On the depth resolution of transmission Kikuchi diffraction (TKD) analysis

Abstract

In this paper, we have analysed the depth resolution that can be achieved by 'on-axis' transmission Kikuchi diffraction (TKD) using a Zr-Nb alloy. The results indicate that the signals contributing to detectable Kikuchi bands originate from a depth of approximately the mean free path of thermal diffuse scattering (λTDS) from the bottom surface of a thin foil sample. This existing surface sensitivity can thus lead to the observation of different grain structures when opposite sides of a nano-crystalline foil are facing the incident electron beam. These results also provide a guideline for the ideal sample thickness for TKD analysis of ≤ 6λTDS, or 21 times the elastic scattering mean free path (λMFP) for samples of high crystal symmetry. For samples of lower symmetry, a smaller thickness ≤ 3λTDS, or ≤ 10λMFP is suggested.

Predicting dwell fatigue life in titanium alloys using modelling and experiment

“…Data were collected on the Zeiss Sigma300 field emission gun scanning electron microscope (FEG-SEM) with an accelerating voltage of 30 kV, working distance of 3 mm and with the sample in the TKD holder normal to the electron beam. High-resolution TKD patterns were obtained in regions of samples (parallel and perpendicular to the loading direction), which showed high dislocation activity covering a hard−soft grain pair in a Zeiss Merlin FEG SEM equipped with a Bruker on-axis Optimus detector 55,56 . The Kikuchi patterns were further subdivided into 20 regions of interest (selected to ignore the area in the centre of the pattern resulting from the direct electron beam) and cross-correlated with reference patterns selected in a region away from any significant interest in the image map as described in refs.…”

Crystallographic Orientation Analysis of Nanocrystalline Tungsten Thin Film Using TEM Precession Electron Diffraction and SEM Transmission Kikuchi Diffraction

“…The sampling depth is the maximum depth from which the diffraction pattern is collected. Recently, Liu et al reported that the sampling depth is linearly proportional to the elastic scattering mean free path ( λ ), and their relationship is d ≈ 3.5 λ in all elements (Liu et al, 2019). As elastic scattering is usually coherent, it is important to determine the sampling depth because both the spot patterns in PED and Kikuchi bands in TKD are generated by the coherent scattering of electrons when electrons escape the sample surface by the final diffraction event.…”

“…If a nanocrystalline film consists mainly of columnar grains, the difference between TEM-PED and SEM-TKD orientation maps would not be so large. For light elements, in particular, the signal from the whole thickness can be collected in both PED and TKD, e.g., d ≈ 80 nm for Al at 30 kV (Sneddon et al, 2017), because λ is only dependent on the element and electron energy but independent of the sample thickness (Liu et al, 2019). However, if a nanocrystalline film consists of heavy elements with multiple overlapping 3D grains along the electron beam path, the difference between TEM-PED and SEM-TKD orientation maps becomes significant due to the different sampling depths.…”

“…Recently, TKD was optimized by changing the geometric configuration from a conventional off-axis to an on-axis detector geometry, which improves the lateral resolution, acquisition speed, and pattern quality (Fundenberger et al, 2015, 2016; Brodu et al, 2017; Yuan et al, 2017). However, as the source of TKD patterns is at the exit surface of the sample, the film surface has a strong influence on the quality of the patterns and thus the reliability of the maps (Keller & Geiss, 2012; Sneddon et al, 2016; Fanta et al, 2018; Liu et al, 2019).…”

Comparison of Orientation Mapping in SEM and TEM

“…It is well known that the diffraction patterns in TKD are produced by the material closest to the exit surface of the electron beam in the sample (Rice et al, 2014; Liu et al, 2019). Recent work has shown that for higher atomic number targets like Pt, the depth from which the TKD patterns arise is between 5 and 12 nm (Sneddon et al, 2017; Liu et al, 2019). This is schematically shown in Figure 6a.…”