Ge effects on silicidation

Nickel and cobalt silicides have formation and/or stability issues when forming in the presence of Ge. Additions of Ge increase the temperature at which a low resistance CoSi2 is formed due to phase separation into CoSi2 and Ge-rich Si-Ge grains. With Ni, additions of Ge decrease the temperature at which NiSi converts to a NiSi2, lead to agglomeration at a lower temperature and lead to germanosilicide formation. Nickel has a “must not exceed” critical temperature that decreases with increasing Ge concentration, and Co has a minimum critical temperature that increases with increasing germanium concentration. Neither silicide is ideal for doped Si-Ge contacts, where the germanium compositions are in excess of 20%. The insertion of a layer of pure Si above the Si-Ge is a promising integration option for forming a robust silicide on Si-Ge substrates. The phase formation and stability of pure Co on Si-20%Ge substrates with Si caps of thickness from 200 to 1000 Å were studied and it was shown that a 400 Å Si cap on Si-20%Ge leads to low resistance CoSi2.