Atom probe tomography (APT) is a nanoscale materials analysis technique that provides 3D (three-dimensional) spatial imaging and chemical composition measurements with high sensitivity.
The technique relies on ionization and subsequent field evaporation of individual atoms/atomic clusters from a specimen surface. The specimen is prepared in the form of a conical tip with an apex radius < 100 nm. The field evaporation occurs as a result of a base DC voltage bias (typically between 1-10 kV) and a pulsed voltage (conductive samples) or pulsed laser (semiconducting and insulating samples). The specimen is also cryogenically cooled down to between 25-80 K to suppress thermal lattice vibrations and improve on spatial positioning.
2D maps (Device level): These are used to provide information about the device structure and to correlate with other techniques.
Field evaporated ions are collected onto a position sensitive detector (PSD) to accurately identify x and y position. The sequence of ions collected is used for z position. The x, y and z together provide the spatial imaging of the specimen tip. The time of flight of the ions is also measured and converted to a mass to charge ratio for chemical composition measurements. The spatial resolution is up to 0.3 nm in depth and 0.5 nm laterally, which is limited to metallic samples. The resolution for semiconducting and insulating samples is lower, typically ~ 1 nm at best. The chemical sensitivity for APT is 10 ppm.
With analysis areas that are ~ 30-50 nm in x and y and between 100-500 nm in z based on material type, this technique is particularly useful for cases where [a] 3D structures/buried interfaces are present, [b] low atomic number (Z) elements are present, either as dopants or in the bulk and [c] nanometer sized clusters are part of the structure. While APT does not provide the same chemical sensitivity as SIMS (secondary ion mass spectrometry), it enables identification of elements within 3D structures that is not possible with SIMS. It also provides higher chemical sensitivity than TEM/STEM (scanning/ transmission electron microscopy) related techniques such as EDX (energy dispersive X-ray spectroscopy) and EELS (electron energy loss spectroscopy). 3D Compositional analysis of nanoscale clusters is routinely carried out using APT, as it is not possible to measure using TEM/STEM.
The LEAP 5000XR at Nanolab is currently the latest in the instrument capability with highest detector efficiency, higher signal to noise ratio and larger volumes captured and analyzed compared to older versions of the instrument. These enable capture of low concentration elements in 3D that are not possible to measure using other techniques.
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