Practical Electron Microscopy and Database

The interpretation of SEM (secondary electron microscopy) images of composite samples may not always be straightforward. This is especially true for SE images, which represent a combination of both the surface and the work function profiles [1]. Many other factors add into or subtract contrast from the images [2-3]. Those factors includes the respective secondary electron (SE) yields of the components, the angular fraction of the secondary electrons (SE)s being collected, possible voltage contact effect between sample and detector, and other effects.

In SEM systems, not all the emitted SEs can be collected but only a fraction of them, kδ(E₀), are. Here, δ(E₀) is yield of secondary electrons at beam energy of E₀. Therefore, the contrasts of SEM images can be basically categorized by topographic, material, and voltage contrast. The topographic and material contrasts originate from yields (SE yield and/or BSE yield), while the voltage contrast originates from variation of the collection efficiency of SEs.

[1] R. J. Keyse, et al., “Introduction to Scanning Transmission Electron Microscopy,” in Microscopy Handbook, Vol. 39, Oxford, England: BIOS Scientific Publishers, (1998).
[2] M. Toth, B.L. Thiel, A.M. Donald, Interpretation of secondary electron images obtained using a low vacuum SEM, Ultramicroscopy 94 (2003) 71–87.
[3] J. Cazaux, On some contrast reversals in SEM: Application to metal/insulator systems, Ultramicroscopy 108 (2008) 1645–1652