Effects of low-energy (1-1.5 kV) nitrogen-ion bombardment on sharply pointed tips: Sputtering, implantation, and metal-nitride formation

Low-energy (1-1.5 kV) nitrogen-ion bombardment of sharply pointed Fe-3 at. % Mo tips is performed, at room temperature and 473 K, by applying a negative voltage to the tip in the presence of nitrogen gas (10^-3-10^-5 Torr) in an ultrahigh vacuum atom-probe field-ion microscope (APFIM). Tip sharpening, as a consequence of sputtering, is observed directly in situ via APFIM. This sharpening phenomenon can be explained by the inertia effect on the trajectories of the field-emitted electrons and nitrogen ions that results in sputtering of the shank of the tip. We also observe that the implanted low-energy (1-1.5 kV) nitrogen ions react with the molybdenum atoms in solid solution in this Fe(Mo) alloy to produce immobile molecular diameter nitrides at room temperature and 473 K. The measured range profile of the implanted 1.5 kV nitrogen species is compared with results calculated using the TRIM.SP Monte Carlo code. This comparison implies that the ionized nitrogen molecules (N₂¹⁺), generated by the field-emitted electrons, obtain the full potential energy of the tip before bombarding the tip's surface and then dissociating into nitrogen ions (N¹⁺) at or near the surface. The experimental nitrogen range profile is quantitatively fitted to the Edgeworth expansion of the normal distribution function.