阅读说明：本技术 防反射膜、光学元件、防反射膜的制造方法及微细凹凸结构的形成方法 (Antireflection film, optical element, method for producing antireflection film, and method for forming fine uneven structure ) 是由 吉弘达矢 板井雄一郎 于 2019-08-27 设计创作，主要内容包括：本发明提供一种能够容易制作且具有更良好的防反射性的防反射膜、具备防反射膜的光学元件、防反射膜的制造方法及微细凹凸结构的形成方法。重复进行多次如下工序：薄膜形成工序,在基材上的成膜面形成包含铝的薄膜；及温水处理工序,通过对薄膜实施温水处理来形成由以氧化铝的水合物为主成分的板状晶体构成的微细凹凸结构。由此,得到包括如下微细凹凸层的防反射膜,该微细凹凸层具有在从作为表面的凸部前端朝向基材侧的厚度方向上逐渐变化且在最靠基材侧的界面处成为最大值的折射率分布,表面的折射率为1.01以下,当将作为防反射对象的光的波长区域中的最长的波长设为λ-(max)时,从表面至沿厚度方向100nm为止的第1折射率梯度为0.4/λ-(max)以下。(The invention provides an antireflection film which can be easily produced and has better antireflection property, an optical element provided with the antireflection film, a method for producing the antireflection film, and a method for forming a fine uneven structure. The following steps are repeated for a plurality of times: a thin film forming step of forming a thin film containing aluminum on a film formation surface on a substrate; and a hot water treatment step of forming a fine uneven structure composed of plate-like crystals mainly composed of a hydrate of alumina by subjecting the thin film to a hot water treatment. Thereby, an antireflection film including a fine uneven layer having a thickness from the tip of the projection as the surface toward the substrate side is obtainedA refractive index distribution gradually changing in direction and having a maximum value at an interface closest to the substrate side, wherein the refractive index of the surface is 1.01 or less, and λ is the longest wavelength in the wavelength region of light to be anti-reflected max When the refractive index gradient of 1 st from the surface to 100nm in the thickness direction is 0.4/lambda max The following.)

1. An antireflection film comprising a fine uneven layer which is disposed on one surface of a substrate and contains a plate crystal of a hydrate of alumina as a main component,

the fine uneven layer has a refractive index distribution which gradually changes in a thickness direction from a tip of a convex portion as a surface toward the base material side and has a maximum value at an interface closest to the base material side, the refractive index of the surface is 1.01 or less, and λ is a longest wavelength in a wavelength region of light to be anti-reflected_maxThe maximum refractive index gradient in the 1 st graded-index region from the surface to 100nm in the thickness direction is 0.4/lambda_maxThe following.

2. The antireflection film according to claim 1,

in the fine uneven layer, a maximum refractive index gradient in a 2 nd graded refractive index region from the 100nm position in the thickness direction to the interface on the substrate side is 0.8/λ_maxThe following.

3. The antireflection film according to claim 1 or 2,

the wavelength region of the light to be anti-reflected is 400nm to 800 nm.

4. The antireflection film according to any one of claims 1 to 3,

the maximum value in the refractive index distribution is 1.5 or more.

5. The antireflection film according to any one of claims 1 to 4,

the thickness of the fine concave-convex layer is more than 550 nm.

6. The antireflection film according to any one of claims 1 to 5,

said substrate is at λ_maxThe refractive index of (b) is more than 1.5.

7. The antireflection film according to any one of claims 1 to 6,

the antireflection film comprises a graded-index layer between the fine uneven layer and the base material,

the graded-index layer has a refractive index distribution in which the refractive index gradually changes in the thickness direction from the interface with the fine uneven layer toward the substrate, and the difference between the refractive index at the interface with the fine uneven layer and the refractive index of the fine uneven layer at the interface is 0.01 or less and the difference between the refractive index at the interface with the substrate and the refractive index of the substrate is 0.01 or less.

8. The antireflection film according to claim 7,

in the graded index layer, the maximum refractive index gradient in the thickness direction is 1.6/lambda_maxThe following.

9. The antireflection film according to claim 7 or 8,

the graded index layer comprises at least one of silicon oxide, silicon nitride, silicon oxynitride and niobium oxide as a main component.

10. The antireflection film according to any one of claims 1 to 6,

the antireflection film is provided with a refractive index matching layer between the fine uneven layer and the base material, and the refractive index matching layer is alternately provided with a high refractive index layer having a relatively high refractive index and a low refractive index layer having a relatively low refractive index.

11. An optical element, comprising: a substrate; and the antireflection film as described in any one of claims 1 to 10, which is disposed on one surface of the substrate.

12. A method for producing an antireflection film, comprising:

a thin film forming step of forming a thin film containing aluminum on a film formation surface on a substrate;

a hot water treatment step of forming a fine uneven structure composed of plate-like crystals mainly composed of a hydrate of alumina by subjecting the thin film to a hot water treatment; and

and a repeating step of repeating the thin film forming step and the hot water treatment step with the surface of the fine uneven structure as the film formation surface.

13. A method of forming a fine textured structure, comprising:

a thin film forming step of forming a thin film containing aluminum on a film formation surface on a substrate;

a hot water treatment step of forming a fine uneven structure composed of plate-like crystals mainly composed of a hydrate of alumina by subjecting the thin film to a hot water treatment; and

and a repeating step of repeating the thin film forming step and the hot water treatment step with the surface of the fine uneven structure as the film formation surface.

Technical Field

The present invention relates to an antireflection film, an optical element provided with the antireflection film, a method for producing the antireflection film, and a method for forming a fine uneven structure.

Background

An antireflection film that suppresses reflection of incident light is provided on an optical surface of an optical element body such as a lens. For example, an antireflection film having a fine uneven structure with a pitch smaller than the wavelength of incident light is known. In order to prevent reflection, it is desirable to reduce the difference in refractive index between the base material and air, but it is difficult to obtain a refractive index of 1.3 or less with a general material. On the other hand, a fine structure having a structural pitch equal to or smaller than the wavelength of light can be regarded as a medium having an effective refractive index corresponding to the volume fraction of a material and air, and therefore a refractive index of 1.3 or less can be obtained. Therefore, when a structure in which the volume fraction continuously changes in the optical axis direction, such as a fine uneven structure, is used, a remarkable antireflection performance can be obtained.

