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Page 2 of 23 Zhou et al. Microstructures 2023;3:2023043 https://dx.doi.org/10.20517/microstructures.2023.38

INTRODUCTION
Atom probe tomography (APT) is an advanced microanalysis technique that offers a remarkable way to
[1-4]
visualize the distribution of atoms within materials at the nanoscale in three dimensions (3-D) . It boasts
an impressive sub-nanometer spatial resolution and a high chemical resolution. The latter enables it to
achieve precision levels of approximately 20 atomic parts per million (at. ppm) for all elements, regardless of
[2,5]
their atomic masses . The combination of these unique abilities allows the research on the distributions of
elements in materials, including those with low atomic mass, such as hydrogen, carbon, and lithium, which
[6-8]
are difficult to analyze using conventional optical or electron microscopes .

In 1955, Müller made a significant breakthrough by developing and utilizing a field ion microscope
(FIM), which laid the groundwork for the APT technique . Using the FIM, they observed tungsten atoms
[9]
on a tungsten tip surface and achieved the world’s first atomic-scale image with an unprecedented
magnification of over 10 times . Later, in 1968, Müller et al. further developed the atom-probe FIM ,
[10]
6
[1]
integrating a mass spectrometer, which enabled the measurement of the time-of-flight (ToF) of evaporated
ions and their mass-to-charge ratios (m/z). This addition provided a chemical resolution to complement the
[12]
[11]
high spatial resolution of the FIM . In 1970, Brenner and McKinney also made a significant contribution
to the development and application of FIM and APT for the advancements in surface science.
Over the following decades, the continuous improvement in computational power facilitated computer-
assisted experimentation, leading to the development of modern APT instruments. The automation of ion
evaporation experiments, data acquisition, and tomographic reconstruction became possible. The first
commercial APT instrument was developed by the research group at the University of Oxford in the early
[13]
1990s , which later led to the foundation of Oxford Nanoscience Ltd. with their products of Position-
Sensitive Atom Probe and the subsequent Energy-Compensated Position-Sensitive Atom Probe. In 2001,
Imago Scientific Instruments introduced the commercial APT module, making the APT technique more
accessible to researchers . In 2006, Oxford Nanoscience was acquired by Imago Scientific Instruments.
[14]
Subsequently, AMETEK acquired Imago Scientific Instruments in 2010 and merged it with CAMECA,
which became the sole APT instrument manufacturer to date. Since Müller’s pioneering work in the 1950s,
APT has found extensive use in various fields of research and development. Figure 1 depicts the increasing
number of APT-related publications by year in the Web of Science database, reflecting the growing
adoption of APT analyses. This technique has made significant contributions to advancements in materials
[15]
[21]
[17]
[20]
[16]
science [7,15] , biology , chemistry , physics [18,19] , geoscience , medicine , and energy science .
As depicted in Figure 2, to induce this field evaporation, a needle-shaped specimen with a tip diameter less
than 100 nm is necessary. The required sharp tips can be created using either electropolishing or focused
[22]
ion beam (FIB) annular milling techniques . In the APT process, a high voltage of 2-15 kilovolts (kV) is
applied to the needle specimen, generating a powerful electric field at the apex of the tip. This electric field is
strong enough to strip the outer-shell electron(s) from the atoms at the apex and ionize them. Once ionized,
the charged particles are directed by the electric field toward a grounded, time-sensitive detector. The time
taken for the ions to travel from the evaporation point to the detector gives the ToF information. To
precisely determine the timing of the evaporation process, controlled pulsing with a marginal energy input
is applied to the APT tip specimen. The tip is at a constant high voltage of several thousand volts, which is
close to the threshold of the level required for field evaporation, and the ion evaporation only takes place
when the additional pulsing energy is applied. This energy pulsing can be achieved by using either of the
two methods -voltage pulsing and laser pulsing using a focused beam shed on the apex of the specimen,
which leads to voltage mode and the laser mode of modern APT, respectively .
[23]