Techniques for Transmission EBSD Mapping of Atom Probe Specimens

Abstract

Transmission Electron Back Scatter Diffraction (t-EBSD)/Transmission Kikuchi Diffraction (TKD) enables crystallographic identification in samples of much smaller size, and with improved spatial resolution, as compared to conventional reflection EBSD. In t-EBSD, the elimination of a severe tilt angle and the collection of primarily low-loss electrons keeps the probe small enough for mapping resolution down to approximately 2 nm [1]. Atom probe tomography (APT) relies on needle shaped specimens with an apex radius of ~50 nm in order to produce the critical electric field for evaporating ions. Such needle shapes are then ideally sized for characterization with t-EBSD. Atom probe data can provide information about segregation at grain boundaries. Combination of this chemical information with crystallographic information from t-EBSD provides a more complete picture of the chemical composition and the microstructure of the sample [2].Here we present strategies to optimize t-EBSD for atom probe applications, including sample holding fixtures and microscope parameters that enable the mapping of multiple specimens without the need to break vacuum. Strategies to avoid contamination of the sample during the mapping process will be presented and optimal map conditions for atom probe specimens will be discussed. Moreover, we will discuss challenges with the application of t-EBSD to the characterization of the inherently threedimensional specimens of interest for APT. Unlike conventional reflection EBSD, the mapped surface in transmission is the bottom surface [3], which can present unique difficulties in a specimen with multiple or overlapping grains that can cause overlapping or blurring of the Kikuchi patterns. We demonstrate this particular phenomenon in an APT specimen by mapping a sample with Atom Probe Assist™ mode with side-by-side grains and introducing a 90 degree rotation, such that only one pattern is visible. One critical element in successfully mapping an APT specimen is overcoming the FIBinduced Ga ion implantation which can cause loss of crystallinity in the region of interest [4]. Cleaning the surface with a low-energy ion beam [5] is demonstrated to significantly improve the image quality of the Kikuchi patterns. Figure 1 shows a typical t-EBSD map obtained from a nickel APT specimen. The inset demonstrates a typical Kikuchi pattern obtained from the specimen.In t-EBSD, like all transmission imaging and diffraction techniques, the sample affects the achievable spatial resolution because the beam broadens and loses energy as the electrons undergo multiple scattering events to reach the exit surface. Beam broadening is particularly important when mapping atom probe specimens, where sample thicknesses may range from tens to a few hundred nanometers. Beam size changes across the cone-shaped APT specimen can be modeled to a first approximation with Monte Carlo simulations of electron trajectories. Our models [3] provide estimates of beam size and effective resolution at each point. Figure 2 shows the electron dis...

Modern Focused-Ion-Beam-Based Site-Specific Specimen Preparation for Atom Probe Tomography

“…As the implementation of APT expands into new application areas, there is a continuing desire to validate or correlate data with other well-established techniques. This correlative information can be used to calibrate or constrain reconstructions (Arslan et al, 2008; Thompson et al, 2009; Cojocaru-Mirédin et al, 2011; Haley et al, 2011; Larson et al, 2011; Hartshorne et al, 2014; Xiong & Weyland, 2014; Grenier et al, 2015; Herbig et al, 2015; Lefebvre et al, 2015), but it can also provide information on grain orientation (Taheri et al, 2010; Cojocaru-Mirédin et al, 2011; Babinsky et al, 2014, 2015; Takahashi et al, 2014; Chen et al, 2015, 2016; Herbig et al, 2015; Rice et al, 2015, 2016; ) and locations of defects and phase boundaries which can improve site-specific ROI targeting and placement (Cojocaru-Mirédin et al, 2011; Babinsky et al, 2014, 2015; Hartshorne et al, 2014; Chen et al, 2015, 2016; Herbig et al, 2015; Rice et al, 2015, 2016). FIB-based specimen preparation certainly provides a means to characterize specimen shapes with scanning electron microscopy (SEM) during manufacture; however, the quality of the available information is often constrained by the diminishing electron-scattering volumes near the apex, which become vanishingly small for the majority of the analyzable region in a finished specimen.…”