Solid solubility of As in CoSi2 and redistribution at the CoSi2/Si interface

Abstract

The solid solubility of As in CoSi2 and the redistribution of As at the CoSi2/Si interface at temperatures between 650 and 950 °C have been investigated. As was implanted in the cap Si layer of mesotaxy samples (epitaxial CoSi2 layers buried in a Si substrate). The As profiles after annealing were measured by secondary ion mass spectrometry. The solubility of As is lower in the cobalt disilicide than in silicon. A detailed description of thermodynamic equilibrium between the solution of As in CoSi2 and the solution of As in Si shows that the distribution coefficient is dependent on the concentration. Within the experimental accuracy, the enthalpy of solution, deduced from the distribution coefficient, is constant in the temperature range and equal to 0.28 eV. The solution of As in CoSi2 can thus be described by Henry’s law, which is valid for dilute solutions. Accumulation of As at both interfaces of the buried CoSi2 layer has been observed and is discussed in terms of segregation and precipitation. A transient enhancement of the As diffusion in the Si substrate has also been found. Simulation of the redistribution process shows that this enhancement is not due to the diffusion of As in CoSi2 but more likely associated with the excess of point defects resulting from ion implantation or from the silicide film. On the basis of both ternary phase diagram calculations and experimental results, it is shown that there is a ternary equilibrium between Si(As), CoSi2, and SiAs in contrast to the equilibrium between Si(As), CoSi2, and CoAs reported previously by other authors. It is shown that the latter equilibrium should decrease the solubility of As in Si, but such a decrease has never been observed experimentally.