Image contrast of ultra HVEM and future ultra HREM
Review of the measurements and interpretation of energy dependence of extinction distance and transmissive power of electrons, which have appeared in the electron microscope images of crystals at the voltages ranging from 50 to 3000 kV, is presented. Applications of two beam dynamical theory of electron diffraction considering absorption effect of electrons made clear the importance of relativistic effect of the mass of electron to the variation of extinction distance and transmissive power at higher voltage and noted that they are proportional to (v/c) and (v/c)2 respectively, where v and c are the velocity of illuminating electrons and light. With elevating accelerating voltage of illuminating electrons, it was noted that many diffracted waves are excited and thus, for understanding of the image contrast and the improvement of transmissive power of electrons, many beam dynamical theory considering absorption of electron waves has to be applied. Detailed analysis of the excitation and absorption of component Bloch waves was carried out and flux (electron current) flow of Bloch waves and total waves was deduced. Improvements of image contrast and visibility of the imperfection in crystalline materials both in thin and thick regions of ultra-high voltages were realized by using the multi-beam imaging (MBI) method. In MBI of very thick crystals, it was noted that most of the inelastically scattered electrons which appeared in the background of the diffraction patterns contribute to the image contrast by the successive elastic scattering after inelastic scattering. It was noted that the improvement of maximum usable thickness at high voltage is highly dependent on the contribution of inelastically scattered electrons. The calculated flux flows of total electrons of various materials in various orientations were calculated for not only ultra-high voltages but low voltages such as 100 kV. It was suggested that the intensity distribution of such flux flow will be recorded by the ultra HREM in the near future and will become useful to identify types of atoms, such as the interface consisting of different kinds of atoms.
Hashimoto
H
Okayama University of Science, Japan.