Reactive diffusion in the Ni-Si system: Phase sequence and formation of metal-rich phases

Abstract

Recent studies on Ni-Si reactions imply the presence of multiple metal-rich phases during annealing, contrary to traditional studies in which only Ni 2Si, NiSi and NiSi2 were detected. In this paper, from in situ x-ray diffraction (XRD) and Rutherford Backscattering Spectroscopy (RBS) of thin and thick Ni films annealed under various conditions, we suggest two explanations for the observed differences. The first difference related to the presence of Ni31Si12 is explained by the fast ramp anneals currently in use in microelectronic applications. The higher temperature reached before all the Ni is consumed to form Ni2Si allows for the Ni31Si12 to nucleate at the interface between Ni 2Si and Ni and grow until the Ni is completely consumed. The faster temperature ramp rate then allows for the coexistence of multiple metal-rich phases in a multilayer stack. The second difference is the presence of the Ni3Si2 phase in either thin or thick films annealed at high or low temperatures. From RBS and in situ XRD measurements, it becomes clear that this phase forms from the reaction of Ni2Si with either NiSi or Si; the important point being it forms when the Ni2Si layer becomes very thin. The formation of Ni3Si2 is the result of a reaction with an unstable Ni2Si compound given that at a given critical thickness, as the film becomes thinner because of NiSi formation, the surface energy will dominate and renders the compound unstable, thus allowing the formation of Ni3Si2.