Thermodynamics of phase formation of the cobalt silicides

Vaporization thermodynamics in the cobalt-silicon binary system has been studied by both reactive and simple Knudsen effusion mass spectrometry. The equilibrium partial pressures of SiO(g) over mixtures of CoSi + CoSi2 and Co2Si + CoSi with SiO2 have been measured over the temperature range ca. 1190-1450 K. The equilibrium vapor pressure of Co(g) over a Co2Si + CoSi two-phase mixture has been measured over the temperature range of ca. 1420-1560 K. Standard reaction enthalpy and entropy changes for the appropriate vaporization reactions, within the temperature ranges of the experiment, were determined from the temperature dependence of the measured vapor pressures. They were used, together with published and estimated heat capacity and standard entropy values, in a third-law evaluation of standard reaction enthalpy changes at 298 K. Standard enthalpies of formation of the intermediate compounds were found by a Hess' law combination of the standard reaction enthalpy values at 298 K. The results obtained are as follows: 1/3 CO2Si, -41.0 +/- 2.0 kJ mol(-1); 1/2 CoSi, -47.3 +/- 2.0 kJ mol(-1);1/3 CoSi2, -32.9 +/- 2.0 kJ mol(-1). These results are compared to previous measurements and estimates and are discussed in relation to processing CoSi2 for microelectronics applications.