Interaction study of nitrogen ion beam with silicon

Focused ion beam technology with light gas ions has recently gained attention with the commercial helium and neon ion beam systems. These ions are atomic, and thus, the beam/sample interaction is well understood. In the case of the nitrogen ion beam, several questions remain due to the molecular nature of the source gas, and in particular, if and when the molecular bond is split. Here, the authors report a cross-sectional scanning transmission electron microscopy (STEM) study of irradiated single crystalline silicon by various doses and energies of nitrogen ionized in a gas field ion source. The shape and dimensions of the subsurface damage is compared to Monte Carlo simulations and show very good agreement with atomic nitrogen with half the initial energy. Thus, it is shown that the nitrogen molecule is ionized as such and splits upon impact and proceeds as two independent atoms with half of the total beam energy. This observation is substantiated by molecular dynamics calculations. High resolution STEM images show that the interface between amorphous and crystalline silicon is well defined to few tens of nanometers.