Antireflection film and optical element

Abstract

To provide a new technique for achieving an antireflection film having optical characteristics of low reflection in a wide band wavelength and excellent transmittance.SOLUTION: An optical element (1) includes a substrate (20) and an antireflection film (10) including a refractive index gradient layer (12). The refractive index gradient layer (12) includes a first layer to an n-th layer whose refractive index gradually decreases from the substrate (20) side. The first layer contains TiO2 particles, and the n-th layer contains SiO2 particles.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an antireflection film and an optical element.

Background Art

[0002] An antireflection film is provided on the surface of a base material, which is an optical element such as a lens and a prism constituting an optical device, for the purpose of improving the light transmittance of the base material.

[0003] Such an antireflection film is conceptually known as an antireflection film having a film structure that does not include an interface where the refractive index changes abruptly. The antireflection film having such a film structure is advantageous in that the wavelength range of reflection reduction is wide and the reflectance does not depend on the incident angle (see, for example, Non-Patent Document 1).

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present invention is to provide a new technology for realizing an antireflection film having optical characteristics of low reflection at broadband wavelengths and excellent transparency.

Means for Solving the Problems

[0006] In order to solve the above problems, an antireflection film according to one aspect of the present invention is an antireflection film provided on a base material constituting an optical member, and the antireflection film includes a refractive index gradient layer having a refractive index that gradually changes from the largest first refractive index to the smallest n-th refractive index (n≧2) in the thickness direction of the antireflection film. The refractive index gradient layer includes a first layer having the first refractive index, an n-th layer having the n-th refractive index, and n-2 layers that are between the first layer and the n-th layer and have a refractive index that gradually decreases from the first refractive index toward the n-th refractive index according to the position in the thickness direction. The first layer contains particles of a first dielectric material, and the n-th layer contains particles of a second dielectric material that exhibits a refractive index lower than that of the first dielectric material.

[0007] Also, in order to solve the above problems, an optical element according to one aspect of the present invention includes a base material constituting an optical member and the above-described antireflection film provided on the base material.

Advantages of the Invention

[0008] According to one aspect of the present invention, it is possible to provide a new technique for realizing an antireflection film having optical characteristics of low reflection at a wide range of wavelengths and excellent transparency.

Brief Description of the Drawings

[0009]

Figure 1

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Figure 9

Embodiments for Carrying Out the Invention

[0010] As a result of intensive research, the present inventors have found that by wet film-forming a high refractive index metal oxide fine particle and either silica sol or hollow silica, or a mixture thereof, the fine refractive index change due to the change in the mixing ratio of the fine particles constituting the layer becomes seamless, and a flat low reflectance is realized in a wide wavelength range, thereby achieving the above problems. Hereinafter, embodiments of the optical element and the antireflection film according to the present invention will be described. In this specification, "~" means a range including the numerical values at both ends thereof.

[0011] 〔Optical Element〕 The layer structure of the antireflection film according to an embodiment of the present invention is schematically shown in FIG. 1. As shown in FIG. 1, the optical element 1 includes a substrate 20 and an antireflection film 10 provided on the substrate 20.

[0012] The substrate 20 constitutes an optical member. The substrate 20 may be made of glass or plastic, and its material is not limited. Examples of the substrate 20 include lenses, prisms (such as color separation prisms and color synthesis prisms), polarizing beam splitters (PBS), and cut filters (for infrared, ultraviolet, etc.). Examples of lenses include interchangeable lenses for single-lens reflex cameras, lenses mounted on digital cameras (DSC), and lenses for digital cameras mounted on mobile phones. The antireflection film 10 will be described later.

[0013] The optical element 1 can be used for a photographing optical element or a projection optical element. Since the optical element 1 is provided with an antireflection film 10, the transmittance of incident light with respect to the optical element 1 can be made substantially 100%, and the optical characteristics are excellent.

[0014] 〔Antireflection film〕 The antireflection film 10 is provided on a base material 20 constituting an optical member. As shown in FIG. 1, the antireflection film 10 has an intermediate layer 11, a refractive index gradient layer (hereinafter, also referred to as "GIAR layer") 12, and a functional layer 13 in this order from the base material 20 side.

[0015] [Intermediate layer] The intermediate layer 11 is located between the base material 20 and the GIAR layer 12. The intermediate layer 11 functions as, for example, one or both of an adhesion layer and an optical interference layer. When the GIAR layer 12 contains a binder described later, the intermediate layer 11 is preferably a layer mainly composed of a compound having good adhesion to the binder. Such an intermediate layer 11 can favorably adhere the GIAR layer 12 to the base material 20 when forming the GIAR layer 12 by a wet film forming method described later, and is suitable from the viewpoint of favorably forming the GIAR layer 12 on the base material 20.

[0016] When the intermediate layer 11 functions not only as an adhesion layer but also as an optical interference layer, by providing the GIAR layer 12 on the intermediate layer 11, the intermediate layer 11, the GIAR layer 12, and the functional layer 13 can function as an integrated antireflection layer. Thus, making the antireflection film 10 a composite layer of the intermediate layer 11, the GIAR layer 12, and the functional layer 13 is preferable from the viewpoint of making the change in refractive index in the thickness direction of the antireflection film 10 a more ideal change in refractive index for exhibiting antireflection performance.

[0017] The material constituting the intermediate layer 11 may be any material that exhibits the functions of the intermediate layer 11 described above. Examples of the material include MgF2, SiO2, Al2O3, Nb2O5, Ta2O5, TiO2, a mixture of La2O3 and TiO2, HfO2, SnO2, ZrO2, a mixture of ZrO2 and TiO2, Pr6O 11A mixture of and TiO2, a mixture of Al2O3 and La2O3, and La2O3 are included.

[0018] <Method for forming intermediate layer> The intermediate layer 11 can be obtained by forming a film of the above materials by various film-forming methods, and can be obtained as a single layer of the layer formed in this way or by laminating a plurality of the formed layers. The method for forming the intermediate layer 11 is not limited, and for example, a vacuum film-forming method or a wet film-forming method may be used.

[0019] The vacuum film-forming method for producing the intermediate layer 11 is preferably a chemical vapor deposition method (CVD method). The chemical vapor deposition method is a method of forming a thin film by using a compound containing the constituent elements of the thin film constituent material (the components constituting the intermediate layer 11) as a source gas, supplying the source gas into a chamber, and causing a chemical reaction in the gas phase or on the substrate surface. When the intermediate layer 11 functions as an optical interference layer, it is preferable to adopt the chemical vapor deposition method because it is easy to control the film thickness and refractive index.

