A short introduction to Atom Probe Tomography

Field ion microscopy (FIM), invented in 1951, utilizes a high electric field to emit tunnelling electrons from a sharp tip cathode, creating an image on a phosphor screen. The resolution is limited due to quantum effects and lateral variations in electron velocity. When the tip is cooled by a cryogen and its polarity is reversed, introducing an imaging gas field ionizes gas ions, producing a projected image of protruding atoms at the tip apex (Müller, 1951). FIM laid the groundwork for atom probe tomography.

The atom probe was first introduced in 1967 by Erwin Wilhelm Müller and J. A. Panitz. They combined FIM with a mass spectrometer capable of single particle detection. For the first time in history, an instrument was capable of detecting the nature of a single atom of a metal surface, selected from neighbouring atoms at the discretion of the observer (Müller et al., 1968).

The 10-cm Atom Probe, invented in 1973, streamlined atom-by-atom analysis by combining a time-of-flight mass spectrometer with a dual-channel plate detector, eliminating previous design challenges (Panitz, 1973) .

The Imaging Atom-Probe (IAP), introduced in 1974, departed from previous atom probe philosophy by focusing on determining the complete crystallographic distribution of a surface species of preselected mass-to-charge ratio, rather than attempting to identify individual surface species (Panitz, 1974).

Modern Atom Probe Tomography (APT) utilizes a position-sensitive detector to deduce the lateral location of atoms. Developed starting in 1983 and culminating in the first prototype in 1986, refinements such as the use of a position-sensitive (PoS) detector and the introduction of the Tomographic Atom Probe (TAP) in 1993 have improved resolution and data acquisition rate (Cerezo et al., 1988; Blavette et al., 1993) . Commercialization by Oxford Nanoscience and CAMECA followed, with subsequent refinements leading to increased field of view, mass, and position resolution. The introduction of the pulsed laser atom probe (PLAP) in 2005 expanded research avenues to include semiconductors and insulating materials (Bunton et al., 2006).

Initially focused on metals, APT applications have expanded to semiconductors, ceramics, and even biological and geological materials. Advanced studies, such as analysing the chemical structure of biological materials like chiton teeth, have provided valuable insights into their composition and structure.

How it works

The analysis of samples using atom probe tomography requires an ultra-high vacuum, with pressures ranging between 10^-8 and 10^-9  Pa. The sample is polished into a very sharp tip and cooled to cryogenic temperatures ranging between 20 and 80 K. The sample is mounted on a stage, capable of three-dimensional movement, and positioned in front of a counter-electrode. The counter-electrode is connected to a direct high-voltage current, with voltage between 1 and 15 kV. As a result, the sharpness of the tip and the high voltage induce an electrostatic field of up to tens of Volts per nm. Then, laser or high voltage pulsing will remove ions from the sample surface on at a time. These released ions are projected onto a position-sensitive time of flight detector, which makes 3D imaging and characterisation of your sample on atom scale possible (Gault et al., 2021).