The combination of a field ion microscope with a time-of-flight mass spectrometer provides the capability for chemical microanalysis at the single atom level. Such an instrument is termed an Atom Probe. Conventionally, the connection between the microscope and the mass spectrometer is made via a small aperture hole in the imaging screen. This defines a region on the specimen, typically about 2nm across, from which the analysis is obtained. The disadvantage of this arrangement is that other regions of the specimen cannot be examined, as ions from all but the selected area strike the image screen and therefore do not pass into the mass spectrometer. In order to overcome this problem, we have developed a version of the Atom Probe which incorporates a wide-angle position sensitive detector system. This instrument, which we have termed the POSAP, is shown schematically in figure 1. Typically, the field of view in this instrument is about 20nm across. The number of ions collected per atom layer removed from the specimen surface is therefore approximately 5,000.
Atomic Electron Tomography: Probing 3D Structure and Material Properties at the Single-Atom Level
