Atom Probe Tomography (APT)
In atom probe tomography (APT), atoms are progressively removed from the surface of a sharp tip specimen by the process of field evaporation. By the application of a high DC voltage (~2-11 kV), a very intense electrostatic field is produced at the apex of the specimen tip, having a radius of curvature of a few tens of nanometres.
In a controlled manner, using voltage pulsing in the LEAP 4000HR instrument, the atoms are subsequently field ionised and then evaporated layer by layer towards a position-sensitive detector, where their chemical identity is determined by time-of-flight mass spectrometry. The evaporated volume is then reconstructed in three dimensions using an inverse projection reconstruction algorithm and the sequence of detected events. The result is a tomographic data set, typically spanning some hundreds of nanometres in depth and containing the spatial coordinates and elemental identities of tens to hundreds of millions of atoms with near atomic resolution.