Table 1 - uploaded by Angus J Wilkinson
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We report the first use of direct detection for recording electron backscatter diffraction patterns. We demonstrate the following advantages of direct detection: the resolution in the patterns is such that higher order features are visible; patterns can be recorded at beam energies below those at which conventional detectors usefully operate; high...
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Citations
... Earlier applications of these direct electron detectors have focused on techniques based in transmission electron microscope (TEM). More recently, the use of direct detectors has also been probed in lower electron energy applications, especially those based in scanning electron microscope (SEM), such as electron backscatter diffraction [3]. Lower incident electron energies will reduce the depth at which electron energy is deposited, and therefore enable the potential use of thinner sensing layers. ...
... Increases in EBSD collection rates and high-quality and high-resolution patterns have been enabled by the development of monolithic active pixel sensors (MAPS) and other direct electron detectors (79)(80)(81)(82)(83). Several research topics that will benefit from the continued development of faster, higher-quality EBSD detectors are (a) rapid high-quality data collection due to the fast readout and sparse sampling, (b) more rapid collection of high-angular-resolution EBSD (HR-EBSD) data, (c) higher-spatial-resolution EBSD data at lower accelerating voltage, and (d) more efficient application to a broader spectrum of materials classes. ...
Advanced experimental and numerical approaches are being developed to capture the localization of plasticity at the nanometer scale as a function of the multiscale and heterogeneous microstructure present in metallic materials. These innovative approaches promise new avenues to understand microstructural effects on mechanical properties, accelerate alloy design, and enable more accurate mechanical property prediction. This article provides an overview of emerging approaches with a focus on the localization of plasticity by crystallographic slip. New insights into the mechanisms and mechanics of strain localization are addressed. The consequences of the localization of plasticity by deformation slip for mechanical properties of metallic materials are also detailed.
... To address these challenges, direct electron detectors (DED) such as hybrid pixel array detector (HPD) or monolithic active pixel sensor (MAPS) can be employed [13], which are based on radiation-hardened solid-state detectors. For the present work, of particular interest is HPD based on the Medipix or Timepix chips operating in the event counting mode and frame readout [14,15] due to the larger electron flux allowed per pixel per unit time and large pixel size. ...
... There have been reports in the literature which have demonstrated the ability to capture Kikuchi patterns generated by EBSD and TKD in SEM with DEDs [13,18]. These initial works have shown significant promise. ...
... Here we apply the multi-exposure fusion technique used in digital light photography, together with a flat field operation that removes the variation in electron scattering yield as a function of scattering angle. The proposed process of obtaining the high dynamic range TKP includes four steps: (1) probing raw event data as a function of scatter angle; 13 (2) weighting of the raw event data; (3) fusing the weighted event data; and (4) flat fielding the fused pattern. ...
Diffraction pattern analysis can be used to reveal the crystalline structure of materials, and this information is used to nano- and micro-structure of advanced engineering materials that enable modern life. For nano-structured materials typically diffraction pattern analysis is performed in the transmission electron microscope (TEM) and TEM diffraction patterns typically have a limited angular range (less than a few degrees) due to the long camera length, and this requires analysis of multiple patterns to probe a unit cell. As a different approach, wide angle Kikuchi patterns can be captured using an on-axis detector in the scanning electron microscope (SEM) with a shorter camera length. These 'transmission Kikuchi diffraction' (TKD) patterns present a direct projection of the unit cell and can be routinely analyzed using EBSD-based methods and dynamical diffraction theory. In the present work, we enhance this analysis significantly and present a multi-exposure diffraction pattern fusion method that increases the dynamic range of the detected patterns captured with a Timepix3-based direct electron detector (DED). This method uses an easy-to-apply exposure fusion routine to collect data and extend the dynamic range, as well as normalize the intensity distribution within these very wide (>95{\deg}) angle patterns. The potential of this method is demonstrated with full diffraction sphere reprojection and highlight potential of the approach to rapidly probe the structure of nano-structured materials in the scanning electron microscope.
... Work by Wilkinson et al. suggests that improved accuracy is possible using direct electron detectors for HR-EBSD analysis [9]. Scintillator-based electron detectors typically rely on a phosphor screen to convert impinging electrons into photons [10][11][12]. ...
The mechanical properties of additive and traditionally manufactured alloys are largely dependent on the characteristics and distribution of dislocation cell networks that develop during the fabrication process. This work demonstrates the ability to quantitatively characterize these dislocation structures by high angular resolution electron backscatter diffraction analysis using a direct electron detector. The defect structures are characterized in terms of the geometrically necessary dislocation density and the associated Burgers vector and line direction. The results are discussed in terms of potential defect formation mechanisms.
