JP2007052345A - Multilayered thin film structure with refractive index gradient and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface structure by which problems of an antireflection film by optical design and problems of an antireflection structure by a sub-wavelength grid are solved at the same time and which has an excellent antireflection function and which can be industrially manufactured. <P>SOLUTION: A multilayered film structure comprises at least three layers of thin film layers each having ≥70% transmissivity at whole visible light regions and ≤380nm thickness. Further a refractive index of each layer at the visible light region is gradually changed between 1.0 and 2.5 in a direction from an uppermost layer being nearest to an external medium to a lowest layer being nearest to a base material so that an apparent refractive index may become larger. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a nanoparticle multilayer thin film that can be used for an optical thin film such as an antireflection film and a method for producing the same. Specifically, a thin film in which the refractive index of each thin film layer formed in the space from the uppermost thin film surface to the base material has an intermediate refractive index from the refractive index 1.0 of air as the external medium to the refractive index of the base material The present invention relates to an antireflection structure utilizing the principle of a subwavelength grating, in which the visible light reflectance is remarkably reduced when it is gradually changed with a large number of layers interposed, and a method for producing the same. The method of the present invention and the gradient refractive index multilayer thin film structure produced thereby are partly the same in design and principle of the conventional antireflection film (AR film), but the system is completely different.

  The development of optical antireflection films on display surfaces, show windows, showcases, display frames, etc. has been actively conducted, and various materials and design methods have been used and commercialized. Especially in the display field, various types of FPD (Flat Panel Display) such as liquid crystal display, plasma display, rear projector, FED (Field Emission Display), OLED (Organic EL) have appeared instead of the conventional curved display CRT. However, there are a wide variety of applications ranging from small displays such as mobile phones to large screen displays such as large televisions. In many of these applications, antireflection processing of screens has been used to improve visibility.

The method for forming the antireflection layer is roughly divided into a dry method (vacuum film forming method) and a wet method (wet film forming method). The dry method is a method of coating a substrate surface mainly with a low-refractive-index metal fluoride or metal oxide by vapor deposition or sputtering. This has high film thickness accuracy and provides a very high performance antireflection effect, but has the disadvantage of low productivity and high cost. On the other hand, in recent years, the wet process technology that enables mass production at low cost by coating has advanced, and the film thickness accuracy has improved to some extent.
Patent Documents 1 to 3 and the like propose inexpensive antireflection films that are used on the display surface to improve contrast and prevent reflection of external light. The problems with the wet method are that coating with a submicron thickness cannot be accurately performed, and the coating layer must not dissolve the coating layer. For example, it is necessary to ensure the adhesiveness of the coating layer.

  The design principle of the antireflection layer that has been conventionally used is briefly described as follows (assuming that the incident angle is zero degree and the optical medium is not scattered or absorbed). This is the principle that is common to the dry and wet AR films.

(1) Reducing the intensity reflectance with a material having a low refractive index (1)
R = (n _f ² −n ₀ n _s ) ² / (n _f ² + n ₀ n _s ) ² (1)

(2) The phase of the reflected light on the front surface and the back surface is shifted by a half wavelength and canceled by the interference effect (2)
(When the refractive index of the thin film is smaller than the base material)
n _f d = λ / 4 (2)

In Formula (1) and Formula (2), n _{0 is} the refractive index of the external medium, n _{f is} the refractive index of the antireflection layer,
n _s : Refractive index of substrate, R: reflectance, d: thickness of antireflection layer, λ: wavelength of incident light.
From formula (1), it can be seen that the reflectance can be lowered by using a material having a low refractive index for the antireflection film.

In order to minimize the reflectance of the transparent body to which the thin film is attached, the effect of canceling out the interference effect by shifting the phase of the reflected light on the front and back surfaces of the antireflection layer by half wavelength is used in combination. Phase condition for this is, when the refractive index of the thin film is smaller than the base material (the low refractive material) n _{f d} = λ / 4, the refractive index of the thin film is greater than the base material (high-refractive index material) is n _f d = λ / 2. The total reflectance is designed to be minimum by forming a thin film layer having a thickness that matches the phase condition. The anti-reflective coating of two or more layers that is frequently used in the dry method, etc. is a wideband anti-reflective body that covers the entire visible light range by flattening the reflectivity curve by combining the wavelengths that minimize the reflectivity of each layer And

  On the other hand, SWW (Sub Wavelength Grating) or sub-wavelength grating is known as a completely different antireflection structure. This is because, when a large number of fine concavo-convex structures having a triangular cross section are formed on the surface, the refractive index is increased in the depth direction when the pitch is less than the wavelength of visible light (380 nm or less) and the depth is 200 to 300 nm or more. This is equivalent to the infinite number of continuously changing layers, and Fresnel reflection does not occur. The reflection of light is mainly caused by a sudden change in the refractive index of the incident surface. Therefore, if there is a structure in which the refractive index changes continuously and smoothly at the boundary where light enters, the incident light will eventually not be reflected. There is such a structure on the compound eye of the nocturnal moth, which gives the effect of capturing the light at night without reflecting it and the effect of avoiding the reflection of the eyes and finding them by natural enemies.

  The effect of reducing the reflectivity by the sub-wavelength grating is large, and the reflectivity characteristic is almost flat at 0.5% or less for the entire wavelength range of visible light. It can be said that this is the principle of the AR film, and the effect of reducing the reflectance with a material having a low refractive index is enhanced to the limit. Other advantages of the sub-wavelength grating include the fact that the wavelength of interest can be set to the entire visible light range because it is not canceled by the phase difference of the reflected light, and the influence of the incident angle is small.

Non-patent document 1 and patent document 4 show a technique using an antireflection body having a subwavelength grating structure.
This is a direct pattern replication technique in which the surface of the arthropod compound eye is pressed against a polymer or the like to transfer the shape of the antireflection structure. Arthropods are not limited to pupae.
Patent Documents 5 and 6 describe structures having a periodic structure with a period smaller than the wavelength of light on the light incident surface. This description relates to the fact that this structure exhibits various optical functions including an antireflection function. Antireflection is said to be possible by creating a conical fine periodic structure. However, this patent does not mention the manufacturing method.

  Patent Document 7 discloses a technique in which low refractive index polymer spherical particles are internally added in a film to exhibit antireflection and antiglare properties. The content is that the film material to be a medium and the refractive index of the spherical particles are selected, and the same optical characteristics as the sub-wavelength grating structure are exhibited.

