Understanding Three-Dimensional Atom Probe Tomography (APT) in One Article

Three-Dimensional Atom Probe (APT), also known as Atom Probe Tomography (APT) or Three-Dimensional Atom Probe (3DAP), is currently the only material analysis technology capable of mapping and precisely measuring chemical composition on a near-atomic scale (z-direction resolution approximately 0.1-0.3 nm, x and y-direction resolution approximately 0.3-0.5 nm). It can accurately characterize the distribution and chemical composition of elements within materials on a near-atomic scale, such as solid solutions, short-range ordering, clusters, nano-precipitates, defects like vacancies/dislocations, and interfaces.

Principle of APT

Atom probe is based on the principle of field emission. Under conditions of ultra-high vacuum and liquid nitrogen-cooled samples, a sufficiently high positive voltage is applied to the sample tip, causing surface atoms to ionize and leave the tip surface. This process is called field emission. By applying an electric field between the sample and a nearby electrode, atoms on the material surface are sequentially removed layer by layer under voltage or laser pulses, and the ejected ions fly toward a detector after passing through a DC electric field. By analyzing the flight time of ions, the mass-to-charge ratio of each ion can be determined. By analyzing the position and order of arrival of ions at the detector, their original positions can be deduced. Through software reconstruction, the three-dimensional spatial distribution of atoms in the material can be restored.

Figure 1. Schematic Diagram of Three-Dimensional Atom Probe Principle

Advantages of APT

APT is currently the only characterization technique capable of obtaining elemental distributions on a three-dimensional scale. It was initially widely used in metallic materials and later extended to semiconductor materials, geological materials, and biological materials. It has the following advantages compared to other characterization techniques:

(1) High characterization limit, typically detecting elements at the ppm level;

(2) Provides information on the three-dimensional orientation of materials, not limited to a single plane;

(3) Can perform point testing of small areas of samples through focused ion beam (FIB) milling;

(4) Can analyze precipitates, interfaces with elemental segregation, defects like vacancies/dislocations, and cluster distributions, enabling quantitative analysis of element distribution and content in materials.

APT Sample Preparation

APT analyzes samples in the form of needle tips, with tip radii typically ranging from 10 to 100 nm. The quality of sample preparation directly affects the quality of the data. Due to the collection of samples under a high electric field (1-10 kV), the samples are subjected to strong stress, which is related to the size of the electric field and the tip radius of the sample. A larger diameter can cause excessive electric field and stress, leading to sample fracture and ejection, making it impossible to continue collecting data. There are generally two methods for APT sample preparation: electrochemical polishing and focused ion beam (FIB) milling.

For non-conductive samples and special samples requiring precise positioning (such as sampling crystal boundaries and precipitate phases), Focused Ion Beam (FIB) technology is typically used for processing. This involves extracting a wedge-shaped sample (side view) from the sample surface using nanomanipulators and Ga ion milling, and then welding the sample onto a substrate using gases such as Pt, W, or C deposited by gas deposition. Subsequently, circular milling is performed using ion beams to fabricate needle-tip samples with dimensions below 100 nm. The FIB sample preparation process is illustrated in Figure 3: (a) determining the sampling position, (b) excavating a trench around the sampling position, (c) depositing platinum on top of the sample for protection, (d) preparing the sample stage for placing the needle-tip sample, (e) placing the excavated sample column into any center of the sample stage, (f) side view, (g) welding the sample column and needle together with platinum and cutting off the excess, (h) completed morphology, (i) side view morphology, (j) top view of the sample, starting thinning, (k) final needle-tip sample prepared.

Figure 2. Schematic Diagram of FIB Sample Preparation

APT Analysis

APT data analysis is an important process for in-depth exploration of data, facilitated by IVAS software. IVAS software is specifically designed for analyzing APT data, allowing the reconstructed and visualized atoms detected by the detector to be presented. APT analysis mainly includes calculations of Image Compression Fraction (ICF), grain boundary orientation difference, mass spectrometry calibration, atomic nearest neighbor distribution analysis, precipitate phase composition analysis, cluster analysis, and one-dimensional line concentration analysis of interfaces.

References

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[2] Wu, Q. (2019). Study on the evolution law of microstructure of Al-Cu alloy during high-pressure torsion processing (Doctoral dissertation, Nanjing University of Science and Technology). DOI: 10.27241/d.cnki.gnjgu.2019.001581.

[3] Wu, X. (2020). Analysis of the distribution law of doping elements in FinFET using three-dimensional atom probe (Doctoral dissertation, Nanjing University of Science and Technology). DOI: 10.27241/d.cnki.gnjgu.2020.001314.

[4] Jiang, Y. (2019). Influence and mechanism of microstructure on hydrogen embrittlement sensitivity of high-strength steel (Doctoral dissertation, University of Science and Technology of China).

[5] Jiao, Z., & Was, G. S. (2010). Novel features of radiation-induced segregation and radiation-induced precipitation in austenitic stainless steels. Acta Materialia, 59(3).