Electron channelling contrast imaging

Last updated

Electron channelling contrast imaging (ECCI) is a scanning electron microscope (SEM) diffraction technique used in the study of defects in materials. These can be dislocations or stacking faults that are close to the surface of the sample, low angle grain boundaries or atomic steps. Unlike the use of transmission electron microscopy (TEM) for the investigation of dislocations, the ECCI approach has been called a rapid and non-destructive characterisation technique [1]

Contents

Mechanism

The word channelling in ECCI, and, similarly, in electron channelling patterns refers to diffraction of the electron beam on its way in the sample. With enough spatial resolution, very small crystal imperfections would change the phase of the incident electron wave-function, and this, in turn, would be reflected in the backscattering probability, showing up as contrast (sharp change in backscattered intensity) close to a dislocation [2]

Background

While we now talk about ECCI being a SEM technique, there was a significant gap of about thirty years between the prediction that defects ought to show up as contrast in the backscattered electron micrographs of the SEM and the development of ECCI as an accessible technique for the user of a standard SEM. [3] In the meanwhile the field emission gun (FEG) had to be developed and integrated in the commercial SEM in order to improve the spatial resolution for channelling applications.

Shortly after, Wilkinson [4] used ECCI to investigate clusters of misfit dislocations lying more than 1 μm underneath the surface at the interface of Si-Ge layers grown on Si. They noted that at this depth the spatial resolution is too low to resolve individual dislocations. However, they still concluded that the invisibility criteria can be applied similarly to TEM. These pioneering ECCI investigations were made on highly tilted samples (), with side-mounted backscattered electron (BSE) detectors, similarly to the electron backscatter diffraction (EBSD) set up.

When applied to metals ECCI tended to be used in a low tilt ( ) configuration. [5] This set up offers a number of advantages: the standard Si-diode detector can be mounted on the pole piece offering a large BSE signal collection angle and the interaction volume is minimised granting higher spatial resolution. The downside to this geometry is the reduction in BSE signal which, for metals, is less of an issue than for semiconductors due to higher atomic numbers. A comprehensive overview of the applications of ECCI for metallic materials has been made by Weidner and Biermann. [6]

From 2006 Trager-Cowan's group [7] showed that using ECCI in the characterisation of nitrides is an excellent idea. Since then ECCI has been used in the forescatter geometry to reveal extended defects and morphological features of GaN samples. [8] Picard et al. [9] also argued that the dislocation type identification criterion can no longer be applied due to surface relaxation. Instead, they used simulations to determine the Burger vectors of dislocations, laying the grounds for a non-destructive dislocation characterisation method.

Literature continues to call ECCI a new technique even though it has been around for almost forty years. There are a number of reasons for this including the fact that it resisted being standardised such that every group has their own method of acquiring ECC-micrographs depending on the material studied, the SEM abilities and the available detectors. Different groups proposed flavours of ECCI to distinguish between procedures. Gutierrez-Urrutia et al. [10] and Zaefferer and Elhami [11] coined the term controlled ECCI (cECCI) for a low tilt geometry ECCI aided by crystallographic information obtained form EBSD maps acquired at tilt. Similarly, Mansour et al. [12] used low tilt ECCI together with high resolution selected area channelling patterns to characterise dislocations in fine-grained Si steel and labelled it accurate ECCI (aECCI).

Related Research Articles

<span class="mw-page-title-main">Scanning electron microscope</span> Electron microscope where a small beam is scanned across a sample

A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image. In the most common SEM mode, secondary electrons emitted by atoms excited by the electron beam are detected using a secondary electron detector. The number of secondary electrons that can be detected, and thus the signal intensity, depends, among other things, on specimen topography. Some SEMs can achieve resolutions better than 1 nanometer.

<span class="mw-page-title-main">Transmission electron microscopy</span> Imaging and diffraction using electrons that pass through samples

Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. The image is then magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a sensor such as a scintillator attached to a charge-coupled device.

<span class="mw-page-title-main">Electron diffraction</span> Bending of electron beams due to electrostatic interactions with matter

Electron diffraction is a general term for phenomena associated with changes in the direction of electron beams due to elastic interactions with atoms. Close to the atoms the changes are described as Fresnel diffraction; far away they are called Fraunhofer diffraction. The resulting map of the directions of the electrons far from the sample is called a diffraction pattern, see for instance Figure 1. These patterns are similar to x-ray and neutron diffraction patterns, and are used to study the atomic structure of gases, liquids, surfaces and bulk solids. Electron diffraction also plays a major role in the contrast of images in electron microscopes.

