Growth Kinetics and Silicon Diffusivity in MoSi<sub>2</sub> and WSi<sub>2</sub> Disilicides

Abstract

Growth kinetics and silicon diffusivities in MoSi2 and WSi2 silicide phases were studied at selected temperatures over the temperature range from 900 to 1500 o C using reaction between dense metal and silicon vapor. Experiments were carried out by electrothermography method [1] using molybdenum and tungsten wires 100 µm in diameter and 8.5 cm length as initial samples directly heated by electrical current. As a silicon source rarefied monosilane was used providing controllable conditions of the silicon supply to the metal surface. To ensure diffusive mode of interaction the partial pressure of silane was selected in the way that certain prevailing of the rate of silicon supply to the surface over the rate of its diffusion into the formed silicide layer was provided in order to hinder the formation of a redundant solid silicon layer (at T < Si melt T ). It is established that for the case of molybdenum two compact uniform layers (MoSi2 and Mo5Si3) were observed in diffusion zone, and the MoSi2 layer was by about one order of magnitude larger in thickness than the Mo5Si3 layer. In the case of tungsten a single-layered diffusive zone is formed, consisting of WSi2 compact and uniform layer; the W5Si3 silicide is not detected at all. In all cases growth kinetics of disilicide layers, up to 5-7 µm thicknesses, conforms well to the parabolic law, which testifies indirectly that the first type boundary condition is realized on the gas medium/MeSi2 boundary. The silicon diffusion coefficients in MoSi2 and WSi2 layers were calculated on the basis of measured parabolic growth constants.