin film forming material is 10 mm.

[0069] Comparative Example 1: Forming a Vapor-Deposited Film and Contacting the Vapor-Deposited Film with a First Acidic Solution The semi-cylindrical first thin film-forming material and the semi-cylindrical second thin film-forming material used in the manufacturing method of Example 2 are used. As shown in FIG. 5, the first thin film-forming material 1 and the second thin film-forming material 2 are arranged so that the plane constituting the semi-circular linear portion 1s of the first thin film-forming material 1 and the plane constituting the semi-circular linear portion 2s of the second thin film-forming material 2 correspond to and contact each other in a plan view. The plane constituting the linear portion 1s of the first thin film-forming material 1 and the plane constituting the semi-circular linear portion 2s of the second thin film-forming material 2 do not have to contact at all points, and at least a portion of the plane of the first thin film-forming material 1 and at least a portion of the plane of the second thin film-forming material may be separated by a distance of less than 1 mm. In the present specification, the arrangement of the first thin film-forming material and the second thin film-forming material in Comparative Example 1 is also referred to as "Arrangement D." A thin film having voids according to Comparative Example 1 was obtained in the same manner as in Example 1, except that the first thin film forming material and the second thin film forming material were used and arranged in arrangement configuration D. The thin film according to Comparative Example 1 was not transparent, and its refractive index and absorptance were not measured. The reason why the thin film according to Comparative Example 1 was not transparent is presumably due to the presence of silicon monoxide remaining in the thin film. The porosity of the thin film according to Comparative Example 1 was 33%.

[0070] The refractive index, porosity, absorptivity, and presence of bright spots on the surface of each thin film (optical thin film) in the Examples and Comparative Examples were confirmed using the following methods. The results are shown in Table 1. Table 1 lists the volume, evaporation surface area, and height of the first thin film-forming material, as well as the volume, evaporation surface area, and height of the second thin film-forming material for Examples 1 to 3 and Comparative Example 1. It also lists the volume ratio of the first thin film-forming material to the second thin film-forming material (referred to as "volume ratio (1st / 2nd)" in the table), and the area ratio of the evaporation surface area of ​​the first thin film-forming material to the evaporation surface area of ​​the second thin film-forming material (referred to as "area ratio (1st / 2nd)" in the table).

[0071] Refractive index measurement method The reflection spectrum of each thin film (optical thin film) of the examples and comparative examples is measured using a spectrophotometer (U-4100, manufactured by Hitachi High-Technologies Corporation, incident angle 5°). The minimum value of the reflected light intensity when the incident light intensity is set to 100 is measured as the reflectance, and the refractive index is calculated from this measured reflectance using the Fresnel coefficient. Because double-sided polished glass was used as the substrate on which the thin film was formed, the reflectance R' obtained from the measurement included multiple repeated reflections, including backside reflection.Since the measured reflectance R' includes multiple repeated reflections, the reflectance R of the thin film can be expressed by the following equation (1).

[0072]

number

[0073] In the formula (1), R o is the reflectance of the substrate (the object on which the film is formed). In the method for measuring the refractive index, the object on which the film is formed is also called the substrate. The reflectance R of the base film or thin film was calculated from the reflectance R' of the base film or thin film that was actually measured based on formula (1). The reflectance R of the base film or thin film is the reflectance that does not take into account reflection from the back surface. The reflectance R of the undercoat or thin film is calculated by using the Fresnel coefficients: m and the refractive index n of the undercoat film or thin film can be expressed using the following formula (2).

[0074]

number

[0075] Here, the refractive index of the air is approximated as 1, and the refractive index of the substrate is n m When the refractive index n of the undercoat film or thin film is larger than the square root of (2), the refractive index n of the undercoat film or thin film can be expressed by the following formula (3).

[0076]

number

[0077] In addition, the refractive index of the substrate, n m When the refractive index n of the undercoat film or thin film is smaller than the square root of (a), the refractive index n of the undercoat film or thin film can be expressed by the following formula (4).

