JP2013109228A - Optical member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical member, in which a fine structure layer comprising a plurality of columnar projections with different heights is formed on a substrate surface, the fine structure layer has a refractive index lower than that of the substrate and thereby, has an antireflection effect, and which can suppress variation in reflectance in a wide wavelength range of light incident to the substrate, although the fine structure layer is a monolayer.SOLUTION: The optical member comprises at least a substrate and a fine structure layer, in which the fine structure layer comprises a plurality of columnar projections with different heights. When the shortest wavelength and the longest wavelength of light incident to the optical member are denoted by λand λ, respectively, a height D of the columnar projection in the fine structure layer satisfies an expression (1): λ/4≤D≤λ/4.

Description

  The present invention relates to an optical member having a microstructure layer composed of a plurality of columnar protrusions having different heights.

  In a display device such as a liquid crystal television, a smartphone, a mobile phone, a notebook PC, or a portable game machine, there is a problem that external light, illumination light, or the like is reflected and visibility is significantly reduced. In addition, noise such as flare and ghost is a big problem in image pickup devices such as digital still cameras and digital video cameras.

  In order to solve such a problem, an optical member with a single-layer or multi-layer antireflection film has been proposed (see Patent Documents 1 to 4).

  Patent Document 1 discloses a porous layer having a plurality of columnar resins formed on a microlens and voids (voids) formed between the columnar resins in an imaging device including a plurality of photoelectric conversion elements and microlenses. Have an antireflection effect. The porous layer has a function as an antireflection film with a reduced refractive index, and reduces noise by reducing re-reflected light and scattered light between the inner surface of the cover glass and the microlens surface of the imaging device.

Patent Document 2 discloses a fine particle laminated thin film having voids provided on a substrate as an antireflection film. In order to form a thin film with a refractive index of 1.25 to 1.30, which can provide a sufficient antireflection effect with a single layer, on a glass or plastic substrate with a refractive index of about 1.52, the refractive index is It is necessary to make the porosity of the fine particle laminated thin film made of 1.48 silica fine particles 43 to 53%. The porosity ρ ₀ is obtained from the following equation (2).
ρ ₀ = 1− (n ₁ ² −n ₀ ² ) / (n ₂ ² −n ₀ ² ) (2)
However, n ₀ is the refractive index of the light incident medium, n ₁ is the refractive index of the fine structure, and n ₂ is the refractive index of the substrate.

Patent Document 3 discloses an example in which a columnar microprojection group is applied as an antireflection film. For example, when the refractive index of the glass substrate is 1.5 and the wavelength of light incident on the substrate is 1550 nm, the ideal antireflective layer has a refractive index of 1.22 and a thickness of about 320 nm. By expanding the air region in the layer composed of the projection group, it is possible to realize a low refractive index layer having a refractive index of 1.3 or less, which is difficult with the conventional continuous film of MgF ₂ used for the antireflection layer of the glass substrate. .

Patent Document 4 discloses an antireflective body that can be manufactured at low cost under atmospheric pressure, has excellent mass productivity, and covers a wide wavelength range. In an antireflective body comprising an intermediate layer and a low refractive index layer provided in this order from the substrate side, the refractive index of the low refractive index layer and the intermediate layer can be obtained by selectively forming recesses on the surface of the substrate. It is equivalently defined by the uneven structure. For example, air having a refractive index of 1.0 is used as the light incident medium, a resin material having a refractive index of 1.5 is used as the base material, the refractive index of the low refractive index layer is 1.2, and the refractive index of the intermediate layer is 1. Set to .4. In this case, the volume density of the base material in the low refractive index layer, that is, the occupation ratio of the convex portion is about 0.35, and the intermediate layer is about 0.77. Therefore, the volume density and porosity of the recesses are about 0.65 in the low refractive index layer and about 0.23 in the intermediate layer.

JP 2002-261261 A JP 2009-113484 A JP 2009-187025 A JP 2010-237419 A

However, the conventional technology has the following problems.

