Contrast Reversal in SEM
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It was reported that the material contrast reversals previously occurred in a primary beam energy range at around or below 1 keV as a result of a crossing of backscattering coefficient curves, η(E0), with energy [1-2]. On the other hand, change of secondary electron yields can also cause contrast reversal. For instance, Figure 4837a shows experimental and theoretical secondary electron yields from quartz [3] and platinum [4]. The theoretical modeling was performed by constant loss model [5 - 6] and parabolic model [7]. In the modeling, the researchers considered some factors such as magnitude for SE attenuation length, s, and SE escape probability, A, of the materials of interest. It is very interesting that the crossing of yield curves, δ(E0), is at around E0 = 2 keV. This crossing suggests a contrast change as well as a contrast reversal in the range of incident beam energy from ~ 1 to ~ 4 keV as shown by the arrows in Figure 4837a.

Secondary yield curves δf(E0) of Pt and quartz

Figure 4837a. Secondary yield curves δf(E0) of Pt and quartz. “exp” represents experimental data. The other ones are simulated data.

Using a linear grey level scale between δ = 0.8 and 2.5, the simulated contrast reversal of field effect transistor are shown in Figure 4837b (E0 = 1 keV) and in Figure 4837b (E0 = 4 keV). It is clear that the contrast of quartz (SiO2) is higher than that of Pt at low beam energy (1 keV) and the contrasts are reversed at higher voltage (4 keV).

Contrast reversal of SEM images of a FET (field effect transistor)

Figure 4837b. Contrast reversal of SEM images of a FET (field effect transistor).

In extreme situations, contrast reversal between the in-lens image and the lateral image may be observed [8].


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[8] J. Cazaux, Recent developments and new strategies in scanning electron microscopy, Journal of Microscopy, 217, (2005) 16–35.




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