Atom probe tomography (APT)

About this technique

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Atom probe tomography (APT) is a new generation characterisation technique that reports the position and chemical identity of atoms within a chosen material. It achieves this through ionising and removing atoms, one at a time, from a sharpened needle-shaped specimen and directing those atoms (now ions) through an electric field to a detector. The atom probe instruments are point-projection microscopes – the ions project in a near perpendicular direction from their original point on the surface of the specimen tip. As a result, atomic resolution may be clearly discerned, corresponding to a magnification of a few million times. The original position of an atom is determined by a position-sensitive detector and the chemical identity determined through time-of-flight mass spectroscopy. The physical mechanism of field-ionisation permits the removal of atoms.

Electrically conductive specimens are typically field-ionised by high-voltage pulse, whereas semi-conducting, or non-conducting specimens, are stimulated to field-ionisation through the use of laser pulses directed at the tip. During the process of data acquisition/field-ionisation the specimen is held at a temperature of approximately 20 K and is contained in an ultra-high vacuum chamber, around 1 x 10^-11 Torr. Conductive materials are commonly prepared for acquisition through electro-chemical polishing a specimen to a needle with an apex of approximately 100 nm. Non-conductive, and specimens with sights of specific interest, are prepared with a dual-beam focused-ion beam instrument. The technique of APT is typically applied to crystalline and amorphous metal alloys and semi-conductor devices but have more recently been used for hard biological samples such as tooth enamel and nacre. It has selective applicability to polymers and softer biological specimens and these sample types are at the frontier of atom probe research. Cryo transfer chambers have now been developed that allow visualisation of highly mobile elements such as hydrogen in materials but also open the way for development of protocols for atom probe analysis of cryogenically frozen biological specimens.

Following the acquisition of data, careful reconstruction ensures APT accurately reproduces a 3D volume of the atoms within the specimen with sub-atomic spatial resolution and the identification of any atom in the periodic table. This data is visualised as an interactive 3D matrix of atoms commonly 150 x 150 x 400 nm in dimension and 40 million atoms in size. Post-processing analysis reveals the richness of information available from this type of data as the quantitative metrics defining atomic clustering, geometries of structures and chemical environments is non-intuitive to visual observation. Investigating the effect of variables involved in treatment or processing parameters (thermal annealing time, composition alterations, etc.) with these metrics of atomic and chemical structuring yield unique and powerful insights into a material's behaviour and understanding of structure to property (e.g. mechanical or electrical) relationships.

It is now also possible to map the orientation of the crystalline structure of matter at the nanoscale in 3D by using atom probe microscopy. Previously directional differences in spatial resolution of atom probe data have made orientation difficult. However, modern detectors now make it possible to distinguish at least three crystallographic planes due to density variations in the atom probe data that correspond to the position of poles. This is sufficient information to determine the orientation of the grain. For this work 3D Hough transforms coupled with fast Fourier transforms are used to obtain crystallographic plane orientations. This technique is highly automated and has the potential to examine large datasets rapidly.