JP2018185392A - Anti-reflection film and optical element having the same, optical system, and optical instrument - Google Patents

Abstract

An antireflection film excellent in wavelength band characteristics and incident angle characteristics, and an optical element, an optical system, and an optical device having the same are provided. An antireflection film is formed on an optical substrate having a refractive index of 1.85 or more and 2.20 or less at a wavelength of 550 nm, and a frustum shape or a substantially frustum shape is regularly arranged at a pitch of 250 nm or less. The fine concavo-convex structure layer has a region in which the refractive index can be regarded as continuously changing over a thickness range of 120 nm or more and 250 nm or less, and the optical substrate and the fine structure layer A dielectric thin film layer having a refractive index of 1.6 to 2.0 is formed therebetween. [Selection] Figure 1

Description

  The present invention relates to an antireflection film formed on the surface of an optical element such as a lens, and an optical element, an optical system, and an optical device having the antireflection film.

  Conventionally, anti-reflection measures have been taken on the surface of optical elements in order to reduce the amount of incident light loss and suppress the generation of unnecessary light such as flare and ghost. For example, a dielectric multilayer film called a multi-coach is widely used as an anti-reflection measure for camera lenses. This is a mechanism to reduce the reflected light by laminating thin films with different refractive indexes at appropriate thicknesses, adjusting the amplitude and phase of the reflected waves generated at the surface and interface of each film, and causing them to interfere with each other. It is. Therefore, it is possible to exhibit excellent antireflection performance with respect to light beams having a specific wavelength and incident angle used in the design. However, interference conditions are lost for light rays with different wavelengths and incident angles from specific light rays, so high antireflection performance is achieved over a wide wavelength range such as the entire visible range and a wide incident angle range such as 0 to 60 °. It was difficult to do.

  On the other hand, digital cameras that have become widespread in recent years use image sensors such as CCD and CMOS, which have a higher surface reflectance than conventional photographic films. For this reason, specific unnecessary light called “digital ghost” is likely to be generated when the light reflected by the sensor surface reaches / reflects the lens surface again and reaches the sensor surface again.

  Furthermore, recent digital camera lenses not only achieve high image quality, but also require high specifications (zoom magnification and brightness) and portability (small and lightweight) at the same time, so anomalous dispersion glass and high refractive index glass are required. , Aspherical lenses and large curvature lenses (lens with a large opening angle) tend to be used frequently. Among these, lenses with a large curvature are incident and output with a large angle of light at the periphery, so the conventional multi-coat cannot reduce the reflection sufficiently, reducing the quality of captured images such as flares and ghosts. In some cases, unnecessary light was generated.

  Against this background, there is a need for the development of high-performance antireflection films that have better wavelength band characteristics and incident angle characteristics than multi-coats. Patent Document 1 provides a low refractive index layer having a lower refractive index than that of the base material on the base material and a periodic structure portion arranged with a period equal to or shorter than the shortest use wavelength, and the refractive index, film thickness, and structure of the low refractive index layer. By appropriately setting the height and shape, it is said that antireflection performance excellent in incident angle characteristics and wavelength band characteristics can be obtained.

International Publication No. 2008/102882

  The antireflection film disclosed in Patent Document 1 describes a range in which performance is improved in terms of film thickness and refractive index, but does not disclose details of specific design values. Moreover, the refractive index range of the base material from which the effects of the invention can be obtained is 1.3 to 1.8, and a method for improving the antireflection performance of a high refractive index base material having a refractive index of 1.85 or more is not disclosed.

  Accordingly, an object of the present invention is to provide an antireflection film having both excellent wavelength band characteristics and incident angle characteristics satisfying << Reflectance standard 1 >> shown below in a high refractive index base material having a refractive index of 1.85 or more, and It is to provide an optical element, an optical system, and an optical apparatus having the same. However, for wavelengths between 400 and 430 nm, a standard value at a wavelength of 400 nm and a standard value at a wavelength of 430 nm are connected by a straight line. The value obtained by connecting the standard value with a straight line is the standard.

