Role of the early stages of Ni-Si interaction on the structural properties of the reaction products

Nickel-silicon compounds, as most of the transition metal silicides, show peculiar thermodynamic and kinetic behaviours. The reason resides in the metastability of a rich variety of different phases, which are frequently favoured by the interaction with the substrate or by the limited amount of atoms available during the reactions (thin films). The large effort devoted to the comprehension of the phenomena governing Ni-Si interaction from the very beginning of the reaction process testifies the widespread interest in the field and it is driven by the need to push as far forward as possible the scaling down of micro/nano-electronics devices. Here, we provide a review on the crucial role of the early stages of the Ni-Si atomic interaction to show how this interaction has a huge impact on the reaction process and on the structural properties of the reaction products. The formation of a Ni-Si mixed layer at the deposition stage, its structure and its role in the further evolution of the reaction couple are discussed on [001] Si and amorphous Si substrates. Controlling the mixed layer properties becomes extremely important in a regime wherein kinetics upsets thermodynamic stability, i.e., in thin films interactions, and during low temperature and/or ultra-rapid thermal processes, as required by the scaling down of the devices. In the review, it is highlighted how the opportunity to control thickness and composition of the mixed (precursor) layer opens the field to tailor new materials possessing intriguing properties, such as the case of transrotational Ni-silicides. Compared to standard poly-Ni silicides, they offer large chemical and structural stability windows as well as a promising electrical behaviour.