Measuring the Crystal Structure

An electron beam incident on a crystal can undergo electron channeling in a shallow near-surface layer which increases the initial beam penetration for certain orientations of the beam relative to the crystal planes. The additional penetration results in a slight reduction in the electron backscattering coefficient, which creates weak crystallographic contrast (a few percent) in SEM images by which differences in local crystallographic orientation can be directly observed: grain boundaries, deformations bands, and so on (e.g., .  Fig. 8).

The backscattered electrons exiting the specimen are subject to crystallographic diffraction effects, producing small modulations in the intensities scattered to different angles that are superimposed on the overall angular distribution that an amorphous target would produce. The resulting “electron backscatter diffraction (EBSD)” pattern provides extensive information on the local orientation, as shown in .  Fig. 8b for a crystal of hematite. EBSD pattern angular separations provide measurements of the crystal plane spacing, while the overall EBSD pattern reveals symmetry elements. This crystallographic information combined with elemental analysis information obtained simultaneously from the same specimen region can be used to identify the crystal structure of an unknown.

Dual-Beam Platforms: Combined

Electron and Ion Beams

A “dual-beam” instrument combines a fully functional SEM with a focused ion beam (FIB), typically gallium or argon. This combination provides a flexible platform for in situ specimen modification through precision ion beam milling and/or ion beam mediated material deposition with sequential or simultaneous electron beam technique characterization of the newly revealed specimen surfaces. Precision material removal enables detailed study of the third dimension of a specimen with nanoscale resolution along the depth axis. An example of ion beam milling of a directionally solidified Al-Cu is shown in .  Fig. 9, as imaged with the SEM column on the dualbeam instrument. Additionally, ion-beam induced secondary electron emission provides scanning ion microscopy (SIM) imaging to complement SEM imaging. For imaging certain specimen properties, such as crystallographic structure, SIM produces stronger contrast than SEM. There is also an important class of standalone SIM instruments, such as the helium ion microscope (HIM), that are optimized for high resolution/high depth-of-field imaging performance (e.g., the same area as viewed by HIM is also shown in .  Fig. 9).

Modeling Electron and Ion

Interactions

An important component of modern Scanning Electron Microscopy and X-ray Microanalysis is modeling the interaction of beam electrons and ions with the atoms of the specimen and its environment. Such modeling supports image interpretation, X-ray microanalysis of challenging specimens, electron crystallography methods, and many other issues. Software tools for this purpose, including Monte Carlo electron trajectory simulation, are discussed within the text. These tools are complemented by the extensive database of Electron-Solid Interactions (e.g., electron scattering and ionization cross sections, secondary electron and backscattered electron coefficients, etc.), developed by Prof. David Joy, can be found in chapter 3 on SpringerLink: http://link.springer. com/chapter/10.1007/978-1-4939-6676-9_3.

References

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Knoll M, Theile R (1939) Scanning electron microscope for determining the topography of surfaces and thin layers. Z Physik 113:260

Oatley C (1972) The scanning electron microscope: part 1, the instrument. Cambridge University Press, Cambridge von Ardenne M (1938) The scanning electron microscope.

Theoretical fundamentals. Z Physik 109:553