≪Reflectance standard 1≫
For light rays with an incident angle of 0 to 45 °,
(1) Reflectance of 0.3% or less in the wavelength range of 430 to 670 nm
(2) Reflectance less than 0.5% at wavelengths of 400nm and 700nm
For rays with an incident angle of 60 °,
(3) Reflectance 2.5% or less in the wavelength range of 430 to 670 nm
(4) Reflectance of 3.5% or less at wavelengths of 400 nm and 700 nm

The present invention is an antireflection film formed on an optical substrate having a refractive index of 1.85 to 2.20 at a wavelength of 550 nm,
A frustum shape or substantially a frustum shape having a fine concavo-convex structure layer regularly arranged at a pitch of 250 nm or less,
The fine concavo-convex structure layer has a region where the refractive index can be regarded as continuously changing over a range of thickness of 120 nm to 250 nm.
A dielectric thin film layer having a refractive index of 1.6 to 2.0 is formed between the optical substrate and the microstructure layer.

  According to the present invention, it is possible to provide an antireflection film having both excellent wavelength band characteristics and incident angle characteristics, and an optical system and an optical apparatus having the antireflection film.

Schematic of refractive index structure of antireflection film of the present invention Schematic sectional view of the antireflection film of the present invention Refractive index structure and reflectance characteristics of antireflection film of Example 1 Refractive index structure and reflectance characteristics of antireflection film of Example 2 Refractive index structure and reflectance characteristics of antireflection film of Example 3 Refractive index structure and reflectance characteristics of antireflection film of Example 4 Refractive index structure and reflectance characteristics of antireflection film of Example 5 Refractive index structure and reflectance characteristics of antireflection film of Example 6 Schematic diagram of the optical apparatus of Example 7 Schematic sectional view of the antireflection film of the present invention (when the shape of the microstructure film is dull) Schematic diagram showing how to replace a dull shaped microstructure with a frustum

  Embodiments of the present invention will be described with reference to the accompanying drawings. All the refractive indexes in the description are values at a wavelength of 550 nm.

  FIG. 1 schematically and schematically shows the refractive index structure of the antireflection film of the present invention. FIG. 2 schematically shows a cross section of the antireflection film of the present invention, and shows an enlarged surface portion of an optical element to which the antireflection film of the present invention is applied on an optical substrate such as a lens. It is.

The optical substrate 11 is a member having a refractive index N _sub of 1.85 to 2.20, and an antireflection film 12 is formed thereon. The antireflection film 12 includes a dielectric thin film layer (first layer) 101 and a fine concavo-convex structure layer 13. The fine concavo-convex structure layer 13 includes an underlayer (second layer) 102, a structural layer (third layer) 103, and the like. It is composed of Here, the fine concavo-convex structure layer 13 is a layer formed by a nanoimprint method or an etching method, the structure layer 103 is a portion where a fine concavo-convex shape is formed (transferred or etched), and the base layer 102 has a structure. It is a layer that remains without being formed. That is, the structural layer 103 and the base layer 102 are the same material.

The first layer 101 is a thin film layer having a refractive index N ₁ of 1.6 to 2.0 and a film thickness D ₁ of 30 to 100 nm. The second layer 102 is a thin film layer having a refractive index N ₂ of 1.38 to 1.62 and a film thickness D ₂ of 90 nm or less. The third layer 103 (the height of the irregularities) thickness D ₃ has the following values 120～300Nm, top bottom refractive index at (substrate side) portion from the value N _3b ranging 1.2 to 1.5 ( It has a region where the refractive index in the (vertex side) portion changes substantially continuously up to a value N _{3t in} the range of 1.12 or less.

  Here, the expression “substantially” does not mean that the refractive index of the medium forming the third layer itself changes continuously, but the “space filling factor” of the concavo-convex structure smaller than the wavelength. By changing continuously in the direction, it means that the “effective refractive index” changes continuously. This is due to the property that light recognizes a concavo-convex structure having a size smaller than its own wavelength as a medium having an “effective refractive index” corresponding to the space occupancy.

The effective refractive index N _eff is the Bruggeman equation, where N _m is the refractive index of the material that forms a concavo-convex structure finer than the wavelength, and ff is the space occupancy of the material.

