The shape of field emitters and the ion trajectories in three-dimensional atom probes

The lateral resolution of three-dimensional atom probes is mainly controlle d by the aberrations of the ion trajectories near the specimen surface. For the first time, a simulation program has been developed to reconstruct the ion trajectories near a sharp hemispherical electrode defined at the atomi c scale. Surface atoms submitted to the highest field were removed one by o ne, The consecutive gradual change of the surface topology was taken into a ccount in the calculation of ion trajectories. As the tip was 'field evapor ated', the initial spherical shape of the emitter was observed to transform gradually into a polygonal shape. When the tip reached its equilibrium sha pe, the field distribution at the tip surface was found to be much more uni form compared to the initial distribution. The calculated distribution of i on impacts on the detector exhibits the presence of depleted zones both at the centre of low index poles and along <001> zone axes. These predictions are in excellent agreement with experiments.