Atom Probe Tomography (APT)

Atom Probe Tomography (APT) is a technique that provides both three-dimensional (3-D) tomographic images and chemical identification at the atomic scale. This technique combines both high spatial resolution and high chemical sensitivity. The spatial resolution is 0.3 nm in depth and 0.5 nm in lateral. The chemical sensitivity is tens of ppm for most of the elements.  

The sample needs to be prepared into a needle shape with the tip radius of 50-100 nm at the apex. The cryogenic cooled needle specimen (25-100K) is biased with DC voltage (2-15 kV). The resulting electrostatic field (tens V/nm) at the apex of needle, in combination with pulsed laser or pulsed voltage, triggers the evaporation of one or more atoms from the surface of the apex. The position of each detected ion is recorded using Position Sensitive Detector (PSD), while the chemical identity is calculated based on the time of flight.

Ideal Uses
  • Map the distribution of elements in 3-D nm-scale structure, including thin films, particles, interfaces
  • Characterize nanoscale structure: the size, composition, and distribution of clusters/precipitates
  • Map and quantify the dopant elements in nm-scale volume
Technical Specifications
  • Signals Detected: positive charged ions
  • Elements Detected: H-U
  • Detection Limits: ~10 ppm
  • Imaging/Mapping: Yes
  • Ultimate Lateral Resolution: 0.5 nm in lateral and 0.3 nm in depth
Strengths
  • High chemical sensitivity (tens ppm)
  • 0.3-0.5nm image resolution
  • Small volume dopant mapping
  • Light element (e.g. Li and B) detection and mapping
Limitations
  • Significant sample preparation time (2-6 hours)
  • Small sampling volumes. Typical analyzed volume is 100-200 nm in diameter and a few hundred nm in depth
  • Some materials are not stable in the high electric field

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