[0078]

number

[0079] The refractive index n of the undercoat film or thin film was calculated based on the above formulas (1) to (4). Regarding the refractive index n of the undercoat film or thin film, reference is made to "Fundamental Theory of Optical Thin Films - Fresnel Coefficients, Characteristic Matrices" by Mitsunobu Kohiyama, published by Optronics Co., Ltd. on February 25, 2011, revised and expanded first printing." In Table 1, when the refractive index is 1.3 or higher, it is recorded as "1.3 or higher."

[0080] Porosity measurement method The porosity (total porosity Vp) of the thin film is calculated using the Lorentz-Lorenz equation shown in the following equation (5). In the following equation (5), n f is the observed refractive index of the thin film, and n b The refractive index of the thin film skeleton is n f is the refractive index of the thin film calculated based on the above formulas (1) to (4). The refractive index n of the thin film skeleton b Since is mainly composed of silicon dioxide (SiO2), the refractive index of silicon dioxide (SiO2) (1.460) is used to calculate the refractive index.

[0081]

number

[0082] How to measure absorption rate The absorptance of a thin film (optical thin film) containing voids is measured using a spectrophotometer (Hitachi High-Technologies Corporation, product name: U-4100, incident angle 5°). The reflected light intensity and transmitted light intensity are measured when the incident light intensity is set to 100, and the value obtained by subtracting these from the incident light intensity of 100 is calculated as the absorptance. Since the absorptance is higher on the short wavelength side, it is taken as the average value from 390 nm to 410 nm. In Table 1, if the absorptance is 1.0% or higher, it is recorded as "1.0 or higher."

[0083] Yield (yield (%) of thin film surface without bright spots) measurement method Using a lighting fixture capable of emitting light with a reflected light illuminance in the range of 1150 lx to 1250 lx, an optical element having a thin film was placed in an area irradiated with the light from the lighting fixture, approximately 10 to 20 cm from the light source of the lighting fixture, and the surface of the thin film (optical thin film) of the optical element was visually confirmed. The surface of the optical thin film was also photographed with a digital camera at a position approximately 5 to 15 cm from the surface of the optical thin film of the optical element. When visually confirming or photographing with a digital camera, the surface of the thin film (optical thin film) of the optical element was irradiated with light with a reflected light illuminance in the range of 1150 lx to 1250 lx in a darkroom. For each thin film (optical thin film) of the Examples and Comparative Examples, the number of thin films that did not have bright spots on the surface of the thin film was counted for each Example or Comparative Example, and the number of thin films that did not have bright spots per 100 was expressed as the yield (%).

[0084] [Table 1]

[0085] In the optical thin film manufacturing methods of Examples 1 and 2, by disposing the first thin film-forming material, which is a solid, and the second thin film-forming material, which is also a solid, at a distance from each other, components contained in the first thin film-forming material and the second thin film-forming material do not react in the solid or liquid phase, and it is possible to suppress the generation of In, a metal that is easily dissociated from components contained in the first thin film-forming material by reacting in the solid or liquid phase. In Examples 1 and 2, the yield of thin films (optical thin films) with no metal present on the surface was 100%.

[0086] The thin films (optical thin films) obtained by the optical thin film manufacturing methods according to Examples 1 and 2 can have a refractive index as low as 1.38 or less. The thin films (optical thin films) obtained by the optical thin film manufacturing methods according to Examples 1 and 2 are irradiated with light whose reflected light luminance is in the range of 1150 lx to 1250 lx, and when visually inspected, thin films (optical thin films) are obtained that are free of bright spots on the surface due to metal reflection. The thin films (optical thin films) obtained by the optical thin film manufacturing methods according to Examples 1 and 2 have a porosity of 50% to 57%, and a porosity in the range of 30% to 90%.

[0087] The thin films (optical thin films) obtained by the optical thin film manufacturing methods of Examples 1 and 2 have sufficiently low absorptance of 0.15% or less, as measured from the transmittance and reflectance measured by a spectrophotometer, and it can be confirmed that the obtained thin films (optical thin films) do not contain indium oxide (I) (InO) and / or metallic indium (In), which have a black body color. It can also be confirmed that the thin films (optical thin films) obtained by the optical thin film manufacturing methods of Examples 1 and 2 do not contain silicon monoxide (SiO), which has a black body color.

