Synthesis of Diamond by Laser-Induced CVD
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SYNTHESIS OF DIAMOND BY LASER-INDUCED CVD K. KITAHAMA* K. HIRATA, H. NAKAMATSU, S. KAWAI, N. FUJIMORI, AND T. IMAI * The institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki 567 Japan **Sumitomo Electric Industries, Ltd., Itami Laboratories, 1-1 Koyakita, l-chome, Itami-shi 664 Japan ABSTRACT Synthesis of diamond thin-films has been tried by an ArF excimer laser-induced chemical vapor deposition (LCVD) technique, using acetylene diluted with hydrogen as a source gas and a silicon wafer as a substrate. In these experiments, irradiation geometry, substrate temperature and laser power density were varied. Upon irradiation by a focused laser beam, deposition of diamond on substrates heated above 400°Cwas observed, and was confirmed by reflection electron diffraction (RED) photographs. Homogeneity of the diamond films was improved by irradiation parallel to the substrate. These facts suggest that the formation of diamond proceeds through multiple photon decomposition of the reactant gas, and that electronic excitation of gas phase rather than that of substrate or adsorbate layer is essential to form diamond. INTRODUCTION Much attention is currently being paid to the synthesis of diamond thinfilms by way of low pressure methods because of its applicability as a hard coating material, heat-sink of VLSI and semiconductor for high temperature use. Synthesis methods studied so far include chemical vapor deposition (CVD) with a tungsten filament [1], plasma CVD [2], ion beam deposition [3] and some modifications or hybrids of these [4-6]. Most of these studies were performed at temperatures around 800'C with methane diluted with hydrogen as a source gas. UV laser-induced CVD could be effective in lowering the substrate temperature, since it can generate chemically active species in high energy states, so that the reaction proceeds by way of paths quite different from those of thermal excitation. Formation of diamond by ArF excimer laser CVD at 193 nm was already confirmed and reported briefly in a letter [7]. This paper is intended to describe the details of diamond formation and to make clear the basic mode of laser excitation for the preparation of diamond. It should be known for laser-induced CVD whether the reaction occurs through a single photon process or a multiple photon process. Therefore, irradiation with and without a lens was performed in the present syntheses. In addition, the reactions in laser-induced CVD are classified into three basic modes of laser excitation, i.e. gas phase excitation, adsorbate excitation, and solid excitation. Although, we did not discriminate the effects of the latter two, present experiments were confined, for simplicity, to the irradiation geometries of both illumination and non-illumination of the substrate surface, i.e. off-parallel and parallel laser irradiation. Laser-induced CVD, of course, requires the reactant gas to absorb the laser light. However, hydrocarbons with low molecular weight exhibit only slight absorption at 193 nm; there is almo
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