Local Electrode Atom Probe Tomography

Atom probe tomography (APT) is one of the most spectacular microscopies that exist. A three-dimensional (3D) image at near atomic scale is produced with single-atom sensitivity where each atom (actually each isotope) in the image is identified. Because the fundamental data format is the 3D position and identity of individual atoms in a volume containing potentially hundreds of millions of atoms, many types of information may be gleaned. Elemental concentration may be determined in any subvolume size or shape simply by counting atoms. Concentration profiles may be calculated in any direction, even radially through a spherical feature or normal to any defined surface. Isoconcentration surfaces can be set to delineate and measure interfaces. Interatomic distribution functions can be determined for studying ordering, dopant interactions, cluster formation, crystal structure, diffusion, and early stages of precipitation. Once the dataset is obtained, the quality and quantity of results derived are limited principally by the intrinsic quality of the data and the microscopist’s ability to direct a computer to extract the information.

In 1965 Müller, the father of the atom probe, once observed, “ … the specimen itself is also the image-forming “lens” and the imaging ion beams originate at the specimen surface” [1]. In other words, the specimen is the primary optic of the atom probe microscope, and as such, specimen preparation forms a critical step in a successful atom probe tomography (APT) analysis. Controlling the tip size and the shape is essential for understanding and manipulating the ionic trajectories from specimen surface to detector.

This chapter describes the LEAP instrument and its hardware components with various levels of detail. Following an introductory section, items are grouped as they are related to the local electrode, elements critical to detection (detection and imaging), transfer of specimens and materials though the various vacuum chambers (transfer and storage of consumables), voltage supplies and laser systems (Field Evaporation Systems), as well as ancillary systems.

Data collection for an atom probe tomography (APT) experiment is the process of (1) introducing an electric field on a specimen in order to initiate ion emission by field evaporation, (2) using the initial ion events to allow fine positioning of the specimen, and (3) increasing and adjusting the voltage (and thus the electric field) to obtain a steady rate of detected ion events. The details of how one can maneuver through these steps safely and efficiently are the focus of this chapter. Because a strong electric field is required to initiate field evaporation, the resulting forces create large stresses that act on the specimen, placing it in continual jeopardy of mechanical failure. Consequently, a successful atom probe experiment is a balancing act of minimizing the chance for specimen failure while at the same time extracting maximum quality from the collected data. After reading this chapter, it becomes clear that prioritize yield over data quality (one must have data before judgments can be made on its quality), but the ultimate decision on this issue rests with the user. Depending on the details of any particular experiment, the user may decide that either specimen yield or data quality takes precedence.

Data processing refers to taking raw atom probe tomography (APT) data and preparing it to be reconstructed into real-space data, while reconstruction takes processed experimental data and converts it into 3D spatial coordinates. Once this has been successfully accomplished, the data can be interrogated for useful nano-structural information.

LEAP data analysis is performed using the CAMECA Integrated Visualization and Analysis Software (IVAS) package. IVAS was introduced in 2004 on a Windows XP^® software platform based on Java™ and Netbeans and continues to require a Windows operating system environment. CAMECA offers several levels of licensing from free and short term use to permanent full versions. Through the years, IVAS development has closely followed a guiding principle that prioritizes ease of use and availability of unencrypted file formats so that analysis can be performed both with IVAS as well as user-developed software. Discussing all the features and concepts pertaining to IVAS would require a separate book (see the IVAS User Guide). Consequently, this chapter describes only some key IVAS topics and features useful for atom probe tomography (APT) practitioners. The features and topics addressed are in no particular order, but each should have some relevance for experienced and novice analysts alike.

Over the past decade there has been a substantial expansion of the applicability of atom probe tomography (APT) to materials of all types. This expansion has been spurred on by major instrumental developments resulting in the achievement of high data collection rates, large fields of view, and renewed utilization of laser pulsing (see Chap.​ 3). Furthermore, the advent of focused ion beam (FIB) methods for specimen preparation (see Chap.​ 2) in APT has had an equally profound impact. FIB-based methods have not only made it possible to create a LEAP specimen from nearly any bulk material type, but also provided the capability to create specimens from specified regions of a sample and in nearly any orientation. At the turn of the century, random site specimen creation was the norm and site-specific specimen creation was a time-consuming process involving many handling steps in conjunction with electropolishing and electron microscopy. Today, FIB-based site-specific specimen creation is routine and electropolishing is often only used when it is more practical. While the use of thermal pulsing is necessary for materials that cannot be evaporated successfully with field pulsing, thermal pulsing also has been found to improve yield for a range of metal applications as well. Certain metal specimens, even high-strength steels, that will not run with adequate yield in voltage pulsing have been found to run with much higher success rate in laser pulsing. With these major advances, applications of APT have blossomed.