[0088] The thin film (optical thin film) obtained by the optical thin film manufacturing method of Comparative Example 1 has a yield of 88% for thin films (optical thin films) with no metal present on the surface, and since the solid first thin film-forming material and the solid second thin film-forming material are arranged in contact with each other, components contained in the first thin film-forming material and components contained in the second thin film-forming material may react in the solid or liquid phase, causing In, a metal that is easily dissociated from the components contained in the first thin film-forming material, to adhere to the surface. Furthermore, the thin film (optical thin film) obtained by the optical thin film manufacturing method of Comparative Example 1 contains silicon monoxide (SiO), which is black in color, and has a refractive index of 1.3 or higher.

[0089] Example 3: Preparing a First Film-Forming Material 9.77 g of indium (III) oxide (In2O3) powder (purity of indium (III) oxide: 99.99% by mass) is weighed out and press-molded to obtain a hollow cylindrical pellet (solid). This pellet (solid) is fired in an air atmosphere (oxygen concentration: approximately 20% by volume, pressure: 101.325 kPa) at a temperature between 1100°C and 1500°C for 12 hours to prepare a hollow cylindrical sintered body, a first thin film-forming material. The placement surface (lower surface) and evaporation surface (upper surface) of the first thin film-forming material are hollow circular in plan view. The area of ​​the placement surface or evaporation surface of the first thin film-forming material is 312 mm 2 The height of the first thin film forming material is 9 mm. The volume of the first thin film forming material (mm 3 ) are listed in Table 2.

[0090] Example 3: Preparing a second film-forming material 0.33 g of silicon monoxide (SiO) powder (silicon monoxide purity: 99.9% by mass) is weighed and press-molded to obtain a cylindrical pellet (solid) of a size that can be placed in the hollow portion of the hollow cylindrical first thin film-forming material at a distance from the first thin film-forming material. This pellet (solid) is fired in an inert atmosphere (argon (Ar): 99.99% by volume) at a temperature in the range of 1100°C to 1200°C for 2 hours to obtain a sintered second thin film-forming material. The second thin film-forming material has a placement surface (lower surface) and evaporation surface (upper surface) that are circular in plan view. The areas of the placement surface and evaporation surface of the second thin film-forming material are 53 mm 2 The height of the second thin film forming material is 5 mm. The volume of the second thin film forming material (mm 3 ) are shown in Table 2. The area ratio of the placement or evaporation surface of the first thin film forming material to the placement or evaporation surface of the second thin film forming material (area ratio of the second thin film forming material to the second thin film forming material (first thin film forming material / second thin film forming material) is 5.9.

[0091] Example 3: Formation of a vapor-deposited film As shown in FIG. 3, a hollow cylindrical first thin film forming material 1 is placed below the substrate in a vapor deposition apparatus, and a cylindrical second thin film forming material 2 is placed in the hollow portion of the first thin film forming material 1, spaced apart from the first thin film forming material 1 in a plan view. In FIG. 3, the distance G separating the first thin film forming material 1 and the second thin film forming material represents the shortest distance separating the first thin film forming material 1 and the second thin film forming material. In this specification, the arrangement of the first thin film forming material and the second thin film forming material in Example 2 is also referred to as "arrangement arrangement C." The distance separating the first thin film forming material and the second thin film forming material is 1 mm. A vapor deposition film is formed in the same manner as in Example 1, except that the first thin film forming material and the second thin film forming material described above are irradiated with an electron beam (G-12100, manufactured by Plasmatech Co., Ltd.) at 600 mA.

[0092] Example 3: Contacting the First Acidic Solution The resulting vapor-deposited film is brought into contact with a first acidic solution to obtain a thin film having voids, in the same manner as in Example 1. The resulting thin film is dried by blowing air onto it using an air duster gun (Kinki Seisakusho Co., Ltd., K-601) for 20 to 30 seconds.

