A new approach to the interpretation of atom probe field-ion microscopy images

The field distribution and the ion trajectories close to the tip surface ar e known to mainly control the contrast of field-ion microscopy and the reso lution of the three-dimensional atom probe. The proper interpretation of im ages provided by these techniques requires the electric field and the ion t rajectories to be determined accurately. A model has been developed in order to compute the ion trajectories close t o a curved emitting surface modelled at the atomic scale. In this model, bo th the gradual change of the tip surface and the chemical nature of atoms w ere taken into account. Predictions and results given by this approach are shown to be in excellent agreement with experiments. The calculated electric field at the tip surfa ce is consistent with field-ion microscopy contrasts. The preferential rete ntion of surface atoms and the order of evaporation were correctly simulate d. The ion trajectories were successfully described. In this way, the cruci al problem of trajectory overlap and local magnification could be investiga ted. These simulations not only lead to a new understanding of the physical basis of image formation, but also have a predictive value. (C) 2001 Elsev ier Science B.V. All rights reserved.