Trajectories of field emitted ions in 3D atom-probe

Abstract

The improvement of the lateral resolution of 3D atom probe requires the ion trajectories to be determined accurately. For the first time, the atomic structure of the emitter (a sharp tip) and the gradual change of its topology during evaporation are taken into account. Atoms submitted to the highest field are removed one by one. Ion trajectories are then simulated step by step after each atom has been removed from the sample surface. The recurrent use of a simulation software (SIMION 3D) for each elemental step makes it possible to model the image transfer function of 3D atom-probe for a non-stationary shape of the emitter. This dynamic model, applied to atom probe data, is shown to correct for the major aberrations present at the centre of low-index poles. The well-known depleted zone present at the pole centre is shown to disappear using this model. An almost constant density of atoms is then observed over the entire analysis area. For the first time, a physical interpretation of these depleted zones is provided and confirmed through simulation and experiments.