Chemical vapor deposition of boron nitride using premixed borontrichloride and ammonia
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Chemical vapor deposition (CVD) of boron nitride (BN) is most readily performed using BC13 and NH 3 , which are brought into the deposition zone through two separate tubes. This causes some problems: inadequate mixing leading to a nonuniform deposit, formation of solid intermediates, etc. To avoid these problems, the process was performed by mixing BC13 and NH 3 at elevated temperatures (120-220 °C) prior to entering the deposition zone. The reaction between them took place by the forming of volatile stoichiometric B - N compounds (trichloroborazine and iminochloroborane), which were then transported through a single tube into a deposition zone. The resulting deposit was found to be hexagonal boron nitride.
I. INTRODUCTION Hexagonal BN (h-BN) is known to be one of the most promising ceramic materials with high thermal conductivity, good electrical insulating properties at high temperature, and good chemical and mechanical stability. Films of h-BN have been prepared most readily by conventional CVD techniques, using two different groups of volatile boron compounds. (1) The first group consists of pairs of compounds, one containing boron and the other nitrogen, which react together to produce BN. As the B containing compound, mainly B-halides (BCI3 or BF3) have been used, with ammonia being the N-containing compound. (2) The second group consists of compounds containing both elements in the stoichiometric ratio, so that BN is formed by their thermal decomposition. Among the compounds used are borazine (B 3 N 3 H 6 ), trichloroborazine (B3N3H3C13), or hexachloroborazine (B3N3C16). The most common approach to form BN coatings by CVD is to use BC13 and NH 3 . The gasses are fed through separate tubes into the deposition zone, where they come in contact and react one with another. As implied in the literature,1"3 it has also been confirmed in our preliminary experiments that the layer quality and the deposition rate are very sensitive to the relative position of the inlet tubes to the sample. That is due to the difficulty of obtaining a proper mixing of these two gases. As CVD of BN is mainly a low-pressure process (approximately 3 mbar —> LPCVD), the speed of the streaming gases in the deposition zone is very high, making their residence time very short. By means of address: IBK-Vinca, Institute of Materials Sciences, P.O. Box 522, 11001 Belgrade, Yugoslavia.
gas flow, temperature, viscosity, and tube geometry, the gas speed and the Reynolds number can be determined. The number indicates that the flow condition is laminar, which makes the difficulties of the mixing problem understandable. Therefore we tried to perform a BN deposition using one of the second group of compounds. Trichloroborazine (TCB) was chosen, applying the deposition procedure as described by Singh.4 The disadvantages of this method are caused by TCB itself: it is very expensive and moisture sensitive so that it has to be handled in a glove box under inert atmosphere; moreover, it is decomposing already at room temperature. Instead of using presynth
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