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The atom probe made one-dimensional compositional maps by combining time-of-flight spectroscopy and field ion microscopy (FIM). The instrument allows the three-dimensional reconstruction of up-to hundreds-of-millions of atoms from a sharp tip (corresponding to specimen volumes of 10,000-1,000,000 nm3).
As in FIM, a sharp tip is made, placed in ultra high vacuum at cryogenic temperature (typically 20-100 K). A region of the tip's surface is selected (sometimes from an FIM image) and placed over a "probe hole" by moving the tip. The atoms at the apex of the tip are ionized, either by a positive pulsed voltage or a laser. These ions are repelled from the tip electrostatically and those passing through the probe hole reach a detector. A fast timing circuit is used to measure the time taken between the pulse and the impact of the ion on the detector, thus allowing the mass-to-charge ratio of the ion to be calculated and; therefore, the corresponding element (or elements) to be identified. From the collection of many of these ions, a chemical profile of the sample can be made with relative position accuracy of less than one atomic spacing.
The imaging atom probe (IAP), invented in 1974 by J. A. Panitz, decreased the need to moving the tip. In the IAP, ions emitted from the surface are recorded and mass analyzed at a detector placed within 12 cm of the tip (to provide a reasonably large field of view). By "time-gating" the detector for the arrival of a particular species of interest its crystallographic distribution on the surface, and as a function of depth, can be determined. Without time-gating all of the species reaching the detector are analyzed.
Atom-probe tomography (APT) uses a position-sensitive detector to deduce the lateral location of atoms. This allows 3-D reconstructions to be generated. The idea of the APT, inspired by J. A. Panitz's patent, was developed by Mike Miller starting in 1983 and culminated with the first prototype in 1986. Various refinements were made to the instrument, including the use of a so-called position-sensitive (PoS) detector by Alfred Cerezo, Terence Godfrey, and George D. W. Smith in 1988. This PoSAP was commercialized by the developers. Since then, there have been many refinements to increase the field of view, mass and position resolution, and data acquisition rate of the instrument. Imago Scientific Instruments (Madison, WI) and Cameca (France) are now the sole commercial developers of APTs.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Atom_probe". A list of authors is available in Wikipedia.|