Cosputtered cobalt silicides on silicon, polycrystalline silicon, and silicon dioxide

Abstract

Silicide formation in cosputtered (Co+Si) films on silicon, polycrystalline silicon, and silicon dioxide (SiO2) has been studied by using the techniques of x-ray diffraction, Rutherford backscattering, and scanning electron microscopy and by measuring the sheet resistance and stress as a function of the composition and sintering temperature. Films with nominal Si/Co atomic ratios of 1 to 3 were investigated. The resistivity and the stress were found to be the lowest for the films with actual Si/Co ratio closest to a value of 2 (i. e., CoSi2). For silicon-rich or silicon-deficient films, sintering led to a Si/Co ratio variation across the thickness of the film. The magnitude of the Si/Co ratio variation was found to depend on the as-deposited Si/Co ratio and the substrate type. The differences in Si/Co ratios at the inner and outer silicide surfaces, the resistivity, and the stress were found to increase with increasing deviations from the CoSi2 stoichiometry (Si/Co=2). At temperatures above 950 °C, an intermixing of CoSi2 and silicon occurred leading to deleterious effects on the properties of the silicide. Also in the absence of silicon (i. e., for CoSi2 films on oxide), sintering in an inert ambient at temperatures ≥ 1000 °C led to decomposition of CoSi2 into CoSi and silicon which was lost in the furnace. The decomposition, however, could be suppressed by capping the silicide with a SiO2 film or by introducing small amounts of oxygen in the annealing ambient. It has been suggested that the observed high temperature instability of cobalt silicides could be associated with (a) very narrow concentration range of stability of the cosputtered cobalt disilicide, (b) tendency of CoSi2 and Si to grow epitaxially on substrate silicon, and (c) permeability of CoSi2 to silicon diffusion at temperatures above 950° C.