FORMATION OF COBALT-SILICIDED P+N JUNCTIONS USING IMPLANT THROUGH SILICIDE TECHNOLOGY

Abstract

This paper investigates electrical and material properties of cobalt silicided p+ n junctions fabricated using implant through silicide (ITS) technology. The annealing procedure was carried out in an open-tube furnace with flowing nitrogen. To prevent residual oxygen in the furnace from reacting with the cobalt, a passivating film of molybdenum was used during the initial stage of annealing. BF2+ ion implantation was employed for the p+ n junction formation. The ITS scheme and the subsequent annealing conditions were evaluated by analysis of the material properties and investigation of the electrical characteristics of the silicided junctions. During high-temperature annealing (greater-than-or-equal-to 900-degrees-C), Co silicide releases its high surface energy via silicon precipitation and film agglomeration. High-temperature stability of the Co silicide can be improved by BF2+ ion implantation, as indicated by the retardation of film agglomeration and decreased degradation of sheet resistance. Cobalt-silicided p+ n junction diodes with 0.1 mum junction depth measured from the silicide/silicon interface were fabricated at a 700-degrees-C annealing and shown to possess excellent electrical properties. The leakage current density measured at -5 V was 0.5 nA/cm2 and a forward ideality factor of 1.006 was obtained. It was found that Co silicide is suitable to serve as an energy barrier and dopant diffusion source for the ITS scheme at low-temperature annealing. For elevated temperatures (greater-than-or-equal-to 900-degrees-C), the use of cobalt silicide as a dopant diffusion source becomes impractical because the dopant evaporation and silicide agglomeration severely degrade junction performance. Nevertheless, the silicide layer can be used as an energy barrier and implantation damage basin for high-temperature processes. No evidence of the electrically activated trapping centers related to the cobalt atoms was observed with BF2+ implantation at energies up to 100 keV. Thus, there is no fundamental limitation prohibiting the formation of cobalt-silicided shallow junctions being fabricated using ITS technology.