WSi2 and CoSi2 as diffusion sources for shallow-junction formation in silicon

The redistribution of B and As ions implanted into thin layers of WSi2 and CoSi2 on poly- or monocrystalline Si and the outdiffusion into the Si substrate during furnace annealing (FA) and rapid thermal processing (RTP) were investigated by several analytical techniques. Shallow junctions (depth xj < 100 nm) with interface concentrations Cint close to the solid solubility of the respective dopant in Si (Cint≳3×1020 cm−3 for As; (Cint ≳ 8 × 1019 cm−3 for B) were obtained with RTP. For FA above 800 °C, the diffusion of B from CoSi2 into Si results in a drop of Cint < 2 × 1019 cm−3 because of strong B segregation and probably reactive loss at the SiO2/CoSi2 interface. No evidence on metal-dopant-compound formation could be found. The dopant redistribution is demonstrated to be a superposition of lattice and grain-boundary diffusion, solubility limits, layer inhomogeneities, dopant segregation at the interface and grain boundaries, and probably phase transformation of the dopants segregated at the SiO2/silicide interface. Electrical results such as, e.g., CoSi2 diode leakage currents (≊1 nA/ cm2) and contact resistances ( 2–5 × 10−7 Ω cm2 for RTP) clearly show that the formation of shallow silicided junctions by diffusion from an implanted silicide is a highly useful technological approach.