According to Patent Document 1, the surface of the transparent resin layer on the microlens is dry-etched in an oxygen plasma atmosphere to form a porous layer having optical voids (voids) in which columnar resins are arranged. The void is represented by a pitch V _p between the columnar resins, and V _p = 0.1 to 0.2 μm so as to be approximately λ / 4 in the visible light region. Also, the thickness _{V t} of the porous layer is set to be a lambda / 4 (0.1 to 0.2 [mu] m) of the optical wavelength in the visible light region (FIG. 4 of Patent Document 1). In the porous layer, the presence of voids reduces the refractive index of the resin layer and functions as an antireflection film. The reflectance of the microlens is a low reflectance of 1/4 compared with that in which the porous layer is not formed. However, Patent Document 1 does not disclose the volume density (void ratio) of voids, nor does it disclose a specific refractive index.

  According to Patent Document 2, in a fine particle laminated film in which silica fine particles having a refractive index of 1.48 are laminated, the refractive index is 1.25 to 1.30 when the porosity is 43 to 53%. However, in the fine particle laminated film, firstly, a process A in which either the electrolyte polymer or the fine particle dispersion is applied to the substrate surface, and then a process B in which the electrolyte polymer or the fine particle dispersion not used in the process A is applied. The process is complicated because the process A and the process B are alternately performed once or more, and further, the process C and the process of applying the alcoholic silica sol solution and at least three processes are performed.

  According to Patent Document 3, a columnar microprojection group aligned in the vertical and horizontal directions and having the same height is applied to the antireflection layer. However, in the case of a single layered columnar microprojection group formed by pressing the mold (FIG. 13 of Patent Document 3), the reflectance varies depending on the wavelength of incident light. In addition, the forming method using the mold does not require photolithography or dry etching as in the semiconductor process, but the ratio of the equivalent diameter of the protrusion to the protrusion height of the columnar minute protrusion group (aspect ratio) is 4 or more. It is difficult to manufacture a mold, and durability against pressing becomes a problem.

According to Patent Document 4, an antireflection body including an intermediate layer and a low refractive index layer provided in order from the base material side exhibits a low reflectance of 0.25% or less in a wide wavelength range of 400 to 750 nm. (FIG. 2 of Patent Document 4). However, when the refractive index of the low refractive index layer and the intermediate layer is equivalently defined by the concavo-convex structure obtained by selectively forming concave portions on the surface of the base material, the assumed refractive index is the low refractive index layer. Is 1.2, and the intermediate layer is 1.4. In this case, the occupied area ratio of the base material of the low refractive index layer is about 0.35, and there is no disclosure of specific numerical values, but the width of the convex portion (reference numeral 26 in FIG. 9) prevents reflection. It is narrower than the pitch x ₁ shorter than the shortest wavelength λ _{min of the power} . For this reason, when the concavo-convex structure is formed by UV nanoimprint or photolithography and dry etching, there is a problem that it is difficult to form a UV nanoimprint mold or a photolithography mask.

  As described above, in the conventional technique, when a columnar resin having a uniform height and a porous layer having voids or a group of columnar microprojections having a uniform height are applied as an antireflection film, a single layer does not transmit incident light. The reflectance varies depending on the wavelength, and there is a problem that the variation in reflectance is large in a wide wavelength range.

  Further, the refractive index of the antireflection film can be lowered by lowering the volume density of the fine particles or the substrate and lowering the porosity, but the production is difficult. Furthermore, if the porosity is too high, there is a problem that the mechanical strength decreases.

  The above-mentioned object can be achieved by the following means by the inventors' extensive research.

An optical member having at least a base material and a fine structure layer, wherein the fine structure layer is composed of a plurality of columnar protrusions having different heights.

When the shortest wavelength of light incident on the optical member is λ _min and the longest wavelength is λ _max , the height D of the columnar protrusions of the microstructure layer satisfies the formula (1).
λ _min / 4 ≦ D ≦ λ _max / 4 (1)

  The interval between the columnar protrusions is irregular.

The columnar protrusions are characterized by being gradually reduced in diameter in an arc shape from the top of the columnar protrusion and the bottom of the columnar protrusion toward the center of the columnar protrusion in the cross-sectional shape in the height direction.

