Atom Probe Tomography (APT)

Atom Probe Tomography

Atom probe tomography (APT) is an analytical technique that can provide images of individual atoms in a sample and enable their positions to be mapped, at nanoscale, in a three-dimensional spatial coordinate system.

The APT test specimen is prepared in the shape of a needle with a sharp conical tip of sub-micron diameter, cryogenically cooled and subjected to a strong electric field of several kilovolts in ultra-high vacuum. While the base voltage is DC voltage a pulsed voltage (for conductive samples) or pulsed laser (for semiconducting and insulating samples) enhances the applied energy. The high electric field at the sharp tip results in evaporation of atoms (as cations) from that location, a process termed field evaporation. The identity of ionic species is determined by time-of-flight Mass Spectrometry, while their original spatial coordinates are inferred using a position sensitive detector, coupled to a digital processing system. The spatial resolution for semiconductor specimens is about 1 nm while for metals it is possible to obtain resolutions of 0.3 nm in depth and 0.5 nm laterally. Depending on the conductivity of the specimen, the applied electric field may also be of voltage-pulse mode or laser assisted mode.

The ability to visualise spatial atom distributions at nano-scales and to distinguish between chemical species, has made APT a valuable analytical tool in the Materials science and semi-conductor industry. New applications of APT have been found in the geological field.

Common Uses of Atom Probe Tomography (APT)

• 3D Chemical mapping of materials at nano-scale
• Dopant Identification and quantification in semiconductors
• Crystal lattice and grain boundary studies
• Study of nanoscale exsolution or heterogeneous inclusions in crystalline materials

Advantages of Atom Probe Tomography (APT)

• All elements including isotopes can be identified
• Cluster formation and compositional variations can be analysed at nano-scale

Limitations of Atom Probe Tomography (APT)

• Challenges exist for low conductivity and insulating materials
• Specimen has to be in the form of sharp tip and this may influence evaporation behaviour thereby distorting the 3D
• Complex sample preparation
• Small sample volumes
• Ion overlap in some samples may result in analytical challenges.
• Maintaining Ultra high vacuum to ensure residual Low molecular weight gases (Hydrogen & Helium)  do not interfere.

Industrial Applications of Atom Probe Tomography (APT)

• Metallurgical Industry
• Semiconductor industry
• Geosciences
• Planetary sciences