Can be used to calculate. That is, in a structure in which the space occupancy continuously changes at a pitch smaller than the wavelength, a film in which the substantial refractive index (effective refractive index) continuously changes can be formed.

  In the present invention, the production method is not particularly limited as long as it is an antireflection film satisfying the above-described refractive index and film thickness.

  For example, the dielectric thin film layer 101 can use a wet method, a vapor deposition method, a sputtering method, or the like. In view of environmental durability and the like, dry film formation such as sputtering is preferable, and further, a silicon oxynitride (SiON) film or a film containing alumina (Al2O3) is preferable.

  For the fine uneven structure layer 13, a nanoimprint method, a method using photolithography and dry etching, or the like can be used. In particular, the nanoimprint method is preferable because the structural layer 103 and the base layer 102 can be formed simultaneously.

  By adopting the refractive index structure as described above, the antireflection film of the present invention realizes high performance that satisfies both the wavelength band characteristics and the incident angle characteristics that satisfy the << reflectance standard 1 >> shown below. Can do.

≪Reflectance standard 1≫
For light rays with an incident angle of 0 to 45 °,
(1) Reflectance of 0.3% or less in the wavelength range of 430 to 670 nm
(2) Reflectance less than 0.5% at wavelengths of 400nm and 700nm
For rays with an incident angle of 60 °,
(3) Reflectance 2.5% or less in the wavelength range of 430 to 670 nm
(4) Reflectance of 3.5% or less at wavelengths of 400 nm and 700 nm

  Here, the reflectance standard on which the present invention is based on high performance will be described. The antireflection film of the present invention places importance on the reflectance characteristics at wavelengths of 430 to 670 nm, and the characteristics at wavelengths of 400 to 430 nm and wavelengths of 670 to 700 nm are relaxed. This takes into account the spectral sensitivity characteristics of image sensors such as CCD and CMOS used in digital cameras. In other words, image sensors for cameras have high sensitivity in the wavelength range of 430 to 670 nm in order to faithfully reproduce images viewed by humans, but low sensitivity at wavelengths of 400 to 430 nm and wavelengths of 670 to 700 nm. It is set to be. Therefore, even if the reflectance characteristics at wavelengths of 400 to 430 nm and wavelengths of 670 to 700 nm are somewhat high, unnecessary light such as flare and ghost hardly affects the captured image, so the reflectance standard for the same wavelength band is relaxed. Is possible.

In the antireflection film of the present invention, the refractive index and film thickness of each film are set.
1.85 ≤ N _sub ≤ 2.20,
1.63 ≤ N ₁ ≤ 2.0, 45.0 nm ≤ D ₁ ≤ 90.0 nm,
1.48 ≤ N ₂ ≤ 1.60, D ₂ ≤ 82.0nm,
1.26 ≤ N _3b ≤ 1.46, 1.015 ≤ N _3t ≤ 1.05, 180nm ≤ D ₃ ≤ 250nm
By setting within the range, it is possible to satisfy << reflectance standard 2 >> which is further superior to <reflectance standard 1 >>.

≪Reflectance standard 2≫
For light rays with an incident angle of 0 to 45 °,
(1) Reflectance of 0.15% or less in the wavelength range of 430 to 670 nm
(2) At wavelengths of 400 nm and 700 nm, the reflectance is 0.35% or less.
(3) Reflectance less than 1.5% in the wavelength range of 430 ~ 670nm
(4) Reflectance less than 2.5% at wavelengths of 400nm and 700nm

  And, by using the optical element on which the antireflection film of the present invention is formed in an optical system such as a camera lens, a high-quality optical system and optical device that suppress the generation of unnecessary light and harmful light such as flare and ghost are suppressed. Can be realized.

  Examples of the present invention will be specifically described below. The refractive index and film thickness of each example are shown in Table 1 as a list.

[Example 1]
FIG. 3 shows the refractive index structure and reflectance characteristics of the antireflection film of Example 1 of the present invention. (1) to (4) show examples in which the present embodiment is applied to four types of optical substrates having a refractive index N _sub of 1.859 to 2.162.