: JP 2001-21706 A : JP 2002-71904 A : JP 2002-55205 A : JP-A-2003-139905 : JP 2002-286906 A : JP 2004-170508 A : JP 2004-26974 A ： Journal of Optical Society of America A, Vol.12, No.2, 333 (1995)

  Antireflection films designed by conventional optical calculations and prepared by the dry method / wet method are widely distributed in the market as actual products, but the antireflection effect is limited in the following points.

(1) As the material of the antireflection film, the lower the refractive index, the higher the effect. However, the refractive index of the low refractive index material is about 1.35 for polymers (cyclic fluororesin) and about 1.37 for inorganic substances (magnesium fluoride), and there is nothing lower than that. Therefore, the value of R is limited from the above equation (1).

(2) The target of equation (2) is coherent monochromatic light. That is, in the case of natural light that is not coherent, the cancellation effect due to the phase difference is incomplete, and when the reflectance is measured with respect to the wavelength, a curve called a reflectance curve is drawn. In order to make this curve as flat as possible, the dry method or the like has been devised such that the antireflection layer has two or more layers and the phase is shifted by 1 / 2λ with the lower high refractive index layer. Actually, in order to obtain a good antireflection effect for the entire visible light wavelength range, 4 to 5 antireflection layers are required, but it is still difficult to flatten the reflectance curve over the entire visible light range. Absent.

(3) A multilayer film composed of layers having different refractive indexes has a problem of not only an adaptive wavelength but also an adaptive incident angle. Equation (2) is effective under conditions close to normal incidence and reduces the reflectivity. However, since natural light is incident from all angles in the actual usage situation, the calculated effect is not necessarily obtained. Absent.

The antireflection structure using the sub-wavelength grating described in paragraph 0009 has the following problems.
(1) Mass production of a large-area sub-wavelength grating is not easy due to the size of the protrusions and the required accuracy. At present, it is difficult to mass-produce the lithography method at a low price, and even when the nanoimprint method is used, the productivity does not increase in view of the availability of the stamper and the pattern transferable area.

(2) Since the tip of the protrusion is sharp and sharp, it is easily damaged, and dirt or the like is likely to adhere and accumulate in the groove between the protrusions. When dirt accumulates in the groove of the sub-wavelength grating, the structure in which the refractive index gradually changes is no longer present around the groove, and the antireflection effect is reduced.
In order to obtain the effect of the sub-wavelength grating while including these various problems, the presence of fine protrusions is indispensable, and the structure needs to be positioned on the outermost surface.

(3) Moreover, the following can be considered as problems found in other existing technologies.
Patent Document 4 has a drawback in that when it is actually industrial production in mind, imprinting is performed using a mold having a very small area, resulting in poor efficiency.
Patent Document 5 and Patent Document 6 do not mention a manufacturing method, and do not clearly show how to realize such an optical structure.
Patent Document 7 is excellent in that a material having a different refractive index is combined to form a flat film having an antireflection / antiglare effect. However, since all the material elements are polymers, this structure The lower limit of the refractive index of the constituent part cannot be so low. Therefore, it is considered that a significant antireflection effect cannot be obtained.

  The present invention solves the above-mentioned problems of the antireflection film by the conventional optical design and the problems of the antireflection structure by the subwavelength grating at the same time, and has an excellent antireflection function and is industrially produced. It is intended to provide a possible surface structure.

  As a means for solving the above problem, when forming a structure composed of a plurality of thin film layers having a graded gradient structure between the external medium and the substrate, it is equivalent to a subwavelength grating, and It has been found that it has an antireflection effect superior to that of the conventional AR film, and since the surface is smooth, it is possible to provide an antireflection structure having greatly improved antifouling properties and durability, which are problems of subwavelength gratings. The present invention has been reached. The present invention includes the following inventions.

(1) A multilayer structure including at least three thin film layers having a transmittance of 70% or more and a thickness of 380 nm or less over the entire visible light region, and the apparent refractive index of each layer in the visible light region A gradient refractive index multilayer thin film characterized in that the apparent refractive index is greatly changed stepwise from 1.0 to 2.5 in the direction from the outermost surface layer closest to the lowermost layer to the base material.

(2) Low refraction in which at least one of the three or more thin film layers constituting the multilayer film structure contains nanoparticles having an average diameter of 380 nm or less prepared on the principle of a sol-gel method using a metal alkoxide The refractive index gradient multilayer thin film according to item (1), which is an index layer.

(3) At least one of the three or more thin film layers constituting the multilayer film structure is composed of magnesium fluoride (refractive index 1.37), lithium fluoride (refractive index 1.39), calcium fluoride ( Item (1) or Item 2, characterized in that it is a layer containing nanoparticles mainly composed of inorganic particles selected from refractive index 1.43) and barium fluoride (refractive index 1.48). The refractive index gradient multilayer thin film described.

(4) At least one of the thin film layers constituting the multilayered film structure is a layer containing nanoparticles mainly composed of hollow or porous inorganic particles, (1) The refractive index gradient multilayer thin film according to any one of items 1 to (3).

(5) The refractive index gradient multilayer thin film according to (3) or (4), wherein the inorganic particles are nanoparticles having an average particle diameter of 3 nm to 380 nm.

(6) The nanoparticle according to any one of items (1) to (5), wherein the nanoparticles are fixed to each other by using a binder or by heat fusion between the particles. Refractive index gradient multilayer thin film.

(7) At least one of the three or more thin film layers constituting the multilayer film structure is a thin film layer formed by applying an organic polymer material having a low refractive index, The refractive index gradient multilayer thin film according to (1) or (2).

(8) The antifouling layer having a thickness of 3 to 10 nm is formed on the outermost surface layer closest to the external medium, according to any one of the items (1) to (7), Refractive index gradient multilayer thin film.

(9) A method of depositing one type selected from the gradient refractive index multilayer thin film according to (1) to (8),
When forming three or more thin film layers forming the gradient refractive index multilayer thin film, for each thin film layer, the secondary aggregation state of the nanoparticles in the coating liquid used at the time of film formation is controlled. By adjusting the particle gap in each thin film layer formed from the coating solution, the average volume porosity is in the range of 10 to 95%, and the apparent refractive index in the visible light region is 1.0 to 2. Forming a layer that is within the range of 5;
Each thin film layer is gradually increased in the visible refractive index in the range of 1.0 to 2.5 from the outermost layer closest to the external medium toward the lowermost layer closest to the substrate. The method for forming a gradient refractive index multilayer thin film is characterized by sequentially stacking layers.

(10) The method according to (9), wherein two or more kinds of nanoparticles having different particle diameters are used as the nanoparticles.