It has at least a base material and a fine structure layer, the porosity of the fine structure layer is ρ ₀ , the refractive index of the light incident medium is n ₀ , the refractive index of the fine structure layer is n ₁ , and the refractive index of the base material When n is ₂ , it is characterized by satisfying the expressions (2) and (3).
ρ ₀ = 1− (n ₁ ² −n ₀ ² ) / (n ₂ ² −n ₀ ² ) (2)
n ₁ = a × √n ₂ (1.11 ≦ a ≦ 1.15) (3)

The columnar protrusions are gradually reduced in diameter in an arc shape from the upper part of the columnar protrusion toward the tip of the columnar protrusion in the cross-sectional shape in the height direction.

  According to the present invention, the refractive index of the microstructure layer formed of a plurality of columnar protrusions having different heights formed on the surface of the base material is lower than the refractive index of the base material and has an antireflection effect, while being a single layer. Variation in reflectance can be suppressed in a wide wavelength range of light incident on the substrate.

  Since the intervals between the columnar protrusions forming the fine structure layer are irregular, uneven color of reflected light due to diffraction can be suppressed.

The fine structure layer can change the refractive index by changing the porosity. The optimum refractive index of the microstructure layer is determined by the refractive index of the substrate. When the porosity is ρ ₀ , the refractive index of the light incident medium existing on the substrate is n ₀ , the refractive index of the fine structure layer is n ₁ , and the refractive index of the substrate is n ₂ , the above formula (2) and Equation (3) holds. If the porosity is too high, the volume density of the columnar protrusions is lowered, which causes a problem that the mechanical strength of the microstructure layer is lowered. Therefore, in the equation (3), by setting the coefficient a to 1.11 or more and 1.15 or less, the porosity can be made to be a refractive index that does not become too high and an antireflection effect can be obtained.

  Since the microstructure layer of the present invention can be formed by dry etching of the substrate, it is compared with conventional techniques formed by repeating complicated processes, pressing by a mold or UV nanoimprint, or photolithography and dry etching. Thus, it is possible to provide an inexpensive optical member that is easy to manufacture.

It is a schematic cross section which shows one Embodiment of the optical member of this invention. It is a schematic cross section which shows the shape of the columnar protrusion which makes the microstructure layer in the optical member of this invention. It is a schematic cross section explaining the height of the columnar protrusion which makes the microstructure layer in the optical member of the present invention. It is a graph which shows the relationship between the refractive index of an anti-reflective film in the refractive index _nsub = 1.59 of a base material, and a reflectance. It is a graph which shows the relationship between the wavelength (lambda) of light in the refractive index _nsub = 1.59 of a base material, and refractive index _nar = 1.26 of an antireflection film, and the optimal film thickness _Dar . It is an electron micrograph which shows one Embodiment of the microstructure layer in the optical member of this invention. It is a model top view which shows one Embodiment of the microstructure layer in the optical member of this invention. It is a graph which shows the transmittance | permeability in the optical member of one Embodiment of this invention. It is a graph which shows the reflectance in the optical member of one Embodiment of this invention.

Hereinafter, embodiments of the present invention will be specifically described.

As shown in FIG. 1, the optical member 1 of the present embodiment includes a base material 2 and a microstructure layer 3 formed on the base material 2. The microstructure layer 3 is composed of a plurality of columnar protrusions 4 having different heights, and there are gaps 5 between the columnar protrusions 4. The refractive index n ₁ of the microstructure layer is lower than the refractive index n _{2 of} the base material, and has an antireflection effect for light incident on the base material.

  The shape of the columnar protrusion 4 will be described with reference to FIG. As shown in FIG. 2A, the columnar protrusion 4 is gradually reduced in diameter in an arc shape from the top toward the tip. Etching progresses from the two directions of the top surface and the side surface of the corner of the tip portion, so that the corner becomes round and becomes an arc shape. Moreover, as shown in FIG.2 (b), the columnar processus | protrusion 4 can also be made into the shape which reduced diameter gradually in circular arc shape toward the center part from the upper part and the bottom part.