[0020] Examples of the wet film-forming method for producing the intermediate layer 11 include dip coating method, spin coating method, spray coating method, alternate (LbL) lamination method, roll coating method, slit coating method, knife coating method, roll coating method, screen printing method, and Langmuir Projet (LB) method. For the wet film-forming method, conventionally known methods and the like may be appropriately adopted according to the shape of the substrate 20, the film thickness to be formed, and the like.

[0021] In the wet film-forming method, the dispersion medium used may be any dispersion medium that does not attack the substrate 20 (has chemical stability against the substrate 20). Examples of the dispersion medium include water; ketone alcohol-based dispersion media such as diacetone alcohol and 3-hydroxy-2-butanone; cellosolve-based dispersion media such as methyl cellosolve and ethyl cellosolve; hydrocarbon-based dispersion media such as n-hexane and n-heptane; perfluoroalkyl alcohol-based dispersion media such as tetrafluoropropanol, octafluoropentanol, and hexafluorobutanol; hydrocarbon-based dispersion media such as cyclohexane, methylcyclohexane, ethylcyclohexane, dimethylcyclohexane, n-butylcyclohexane, t-butylcyclohexane, and cyclooctane; ether-based dispersion media such as diisopropyl ether and dibutyl ether; and hydroxyl ester-based dispersion media such as methyl lactate, ethyl lactate, and methyl isobutyrate. The dispersion medium may be used alone or in combination of two or more.

[0022] Note that the refractive index and film thickness of the intermediate layer 11 can be appropriately set to appropriate values from the viewpoint of exhibiting antireflection performance. The optical properties of the intermediate layer 11 are preferably appropriately designed by the matrix method.

[0023] <Surface treatment of the intermediate layer> Also, it is preferable that the intermediate layer 11 is subjected to a surface treatment for enhancing the adhesion to the GIAR layer 12 (wettability with respect to the material liquid of the GIAR layer 12 described later). By subjecting the intermediate layer 11 to the surface treatment, the adhesion of the GIAR layer 12 to the intermediate layer 11 can be made better. Examples of the surface treatment include plasma treatment and UV cleaning treatment. More specifically, examples of the surface treatment include, when the intermediate layer 11 is formed by a vacuum film-forming method, a method of irradiating the surface of the intermediate layer 11 with plasma using argon gas, hydrogen gas, helium gas, or oxygen gas as a treatment gas without leaking the chamber.

[0024] According to this method, it is preferable because surface treatment can be immediately performed after film formation using the vacuum film forming apparatus used when forming the intermediate layer 11. Further, the wettability on the surface of the intermediate layer 11 may be improved by performing atmospheric pressure plasma treatment or UV cleaning treatment on the formed intermediate layer 11. The contact angle of the above material liquid with respect to the intermediate layer 11 is preferably within a range where sufficient wettability can be obtained regardless of the presence or absence of surface treatment. Such surface treatment of the intermediate layer 11 is even more effective from the viewpoint of enhancing the antireflection performance of the antireflection film 10.

[0025] The intermediate layer 11 can be mainly composed of an organometallic compound having good adhesion to the resin film, and can have wettability with respect to the material liquid used when forming the GIAR layer 12. By disposing such an intermediate layer 11 on the substrate 20, when the GIAR layer 12 is formed by a wet film forming method, the film can be favorably formed on the surface of the intermediate layer 11, and the GIAR layer 12 can be firmly adhered to the substrate 20 via the intermediate layer 11.

[0026] [Gradient Index Antireflection (GIAR) Layer] The GIAR layer 12 has a refractive index that gradually changes from the largest first refractive index to the smallest n-th refractive index (n ≧ 4) in the thickness direction of the antireflection film (hereinafter, also simply referred to as the "thickness direction"). More specifically, the GIAR layer 12 is configured by forming n thin films. That is, the GIAR layer 12 includes a first layer having a first refractive index, an n-th layer having an n-th refractive index, and n - 2 layers having a refractive index that gradually decreases from the first refractive index to the n-th refractive index according to the position in the thickness direction between the first layer and the n-th layer.

[0027] The number (n) of layers with different refractive indices in the GIAR layer 12 is 4 or more. For example, in the GIAR layer 12, there are four or more different refractive indices that gradually increase or gradually decrease as the depth from the surface of the GIAR layer 12 increases. The refractive index at a specific depth in the GIAR layer 12 can be obtained by measuring the exposed surface of the GIAR layer 12 after etching to the specific depth with, for example, a thickness / film thickness / optical constant measuring device (manufactured by NXT). The "specific depth" may be any depth that reaches a layer with a different refractive index in the GIAR layer 12 from the surface of the GIAR layer 12. For example, it may be four or more randomly determined depths in the thickness direction of the GIAR layer 12, or each depth when the film thickness of the GIAR layer 12 is equally divided into four or more parts, or a depth at regular intervals such as 10 nm. From the perspective of realizing a flat low reflectance for incident light in a wide wavelength range, the number (n) of layers with different refractive indices in the GIAR layer 12 is preferably as large as possible within the achievable range. For example, it is preferably 10 to 40, and more preferably 15 to 30.

[0028] When the antireflection film 10 is disposed on the substrate 20, the first layer may be the layer on the substrate 20 side in the thickness direction or the opposite side (the functional layer 13 side in FIG. 1). Similarly, the nth layer may be on the functional layer 13 side or the substrate 20 side in the thickness direction.

[0029] When the first layer is the layer closest to the substrate 20 side in the GIAR layer 12, from the perspective of suppressing reflection at the interface on the substrate 20 side in the GIAR layer 12, the difference between the refractive index of the first layer (i.e., the first refractive index) and the refractive index of the substrate 20 is preferably small. From such a perspective, the difference between the first refractive index and the refractive index of the substrate 20 is preferably 0.05 or less, and more preferably 0.02 or less.

[0030] Similarly, when the n-th layer is the layer located on the side of the GIAR layer 12 that is farthest from the substrate 20, from the viewpoint of suppressing reflection at the interface on the side of the GIAR layer 12 opposite to the substrate 20, it is preferable that the difference between the n-th refractive index and the refractive index of the medium around the antireflection film is small. The "medium around the antireflection film" is the medium around the antireflection film in the environment where the antireflection film is used, and is the medium through which the light incident on the antireflection film 10 toward the substrate 20 passes. This medium is, for example, air when the optical element is used in air, and water when the optical element is used in water. From the above viewpoints, the difference between the n-th refractive index and the refractive index of the medium around the antireflection film is preferably 0.25 or less, and more preferably 0.20 or less.

