THERMODYNAMIC APPROACH TO BORON CHEMICAL VAPOR-DEPOSITION BASED ON A COMPUTER MINIMIZATION OF THE TOTAL GIBBS FREE-ENERGY

A thermodynamic approach based on the minimization of the total Gibbs free energy of the system is used to study the chemical vapour deposition (CVD) of boron from BCl3-H2 or BBr3-H2 mixtures on various types of substrates (at 1000 < T < 1900 K and 1 atm). In this approach it is assumed that states close to equilibrium are reached in the boron CVD apparatus. For an inert substrate (excess of boron) the H2 reduction of BBr3 leads to elemental boron and to a small amount of BHBr2 by-product. There remains unreacted BBr3 at equilibrium. The thermodynamic yield in boron is high and only slightly dependent on temperature and composition. The use of BCl3 is less interesting; it leads to a higher proportion of by-products (BHCl2 associated with BCl2 and BCl at the highest temperatures) and unreacted BCl3. The yield in boron is smaller and strongly dependent on both temperature and composition. An excess of H2 increases the conversion per pass of BBr3 (or BCl3) into boron and decreases the amount of unreacted boron(III) halide at equilibrium. This thermodynamic approach is also applied to boron CVD on a metal substrate M where two reducing agents (H2 and M) are in competition. For a very reactive metal (such as titanium), the part taken by M in the reduction of BCl3 (or BBr3) is predominant under the conditions commonly used in boron CVD with strong chemical attack of the substrate. For tantalum both reducing agents (H2 and tantalum) are active and finally for tungsten BCl3 is mainly reduced by H2 with no corrosion of the substrate. Thus tungsten appears to be the best substrate metal for boron CVD. However, reactive substrates (titanium) can be used if special care is taken at the very beginning of the CVD run. © 1979.