(11) The coating liquid used at the time of film formation contains a binder component, and the content of the binder component between the nanoparticles is changed so that the average volume porosity is in the range of 10 to 95% and visible. The method for forming a gradient refractive index multilayer thin film according to (9) or (10), wherein a thin film layer having an apparent refractive index in the optical region of 1.0 to 2.5 is formed.

(12) By combining the methods described in the items (10) and (11), the apparent refractive index in the visible light region is 1.0 to 2.5 in the range of the average volume porosity of 10 to 95%. A method for forming a gradient refractive index multilayer thin film according to item (9), wherein a thin film layer between the layers is formed.

(13) A means for controlling the secondary aggregation state of the nanoparticles in the coating liquid used at the time of film formation and adjusting the particle gap in each thin film layer formed from the coating liquid is a metal alkoxide sol-gel The method for forming a gradient refractive index multilayer thin film according to any one of (9) to (13), wherein the method is applied to control the porosity between the particles.

(14) The metal alkoxide sol-gel method includes adding a metal alkoxide to water, hydrolyzing it as an aqueous solution, adding an organic solvent having low compatibility to the aqueous solution, and vigorously stirring the aqueous phase into the organic solvent phase. The method of forming a graded refractive index multilayer thin film according to (13), wherein the dispersion vesicles are dispersed into O / W emulsions and polymerization is carried out by dehydration condensation between metal alkoxide molecules in each dispersion vesicle.

  It was shown that the structure having a graded gradient structure in the apparent refractive index obtained by the method described in the present invention has an antireflection effect equivalent to that of the subwavelength grating. This is an antireflection structure that has antireflection properties superior to those of conventional AR films and has a smooth surface, and thus has significantly improved antifouling properties and durability, which are problems of subwavelength gratings. Further, unlike the conventional AR film, the phase of incident light and reflected light is shifted by a half wavelength and a method for enhancing the antireflection effect by interference is not involved, so that there is little wavelength dependence and an antireflection effect can be obtained for the entire visible light region. .

According to the present invention, several antireflection layers that gradually increase the apparent refractive index are formed in order from the outermost layer of the light incident surface, and the refractive index of the substrate is gradually increased from the refractive index 1.0 of the external medium (air). The above-mentioned problem can be solved by forming a structure that approaches the surface.
Specifically, for example, in the case of a five-layer structure formed on a quartz substrate [refractive index 1.45 (λ = 600 nm)], a low refractive index layer having an apparent refractive index of about 1.1 is formed on the uppermost layer. Next, a second layer of about 1.2 and a third layer of about 1.3 continue, and a fourth layer having an apparent refractive index of about 1.4 is disposed on the surface in contact with the quartz substrate. The refractive index of the space from the substrate to the substrate is tilted. This intermittent refractive index gradient structure is equivalent to the continuous refractive index gradient structure which is the principle of the sub-wavelength grating, and finally makes Fresnel reflection almost zero. The target apparent refractive index of each layer of the antireflection structure is preferably a value obtained by dividing the difference in refractive index between the external medium and the substrate by the number of layers, and has a substantially linear refractive index gradient.

  The thickness of each layer is preferably equal, and the total thickness of the antireflection structure formed by each coating layer is at least 150 nm (nanometers), preferably 200 to 2000 nm. The reason is that if the thickness of each layer is not uniform, a linearly inclined structure having a refractive index similar to the principle of the sub-wavelength grating cannot be obtained, and the total thickness of the antireflection structure is less than 150 nm. In the case where it is small, the refractive index changes abruptly in view of the wavelength of visible light, and the effect of the sub-wavelength grating cannot be obtained, resulting in Fresnel reflection. In principle, the effect of canceling the phase of incident light and reflected light by shifting them by half a wavelength as in the case of a normal AR film is not performed by the sub-wavelength grating and the gradient index structure of the present invention. No exact calculation is required for the thickness of each layer. However, it is confirmed by optical calculation that about 100 nm gives a good result.

  In the present invention, inorganic nanoparticles are used as a component constituting the antireflection layer. When the particle size of the inorganic nanoparticles is larger than the wavelength of visible light, specifically, the particle size is larger than 380 nm, the reflected light is scattered (diffuse reflection), so the visible light transparency (transparency) of the film is lowered. It is not preferable. Accordingly, it is desirable that the inorganic particles used in the present invention have a diameter of not more than the wavelength of visible light, specifically, a diameter of 100 nm or less, and more specifically a diameter of 50 nm or less. Those having a particle size in this range are so-called nanoparticles. The diameter of the inorganic nanoparticles of the present invention is substantially 3 nm. The material used for the particles need not be optically transparent in the form of a macro lump. If the particle diameter satisfies the specified size, the particle size is sufficiently smaller than the wavelength of visible light, so that no scattering occurs and the film becomes optically transparent.

  The apparent refractive index of each layer formed by the inorganic particles described above is determined by the following factors. That is, the intrinsic refractive index of the solid substance constituting the particle, and the porosity inside and between the particles. The apparent refractive index of each layer is nothing but an arithmetic mean using the volume ratio of the refractive index of the air contained in the void (n = about 1.00) and the refractive index of the solid material constituting the particle, By controlling, each layer can be designed to have an intended refractive index.

  As a specific measure for producing a coating layer having a necessary refractive index, the present invention proposes a method of controlling the porosity between the particles by applying a sol-gel method of metal alkoxide. That is, a metal alkoxide is added to water to make an aqueous solution and hydrolyzed. Although both acid and alkali can be used as a catalyst for hydrolysis, hydrolysis of aminosilane-based metal alkoxide proceeds sufficiently even without a catalyst. By adding an organic solvent with low compatibility to this aqueous solution and stirring vigorously, the aqueous phase is dispersed as fine dispersed cells in the organic solvent phase to form an O / W emulsion, and dehydration condensation between metal alkoxide molecules in each dispersed cell. Proceed with polymerization. In the polymerization process, the metal alkoxide has a particle size with increasing molecular weight in the order of water-soluble component → sol → gel. The particle size of the emulsion is affected by the number of rotations of stirring, the stirring time, the shape of the container, etc., but the particle size of a normal emulsion is about 400 nm or more. If an emulsion having a particle size of 400 nm contains 0.04 vol% of a metal alkoxide component, the diameter of the particles after gelation / drying is 30 nm, which is the size of nanoparticles.

  As the metal alkoxide used in the application of the sol-gel method of the metal alkoxide, functional silane (silylating agent), so-called silane coupling agent, or the like is used. For example, tetramethoxysilane (refractive index 1.37), tetraethoxysilane (refractive index 1.38), monomethyltrimethoxysilane (refractive index 1.37), monomethyltriethoxysilane (refractive index 1.38), dimethyldimethoxy Silane (refractive index 1.37), dimethyldiethoxysilane (refractive index 1.38), phenyltrimethoxysilane (refractive index 1.47), 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3 -Glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, etc. are mentioned. The core metal does not have to be silicon, but may be other materials such as titanate.