  In each figure, (a) is the refractive index profile, (b) is the reflectance characteristic (full scale 3.5%), and (c) is the reflectance characteristic (full scale 0.5%). The details of the refractive index and film thickness of each film are as shown in Table 1. The optical substrates are (1) L-LAH85 manufactured by OHARA, (2) S-LAH58, (3) S-LAH79, and (4) K-PSFn173 manufactured by Sumita Optical Glass. Each example exhibits high antireflection performance, (1) satisfies << reflectance standard 2 >>, and (2), (3), and (4) satisfy << reflectance standard 1 >>.

  The first layer (dielectric thin film) 101 in this embodiment uses silicon oxynitride (SiON). The material of the first layer is not limited to silicon oxynitride as long as the refractive index shown in Table 1 can be realized. However, silicon oxynitride can be arbitrarily adjusted within the range of 1.4 to 2.0 by adjusting the mixing ratio of oxygen and nitrogen. It is preferable because it can be set. Silicon oxynitride is also a preferable material because of its high ability to suppress moisture permeation from the outside and elution of alkali components from the inside of the glass material (optical substrate).

  The method for forming the first layer is not limited, but any method such as a vacuum deposition method, a sputtering method, a molecular beam epitaxy (MBE) method, or a plasma CVD method can be used. In forming a dielectric thin film, even if the material is the same, the refractive index and the refractive index dispersion slightly vary depending on the film forming conditions (substrate temperature, ultimate vacuum, etc.). In such a case, substantially the same high performance can be obtained by correcting the film thickness so that the optical film thickness (refractive index × physical film thickness) value of the above film matches.

  The fine concavo-convex structure layer 13 is a film formed by transferring a fine concavo-convex shape to a resin (refractive index: 1.493) mainly composed of acrylic by a nanoimprint method. The third layer (structure layer) 103 is a portion to which the fine concavo-convex shape has been transferred, and the second layer (base layer) 102 is a base layer that remains under the structure without being transferred.

This embodiment is characterized in that the film thickness of the second layer 102 is fixed at 20.0 nm. The concave-convex shape of the third layer 103 is a regular arrangement of frustum shapes (shapes obtained by cutting off the apexes of a cone, a quadrangular pyramid, or a hexagonal pyramid) with a pitch of 250 nm or less. By making the space occupancy (FF _b ) at the bottom part 90% and the space occupancy (FF _t ) at the top part 4%, the refractive index changes continuously from 1.443 to 1.018. Yes.

In order to obtain good reflectance characteristics, FF _t is preferably a small value. This is because if the top portion is conical, the refractive index is 1.0, the same as air, and the reflection there can be zero. However, in molding such as nanoimprint, it is practically difficult to achieve FF _t close to 0% considering the mold manufacturing method, durability, and mold release after molding. Therefore, in the present invention, FFt is set to 1% or more, more preferably 4% or more.

Further, as the height (thickness) of the unevenness of the fine structure layer 14 is increased, the antireflection performance (particularly performance at a large incident angle) can be increased. However, if the height of the unevenness is increased, the releasability at the time of molding is lowered.
D ₃ ≤ 250nm
It is set to become. From the viewpoint of releasability, not only the height but also the aspect ratio (height / pitch) is important. As the aspect ratio increases, the releasability decreases, so the pitch is preferably in the range of 100 to 250 nm.

  Similarly, considering the mold manufacturing method in nanoimprint, it is difficult to realize a strict frustum shape. In fact, as shown in FIG. 10, the bottom part and the top part are dull and rounded. turn into. In such a case, as shown in FIG. 11, a frustum 202 having the same volume that can be formed by extending the inclination of the cone as it is is formed above the point 201 where deviation starts from the shape of the frustum. By substituting with a middle broken line), it can be converted into an almost equivalent frustum shape. The same idea can be applied to the bottom part. That is, when the shape of the microstructure layer is rounded, the effect of the present invention can be obtained by setting the frustum shape replaced by the above method to be in the numerical range as described above. be able to.

