3. From Field Desorption Microscopy to Atom Probe Tomography

As mentioned in Chap. 1, atom probe tomography (APT) exploits the principle of field evaporation to successively remove those atoms at the apex of a needle-shaped specimen. Field evaporation involves the ionisation of these surface atoms, whereupon they are subjected to an electric field force that causes them to accelerate towards a detector under a particular projection. The evaporation event follows immediately after the ionisation of the surface atoms, and this ionisation is induced by the combined effects of a standing (DC) electrostatic field and either high-voltage or laser pulses transmitted to the surface atoms of the specimen. The first technique that exploited this process was field desorption microscopy (FDM), which subsequently evolved into the modern atom probe. A general summary of field evaporation theory is first presented, before a more detailed description of FDM and APT and the specific pulsing techniques.