Field Ion Emission Mechanisms
Résumé
There are two ways to explore a material in three dimensions: “seeing through” or “dig into.” “Seeing through” is the method used, for instance, in electron tomography or in X-ray tomography. In this case the quality of the produced images is related to our ability to control and exploit the imaging beam (electron or photon). In atom probe tomography, a “dig” process is involved, named “field evaporation.” As the brush of an archaeologist, field evaporation is used to reveal the features of interest, layer after layer, inside the sample. The power of this physical mechanism is that the erosion is made at the ultimate scale of the atom, in a way so delicate that depth exploration of complex nanostructured materials can be achieved. This chapter is centered on this physical and central mechanism of field evaporation. The chapter describes the field evaporation and field ionization theories in a scholarly approach from the production of the required high electric field at the sample surface to the first steps of flight of the produced ions. This understanding is of fundamental importance to better evaluate the limits and field of application of the atom probe as a nanoanalyzing instrument.