In jp 2010-156844 a (hereinafter, patent document 1), international publication No. 2016/006651 (hereinafter, patent document 2), and jp 2014-021146 a (hereinafter, patent document 3), in order to obtain higher antireflection performance, a refractive index distribution in the thickness direction of an antireflection film having a fine uneven structure is studied. In particular, patent documents 1 and 2 propose a structure having a concave portion or a convex portion in a tapered shape or a truncated cone shape.

As a method for forming a fine uneven structure, an imprint method is known. The method comprises the following steps: a fine uneven structure is formed on a metal or resin mold and transferred to an optical element to be antireflection-treated. The imprint method is put to practical use as a method for forming an antireflection structure of a thin film for a flat panel display, for example. Further, according to the imprint method, the shapes of the convex portion and the concave portion can be relatively easily controlled, and the shapes proposed in patent documents 1 and 2 and the like can be realized.

However, when the imprint method is applied to a curved surface such as a glass lens, for example, there are technical difficulties, and for example, a mold for each curvature of the lens is required, and alignment with high accuracy is required, which causes an increase in cost.

On the other hand, as a fine uneven structure that can be formed on a curved surface at low cost, a fine uneven structure containing a plate crystal of a hydrate of alumina as a main component is known. Patent document 3, japanese patent No. 4182236 (hereinafter, patent document 4) and japanese patent No. 4520418 (hereinafter, patent document 5) propose an antireflection film having such a fine uneven structure containing plate crystals of an alumina hydrate as a main component. Patent documents 3 to 5 propose a structure including a thin film layer for alleviating a refractive index step difference between a fine uneven structure mainly composed of plate crystals of alumina hydrate and a base material.

Disclosure of Invention

Technical problem to be solved by the invention

However, it is difficult to control the shape of a fine uneven structure mainly composed of plate crystals of alumina hydrate in detail. For example, patent document 4 describes that "the height of the fine unevenness is 0.005 μm to 5.0 μm". However, in the examples of patent document 4, only examples up to a height of 0.3 μm are described. Similarly, patent document 5 describes that "the thickness of the plate crystal layer is 20nm or more and 1000nm or less", but the maximum thickness of the plate crystal layer described in the examples is 500 nm.

As for the fine uneven structure mainly composed of the plate crystal of the alumina hydrate, the difficulty of controlling the fine uneven structure has not been sufficiently studied for realizing a more desirable refractive index distribution of the antireflection property.

The antireflection film is generally designed to minimize reflectance against incidence from an incidence angle of 0 ° (normal direction) with respect to an incidence plane of the optical element. Therefore, the low reflectance at the incident angle of 0 ° is a performance required for granted.

On the other hand, it is undesirable that the reflectance for oblique incidence such as an incident angle of 45 ° or 60 ° is greatly increased with respect to the reflectance for an incident angle of 0 °. Therefore, low reflectance is required as the antireflection property, and the reflectance at oblique incidence increases slightly compared to that at normal incidence.

In view of the above circumstances, an object of the present invention is to provide an antireflection film, an optical element, a method for producing an antireflection film, and a method for forming a fine uneven structure, which can be easily produced and have a low reflectance and in which an increase in reflectance at oblique incidence is suppressed.

Means for solving the technical problem

Specific means for solving the above problems include the following means.

<1> an antireflection film comprising a fine uneven layer which is disposed on one surface of a substrate and contains a plate-like crystal of a hydrate of alumina as a main component,

the fine uneven layer has a refractive index distribution which gradually changes in a thickness direction from a tip of a convex portion as a surface toward the base material side and has a maximum value at an interface closest to the base material side, the refractive index of the surface is 1.01 or less, and λ is a longest wavelength in a wavelength region of light to be anti-reflected_maxThe maximum refractive index gradient in the 1 st graded-index region from the surface to 100nm in the thickness direction is 0.4/lambda_maxThe following.

<2>According to<1>The antireflection film described above, wherein in the fine uneven layer, the maximum refractive index gradient in the 2 nd graded refractive index region from the 100nm position in the thickness direction to the interface on the substrate side is 0.8/λ_maxThe following.

<3> the antireflection film according to <1> or <2>, wherein a wavelength region of the light to be antireflection-treated is 400nm to 800 nm.

<4> the antireflection film according to any one of <1> to <3>, wherein the maximum value in the refractive index distribution is 1.5 or more.

<5> the antireflection film according to any one of <1> to <4>, wherein the thickness of the fine uneven layer is 550nm or more.

<6>According to<1>To<5>The antireflection film according to any of the above, wherein the substrate has a refractive index of λ_maxThe refractive index of (b) is more than 1.5.

<7> the antireflection film according to any one of <1> to <6>, which comprises a graded-refractive-index layer between the fine uneven layer and the base material,

the graded-index layer has a refractive index distribution in which the refractive index gradually changes in the thickness direction from the interface with the fine uneven layer toward the substrate, and the difference between the refractive index at the interface with the fine uneven layer and the refractive index of the fine uneven layer at the interface is 0.01 or less and the difference between the refractive index at the interface with the substrate and the refractive index of the substrate is 0.01 or less.

<8>According to<7>The antireflection film described above, wherein, in the graded-index layer, the maximum refractive index gradient in the thickness direction is 1.6/λ_maxThe following.

<9> the antireflection film according to any one of <7> and <8>, wherein the graded-index layer contains at least one of silicon oxide, silicon nitride, silicon oxynitride, and niobium oxide as a main component.

<10> the antireflection film according to any one of <1> to <6>, wherein a refractive index matching layer is provided between the fine uneven layer and the base material, and the refractive index matching layer alternately includes a high refractive index layer having a relatively high refractive index and a low refractive index layer having a relatively low refractive index.