<span class="mw-page-title-main">Dislocation</span> Linear crystallographic defect or irregularity

In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to slide over each other at low stress levels and is known as glide or slip. The crystalline order is restored on either side of a glide dislocation but the atoms on one side have moved by one position. The crystalline order is not fully restored with a partial dislocation. A dislocation defines the boundary between slipped and unslipped regions of material and as a result, must either form a complete loop, intersect other dislocations or defects, or extend to the edges of the crystal. A dislocation can be characterised by the distance and direction of movement it causes to atoms which is defined by the Burgers vector. Plastic deformation of a material occurs by the creation and movement of many dislocations. The number and arrangement of dislocations influences many of the properties of materials.

<span class="mw-page-title-main">Electron backscatter diffraction</span> Scanning electron microscopy technique

Electron backscatter diffraction (EBSD) is a scanning electron microscopy (SEM) technique used to study the crystallographic structure of materials. EBSD is carried out in a scanning electron microscope equipped with an EBSD detector comprising at least a phosphorescent screen, a compact lens and a low-light camera. In this configuration, the SEM incident beam hits the tilted sample. As backscattered electrons leave the sample, they interact with the crystal's periodic atomic lattice planes and diffract according to Bragg's law at various scattering angles before reaching the phosphor screen forming Kikuchi patterns (EBSPs). EBSD spatial resolution depends on many factors, including the nature of the material under study and the sample preparation. Thus, EBSPs can be indexed to provide information about the material's grain structure, grain orientation, and phase at the micro-scale. EBSD is applied for impurities and defect studies, plastic deformation, and statistical analysis for average misorientation, grain size, and crystallographic texture. EBSD can also be combined with energy-dispersive X-ray spectroscopy (EDS), cathodoluminescence (CL), and wavelength-dispersive X-ray spectroscopy (WDS) for advanced phase identification and materials discovery.

<span class="mw-page-title-main">Scanning transmission electron microscopy</span> Scanning microscopy using thin samples and transmitted electrons

A scanning transmission electron microscope (STEM) is a type of transmission electron microscope (TEM). Pronunciation is [stɛm] or [ɛsti:i:ɛm]. As with a conventional transmission electron microscope (CTEM), images are formed by electrons passing through a sufficiently thin specimen. However, unlike CTEM, in STEM the electron beam is focused to a fine spot which is then scanned over the sample in a raster illumination system constructed so that the sample is illuminated at each point with the beam parallel to the optical axis. The rastering of the beam across the sample makes STEM suitable for analytical techniques such as Z-contrast annular dark-field imaging, and spectroscopic mapping by energy dispersive X-ray (EDX) spectroscopy, or electron energy loss spectroscopy (EELS). These signals can be obtained simultaneously, allowing direct correlation of images and spectroscopic data.

<span class="mw-page-title-main">Slip (materials science)</span> Displacement between parts of a crystal along a crystallographic plane

In materials science, slip is the large displacement of one part of a crystal relative to another part along crystallographic planes and directions. Slip occurs by the passage of dislocations on close/packed planes, which are planes containing the greatest number of atoms per area and in close-packed directions. Close-packed planes are known as slip or glide planes. A slip system describes the set of symmetrically identical slip planes and associated family of slip directions for which dislocation motion can easily occur and lead to plastic deformation. The magnitude and direction of slip are represented by the Burgers vector, b.

<span class="mw-page-title-main">Low-energy electron diffraction</span> Technique for determining surface structures

Low-energy electron diffraction (LEED) is a technique for the determination of the surface structure of single-crystalline materials by bombardment with a collimated beam of low-energy electrons (30–200 eV) and observation of diffracted electrons as spots on a fluorescent screen.

Diffraction topography is a imaging technique based on Bragg diffraction. Diffraction topographic images ("topographies") record the intensity profile of a beam of X-rays diffracted by a crystal. A topography thus represents a two-dimensional spatial intensity mapping of reflected X-rays, i.e. the spatial fine structure of a Laue reflection. This intensity mapping reflects the distribution of scattering power inside the crystal; topographs therefore reveal the irregularities in a non-ideal crystal lattice. X-ray diffraction topography is one variant of X-ray imaging, making use of diffraction contrast rather than absorption contrast which is usually used in radiography and computed tomography (CT). Topography is exploited to a lesser extends with neutrons, and has similarities to dark field imaging in the electron microscope community.