... 200 to 300 keV, due to the lower lateral spread of electrons at higher beam energies in a thin sensitive layer. However, differently designed MAPS detectors have been demonstrated that are optimized for lower electron beam energies (5-30 keV) [20,21], and also very high electron beam energies (1 MeV) [22]. ...
... The use of a direct detector for EBSD was first reported in 2013 by Wilkinson et al [20]. In this case a 1024 × 1024 pixel MAPS based detector was employed. ...
... 200-300 keV). However, Wilkinson et al were able to develop a mechanically thinned MAPS sensor that was sensitive to low energy electrons when back-illuminated [20]. The detector showed promise, even at a beam energy of 5 keV, which is below the energy at which conventional EBSD detectors perform well, but the direct detector was limited to a relatively low frame rate of 28 frames per second [20]. ...
The past decade has seen rapid advances in direct detector technology for electron microscopy. Direct detectors are now having an impact on a number of techniques in transmission electron microscopy (TEM), scanning electron microscopy, and scanning TEM (STEM), including single particle cryogenic electron microscopy, in situ TEM, electron backscatter diffraction, four-dimensional STEM, and electron energy loss spectroscopy. This article is intended to serve as an introduction to direct detector technology and an overview of the range of electron microscopy techniques that direct detectors are now being applied to.
... This indirect conversion process introduces absorption and scattering effects that reduce the available signal whilst also causing an intrinsic loss of spatial resolution [5]. Recent developments have therefore focused on the use of direct electron detection sensors, as first reported by Wilkinson et al. [6]. By using a back-thinned active pixel sensor architecture, high precision EBSPs were achieved at accelerating voltages as low as 5 kV and speeds of up to 28 pps. ...
Performing EBSD with a horizontal sample and a parallel EBSD detector sensor, enables safer specimen movements for data collection of large specimen areas and improves the longitudinal spatial resolution. The collection of electron backscattering patterns (EBSPs) at normal incidence to the electron beam has been revisited via the use of a direct electron detection (DED) sensor. In this article we present a fully operational DED EBSD detection system in this geometry, referred to as the tilt-free geometry. A well-defined Σ=3[101]{121} twin boundary in a Molybdenum bicrystal was used to measure the physical spatial resolution of the EBSD detector in this tilt-free geometry. In this study, two separate methods for estimating the spatial resolution of EBSD, one based on a pattern quality metric and the other on a normalised cross correlation coefficient were used. The spatial resolution was determined at accelerating voltages of 8 kV, 10 kV, 12 kV, 15 kV and 20 kV ranging from ~22− 38 nm using the pattern quality method and ~31− 46 nm using the normalised cross correlation method.
... This electron diffraction technique is, of course, slower compared to the time required to acquire TEM images, and typical acquisition time is on an hour scale for a 1000 × 1000-pixel image. However, recent advances in the EBSD camera technology [17,18] The phases used for indexing were austenite fcc with space group 225 (Fm 3m, a, b, c = 3:66 Å) and martensite bct with space group 139 (I 4/mmm, a, b = 2:847 Å, c = 3:018 Å) both implemented in the Esprit software. The martensite lattice parameters were extracted from the Joint Committee on Powder Diffraction Standards (JCPDS) PDF card #00-044-1293. ...
The microstructures of quenched and tempered steels have been traditionally explored by transmission electron microscopy (TEM) rather than scanning electron microscopy (SEM) since TEM offers the high resolution necessary to image the structural details that control the mechanical properties. However, scanning electron microscopes, apart from providing larger area coverage, are commonly available and cheaper to purchase and operate compared to TEM and have evolved considerably in terms of resolution. This work presents detailed comparison of the microstructure characterization of quenched and tempered high-strength steels with TEM and SEM electron channeling contrast techniques. For both techniques, similar conclusions were made in terms of large-scale distribution of martensite lath and plates and nanoscale observation of nanotwins and dislocation structures. These observations were completed with electron backscatter diffraction to assess the martensite size distribution and the retained austenite area fraction. Precipitation was characterized using secondary imaging in the SEM, and a deep learning method was used for image segmentation. In this way, carbide size, shape, and distribution were quantitatively measured down to a few nanometers and compared well with the TEM-based measurements. These encouraging results are intended to help the material science community develop characterization techniques at lower cost and higher statistical significance.
... This indirect conversion process introduces absorption and scattering effects that reduce the available signal whilst also causing an intrinsic loss of spatial resolution [5]. Recent developments have therefore focused on the use of direct electron detection sensors, as first reported by Wilkinson et al. [6]. By using a back-thinned active pixel sensor architecture, high precision EBSPs were achieved at accelerating voltages as low as 5 kV and speeds of up to 28 pps. ...
