The Time-of-Flight Atom-Probe and Its Application to Surface Analysis and Gas-Surface Interactions

Abstract

The time-of-flight (ToF) atom-probe field-ion microscope (FIM) is a micro- analytical tool with sensitivity capable of detecting single atoms [12.1–4]. It is a combination of a field-ion microscope and a time-of-flight mass spectrometer. The tip is mounted on either an external or an internal gimbal system, and a small probe hole covering a few atom image diameters is open at the screen assembly. Behind the probe hole is a ToF tube and an ion detector. To operate the atom-probe, the tip orientation is adjusted until the image of the atoms intended for identification falls on the probe hole. A nanosecond-duration high-voltage pulse is applied to field evaporate these surface atoms as well as others. However, only the field-evaporated ions of these surface atoms can pass through the probe hole, enter the flight tube, and reach the ion detector. From the flight times measured, the chemical identity of these atoms can be determined. This high-voltage pulse atom-probe is very useful for the microanalysis of materials, especially for metals and alloys where field evaporation proceeds from plane edges, atom by atom. Thus, atomic layer by atomic layer compositional analysis can be carried out, as will be discussed in Sect.12.2.2. There are, however, many insurmountable problems in applying this instrument to the study of gas-surface interactions. Adsorbed atoms or molecules often cannot be field desorbed without also field evaporating the substrate atoms.