Transmitted electron mode

Specimens prepared and mounted similarly to those for TEM can be examined in the SEM if the electron detector is placed on the opposite side of the specimen from the incident electron beam. Useful, high-contrast, images can be obtained by this form of scanning transmission electron microscopy, with effective electron penetration through non-crystalline materials equivalent to a TEM operating at a higher gun voltage, (Figure 5.27). Although, unlike the TEM, the image is not affected by electron energy losses in passing through the specimen, there is a top/bottom effect whereby detail is resolved better at the electron input side than the exit side because of the increase in beam size by multiple scattering.

The interpretation of images in this mode is very similar to that already described for the TEM. STEM images may readily be produced by either bright- or dark-field techniques (Chapter 7, p. 303). There is no difficulty in producing high-contrast images - only in reproducing them satisfactorily on paper afterwards.

X-ray emissive mode

In this mode information about the elemental composition of the specimen can be presented as a spot elemental anlysis or an elemental distribution map. These will be described in more detail in Chapter 6.

Application of several examination modes to the same specimen normally produces much complementary information, although it may not be straightforward to explain all the observed features in detail.