The refractive index _nar of the antireflection film that provides a high antireflection effect (non-reflection) with a single layer is such that the refractive index of the substrate is n _sub and the refractive index n _{air of the} light incident medium (air) is 1.00. Is obtained from the equation (4). Further, the optimum film thickness _Dar is obtained from Equation (5), where λ is the wavelength of light incident on the substrate. As shown in equation (5), when the film thickness of the antireflection film is set to ¼ times the wavelength λ of the light, the phase of the reflected light on the film surface and the reflected light on the film / substrate interface is reversed and reflected. Is suppressed. The graph of FIG. 4 shows the relationship between the refractive index and the reflectance of the antireflection film at the refractive index n _sub = 1.59 of the substrate (polycarbonate). In FIG. 2, the horizontal axis represents the refractive index n _{ar of} the antireflection film, and the vertical axis represents the reflectance. As can be seen from FIG. 2, the antireflection effect is obtained when the refractive index of the antireflection film is lower than the refractive index of the substrate. The reason why the reflectance is theoretically 0% is n _ar = 1.26 from the equation (4). However, such a low refractive index material does not normally exist, and studies on an antireflection film or an antireflection structure as in Patent Documents 1 to 4 have been made. The graph of FIG. 5 shows the relationship between the wavelength λ of the incident light and the optimum film thickness D _ar at the refractive index n _sub = 1.59 of the substrate and the refractive index n _ar = 1.26 of the antireflection film. The horizontal axis in FIG. 5 is the wavelength λ of the incident light, and the vertical axis is the optimum film thickness _Dar . As can be seen from FIG. 5, the optimum film thickness varies depending on the wavelength of light.
n _ar = √ (n _air × n _sub ) = √n _sub (4)
D _ar = λ / (4 × n _ar ) (5)

  The plurality of columnar protrusions having different heights are formed by directly dry-etching the substrate without forming a mask layer by photolithography or UV nanoimprint. “Different height” means that when the microstructure layer is observed with a scanning electron microscope (SEM) as shown in FIG. 6, for example, the maximum height of the columnar protrusions is 200 nm, the minimum height is 50 nm, and the maximum The state where the height of each columnar protrusion is irregular within the minimum height range is shown. As described above, the fine structure layer including a plurality of columnar protrusions having different heights includes columnar protrusions having a height range corresponding to ¼ of the wavelength range of incident light (for example, 400 nm to 700 nm). Further, the intervals (pitch) between the columnar protrusions formed by dry etching are also irregular. The pitch is about 100 nm or less, and is sufficiently smaller than at least the wavelength range of incident light (for example, 400 nm to 700 nm). The height and thickness of the columnar protrusions can be changed depending on the dry etching conditions. The porosity of the microstructure layer is also changed depending on the dry etching conditions.

  As shown in FIG. 3, the height D of the columnar protrusion is defined as a vertical interval (reference numeral 6) between the bottom surface of the concave portion around the convex portion of the columnar protrusion 4 and the vertex of the convex portion. Arrow). When the bottom surface of the concave portion is not flat but arcuate, it is defined by the point where the distance is widest. By defining in this way, the height of the columnar protrusion can be defined even when the bottom surface of the recess is not flush and the vertex of the projection is not flush. When the bottom surface of the concave portion around a certain convex portion is not flush with each other, when the cross section is viewed, the intermediate position M in the height direction between the two bottom surface of the concave portion and the top surface is defined as the height.

As an apparatus used for dry etching, dry etching used for manufacturing a semiconductor device can be used as appropriate. In addition to the reactive ion etching and ion beam etching apparatuses, the reverse sputtering function of the sputtering apparatus can be used. As a plasma generation mechanism of an apparatus used for dry etching, a parallel plate type, an ECR type, an ICP type, or the like may be used as appropriate. A gas used for dry etching is argon, nitrogen, oxygen, a fluorine-based gas, or a mixed gas of these gases.

Since the fine structure layer is formed by directly dry-etching the base material, it is made of the same kind of material as the base material. In addition, when dry etching the substrate, the holder for holding the substrate, the stage on which the holder is placed, and the deposition plate in the apparatus chamber are scraped together with the substrate, and a very small part of it becomes a fine structure layer. May adhere. When the microstructure layer is analyzed by X-ray photoelectron spectroscopy (XPS, ESCA), for example, aluminum as a component of the holder, iron or chromium constituting the stage is detected, and is included in the microstructure layer Conceivable. Furthermore, when a fluorine-based gas is used for dry etching, it is considered that fluorine is detected and contained in the fine structure layer. Other gases may also be contained in the fine structure layer, but are not detected by ESCA or are difficult to separate from those derived from the substrate even if detected. These components have a slower etching rate than the base material, and function as a mask when the base material is etched. Moreover, it functions as a protective film for the side wall of the convex portion formed at the initial stage of etching, and the columnar protrusion grows in the height (depth) direction by the etching of the concave portion other than the convex portion.

