Overview
Atom probe samples are shaped to implicitly provide a highly curved electric potential to induce the resultant magnification, as opposed to direct use of lensing, such as via magnetic lenses. Furthermore, in normal operation (as opposed to a field ionization modes) the atom probe does not utilize a secondary source to probe the sample. Rather, the sample is evaporated in a controlled manner (field evaporation) and the evaporated ions are impacted onto a detector, which may be up to several meters from the specimen.
The samples used in atom probe are usually a metallic or semi-conducting material, with the needle geometry produced by electropolishing, or focused ion beam methods. Preparation is done, often by hand, to manufacture a tip radius sufficient to induce a high electric field, with radii on the order of 100 nm.
To conduct an atom probe experiment, such a needle is placed in an ultra high vacuum chamber. After introduction into the vacuum system, the sample is reduced to cryogenic temperatures (typically 20-100 K) and manipulated such that the needle's point is aimed towards an ion detector. A pulsed high voltage source (typically 1-2 kV) is generated and applied to the specimen, with pulse repetition rates in the hundreds of kilohertz range. The application of the pulsed voltage to the sample allows for individual ions at the sample surface to have their electric field, and hence atomic bonding, temporarily disrupted. This results in ejection of an ionised atom from the sample surface at a known time. The delay between application of the pulse and detection of the ion allows for the computation of a mass-to-charge ratio.
Whilst the uncertainty in the atomic mass computed by time-of-flight methods in atom probe is sufficiently small to allow for detection of individual isotopes within a material this uncertainty may still, in some cases, confound definitive identification of atomic species. Effects such as superposition of differing ions with multiple electrons removed, or through the presence of complex species formation during evaporation may cause two or more species to have sufficiently close time-of-flights to make definitive identification impossible.
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