  Since a phenomenon occurs in which the water-soluble component in each dispersion vesicle gradually gels, a plurality of nanoparticles are generated simultaneously in the entire dispersion. At this time, the organic solvent is volatilized in parallel with the gelation of the metal alkoxide. Finally, water and the organic solvent are completely vaporized and removed, and the nanoparticles are bonded and formed into a film at a low density. When the gelation state is adjusted in advance so that dehydration condensation can still be performed when the nanoparticles are in contact with each other, the nanoparticles are automatically bonded together with the drying of the solvent.

  In order to control the porosity and apparent refractive index in the coating layer space thus obtained, the type of metal alkoxide, the concentration of metal alkoxide in the aqueous phase, the size of the aqueous emulsion in the organic solvent, the aqueous phase And adjust the ratio of organic solvent phase. For example, when the size of the generated nanoparticles is increased by increasing the metal alkoxide concentration in the aqueous phase in the dispersion, the porosity of the film after drying is relatively large and the apparent refractive index is small. Conversely, when the size of the nanoparticles formed by reducing the concentration of the metal alkoxide in the aqueous phase is reduced, the film formation after drying becomes relatively dense and the porosity decreases, so the apparent refractive index is the metal alkoxide used. Close to the value of the bulk.

  Considering the filling of spheres, the density should not change even if the particle size changes. However, in the actual sol-gel method, as the particle size is smaller as described above, a denser and denser layer is formed. In the method in which the generated nanoparticles are bonded before they are completely gelled, the grain boundary is not point bonded but the surface of the particle is surface bonded using a certain area. Therefore, when the particle size of the nanoparticles is small, the number of particles increases, the total area of the particle surface used for adhesion increases, and the porosity in the coating layer decreases. On the other hand, when the particle size of the nanoparticles is large, the number of particles decreases, and the total area of the particle surfaces used for adhesion does not increase so much. By using this principle, it is possible to adjust the density of the coating layer made of nanoparticles prepared by the sol-gel method.

  The material obtained by application of the sol-gel method is applied by spin coating, slit die coating, dip coating, cap coating, etc., and thinned on the surface of the substrate. For example, the spin coating method can accurately control the coating thickness from several nanometers to several hundred nanometers and has good reproducibility, but it is a batch system that is not suitable for large-area coatings. There are also disadvantages such as not suitable. For example, the slit die coating method is suitable for large areas and mass production because continuous coating is possible, but has a drawback that a coating layer having a thickness of 100 nanometers or less cannot be formed with high accuracy. Any method or a combination of these methods may be used, but the coating is sequentially performed in layers so as not to dissolve the undercoat layer from the first layer. This is because if the new coating layer dissolves the undercoat layer, the film thickness accuracy cannot be maintained, and the designed antireflection effect cannot be obtained. For this purpose, the coating layer must be completely gelled and fixed for each layer.

In the present invention, the entire refractive index gradient structure can be formed by further combining the following optical layer with the low refractive index layer to which the sol-gel method is applied. Sunahachi,
(1) Layer coated with low refractive index organic polymer material such as fluorine resin, silicone resin, polyester, acrylic, olefin resin, etc. (2) Magnesium fluoride (refractive index 1.37), lithium fluoride Nanoparticles such as (refractive index 1.39), calcium fluoride (refractive index 1.43), barium fluoride (refractive index 1.48), silicon dioxide (refractive index 1.44) with organic or inorganic binder Fixed or heat-sealed layer

(3) Magnesium fluoride (refractive index 1.37), lithium fluoride (refractive index 1.39), calcium fluoride (refractive index 1.43), barium fluoride (refractive index 1.48), silicon dioxide ( A layer in which a low refractive index inorganic substance such as a refractive index of 1.44) is formed into a thin film by means such as vacuum deposition or sputtering. (4) Hollow or porous nanoparticles such as nanoporous silica (refractive index of 1.27) Or a layer fixed with an inorganic binder or heat-sealed,
Is combined with the low refractive index layer by the sol-gel method. Details are described below.

  As a material for forming the antireflection layer of the present invention, fluorine resin, silicone resin, polyester, acrylic, olefin resin and the like can be used. For example, tetrafluoroethylene-perfluorodioxole copolymer (refractive index 1.35), tetrafluoroethylene-hexafluoropropylene copolymer (refractive index 1.34), polytrifluoroethyl methacrylate (refractive index 1). .42), polytetrafluoroethylene (refractive index 1.35 to 1.38), polyvinyl acetate (refractive index 1.45 to 1.47), polyisobutyl methacrylate (refractive index 1.49), poly-4 -Methylpentene-1 (refractive index 1.47) can be mentioned as a specific example, but is not limited thereto.

  When such an organic polymer material is used, since the refractive index is only about 1.34, it is necessary to complete the entire refractive index gradient structure in combination with another low refractive index layer. In order to make these low-refractive-index organic polymer materials into a thin film as an antireflection layer, they are dissolved in an appropriate solvent at a predetermined concentration, and the spin coating method, slit die coating method, dip coating method, cap coating method already described, Apply by the bar coat method.

  As a material for forming the antireflection layer of the present invention, inorganic powder, inorganic particles, inorganic fine particles, or nanoparticles can be used. For example, magnesium fluoride (refractive index 1.37), lithium fluoride (refractive index 1.39), calcium fluoride (refractive index 1.43), barium fluoride (refractive index 1.48), silicon dioxide (refractive) The rate 1.44) can be cited as some specific examples, but is not limited thereto. When such an inorganic material is used, since the refractive index is only about 1.37, it is necessary to complete the entire refractive index gradient structure in combination with another low refractive index layer.

  When the antireflection layer is formed using the above-described inorganic particles, the reflected light is scattered (diffuse reflection) when the particle diameter is larger than the wavelength of visible light, specifically, when the particle diameter is larger than 380 nm. Therefore, the visible light transmittance (transparency) of the film is lowered, which is not preferable. Therefore, it is desirable that the inorganic particles used in the present invention have a diameter less than the wavelength of visible light, specifically a diameter of 100 nm or less, more specifically a diameter of 50 nm or less. Is done. The diameter of the inorganic nanoparticles of the present invention is substantially 3 nm.

