Gas-Phase and Surface Reactions of Decomposed Species in Catalytic CVD
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GAS-PHASE AND SURFACE REACTIONS OF DECOMPOSED SPECIES IN CATALYTIC CVD Namiko Honda, Atsushi Masuda and Hideki Matsumura School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan ABSTRACT Gas-phase and surface reactions of transported species decomposed on the catalyzer were investigated in catalytic CVD (Cat-CVD), often called hot-wire CVD, using a specially designed reactor tube. The phenomena were comparatively studied using H2- or He-diluted SiH4. It turned out that the control of gas flow through the catalyzer between the gas showerhead and the substrate is a key factor to obtain high uniformity in not only the film thickness but also the crystallinity for Si films prepared by Cat-CVD using the gas pressure above about 10 Pa and the gas-flow velocity faster than several m/s. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) and polycrystalline silicon (poly-Si) films are widely used as the materials for solar cells and thin-film transistors (TFTs) in liquid crystal displays (LCDs). Recently, in order to lower the cost of these devices, high-rate, lowtemperature and large-area deposition techniques are required. Catalytic CVD (Cat-CVD), often called hot-wire CVD, is one of the promising candidates for obtaining high quality Si-based films and satisfies the above-mentioned requirements [1-3]. In this method, deposition precursors are generated on the heated catalyzer through catalytic cracking reactions. It is essentially important to understand the transport mechanism of deposition precursors from the catalyzer to substrates in order to reveal factors determining quality and uniformity of the films. Crystalline fraction is one of the most important factors of poly-Si film quality. It is wellknown that the crystalline fraction depends on the amount of atomic hydrogen on the growing surface. Characters of precursors in surface reaction are important to determine film properties. Characters of precursors change through the gas-phase reaction such as collision with other molecules. In Cat-CVD, decomposed species such as deposition precursors are formed on limited area, the catalyzer surface, and are transported to the substrate surface by thermal diffusion and/or gas flow. Therefore it seems that the density and the character of deposition precursors and other species are strongly depend on the geometrical position of the catalyzer. The authors recently reported that the gas flow in the chamber strongly influences the thickness distribution in Cat-CVD for the pressure above about 10 Pa and the gas velocity faster than several m/s [4]. In this study the spatial distributions of both the film thickness and the crystalline fraction for a-Si:H films and poly-Si films were investigated as a function of the position from the catalyzer using the specially designed reactor tube. Mechanisms for decomposition of SiH4 and crystallization of Si films are also discussed.
EXPERIMENTAL Figure 1 shows a schematic of the experimental system. A quartz
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