APT: Atom Probe Tomography
Definition:Atom probe tomography [1] is a destructive characterization method that enables measuring the three dimensional (3D) distribution of elements at near-atomic spatial resolution with a chemical sensitivity, equal for all elements, in the range of a few ppm. Typical measurement volumes are 50 nm × 50 nm × 200 nm. The technique is ideal for measuring nano precipitates, the chemistry of an interface, as well as the segregation in the nm scale.
Explanation:APT samples are in a needle-shape, which is generally prepared by electrochemical methods or focused ion beam (→FIB) milling. The measurement is performed between 20 and 100 K under a ultra-high vacuum condition. A high base voltage is applied between sample and a hollow-cone-shaped local electrode which is placed closely above the tip apex. At frequencies of 100–250 kHz, either the voltage is raised cyclically (voltage mode, more accurate, but less sensitive to the samples) or the apex is locally heated by a laser (laser mode, for materials that tend to fracture in the voltage mode) just above the threshold of field evaporation of ions from the tip apex where the curvature is the highest and thus, the electric field is the strongest. In the ideal case, one individual atom is ionized every 100-1000 pulse and accelerated towards a detector where its position and time of flight are measured. By converting the time of flight to a mass to charge ratio, the ions can be identified. The additional positional information is combined with the information on the sequence of the detector hitting events, which enables to reconstruct the 3D distribution of the atoms in the sample.
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Figure 1: 3D atom map of an Fe-Mn-Al-C alloy. The carbon-rich precipitates are highlighted using 9 at% carbon isoconcentration surfaces.
SFB-Link:In project C8, we have developed an experimental approach for performing correlative transmission electron micrography on atom probe tips, and investigated the potential existence of Mn-C clusters as well as the segregation to nano-twins.
References:[1] T. F. Kelly and D. J. Larson, Annual Review of Materials Research, Vol 42 42, 1 (2012)