Performing EBSD with a horizontal sample and a parallel EBSD detector sensor, enables safer specimen movements for data collection of large specimen areas and improves the longitudinal spatial resolution. The collection of electron backscattering patterns (EBSPs) at normal incidence to the electron beam has been revisited via the use of a direct electron detection (DED) sensor. In this article we present a fully operational DED EBSD detection system in this geometry, referred to as the tilt-free geometry. A well-defined Σ=3[101]{121} twin boundary in a Molybdenum bicrystal was used to measure the physical spatial resolution of the EBSD detector in this tilt-free geometry. In this study, two separate methods for estimating the spatial resolution of EBSD, one based on a pattern quality metric and the other on a normalised cross correlation coefficient were used. The spatial resolution was determined at accelerating voltages of 8 kV, 10 kV, 12 kV, 15 kV and 20 kV ranging from ∼22−38 nm using the pattern quality method and ∼31−46 nm using the normalised cross correlation method.
... The development of modern direct detectors has led to breakthroughs in transmission electron microscopy (TEM) for techniques such as single particle cryo-electron microscopy (using MAPS) [35,36] and 4D-STEM (using both MAPS and PAD) [37][38][39][40][41][42][43][44]. Direct detectors have also been employed for EBSD applications and their utility have been demonstrated in terms of acquiring high quality diffraction patterns, as well as the application of energy filtering to EBSD [45,46]. The first implementation of direct detectors for EBSD by Wilkinson et al. [45] used a CMOS sensor that is backthinned (i.e. a MAPS) to improve the detection of low-energy electrons in SEM, and several subsequent studies used the Medipix2 detector (i.e. a PAD) [46][47][48]. ...
... Direct detectors have also been employed for EBSD applications and their utility have been demonstrated in terms of acquiring high quality diffraction patterns, as well as the application of energy filtering to EBSD [45,46]. The first implementation of direct detectors for EBSD by Wilkinson et al. [45] used a CMOS sensor that is backthinned (i.e. a MAPS) to improve the detection of low-energy electrons in SEM, and several subsequent studies used the Medipix2 detector (i.e. a PAD) [46][47][48]. For practical EBSD mapping, the acquisition rate of EBSD patterns on the current direct detection cameras is relatively slow (Table 1), which was ascribed to limitations in the readout system, not the fundamental sensor performance [47]. ...
A monolithic active pixel sensor based direct detector that is optimized for the primary beam energies in scanning electron microscopes is implemented for electron back-scattered diffraction (EBSD) applications. The high detection efficiency of the detector and its large array of pixels allow sensitive and accurate detection of Kikuchi bands arising from primary electron beam excitation energies of 4 keV to 28 keV, with the optimal contrast occurring in the range of 8–16 keV. The diffraction pattern acquisition speed is substantially improved via a sparse sampling mode, resulting from the acquisition of a reduced number of pixels on the detector. Standard inpainting algorithms are implemented to effectively estimate the information in the skipped regions in the acquired diffraction pattern. For EBSD mapping, an acquisition speed as high as 5988 scan points per second is demonstrated, with a tolerable fraction of indexed points and accuracy. The collective capabilities spanning from high angular resolution EBSD patterns to high speed pattern acquisition are achieved on the same detector, facilitating simultaneous detection modalities that enable a multitude of advanced EBSD applications, including lattice strain mapping, structural refinement, low-dose characterization, 3D-EBSD and dynamic in situ EBSD.
... While this article mainly focuses on detectors in the scanning transmission electron microscope, it should be noted that direct detection of electrons is potentially also of use in scanning electron microscopes 66 to perform electron backscatter diffraction. 67 It has been shown that higher quality patterns were produced over wider angular ranges, 68 and this has been recently been used for orientation mapping beam sensitive materials. 69 In TEM or STEM, these 4D-STEM and scanned diffraction techniques have been well reviewed recently by Ophus et al. 70 A brief list of some of these techniques is given here, approximately in the order of increasing beam convergence angle. ...
Detectors are revolutionizing possibilities in scanning transmission electron microscopy because of the advent of direct electron detectors that record at a high quantum efficiency and with a high frame rate. This allows the whole back focal plane to be captured for each pixel in a scan and the dataset to be processed to reveal whichever features are of interest. There are many possible uses for this advance of direct relevance to understanding the nano- and atomic-scale structure of materials and heterostructures. This article gives our perspective of the current state of the field and some of the directions where it is likely to go next. First, a wider overview of the recent work in this area is given before two specific examples of its application are given: one is imaging strain in thin films and the other one is imaging changes in periodicity along the beam direction as a result of the formation of an ordered structure in an epitaxial thin film. This is followed by an outlook that presents future possible directions in this rapidly expanding field.