  The material used for the particles need not be optically transparent in the form of a macro lump. If the particle diameter satisfies the specified size, the particle size is sufficiently smaller than the wavelength of visible light, so that no scattering occurs and the film becomes optically transparent. The particles may have any shape, but the aspect ratio of the major axis to the minor axis is preferably within 10 to obtain optical isotropy. In the present invention, in order to form an antireflection layer having a target refractive index, two or more kinds of inorganic nanoparticles having different refractive indexes may be mixed and used at a predetermined ratio.

Such inorganic nanoparticles are filled at a low density to form a low refractive index layer. A coating layer having a required apparent refractive index is prepared by controlling the density. This is because air (refractive index of 1.0) occupies the space remaining between the nanoparticles, so that the apparent refractive index of the entire layer is lowered. In order to form a low-density nanoparticle layer, monodispersed nanoparticles are applied in a dilute binder liquid, and the dispersion medium is evaporated to adhere and fix the particles with a binder component. During the drying process, the binder component remaining at the grain boundaries of the inorganic nanoparticles binds the particles together.
When inorganic nanoparticles form secondary aggregates in the course of evaporation, the aggregates may be clogged with binder components, resulting in substantial increase in particle size, loss of optical transparency, and low density. Therefore, a low refractive index cannot be realized. In addition, the accuracy of the film thickness is lowered, which is not preferable. Therefore, the dispersion medium must be vaporized while maintaining the monodispersed state as much as possible.

  In order to form a monodispersed layer, it is effective to vaporize the dispersion medium while controlling the zeta potential in the vicinity of the particles so that the repulsive force acting between the particles is always maintained. It is a simple method. For this reason, a small amount of zeta potential control agent is added to control repulsive force between particles. This method is advantageous for forming an antireflection layer having a low density and a low refractive index because it delays the time until the particles aggregate in the process of removing the solvent and the filling rate of the particles in the layer decreases. .

  As another means for controlling the density, there is a method in which two or more kinds of particles having different particle diameters are mixed and used as an antireflection layer. In this method, when the antireflection layer is composed only of particles having a relatively large particle size, the apparent refractive index is low because the porosity is high, and when particles having a relatively large particle size are combined with small particles, The apparent refractive index increases because the porosity decreases. The size of the particle diameter here refers to a difference in relative size and need not be specifically described. However, when the particle size is about 5 nm or less, a phenomenon occurs in which the grain boundary disappears or becomes unclear when the solvent of the dispersion is dried and vaporized. In other words, the film-forming property appears. Although such a particle size is called a critical particle size, when only particles satisfying this condition are used, the void ratio of the antireflection layer is remarkably reduced as a result, so that the apparent refractive index is increased.

  In addition, there is a method of adjusting the porosity between particles by changing the amount of the binder material remaining between the nanoparticles. This is to determine the blockage rate of the interparticle voids in each coating layer by setting several concentrations of the binder material to be used in advance and controlling the residual solid content after vaporizing the dispersion medium.

  A sufficient amount of the binder material is required to adhere the grain boundaries of the nanoparticles forming each antireflection layer, but it should not be used in excess. This is because when the binder component becomes a lump and has a diameter larger than the wavelength of visible light, influences such as coloring of the antireflection layer, generation of scattered light, and reduction of the antireflection effect appear. Further, the binder component may move mainly in the thickness direction due to migration during vaporization of the dispersion medium and in the in-plane direction depending on the drying conditions. Since this occurs when the binder liquid moves to a place close to the heat source during drying, that is, a place where the drying speed is partially high, it is basically inevitable whether it is hot air drying or air drying. However, in the case of hot air drying, migration can be reduced by lowering the drying temperature, and migration can be prevented more effectively by using air drying. The most ideal method is to sublimate the dispersion medium by freeze drying. In this case, the migration is considered to be zero, but it is not so realistic because the production efficiency is low.

  A metal alkoxide can be used as a binder (adhesive) component used for bonding the inorganic nanoparticles to form an antireflection layer. For example, tetramethoxysilane (refractive index 1.37), tetraethoxysilane (refractive index 1.38), monomethyltrimethoxysilane (refractive index 1.37), monomethyltriethoxysilane (refractive index 1.38), dimethyldimethoxysilane Examples thereof include functional silanes (silylating agents) such as (refractive index 1.37), dimethyldiethoxysilane (refractive index 1.38), and phenyltrimethoxysilane (refractive index 1.47). The core metal does not have to be silicon, and may be other materials such as titanate.

  Examples of the organic material binder that binds the inorganic nanoparticles include acrylic, epoxy, vinyl acetate, silicone, and polyurethane adhesives. However, the form of the binder that can be used in the present invention is limited to the solution type. When the binder is an emulsion type, the particle size is usually 400 nm or more, so that a lump of the solid component of the adhesive appears in the inorganic nanoparticles after solvent drying and may scatter visible light from its diameter. is there.

  Inorganic nanoparticles are dispersed together with a binder component in a dispersion medium before coating. As the dispersion medium at this time, water, an organic solvent, or a mixed system of water and an organic solvent is used. Organic solvents mixed with water include alcohols such as methanol, ethanol, isopropanol, ethylene glycol, diethylene glycol, triethylene glycol, butylene glycol, glycerin, N-methylpyrrolidone, dimethyl sulfoxide, phenols, amines, ketones, etc. However, the present invention is not particularly limited thereto. The organic solvent mixing ratio in the coating liquid dispersion medium is usually 50% or less, preferably 30% or less. When the nanoparticles are inorganic, water or a mixed system of water and an organic solvent is used. Further, regardless of whether the nanoparticles are organic or inorganic, the dispersion medium must not dissolve the dispersion (particles) excessively, and the solubility product of the dispersion medium with respect to the dispersion (particles) is preferably about 0.01 or less. .

  In any case, the low refractive index layer can be easily formed by combining with the application of the sol-gel method described above. This is because, for example, when an organic polymer material is coated with a solvent, the refractive index is only about 1.35 at least, so that a low refractive index layer having an apparent refractive index of about 1.1 is formed on the surface layer. This is because it is effective to combine the application of the sol-gel method of the present invention. When a low density layer is formed using nanoparticles or hollow nanoparticles, a certain low refractive index layer can be formed. However, since the application of the sol-gel method of the present invention is excellent with respect to the ease of controlling the refractive index, it is possible to more easily create a laminated structure having a refractive index near the outermost layer near 1.1.