<11> an optical element comprising: a substrate; and the antireflection film described in any one of <1> to <10>, which is provided on one surface of the substrate.

<12> a method for producing an antireflection film, comprising:

a thin film forming step of forming a thin film containing aluminum on a film formation surface on a substrate;

a hot water treatment step of performing a hot water treatment on the thin film to form a fine uneven structure composed of plate-like crystals mainly composed of a hydrate of alumina; and

and a repeating step of repeating the thin film forming step and the hot water treatment step with the surface of the fine uneven structure as the film formation surface.

<13> a method for forming a fine uneven structure, comprising:

a thin film forming step of forming a thin film containing aluminum on a film formation surface on a substrate;

a hot water treatment step of performing a hot water treatment on the thin film to form a fine uneven structure composed of plate-like crystals mainly composed of a hydrate of alumina; and

and a repeating step of repeating the thin film forming step and the hot water treatment step with the surface of the fine uneven structure as the film formation surface.

Effects of the invention

According to the present invention, an antireflection film which can be easily produced and has a low reflectance and in which an increase in reflectance at oblique incidence is suppressed can be provided.

Drawings

Fig. 1 is a schematic view showing a schematic structure and a refractive index distribution of an antireflection film and an optical element according to embodiment 1.

Fig. 2 is a view showing a process for producing an antireflection film.

Fig. 3 is a schematic view showing the schematic structure and refractive index distribution of the antireflection film and the optical element according to embodiment 2.

Fig. 4 is a schematic view showing the schematic structures and refractive index distributions of the antireflection film and the optical element according to embodiment 3.

Fig. 5 is a view showing a refractive index distribution of the fine uneven layer of example 1.

Fig. 6 is a graph showing the wavelength dependence of the reflectance of the antireflection film of example 1.

Fig. 7 is a graph showing the wavelength dependence of the reflectance of the antireflection film of example 2.

Fig. 8 is a graph showing the wavelength dependence of the reflectance of the antireflection film of example 2.

Fig. 9 is a graph showing the wavelength dependence of the reflectance of the antireflection film of example 4.

FIG. 10 is a graph showing the wavelength dependence of the reflectance of the anti-reflection film of example 5.

FIG. 11 is a graph showing the wavelength dependence of the reflectance of the anti-reflection film of example 6.

FIG. 12 is a graph showing the wavelength dependence of the reflectance of the anti-reflection film of example 7.

Fig. 13 is a view showing a refractive index distribution of the fine uneven layer of example 1.

Fig. 14 is a graph showing the wavelength dependence of the reflectance of the anti-reflection film of comparative example 1.

Fig. 15 is a graph showing the wavelength dependence of the reflectance of the anti-reflection film of comparative example 2.

Fig. 16 is a scanning electron microscope image of a cross section of the fine uneven layer of example 1.

Fig. 17 is a scanning electron microscope image of a cross section of the fine uneven layer of comparative example 1.

Detailed Description

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Fig. 1 is a schematic cross-sectional view showing a schematic structure of an optical element 1 including an antireflection film 11 according to embodiment 1 of the present invention, and a view showing a refractive index distribution in a thickness direction.

As shown in fig. 1, the antireflection film 11 of the present embodiment includes a fine uneven layer 20 mainly composed of plate-like crystals of hydrated alumina disposed on one surface of a substrate 10. The optical element 1 includes a substrate 10 and an antireflection film 11 disposed on one surface thereof.

The substrate 10 is a transparent optical member, such as a flat plate, a concave lens or a convex lens, which is mainly used in an optical device, or a flexible transparent film. As a material of the substrate, glass, plastic, or the like can be used. Here, "transparent" means that the internal transmittance is substantially 10% or more with respect to the wavelength of light (light subject to reflection) to be prevented from being reflected in the optical member. However, the substrate on which the antireflection film is formed is not limited to a transparent substrate, and is not particularly limited as long as it has a surface intended to prevent reflection.

The fine uneven layer 20 is a layer containing a plate crystal of a hydrate of alumina as a main component, at least the surface of which has fine unevenness. The hydrate of alumina is boehmite (denoted as Al) as alumina monohydrate₂O₃·H₂O or AlOOH. ) Bayerite (denoted as Al) as alumina trihydrate (aluminum hydroxide)₂O₃·3H₂O or Al (OH)₃. ) And the like. The phrase "mainly containing plate-shaped crystals of alumina hydrate" means that the plate-shaped crystals of alumina hydrate in the fine uneven layer 20 are 80 mass% or more of the components constituting the fine uneven layer 20.

In fig. 1, a fine uneven layer 20 is shown in which uniform convex portions having a large height difference are arranged in a row. However, the fine uneven layer 20, which is actually composed mainly of plate crystals of alumina hydrate, has a random structure in which plate crystals are superposed on each other, and specifically, has a cross section as shown in fig. 16 described later.

The fine uneven layer 20 has a refractive index distribution that gradually changes in the thickness direction from the front end of the projection as the surface toward the substrate 10 side and has a maximum value at the interface closest to the substrate 10 side. The gradual change in refractive index means that there is no refractive index step exceeding 0.01 in the refractive index distribution. The region from the surface of the fine uneven layer 20 to 100nm in the thickness direction is defined as a 1 st graded index region 21, and the region from the position of 100nm in the thickness direction to the interface closest to the substrate side is defined as a 2 nd graded index region 22. In this specification, the refractive index is a refractive index at a wavelength of 540nm unless otherwise specified.

The refractive index distribution in the thickness direction of the fine uneven layer 20 will be described.

The most important is the refractive index distribution at the tip side of the convex portion. If reflection occurs at a surface portion on which light is incident, an interference action for canceling the reflection needs to be generated in a subsequent structure. If the structure that causes interference is provided, the antireflection performance is degraded when light obliquely enters the antireflection film. Therefore, it is most important to prevent the reflection from occurring in the front end portion.