[0031] In the GIAR layer 12, the first layer contains particles of a first dielectric material (hereinafter also referred to as "first dielectric material particles"), and the n-th layer contains particles of a second dielectric material that exhibits a refractive index lower than that of the first dielectric material (hereinafter also referred to as "second dielectric material particles"). The first dielectric material particles may be any particles capable of exhibiting the aforementioned first refractive index, and the second dielectric material particles may be any particles capable of exhibiting the aforementioned n-th refractive index. The first dielectric material particles and the second dielectric material particles can be appropriately determined from known dielectric material particles according to the desired first refractive index of the first layer and the desired n-th refractive index of the n-th layer in the GIAR layer 12.

[0032] When the first layer is the layer on the substrate 20 side and the n-th layer is the layer on the air atmosphere side, usually, the first dielectric material particles are dielectric material particles that exhibit a larger refractive index, and the second dielectric material particles are dielectric material particles that exhibit a smaller refractive index. In such a case, preferably, the first dielectric material particles are metal oxide particles, and the second dielectric material is silicon dioxide particles.

[0033] Examples of the dielectric material in the first dielectric material particles or the second dielectric material particles include silica (SiO2), aluminum oxide (Al2O3), niobium pentoxide (Nb2O5), tantalum pentoxide (Ta2O5), titanium oxide (TiO2), lanthanum oxide (La2O3), yttrium oxide (Y2O3), hafnium oxide (HfO2), tin oxide (SnO2), zirconium oxide (ZrO), zirconia (ZrO2), and magnesium fluoride (MgF2). Further, the first dielectric material particles or the second dielectric material particles may be particles that exhibit a desired refractive index according to the form of the particles. For example, the second dielectric material particles may be silica sols having voids such as porous silica sols and hollow silica sols.

[0034] Both the first dielectric material particles and the second dielectric material particles may be single - type particles, or a mixture of two or more types of particles, within the range of exhibiting a desired refractive index in each layer from the first layer to the nth layer. Each layer may contain the same kind of dielectric material particles in two or more different layers within the range where the refractive index of each layer is different.

[0035] The refractive index of each layer from the first layer to the nth layer can be achieved by mixing different dielectric material particles that exhibit different refractive indices. Also, the refractive index of each layer from the first layer to the nth layer can be achieved by mixing the same kind of dielectric material particles having different particle diameters.

[0036] For example, if the first layer is the layer on the side of the base material 20 and the nth layer is the layer on the air - atmosphere side, one or more layers from the first layer to the nth layer may be composed of mixed particles of the aforementioned metal oxide particles and silicon dioxide particles. Also, one or more layers other than the first layer from the first layer to the nth layer may be composed of mixed particles of two or more types of silicon dioxide particles having different particle diameters. Further, one or more layers other than the first layer from the first layer to the nth layer may be composed of mixed particles of either or both of hollow silicon dioxide particles and porous silicon dioxide particles.

[0037] If the thickness of the GIAR layer 12 is too thin, the suppression of reflection of incident light may be insufficient. From the viewpoint of sufficiently expressing the antireflection characteristics of incident light in the antireflection film, the thickness of the GIAR layer 12 is preferably 1 / 4 times or more of the wavelength of light according to the purpose of the substrate 20 (optical element 1). According to the above viewpoint, the thickness of the GIAR layer 12 may be thick, but from the viewpoint of light scattering intensity, it is preferably thin to some extent. From such a viewpoint, the thickness of the GIAR layer 12 is preferably 1 μm or less, for example.

[0038] Hereinafter, one embodiment of the refractive index gradient layer in the present invention will be described in more detail. An example of the structure of the GIAR layer 12 and the partial refractive index is schematically shown in FIG. 2. The GIAR layer 12 has, for example, a structure in which three layers of a high refractive index layer 12a, a medium refractive index layer 12b, and a low refractive index layer 12c overlap in this order on the intermediate layer 11, starting from the intermediate layer 11 side. Each of the three layers of the high refractive index layer 12a, the medium refractive index layer 12b, and the low refractive index layer 12c is composed of a plurality of thin films (first layer to nth layer) having slightly different refractive indices and gradually changing in the thickness direction. The thickness of the thin film to be produced is, for example, 15 to 40 nm.

[0039] For example, as shown in FIG. 2, when the refractive index of the substrate 20 is n0, the refractive index of the first layer of the GIAR layer 12 is n0 ± 0.05, the refractive index of the second layer is n0 - 0.03 ± 0.05, and the refractive index of the third layer is n0 - 0.06 ± 0.05. Also, the refractive index of the nth layer is 1.20 ± 0.02, and the refractive index of the n - 1th layer is 1.23 ± 0.03. In this way, the refractive indices of each of the first layer to the nth layer are designed so that the refractive index difference between adjacent layers in the thickness direction is 0.03. Note that the GIAR layer 12 in FIG. 2 is a case where the medium around the antireflection film 10 is air. For example, the nth layer is composed of hollow silica sol. In this way, the nth layer is configured to have a refractive index as small as possible as the refractive index of the layer by dielectric particles.

[0040] Each of the three layers, i.e., the high refractive index layer 12a, the medium refractive index layer 12b, and the low refractive index layer 12c, can be confirmed by observing the cross-section of the GIAR layer 12 with a scanning electron microscope (SEM). In SEM, dielectric particles can be observed with contrast corresponding to the magnitude of the atomic number. Therefore, each of the three layers, i.e., the high refractive index layer 12a, the medium refractive index layer 12b, and the low refractive index layer 12c, can be confirmed by representative dielectric particles in each layer, or characteristic distributions of particles brought about by such particles, etc.

[0041] For example, the high refractive index layer 12a can be defined as the portion where metal oxide particles contained in the first layer in the GIAR layer 12 are observed from the SEM image in the thickness direction, and the low refractive index layer 12c can be defined as the portion where the characteristics of the hollow silica sol in the GIAR layer 12 are observed from the SEM image. Also, the medium refractive index layer 12b can be defined as the portion between the high refractive index layer 12a and the low refractive index layer 12c in the thickness direction.

[0042] <High refractive index layer> The high refractive index layer 12a is located closest to the substrate 20 side in the thickness direction among the three layers of the high refractive index layer 12a, the medium refractive index layer 12b, and the low refractive index layer 12c. The high refractive index layer 12a includes at least the first layer among the first layer to the nth layer described above. The first layer has a sufficiently small difference (e.g., 0.05 or less) between the first refractive index and the refractive index of the substrate 20 as described above. Therefore, the high refractive index layer 12a has a refractive index that forms the above refractive index difference at the portion closest to the substrate 20 side in the thickness direction and gradually decreases toward the medium refractive index layer 12b.