  The antireflection film of the present invention is provided with an antifouling layer on the coating layer having the lowest refractive index, which is the uppermost layer, after laminating all the layers of the gradient refractive index multilayer thin film, if necessary. be able to. The antifouling layer has a thickness of 10 nm or less, preferably 3 to 5 nm, and is formed by forming a film by a coating method or a vapor deposition method. The thickness of the antifouling layer is important. By setting the thickness to 3 to 5 nm, the antifouling layer is an optically negligible layer that is located on the outermost surface and does not participate in the antireflection effect. If the thickness exceeds the above numerical value, an optical influence such as reflection / scattering appears, which is not preferable.

    Since the material for forming the antifouling layer is a layer formed on the outermost surface, it is required to have chemical resistance, wiping properties, weather resistance, hardness, etc., but it is an extremely thin layer. The minimum performance required for this layer is wipeability and hardness. Widely used layer-forming materials are fluororesins and metal alkoxysilanes, and these are applied as a dilute solution on the top layer by a coating method. In addition, a metal or a metal oxide may be laminated as a thin film by a vapor deposition method or a sputtering method. By providing such an antifouling layer, the surface strength can be improved in addition to preventing contamination. In particular, since the uppermost layer of the antireflection layer aiming at a refractive index of about 1.1 is formed of nanoparticles in a low density state, there is a concern about insufficient strength. However, such an antifouling layer should be provided thereon. Therefore, it is possible to prevent particles from falling off, prevent damage to low-density structures, prevent entry of dirt substances such as sebum between the particles, and other anti-fixing effects of surface dirt that can be considered in use. The structure can be finished into a stronger practical product.

  It is also effective to use an organic / inorganic primer, a silane coupling agent, or the like in order to increase the adhesion between the layers of the gradient refractive index multilayer thin film structure of the present invention. However, when using it, it is important to check in advance whether there is adhesion with each layer, and the use should be performed in a very dilute state to form an extremely thin interlayer adhesive layer close to a monomolecular layer. Is ideal. This is because when the thickness of the adhesive layer has an optical influence, the refractive index gradient structure as calculated is not obtained.

  In order to explain specific features of the present invention in detail, examples will be described below. Note that the present invention is not limited to these examples as long as the gist of the present invention is reproduced. Moreover, the part in each Example and a comparative example shows a mass part.

Example 1
(First layer)
Monomethyltrimethoxysilane was added to water at 0.001% by volume and hydrolyzed with acetic acid for 1 hour. This aqueous solution and methyl ethyl ketone were mixed at 35 parts to 65 parts and stirred vigorously with a homogenizer to obtain an emulsion (strong stirring dispersion). Thereafter, the number of rotations was weakened and the temperature was maintained at 60 ° C. for 3 hours while continuing the stirring, so that the metal alkoxide was gelled in the aqueous phase. When gelation progressed to some extent, a dispersion was applied onto an optical quartz substrate with a spin coater at 1500 rpm to form a film. Next, the quartz substrate was heated and dried at 120 ° C. for 5 minutes on a heater to continue the gelation of the metal alkoxide and vaporize the solvent and water. The first layer thus obtained had an average coating layer thickness of about 97 nm and an apparent refractive index of 1.36 (measured by a single beam ellipsometer). Details of the first layer are shown in Table 1.

(Second layer)
Monomethyltrimethoxysilane was added to water at 0.003% by volume and hydrolyzed with acetic acid for 1 hour. This aqueous solution and methyl ethyl ketone were mixed in 25 parts to 75 parts and stirred vigorously with a homogenizer to obtain an emulsion (strong stirring dispersion). Thereafter, the number of rotations was weakened and the temperature was maintained at 60 ° C. for 3 hours while continuing the stirring, so that the metal alkoxide was gelled in the aqueous phase. When gelation progressed to some extent, the dispersion was applied onto the first layer on the optical quartz substrate with a spin coater at 1500 rpm to form a film. Next, the quartz substrate was heated and dried at 120 ° C. for 5 minutes on a heater, followed by gelation of the metal alkoxide and a step of vaporizing the solvent and water. The second layer thus obtained had an average coating layer thickness of about 99 nm and an apparent refractive index of 1.30 (measured with a single beam ellipsometer). Details of the second layer are shown in Table 1.

(3rd layer)
Monomethyltrimethoxysilane was added to water at 0.009% by volume and hydrolyzed with acetic acid for 1 hour. This aqueous solution and methyl ethyl ketone were mixed in 17 parts to 83 parts and stirred vigorously with a homogenizer to obtain an emulsion (strong stirring dispersion). Thereafter, the number of rotations was weakened and the temperature was maintained at 60 ° C. for 3 hours while continuing the stirring, so that the metal alkoxide was gelled in the aqueous phase. When gelation progressed to some extent, the dispersion was applied onto the second layer on the optical quartz substrate by a spin coater at 1500 rpm to form a film. Next, the quartz substrate was heated and dried at 120 ° C. for 5 minutes on a heater, followed by gelation of the metal alkoxide and a step of vaporizing the solvent and water. The third layer thus obtained had an average coating layer thickness of about 103 nm and an apparent refractive index of 1.23 (measured by a single beam ellipsometer). Details of the third layer are shown in Table 1.

(Fourth layer)
Monomethyltrimethoxysilane was added to water at 0.02% by volume and hydrolyzed with acetic acid for 1 hour. This aqueous solution and methyl ethyl ketone were mixed in 12 parts to 88 parts and stirred vigorously with a homogenizer to obtain an emulsion (strong stirring dispersion). Thereafter, the number of rotations was weakened and the temperature was maintained at 60 ° C. for 3 hours while continuing the stirring, so that the metal alkoxide was gelled in the aqueous phase. When gelation progressed to some extent, the dispersion was applied onto the third layer on the optical quartz substrate by a spin coater at 1500 rpm to form a film. Next, the quartz substrate was heated and dried at 120 ° C. for 5 minutes on a heater, followed by gelation of the metal alkoxide and a step of vaporizing the solvent and water. The fourth layer thus obtained had an average coating layer thickness of about 100 nm and an apparent refractive index of 1.16 (measured by a single beam ellipsometer). Details of the fourth layer are shown in Table 1.

(5th layer)
Monomethyltrimethoxysilane was added to water at 0.04% by volume and hydrolyzed with acetic acid for 1 hour. This aqueous solution and methyl ethyl ketone were mixed in a ratio of 7 parts to 93 parts, and stirred vigorously with a homogenizer to obtain an emulsion (strong stirring dispersion). Thereafter, the number of rotations was reduced and the temperature was kept at 60 ° C. for 3 hours while continuing the stirring, so that the metal alkoxide was gelled in the aqueous phase. When gelation progressed to some extent, the dispersion was coated on the fourth layer on the optical quartz substrate with a spin coater at 1500 rpm to form a film. Next, the quartz substrate was heated and dried at 120 ° C. for 5 minutes on a heater, followed by gelation of the metal alkoxide and a step of vaporizing the solvent and water. The fifth layer thus obtained had an average coating layer thickness of about 102 nm and an apparent refractive index of 1.09 (measured with a single beam ellipsometer). Details of the fifth layer are shown in Table 1.