The refractive index of the surface of the fine uneven layer 20 is 1.01 or less, and the longest wavelength in the wavelength region of light to be reflected is represented by λ_maxThe maximum refractive index gradient in the 1 st graded-index region 21 from the surface to 100nm in the thickness direction is 0.4/lambda_maxThe following.

The light to be anti-reflected is generally light in the visible light region, and may be light in the infrared region as necessary, depending on the application. In the present specification, the visible light region refers to a wavelength of 400nm to 800 nm. Therefore, when the visible light region is an antireflection object, the longest wavelength λ in the wavelength region of light to be an antireflection object is_max800nm, shortest wavelength lambda_minIs 400 nm.

When the maximum refractive index gradient in the 1 st graded-index region 21 from the surface to 100nm in the thickness direction is 0.4/lambda_maxHereinafter, reflection at the tip portion can be sufficiently suppressed. The maximum refractive index gradient in the range from the surface 10Onm was 0.4/lambda_maxIn the following range, the refractive index gradient does not need to be constant, and can be increased or decreased.

When the refractive index gradient is determined from the refractive index distribution, the shortest wavelength λ in the wavelength region of light to be anti-reflected is_minThe variation of the refractive index gradient in the nm range below 1/20 is negligible. Therefore, in the present specification, the refractive index gradient is set to λ_minThe refractive index gradients in the range of/20 are averaged. I.e. will be at λ centered around each position_minThe refractive index gradient averaged in the range of/20 is defined as the refractive index gradient at each position.

Therefore, in the 1 st graded-index region 21, even microscopicAbove 0.4/lambda_maxThe position of the gradient (D) of (2) is also determined by the position of λ centered on the position_minThe value obtained by averaging the refractive index gradients in the range of/20 is positive and 0.4/lambda_maxThe following may be used. Therefore, as a whole, as shown in the lower graph of fig. 1, the refractive index n up to a position 100nm from the front end₁Until now, the refractive index gradually increases. Since the maximum refractive index gradient in this range is 0.4/lambda_maxTherefore, when the light to be reflected is visible light, n₁Is 1.05 or less. In addition, in the 1 st graded index region 21, the maximum refractive index gradient is more preferably 0.2/λ_maxThe following.

The surface of the fine uneven layer 20 can be clearly defined by examining the refractive index distribution of the fine uneven layer 20 in the thickness direction. When there is a step in the refractive index in the interface region with air, the step is the surface of the fine uneven layer 20. When there is no step difference in refractive index in the interface region with air, the refractive index decreases from the substrate side toward the thickness direction, and a portion where the refractive index becomes 1.0 is the surface.

The maximum gradient of refractive index in the 2 nd graded-refractive-index region 22 from the position of 100nm in the thickness direction of the fine uneven layer 20 to the interface closest to the substrate 10 side is preferably 0.8/lambda_maxThe following. In this case, even if more than 0.8/lambda exists on a microscopic scale_maxThe position of the gradient (D) of (2) is also determined by the position of λ centered on the position_minThe value obtained by averaging the refractive index gradients in the range of/20 is 0.8/lambda_maxThe following may be used.

In the case where the fine uneven layer 20 is directly provided on one surface of the substrate 10 as in embodiment 1, the refractive index n of the interface closest to the substrate side of the 2 nd graded refractive index region 22, which is the maximum value in the refractive index distribution of the fine uneven layer 20, is preferably set to be the maximum value in the refractive index distribution of the fine uneven layer 20₂And refractive index n of one surface of the substrate 10_SThe difference is 0.01 or less, and particularly preferably equal to each other.

The thickness h2 of the 2 nd graded-index region 22 is preferably 200nm or more, and preferably 600nm or less. The thickness h2 of the 2 nd graded-index region 22 is particularly preferably 450nm or more. That is, the thickness h of the entire fine uneven layer 20 is preferably 300nm or more, preferably 700nm or less, and particularly preferably 550nm or more.

By providing the fine uneven layer 20 having the refractive index distribution, reflection on the surface can be sufficiently suppressed, and a good antireflection property can be obtained. The fine uneven layer 20 having such a refractive index distribution can achieve a low reflectance even with obliquely incident light, and can sufficiently suppress a change in reflectance due to an incident angle (refer to examples described later).

The fine uneven layer having the refractive index distribution can be obtained by repeating the film formation of the thin film containing aluminum and the hot water treatment twice or more. A method for forming a fine uneven structure, that is, a method for manufacturing the antireflection film 11 will be described with reference to fig. 2.

A thin film 25 containing an aluminum element is formed on the surface of the substrate 10 (step 1). The thin film containing an aluminum element contains a mixture of one or more selected from metallic aluminum, aluminum oxide (aluminum), aluminum nitride, and aluminum fluoride, for example. The thin film 25 containing aluminum element can be obtained by vapor-phase film formation or liquid-phase film formation, but from the viewpoint of ease of forming a thin film on a structure including a plurality of curved surfaces, gas-phase film formation is particularly preferable. The thickness of the thin film can be 1nm to 100nm, and more preferably 20nm to 80 nm.

After the film forming process of step 1, the film 25 is immersed in warm water 50 together with the substrate 10 (step 2). The temperature of the hot water in the hot water treatment step is preferably 60 ℃ or higher and boiling point temperature or lower, and the immersion time is preferably 1 minute or higher and 10 minutes or lower. The temperature of the warm water is more preferably 95 ℃ or higher, and the immersion time is more preferably 3 minutes or longer. Further, as the hot water treatment liquid, pure water is preferably used, and ultrapure water having a specific resistance of 14 M.OMEGA.m or more at a water temperature of 25 ℃ is particularly preferably used.

By the above-described warm water treatment step, the thin film 25 becomes a fine uneven layer 26 composed of plate-like crystals of alumina hydrate (step 3). A drying treatment is performed to remove warm water from the surface of the fine uneven layer 26 of the base material taken from the warm water 50. As the drying treatment, warm air drying, infrared drying, hot plate drying, suction drying, vacuum drying, steam drying, warm water pull-up drying (hot water pull-up drying), marangoni drying, air blow drying (air blow drying), spin drying, and the like used in a drying step after industrial cleaning can be used.