The porosity of the microstructure layer was calculated from the area ratio of the recesses without columnar protrusions. FIG. 7 is a schematic view of the top surface of the microstructure layer, and the black portions represent the recesses. The value calculated by dividing the area of the black part by the area (area of one image) obtained by adding the white part representing the columnar protrusion and the black part (area of one image) and multiplying by 100 is defined as the porosity (unit:%). In addition, since the fine structure layer is composed of columnar protrusions having different heights, there is a concern that the porosity may change depending on the position in the height direction. However, for example, the refractive index n ₁ of the fine-structure layer having the obtained void ratio ρ ₀ = 33.8% is the value obtained from the equation (2) and the ellipsometer (DHA-OLX, Mizoji Optical Co., Ltd.). Manufactured, laser wavelength: 632.8 nm) are almost the same (calculated value: 1.41, measured value: 1.42), and the influence of the change in the position in the height direction is small.

As a base material used for the optical member of the present embodiment, it is desirable that the substrate is transparent and columnar protrusions are formed by dry etching. For example, resin materials such as polystyrene, polypropylene, polyurethane, polymethyl methacrylate (acrylic resin), modified acrylic, cellulose acetate, polyethylene terephthalate, polyethylene naphthalate, polycarbonate, and polyolefin can be used. The substrate may be a film having a thickness of 0.3 mm or less, or a plate having a thickness exceeding 0.3 mm. Alternatively, an optical element such as a lens, a microlens, or an optical waveguide may be used as the base material. Furthermore, it is possible to apply to the hard coat used for the protective film of displays, such as a smart phone, and the back surface (non-conductive surface) side of a transparent conductive film. When applied to a hard coat, visibility can be improved by suppressing reflection of external light. Moreover, when it applies to the back surface of a transparent conductive film, reflection of incident light can be suppressed and improvement in transmittance can be expected. Thus, it is also possible to apply to only one of the front and back surfaces, such as a functional film such as a protective film and a transparent conductive film, and a housing glass or a plastic such as a display.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

[Example 1]
A first embodiment of the present invention using a polycarbonate plate having a thickness of 0.8 mm (Polyca Ace manufactured by Sumitomo Bakelite Co., Ltd.) as a substrate will be described. For dry etching, a reverse sputtering function of a sputtering apparatus C-7960M (former Anelva Co., Ltd., current Canon Anelva Co., Ltd.) was used. First, the substrate was fixed to an aluminum holder and conveyed into the chamber. The gas is a mixed gas of argon and oxygen, and the flow rates are 45 sccm and 5 sccm, respectively. The pressure in the chamber was adjusted to 1.4 Pa by controlling the opening of the main valve. The applied RF power is 400W. Under these conditions, the substrate was dry-etched for 7 minutes. As a result of this treatment, a microstructure layer composed of a plurality of columnar protrusions having different heights as shown in FIG. 6 was formed, and an optical member was manufactured. At this time, the porosity was 33.8%. Further, the height D of the columnar protrusions varied between a maximum of 200 nm and a minimum of 50 nm. The substrate etching rate was about 30 nm / min.

The refractive index n ₁ of the fine structure layer with the porosity ρ ₀ = 33.8% is n ₁ = 1.41 from the equation (2). Therefore, the coefficient a = 1.12, which satisfies Expression (3) (an example of an optical member having a microstructure layer that satisfies Expression (3)). Here, the light incident medium is air and has a refractive index n ₀ = 1.00, and the base material is polycarbonate and has a refractive index n ₂ = 1.59.

  FIG. 8 shows the result of measuring the transmittance of the optical member having the fine structure layer of Example 1 with an ultraviolet-visible spectrophotometer (V-660, manufactured by JASCO Corporation). The horizontal axis of FIG. 8 represents the wavelength λ (unit: nm) of light, and the measurement wavelength is 400 nm to 700 nm (an example of the wavelength range of incident light that should be prevented from being reflected). At this time, the height D of the columnar protrusion changes between 200 nm and 50 nm, and satisfies the formula (1) (100 nm ≦ D ≦ 175 nm, an example of an optical member having a microstructure layer satisfying the formula (1) .) The vertical axis represents the transmittance (unit:%). The fine structure layer is formed only on one surface of the substrate, light is incident from the fine structure layer side, and the transmittance is calculated from the intensity ratio of the light emitted from the other substrate surface. The incident angle of light is 0 ° (perpendicular incidence on the substrate surface). The transmittance of the optical member of Example 1 is higher than the transmittance of the base material, and it can be considered that the improved transmittance is the reduced reflectance.