With respect to the optical quartz substrate on which the coating layer having a five-layer structure manufactured according to the above method was formed, the reflectance over the entire visible light range was measured. The results are shown in FIG. From the reflectance measurement result of FIG. 1, the reflection intensity of the product manufactured in Example 1 of the present invention is 3% or less with respect to the entire visible light region, and it can be evaluated that the optical characteristics of very low reflectance are exhibited. In addition, unlike ordinary antireflection films treated with AR coating, the reflectance in both the low wavelength region and the high wavelength region is significantly reduced. Such a characteristic that the reflectance is not wavelength-dependent is seen in the sub-wavelength grating, and it can be seen that Example 1 of the present invention gives the same effect as the sub-wavelength grating by the coating method. In other words, it can be seen that the sub-wavelength grating effect can be obtained by combining the method of applying the gelation of the metal alkoxide with the multilayer coating.
The thickness and apparent refractive index of each single layer from the first layer to the fifth layer are the same as the formation conditions of each layer in the case of forming a laminated structure, and each single layer is formed on the optical quartz substrate. And measured.

Example 2
(First layer)
Silica particles having an average particle diameter of 20 nm and a refractive index of 1.44 were dispersed in water at 1% by volume using a homogenizer, and 0.14 volume of monomethoxysilane hydrolyzed with 1.0 ppm of acetic acid for 1 hour. % To give a coating solution. This coating solution is spin-coated on an optical quartz plate (thickness: 1.5 mm) at 1500 rpm, water as a solvent is removed by air drying, and alkoxysilane used as a binder is reacted with inorganic particles. Heat at 5 ° C. for 5 minutes. The first layer thus formed had an average coating layer thickness of about 102 nm and an apparent refractive index of 1.35 (measured with a single beam aepsometer). Details are shown in Table 2.

(Second layer)
Silica particles having an average particle size of 20 nm and a refractive index of 1.44 were dispersed in water at 1% by volume using a homogenizer, and monomethyltrimethoxysilane hydrolyzed with 1.0 ppm of acetic acid for 1 hour to 0.13. It added by volume% and was set as the coating liquid. This coating liquid is spin-coated on an optical quartz substrate (thickness 1.5 mm) at 1500 rpm, water as a solvent is removed by air drying, and alkoxysilane used as a binder is reacted with inorganic particles. Heating was performed at 5 ° C. for 5 minutes. The second layer thus obtained had an average coating layer thickness of about 99 nm and an apparent refractive index of 1.27 (measured by a single beam ellipsometer). Details are shown in Table 2.

(3rd layer)
A nanoporous silica particle having an average particle diameter of 20 nm and an apparent refractive index of 1.27 was dispersed in water at 1% by volume using a homogenizer, and monomethyltrimethoxysilane hydrolyzed with 1.0 ppm of acetic acid for 1 hour to 0%. It was added at 12% by volume to obtain a coating solution. This coating liquid is spin-coated on an optical quartz substrate (thickness 1.5 mm) at 1500 rpm, water as a solvent is removed by air drying, and alkoxysilane used as a binder is reacted with inorganic particles. Heating was performed at 5 ° C. for 5 minutes. The third layer thus obtained had an average coating layer thickness of about 98 nm and an apparent refractive index of 1.19 (measured by a single beam ellipsometer). Details are shown in Table 2.

(Fourth layer)
A nanoporous silica particle having an average particle size of 20 nm and a refractive index of 1.27 was dispersed in water at 1% by volume using a homogenizer, and monomethyltrimethoxysilane hydrolyzed with 1.0 ppm of acetic acid for 1 hour was added to 0.1%. It added by 14 volume% and was set as the coating liquid. This coating liquid is spin-coated on an optical quartz substrate (thickness 1.5 mm) at 1500 rpm, water as a solvent is removed by air drying, and alkoxysilane used as a binder is reacted with inorganic particles. Heating was performed at 5 ° C. for 5 minutes. The fourth layer thus obtained had an average coating layer thickness of about 100 nm and an apparent refractive index of 1.11 (measured with a single beam ellipsometer). Details are shown in Table 2.

  The thickness and the apparent refractive index of each single layer from the first layer to the fourth layer are the same as the formation conditions of each layer in the case of forming a laminated structure, and each single layer is formed on the optical quartz substrate. And measured.

In Example 2, the materials described below were used as materials such as silica and nanoporous silica.
(Silica particles)
Product Name: Cataloid S1-50
Manufacturer: Catalyst Kasei Co., Ltd.
Average particle diameter: about 20 nm
50% aqueous dispersion

(Nanoporous silica)
Manufacturer: Sumitomo Osaka Cement Co., Ltd.
Average particle size: about 30nm
7.7% aqueous dispersion

With respect to the optical quartz substrate on which the coating layer having a four-layer structure manufactured according to the above method was formed, the reflectance over the entire visible light range was measured. The results are shown in FIG.
From the reflectance measurement results of FIG. 2, it can be said that the reflection intensity of Example 2 of the present invention is 3% or less with respect to the entire visible light range, and exhibits very low reflectance optical characteristics. Also, unlike ordinary AR-coated antireflection films, the reflectance at low and high wavelengths is significantly reduced. Such a characteristic that the reflectance is not wavelength-dependent is seen in the sub-wavelength grating, and it can be seen that Example 2 of the present invention gives the same effect as the sub-wavelength grating by the coating method. In other words, the subwavelength grating effect could be obtained by combining commercially available nanoparticles and multilayer coating.

Comparative Example 1
Obtain a commercially available AR coating film [trade name, KAYACOAT ARS-D250TG-125 (manufactured by Nippon Kayaku Co., Ltd.) (single layer type: this film is made by the wet coating method)], and reflectivity characteristics with MCPD100 manufactured by Otsuka Electronics As a result of the measurement, a reflectance curve as shown in FIG. 3 was obtained. As is apparent from FIG. 3, the reflectance on the low wavelength / high wavelength side is high and wavelength dependence is seen, so that the antireflection effect is not high in all wavelength regions.