Then, the thin film forming step and the hot water treatment step are repeated with the fine uneven layer 26 as a film forming surface. Specifically, the thin film 25 containing an aluminum element is formed on the surface of the fine uneven layer 26 (step 4), and is immersed in warm water 50 together with the base material 10 (step 5).

Through the above steps, the fine uneven structure having the refractive index distribution can be formed on the substrate 10, and the antireflection film 11 having the fine uneven layer 20 can be produced (step 6). When the ratio of metallic aluminum, aluminum oxide (aluminum), aluminum nitride, and aluminum fluoride in the thin film containing an aluminum element is 99% or more, the hydrate of aluminum oxide in the obtained fine uneven layer 20 has a purity of 95% or more. Contains less than 5% of magnesium or sodium as impurities.

The thin film forming step and the hot water treatment step may be performed not only twice but also three times or more.

According to the studies of the present inventors, a fine uneven layer having a structure height (thickness) of more than 500nm cannot be obtained by film formation of a thin film containing aluminum and hot water treatment only once. Here, the thickness of the fine uneven layer refers to the height (thickness) of a region in which the refractive index changes. For example, even when an aluminum thin film having a thickness of 80nm is formed and subjected to a hot water treatment, a fine uneven layer having a structure height of more than 500nm and comprising plate-like crystals cannot be obtained. Further, an aluminum thin film having a thickness of 100nm was formed and subjected to warm water treatment, and as a result, a part of the substrate side was opaque, that is, a layer in a metal state was left without being changed to a hydrate of alumina. On the other hand, according to the manufacturing method of one embodiment of the present invention, a fine uneven layer having a structure height of more than 500nm can be easily obtained, and particularly, a refractive index gradient of 0.4/λ in the 1 st graded refractive index region at the front end can be obtained_maxFine uneven structure having refractive index distribution of。

An antireflection film having a fine uneven layer composed of plate-like crystals mainly composed of a hydrate of alumina has been proposed so far, but only the use of a fine uneven layer formed by thin film formation and hot water treatment at one time has been studied so far. In the method for forming a fine uneven structure and the method for producing an antireflection film according to one aspect of the present invention, the thin film formation and the hot water treatment are repeated twice or more. This makes it possible to obtain a fine uneven layer comprising plate-like crystals having a structural height that has not been obtained in the past and a fine uneven layer having a refractive index distribution in which the refractive index gradient at the tip of a convex portion is extremely small.

Therefore, the antireflection film 11 capable of sufficiently suppressing reflection on the surface and obtaining a good antireflection property can be manufactured easily and inexpensively.

Fig. 3 is a schematic cross-sectional view showing a schematic structure of an optical element 2 including an antireflection film 12 according to embodiment 2 of the present invention, and a view showing a refractive index distribution in a thickness direction.

As shown in fig. 3, the antireflection film 12 of the present embodiment includes a graded-index layer 30 as an intermediate layer between the fine uneven layer 20 and the substrate 10. The fine uneven layer 20 is the same as the fine uneven layer of the antireflection film 11 of embodiment 1 described above.

When the refractive index n of the 2 nd graded refractive index region of the fine concavo-convex layer 20 at the interface closest to the base material side₂And refractive index n of substrate 10_SWhen the difference exceeds 0.01, it is preferable to provide a graded-index layer 30 for compensating the difference in refractive index between the two layers.

The refractive index of the substrate 10 exceeds 1.5, and may be 1.6 or more. In this case, the refractive index n of the 2 nd graded index region₂Preferably 1.5 or more.

The graded refractive index layer 30 has a refractive index distribution in which the refractive index gradually changes in the thickness direction from the interface with the fine uneven layer 20 toward the substrate 10, the refractive index at the interface with the fine uneven layer 20 and the refractive index n of the fine uneven layer 20 at the interface₂A difference of 0.01 or less between the refractive index at the interface with the substrate 10 and the refractive index of the substrate 10Refractive index n_SThe difference is 0.01 or less. Further, it is preferable that the maximum refractive index gradient in the thickness direction is 1.6/λ_maxThe following.

The graded-index layer 30 is a thin film layer containing at least two or more materials, and is a layer in which the ratio of the materials is continuously changed in the thickness direction of the thin film to obtain a continuous change in refractive index. For example, a change in the ratio of oxygen to nitrogen in a silicon oxynitride (SiON) film caused by reactive sputtering can be used. Furthermore, silicon oxide (SiO) in the Meta mode (Meta mode) sputtering can be used₂) With titanium oxide (TiO)_x) Mixed film of (2), SiO₂With niobium oxide (Nb)₂O₅) And SiON and Nb₂O₅The element ratio of the mixed film of (1). In addition, as another method, a desired graded-index thin film can be obtained by sequentially coating a plurality of solutions having different refractive indices on a substrate. Examples of the coating method include spin coating, dip coating, spray coating, and inkjet method. Here, the continuous refractive index change means that the amount of change in refractive index of adjacent layers is 0.01 or less.

The antireflection film according to embodiment 2 can be produced by: the graded index layer 30 is formed on the substrate by the above method, and the fine uneven layer 20 is formed by the above-described method for forming a fine uneven structure with the surface of the graded index layer 30 as a film formation surface.

According to the antireflection film of embodiment 2, since the fine uneven layer 20 is provided as in embodiment 1, the same effects as in embodiment 1 can be obtained, and also, even when the refractive index of the base material exceeds 1.5, and is further as large as 1.6 or more, very good antireflection properties can be obtained.

Fig. 4 is a schematic cross-sectional view showing a schematic structure of an optical element 3 including an antireflection film 13 according to embodiment 3 of the present invention, and a view showing a refractive index distribution in a thickness direction.