[0093] Example 3: Contacting the Second Acidic Solution A nitric acid solution with a pH of 0.8 is prepared as the second acidic solution. The concentration of nitric acid in the second acidic solution is 0.63% by mass. The substrate on which the thin film having voids is formed is immersed in the second acidic solution at room temperature (20°C to 25°C), and the thin film having voids is brought into contact with the second acidic solution, thereby dissolving the indium (III) oxide (In2O3) remaining in the thin film from the thin film, resulting in a thin film having voids. The contact time (immersion time) between the thin film and the second acidic solution is 45 minutes, and a thin film having voids and a refractive index of 1.38 or less is obtained.

[0094] Examples 4 to 18 A thin film having voids can be obtained by the optical thin film manufacturing method of Examples 4 to 18 in the same manner as in Example 3, except that the height of the first thin film forming material similar to that of Example 3 and the height of the second thin film forming material similar to that of Example 3 are changed as shown in Table 2.

[0095] For each thin film (optical thin film) in the Examples and Comparative Examples, the refractive index of each thin film (optical thin film) after contact with the first acidic solution but before contact with the second acidic solution, and the refractive index of each thin film (optical thin film) after contact with the first acidic solution but before contact with the second acidic solution, were measured using the method described above, and the yield of thin films with no bright spots on the surface was measured using the method described above. The results are shown in Table 2. Table 2 lists the volume and height of the first thin film-forming material and the volume and height of the second thin film-forming material for Examples 3 to 18. Also shown are the volume ratio of the volume of the first thin film-forming material to the volume of the second thin film-forming material (referred to as "volume ratio (1st / 2nd)" in the table), the refractive index of each thin film (optical thin film) after contact with the first acidic solution but before contact with the second acidic solution (refractive index after contact with the first acidic solution), the refractive index of each thin film (optical thin film) after contact with the first acidic solution but after contact with the second acidic solution (refractive index after contact with the second acidic solution), the yield (%) of the number of thin films without bright spots after contact with the first acidic solution but after contact with the second acidic solution, and the porosity (%). The refractive index, yield, and porosity are measured using the methods described above.

[0096] SEM image A scanning electron microscope (SEM) was used to obtain an SEM photograph of the cross section of the thin film according to Example 3. Fig. 4 shows SEM photographs of the cross sections of the film-forming object and the thin film obtained by the manufacturing method of Example 3.

[0097] [Table 2]

[0098] In the optical thin film manufacturing method according to Examples 3 to 18, when the first thin film-forming material, which is a solid, and the second thin film-forming material, which is also a solid, are arranged at a distance from each other and the volume ratio of the first thin film-forming material to the second thin film-forming material (first thin film-forming material / second thin film-forming material) is within the range of 1 to 24, oxygen (O) dissociated from the raw materials contained in the first thin film-forming material can be sufficiently supplied to silicon monoxide (SiO) contained as a main component in the second thin film-forming material in the gas phase, and the reaction of silicon monoxide (SiO) contained in the second thin film-forming material can be promoted in the gas phase, making it possible to form a vapor-deposited film containing a sufficient amount of silicon dioxide (SiO2) as the skeleton, distributed approximately uniformly on the object to be formed. In the optical thin film manufacturing method according to Examples 3 to 18, the resulting vapor-deposited film is contacted with a first acidic solution to preferentially dissolve indium oxide (In2O) contained in the vapor-deposited film at a rate that does not destroy the silicon dioxide (SiO2) skeleton in the vapor-deposited film, thereby maintaining the silicon dioxide (SiO2) skeleton and producing a thin film (optical thin film) with a refractive index of 1.38 or less. By further contacting the thin film obtained after contact with the first acidic solution with a second acidic solution with a pH lower than the first acidic solution, that is, less than 1.0, the indium oxide (In2O3) contained in the thin film is dissolved, thereby producing a thin film with a low refractive index of 1.30 or less.

[0099] 4, the optical thin film 10 obtained after contact with the second acidic solution by the production method of Example 3 has a plurality of columnar structures 11 extending from the surface of the film-forming object 13 to the surface of the optical thin film 10, and voids 12 between the columns of the columnar structures 11. The columnar structures 11 are the skeleton of the optical thin film 10 and are made of silicon dioxide (SiO2).

[0100] The present disclosure may include the following aspects.