Comparative Example 2
When a commercially available AR coat film [trade name, AR1-1215 (PET base: multi-layer type) manufactured by Sony Chemical Co., Ltd.] was obtained and the reflectance characteristics were measured with MCPD100 manufactured by Otsuka Electronics, a reflectance curve as shown in FIG. 4 was obtained. It was. This film is an AR film prepared by a sputtering method, and the region where the reflectivity is about 1% is a range of 430 to 600 nm, which is wider than that of Comparative Example 1, but it is still low wavelength and high Since the reflectance on the wavelength side is remarkably high and wavelength dependency is seen, it cannot be said that the antireflection effect is high in all wavelength regions.

Comparative Example 3
A sub-wavelength grating structure [test product manufactured by NTT-AT (PC base)] prepared by the stamp-type nanoprint method was obtained, and its reflectance characteristics were measured with an MCPD100 manufactured by Otsuka Electronics. A curve was obtained. This film shows no wavelength dependence and realizes a low reflectance of 2% or less over the entire visible light range. However, the fabrication of this structure requires a mold with nano-level surface unevenness processing, and also requires an operation of pressing and transferring the uneven surface of the mold surface to the substrate surface for a certain period of time. For some reason, there is a problem that it is not suitable for production on a commercial scale because of its high cost and slow production speed.

Comparative Example 4
The multilayer AR coating film (multilayer structure) published by Tokai Optical Co., Ltd. on the Internet is said to have the reflectance shown in FIG. This film is an AR film prepared by a vapor deposition method, and a region having a reflectance of 1% or less is in a relatively wide range of 420 to 640 nm. However, when the entire visible light region of 400 to 800 nm is targeted, the reflectance in the low wavelength region and the high wavelength region is still high and the wavelength dependence is seen, so it cannot be said that the reflectance in the entire wavelength region is low. .

  As is clear from the above examples and comparative examples, the gradient refractive index multilayer thin film structure of the present invention has antireflection characteristics superior to those of conventional AR films, and is a layer surface formed by a coating method. Is an anti-reflection structure with significantly improved antifouling properties and durability, which are problems of sub-wavelength gratings. Further, unlike the conventional AR film, the phase of incident light and reflected light is shifted by a half wavelength and a method for enhancing the antireflection effect by interference is not involved, so that there is little wavelength dependence and an antireflection effect can be obtained for the entire visible light region. .

The figure which shows the reflective characteristic of the refractive index gradient multilayer thin film produced in Example 1 The figure which shows the reflective characteristic of the refractive index gradient multilayer thin film produced in Example 2 The figure which shows the reflective characteristic of AR coat film of the comparative example 1 The figure which shows the reflective characteristic of AR coat film of the comparative example 2 The figure which shows the reflective characteristic of the subwavelength structure of the comparative example 3 The figure which shows the reflective characteristic of AR coat film of the comparative example 4

Claims (13)

  A multilayer film structure including at least three thin film layers having a transmittance of 70% or more over the entire visible light range and a thickness of 380 nm or less, and the apparent refractive index of each layer in the visible light region is closest to the external medium A gradient refractive index multilayer thin film characterized in that the apparent refractive index is greatly changed stepwise from 1.0 to 2.5 in the direction of the lowermost layer closest to the base material from the outermost layer.   Low refractive index layer containing nanoparticles having an average particle size of 380 nm or less, wherein at least one of the three or more thin film layers constituting the multilayer structure is prepared by the principle of a sol-gel method using a metal alkoxide The gradient refractive index multilayer thin film according to claim 1, wherein   At least one of the three or more thin film layers constituting the multilayer structure is composed of magnesium fluoride (refractive index 1.37), lithium fluoride (refractive index 1.39), calcium fluoride (refractive index 1). .43) and a nanoparticle containing inorganic particles selected from barium fluoride (refractive index: 1.48) as a main constituent, the refractive index gradient according to claim 1 or 2. Multilayer thin film.   At least one of the thin film layers constituting the multilayer film structure is a layer containing nanoparticles mainly composed of hollow or porous inorganic particles. The refractive index gradient multilayer thin film according to any one of the above items.   The refractive index gradient multilayer thin film according to claim 3 or 4, wherein the inorganic particles are nanoparticles having an average particle diameter of 3 nm to 380 nm.   The refractive index gradient multilayer thin film according to any one of claims 1 to 5, wherein the nanoparticles are fixed to each other by using a binder or by thermal fusion of the particles.   2. The thin film layer formed by coating at least one of the three or more thin film layers constituting the multilayer film structure with an organic polymer material having a low refractive index. The refractive index gradient multilayer thin film of any one of -6. A method for forming a film selected from the gradient refractive index multilayer thin film according to claim 1,
When forming three or more thin film layers forming the gradient refractive index multilayer thin film, for each thin film layer, the secondary aggregation state of the nanoparticles in the coating liquid used at the time of film formation is controlled. By adjusting the particle gap in each thin film layer formed from the coating solution, the average volume porosity is in the range of 10 to 95%, and the apparent refractive index in the visible light region is 1.0 to 2. Forming a layer that is within the range of 5;
Each thin film layer is gradually increased in the visible refractive index in the range of 1.0 to 2.5 from the outermost layer closest to the external medium toward the lowermost layer closest to the substrate. The method for forming a gradient refractive index multilayer thin film is characterized by sequentially stacking layers.   9. The method according to claim 8, wherein two or more kinds of nanoparticles having different particle diameters are used as the nanoparticles.   The coating liquid used at the time of film formation contains a binder component, and the average volume porosity is in the range of 10 to 95% by changing the content of the binder component between the nanoparticles. 10. The method for forming a graded refractive index multilayer thin film according to claim 8, wherein a thin film layer having an apparent refractive index of 1.0 to 2.5 is formed.   A thin film having an average volume porosity in the range of 10 to 95% and an apparent refractive index in the visible light region of 1.0 to 2.5 by combining the methods according to claim 9 and claim 10 9. The method for forming a graded refractive index multilayer thin film according to claim 8, wherein a layer is formed.   A means for controlling the secondary aggregation state of the nanoparticles in the coating liquid used at the time of forming the film and adjusting the particle gap in each thin film layer formed from the coating liquid is based on the metal alkoxide sol-gel method. The method for forming a gradient refractive index multilayer thin film according to any one of claims 8 to 10, wherein the method is a method of controlling the porosity between the particles. In the sol-gel method of the metal alkoxide, a metal alkoxide is added to water, hydrolyzed as an aqueous solution, an organic solvent having low compatibility is added to the aqueous solution, and the aqueous phase is finely dispersed in the organic solvent phase. The method of forming a gradient refractive index multilayer thin film according to claim 12, wherein the film is dispersed as an O / W emulsion and polymerization is performed by dehydration condensation between metal alkoxide molecules in each dispersion cell.

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