As shown in fig. 4, the antireflection film 12 of the present embodiment includes a refractive index matching layer 40 as an intermediate layer between the fine uneven layer 20 and the substrate 10 in the antireflection film 11 of fig. 1. The fine uneven layer 20 is the same as the fine uneven layer of the antireflection film 11 of embodiment 1 described above.

When the refractive index n of the substrate 10_SAnd refractive index n of the fine uneven layer 20₂If the difference exceeds 0.01, a refractive index matching layer 40 may be provided as in the present embodiment, instead of the graded index layer 30 of embodiment 2. The reflection that cancels the reflection caused by the difference in refractive index between the substrate 10 and the fine uneven layer 20 is generated by the refractive index matching layer 40, and thus the antireflection performance can be improved.

The refractive index matching layer 40 has a laminated structure in which a high refractive index layer 41 having a relatively high refractive index and a low refractive index layer 42 having a relatively low refractive index are alternately laminated, and the ratio of the relatively high refractive index to the relatively low refractive index is 1.1 or more. Also, "having a relatively high refractive index" and "having a relatively low refractive index" refer to the relative relationship of the high refractive index layer and the low refractive index layer, and refer to the high refractive index layer having a higher refractive index than the low refractive index layer, and the low refractive index layer having a lower refractive index than the high refractive index layer.

In the refractive index matching layer 40 of the antireflection film 13 shown in fig. 4, a total of 8 high refractive index layers 41 and low refractive index layers 42 are alternately stacked so that the high refractive index layers 41 are disposed on the most substrate side, but the stacking order and the number of layers are not limited to this. That is, the low refractive index layer 42 may be laminated on the substrate side, and the total number of layers of the laminated structure of the high refractive index layer 41 and the low refractive index layer 42 may be at least two, and preferably four or more.

Refractive index n of high refractive index layer 41_HPreferably 1.7 or more, more preferably 1.9 or more. Refractive index n of low refractive index layer 42_LPreferably 1.6 or less, more preferably 1.55 or less. In addition, the refractive index n of the low refractive index layer 42 is preferably set_LRefractive index n lower than that of the substrate 10_SAnd the refractive index n of the high refractive index layer 41_HRefractive index n higher than that of the substrate 10_S。

The high refractive index layers 41 and the low refractive index layers 42 do not have to have the same refractive index and be made of the same material, but it is preferable to have the same refractive index and the same material from the viewpoint of suppressing the material cost and the film formation cost.

Examples of the material of the high refractive index layer 41 include an oxide of any one of aluminum, titanium, tantalum, zirconium, niobium, magnesium, and lanthanum, aluminum nitride, aluminum oxynitride, silicon nitride, silicon oxynitride, and a mixture thereof.

Examples of the material of the low refractive index layer 42 include silicon oxide, silicon oxynitride, magnesium fluoride, and a mixture thereof, and a mixture of aluminum oxide and a mixture of silicon oxide, silicon oxynitride, and magnesium fluoride.

Each layer of the refractive index matching layer 40 can be formed by a vapor deposition method such as a sputtering method, an electron beam evaporation method, or a chemical vapor deposition method.

The antireflection film according to embodiment 3 can be produced by: the refractive index matching layer 40 is formed on the substrate by the above-described method, and the fine uneven layer is formed by the above-described method for forming the fine uneven structure using the surface of the refractive index matching layer 40 as a film formation surface.

According to the antireflection film of embodiment 3, since the fine uneven layer 20 is provided as in embodiment 1, the same effects as in embodiment 1 can be obtained, and also, even when the refractive index of the base material exceeds 1.5, and is further as large as 1.6 or more, very good antireflection properties can be obtained.

Examples

Hereinafter, examples of the present invention and comparative examples will be described. Here, the light to be antireflection is visible light. Thus, λ_maxIs 800 nm.

[ example 1]

Alumina (Al) of 70nm was formed on a glass substrate and a single crystal Si substrate of Eagle XG (registered trademark) manufactured by Corning Incorporated co₂O₃) A film. Next, ultrapure water having a resistivity of 14 M.OMEGA.cm or more at a water temperature of 25 ℃ was heated to 100 ℃ to impregnate the two substrates on which the alumina thin films were formed for 3 minutes. Thereby obtaining oxygenAfter the fine uneven structure mainly composed of plate crystals made of aluminum hydrate, hot water was dried by air-blown drying. Subsequently, an alumina thin film of 70nm was formed again by the DC sputtering method. Further, the two substrates on which the alumina thin films were formed were immersed for 3 minutes by heating ultrapure water to 100 ℃. This provides a fine uneven layer which is thicker than the fine uneven structure formed by the first hot water treatment and contains a hydrate of alumina as a main component. That is, the antireflection film of example 1 in which the fine uneven layer was disposed on the glass substrate was produced by repeating the film formation of the alumina thin film and the hot water treatment twice to form the fine uneven structure. Here, the purity of the alumina thin film formed by the DC sputtering method was measured by X-ray photoelectron spectroscopy, and as a result, the purity was 99% or more. Further, the following measurement of refractive index distribution was performed using a sample prepared at the same time and having a fine uneven layer disposed on a single crystal Si substrate.

< measurement of refractive index distribution >

With respect to the obtained structure, a refractive index distribution (refractive index distribution) of the fine uneven layer in the thickness direction was obtained by spectroscopic ellipsometry measurement. The obtained refractive index profile is shown in fig. 5. The horizontal axis represents the thickness from the surface of the base material with the position of the surface of the base material set to 0, and the vertical axis represents the refractive index. The refractive index changes continuously from the surface of the substrate and gradually decreases to the refractive index 1 of air. As shown in fig. 5, there is no refractive index step at the interface with air. Therefore, a point where the refractive index matches the refractive index 1 of air is regarded as the surface of the fine uneven layer. The thickness of the fine relief layer is 590 nm.

The refractive index at a position of 100nm in the thickness direction from the surface of the fine uneven structure was 1.045. Therefore, the refractive index gradient in the 1 st graded index region was 0.045/100nm and satisfied 0.4/λ_maxThe following. The thickness of the 2 nd graded index region was 490nm, and the refractive index was changed from 1.045 to 1.52 until the surface of the substrate. Thus, the refractive index gradient in the 2 nd graded index region was 0.475/490 nm.