[0043] The high refractive index layer 12a can be configured with mixed particles obtained by mixing multiple types of metal oxide particles having different refractive indices, so that the average refractive index of each layer (thin film) such as the first layer in the high refractive index layer 12a can be appropriately adjusted. Configuring each layer with two or more types of dielectric particles having different refractive indices in this way is preferable from the viewpoint of enabling the stacking of thin films with slightly lower refractive indices and repeating the process. As the most preferable form, each layer in the high refractive index layer 12a is formed by changing the mixing ratio of a specific high refractive index dielectric particle (such as titania sol) that forms a thin film layer closest to the refractive index of the substrate 20 and silica sol (SiO2), and this is the form of forming the high refractive index layer 12a. This form is preferable from the viewpoints of manufacturing convenience and chemical stability, that is, long-term stability.

[0044] <Medium refractive index layer> The medium refractive index layer 12b is located between the high refractive index layer 12a and the low refractive index layer 12c in the thickness direction. The medium refractive index layer 12b includes at least one layer between these layers, excluding the aforementioned first layer and the nth layer. The medium refractive index layer 12b can be formed using the same material (such as silica sol).

[0045] Generally, the refractive index of a thin film layer composed of fine particles is represented by the average refractive index due to high refractive index particles and the voids existing between the particles. Therefore, usually, when the voids become larger, the average refractive index of the thin film layer becomes lower, and when the voids become smaller, the average refractive index of the thin film layer becomes higher. Therefore, it is possible to adjust the total amount of voids between particles by laminating dielectric material particles of the same composition but different particle sizes, making the refractive index boundary between layers unclear and suppressing the attenuation of transmitted light due to reflection within the layer. In particular, silica sol is suitable for forming a refractive index gradient layer by this method because of its rich variation in particle size.

[0046] <Low refractive index layer> The low refractive index layer 12c is located on the side farthest from the base material 20 in the thickness direction among the three layers of the high refractive index layer 12a, the medium refractive index layer 12b, and the low refractive index layer 12c. The low refractive index layer 12c includes the n-th layer that can be the outermost layer located farthest from the base material 20 in the GIAR layer 12. From the viewpoint of reducing the reflection occurring at the outermost layer, if the medium around the antireflection film 10 is, for example, air, it is preferable that the refractive index of the n-th layer is as close as possible to 1.0. From such a viewpoint, it is preferable that the n-th layer is composed only of a silica sol having voids.

[0047] In the layer on the base material 20 side of the n-th layer in the low refractive index layer 12c, the average refractive index of each layer can be adjusted by mixing a silica sol having voids and a silica sol not having voids. Thereby, it becomes possible to form a refractive index gradient layer by disposing a thin film having a slightly larger refractive index described above under the outermost layer.

[0048] When the number (n) from the first layer to the n-th layer is sufficiently large, a GIAR layer 12 is formed in which the refractive index gradually changes apparently seamlessly in the thickness direction. Further, regardless of the number (n) from the first layer to the n-th layer, when the boundary of each layer interface is unclear, a GIAR layer 12 is formed in which the refractive index gradually changes apparently seamlessly in the thickness direction.

[0049] <Film formation method of GIAR layer> The GIAR layer 12 (from the first layer to the n-th layer) can be manufactured by a wet film formation method. For the wet film formation method for manufacturing the GIAR layer 12, the same method and dispersion medium as the wet film formation method for forming the intermediate layer 11 described above can be adopted.

[0050] <Particle size of particles constituting GIAR layer> The scattering phenomenon when the particle size is sufficiently smaller than the wavelength of light is represented by the Rayleigh scattering formula (Formula 1 below). The meanings of the characters in Formula 1 above are shown below. C sca =8 / 3×{2πn p r / λ} 4 ×[{(np / n m ) 2 -1} / {(n p / n m ) 2 +2}] 2 ×πr (Equation 1) C sca : Scattering cross-section n p : Refractive index of the particulate material n m : Refractive index of the base material (air) r: Radius of the fine particle λ: Wavelength of light

[0051] The extinction coefficient α of a thin film composed of the volume fraction η of fine particles present in the optical path d obtained from Equation 1 sca is represented by the following equation (Equation 2). α sca =3ηC sca / 4πr 3 (Equation 2)

[0052] Here, assuming that the material of the base material is air (refractive index 1.0), the wavelength of light is 550 nm, and the film thickness of the high refractive index layer 12a in the GIAR layer 12 is 100 nm, the particle diameter and the extinction coefficient α when substituting the refractive index of the material that can be used for the high refractive index layer 12a sca The relationship with is shown in FIG. 3.

[0053] For example, for silica (refractive index 1.46), if the particle diameter is 40 nm or less, the extinction coefficient α sca becomes 0.001 or less. If it is used as the fine particles constituting the GIAR layer 12, it is desirable that the particle diameter be 40 nm or less. However, the extinction coefficient α of the silica sol having voids sca shows a different correlation from the extinction coefficient α sca that substantially has no such voids.

[0054] Similarly, in the above case, the particle diameter of antimony pentoxide (1.65) is preferably 25 nm or less, the particle diameter of zirconium oxide (2.05) is preferably 16 nm or less, the particle diameter of tantalum pentoxide (2.275) is preferably 12 nm or less, and the particle diameter of titanium oxide (2.50) is preferably 10 nm or less.

[0055] From the viewpoint of suppressing scattered light, the particle diameter of the metal oxide particles is preferably 25 nm or less, more preferably 16 nm or less, and even more preferably 10 nm or less in terms of the average particle diameter of the primary particles.

[0056] In addition, according to the same calculation, the particle diameter of aluminum oxide (1.76) is 23 nm or less, the particle diameter of niobium pentoxide (2.32) is 13 nm or less, the particle diameter of lanthanum oxide (1.84) is 20 nm or less, the particle diameter of yttrium oxide (1.88) is 19 nm or less, the particle diameter of hafnium oxide (1.91) is 18 nm or less, the particle diameter of tin oxide (2.00) is 16 nm or less, the particle diameter of zirconium oxide (2.10) is 16 nm or less, and the particle diameter of zirconia (2.05) is preferably 16 nm or less.

[0057] Moreover, the average particle diameter of the primary particles of silica sols having different particle diameters is preferably 2 nm or more.

[0058] From the viewpoint of suppressing scattered light, the average particle diameter of the primary particles of silica sols having different particle diameters is preferably 40 nm or less. However, this is not the case when the shape of the silica particles is hollow or the particles are porous particles, because the average refractive index of the particles varies depending on the structure of the particles.

