Backscattered electrons, information

Electron backscatter diffraction (EBSD) — The focused electron beam of Scanning Electron Microscopes (SEM) can be used to detect the crystallographic orientation of the top layers of a sample. The backscattered electrons (information depth 40-70 nm at 25 kV accelerating potential, lateral resolution around 200 nm) provide characteristic diffraction patterns (Kikuchi lines) on a phosphor screen. The patterns are recorded by a CCD-camera and interpreted by software. The position of the unit cell of the sample is determined by the corresponding Euler angles. In scanning mode, the software produces a surface orientation mapping that consists of... 

The SEM can also be used to provide crystallographic information. Surfaces that to exhibit grain structure (fracture surfaces, etched, or decorated surfaces) can obviously be characterized as to grain size and shape. Electrons also can be channeled through a crystal lattice and when channeling occurs, fewer backscattered electrons can exit the surface. The channeling patterns so generated can be used to determine lattice parameters and strain. 

Backscattered electrons, however, do give some elemental information about the sample because they are more energetic than secondary electrons and escape from farther within the sample [45,46], On the molecular level, the electron beam can interact with the nucleus of an atom and be scattered with minimal loss of energy. These incident electrons may be scattered more than once and then ejected from the sample as backscattered electrons. The back-scattered electrons originate from a greater depth within the sample and are... 

Particle analysis is the most informative method to date for the identification of FDR particles. It does, however, suffer from several major disadvantages including high cost of instrumentation and lengthy and tedious procedures requiring specialized staff Since its introduction serious attempts have been made to solve the time problem. These include the use of backscattered electron images, automation of the search procedure, and sample manipulation to pre-concentrate the sample prior to SEM examination.145151... 

The number of backscattered electrons (BSE) that are produced from a given atom is proportional to the atomic number. That is, materials composed of heavy atoms will backscatter more electrons, resulting in brighter gray tones in the image relative to less dense materials. Hence, BSE produce an image that is related to material composition, providing both spatial and chemical information. 

Figure 4.16 Comparison between (a) a secondary electron image and (b) a backscattered electron image for the same area of nickel alloy. Additional compositional information is obtained from the backscattered image. (Reproduced with kind permission of Springer Science and Business Media from J.I. Goldstein et al, Scanning Electron Microscopy and X-ray Microanalysis, 2nd ed., Plenum Press, New York. 1992 Springer Science.)...

As stated, the high energetic backscattered electrons result from a single interaction and are therefore most likely to come from the upper layer of the specimen. So if only the high energetic BSE are detected the information depth is smaller than the penetration depth. If a non-dispersive detector is used all BSE are detected simultaneously, so the information depth becomes a substantial part of the penetration depth of the primary electron beam. 

Assuming that all generated x-rays can escape from the sample, the information depth is related to the primary beam energy, reflecting the interaction volume of the primary and the backscattered electrons. The information depth is also related to the characteristic radiation of interest. By decreasing E the interaction volume is decreased as well (thus the information... 

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