< reflectance >

The single-sided reflectance of the S-polarized light and the P-polarized light in the visible light (wavelength 400 to 800nm) ranges at incident angles of 0 °, 45 °, and 60 ° with respect to the substrate surface was obtained by numerical calculation using the refractive index distribution obtained in the above manner. The numerical calculation uses the software Essential mac. Fig. 6 shows reflection dependencies of the refractive indices at the incident angles 0 °, 45 °, and 60 ° obtained by numerical calculation. The average reflectance at incident angles of 0 °, 45 ° and 60 ° was determined for wavelengths within the range of 400 to 800 nm. The average reflectance is summarized in table 3 below together with the results of other examples and comparative examples.

The average reflectance obtained at an incident angle of 60 ° was 1.3%, and a good value was obtained even at oblique incidence.

[ example 2]

Silicon oxynitride was formed on the glass substrate of S-LAH55V manufactured by OHARA inc. The refractive index of silicon oxynitride can be controlled according to the gas flow rate during reactive sputtering. Accordingly, the graded index layer was formed by continuously changing the refractive index from 1.84 to 1.52 from the substrate side toward the surface side. The thickness of the graded index layer was set at 1280nm and the 3 rd refractive index gradient was set at 0.00025/nm. A fine uneven layer was formed on the graded index layer in the same manner as in example 1, and an antireflection film of example 2 in which the graded index layer and the fine uneven layer were disposed on a glass substrate was produced.

With respect to the obtained structure, the wavelength dependence of the reflectance and the average reflectance were determined in the same manner as in example 1, using the refractive index distribution of the fine concave-convex layer determined in example 1 for the fine concave-convex layer formed in the same manner as in example 1, and further using the refractive index distribution of the graded-index layer. Fig. 7 shows the reflection dependency of the refractive index at the incident angles of 0 °, 45 °, and 60 ° obtained by numerical calculation, and table 3 shows the average reflectance.

The average reflectance obtained at an incident angle of 60 ° was 1.3%, and a good value was obtained even at oblique incidence.

[ example 3]

An antireflection film of example 3 was produced in the same manner as in example 2, except that the thickness of the graded-index layer was 640nm and the refractive index gradient was 0.0005/nm.

The wavelength dependence of the reflectance and the average reflectance were determined in the same manner as in example 2. Fig. 8 shows the reflection dependency of the refractive index at the incident angles of 0 °, 45 °, and 60 ° obtained by numerical calculation, and table 3 shows the average reflectance.

The average reflectance obtained at an incident angle of 60 ° was 1.2%, and a good value was obtained even at oblique incidence.

[ example 4]

An antireflection film of example 4 was produced in the same manner as in example 2, except that the thickness of the graded-index layer was 320nm and the refractive index gradient was 0.001/nm.

The wavelength dependence of the reflectance and the average reflectance were determined in the same manner as in example 2. Fig. 9 shows the reflection dependency of the refractive index at the incident angles of 0 °, 45 °, and 60 ° obtained by numerical calculation, and table 3 shows the average reflectance.

The average reflectance obtained at an incident angle of 60 ° was 1.2%, and a good value was obtained even at oblique incidence.

[ example 5]

An antireflection film of example 5 was produced in the same manner as in example 2, except that the thickness of the graded-index layer was 160nm and the refractive index gradient was 0.002/nm.

The wavelength dependence of the reflectance and the average reflectance were determined in the same manner as in example 2. Fig. 10 shows the reflection dependency of the refractive index at the incident angles of 0 °, 45 °, and 60 ° obtained by numerical calculation, and table 3 shows the average reflectance.

The average reflectance obtained at an incident angle of 60 ° was 1.1%, and a good value was obtained even at oblique incidence.

[ example 6]

A refractive index matching layer having a laminated film of two types of silicon oxynitride (SiON) and silicon nitride (SiN) as an intermediate layer was formed on an S-LAH55V glass substrate manufactured by OHARA inc. Table 1 shows the layer structure of the antireflection film of example 6. The film thickness of each layer of the index matching layer is shown in table 1. Then, a fine uneven layer was formed on the laminated film by the same method as in example 1, and the antireflection film of example 6 in which the fine uneven layer was disposed on the refractive index matching layer was produced.

[ Table 1]

With respect to the obtained structure, the wavelength dependence of the reflectance and the average reflectance were determined in the same manner as in example 1, using the refractive index distribution of the fine uneven layer determined in example 1 for the fine uneven layer formed in the same manner as in example 1, and further using the refractive index distribution of the refractive index matching layer. Fig. 11 shows the reflection dependency of the refractive index at the incident angles of 0 °, 45 °, and 60 ° obtained by numerical calculation, and table 3 shows the average reflectance.

The average reflectance obtained at an incident angle of 60 ° was 1.1%, and a good value was obtained even at oblique incidence.

[ example 7]

An antireflection film of example 7 was produced in the same manner as in example 2, except that the thickness of the graded-index layer was 80nm and the refractive index gradient was 0.004/nm.

The wavelength dependence of the reflectance and the average reflectance were determined in the same manner as in example 2. Fig. 12 shows the reflection dependency of the refractive index at the incident angles of 0 °, 45 °, and 60 ° obtained by numerical calculation, and table 3 shows the average reflectance. The average reflectance at the incident angle of 0 °, 45 ° is large as compared with the other examples, but the increase rate of the average reflectance at the incident angle of 60 ° from the increase of the incident angle of 0 ° is suppressed as compared with the other examples.

