NIST has recently begun a multi-year joint effort between the Material Measurement Laboratory (MML) and the Physical Measurement Laboratory (PML) aimed at revolutionizing 3D chemical mapping at the atomic scale by dramatically reducing the uncertainty in measured composition and improving the reconstruction process in atom probe tomography.
Subnanometer-resolved 3-D chemical mapping of any atom in any solid continues to be an unattainable goal of materials research. If possible, it would have a profound economic impact on U.S. industries collectively worth hundreds of billions of dollars, such as automotive, aerospace, microelectronics, renewable energy, advanced manufacturing, advanced electronics, nuclear storage, and biomaterials. All of these diverse, materials-intensive industries depend on microstructural features, elemental composition control, and chemical processes occurring at subnanometer dimensions. Thus, for process development, verification and qualification, quality assurance, failure analysis, and competitive engineering, U.S. industry has a very real and pressing need for the development of metrology tools that can provide subnanometer-resolved 3-D chemical maps of any elemental isotope in a complex heterogeneous nanostructure with high analytical sensitivity.
Conventional laser-pulsed atom probe tomography (L-APT) combines a point projection microscope with time of flight (TOF) mass spectroscopy, and operates by means of laser-assisted thermal field ion evaporation. A high standing voltage is applied between the specimen and detector and held just below the threshold for ionic field evaporation. A pulsed laser imparts a weak thermal transient that triggers field ion evaporation, progressively deconstructing the specimen, ideally one atom at a time. The ions are collected on a 2-D position sensitive detector and isotopically identified by their TOF. 3-D “reconstructions” are then computed using a sequential back projection algorithm. APT can in principle detect chemical concentrations as low as tens of ppm while simultaneously rendering 3-D isotopically identified chemical maps of any element (including hydrogen) with subnanometer spatial resolution.
Even though L-APT is the state-of-the-art for 3D chemical mapping at the nanoscale, it suffers from several fundamental limitations that drastically restrict its performance and utility, can lead to high uncertainty in measured composition, hinder the absolute quantification of its data, and prevent a high-fidelity reconstruction of the original specimen. The extreme atom probe tomography project at NIST aims to address some these limitations by improving on the mechanism of thermal field ion evaporation and specimen reconstruction.