<span class="mw-page-title-main">Electron tomography</span>

Electron tomography (ET) is a tomography technique for obtaining detailed 3D structures of sub-cellular, macro-molecular, or materials specimens. Electron tomography is an extension of traditional transmission electron microscopy and uses a transmission electron microscope to collect the data. In the process, a beam of electrons is passed through the sample at incremental degrees of rotation around the center of the target sample. This information is collected and used to assemble a three-dimensional image of the target. For biological applications, the typical resolution of ET systems are in the 5–20 nm range, suitable for examining supra-molecular multi-protein structures, although not the secondary and tertiary structure of an individual protein or polypeptide. Recently, atomic resolution in 3D electron tomography reconstructions has been demonstrated.

<span class="mw-page-title-main">Kikuchi lines (physics)</span> Patterns formed by scattering

Kikuchi lines are patterns of electrons formed by scattering. They pair up to form bands in electron diffraction from single crystal specimens, there to serve as "roads in orientation-space" for microscopists uncertain of what they are looking at. In transmission electron microscopes, they are easily seen in diffraction from regions of the specimen thick enough for multiple scattering. Unlike diffraction spots, which blink on and off as one tilts the crystal, Kikuchi bands mark orientation space with well-defined intersections as well as paths connecting one intersection to the next.

The multislice algorithm is a method for the simulation of the elastic scattering of an electron beam with matter, including all multiple scattering effects. The method is reviewed in the book by John M. Cowley, and also the work by Ishizuka. The algorithm is used in the simulation of high resolution transmission electron microscopy (HREM) micrographs, and serves as a useful tool for analyzing experimental images. This article describes some relevant background information, the theoretical basis of the technique, approximations used, and several software packages that implement this technique. Some of the advantages and limitations of the technique and important considerations that need to be taken into account are described.

<span class="mw-page-title-main">Stacking fault</span>

In crystallography, a stacking fault is a planar defect that can occur in crystalline materials. Crystalline materials form repeating patterns of layers of atoms. Errors can occur in the sequence of these layers and are known as stacking faults. Stacking faults are in a higher energy state which is quantified by the formation enthalpy per unit area called the stacking-fault energy. Stacking faults can arise during crystal growth or from plastic deformation. In addition, dislocations in low stacking-fault energy materials typically dissociate into an extended dislocation, which is a stacking fault bounded by partial dislocations.

<span class="mw-page-title-main">Precession electron diffraction</span> Averaging technique for electron diffraction

Precession electron diffraction (PED) is a specialized method to collect electron diffraction patterns in a transmission electron microscope (TEM). By rotating (precessing) a tilted incident electron beam around the central axis of the microscope, a PED pattern is formed by integration over a collection of diffraction conditions. This produces a quasi-kinematical diffraction pattern that is more suitable as input into direct methods algorithms to determine the crystal structure of the sample.

<span class="mw-page-title-main">Ben Britton</span> British materials scientist and engineer

Thomas Benjamin Britton is a materials scientist and engineer based at The University of British Columbia. He is a specialist in micromechanics, electron microscopy and crystal plasticity. In 2014 he was awarded the Silver Medal of the Institute of Materials, Minerals and Mining (IOM3), a society of which he then became a Fellow in 2016.

Carol Trager-Cowan is a Scottish physicist who is a Reader in physics and Science Communicator at the University of Strathclyde. She works on scanning electron microscopy, including Electron backscatter diffraction (EBSD), diffraction contrast and cathodoluminescence imaging.

Angus J Wilkinson is a professor of materials science based at University of Oxford. He is a specialist in micromechanics, electron microscopy and crystal plasticity. He assists in overseeing the MicroMechanics group while focusing on the fundamentals of material deformation. He developed the HR-EBSD method for mapping stress and dislocation density at high spatial resolution used at the micron scale in mechanical testing and micro-cantilevers to extract data on mechanical properties that are relevant to materials engineering.

Transmission Kikuchi Diffraction (TKD), also sometimes called transmission-electron backscatter diffraction (t-EBSD), is a method for orientation mapping at the nanoscale. It’s used for analysing the microstructures of thin transmission electron microscopy (TEM) specimens in the scanning electron microscope (SEM). This technique has been widely utilised in the characterization of nano-crystalline materials, including oxides, superconductors, and metallic alloys.

Dark-field X-ray microscopy is an imaging technique used for multiscale structural characterisation. It is capable of mapping deeply embedded structural elements with nm-resolution using synchrotron X-ray diffraction-based imaging. The technique works by using scattered X-rays to create a high degree of contrast, and by measuring the intensity and spatial distribution of the diffracted beams, it is possible to obtain a three-dimensional map of the sample's structure, orientation, and local strain.

