Atom Probe

Atom Probe Tomography is a powerful microstructural technique using position-sensitive time of flight mass spectrometry. A sharp needle of the desired material is produced using either electropolishing or focused ion beam (FIB). This needle is loaded into the atom probe and pumped to better than 3×10^-11 Torr and cryogenically cooled to 20-70K. A high voltage is applied between the specimen tip and a local electrode. Then a pulse of either higher voltage or a laser beam is applied, enough to field evaporate 1 atom at a time into the detector. The time of flight between the pulse and the atom hitting the detector gives the mass/charge ratio and hence elemental species of the evaporated atom, and the position on the detector gives spatial information about the position of the atom within the needle. Over hours of analysis time and millions of evaporated atoms, a 3-dimensional atom-by-atom image of the chemical composition of the specimen can be reconstructed.