Photochemical Moisture Proof Coating on Nonlinear Optical Crystal by ArF Excimer Laser
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Photochemical Moisture Proof Coating on Nonlinear Optical Crystal by ArF Excimer Laser M. Kojima and M. Murahara: Department of Electrical Engineering, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan ABSTRACT Silicone oil was photo-chemically oxidized to change into SiO 2 on the crystal by using an ArF excimer laser; the protective moistureproof film has been developed for a nonlinear optical crystal that is deliquescent. The nonlinear optical crystals such as CsLiB6 O10 (CLBO) and KH2 PO4 (KDP) are deliquescent, which causes their surfaces to be cloudy by absorbing moisture in the air. We, therefore, demonstrated the growth of the SiO 2 film directly on the crystal so as to be moistureproof. Firstly, dimethylsiloxane silicone oil (-O-Si(CH3 )2 -O-)n was poured on the substrate and coated by a spinner for making the silicone oil thin layer. Then, the ArF excimer laser was vertically irradiated on the sample in oxygen atmosphere. The O atom on the substrate surface was photo-excited by the laser to generate a high active O atom. At the same time, the Si-CH3 bond of the silicone oil was photo-dissociated and the dangling bond of Si was linked with the active O atom to form a SiO 2 film on the crystal surface. In short, the film formed by the new technology can be used as a protective coating, which has the moisture resistance and the UV permeability, for a nonlinear optical crystal. INTRODUCTION The SiO 2 films have been widely used as the transparent protective film, the protective chemicalproof film and the insulated film, which is used for semiconductors or electric parts. The thermal oxide, chemical vapor deposition (CVD), vacuum vapor deposition and sputtering methods are generally used for making the optical thin films. M.Ts uji of Kyushu Univ. has fabricated the SiO 2 film by LCVD (laser- induced chemical vapor deposition) method in the presence of mixtures of SiH 4 and N2 O at substrate temperature of 30-500 degrees centigrade [1]. G.Reisse Engineering Mittweida Univ. has also fabricated the SiO 2 film by LCVD method in the presence of mixtures of SiH4 and N2 O [2]. E.Fogarassy has fabricated the SiO 2 film by laser ablation from silicon and silicon monoxide
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targets, performed under vacuum and in oxygen atmosphere at substrate temperature of 20-450 degrees centigrade [3]. K.Sano of Suzuki Motor Co. has fabricated the SiO 2 film by TEOS/O 2 ECR plasma at substrate temperature of 25-350 degrees centigrade [4]. However, these methods need degree of vacuum to use the reaction gases, which are used to form the film. In addition, the substrate is heated at certain amount of temperature. Therefore, the difference between the substrate and the film in thermal expansion rate causes cracking. On the other hand, our method does not need the degree of vacuum and the heat. We have developed the new film formation method to inhibit cracking by irradiating the ArF laser on the silicone oil to be photo-oxidized in the air [5-7]. Because the nonlinear optical crystals are deliquescent
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