The effects of thermal diffusion of nitrogen gas on silicon nitridation rate

Abstract

Recently Mangels [1] has reported that a more complete nitridation and generally higher nitriding rate were obtained by using the variable atmosphere in the nitrogen-demand nitriding runs and explained this behaviour with improved nitridation by decreasing nitrogen partial pressure, which was noted earlier by Atkinson et al. [2] who found that, using an isothermal nitriding run, a more complete nitridation was obtained with reduced nitrogen partial pressures. Present attention is drawn with these results to the higher nitriding rates and more complete nitridation obtained by using helium compared with those obtained by using other gases as the variable atmosphere. For example, the variable helium atmosphere (4% H2, 50% He, balance N 2 in the final gas composition) had the highest nitriding rate (34.6 g Si per hour) and this nitriding rate is higher than that (27 g Si per hour) of the variable argon atmosphere (4% H2, 50% Ar, balance N2 in final gas composition), moreover, even the constant helium atmosphere (7% He/N2) had the nitriding rate (28.69g Si per hour) surprisingly higher than only that (17.6 g Si per hour) of the variable hydrogen (25% H2/N2 in final composition) and also higher than that of the variable argon atmospheres. Since the higher the reaction temperature the higher the reaction rate becomes, the presence of low thermal conductivity gases in the nitriding atmosphere increases the reaction rate more than the presence of high thermal conductivity gases do due to the retardation of heat dissipation generated by the exothermic reaction. However, argon's thermal conductivity is less than nitrogen's and that of hydrogen and helium are greater than that of nitrogen and helinm which are slightly less than hydrogen's. Therefore, these can explain the formation of RBSN with a uniform microstructure but cannot explain the reason for an increase in the nitridation rates by using helium. Moreover, hydrogen has the function of aiding the removal of the reaction-inhibiting silica film but helium has not. On the other hand, this can be explained if more reduced partial pressure of nitrogen around the silicon compact is obtained with additions of helium as compared with other gases. In order to solve this question, we are studying the effect that the thermal segregations by the Soret effect [3]; inside a nitriding furnace containing the various binary gas mixtures, have on the nitridation rate and some brief preliminary results are described here . Diffusion is most frequently associated with a nonuniform ~ composition, but it can also arise from a nonuniform temperature. Enskog and Chapman [4], independently developed the kinetic theory of gases in nonuniform states, and showed that in general a temperature gradient should cause diffusion. Therefore, because of thermal diffusion known as Soret effect, when a temperature gradient is set up in a mixture which is initially uniform in composition, a concentration gradient develops, and this increses until the separating effect of thermal diffusion is balanced by the diffusion which tends to equalize composition. In gas mixtures at normal temperatures, the heavier molecules usually diffuse down the temperature gradient, leading to a higher concentration in the colder region rather than the warmer one. Accordingly, when the nitridation of silicon is performed in a closed tube or cold wall furnace which contains the nitriding atmosphere of the binary gas mixture, in N2-H2 and N2-He mixtures, the nitrogen gas migrates to the cold region and hydrogen or helium gas to the hot region, leading to a lower concentration of nitrogen gas in the hot region than the cold region, whereas in N2-Ar mixtures the nitrogen gas migrates to the hot region and argon gas to the cold region, leading to a higher concentration of nitrogen gas in the hot region than in the cold region.