This study discusses atom probe tomography (APT) and the feasibility of using a novel script-controlled focused ion beam-scanning electron microscopy (FIB-SEM) system for tip fabrication to improve the reliability and reproducibility of the APT technique.
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APT has gained significant prominence for microstructural analysis of different alloys and devices, such as semiconductors and insulators, due to the development of a site-specific tip preparation method using a FIB with an SEM system and extensive utilization of a laser-assisted field evaporation method.
The three-dimensional (3D) distribution of atoms in materials can only be visualized using the APT technique. Although the volume size can be analyzed at the nanoscale using APT, the technique possesses a high chemical sensitivity with quantitative analytical capability and high spatial resolution close to atomic resolution.
In APT analysis, voltage or laser pulses are applied on the tip/needle-shaped specimen under a high electric field to ionize atoms from the tip surface while measuring the position and mass of those ions simultaneously using a position-sensitive detector.
Specifically, the use of laser pulses in laser-assisted field evaporation has substantially reduced the tip fracture frequency during APT measurement, which has been a major challenge in APT analysis. Thus, the laser-assisted field evaporation significantly improved the measurable volume size and analysis yield.
Studies have also demonstrated that the use of femtosecond laser pulses is more effective compared to nanosecond voltage pulses in improving mass resolution. However, the laser-assisted evaporation can result in a mass tail issue in the mass spectra, which can adversely affect the mass resolution.
Effect of the APT Tip Shape on the Reproducibility of the APT Technique
The mass resolution in the laser mode depends on multiple factors, including the thermal conductivity of the material, laser irradiation conditions, and the tip shape. Although commercial instruments such as CAMECA LEAP instruments allow extremely accurate control of measurement/analysis conditions such as laser power, evaporation rate, beam position, and wavelength, the tip fabrication for APT analysis still depends on the experience and skill of the FIB-SEM operator.
Moreover, maintaining a constant tip shape is crucial to obtain the tomography data with good reproducibility, as the analytical volume primarily depends on the tip shape. These factors necessitate the development of a novel method to reproducibly prepare tips to improve the reliability and reproducibility of the APT data.
In recent years, several studies have demonstrated that a scripting language can automatically control FIB-SEM systems, and samples for scanning transmission electron microscopy (S/TEM) can be prepared using an automated/semi-automated method.
Automated APT Tip Shape Fabrication Using a Script-Controlled FIB-SEM System
In a study recently published in the journal Ultramicroscopy, researchers have developed a novel method to fabricate tips in a desired shape automatically with high reproducibility and accuracy using a script-controlled FIB-SEM system.
A Thermo Fisher Scientific Helios 5UX FIB-SEM system equipped with an AutoScript 4 system was employed to fabricate the APT tips. The AutoScript 4 system can control the FIB-SEM system using a Python-based scripting language.
Python 3.6-based scripting allows the control of different SEM and FIB conditions, image acquisition, stage movement, and processing fabrications without any operator interactions. Researchers performed the lift-out process manually to control the tip shape in place of using the semi-automatic lift-out method.
Researchers performed APT analyses using the fabricated tips with different shapes to investigate the effect of the tip shape on the APT data. CAMECA LEAP 5000XS was utilized for APT analysis with a 355 nm wavelength picosecond laser pulsing mode at 500 kHz repetition rate with 100 pJ laser pulse energy at 50 K specimen base temperature.
The detection rate was maintained at 2.0 %/one ion per 50 laser pulses. All tips were mounted on silicon support with platinum FIB deposition, and the sample height/distance from the tip apex to the silicon support was approximately 3.0 µm. CAMECA AP Suite Software 6.1 was employed to analyze the obtained APT data.
Three alloys, including iron-based nanocrystalline soft magnetic material, commercial SUS304 stainless steel, and Corson alloy, were used in the study.
The properties of these alloys primarily depend on the nanoscale 3D structure, which makes APT a suitable tool for understanding the relationship between their microstructures and their properties.
Additionally, stainless steel consists of several trace elements and is a suitable material for comparing the differences in chemical sensitivity and mass resolution in APT analysis.
Researchers successfully fabricated the APT tips with three different shapes from a Fe-based nanocrystalline soft magnetic material, Corson alloy, and stainless steel using the novel script-controllable FIB-SEM system.
In the laser mode, the mass resolution differed significantly based on the tip shape, even when the tips from the same alloy were used under the same measurement condition. Both calculation and experimental results indicated that the automated tip fabrication method can effectively improve the reliability and reproducibility of the APT technique.
To summarize, the automated fabrication of APT tips using a script-controlled FIB-SEM system can play a crucial role in reproducibly fabricating tips with specified shapes without manual intervention and improve the overall reliability of the APT technique.
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References and Further Reading
Uzuhashi, J., Ohkubo, T., Hono, K. (2023). Development of automated tip preparation for atom probe tomography by using script-controlled FIB-SEM. Ultramicroscopy, 247, 113704. https://doi.org/10.1016/j.ultramic.2023.113704
M. Kodzuka., T. Ohkubo., K. Hono. (2011) Laser assisted atom probe analysis of thin film on insulating substrate. Ultramicroscopy, 111, 557. https://doi.org/10.1016/j.ultramic.2010.11.008
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