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The atomic scale Silicidation of Ni-Si System by In-situ TEM Investigation could be divide into three steps. At the first step (250 °C), Ni would diffuse into Si substrate to synthesize triangular Ni 2 Si. Afterwards, continuous NiSi thin films would be synthesized at 400 °C. At last, trapezoid NiSi would react with Si to form NiSi 2 at third step (600 °C). Furthermore, the electric electrical properties of Ni-Si system were identified by four-point probe measurements; the resistivities of NiSi, Ni 2 Si and NiSi 2 were 14.13 µΩ∙cm, 22.80 µΩ∙cm and 37.77 µΩ∙cm, respectively. • The diffusion behaviour of Ni-Si system was investigated through in situ HRTEM. • Ni 2 Si and NiSi were diffusion controlled and NiSi 2 was nucleation controlled. • The resistivities of NiSi, Ni 2 Si and NiSi 2 were 14.1, 22.8, and 37.7 µΩ∙cm, respectively. • The diffusion rate could be calculated by in-situ TEM observation. • This work provides a novel method to obtain low resistivity silicide for IC devices. Nickel silicide has many advantages, such as low resistivity and low formation temperature; therefore, it has been widely used in the fields of solar cells, transistors and complementary metal-oxidesemiconductor (CMOS) devices. To obtain high-quality nickel-silicide thin film, solid-state reaction is a convenient and efficient fabrication method. For better understanding of the dynamic formation mechanism, we used in-situ transmission electron microscopy (TEM) to record the diffusion behavior during the heating process. In this work, three-steps annealing process to synthesize different nickel silicides corresponding to the various formation temperatures were investigated systematically. At 250 °C, the product of the first-step annealing was inverted-triangle Ni 2 Si, embedded in the Si substrate. Then, well-distributed NiSi thin film was synthesized, having the lowest resistivity among Ni-Si system at 400 °C. Finally, NiSi 2 , a Si-rich product, would form during the third-step annealing at 600 °C. NiSi 2 product and Si substrate have small lattice mismatch; thus, the epitaxial relationship would be observed. We provide the evidence of diffusion behaviors and structural identification of Ni-Si system. Furthermore, these results are beneficial for the formation of specific nickel silicides, which is expected to optimize the fabrication of microelectronics. [ABSTRACT FROM AUTHOR]