FIG. 9 shows the reflectance of the optical member having the fine structure layer of the first embodiment. The reflectance is the reflectance when the fine structure layer is formed. The horizontal axis represents light wavelength λ (unit: nm), and the vertical axis represents reflectance (unit:%). This reflectance is a value obtained by subtracting the transmittance improvement of each wavelength in FIG. 8 from the reflectance (5.2%) on one side calculated from the refractive index n ₂ = 1.59 of the polycarbonate polycarbonate. It has become. As shown in FIG. 9, the fine structure layer that functions as the antireflection layer of the optical member of Example 1 is a single layer, but has a small variation in reflectance in a wide wavelength range. Table 1 shows the average reflectance and the standard deviation σ. The average reflectance at 7 points from λ = 400 nm to 700 nm every 50 nm was 1.8%, and the standard deviation σ was 0.21%.

[Example 2]
A second embodiment of the present invention will be described. In addition, since the apparatus used for the base material and dry etching, the transmittance measuring apparatus, the calculation method of the reflectance, and the like are the same as those in the first embodiment, the description thereof is omitted. The dry etching conditions were the same, but the processing time was 6 minutes and 40 seconds. The height D of the columnar protrusions varied between a maximum of 190 nm and a minimum of 40 nm (an example of an optical member having a microstructure layer satisfying the formula (1)). In Example 2, the porosity of the microstructure layer is ρ ₀ = 26.3%, and the refractive index n _{1 is} 1.45. Further, the coefficient a = 1.15 (an example of an optical member having a fine structure layer satisfying the expression (3)). The average reflectance is 2.4%, and the standard deviation σ of the reflectance is 0.29% (FIG. 9, Table 1).

[Example 3]
A third embodiment of the present invention will be described. In addition, since the apparatus used for the base material, dry etching, the transmittance measuring apparatus, the reflectance calculation method, and the like are the same as those in the first and second embodiments, the description thereof is omitted. The dry etching conditions were the same for the mixed gas of argon and oxygen, but the flow rate and the opening of the main valve were adjusted to maintain the pressure at 0.7 Pa while maintaining the mixing ratio. The processing time was 7 minutes. The height D of the columnar protrusions varied between a maximum of 200 nm and a minimum of 60 nm (an example of an optical member having a microstructure layer satisfying the formula (1)). In Example 3, the porosity of the microstructure layer is ρ ₀ = 37.8%, and the refractive index n ₁ = 1.39. Further, the coefficient a = 1.11 (an example of an optical member having a fine structure layer satisfying the formula (3)). The average reflectance is 1.4%, and the standard deviation σ of the reflectance is 0.30% (FIG. 9, Table 1).

  Thus, according to Examples 1 to 3, the optical member of the present invention has a reflectance lower than that of the base material and has a small variation in reflectance over a wide light wavelength range even though it is a single layer (σ ≦ 0.3). %).

[Comparative Example 1]
Comparative Example 1 will be described. In addition, since the apparatus used for the base material, the dry etching, the transmittance measuring apparatus, the calculation method of the reflectance, and the like are the same as those in the first to third embodiments, the description thereof is omitted. The dry etching conditions are the same as in Examples 1 to 3. The processing time was 10 minutes and 30 seconds. The height D of the columnar protrusions varied between a maximum of 300 nm and a minimum of 180 nm. In this comparative example 1, the porosity of the microstructure layer is ρ ₀ = 39.5%, and the refractive index n ₁ = 1.38. The coefficient a = 1.10. The average reflectance is 4.3%, and the standard deviation σ of the reflectance is 0.71% (FIG. 9, Table 1). The columnar protrusions of Comparative Example 1 have a maximum height of 300 nm, and the columnar protrusions collapse and the columnar protrusions aggregate. Therefore, the porosity is high. Therefore, the coefficient a is lower than that in the third embodiment. However, since the maximum height of the columnar protrusions exceeds 200 nm and the range of height change does not satisfy the formula (1), the effect of reducing the reflectance is small, and the variation in reflectance is large over a wide wavelength range of light. .

