Depth resolution in on-axis TKD 1101
the patterns 2 and 8 in Figure 4. Very shortly after point 2, while the diffraction spots are still progressively developing with the increase in thickness of the bottom crystal, a faint Kikuchi band contrast associated to the bottom crystal appears as well. The thickness interval inside which the diffraction from the bottom crystal consists only of spots is not measurable because it is too narrow (i.e., the Kikuchi diffraction very closely follows the spots). The Kikuchi dif- fraction associated to the bottom crystal then builds very progressively as the thickness of the bottomcrystal increases. In the meantime, while the twin boundary gets further away from the bottom surface, the Kikuchi diffraction associated to the top crystal progressively decreases in contrast and fades. There is thus, a bottom crystal thickness for which the Kikuchi diffractions associated to the two crystals are somehow equivalent in contrast. This point of balance seems to be reached somewhere around the point 4 in Figure 4 and seems to correspond roughly to half of the depth resolution. This thickness, for which the two Kikuchi diffractions of these two crystals are equivalent in contrast, will typically be the one for which the indexing will swing from one orien- tation to the other in orientation maps (note that the pro- gressive generation of the Kikuchi diffraction associated to the bottom crystal and the fading of the Kikuchi diffraction of the top crystal are most likely the product of two very separate physical mechanisms, which means there is no reason for this point of balance to necessarily be halfway). With further increasing thickness of the bottom crystal, there is finally a point where the Kikuchi diffraction associated to the top crystal is no longer visible in patterns. It corresponds to point 6. Here, we consider that the depth resolution is equal to the thickness of the bottomcrystal. At point 6, there remains enough top crystal to produce Kikuchi diffraction (about 40 nmfor the particular line scan in Figure 4, but even more for many other line scans, see Fig. 5), and still, no information in the forms of a Kikuchi diffraction associated to the top crystal is visible in patterns. Only the spots asso- ciated to the top crystal remain visible, until point 8, which is much further in terms of lateral positioning of the beam. The depth resolution of the Kikuchi diffraction in TKD is here evidenced without ambiguity, and as expected, TKD does not see the crystal that is above the depth resolution. While this aspect of the depth resolution is made clear, a
new question emerges and remains open: what does TKD see inside that depth resolution layer? The description above of the evolution of the diffraction pattern along a line scan showed that two mechanisms are active inside the depth resolution layer: fading but also generation of Kikuchi diffraction. While the fading is here well studied and its dependence with energy identified (see Towards a Physical Model for the Depth Resolution section), the progressive generation with increasing crystal thickness could not be determined with the same accuracy and its dependence with incident energy remains unknown as of now. Consequently,we do not know how these two characteristic lengths interact. While it seems that the generation of Kikuchi diffraction occurs over a thickness shorter than is needed for absorption, maybe
Figure 5. Depth resolution dependence with sample thickness and incident energy by on-axis transmission Kikuchi diffraction (TKD) in scanning electron microscope for cubic Si. For each incident energy, the depth resolution was measured at different locations on the Si lamella, with the objective to vary the local sample thickness. The red dots are the local sample thickness at the point of measurement of the depth resolution. The depth resolution in on-axis TKD appears to have a close to linear depen- dence with incident energy, but no dependence with sample thick- ness. The elastic and plasmon mean free paths are taken from Mayol & Salvat (1997) and Shinotsuka et al. (2015), respectively, and the mean absorption coefficient is obtained from Reimer (1997: 309).
there could be a scenario (i.e., atomic number and energy) for which itwould be the
opposite.Maybe in that case only blurred, partially generated patterns could be produced, even by a per- fect single crystal. As of now, before further studies, a very approximate answer to the question “what does TKD see inside that depth resolution layer” is: a portio of all crystals/ grains in variable proportions depending on their size, because it determines the generation part, and their in-depth position inside the depth resolution layer, because it determines the fading part. In particular, a crystal/grain right at the back sur- face will not always necessarily be the one contributing the most to the Kikuchi pattern. For example, a small grain right at the back surface can contribute less than a bigger grain further away from the back surface. It was precisely the case between point 2 and 4 in Figure 4, where the most visible crystal was actually the top crystal even though the bottom crystal is pre- sent right at the back surface.
Depth Resolution Dependence With Sample Thickness and Incident Energy The depth resolution in on-axis TKD as a function of sample thickness and incident energy determined using the
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