<span class="mw-page-title-main">Weak-beam dark-field microscopy</span> Electron microscopy technique

Weak beam dark field (WBDF) microscopy is a type of transmission electron microscopy (TEM) dark field imaging technique that allows for the visualization of crystal defects with high resolution and contrast. Specifically, the technique is mainly used to study crystal defects such as dislocations, stacking faults, and interfaces in crystalline materials. WBDF is a valuable tool for studying the microstructure of materials, as it can provide detailed information about the nature and distribution of defects in crystals. These characteristics can have a significant impact on material properties such as strength, ductility, and corrosion resistance.

References

  1. ↑ SSD group, University of Strathclyde, University of Strathclyde, Glasgow. Retrieved on 03 March 2019.
  2. ↑ .Booker, G. R.; Shaw, A. M. B.; Whelan, M. J.; Hirsch, P. B. (1967). "Some comments on the interpretation of the 'Kikuchi-like reflection patterns' observed by scanning electron microscopy". Philosophical Magazine. 16. 16 (144): 1185–1191. Bibcode:1967PMag...16.1185B. doi:10.1080/14786436708229969.
  3. ↑ Czernuszka, J. T.; Long, N. J.; Boyes, E.D.; Hirsch, P.B. (1990). "Imaging of dislocations using backscattered electrons in a scanning electron microscope". Philosophical Magazine Letters. 62. 62 (4): 227–232. Bibcode:1990PMagL..62..227C. doi:10.1080/09500839008215127.
  4. ↑ Wilkinson, A. J.; Anstis, G. R.; Czernuszka, J. T.; Long, N. J.; Hirsch, P. B. (1993). "Electron channelling contrast imaging of interfacial defects in strained silicon-germanium layers on silicon". Philosophical Magazine A. 68. 1 (1): 59–80. Bibcode:1993PMagA..68...59W. doi:10.1080/01418619308219357.
  5. ↑ Simkin, B. A.; Crimp, M. A. (1999). "An experimentally convenient configuration for electron channeling contrast imaging". Ultramicroscopy. 77. 1–2 (1–2): 65–75. doi:10.1016/S0304-3991(99)00009-1.
  6. ↑ Weidner, A; Biermann, H (2015). "Case studies on the application of high resolution electron channelling contrast imaging–investigation of defects and defect arrangements in metallic materials". Philosophical Magazine. 95. 95 (7): 759–793. Bibcode:2015PMag...95..759W. doi: 10.1080/14786435.2015.1006296 . S2CID   135707746.
  7. ↑ Trager-Cowan, C.; Sweeney, F.; Winkelmann, A.; Wilkinson, A. J.; Trimby, P. W.; Day, A. P.; Gholinia, A.; Schmidt, N. H.; Parbrook, P. J.; Watson, I. M. (2006). "Characterisation of nitride thin films by electron backscatter diffraction and electron channelling contrast imaging". Materials Science and Technology. 22. 22 (11): 1352–1358. Bibcode:2006MatST..22.1352T. doi:10.1179/174328406X130957. S2CID   136474611.
  8. ↑ Picard, Y. N.; Twigg, M. E.; Caldwell, J. D.; Eddy, C. R.; Neudeck, P. G.; Trunek, A. J.; Powell, J. A. (2007). "Electron channeling contrast imaging of atomic steps and threading dislocations in 4 H-Si C". Applied Physics Letters. 90. 90 (23): 234101. Bibcode:2007ApPhL..90w4101P. doi:10.1063/1.2746075.
  9. ↑ Picard, Y. N.; Liu, M.; Lammatao, J.; Kamaladasa, R; De Graef, M. (2014). "Theory of dynamical electron channeling contrast micrographs of near-surface crystal defects". Ultramicroscopy. 146: 71–8. doi:10.1016/j.ultramic.2014.07.006. PMID   25127516.
  10. ↑ Gutierrez-Urrutia, I.; Zaefferer, S.; Raabe, D. (2009). "Electron channeling contrast imaging of twins and dislocations in twinning-induced plasticity steels under controlled diffraction conditions in a scanning electron microscope". Scripta Materialia. 61. 61 (7): 737–740. doi:10.1016/j.scriptamat.2009.06.018.
  11. ↑ Zaefferer, S.; Elhami, N. (2014). "Theory and application of electron channelling contrast imaging under controlled diffraction conditions". Acta Materialia. 75: 20–50. Bibcode:2014AcMat..75...20Z. doi:10.1016/j.actamat.2014.04.018.
  12. ↑ Haithem, M.; Crimp, M. A.; Gey, N.; Maloufi, N. (2015). "Accurate electron channeling contrast analysis of a low angle sub-grain boundary" (PDF). Scripta Materialia. 109: 76–79. doi:10.1016/j.scriptamat.2015.07.023.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.