[0101] [Section 1] preparing a first thin film forming material which is a solid material containing at least one selected from the group consisting of indium oxide and zinc sulfide, and a second thin film forming material which is a solid material containing silicon oxide; disposing the first thin film forming material and the second thin film forming material at a distance from each other; A method for manufacturing an optical thin film, comprising heating the first thin film forming material and the second thin film forming material by vapor deposition in a non-oxidizing atmosphere to form a vapor-deposited film on an object to be film-formed. [Section 2] Item 1. The method for manufacturing an optical thin film according to item 1, wherein, in arranging the first thin film forming material and the second thin film forming material, the distance separating the first thin film forming material and the second thin film forming material is in the range of 1 mm or more and 4 mm or less. [Section 3] Preparing two of the first thin film forming materials and one of the second thin film forming materials; Item 3. The method for producing an optical thin film according to item 1 or 2, comprising disposing the second thin film forming material between two of the first thin film forming materials. [Section 4] 4. The method for manufacturing an optical thin film according to claim 1, wherein the first thin film forming material is hollow cylindrical, and the second thin film forming material is cylindrical so as to be able to be placed in the hollow portion of the first thin film forming material. [Section 5] Item 5. The method for producing an optical thin film according to any one of items 1 to 4, wherein the volume of the first thin film-forming material is in the range of 1 to 24 times the volume of the second thin film-forming material. [Section 6] Item 6. The method for producing an optical thin film according to any one of items 1 to 5, wherein the area of ​​the evaporation surface of the first thin film forming material is in the range of 1 to 8 times the area of ​​the evaporation surface of the second thin film forming material. [Section 7] Item 7. The method for producing an optical thin film according to Item 6, wherein the height of the first thin film forming material is in the range of 0.5 to 3 times the height of the second thin film forming material. [Section 8] Item 8. The method for producing an optical thin film according to any one of items 1 to 7, comprising contacting the vapor-deposited film with a first acidic solution having a pH in the range of 1.0 to 3.0 to obtain a thin film having voids. [Section 9] Item 9. The method for producing an optical thin film according to Item 8, wherein the first acidic solution contains at least one selected from the group consisting of oxalic acid and nitric acid. [Section 10] Item 8 or 9. The method for producing an optical thin film according to Item 8 or 9, wherein the time for which the vapor-deposited film is in contact with the first acidic solution is 30 minutes or more and 90 minutes or less, and the temperature of the first acidic solution when the vapor-deposited film is in contact is in the range of 15°C or more and 28°C or less. [Section 11] Item 11. The method for producing an optical thin film according to any one of items 8 to 10, comprising contacting the thin film with a second acidic solution having a pH of less than 1.0. [Section 12] Item 12. The method for producing an optical thin film according to Item 11, wherein the second acidic solution contains nitric acid. [Section 13] Item 13. The method for producing an optical thin film according to Item 11 or 12, wherein the thin film is contacted with the second acidic solution for 30 minutes or more and 240 minutes or less. [Section 14] Item 14. The method for producing an optical thin film according to any one of items 11 to 13, wherein the temperature of the second acidic solution when contacting the thin film is in the range of 50° C. or more and 80° C. or less. [Section 15] After contact with the first acidic solution or after contact with the second acidic solution, An optical thin film containing silicon oxide and having a refractive index of 1.38 or less, and an object to be coated, Item 15. The method for producing an optical thin film according to any one of Items 8 to 14, wherein the optical thin film has a plurality of columnar structures extending from the surface of the film-forming object to the surface of the optical thin film, and gaps between the columns of the columnar structures, and is irradiated with light having an illuminance of reflected light in the range of 1150 lx to 1250 lx, and is visually inspected such that no bright spots resulting from metal reflection are present on the surface. [Section 16] Item 16. The method for producing an optical thin film according to Item 15, wherein the porosity of the optical thin film is in the range of 30% to 90%. [Industrial Applicability]

[0102] According to a method for producing an optical thin film according to one embodiment, an optical thin film can be produced in which the presence of metal on the surface of the thin film is reduced, and an optical thin film that can be used not only for camera lenses but also for high-definition liquid crystal panels, etc. The optical member according to one embodiment can be used in optical members such as astronomical telescopes, eyeglass lenses, cameras, disc drive devices equipped with optical pickup components such as bandpass filters and beam splitters, display devices equipped with high-definition liquid crystal panels, and light-emitting devices. [Explanation of symbols]

[0103] 1: first thin film forming material, 1s: straight part, 2: second thin film forming material, 2s: straight part, 2c: curved part, 10: optical thin film, 11: columnar structure, 12: void, 13: film-forming object.