[Comparative Example 2]
Comparative example 2 will be described. In addition, since the apparatus used for the base material and dry etching, the transmittance measuring apparatus, the calculation method of the reflectance, and the like are the same as those in Examples 1 to 3 and Comparative Example 1, description thereof is omitted. The dry etching conditions were the same for the mixed gas of argon and oxygen, but the pressure was adjusted to 4.0 Pa by adjusting the flow rate and the opening of the main valve while maintaining the mixing ratio. The processing time was 3 minutes 30 seconds. The height D of the columnar protrusions varied between a maximum of 90 nm and a minimum of 20 nm. In Comparative Example 2, the porosity of the microstructure layer is ρ ₀ = 25.0%, and the refractive index n ₁ = 1.46. Further, the coefficient a = 1.16. The average reflectance is 3.9%, and the standard deviation σ of the reflectance is 0.40% (FIG. 9, Table 1). Since the porosity is small and the refractive index is small, the effect of reducing the reflectance is small. Further, since the maximum height of the columnar protrusion is less than 100 nm and the range of the height change does not satisfy the formula (1), the variation in reflectance is large in a wide wavelength range of light.

  As mentioned above, although this invention was demonstrated according to embodiment, this invention is not limited to the said embodiment. For example, the columnar protrusions may be gradually reduced in diameter in a circular arc shape from the top of the columnar protrusion and the bottom of the columnar protrusion toward the center of the columnar protrusion in the cross-sectional shape in the height direction. The above shape can be obtained by changing the pressure in the chamber under dry etching conditions. For example, the above shape was obtained by changing the pressure from 1.4 Pa to 2.6 Pa. In this case, since the thickness of the bottom of the columnar protrusion is not reduced, the columnar protrusion is not collapsed or the protrusions are not aggregated. Further, since the upper part of the columnar protrusion is not thinned, the porosity is not increased unnecessarily. As the porosity of the central part of the columnar protrusion increases, the refractive index decreases and the effect of reducing the reflectance increases.

  In addition, the columnar protrusions are gradually reduced in diameter in an arc shape from the upper part of the columnar protrusion toward the tip of the columnar protrusion in the cross-sectional shape in the height direction. Etching progresses from the two directions of the top surface and the side surface of the corner of the tip portion, so that the corner becomes round and becomes an arc shape. In this case, since the refractive index gradually increases from the light incident medium (air) at the upper part of the columnar protrusion, an antireflection effect is obtained.

DESCRIPTION OF SYMBOLS 1 Optical member 2 Base material 3 Fine structure layer 4 Columnar protrusion 5 Space | gap 6 Arrow symbol M which shows the space | interval (height) of a recessed part bottom face and a convex part top The intermediate position between two convex part surrounding recessed parts (point)

Claims (5)

An optical member having at least a base material and a fine structure layer, wherein the fine structure layer comprises a plurality of columnar protrusions having different heights. The height D of the columnar protrusions of the microstructure layer satisfies the formula (1) when the shortest wavelength of light incident on the optical member is λ _min and the longest wavelength is λ _max. The optical member according to 1.
λ _min / 4 ≦ D ≦ λ _max / 4 (1)
  The optical member according to claim 1, wherein the interval between the columnar protrusions is irregular. The optical member according to any one of claims 1 to 3, wherein the columnar protrusion has a reduced diameter at a center portion of the columnar protrusion with respect to the upper portion of the columnar protrusion and the bottom portion of the columnar protrusion in a cross-sectional shape in the height direction. . It has at least a base material and a fine structure layer, the porosity of the fine structure layer is ρ ₀ , the refractive index of the light incident medium is n ₀ , the refractive index of the fine structure layer is n ₁ , and the refractive index of the base material when was the n _2, the optical member according to claims 1 to 4 or to satisfy the equation (2) and (3).
ρ ₀ = 1− (n ₁ ² −n ₀ ² ) / (n ₂ ² −n ₀ ² ) (2)
n ₁ = a × √n ₂ (1.11 ≦ a ≦ 1.15) (3)