Thermal spike mixing in cobalt silicide formation

Abstract

Ion beam mixing in cobalt silicide has been studied using silicon, argon and helium ions. Both Co-Si and Co-SiO2-Si systems have been studied, the latter in the form of coated silicon wafers which have not had their native oxide removed. Dose and temperature dependence and the effects of post-bombardment annealing have been studied. Oxide free samples show both temperature dependent and temperature independent mixing regimes. Mixing is retarded in samples with an oxide layer and this is reflected in the mixing activation energies which for Si+ ions are 0.90+or-0.28 eV and 2.17+or-0.79 eV for the two types of sample. There is evidence that implanted Si dilutes the interfacial barrier and enhances migration. The results indicate that the chemical species of the ion is important in determining the degree of mixing. Argon ion dose dependence experiments indicate a minimum dose of 1*1016 ions cm-2 for the onset of mixing (at 350 degrees C) in both types of sample. Temperature dependence studies have yielded mixing activation energies for Ar+ ions of 0.46+or-0.10 eV from the Co-Si samples and 0.64+or-0.18 eV for the samples with oxide present. These values are intermediate between those obtained for Si and Xe bombarded samples. He+ ion bombardment studies indicate that mixing is not associated with ionization spikes. Considerations of ion concentration, bombardment flux, silicide thickness and activation energies favour thermal spike mixing rather than models based on radiation enhanced diffusion.