Atom-probe field-ion microscopy (APFIM)

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.