[0059] Note that the particle diameter of the dielectric particles described above is the average particle diameter of the primary particles. The average particle diameter can be measured by a particle size distribution meter. The particle diameter of the dielectric particles described above may be an actual measured value or a catalog value. The average particle diameter of the primary particles of the dielectric particles in the refractive index gradient layer can be measured using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Also, in the formation of the aforementioned GIAR layer (wet film formation method), the average particle diameter of the primary particles of the dielectric particles can be measured by the dynamic light scattering method using a particle size distribution meter of the dispersion liquid of the dielectric particles used in the wet film formation method. Also, in the formation of the GIAR layer, the average particle diameter of the primary particles of the dielectric particles can be adjusted by using a classified product of the particles or a mixture thereof, or by removing coarse particles by filtration of the dispersion liquid.

[0060] <Hardening treatment of GIAR layer> The particles constituting the GIAR layer 12 (from the first layer to the nth layer) may be bonded to each other by a binder. By bonding the particles to each other with a binder, the particles are fixed to each other, and the GIAR layer 12 itself is further strengthened.

[0061] The binder may be, for example, one or more compounds selected from the group consisting of silane compounds, organosilicon compounds, their hydrolyzates, and their polycondensates. Examples of silane compounds include silica sol. Examples of organosilicon compounds include silane coupling agents. Examples of other binders include polyvinyl alcohol, polyoxyethylene, polymethyl methacrylate, polymethyl acrylate, cyanoacrylate, diacetyl cellulose, triacetyl cellulose, nitrocellulose, polyester, alkyd resin, fluoroacrylate, vinyl acetate resin, phenol resin, epoxy resin, and formaldehyde. The binder may be one kind or more than one kind.

[0062] When the GIAR layer 12 contains a silane compound as the above binder component, both the binder component that constitutes the GIAR layer 12 and the silica sol that constitutes each layer (the first layer to the nth layer) of the GIAR layer 12 are excellent in durability and stability. Therefore, it is possible to further enhance the durability and stability of the GIAR layer 12.

[0063] In the case of heating the GIAR layer in the above curing treatment and when the substrate 20 contains a resin material, it is desirable that the heating temperature in the curing treatment be a temperature equal to or lower than the heat distortion temperature of the resin material contained in the substrate 20. Further, when the linear thermal expansion coefficient of the substrate 20 is large, from the viewpoint of suppressing the occurrence of cracks in the cured GIAR layer, it is more preferable to further perform an annealing treatment within a temperature range equal to or lower than the heat distortion temperature of the resin material.

[0064] [Functional layer] The functional layer 13 is a transparent ultrathin film that does not optically affect the antireflection performance of the GIAR layer 12. The functional layer 13 has a function of enhancing physical or chemical properties such as the surface hardness, scratch resistance, heat resistance, weather resistance, solvent resistance, water repellency, oil repellency, antifogging property, hydrophilicity, antifouling resistance, and conductivity of the antireflection film 10. The functional layer 13 is, for example, a transparent layer having a refractive index of 1.30 or more and 2.35 or less and a physical film thickness of 0.1 nm or more and 30 nm or less. Such a functional layer 13 can substantially suppress the optical influence of the functional layer 13 on the antireflection effect by the GIAR layer 12.

[0065] When the refractive index of the functional layer 13 exceeds the above range, the functional layer 13 may optically affect the antireflection characteristics of the GIAR layer 12. Further, when the physical film thickness of the functional layer 13 is less than 1 nm, the manifestation of the functions required of the functional layer 13 may be insufficient. Also, when the physical film thickness of the functional layer 13 exceeds 10 nm, even if the refractive index of the functional layer 13 is within the above range, the functional layer 13 may optically affect the antireflection characteristics of the GIAR layer 12.

[0066] As the material constituting the functional layer 13, a transparent material having a refractive index of 1.30 or more and 2.35 or less can be used. As long as the refractive index is within the above range and the material is transparent, an appropriate material may be selected as appropriate according to the function to be imparted to the surface of the antireflection film 10. Examples of the transparent inorganic material having a refractive index within the above range include SiO x N y (both x and y are greater than 0), SiO2, SiO x , Al2O3, a mixture of ZrO2 and TiO2, a mixture of La2O3 and TiO2, SnO2, ZrO2, a mixture of La2O3 and Al2O3, Pr2O5, ITO (indium tin oxide), and AZO (aluminum zinc oxide). Examples of the material of the functional layer 13 include DLC (diamond-like carbon), epoxy-based resins, acrylic-based resins, fluorine-based resins, and silicon-based resins.

[0067] In the production of the functional layer 13, various hard coat agents containing the above materials may be used. In the production of the functional layer 13, an appropriate film formation method can be adopted as appropriate according to the material and film thickness.

[0068] 〔Other Embodiments〕 The antireflection film according to the present invention may contain components other than those described in the above embodiments within the range where the effects of the present invention can be obtained.

[0069] For example, the antireflection film according to the present invention may not have an intermediate layer and a functional layer. In the case of not having an intermediate layer, a treatment for enhancing the adhesion to the GIAR layer, such as a surface treatment for hydrophilization, may be applied to the surface of the substrate.

[0070] Further, the intermediate layer may be a single layer or a laminated structure of a plurality of layers, and the number of layers in the case of a laminated structure is not limited.

[0071] The three layers of the high refractive index layer, the medium refractive index layer, and the low refractive index layer in the GIAR layer may be set according to each range of the refractive index after arbitrarily dividing the range of the refractive index in the thickness direction of the GIAR layer into three parts.

[0072] The GIAR layer may be composed of only a high refractive index layer and a low refractive index layer.

[0073] 〔Summary〕 A first aspect of the present invention is an antireflection film provided on a substrate constituting an optical member. The antireflection film includes a refractive index gradient layer having a refractive index that gradually changes from the largest first refractive index to the smallest n-th refractive index (n≥4) in the thickness direction of the antireflection film. The refractive index gradient layer includes a first layer having the first refractive index, an n-th layer having the n-th refractive index, and n - 2 layers having a refractive index that gradually decreases from the first refractive index toward the n-th refractive index according to the position in the thickness direction between the first layer and the n-th layer. The first layer contains particles of a first dielectric substance, and the n-th layer contains particles of a second dielectric substance that exhibits a refractive index lower than that of the first dielectric substance. According to the first aspect, it is possible to provide a new technology for realizing an antireflection film having optical characteristics of low reflection in a wide wavelength band and excellent transparency.

[0074] A second aspect of the present invention is, in the first aspect, that the refractive index gradient layer has the first layer at the position closest to the substrate in the thickness direction of the antireflection film, and the difference between the first refractive index and the refractive index of the substrate is 0.05 or less. The second aspect is more effective from the viewpoint of providing an apparently seamless refractive index change and suppressing attenuation of transmitted light due to internal reflection in the layer.

