Display devices for use in TVs, cell phones, etc., and optical elements, such as camera lenses, etc., usually adopt an antireflection technique in order to reduce the surface reflection and increase the amount of light transmitted therethrough. This is because, when light is transmitted through the interface between media of different refractive indices, e.g., when light is incident on the interface between air and glass, the amount of transmitted light decreases due to, for example, Fresnel reflection, thus deteriorating the visibility.
An antireflection technique which has been receiving attention in recent years is forming over a substrate surface a very small uneven pattern in which the interval of recessed portions or raised portions is not more than the wavelength of visible light (λ=380 nm to 780 nm). See Patent Documents 1 to 4. The two-dimensional size of a raised portion of an uneven pattern which performs an antireflection function is not less than 10 nm and less than 500 nm.
This method utilizes the principles of a so-called moth-eye structure. The refractive index for light that is incident on the substrate is continuously changed along the depth direction of the recessed portions or raised portions, from the refractive index of a medium on which the light is incident to the refractive index of the substrate, whereby reflection of a wavelength band that is subject to antireflection is prevented.
The moth-eye structure is advantageous in that it is capable of performing an antireflection function with small incident angle dependence over a wide wavelength band, as well as that it is applicable to a number of materials, and that an uneven pattern can be directly formed in a substrate. As such, a high-performance antireflection film (or antireflection surface) can be provided at a low cost.
As the method of forming a moth-eye structure, using an anodized porous alumina layer which is obtained by means of anodization of aluminum has been receiving attention (Patent Documents 2 to 4).
Now, the anodized porous alumina layer which is obtained by means of anodization of aluminum is briefly described. Conventionally, a method of forming a porous structure by means of anodization has been receiving attention as a simple method for making nanometer-scale micropores (very small recessed portions) in the shape of a circular column in a regular arrangement. An aluminum base is immersed in an acidic electrolytic solution of sulfuric acid, oxalic acid, phosphoric acid, or the like, or an alkaline electrolytic solution, and this is used as an anode in application of a voltage, which causes oxidation and dissolution. The oxidation and the dissolution concurrently advance over a surface of the aluminum base to form an oxide film which has micropores over its surface. The micropores, which are in the shape of a circular column, are oriented vertical to the oxide film and exhibit a self-organized regularity under certain conditions (voltage, electrolyte type, temperature, etc.). Thus, this anodized porous alumina layer is expected to be applied to a wide variety of functional materials.
A porous alumina layer formed under specific conditions includes cells in the shape of a generally regular hexagon which are in a closest packed two-dimensional arrangement when seen in a direction perpendicular to the film surface. Each of the cells has a micropore at its center. The arrangement of the micropores is periodic. The cells are formed as a result of local dissolution and growth of a coating. The dissolution and growth of the coating concurrently advance at the bottom of the micropores which is referred to as a barrier layer. As known, the interval between adjacent micropores (the distance between the centers), is approximately twice the thickness of the barrier layer, and is approximately proportional to the voltage that is applied during the anodization. It is also known that the diameter of the micropores depends on the type, concentration, temperature, etc., of the electrolytic solution but is, usually, about ⅓ of the size of the cells (the length of the longest diagonal of the cell when seen in a direction vertical to the film surface). Such micropores of the porous alumina may constitute an arrangement which has a high regularity (periodicity) under specific conditions, an arrangement with a regularity degraded to some extent depending on the conditions, or an irregular (non-periodic) arrangement.
Patent Document 2 discloses a method of producing an antireflection film (antireflection surface) with the use of a stamper which has an anodized porous alumina film over its surface.
Patent Document 3 discloses the technique of forming tapered recesses with continuously changing pore diameters by repeating anodization of aluminum and a pore diameter increasing process.
The applicant of the present application discloses, in Patent Document 4, the technique of forming an antireflection film with the use of an alumina layer in which very small recessed portions have stepped lateral surfaces.
As described in Patent Documents 1, 2, and 4, by providing an uneven structure (macro structure) which is greater than a moth-eye structure (micro structure) in addition to the moth-eye structure, the antireflection film (antireflection surface) can be provided with an antiglare function. The two-dimensional size of a raised portion of the uneven structure which is capable of performing the antiglare function is not less than 1 μm and less than 100 μm.
Utilizing an anodized porous aluminum film can facilitate the manufacture of a mold which is used for formation of a moth-eye structure over a surface (hereinafter, “moth-eye mold”). In particular, as described in Patent Documents 2 and 4, when the surface of the anodized aluminum film as formed is used as a mold without any modification, a large effect of reducing the manufacturing cost is achieved. The structure of the surface of a moth-eye mold which is capable of forming a moth-eye structure is herein referred to as “inverted moth-eye structure”.
