Photochemical Deposition of Transparent low Refractive Index SiO 2 Topcoat for Laser Head at Room Temperature
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PHOTOCHEMICAL DEPOSITION OF TRANSPARENT LOW REFRACTIVE INDEX SiO2 TOPCOAT FOR LASER HEAD AT ROOM TEMPERATURE Y.Tezuka and M.Murahara Department of Electrical Engineering, Tokai University 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292, JAPAN ABSTRACT A transparent, low refractive index SiO 2 film was photo-chemically laminated on a glass slab laser head by the Xe2 * excimer lamp in the atmosphere of NF3 and O2 mixed gas at room temperature; which made it possible to inhibit the decrease in the laser output power caused by the evanescent wave leakage. The transparent SiO 2 film of 260nm thickness was laminated on the non-heated substrate by the lamp irradiation for one hour. The hardness of the film before annealing was 3 by Mohs’ Scale of Hardness, and its hardness improved to 5 after annealing at 250 degrees centigrade for one hour. The refractive index of the film was 1.42, being lower than the index of silica glass that is 1.46. Furthermore, the transmittance in the visible region increased by 2% with its antireflection coating. INTRODUCTION The evanescent waves arise on the boundary between cooling water and laser head when the lights reflect perfectly from the laser head, of which leakage causes the decrease in the laser output power. To inhibit the decrease, we have tried to laminate the thin protective coat of 2-micron meter with low refractive index on the contact surfaces of cooling water and glass slab laser head. For a protective coat formation, there are a vacuum deposition method as a dry process and a spin coating method as a wet process. The vacuum deposition method requires the temperature of 500 degrees centigrade and above; therefore, the coat becomes hard but the thermal denaturation of the substrate cannot be avoided. The spin coating method cannot attain a good adhesion to the substrate due to its low density of the topcoat. C. Calmes of Univ. Paris has fabricated the Si-Ge-C film by ultra high vacuum chemical vapor deposition [1]. This method, however, requires high temperature. In addition, T. Kohoutek of Univ. of Pardubice has fabricated amorphous Ag–As–S films by standard spin-coating techniques from their organic solution [2]. However, Kohoutek’s study doesn’t refer to
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adhesion to the substrate. For this reason, a new method to form a hard protective coat at room temperature is required. We have successfully fabricated a transparent and low refractive index SiO 2 film at room temperature by using Xe2 * excimer lamp [3-8]. In this paper, we reported the improvement of film hardness with annealing. A GROWTH PROCESS OF THE SiO2 FILM Figure 1 shows a growth process of the SiO 2 film. The NF3 and O2 mixed gas was photo-dissociated by the Xe2 * excimer lamp to generate NO2 and F radicals because the photon energy of Xe2 * excimer lamp was higher than the binding energy of N-F and O=O. The F radicals etched the Si wafer and produce SiF 4 , which adheres on the sample substrate. The SiF 4 adhered reacted with the NO2 photo-dissociated, laminating a SiO 2 monomolecular layer on th
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