Patent Application
20140327966
This application is a Continuation of PCT International Application No. PCT/JP2013/000280 filed on Jan. 22, 2013, which claims priority under 35 U.S.C §119 (a) to Japanese Patent Application No. 2012-011634 filed on Jan. 24, 2012. Each of the above applications is hereby expressly incorporated by reference, in its entirety, into the present application.
The present invention relates to an antireflection film having a two-layer structure including a first layer and a second layer formed on a substrate.
Conventionally, reflection on surfaces of optical elements cause decrease of light transmittance of lenses, etc., ghost or flare, reflection of external light on display screens, etc., and therefore high-performance antireflection is desired.
As a method for achieving such antireflection, a method where a multi-layer film having a single-layer, two-layer, three-layer or more layer structure is formed on a surface of an optical element to reduce reflection by utilizing an interference effect between the layers is proposed.
For example, in a case where an antireflection film is formed by a single layer, the single layer is formed to have a refractive index of n1/2 relative to a refractive index n of the substrate and have a film thickness of λ0/4 relative to a design wavelength λ0. In this case, however, the reflection largely increases if the wavelength of incoming light deviates from the design wavelength λ0.
Further, an antireflection film having a two-layer structure including a layer of a high-refractive index material and a layer of a low-refractive index material formed from the substrate side is proposed. However, this antireflection film also has the problem of increase of reflection when the wavelength of incoming light deviates from a design wavelength, and has a narrow antireflection band.
Still further, an antireflection film having a three-layer or more layer structure including a layer of a high-refractive index material and a layer of a low-refractive index material is proposed. However, such an antireflection film has a drawback that the reflectance increases when light enters obliquely.
Yet further, an antireflection film having a so-called moth-eye structure, where an average refractive index is gradually changed from the substrate side, is proposed. This type of antireflection film, however, requires a certain film thickness, and has a weak mechanical strength, resulting in an unstable structure.
Further, each of Japanese Unexamined Patent Publication Nos. 10(1998)-268103, 10(1998)-227902 and 2010-281876 (hereinafter, Patent Documents 1, 2 and 3 respectively), for example, proposes an antireflection film including two layers that have refractive indices decreasing in a stepwise mariner from the substrate side; however, preferred refractive indices and film thicknesses thereof are not clear.
In view of the above-described circumstances, the present invention is directed to providing an antireflection film that can reduce reflectance in a wavelength range near a design wavelength when compared to conventional techniques, and has a stable structure that can be formed in a simple manner.
The antireflection film of the invention is an antireflection film comprising a first layer and a second layer formed on a substrate in this order from the substrate side, the second layer being formed to be in contact with an ambient medium, wherein the first layer and the second layer are formed such that an average refractive index n1 of the first layer at a given wavelength λ0, an average refractive index n2 of the second layer at the given wavelength λ0, a film thickness d1 of the first layer, and a film thickness d2 of the second layer satisfy conditional expressions below:
0.96<n1/(n0×n0×n3)1/3<1.04,
0.96<n2/(n0×n3×n3)1/3<1.04,
0.8<d1×n1/(λ0/6)<1.2, and
0.8<d2×n2/(λ0/6)<1.2,
where n0 is a refractive index of the substrate at the given wavelength λ0, and n3 is a refractive index of the ambient medium at the given wavelength λ0.
In the antireflection film of the invention, the first layer and the second layer may be formed such that the average refractive index n1 of the first layer at the given wavelength λ0, the average refractive index n2 of the second layer at the given wavelength λ0, the film thickness d1 of the first layer, and the film thickness d2 of the second layer satisfy conditional expressions below:
0.98<n1/(n0×n0×n3)1/3<1.02,
0.98<n2/(n0×n3×n3)1/3<1.02,
0.9<d1×n1/(λ0/6)<1.1, and
0.9<d2×n2/(λ0/6)<1.1.
