Ion imaging in a Focused Ion Beam microscope: modeling the channeling contrast to construct EBSD ‐like orientation maps

Field: Materials Science (Physical Sciences)

Electron backscatter diffraction (EBSD) is routinely employed as a characterization tool to obtain individual grain orientations, local texture and phase identification. Efforts are currently being made to optimize the compromise between speed of indexation, spatial and angular resolution, quality of phase recognition and dimensions of the field‐of‐view. In this context, any new technique that can lead to a better compromise would be welcome. With this in mind, we proposed recently the iCHORD method (for ion CHanneling ORientation Determination), aiming at constructing orientation maps based on the well‐known channeling contrast phenomenon observed in a rotation series of ionic images (see figure 1) [1]. The proof‐of‐concept of the iCHORD method was obtained by predicting the intensity loss received by a detector for specific orientations of a crystal thanks to crystallographic calculations. However, the number of parameters to adjust when applying the technique to different materials was quite high. In order to improve the method and make it more versatile, a new modelisation of the channeling contrast is proposed in the present study. Experimentally and theoretically, an intensity loss is observed for a crystal when the ion beam arrives parallel to some low index crystallographic planes, and particularly when the sample is in a zone axis. Therefore, if the atomic structure of the crystal is projected onto a surface perpendicular to the ion beam, an intensity loss will corresponds to large “free spaces” between the atomic columns (see figure 2). In other words, the intensity loss for a specific crystal orientation is closely related to the sum of the grey levels of the pixels constituting the projection. More quantitatively, to avoid some projection artefacts, it is necessary to adjust the number and disposition of atoms to be considered, as well as their interactions with the ion beam as a function of their depth to obtain an efficient model of the channeling effect. As the atomic positions of atoms are used, only the structure file of the material under consideration has to be provided, without specific calculations for cubic or hexagonal structures for instance. Only the parameters regarding the ponderation function have to be adjusted, which is quite straightforward, hence meeting the versatility required. In this framework, theoretical intensity profiles obtained by varying the orientation of a crystal (see figure 3) fit quite well with experimental profiles obtained by varying the sample orientation in the same way. This improvement in the modelisation of the channeling contrast has been embedded in the workflow aiming at obtaining orientation maps without the use of the EBSD technique. Together with new developments regarding the sampling of the orientation space and the indexation algorithm, a new version of the iCHORD method is released. The impact on the angular resolution as well as speed of indexing are discussed. The benefits of using the iCHORD method in place of the EBSD technique are also summarized.