The present application relates to an optical element, a replica substrate configured to form an optical element, and methods for producing the optical element and the replica substrate. Specifically, the present application relates to an optical element having many structures each being in the form of a projection or depression on a surface of a base.
In an optical element including a transparent substrate composed of a material such as glass or plastic, in order to reduce surface reflection of light to improve light transmission characteristics, a method for forming fine and dense projections and depressions (subwavelength structure: moth-eye structure) on a surface of the optical element is employed. Generally, in the case where periodic projections and depressions are provided on a surface of an optical element, light passes through the projections and depressions and is diffracted, significantly reducing the amount of light components that travel in straight lines. However, in the case where the pitch of the projections or depressions is smaller than the wavelength of light transmitted, diffraction does not occur. For example, in the case where each of the projections is in the form of a cone, an effective antireflection effect and excellent transmission characteristics are obtained for single-wavelength light corresponding to the pitch and the height of the projections.
For example, Non-Patent Document 1 (NTT Advanced Technology Corporation site (accessed May 21, 2006), “Master Mold for Antireflective (Moth Eye) Structure Independent of Wavelength”) describes a method for producing the foregoing optical element. A photoresist on a Si substrate is formed into an uneven photoresist pattern by electron beam lithography. The Si substrate is subjected to etching with the resulting uneven photoresist pattern as a mask, thereby forming a Si master mold having subwavelength structures each being in the form of a fine cone (pitch: about 300 nm, depth: about 400 nm) as shown in FIG. 15.
The resulting Si master mold described above can also have an antireflection effect on light having a wide wavelength range. As shown in FIG. 16, the arrangement of the subwavelength structures in a hexagonal lattice results in an extremely high antireflection effect (reflectivity: 1% or less) in the visible region (see FIG. 17). In FIG. 17, 11 represents the reflectivity of a flat portion of the Si master mold, and 12 represents the reflectivity of the patterned portion of the Si master mold.
As shown in FIG. 18, a Ni electroformed stamper of the Si master mold is produced. As shown in FIG. 19, the stamper has an uneven pattern in a predetermined region RI of a surface of the stamper, the pattern being the inverse of the pattern of the surface of the Si master mold. The uneven pattern is transferred to a transparent polycarbonate resin with the stamper, thereby affording a target optical element (replica substrate). The resulting optical element also has a high antireflection effect (reflectivity: 1% or less) (see FIG. 20). In FIG. 20, 13 represents the reflectivity of a portion other than the pattern, and 14 represents the reflectivity of the pattern. However, Non-Patent Document 1 does not describe the transmission characteristics of the optical element or improvement in the light-guiding performance of a display or the like with the optical element.
In recent years, attempts have been made to apply uneven surfaces of subwavelength structures to antireflection elements and high-transmittance elements used for various optical devices such as displays, optoelectronics devices, optical communication devices, solar cells, and lighting systems. Hereinafter, liquid crystal displays as an example of the application of the uneven surfaces of subwavelength structures will be described below.
In recent years, ultrathin displays, such as liquid crystal displays (LCDs) and plasma display panels (PDPs), have been practically used instead of cathode-ray tube (CRTs) displays. In particular, LCDs are spreading at an accelerating pace because of low-power-consumption operation and a reduction in the cost of large color LCD panels.
In liquid crystal displays, backlight systems in which transmissive color LCD panels are illuminated from the back side of the panels by backlights to display color images are mainly employed. As light sources of backlight systems, cold-cathode fluorescent lamps (CCFLs) that emit white light are often used.
Cold-cathode fluorescent lamps contain mercury in their fluorescent tubes and may adversely affect the environment. Thus, as light sources of backlight systems, light sources to replace cold-cathode fluorescent lamps may be needed. Since blue-light-emitting diodes have been recently developed, light-emitting diodes (LEDs) emitting red light, green light, and blue light, which are the primary colors of light, have been commercially available. Thus, LEDs hold promise for light sources to replace cold-cathode fluorescent lamp. Mixing red light, green light, and blue light emitted from LEDs results in white light with excellent color purity.
The use of LEDs as light sources of backlight systems enhances the color purity of colored light through color liquid crystal display panel. Thus, the color reproduction range is expected to extend to the extent specified by the NTSC (national television system committee) system or to the extent more than specified.
In the case where LEDs emitting red light, green light, and blue light are used as light sources of backlight systems configured to illuminate color LCD panels, a technique for efficiently transmitting each colored light may be required. As described above, therefore, attempts have been made to apply uneven surfaces of subwavelength structures to LCDs.
For example, Japanese Patent No. 3723843 discloses an antireflection filter (subwavelength structure) formed by subjecting a resist film to electron beam lithography and then development to form a resist pattern and directly subjecting a second semiconductor layer to etching with the resist pattern using a predetermined etching gas. The patent document also describes that a semiconductor light-emitting element having the antireflection filter has an emission intensity 30% higher than that of a semiconductor light-emitting element not having the antireflection filter.
However, electron beam exposure disadvantageously requires a long operation time and thus is not suitable for industrial production. For example, in the case where an electron beam with a beam current of 100 pA, which is used for the finest pattern, is used for a resist such as calixarene requiring a dose of several tens of mC/cm2, the exposure of a square with a side length of 200 μm is not completed even after 24-hour exposure. In the case where the area of a 2.5-inch display (50.8 mm×38.1 mm), which is commonly used for mobile phones, is exposed, it takes about 20 days.
A method described in Non-Patent Document 2 (National Institute of Advanced Industrial Science and Technology site. http://aist.go.jp/aist_i/press_release/pr2006/pr20060306/pr20060306.html (accessed May 21, 2006), “Development of Desktop Apparatus Enabling Nanometer-Scale Microfabrication”) utilizes optical-disk-recording technology. Thus, a large-area optical element can be produced at high speed and low cost. However, the resulting optical element produced by the method has poor wavelength-dependent characteristics of reflectivity and a reflectivity exceeding 1%; hence, the optical element is not practical for an antireflection structure. This may because the density (aperture ratio) of the nanodot pattern is low (50% or less).
In addition to the foregoing subwavelength structures, attempts have been made to apply inorganic multilayer films to LCDs. For example, Japanese Unexamined Patent Application Publication No. 2006-145885 discloses that low-refractive-index dielectric layers composed of silicon oxide (SiO2) and high-refractive-index dielectric layers composed of niobium pentoxide (Nb2O5) are stacked to form a 24-layer optical filter having the foregoing optical properties, thereby increasing transmittances of red light (640 nm), green light (530 nm), and blue light (450 nm) to 80%, 80%, and 50%, respectively. However, such an optical filter does not sufficiently improve transmittance characteristics. Thus, further improvement in transmittance characteristics, in particular, improvement in transmittance characteristics of blue light is desired.