The first layer may be a continuous film uniformly formed using a homogeneous material, and the second layer may include first areas and second areas having different refractive indices and alternately arranged along in-plane directions.
The first areas or the second areas of the second layer may be air gaps.
The first layer or the second layer may have a relief structure with a pitch not greater than the given wavelength λ0.
Each projecting area of the relief structure may have side surfaces that are formed perpendicular to a surface on which the projecting area is foamed.
The first layer may be made of MgF.
The second layer may be made of a resin.
The optical element of the invention is provided with the above-described antireflection film of the invention.
In the optical element of the invention, the given wavelength may be near the center of an operating wavelength range.
According to the antireflection film and the optical element of the invention, the antireflection film includes a first layer and a second layer formed on a substrate in this order from the substrate side, the second layer being formed to be in contact with an ambient medium, wherein the first layer and the second layer are formed such that an average refractive index n1 of the first layer at a given wavelength λ0, an average refractive index n2 of the second layer at the given wavelength λ0, a film thickness d1 of the first layer, and a film thickness d2 of the second layer satisfy conditional expressions below:
0.96<n1/(n0×n0×n3)1/3<1.04,
0.96<n2/(n0×n3×n3)1/3<1.04,
0.8<d1×n1/(λ0/6)<1.2, and
0.8<d2×n2/(λ0/6)<1.2,
where n0 is a refractive index of the substrate at the given wavelength λ0, and n3 is a refractive index of the ambient medium at the given wavelength λ0. The thus formed antireflection film of the invention can achieve wider-band antireflection than the conventional antireflection films having a two-layer structure. In particular, the antireflection film of the invention has a local minimum point of reflectance around the design wavelength, thereby achieving further reduction of reflectance near the center of the operating wavelength range. It should be noted that the basis for the numerical values in conditional expressions (1) to (4) will be described in detail later.
In particular, when the antireflection film of the invention is used in the visible light range, further reduction of reflectance around green light, which is highly visible to human eyes, can be achieved, thereby allowing further reduction of visually recognizable reflection light.
Further, the film thicknesses set as shown by the above conditional expressions allow reducing the film thicknesses.
In the case where the first layer or the second layer has a relief structure, for example, higher water repellency can be provided, which in turn provides an antifouling effect.
Further, since the structure of the antireflection film of the invention can be made to have a low aspect ratio when compared to a moth-eye structure, a structure with high mechanical strength can be achieved. This also allows improving mold releasability, thereby improving manufacturing suitability.
Hereinafter, one embodiment of an antireflection film of the present invention will be described in detail with reference to the drawings.
As shown in
First, the basic design concept of the antireflection film 1 of this embodiment is described using
Then, a film thickness d1 of the first layer 12 and a film thickness d2 of the second layer 14 are set such that there is an optical path length difference of (⅔)·λ0 between reflection light R1 of incoming light L from the interface between the first layer 12 and the glass substrate 10 and reflection light R3 of the incoming light L from the interface between the air and the second layer 14, and there is an optical path length difference of (⅓)·λ0 between reflection light R2 of the incoming light L from the interface between the second layer 14 and the first layer 12 and the reflection light R3 of the incoming light L from the interface between the air and the second layer 14. That is, the first layer 12 and the second layer 14 are formed such that the value of n1×d1 is near the value of λ0/6 and the value of n2×d2 is near the value of λ0/6. It should be noted that the reflection light R1, the reflection light R2 and the reflection light R3 have almost the same intensities (amplitudes).
Based on the above-described design concept, the first layer 12 and the second layer 14 of the antireflection film 1 of this embodiment are formed such that the average refractive index n1 of the first layer 12 at the design wavelength λ0 and the average refractive index n2 of the second layer 14 at the design wavelength λ0 satisfy conditional expressions (1) and (2) below, and the film thickness d1 of the first layer 12 and the film thickness d2 of the second layer 14 satisfy conditional expressions (3) and (4) below (the basis for the numerical values in conditional expressions (1) to (4) will be described in detail later):
0.96<n1/(n0×n0×n3)1/3<1.04 (1),
0.96<n2/(n0×n3×n3)1/3<1.04 (2),
0.8<d1×n1/(λ0/6)<1.2 (3),
0.8<d2×n2/(λ0/6)<1.2 (4).
