Atomic-scale boron redistribution during reactive diffusion in Ni–Si

Abstract

The redistribution of boron during the formation of the Ni silicides was investigated using atom probe tomography and transmission electron microscopy. A 7 nm amorphous intermixed region was found after deposition of a 30 nm thick Ni film at room temperature. The formation of this Ni–Si layer was found to have almost no influence on the boron implantation profile. After heating at 290 °C for 1 h, three types of silicides (Ni2Si, NiSi, and NiSi2) were identified below a thin remaining film of Ni (8 nm). The unexpected presence of the silicon-rich NiSi2 phase at this temperature may be caused by the presence of a thin silicon oxide (SiO2) observed at the Ni/Ni2Si interface that may act as a diffusion barrier. The average boron profile in NiSi2 and NiSi silicides is similar to the profile in the silicon substrate before reaction. A segregation of boron at several interfaces was detected. Small boron clusters (1.5 at. %) were found in NiSi, NiSi2, and Si phases but not in Ni2Si. After a 1 min heat treatment at 450 °C, the NiSi phase is the only silicide present. Boron clusters with a platelet shape and a concentration of 3 to 5 at. % of boron were found in both NiSi and Si. The presence of boron in the Ni silicide and its precipitation in the form of tiny clusters is likely to affect the electrical properties of the contacts.