A known antireflection film production method with the use of a moth-eye mold uses a photocurable resin. Firstly, a photocurable resin is applied over a substrate. Then, an uneven surface of a moth-eye mold which has undergone a mold release treatment is pressed against the photocurable resin in vacuum, whereby the uneven structure at the surface of the moth-eye mold is filled with the photocurable resin. Then, the photocurable resin in the uneven structure is irradiated with ultraviolet light so that the photocurable resin is cured. Thereafter, the moth-eye mold is separated from the substrate, whereby a cured layer of the photocurable resin to which the uneven structure of the moth-eye mold has been transferred is formed over the surface of the substrate. The method of producing an antireflection film with the use of the photocurable resin is disclosed in, for example, Patent Document 4.
The above-described moth-eye mold can be fabricated using an aluminum base, such as typically a substrate made of aluminum or a cylinder made of aluminum, or an aluminum film formed on a support that is made of a non-aluminum material, such as typically a glass substrate. However, in a moth-eye mold manufactured using an aluminum film formed on a glass substrate or plastic film, the adhesive property between the aluminum film (part of which is an anodized film) and the glass substrate or plastic film deteriorates in some cases. The applicant of the present application found that, by forming an inorganic underlayer (e.g., SiO2 layer) and a buffer layer containing aluminum (e.g., AlOx layer) on a surface of a base which is made of glass or plastic, the above-described deterioration of the adhesive property is prevented. This is disclosed in Patent Document 5.
The applicant of the present application developed a method for efficiently producing an antireflection film using a moth-eye mold in the form of a cylinder (roll) according to a roll-to-roll method (e.g., WO 2011/105206). The moth-eye mold in the form of a cylinder can be formed by, for example, forming an organic insulating layer over an outer perimeter surface of a metal cylinder, forming an aluminum film on this organic insulating layer, and alternately and repeatedly performing anodization and etching on the aluminum film. In this case also, the adhesive property can be improved by forming the inorganic underlayer and the buffer layer disclosed in Patent Document 5.
According to further researches carried out by the present inventors, an aluminum film formed on an organic insulating layer contains abnormal grains in many cases. The abnormal grains are formed by abnormal growth of a crystal of aluminum. The aluminum film is an aggregation of crystal grains whose average grain diameter (average grain size) is about 200 nm. On the other hand, the grain diameter of the abnormal grains is larger than the average grain diameter, e.g., not less than 500 nm in some cases. The organic insulating layer has a lower thermal conductivity than the other materials (metal materials and inorganic insulating films), and therefore, the aluminum film readily reaches a relatively high temperature in the process of depositing the aluminum film (e.g., sputtering or vapor deposition). As a result, it is inferred that abnormal growth of crystal grains is likely to occur, i.e., abnormal grains are likely to be produced. Note that such a phenomenon can also occur when an aluminum film is directly deposited on a surface of an aluminum pipe (e.g., the thickness is not less than 1 mm).
When a moth-eye mold is manufactured using an aluminum film in which abnormal grains are present, structures corresponding to the abnormal grains are formed in the surface of a porous alumina layer of the moth-eye mold. When an antireflection film is formed using such a moth-eye mold, the structures corresponding to the abnormal grains are transferred to the surface of the antireflection film. Therefore, light is scattered by the structures transferred to the surface of the antireflection film which are attributed to the abnormal grains. That is, the antireflection film has a haze. In the case where the antireflection film is provided with an antiglare function as described above, no problem occurs in some cases even when the antireflection film has a haze which is attributed to the abnormal grains. However, there is such a problem that an antireflection film which does not have an antiglare function cannot be producing. Further, it is difficult to control the formation density (frequency of occurrence) of abnormal grains, and therefore, from the viewpoint of mass productivity, preventing production of abnormal grains is preferred.
The present inventors disclose in the international patent application (PCT/JP2012/058394, WO 2012/137664) that an aluminum alloy layer which contains aluminum and such a metal element that the absolute value of the difference between the standard electrode potential of the metal element and the standard electrode potential of aluminum is not more than 0.64 V (for example, the metal element may be Ti, Nd, Mn, Mg, Zr, V, and Pb, and the content of the metal element in the entire aluminum alloy layer is less than 10 mass %) scarcely contains abnormal grains, and as a result, a mold can be obtained which is capable of producing an antireflection film that does not have an undesirable haze.
The entire disclosures of Patent Documents 1, 2, 4, and 5 and the above-identified international patent application are herein incorporated by reference.