Stress-induced precipitation of dopants diffused into Si and TiSi₂and CoSi₂implanted layers

As, P and B ions were implanted into TiSi2 and CoSi2 thin layers and subsequently diffused into the underlying silicon substrate either by a rapid thermal process or by a long annealing in a furnace. For all the species, shallow junctions (20-400 nm depth) were obtained with a high dopant concentration (1019-1020 cm-3) at the silicide/silicon interface. In the case of As, the comparison between the amount of diffused substitutional arsenic and the carrier profile indicates the presence of inactive substitutional atoms at the silicide/silicon interface. These As atoms form coherent precipitates at the silicide/silicon interface as evidenced by transmission electron microscopy cross sections. Precipitates were also found for P and B dopants, but for this latter species they were located into the silicon substrates at 30 nm from the interface. The precipitate formation has been associated with the high tensile stress induced by the silicide layer on the surface silicon region and to its influence on the solid solubility of the dopant. The precipitate density decreases if the silicide thickness is reduced, i.e. if the tensile stress produced by silicide layer becomes lower. These precipitates are easily dissolved during high-temperature annealing after removal of the silicide layer.