Direct Solid State Phase Transformation from Co to Epitaxial CoSi₂ in Co / Thin Ti / (100) Si Structure and its Application for Shallow Junction Formation

Abstract

Direct epitaxial CoSi₂ formation from Co, which is contrary to the reported silicidation process: Co→Co₂Si→CoSi→CoSi₂, has been found during anneal of Co / Ti / (100) Si system. 2 nm thick Ti and 15 nm thick Co films were sputter deposited, and then annealed for 30 min at temperatures between 375°C and 900°C. At room temperature, the 2 nm Ti immediately forms an amorphous Ti-Si-Co layer between the Co and Si. Epitaxial CoSi₂ begins to form at 400°C, while the amorphous layer continues to act as both a Co diffusion retardant and Si diffusion suppressant even at 900°C. This retarded diffusion of Co reduces the growth rate of the CoSi₂ over the entire temperature range studied. Superiority of the epitaxial to polycrystalline suicide has been demonstrated. In self aligned structures, an epitaxial CoSi₂ film is formed by a single-step anneal without any overgrowth onto adjacent field oxide areas utilizing the amorphous diffusion controlling layer. A p⁺/n junction of 40 nm depth with reduced leakage and low ideality factor has been obtained by impurity diffusion from epitaxial CoSi₂.