From Northwestern University Center for Atom-Probe Tomography
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In 1988 Cerezo, Godfrey and Smith at the University of Oxford applied a position sensitive detector to a time-of-flight atom probe, thus determining both the mass to charge ratio and the position of the ions simultaneously. This new instrument, which they called the position sensitive atom probe (PoSAP), enabled two dimensional element mapping with a subnanometer spatial resolution. Reconstruction of a series of two dimensional maps makes it possible to reconstruct a three dimensional element map, and hence this new generation of atom probes are generally called three dimensional atom probe (3DAP).

Details about the 3DAP (poster by Jason Sebastian)

By measuring the time-of-flight and the coordinates of ions using a PSD, it is possible to map out two dimensional elemental distribution with a near-atomic resolution. The lateral spatial resolution is limited by the evaporation aberration that occurs during the ionization of atoms on the surface. However, the error originated from the evaporation aberration does not exceed 0.2 nm, and this is still the lowest of the errors achieved by any existing analytical instruments. The elemental maps can be extended to the depth direction by ionizing atoms from the surface of the specimen continuously, by which atom distribution can be reconstructed in a three dimensional real space. Since field evaporation occurs layer-by-layer in low index planes, the reconstructed 3D elemental map shows the layers corresponding to the atomic planes in the depth direction.