Advanced Electron Microscopy For Materials Science
Feb 18, 2024
3. Extension of In Situ Studies on Collective Motion of Secondary Electrons
Based on these studies on the distribution of secondary electrons on the charged insulating films, in situ experiments can be conducted to investigate the interaction of insulating specimens. Figure 5 shows the results of an in situ study of the electric field variations and the change in secondary electron distribution between two insulating materials: a square pillar of epoxy resin prepared by the FIB method with Ga+ ions, and a flake of kidney tissue embedded in epoxy resin and prepared by microtomy.70) The geometric configurations of the square pillar and kidney flake are shown in Fig. 5(c). The square pillar of epoxy resin set in the piezo driving probe of a TEM specimen holder56) approached the kidney flake gradually.
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In the reconstructed phase images of Fig. 5(a), because of mutual irradiation with secondary electrons, the electric potential of both specimens decreases as their separation decreases. In the reconstructed amplitude images in Fig. 5(b), red- and yellow-colored regions are observed around the kidney flake but not around the square pillar, which is consistent with the observations presented in Fig. 4. When the separation of the specimens fell below approximately 1 µm, most of the red- and yellow-colored regions disappeared, indicating a decrease in the number of secondary electrons around the kidney flake. Nonetheless, faint color regions remain in the lower-right region, some distance from the pillar of epoxy resin. The visibilities of the interference fringes in the two green "f " and yellow "e" square regions indicated in the leftmost image of Fig. 5(a) are compared as functions of the separation between the specimens, as shown in Fig. 5(d), as the pillar moves toward ( ) or away from ( ) the kidney flake. A prominent increase in visibility is observed in the yellow square near the kidney flake when the square pillar of epoxy resin approaches the kidney flake surface. The decrease in the number of secondary electrons on the surface of the kidney flake results from the presence of a conductive layer on the surface of the epoxy resin, through which secondary electrons were transferred.
As noted, the accumulation and collective motions of electrons around various positively charged insulators have been observed through the amplitude reconstruction process in electron holography. Those experiments were performed by TEM with a thin insulating specimen surface. Here, we show that similar effects of secondary electrons, which are attracted even to bulk surfaces of insulators, can be observed by scanning ion microscopy (SIM) with a FIB system. The common feature of the phenomena observed by TEM and SIM is the positive charging effect of insulators. In the latter case, during SIM observations, a Ga+-ion probe is used to scan the bulk insulator surface. At each point in the scanned area, secondary electrons are emitted from the bulk surface, whereas Ga+ ions accumulate in the bulk specimen, resulting in positive charging. Figure 6 shows a bulk specimen in which W islands were deposited on a mica substrate to form the character "e" and a superscript minus symbol, "¹". Figure 6(a) corresponds to the initial state where W islands are not touched by the +5 V-biased probe needle. When the probe touched one of the islands, "¹", the other island, "e", also became brighter (Fig. 6(b)), implying the charge transfer from the island "¹" to the island "e" with the existence of metallic elements Ga and W on the mica substrate like the case of the epoxy thin film in Fig. 4. The existence of W results from its spread during the deposition process. When the probe directly contacted the character "e", it became much brighter (Fig. 6(c)). On the other hand, in Fig. 6(d), the character "¹" again became brighter when touched by the probe but the "e" did not become bright. The SIM observation was continued for 48 s from scene (b) to scene (d), and the nominal Ga+ dose during this period was 0.027 (C m¹2 ) or 0.17 (ion nm¹2 ). In Fig. 6(d), it is considered that remarkable sputtering of the specimen surface by Ga+ ions results in different surface conditions from those of Fig. 6(b) and (c).
4. Summary
Research activity related to microstructure and electromagnetic field analyses using TEM continues to increase. The new trends in these microstructure and electromagnetic field analyses were discussed at the 27th Meeting of the Materials Science and Engineering Research Society, held at the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, with the theme of "Recent Trends in Materials Properties and Microscopy Techniques". Based on this meeting, some recent trends related to TEM analyses were published in October 2019 in a special issue of Materials Transactions (Vol. 60, No. 10) under the title "Development and Application of Advanced Electron Microscopy Techniques for Materials Science". As an extension of electron holography on secondary electrons around charged insulating specimens, the interaction between such specimens has been clarified through in situ experiments. In addition, similar effects of secondary electrons, which are attracted to the bulk surface of insulators, have been also observed by SIM with an FIB system. These in situ experiments on secondary electron distributions are expected to be carried out widely and analyzed quantitatively shortly.
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