The Conventional One-Dimensional Atom Probe

An FIM may be modified so that the imaged atom chosen for analysis can be positioned over a small aperture in the phosphor-coated screen. If the electric field is raised to a sufficiently high value, material may be removed from the surface by field evaporation. The specimen is subjected to a high-voltage pulse, which causes a number of atoms on the specimen surface to field evaporate as positive ions. Only the atom that was imaged over the aperture (or probe hole ) passes into a time-of-flight mass spectrometer, all the other atoms being blocked off by the screen. The applied 

The mass-to-charge ratio, m/n, of those ions that pass through the probe aperture and are analysed in the mass spectrometer is calculated from the equivalence between the potential energy of the atom on the specimen surface at voltage V0, and the kinetic energy that the atom acquires during acceleration to the grounded 

Since only one or two charge states are observed for any element, the elemental identity of each ion is determined from its mass-to-charge ratio by consulting a table of known isotope abundances. 

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Figure 1.4. Schematic diagram of the basic principles of the conventional (one-dimensional) atom probe, with a reflection energy compensator. (Reproduced by permission of Hono 1999.)...

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