Spatial Resolution(s) in Atom Probe Tomography

Atom probe tomography (APT) is often quoted to provide "atomic-scale" analysis of materials in three-dimensions. Despite efforts to quantify APT's spatial resolution, misunderstandings remain regarding its true spatial performance. If the depth resolution was once reported to be 20 pm, quoting this value outside of its specific context is misleading and should be avoided. The resolution achievable in pure metals, at one specific location, within one reconstructed dataset, does not generally apply across materials or analysis conditions, or even throughout a single tomographic reconstruction. Here, we review various efforts at defining and measuring the spatial resolution in the study of single phase and single element materials-i.e., pure metals-in field-ion microscopy (FIM) and APT. We also report on the degradation of the resolution arising from ion optical devices used to improve the mass-resolution. We aim to offer some perspective as to how reported resolutions may be or may not be of any relevance to most of the materials characterization efforts by APT, including cases of precipitates in a matrix that emphasize the need to consider an effective resolution. Finally, we discuss concepts to improve the spatial accuracy of the technique in a relatively distant future.