5.9 CONCLUSIONS

The use of NiSi as contacts to source, drain and gate regions of CMOS devices depends first on the possible extendibility of CoSi ₂ as the device dimensions continue to decrease, as the available Si thickness for the formation of the silicide is further limited and as the substrate material may be modified with the addition of Ge. The limitations on the current CoSi ₂ process associated with these dimensional and material constraints were described. Although some clever modifications to the process or slight adjustment of the material may extend the use of CoSi ₂ contacts for a limited number of generations, fundamental material limitations will demand the introduction of either a raised-source drain process or the implementation of a new contact material such as NiSi.

Among the advantages of NiSi contacts are the lower thermal budget, the lower resistivity and lower Si consumption, the formation being controlled by diffusion of Ni instead of nucleation, and the possible formation of a low resistivity phase on SiGe substrates. Special attention was drawn both to the anisotropy in thermal expansion of NiSi and to the unexpected texture in films on single crystal Si. As many properties of NiSi differ from its CoSi ₂ counterpart, there are serious challenges to the use of this material. Such challenges reside in the more complex phase formation sequence at low temperatures, in the control and limitation of Ni diffusion into the Si, in the avoidance of high temperature formation of NiSi