UTILIZATION OF ELECTRON CHANNELLING CONTRAST IMAGING TO DISPLAY CRYSTAL LATTICE ORIENTATION IN SCANNING ELECTRON MICROSCOPY

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Abstract and oral prezentation on 19th International Microscopy Congress. 9 – 14 September 2018. Recently, the backscattered electron diffraction (EBSD) method, which is closely associated with scanning electron microscopy, has been successfully used to analyze the crystal lattice of polycrystalline materials. The EBSD method is applied to bulk samples and provides, among other things, information on the orientation of crystal lattices of individual grains, texture, deformations, or information on the representation and distribution of individual phases. The big advantage of this method is the speed of acquisition and processing of measured diffraction patterns, this is mainly due to the computing power of current computers, improving the algorithms for indexing the diffraction bands. A new method based on the well-known channelling contrast effect can also be used to map orientation. This method is based on the interaction of primary electrons in the material. The method is based on the modulation of the backscattered electron signal. The modulation of the backscattered electron signal is due to the channelling effect. If the signal intensity is plotted against the rotation of the sample, then it is possible to determine the orientation of the crystal lattice at that location from this curve. The limit of the ECCI method is scanning at a very low magnification, where it is also necessary to consider the deflection of the beam, whose large values are reflected in the mutual angle between the electron beam and the atomic lattice. Conversely, the great advantage of this method is the simplicity of the geometry (tilt of approximately 10 ° and a working distance of approximately 4.5 mm), which allows sample movement and mapping of larger areas. Another big advantage is that only a SEM with a BSE detector and a turntable are required to measure. In the case of investigating large single crystals under conditions of long working distance (WD) and large oscillation of the primary electron beam, it is possible to record the strip structures similar to EBSD patterns by the backscattered electron detector. These are the so-called electron channeling patterns – ECPs (Electron Channeling Patterns). The formation of these lines is due to a change in the orientation of the beam towards the crystal. It is therefore necessary, unlike methods where similar patterns arise in a static beam (EBSD patterns, Kossel patterns), to invoke ECPs to rasterize the beam across the surface. By analyzing the ECP structures, crystal orientation information can be obtained, similar to the EBSD diffraction pattern. In cases where sufficiently large single crystals are not available, such as fine-grained polycrystalline materials, the rocking beam method (SACP) must be used to initiate the ECP. While the electron channel patterns (ECPs) are recorded by the backscattered electron detector at high beam amplitude and long working distances, only a small fraction of the overall ECP image is detected by the BSE detector at small working distances and small beam amplitudes. ECP consists of bands of varying intensity. The width of each strip corresponds to twice the Bragg angle and the angles between the stripes correspond to the angles between the crystal planes. Thus, ECPs exhibit crystal lattice symmetry. If the crystal is subsequently tilted or rotated, the ECP will move or rotate accordingly. However, the translational movement of the crystal has no effect on the ECP. If the sample is rotated around one point, the detected (measured) intensity point will move along the ECP pattern and the resulting intensity pattern as a function of the sample rotation will show the structure of the bands contained in the ECP pattern.

Abstract and oral prezentation on 19th International Microscopy Congress. 9 – 14 September 2018. Recently, the backscattered electron diffraction (EBSD) method, which is closely associated with scanning electron microscopy, has been successfully used to analyze the crystal lattice of polycrystalline materials. The EBSD method is applied to bulk samples and provides, among other things, information on the orientation of crystal lattices of individual grains, texture, deformations, or information on the representation and distribution of individual phases. The big advantage of this method is the speed of acquisition and processing of measured diffraction patterns, this is mainly due to the computing power of current computers, improving the algorithms for indexing the diffraction bands. A new method based on the well-known channelling contrast effect can also be used to map orientation. This method is based on the interaction of primary electrons in the material. The method is based on the modulation of the backscattered electron signal. The modulation of the backscattered electron signal is due to the channelling effect. If the signal intensity is plotted against the rotation of the sample, then it is possible to determine the orientation of the crystal lattice at that location from this curve. The limit of the ECCI method is scanning at a very low magnification, where it is also necessary to consider the deflection of the beam, whose large values are reflected in the mutual angle between the electron beam and the atomic lattice. Conversely, the great advantage of this method is the simplicity of the geometry (tilt of approximately 10 ° and a working distance of approximately 4.5 mm), which allows sample movement and mapping of larger areas. Another big advantage is that only a SEM with a BSE detector and a turntable are required to measure. In the case of investigating large single crystals under conditions of long working distance (WD) and large oscillation of the primary electron beam, it is possible to record the strip structures similar to EBSD patterns by the backscattered electron detector. These are the so-called electron channeling patterns – ECPs (Electron Channeling Patterns). The formation of these lines is due to a change in the orientation of the beam towards the crystal. It is therefore necessary, unlike methods where similar patterns arise in a static beam (EBSD patterns, Kossel patterns), to invoke ECPs to rasterize the beam across the surface. By analyzing the ECP structures, crystal orientation information can be obtained, similar to the EBSD diffraction pattern. In cases where sufficiently large single crystals are not available, such as fine-grained polycrystalline materials, the rocking beam method (SACP) must be used to initiate the ECP. While the electron channel patterns (ECPs) are recorded by the backscattered electron detector at high beam amplitude and long working distances, only a small fraction of the overall ECP image is detected by the BSE detector at small working distances and small beam amplitudes. ECP consists of bands of varying intensity. The width of each strip corresponds to twice the Bragg angle and the angles between the stripes correspond to the angles between the crystal planes. Thus, ECPs exhibit crystal lattice symmetry. If the crystal is subsequently tilted or rotated, the ECP will move or rotate accordingly. However, the translational movement of the crystal has no effect on the ECP. If the sample is rotated around one point, the detected (measured) intensity point will move along the ECP pattern and the resulting intensity pattern as a function of the sample rotation will show the structure of the bands contained in the ECP pattern.

Scanning electron microscopy, ECCI, EBSD, backscattered electrons

09.09.2018

IMC19

Sydney

2560