[0075] A third aspect of the present invention is, in the second aspect, that the difference between the n-th refractive index and the refractive index of the medium surrounding the antireflection film is 0.25 or less. The third aspect is even more effective from the viewpoint of providing an apparently seamless refractive index change and suppressing attenuation of transmitted light due to internal reflection in the layer.

[0076] In the fourth aspect of the present invention, in any of the first to third aspects, the first dielectric material particles are metal oxide particles, the second dielectric material particles are silicon dioxide particles, and one or more layers from the first layer to the nth layer are composed of mixed particles of metal oxide particles and silicon dioxide particles. The fourth aspect is more effective from the viewpoint of suppressing attenuation of transmitted light due to reflection between the base material and the antireflection film.

[0077] In the fifth aspect of the present invention, in any of the first to fourth aspects, the second dielectric material particles are silicon dioxide particles, and one or more layers other than the first layer from the first layer to the nth layer are composed of mixed particles of two or more types of silicon dioxide particles having different particle diameters. The fifth aspect is more effective from the viewpoint of realizing an apparently seamless refractive index change at the central portion in the thickness direction of the refractive index gradient layer.

[0078] In the sixth aspect of the present invention, in the fifth aspect, the average particle diameter of the primary particles of the silicon dioxide particles having different particle diameters is 2 nm or more and 40 nm or less. The sixth aspect is more effective from the viewpoint of realizing an apparently seamless refractive index change by silica sol.

[0079] In the seventh aspect of the present invention, in any of the first to sixth aspects, the second dielectric material particles are silicon dioxide particles, and one or more layers other than the first layer from the first layer to the nth layer are composed of mixed particles of either one or both of hollow silicon dioxide particles and porous silicon dioxide particles. The seventh aspect is more effective from the viewpoint of suppressing attenuation of transmitted light due to reflection on the surface of the antireflection film.

[0080] In the eighth aspect of the present invention, in any of the first to seventh aspects, the thickness of the refractive index gradient layer is 1 / 4 times or more of the wavelength of light according to the purpose of the base material. The eighth aspect is more effective from the viewpoint of sufficiently suppressing reflection of incident light in the antireflection film.

[0081] The ninth aspect of the present invention is that, in any one of the first to eighth aspects, the first dielectric material particles are metal oxide particles, and the average particle diameter of the primary particles of the metal oxide particles is 25 nm or less. The ninth aspect is more effective from the viewpoint of adjusting the refractive index within the refractive index gradient layer.

[0082] The tenth aspect of the present invention is that, in any one of the first to ninth aspects, the particles in one or more layers from the first layer to the nth layer are bonded by a binder. The tenth aspect is more effective from the viewpoint of imparting or enhancing the durability of the antireflection film, which is less likely to be damaged by an external force due to the enhanced strength of the refractive index gradient film.

[0083] The eleventh aspect of the present invention is that, in the tenth aspect, the binder is a silane compound. The eleventh aspect is even more effective from the viewpoint of imparting or enhancing the durability of the antireflection film, which is less likely to be damaged by an external force due to the enhanced strength of the refractive index gradient film.

[0084] The twelfth aspect of the present invention is an optical element including a substrate constituting an optical member and an antireflection film according to any one of the first to eleventh aspects. According to the twelfth aspect, as in the first aspect, it is possible to provide a new technology for realizing an antireflection film having optical characteristics of low reflection at a wide range of wavelengths and excellent transparency.

[0085] According to the present invention, since the substrate, which is an optical member, includes the antireflection film according to the present invention described above, the transmittance of incident light with respect to the resulting optical element can be made substantially 100%, and an optical element having excellent optical characteristics can be obtained. The present invention having such an effect is expected to contribute to the achievement of, for example, Goal 9, "Build the infrastructure for industry and innovation," of the Sustainable Development Goals (SDGs) proposed by the United Nations.

[0086] In addition, the refractive index gradient layer that can be formed by the wet film-forming method according to the present invention overcomes the following drawbacks in the moth-eye structure, which is one of the conventional refractive index gradient structures, and is also excellent in this regard. · It is difficult to directly form the structure on a substrate with a large curvature and a large-area substrate. · Expensive equipment is required to produce a mold for transferring the moth-eye structure. · The mold must be produced for each lens part number. · The durability of the mold is shorter than that of a normal mold, and as a result, the cost may increase.

[0087] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

Example

[0088] An example of the present invention will be described below. In this example, an in-house dip coater was used, and a thin film was formed on a transparent substrate according to the following procedure. As the transparent substrate, a BK7 (borosilicate crown glass) substrate with a planar diameter φ of 30 mm and a thickness t of 1.0 mm (refractive index n = 1.52) was used.

[0089] The reagents, concentrations of the coating solution, and average particle diameters of the particles in the coating solution used in this example are shown in Table 1. In the following table, the "average particle diameter" is the average particle diameter of the primary particles. Also, "Tinok", "Cataloid", and "Adelite" are all registered trademarks.

[0090]

Table 1

[0091] A hollow silica sol, a silica sol, and a titanium oxide (titania) sol were diluted with a solvent to prepare a particulate dispersion having the composition shown in Table 1 as a coating liquid. Then, a specific coating liquid was applied to BK7 and dried to form a film of about 100 nm consisting of a single composition on BK7 for each of Samples #1 to #8. For each of the formed Samples #1 to #8, the film thickness and refractive index were measured with an ellipsometer. The coating liquids used and the measurement results of the obtained samples are shown in Tables 2 and 3. In the following tables, the refractive index n is the refractive index of light with a wavelength of 550 nm.

[0092]

Table 2

[0093]

Table 3

[0094] From Table 3, it was found that the refractive index of the thin film changes linearly according to the average particle diameter of the silica sol.

[0095] The coating liquid of Sample #2 in Table 2 and the coating liquid of Sample #6 in Table 3 were mixed at a specific ratio to newly prepare a coating liquid. The prepared coating liquid was applied to BK7 and dried to form a film of about 40 to 100 nm consisting of a single composition on BK7 for each of Samples #9 and #10. For each of the formed Samples #9 and #10, the film thickness and refractive index were measured with an ellipsometer. The composition of the coating liquid and the measurement results of the samples are shown in Table 4.

[0096]

Table 4

[0097] From Table 4, it was found that in the thin film formed using the mixed dispersion of silica sol and titania sol, the refractive index of the thin film changes linearly according to the mixing ratio of each particle in the mixed dispersion.

