The second revolution in atom probe tomography
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Introduction Atom probe tomography (APT) is one of the highest performance microscopies yet devised. A three-dimensional, atomicscale image is produced with single-atom sensitivity, where each atom (actually each isotope) in the image is identifiable. APT is used to study all types of inorganic materials and is gradually demonstrating capabilities for analysis of organic materials. Increasingly, these qualities make APT an essential tool for any modern materials characterization laboratory. There has been a dramatic improvement in atom probe instrumentation in the past decade that has been driven by the availability of vastly superior commercial instruments, most notably the local-electrode atom probe (LEAP). The atom probe was invented in 1967 at Pennsylvania State University by Müller, Panitz, and McLane1 as a one-dimensional analytical instrument that collected hundreds of atoms per day in 1 nm fields of view. There was only depth registry of atom positions in the data, no lateral positioning information, and the data collection required minutes per atom. By
1973, the origins of tomographic analysis were present in Panitz’s 10 cm atom probe.2 The first revolution came when a true three-dimensional imaging instrument was developed by Cerezo, Godfrey, and Smith3 that could record hundreds of atoms per minute in up to ∼15 nm wide fields of view. We consider the LEAP4 and related instruments such as the scanning atom probe5 combined with focused ion beam (FIB)-based specimen preparation6 to be the second revolution in the development of the atom probe. With a LEAP, millions of atoms per minute can be collected with 200 nm wide fields of view with high mass resolution (1/1500, full width at half maximum) on many types of materials. It is helpful to review the history of APT so that one can understand what the LEAP is and how it evolved from its predecessors. This article is intended to be historical in nature and focuses on the people and machines that have led to the current generation of atom probe instruments. In Figure 1, a flowchart of predecessor instruments in the lineage of the modern atom probe is presented. The history will be recounted below with reference to this chart.
Thomas F. Kelly, Cameca Instruments, Inc., Madison, WI; [email protected]. David J. Larson, Cameca Instruments, Inc., Madison, WI; [email protected]. DOI: 10.1557/mrs.2012.3
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MRS BULLETIN • VOLUME 37 • FEBRUARY 2012 • www.mrs.org/bulletin
© 2012 Materials Research Society
THE SECOND REVOLUTION IN ATOM PROBE TOMOGRAPHY
Figure 1. Ancestry of the local-electrode atom probe.
Ancestry of the local-electrode atom probe APT is the name of the technique. There are many types of instruments that may be one-dimensional atom probes (1DAP) or three-dimensional atom probes (3DAP). Some instruments that are 3DAPs were given names such as the position-sensitive atom probe (PoSAP)3 and tomographic atom probe (TAP).7 Instruments that feature a local electrode, which is a counter electrode for the specimen that produces higher electric field
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