A Brief Overview of Atom Probe Tomography Research

Atom probe tomography (APT) stems from the field-ion microscope (Müller, 1956) equipped with a time-of-flight mass spectrometer developed by Erwin Müller and his coworkers in 1967 to 1968 (Müller et al., 1968). The technique is henceforth about to celebrate its 50th anniversary. Field ion microscopy makes use of an intense electric field, in the rage of 10 Vm, to ionize rare gas atoms in the vicinity of the specimen surface. The specimen is shaped as a sharp needle, with a radius of curvature R in the range of 30 to 200 nm, which enables the generation of electric fields of the appropriate magnitude by biasing the specimen to a few kilovolts (the field is proportional to V/R). The specimen is maintained at low temperature (20~80 K) to prevent surface migration and improve the control over the field evaporation process, and the analysis takes place in ultra-high vacuum conditions (≈10 Pa), prior to the introduction of the imaging gas (≈10 Pa). The beams of ions generated near the surface are projected, under the effect of the very intense electric field, onto a screen and the image formed reveals the topography of the specimen surface down to the atomic-level (Müller & Bahadur, 1956), allowing to image lattice defects (Beavan et al., 1971; Dagan et al., 2015; Wilde et al., 2000) as well as secondary phases (Faulkner & Ralph, 1972). In atom probe experiments, no imaging gas is used, and it is the atoms from the specimen itself that are successively removed in the form of ions. The prevalent theory of field evaporation from metal surfaces involves atoms escaping while their charge is progressively drained back into the surface (Forbes, 1995), and as the singly-charged ion is accelerated away from the surface, it can undergo one or more successive electricfield-induced ionization (Kingham, 1982). In order to achieve time-control of the field evaporation process, high-voltage (Müller et al., 1968) or laser pulses (Bunton et al., 2007; Gault et al., 2006; Kellogg & Tsong, 1980) are superimposed to a direct current high voltage (VDC) field, as depicted in Fig. 1, to trigger the departure of the ions. This, in turns, allows for elemental identification of each evaporated ion by timeof-flight mass spectrometry. The emission of the ions takes place during the pulse, and in the case of the voltage-pulsed instrument, and although the pulse is in the nanosecond time range, ions are emitted with a range of energies, that cause a spread of the times-of-flight for ions of a single element, which limit the mass resolving power of the technique.