Claims

1. preparing a first thin film forming material which is a solid material containing at least one selected from the group consisting of indium oxide and zinc sulfide, and a second thin film forming material which is a solid material containing silicon oxide; disposing the first thin film forming material and the second thin film forming material at a distance from each other; A method for manufacturing an optical thin film, comprising: heating the first thin film forming material and the second thin film forming material by vapor deposition in a non-oxidizing atmosphere to form a vapor-deposited film on an object to be film-formed.

2. 2. The method for manufacturing an optical thin film according to claim 1, wherein, when arranging the first thin film forming material and the second thin film forming material, the distance separating the first thin film forming material and the second thin film forming material is in the range of 1 mm or more and 4 mm or less.

3. Preparing two of the first thin film forming materials and one of the second thin film forming materials; The method for producing an optical thin film according to claim 1 , further comprising disposing the second thin film forming material between two of the first thin film forming materials.

4. 2. The method for producing an optical thin film according to claim 1, wherein the first thin film-forming material is hollow cylindrical, and the second thin film-forming material is cylindrical so as to be able to be placed in the hollow portion of the first thin film-forming material.

5. The method for producing an optical thin film according to claim 1 , wherein the volume of the first thin film-forming material is within a range of 1 to 24 times the volume of the second thin film-forming material.

6. The method for producing an optical thin film according to claim 1 , wherein the evaporation surface area of ​​the first thin film material is within a range of 1 to 8 times the evaporation surface area of ​​the second thin film material.

7. 7. The method for producing an optical thin film according to claim 6, wherein the height of the first thin film-forming material is within a range of 0.5 to 3 times the height of the second thin film-forming material.

8. The method for producing an optical thin film according to claim 1 , comprising contacting the vapor-deposited film with a first acidic solution having a pH in the range of 1.0 to 3.0 to obtain a thin film having voids.

9. The method for producing an optical thin film according to claim 8 , wherein the first acid solution contains at least one selected from the group consisting of oxalic acid and nitric acid.

10. 10. The method for producing an optical thin film according to claim 8, wherein the time for which the vapor-deposited film is in contact with the first acidic solution is 30 minutes or more and 90 minutes or less, and the temperature of the first acidic solution when the vapor-deposited film is in contact with the first acidic solution is in the range of 15°C or more and 28°C or less.

11. The method for producing an optical thin film according to claim 8 , further comprising contacting the thin film with a second acidic solution having a pH of less than 1.

0.

12. The method for producing an optical thin film according to claim 11 , wherein the second acidic solution contains nitric acid.

13. The method for producing an optical thin film according to claim 11 or 12, wherein the thin film is brought into contact with the second acidic solution for a period of 30 minutes or more and 240 minutes or less.

14. 13. The method for producing an optical thin film according to claim 11, wherein the temperature of the second acidic solution when the thin film is brought into contact with the second acidic solution is in the range of 50°C or more and 80°C or less.

15. After contact with the first acidic solution or after contact with the second acidic solution, An optical thin film containing silicon oxide and having a refractive index of 1.38 or less, and a film-forming object, 12. The method for producing an optical thin film according to claim 8 or 11, wherein the optical thin film has a plurality of columnar structures extending from the surface of the film-forming object to the surface of the optical thin film, and gaps between the columns of the columnar structures, and when irradiated with light having an illuminance of reflected light in the range of 1150 lx to 1250 lx and confirmed visually, no bright spots due to metal reflection are present on the surface.

16. The method for producing an optical thin film according to claim 15, wherein the porosity of the optical thin film is in the range of 30% to 90%.

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