Comparative example 1

An Al thin film of 40nm was formed on a glass substrate and a single crystal Si substrate of Eagle XG (registered trademark) manufactured by Corning Incorporated co. Next, the same ultrapure water as in example 1 was heated to 100 ℃, and the two substrates on which the Al films were formed were immersed for 3 minutes, thereby obtaining a fine uneven layer mainly composed of plate-like crystals made of alumina hydrate. The antireflection film of comparative example 1 was produced in which a fine uneven layer formed by performing the Al thin film formation and the warm water treatment only once was disposed on a glass substrate. Here, the purity of the Al film formed by the DC sputtering method was measured by X-ray photoelectron spectroscopy, and as a result, the purity was 99% or more. Further, the following measurement of refractive index distribution was performed using a sample prepared at the same time and having a fine uneven layer disposed on a single crystal Si substrate.

With respect to the obtained structure, a refractive index distribution (refractive index distribution) of the fine uneven layer in the thickness direction was obtained by spectroscopic ellipsometry measurement. The obtained refractive index profile is shown in fig. 13. The horizontal axis represents the thickness from the surface of the base material with the position of the surface of the base material set to 0, and the vertical axis represents the refractive index. As shown in fig. 13, there is no refractive index step at the interface with air. Therefore, a point where the refractive index matches the refractive index 1 of air is regarded as the surface of the fine uneven layer. The thickness of the fine concavo-convex layer is 325 nm.

The refractive index at a position of 100nm in the thickness direction from the surface of the fine uneven layer was 1.09. Therefore, the refractive index gradient in the 1 st graded-index region is 0.09/100nm and exceeds 0.4/lambda_max. The 2 nd graded index region had a thickness of 225nm, a refractive index having a peak value from 1.1 to the surface of the substrate, and then dropped to around 1.3.

In the same manner as in example 1 except for using the refractive index distribution obtained in the above manner, the single-sided reflectances of the S-polarized light and the P-polarized light in the visible light (wavelength of 400 to 800nm) ranges at incident angles of 0 °, 45 °, and 60 ° with respect to the substrate surface were obtained by numerical calculation. Fig. 14 shows reflection dependencies of refractive indices at incident angles of 0 °, 45 °, and 60 ° obtained by numerical calculation. Table 3 shows the average reflectance.

The average reflectance at an incident angle of 60 ° was 2.2%, which is a relatively large value.

Comparative example 2

A laminated film composed of two kinds of silicon oxynitride and silicon nitride was formed as an index matching layer on an S-LAH55V glass substrate manufactured by OHARA inc. Table 2 shows the layer structure of the antireflection film of comparative example 2. The film thickness of each layer of the index matching layer is shown in table 2. Then, a fine uneven layer was formed on the laminated film by the same method as in comparative example 1, and an antireflection film of comparative example 2 in which the fine uneven layer was disposed on the refractive index matching layer was produced.

[ Table 2]

With respect to the obtained structure, the wavelength dependence of the reflectance and the average reflectance were determined in the same manner as in comparative example 1, using the refractive index distribution of the fine uneven layer determined in comparative example 1 for the fine uneven layer formed in the same manner as in comparative example 1, and further using the refractive index distribution of the refractive index matching layer. Fig. 15 shows the reflection dependency of the refractive index at the incident angles of 0 °, 45 °, and 60 ° obtained by numerical calculation, and table 3 shows the average reflectance.

A very low average reflectance was obtained at an incident angle of 0 °, but the average reflectance of 1.6% and the value at oblique incidence were large compared to the examples, which were obtained at an incident angle of 60 °.

[ Table 3]

As is clear from the results of example 1 and comparative example 1, which have no intermediate layer, the fine uneven layer treated with hot water twice can have a lower average reflectance at any incident angle than the fine uneven layer treated with hot water once, and thus can have a good anti-reflection property.

As shown in table 3, in examples 1 to 6, a good average reflectance of 0.2% or less was obtained at an incident angle of 0 °. Even when the incident angle is increased to 45 ° and 60 °, an average reflectance of 1.5% or less is obtained, and the increase in reflectance when the light is obliquely incident is suppressed to 10 times or less with respect to the incident angle of 0 °, whereby good antireflection properties are obtained. In example 7, although the average reflectance at the incident angle of 0 ° is slightly higher than that of the other examples, the increase in the average reflectance at the incident angles of 45 ° and 60 ° with respect to the incident angle of 0 ° is suppressed to 4 times or less, the change width of the reflectance with respect to oblique incidence is small, and good antireflection properties are obtained. In comparative example 2 having a structure including a fine uneven layer obtained by a single hot water treatment and a refractive index matching layer composed of a laminated film of high and low refractive index layers, a very low average reflectance was obtained at an incident angle of 0 °. On the other hand, in comparative example 2, the average reflectance at the incident angle of 45 ° was increased by 5 times or more and the average reflectance at the incident angle of 60 ° was increased by 20 times with respect to the incident angle of 0 °, and the antireflection performance was significantly reduced by oblique incidence.

Fig. 16 and 17 are Scanning Electron Microscope (SEM) images of cross sections of the fine uneven layer formed on the Si substrate in example 1 and comparative example 1, respectively. The multiplying power is 5 ten thousand times. The fine uneven layer of example 1 shown in fig. 16 has a thickness of 590nm, is composed of plate-like crystals from the surface of the base material to the surface of the fine uneven layer, and has fine unevenness at least on the surface. On the other hand, the fine uneven layer of comparative example 1 shown in fig. 17 has a thickness of 325 nm. As shown in fig. 16, it is understood that the fine uneven layer of example 1, even if it is thick, is composed of plate-like crystals from the surface of the fine uneven layer to the surface of the Si substrate. That is, by repeating the film formation of the thin film layer containing aluminum and the hot water treatment twice or more, a fine uneven layer having a film thickness of more than 500nm was successfully formed without leaving a thin film layer that has not been hydrated on the substrate side. Heretofore, there has been no previous example of actually producing a fine uneven layer comprising plate-like crystals of hydrated alumina having a thickness of more than 500 nm.

The entire disclosure of japanese application No. 2018-182735, filed on 27.9.2018, is incorporated herein by reference.

All documents, patent applications, and technical standards described in the present specification are incorporated by reference into the present specification to the same extent as if each document, patent application, and technical standard incorporated by reference was specifically and individually described.