Formation of thin films of CoSi₂: Nucleation and diffusion mechanisms

Abstract

The reaction of thin cobalt films with their silicon substrates leads to the formation (in sequence) of Co2Si, CoSi and CoSi2. The formation of the first two compounds has already been studied in detail, but the formation of the disilicide remains to be elucidated. It is important to understand better the formation of CoSi2 because of its similarities with NiSi2 which is well known to form via nucleation-controlled processes. With very thin films, only 20-30 nm of cobalt, clear evidence for nucleation phenomena has been obtained. It is believed that nucleation explains, at least partly, the complex kinetics which are observed during the formation of thick (several hundred nanometers) layers of CoSi2. The coincidence of the temperatures where nucleation and diffusion take place makes it impossible to identify clearly the diffusion kinetics as, for example, with Co2Si, CoSi or NiSi, or the nucleation process as, for example, with NiSi2. As with other compounds the importance of the nucleation effects results from the small driving force for the transition from CoSi to CoSi2. An attempt is made to compare quantitatively the role of nucleation in the formation of CoSi2 over crystalline silicon and over amorphous silicon. An experiment with an implanted marker confirms the expectation that the formation of CoSi2, like that of the isomorphous NiSi2, occurs mostly through the motion of the metal atoms. © 1985.