[0098] The coating liquid of Sample #3 in Table 2 and the coating liquid of Sample #4 in Table 3 were mixed at a specific ratio to newly prepare a coating liquid. The prepared coating liquid was applied to BK7 and dried, and thin films of about 90 to 300 nm consisting of a single composition were formed on BK7 to obtain Samples #11 and #12. For each of the formed Samples #11 and #12, the film thickness and refractive index were measured with an ellipsometer. The composition of the coating liquid and the measurement results of the samples are shown in Table 5.

[0099]

Table 5

[0100] From Table 5, it was found that in the thin film formed using the mixed dispersion of silica sol and antimony oxide sol, the refractive index of the thin film changed linearly according to the mixing ratio of each particle in the mixed dispersion.

[0101] The coating liquid of Sample #1 in Table 2 and the coating liquid of Sample #8 in Table 3 were mixed at a specific ratio to newly prepare a coating liquid. The prepared coating liquid was applied to BK7 and dried, and thin films of about 100 nm consisting of a single composition were formed on BK7 to obtain Samples #13, #14, and #15. For each of the formed Samples #13, #14, and #15, the film thickness and refractive index were measured with an ellipsometer. The composition of the coating liquid and the measurement results of the samples are shown in Table 6.

[0102]

Table 6

[0103] From Table 6, it was found that in the thin film formed using the mixed dispersion of silica sol and hollow silica sol, the refractive index of the thin film changed linearly according to the mixing ratio of each particle in the mixed dispersion.

[0104] The thin films of Samples #4 to #8 shown in Table 3 above were overcoated with a diluted solution of ethyl silicate and annealed at 120°C for 3 hours. By performing the curing treatment in this way, cured Samples #4 to #8 were obtained. For each of Samples #4 to #8 after the curing treatment, the film thickness and refractive index were measured with an ellipsometer. The measurement results of the samples after the curing treatment are shown in Table 7.

[0105]

Table 7

[0106] Similarly, the thin films of Samples #3, #11, #12, and #4 shown in Table 5 above were overcoated with a diluted solution of ethyl silicate and annealed at the above-mentioned 120°C to obtain cured Samples #3, #11, #12, and #4. For each of Samples #3, #11, #12, and #4 after the curing treatment, the film thickness and refractive index were measured with an ellipsometer. The measurement results of the samples after the curing treatment are shown in Table 8.

[0107]

Table 8

[0108] Similarly, the thin films of Samples #8, #13, #14, #15, and #1 shown in Table 6 above were overcoated with a diluted solution of ethyl silicate and annealed at the above-mentioned 120°C to obtain cured Samples #8, #13, #14, #15, and #1. For each of Samples #8, #13, #14, #15, and #1 after the curing treatment, the film thickness and refractive index were measured with an ellipsometer. The measurement results of the samples after the curing treatment are shown in Table 9.

[0109]

Table 9

[0110] 〔Example 1〕 The coating liquid of Sample #1 in Table 2 and a new coating liquid prepared by mixing the particles shown in Table 10 at a specific ratio according to the target refractive index were prepared. The compositions of the prepared coating liquids and the target refractive indices are shown in Table 10.

[0111]

Table 10

[0112] First, both sides of a BK7 substrate with a diameter of 30 mm and a thickness of 1 mm were subjected to hydrophilic treatment by vacuum plasma irradiation. Next, the coating liquids of Samples #16 to #31 in Table 10 were used in descending order of the target refractive index, and films were formed by the dip coating method so that the film thickness of the thin films formed by each coating liquid was 20 nm. In this way, a refractive index gradient layer with a thickness of 20 nm per layer, 17 layers, and an estimated film thickness of 340 nm was formed on the surface of the BK7 substrate. Thus, an optical element including the BK7 substrate and an antireflection film containing the above refractive index gradient layer on its surface was fabricated. The optical element was designated as "GIAR Ti#1". The layer structure of GIAR Ti#1 is schematically shown in FIG. 4.

[0113] 〔Example 2〕 Except that the thickness of each layer in the refractive index gradient layer was set to 15 nm, in the same manner as in Example 1, a refractive index gradient layer with a thickness of 15 nm per layer, 16 layers, and an estimated film thickness of 240 nm was formed on the surface of the BK7 substrate. Thus, an optical element including the BK7 substrate and an antireflection film containing the above refractive index gradient layer on its surface was fabricated. The optical element was designated as "GIAR Ti#2".

[0114] 〔Example 3〕 The coating liquids of Samples #1 and #3 in Table 2, the coating liquids of Samples #4 to #8 in Table 3, and a new coating liquid prepared by mixing the particles shown in Table 11 at a specific ratio according to the target refractive index were prepared. The compositions of the prepared coating liquids and the target refractive indices are shown in Table 11.

[0115]

Table 11

[0116] First, both sides of a BK7 substrate with a diameter of 30 mm and a thickness of 1 mm were subjected to hydrophilic treatment by vacuum plasma irradiation. Next, the coating liquids of Samples #1, #3 to #8, #11, #12, and #33 to #36 in Table 11 were used in descending order of the target refractive index, and thin films were formed by the dip coating method so that the film thickness of each thin film by each coating liquid was approximately 20 nm except for the outermost thin film. The film thickness of the outermost thin film was formed to be approximately 100 nm. Next, the annealing treatment at 120 °C described above was performed. Thus, a refractive index gradient layer with 13 layers and an estimated film thickness of 340 nm was formed on the surface of the BK7 substrate. Thus, an optical element including the BK7 substrate and an antireflection film including the above refractive index gradient layer on its surface was fabricated. The optical element was designated as "GIAR Sb#1". The layer configuration of GIAR Sb#1 is schematically shown in FIG. 5.

[0117] 〔Example 4〕 A refractive index gradient layer with 13 layers and an estimated film thickness of 460 nm was formed on the surface of the BK7 substrate in the same manner as in Example 1 except that the thickness of each layer other than the outermost layer in the refractive index gradient layer was set to approximately 30 nm. Thus, an optical element including the BK7 substrate and an antireflection film including the above refractive index gradient layer on its surface was fabricated. The optical element was designated as "GIAR Sb#2".

[0118] 〔Comparative Example 1〕 First, both sides of a BK7 substrate with a diameter of 30 mm and a thickness of 1 mm were subjected to hydrophilic treatment by vacuum plasma irradiation. Next, the coating liquid of Sample #20 in Table 10 was formed on the surface of the BK7 substrate by the dip coating method so that the film thickness of the thin film by the coating liquid was approximately 90 nm. Next, on the fabricated thin film, the coating liquid of Sample #1 in Table 10 was used, and a thin film was formed by the dip coating method so that the film thickness of the thin film by the coating liquid was approximately 105 nm. Thus, an optical element including the BK7 substrate and an antireflection film by the above optical interference on its surface was fabricated. The optical element was designated as "DL" (dual layer). The layer configuration of DL is schematically shown in FIG. 6.