  As for the arrangement of the fine structure, any arrangement such as a hexagonal arrangement and a square arrangement may be used as long as the above refractive index range is satisfied. The frustum shape may be any shape such as a frustum, an elliptic frustum, or a polygonal frustum as long as the refractive index range is satisfied.

 In this embodiment, a resin mainly composed of acrylic is used as the material of the fine concavo-convex structure layer 13, but the present invention is not limited to this. Any material other than acrylic, such as polycarbonate, polystyrene, cycloolefin polymer, etc., that satisfies the above-mentioned refractive index conditions and film thickness conditions and can transfer and form a microstructure can be used. Can be expressed. The curing method may be any method such as an ultraviolet curable resin, a thermosetting resin, or a thermoplastic resin. A sol-gel material such as spin-on-glass (SOG) may be used.

[Example 2]
FIG. 4 shows the refractive index structure and reflectance characteristics of the antireflection film of Example 2 of the present invention. (1) to (3) show examples in which the present invention is applied to three types of optical substrates having a refractive index N _sub of 1.859 to 2.011.

  Details of the refractive index and film thickness of each film are as shown in Table 1. In this embodiment, the thickness of the second layer 102 is 0 nm. That is, there is no second layer, and the third layer (structure layer) 103 is formed directly on the first layer 101. By setting the refractive index and film thickness as described in Table 1, (1) and (2) satisfy << Reflectance standard 2 >>, and (3) satisfies << Reflectance standard 1 >>. The manufacturing method of the optical substrate and each film is the same as that described in the first embodiment, and will be omitted.

[Example 3]
FIG. 5 shows the refractive index structure and reflectance characteristics of the antireflection film of Example 3 of the present invention. (1) to (4) show examples in which the present invention is applied to four types of optical substrates having a refractive index N _sub of 1.859 to 2.162.

  Details of the refractive index and film thickness of each film are as shown in Table 1. This embodiment is characterized in that the thickness of the second layer 102 is set to 70 to 80 nm. By setting the refractive index and film thickness as shown in Table 1, all of the antireflection films (1) to (4) satisfy << Reflectance standard 2 >>. The manufacturing method of the optical substrate and each film is the same as that described in the first embodiment, and will be omitted.

[Example 4]
FIG. 6 shows the refractive index structure and reflectance characteristics of the antireflection film of Example 4 of the present invention. (1) to (4) show examples in which the present invention is applied to four types of optical substrates having a refractive index N _sub of 1.859 to 2.162. Details of the refractive index and film thickness of each film are as shown in Table 1. The present embodiment is characterized in that a resin (refractive index: 1.588) containing polycarbonate as a main component is used as the material of the fine concavo-convex structure layer 13. By setting the refractive index and film thickness as shown in Table 1, all of the antireflection films (1) to (4) satisfy << Reflectance standard 2 >>. The manufacturing method of the optical substrate and each film is the same as that described in the first embodiment, and will be omitted.

[Example 5]
FIG. 7 shows the refractive index structure and reflectance characteristics of the antireflection film of Example 5 of the present invention. (1) to (4) show examples in which the present invention is applied to an optical substrate having a refractive index N _sub of 1.888. Details of the refractive index and film thickness of each film are as shown in Table 1. This embodiment is characterized in that the space occupancy (FF _t ) at the top portion of the structural layer (third layer) 103 is changed from 1% to 25%. By setting the refractive index and film thickness as shown in Table 1, (1) and (2) satisfy << Reflectance standard 2 >>, and (3) and (4) satisfy << Reflectance standard 1 >>. Yes. The manufacturing method of the optical substrate and each film is the same as that described in the first embodiment, and will be omitted.

From this example, the space occupancy (FF _t ) at the top portion of the structural layer (third layer) 103 is set between 1 and 25%, and the refractive index (N _3t ) at the top portion is 1.12 or less. It can be seen that it is preferable. More preferably, the refractive index (N _3t ) of the top portion should be 1.05 or less, and for this purpose, the space occupancy (FF _t ) in the top portion should be set to 10% or less.

