Photochemical Lamination of Low Refractive Index Transparent SiO 2 Film at Room Temperature for Antireflection Coating
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PHOTOCHEMICAL LAMINATION OF LOW REFRACTIVE INDEX TRANSPARENT SiO2 FILM AT ROOM TEMPERATURE FOR ANTIREFLECTION COATING Y. Ogawa and M. Murahara Department of Electrical Engineering, Tokai University 1117 Kitakaname, Hiratsuka-shi, Kanagawa 259-12, Japan ABSTRACT A transparent low refractive index SiO 2 film laminated on a glass substrate at room temperature by photochemical reactions with the Xe2 * excimer lamp (172nm). This SiO 2 film grown on the fused silica glass was proved to avoid reflection of light. A refractive index of the SiO 2 film was 1.36. After annealing the film for one hour at 200 degrees centigrade, the refractive index increased to 1.42. The refractive index increased as the F atom density in the SiO 2 film decreased. INTRODUCTION Low refractive index films such as MgF 2 film and TiO 2 film have been widely used for antireflection coating and clad materials for optical fibers. In addition to them, the SiO 2 film, which was doped the fluorine atom or boron atom, was used as antireflection coating for silica glasses. Kawamata of Olympus Optical Co. has fabricated the MgF 2 film by the sputtering method using the MgF 2 target in the presence of O2 gas [1]. This method, however, employed the different material from the glass substrate. In addition, Miyajima of Toshiba Co. has fabricated the fluorine atom doped SiO 2 film at 400 degrees centigrade substrate temperature by PE-CVD (plasma-enhanced chemical vapor deposition) method in the presence of tetraethylorthosilicate (TEOS), O2 and CF4 mixed gases [2]. Yoo of Novellus Systems Inc. has also fabricated the fluorine atom doped SiO 2 film at from 300 to 400 degrees centigrade substrate temperature by PE-CVD method in the presence of TEOS, O2 and CF4 mixed gases [3]. However, neither the Miyajima’s study report nor Yoo’s did refer to transparency. We have successfully fabricated a transparent and low refractive index SiO 2 film at room temperature by using Xe2 * excimer lamp [4-8]. In this paper, we reported on the new method, in which the deposition rate was enhanced with cooling the substrate.
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PRINCIPLE OF SiO2 FILM LAMINATION The Xe2 * excimer lamp light was irradiated to the NF3 and O2 mixed gases in which a glass substrate and a piece of Si wafer were placed. The wavelength of the lamp is 172nm, and the photon energy is 166.1 kcal/mol. The binding energies of NF3 and O2 are 69 kcal/mol and 119.1 kcal/mol, respectively. Since the NF3 gas and the O2 gas have a strong absorption band in the wavelength of 172nm(Fig.1), they were photo-dissociated to produce F radicals and NO2 . The F radicals etched the surface of the Si wafer, producing SiF 4 , which was adsorbed on the glass substrate surface to produce one molecular layer of SiF 4 . The SiF 4 layer was then oxidized with the NO2 to grow one molecular layer of SiO 2 . By the adsorption of SiF 4 and oxidized reaction with NO2 , the SiO 2 film was automatically laminated on the glass substrate. Then, the fluorine atoms were doped in the SiO 2 film because the SiF 4 , which did not reac
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