[0119] 〔Comparative Example 2〕 A BK7 substrate with a diameter of 30 mm and a thickness of 1 mm was prepared as Comparative Example 2 of the optical element. This comparative example is referred to as "BK7".

[0120] 〔Evaluation〕 The antireflection films of the above examples and comparative examples were evaluated by the following method.

[0121] [Evaluation Method] (1) Antireflection Characteristics In the wavelength range of 400 to 1100 nm, the spectral reflectance of specular reflection on the surface of each optical element was measured. For the measurement of the spectral reflectance, a reflection spectroscopic film thickness meter FE-3000 manufactured by Otsuka Electronics Co., Ltd. was used.

[0122] Also, in the wavelength range of 400 to 1600 nm, the transmittance of each optical element was measured. For the measurement of the spectral transmittance, a spectroscopic altimeter U-4100 manufactured by Hitachi High-Technologies Corporation was used.

[0123] (2) Microscopic Structure of the Antireflection Film The cross-sections of the antireflection films in Examples 1 to 4 and Comparative Example 1 were observed using a scanning electron microscope (SEM: Scanning Electron Microscope), and the array structure of the fine particles constituting the refractive index gradient layer in each antireflection film was examined. For the SEM observation, an S-5500 manufactured by Hitachi High-Tech was used.

[0124] [Evaluation Results] (1) Antireflection Characteristics Fig. 7 shows the measurement results of the surface spectral reflectance of the optical elements in Examples 1 to 4 and Comparative Examples 1 and 2. The measurement results of the spectral reflectance of the antireflection film produced are shown. For all of the optical elements in Examples 1 to 4 and Comparative Example 1, the spectral reflectance of specular reflection with respect to incident light in the wavelength range of 400 to 700 nm was 1.1% or less.

[0125] Fig. 8 shows the measurement results of the transmittance of the optical elements of Examples 1 to 4 and Comparative Examples 1 and 2. For all of the optical elements of Examples 1 to 4 and Comparative Example 1, the transmittance of incident light in the wavelength range of 400 to 1600 nm was 96% or more. In particular, for the optical elements of Examples 1 to 4, the degree of reduction in transmittance on the long wavelength side was smaller than that of the optical element of Comparative Example 1.

[0126] (2) Microscopic structure of the antireflection film An SEM photograph of the cross section of the optical element of Example 4 is shown in Fig. 9. In the figure, “(a)” shows a mixed layer of silica sol and antimony oxide sol (high refractive index layer), “(b)” shows a mixed layer of silica sols with different particle diameters (medium refractive index layer), and “(c)” shows a mixed layer of silica sol and hollow silica sol mixed layer (low refractive index layer). In this SEM photograph, a material with a larger atomic number is observed brighter. Therefore, the contrast between antimony oxide sol and silica sol is clear. According to the above SEM photograph, the three zones of (a) to (c) above were clearly confirmed. Also, each of these layers is composed of an overlap of thin films with a plurality of different refractive indices, but the boundary of the thin film is not observed in each layer. Thus, although the refractive index gradient film of Example 4 apparently includes three layers, layers corresponding to more finely divided thin films having different refractive indices are not confirmed from the SEM observation.

[0127] From the above, it is clear that in the antireflection film of the present invention, the composition of the fine particle layer laminated on the substrate gradually changes to form a refractive index gradient layer, and high antireflection performance can be realized in a wide wavelength range.

Industrial applicability

[0128] Since the antireflection film according to the present invention has high antireflection performance, it can be suitably used as an antireflection film for various optical elements.

Explanation of symbols

[0129] 1 Optical element 10 Antireflection film 11 Intermediate layer 12 Refractive index gradient layer 12a High refractive index layer 12b Medium refractive index layer 12c Low refractive index layer 13 Functional layer 20 Substrate

Claims

1. An antireflection film provided on a substrate constituting an optical member, wherein the antireflection film includes a refractive index gradient layer having a refractive index that gradually changes from a largest first refractive index to a smallest n-th refractive index (n ≥ 4) in the thickness direction of the antireflection film, the refractive index gradient layer includes a first layer having the first refractive index, an n-th layer having the n-th refractive index, and n - 2 layers having a refractive index that gradually decreases from the first refractive index toward the n-th refractive index and that are between the first layer and the n-th layer and depend on the position in the thickness direction, the first layer contains particles of a first dielectric material, and the n-th layer contains particles of a second dielectric material that exhibits a lower refractive index than the first dielectric material, Antireflection film.

2. the refractive index gradient layer has the first layer at a position closest to the substrate in the thickness direction of the antireflection film, and the difference between the first refractive index and the refractive index of the substrate is 0.05 or less, The antireflection film according to claim 1.

3. The antireflection film according to claim 2, wherein the difference between the n-th refractive index and the refractive index of the medium surrounding the antireflection film is 0.25 or less.

4. the particles of the first dielectric material are metal oxide particles, the particles of the second dielectric material are silicon dioxide particles, and one or more layers from the first layer to the n-th layer are composed of mixed particles of the metal oxide particles and the silicon dioxide particles. The antireflection film according to claim 1.

5. the particles of the second dielectric material are silicon dioxide particles, and one or more layers other than the first layer from the first layer to the n-th layer are composed of mixed particles of two or more types of silicon dioxide particles having different particle diameters. The antireflection film according to claim 1.

6. The antireflection film according to claim 5, wherein the average particle diameter of the primary particles of the silicon dioxide particles having different particle diameters is 2 nm or more and 40 nm or less.

7. the particles of the second dielectric material are silicon dioxide particles, and one or more layers other than the first layer from the first layer to the n-th layer are composed of mixed particles of one or both of hollow silicon dioxide particles and porous silicon dioxide particles. The antireflection film according to claim 1.

8. The antireflection film according to claim 1, wherein the thickness of the refractive index gradient layer is 1 / 4 times or more the wavelength of light according to the purpose of the substrate.

9. the particles of the first dielectric material are metal oxide particles, The antireflection film according to claim 1, wherein the average particle diameter of the primary particles of the metal oxide particles is 25 nm or less.

10. The antireflection film according to claim 1, wherein the particles in one or more layers from the first layer to the nth layer are bonded by a binder.

11. The antireflection film according to claim 10, wherein the binder is a silane compound.

12. An optical element comprising a substrate constituting an optical member and the antireflection film according to any one of claims 1 to 11 provided on the substrate.

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