[Example 6]
FIG. 8 shows the refractive index structure and reflectance characteristics of the antireflection film of Example 6 of the present invention. (1) to (4) show examples in which the present invention is applied to an optical substrate having a refractive index N _sub of 1.888. Details of the refractive index and film thickness of each film are as shown in Table 1. This embodiment is characterized in that the thickness of the dielectric thin film layer (first layer) 101 is changed from 32 nm to 95 nm. By setting the refractive index and film thickness as described in Table 1, (2) and (3) satisfy << Reflectance standard 2 >>, and (1) and (4) satisfy << Reflectance standard 1 >>. Yes. The manufacturing method of the optical substrate and each film is the same as that described in the first embodiment, and will be omitted.

  From this example, it can be seen that the thickness of the first layer 101 is preferably set between 30 and 100 nm. More preferably, it may be set in the range of 45 to 95 nm.

[Example 7]
FIG. 9 is a schematic view when the optical element (lens) on which the antireflection film of the present invention is formed is applied to an optical apparatus (camera). In FIG. 9, since the digital camera 2001 uses the lens 1001 having the antireflection film of the present invention to reduce the generation of harmful light such as flare and ghost, a high-quality image can be obtained. it can.

11 Optical substrate (lens)
12 Anti-reflective coating
13 Fine relief structure layer
101 First layer (dielectric thin film layer)
102 Second layer (underlayer)
103 3rd layer (Structure layer)
1001 optical system
2001 Optical device (camera)

Claims (9)

An antireflection film formed on an optical substrate having a refractive index of 1.85 or more and 2.20 or less at a wavelength of 550 nm,
The antireflection film has a frustum shape or a substantially uneven structure layer in which the frustum shape is regularly arranged at a pitch of 250 nm or less,
The fine concavo-convex structure layer has a region where the refractive index can be regarded as continuously changing over a thickness of 120 nm to 250 nm.
An antireflection film, wherein a dielectric thin film layer having a refractive index of 1.6 to 2.0 is formed between an optical substrate and the microstructure. The antireflection film according to claim 1,
The refractive index of the optical substrate is N _sub, and in order from the optical substrate side,
A first layer having a refractive index of N ₁ and a thickness of D ₁ ;
A second layer (underlayer) having a refractive index of N ₂ and a film thickness of D ₂ ;
The third layer (structural layer) whose period is 250 nm or less and whose film thickness (height of the fine concavo-convex structure) is D ₃ is integrally formed (same material),
When the space occupancy at the bottom of the fine relief structure in the third layer is FF _b , the effective refractive index is N _3b , the space occupancy at the top is FF _t , and the effective refractive index is N _3t ,
An antireflection film characterized in that the refractive index and film thickness of each layer satisfy the following conditions.
1.85 ≤ N _sub ≤ 2.20,
1.60 ≤ N ₁ ≤ 2.0, 30 nm ≤ D ₁ ≤ 100 nm
1.38 ≤ N ₂ ≤ 1.62, D ₂ ≤ 90nm
1.20 ≤ N _3b ≤ 1.50, 1.004 ≤ N _3t ≤ 1.12, 120nm ≤ D ₃ ≤ 250nm 3. The antireflection film according to claim 2, wherein the refractive index and film thickness of each layer satisfy the following conditions.
1.85 ≤ N _sub ≤ 2.20,
1.63 ≤ N ₁ ≤ 2.0, 45nm ≤ D ₁ ≤ 90nm
1.48 ≤ N ₂ ≤ 1.60, D ₂ ≤ 82nm
1.26 ≤ N _3b ≤ 1.46, 1.015 ≤ N _3t ≤ 1.05, 180nm ≤ D ₃ ≤ 250nm The reflection according to any one of claims 1 to 3, wherein the first layer is any one of a film containing silicon oxynitride (SiON) or aluminum oxide (Al ₂ O ₃ ). Prevention film. The antireflection film according to any one of claims 1 to 4, wherein the second layer and the third layer are made of an energy curable resin material. The antireflection film according to any one of claims 1 to 4, wherein the second layer and the third layer are inorganic materials formed by a sol-gel method. An optical element, wherein the antireflection film according to claim 1 is formed. An optical system comprising the optical element according to claim 7. An optical apparatus comprising the optical system according to claim 8.