APT is a technique providing quantitative three-dimensional elemental analysis of solids at the atomic scale. The method utilizes the phenomenon of field evaporation, whereby atoms are successively ionized and removed from the surface of a needle-shaped specimen and further accelerated toward the position-sensitive detector. The required electric field is created by combination of standing voltage combined with either voltage or nanosecond laser pulses, enabling characterization of materials with a wide conductivity range (from metals to insulators). Software reconstruction algorithms utilize the detector hit positions and recorded time-of-flight of ions to restore 3D atomic distribution inside the specimen with sub-nanometer accuracy.
LEAP 4000X HR, Cameca Scientific Instruments (Madison, WI, USA).
Features and capabilities:
A typical field-of-view is about 100 × 100 nm with analyzed depth is a few hundreds of nm. The corresponding analysis volume contains a few millions to a few hundreds of millions of atoms.
Spatial resolution: lateral down to 0.2 nm, depth down to 0.1 nm. Mass resolving power m/m > 1000.
Elemental sensitivity down to 10 ppm with equal sensitivity to all elements in the periodic table (hydrogen quantification is challenging due to technical limitations). No standard reference samples required.
The combination of its characteristics makes APT a unique method especially for (but not limited to) the quantitative analysis of elemental clustering and segregation at buried microstructural features such as dislocations and grain boundaries.
Materials must withstand stresses up to 1 GPa during the analysis due to strong electrostatic pressure. Some “weak” materials undergo premature sample fracture and cannot be analyzed.
APT samples are basically prepared using focused ion beam methods. Materials must thus tolerate ion irradiation with energies of at least a few keV without unacceptable structural changes in the region of interest. Protective coatings can be however used to minimize ion-induced damage.
Most organic and, in particular, biological materials are not suitable for APT analysis, with exception of limited material classes (e.g. bone tissues).
Despite high spatial resolution, no crystallographic information can be retrieved from APT measurements, with exceptions of highly ordered conducting materials (e.g. intermetallics). Complementary techniques such as TEM should be applied for that purpose.