Atom Probe Tomography of Thin Films

 

K.T. Henry, G.B. Thompson, B. Geiser, M.P. Moody, S.P. Ringer

 

 

Atom probe tomography is a three-dimensional reconstruction technique which, in general, is based upon a simple reconstruction procedure that incorporates a single evaporation field. As a consequence of this approximation, species that have dramatically different evaporation field strengths suffer from trajectory aberrations, local magnification effects and preferential evaporation artifacts.  To improve the fidelity of the reconstruction, atom probe data sets can be compared to TEM tomography reconstructions and/or simulations of the evaporation sequence. In terms of the latter, simulations provide insights into how the aberrations develop and forward-feeding modeling for field evaporation voltages that can be applied to the experimental data to minimize, or even eliminate, the artifacts in the reconstruction.  In the present study, the evaporation behavior, with its accompanying artifacts, has been quantified through comparison of experimental data to evaporation simulations. The ordered L10 phase of FePt was selected because the elemental species of Fe (33 V/nm) and Pt (44 V/nm) have sufficiently large differences in evaporation fields, which would promote the aforementioned issues. The L10 structure consists of alternating atomic planes of Fe and Pt in the [001]. An advantage of thin films, for this type of study, is that the crystallographic texture can be controlled through proper substrate selection and deposition conditions and the degree of order can be controlled by the annealing temperature. Collectively, this provides tunable variables to study materials-dependent field evaporation behavior.

A depletion of Fe at the pole and along the zone lines was observed in the atom maps for these experiments. Possible mechanisms that explain the reconstructed depletion of Fe near poles include: (1) local electrostatic effects because of the extremely high evaporation field differences, (2) local magnification effects, or (3) surface diffusion during data acquisition. To determine if surface diffusion contributed to this artifact, field ion microscopy images using He were taken. No qualitative observable migration of Pt on the surface was observed. Thus, local electrostatic effects are believed to be the major contributor. The evaporation simulation results were in good agreement with the experimental data, i.e. Fe depletion (Pt enrichment) at the poles and zone lines. As expected, preferential evaporation of Fe was noted since its evaporation field is much less than Pt. The simulations showed that subsequent Fe evaporation after Pt resulted in a shift of the lattice spacing for the Fe sublattice. Though this evaporation issue was noted, interestingly, the experimental registry of the spatial frequency in the spatial distribution maps was not significantly affected.  Compositional analysis radially from the pole showed a linear decrease in Pt towards the ideal composition. The composition of the film was found to be dependent upon the detection type, i.e. single ions detected yielded a composition of Fe55Pt45 whereas multiple ion detection yielded a composition of Fe46Pt54. Through the simulation, the experimental density variations observed have been rationalized as a consequence of local magnification effects. The collective results simulation has been used to improve the final reconstruction by selecting an appropriate evaporation field.