It should be noted that the ambient medium is not limited to air and may be any other medium.
Next, one example of the antireflection film 1 of this embodiment is described with reference to
The antireflection film 1 of this example is configured to achieve antireflection for the visible light range (400 nm to 700 nm), and the design wavelength is the center wavelength λ0=550 nm of the visible light range.
As a material forming the glass substrate 10, N-SK16, available from SCHOTT AG, is used. The glass substrate 10 has a refractive index of 1.622 at the design wavelength λ0, and a reflectance of 5.6% when the first layer 12 and the second layer 14 are not provided. It should be noted that this reflectance is normal incidence reflectance at the air interface.
As shown in
As shown in
The relief structure of the second layer 14 is formed, specifically, by arranging projecting areas 14a at a pitch of 200 nm (i.e., “P” shown in
Since the pitch of the relief structure of the second layer 14 is not greater than any wavelength in the visible light range, as described above, the second layer 14 is optically regarded as a film having an average refractive index that is determined by a volume ratio between the projecting areas 14a and the depressed areas, which are air gaps. Therefore, in the case where the relief structure is formed using a resin material having a refractive index of 1.51, as described above, the second layer 14 can be regarded as a film having an average refractive index of 1.177 (the refractive index n2).
Values with respect to conditional expressions (1) to (4) shown above of the thus formed antireflection film 1 are as follows:
n1/(n0×n0×n3)1/3=1.0018,
n2/(n0×n3×n3)1/3=1.0018,
d1×n1/(λ0/6)=0.9958,
d2×n2/(λ0/6)=1.0015.
Next, the basis for the values in conditional expressions (1) to (4) is described.
As can be seen from the results shown in
It should be noted that an even higher antireflection effect can be provided when the first layer 12 and the second layer 14 are formed to satisfy values of conditional expressions (5) to (8) below:
0.98<n1/(n0×n0×n3)1/3<1.02 (5),
0.98<n2/(n0×n3×n3)1/3<1.02 (6),
0.9<d1×n1/(λ0/6)<1.1 (7),
0.9<d2×n2/(λ0/6)<1.1 (8).
It should be noted that, although the relief structure of the second layer 14 of the antireflection film 1 of the above-described example is formed using nanoimprinting, this is not intended to limit the invention. The relief structure may be formed using any other existing process. Further, not only the second layer 14 but also the first layer 12 may have a relief structure similarly to the second layer 14.
Further, although the second layer 14 of the antireflection film 1 of the above-described example is formed by the projecting areas 14a and the depressed areas which are air gaps, the depressed areas may not necessarily be air gaps, and may be filled with a material having a refractive index that is different from the refractive index of the projecting areas 14a, so as to satisfy the refractive index n2 of the above-described conditional expression.
The method used to form the low-refractive index layer is not limited to the above-described method for forming the relief structure. For example, the low-refractive index layer may be formed using a material having a low average refractive index, such as a material containing silica aerogel or hollow particles, or using a film having a low average refractive index, such as a boehmite film. Further, a plurality of such low-refractive index layers may be formed in the form of a layer stack. Still alternatively, the low-refractive index layer may be formed by etching a surface of a base material to form the two-level relief structure.
The antireflection film 1 of the above-described embodiment can be formed on an optical element. Examples of the optical element include lenses, prisms, filters, window materials, etc. The antireflection film 1 suitable for the optical element can be formed by setting the above-described design wavelength λ0 near the center of the operating wavelength range of the optical element.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-011634 | Jan 2012 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2013/000280 | Jan 2013 | US |
| Child | 14338547 | US |
