Vol.1 No.1 2008

31/76

Research paper : A challenge to the low-cost production of highly functional optical elements (J. Nishii)−28−Synthesiology - English edition Vol.1 No.1 (2008) level cause the diffraction of incident light. In the case of the structures with the period sufficiently smaller than the wavelength, volume fraction of air was greater than the glass at the top area of the structure, and their volume fractions gradually reversed as the incident light traveled further into the structure. Thus the interface of glass and air seemed nonexistent for the incident light coming into the structure from various angles. Also, two-dimensionally isotropic arrangement canceled out the polarization dependence.Although several theoretical analyses and the fabrication researches have been published for sub-wavelength antireflection structures, most of them dealt with resins such as acrylic[7], and never got beyond prototypes. There were some researches using electron beam lithography and dry etching for the fabrication of antireflection structure on the surface of glass [8, 9]. However, the mass production was difficult because of the prolonged fabrication time. Therefore, we decided to fabricate such antireflection structures using the glass molding process.In this paper, I will describe the study in which silica glass was used as heat resistant mold. The metal thin-film coated on the silica substrate was patterned by electron beam lithography, and then the desired periodic structure was formed on the substrate by a dry etching process. The shape of mold was designed so as to minimize the reflection on the surface of glass. Figure 6(a) shows a two-dimensional periodic structure with period of 300 nm and the height about 550 nm. Using a vacuum coating process, a thin film was coated on the mold surface to prevent the thermal adhesion between the mold and the glass. A phosphate optical glass with refractive index of 1.6 was pressed at about 500 °C. Finally, an inverted periodic structure with height about 500 nm was successfully obtained as shown in Figure 6(b)[10]. Here, the important point was the temperature of demolding. The reflectivity on the surface of optical glass with periodic structure was measured precisely using an integrating sphere, which was 0.56 % at the perpendicular incidence and at the wavelength 462 nm. This value was lower than that of a single antireflection thin film coated on the glass surface. Therefore, the performance of this antireflection structure was in the practical level. Figure 6(c) is a photograph of the prototype. The surface reflectivity decreased with optimization of molding condition, and the characters under the glass plate could be read clearly from a tilted angle. A current target of this research is the pickup lens for optical disc drive and digital still camera lens. The development of curved surface mold using ultrahard materials in place of silica is in progress. This research will open a possibility of overcoming the problems such as low cost and mass production that delayed the practical uses of antireflection structures.5 DiscussionVertical collaboration between companies, AIST and universities gathered from the three fields, i.e., materials, microfabrication, and devices, was effective beyond expectation. The optical glass technology in Japan is at the top level of the world. Actually, some glass materials suitable for imprinting are commercially available. However, many more hurdles must be cleared from the aspect of mass production. Glass companies are working efficiently on the three research issues of composition improvement, molding, and device characterization, with collaboration of AIST, mold material companies, and home appliance companies. However, since mold materials are not developed for the purpose to fabricate subwavelength structures with microfabrication process, the mold material and its production method must be optimized through close cooperation with manufacturers. If the results are adequately patented, these material technologies might be protected from the catch-up of neighboring countries. On the other hand, it is known that the glass materials at around the molding temperature range show viscoelastic behavior, but currently there are few data on such high temperature properties required for the molding simulation. In some cases, it is necessary to develop the measurement equipment of such properties, which will also be patented just like the materials.On the other hand, three processes including the mold fabrication, the demolding thin film coating and the precision molding are faced with hard problems in terms of research strategy. Even if the technologies related to these processes could be patented, it would be extremely difficult to protect them from the catch-up of neighboring countries. Some believe that it may be of best interest to leave them as “black boxes”. Recently, there are moves to commercialize the technical know-how on fabrication methods as a recipe packaged with the production instruments for processing, coating, molding, or others. Actually, some recipes are packaged with the equipments used in our research project. Therefore, anyone can fabricate the conventional devices using such recipes, which are based on the knowledge accumulated in the collaborative work between the equipment companies and the optical device companies. In the future, it will be extremely important (a)(b)(c)Fig. 6 SEM photographs of (a)mold and (b) molded antireflection structure. (c) External shot of molded glass.

※このページを正しく表示するにはFlashPlayer9以上が必要です