The development and evaluation of EBSD detection systems
Creators: name: MARSHALL, ANDREW LEWIS
Publisher: name: University of Surrey
Year: 2022

description: Electron backscatter diffraction (EBSD) is the principal technique used to obtain crystallographic information
description: in the scanning electron microscope (SEM). EBSD provides microstructural and crystallographic
description: information at high speeds with a large angular range and a high spatial resolution, whilst having a
description: high level of automation. As a result, EBSD is a statistically reliable technique, capable of producing
description: microstructural data over large areas. Whilst the technique is becoming increasingly widespread,
description: new technological developments are continuously emerging that offer advantages for aspects such as;
description: accessibility, experimental workflows, spatial resolution and applications development.<br>
description: <br>
description: This work aims to evaluate and characterise some of the latest developments for a complete EBSD
description: detection system, to further the understanding of the capabilities of the technique. The evaluation
description: considers: i) the capture of electron backscattering patterns (EBSPs) using a novel Timepix-based
description: detector in a reflection Kikuchi diffraction (RKD) geometry, ii) crystal orientation determination via
description: a novel Hough-based indexing algorithm and iii) the segmentation of orientation data utilising two
description: contrasting grain reconstruction algorithms. These developments represent three of the main stages of
description: an EBSD workflow; pattern capture, crystal indexing and post-processing. Particular attention is given
description: to the standardisation of evaluation procedures for each stage of the technique using both simulated
description: and experimental data.<br>
description: <br>
description: The collection of EBSPs at normal incidence to the electron beam was revisited via the use of a fully
description: operational Timepix EBSD detection system in a novel geometry. The detection system consisted of
description: all necessary hardware, firmware and processing software. The Timepix-based EBSD detection system,
description: with a small form factor and zero readout noise, demonstrated the capability to collect high quality
description: EBSPs in the RKD geometry despite a lower backscatter electron yield. The use of an appropriate
description: curve fit, and corresponding Gaussian FWHM measurement permitted a first direct physical spatial
description: resolution comparison between pattern quality (PQ) and normalised cross correlation (NCC) based
description: metrics. The optimum physical spatial resolution for a molybdenum bicrystal was 22 nm and 31 nm
description: for the PQ and NCC metrics respectively, when using an accelerating voltage of 12 kV. The automation
description: of the collection of EBSD data sets, inclusive of a flatfield image, with varying accelerating voltage,
description: working distance, exposure time and energy threshold was also possible. The optimum parameters for
description: the RKD geometry for an iron specimen were found to be: an accelerating voltage range of 8 – 12 kV,
description: working distance range of 10 – 14 mm and energy threshold ∼ 5 kV below the incident energy.<br>
description: <br>
description: An indexing accuracy better than 0.2◦ is consistently achieved for a novel Hough-based indexing
description: algorithm that utilises a 2-D spherical mapping and generalised Voronoi diagram for band-reflector pair
description: correspondence searching. A mean accuracy for 2500 EBSPs approached 0.1◦ under optimum experimental
description: conditions. The simultaneous fitting of Kikuchi band normals produced a low sensitivity with
description: respect to the pattern centre calibration in the Z-direction (PCz), with an increased sensitivity in PCx
description: and PCy. The impact of EBSP noise level, image size and detector-specimen distance on the accuracy
description: of the indexer was also evaluated. Mis-indexing, as caused by pseudo-symmetry in quartz, is consistently
description: avoided when 13 or more bands are used for indexing. The accuracy of two contrasting EBSD
description: grain reconstruction algorithms; referred to as the flood-fill and clustering algorithms, were found to
description: be 0.0 – 0.7 % and 0.0 – 113.7 % respectively, for a range of synthetically generated microstructures.
description: The clustering algorithm was observed to over-segment grains under certain conditions, however, the
description: observation of low-angle grain boundaries for sub-grain analyses is possible using this approach. <br>