1098 Etienne Brodu and Emmanuel Bouzy


Figure 2. a: Side view of the Si lamella being excavated by focused ion beam. The solid green line indicates the position- ing of the twin boundary, which is tilted by 8° relative to the surface of the lamella. b: Direct view of the twin boundary with backscattered electrons (BSD detector) in the same area, before excavation. The arrows indicate details on the surface that were used to track the position of the twin boundary. c: Same view as (b) but with secondary electrons (InLens detector); the boundary is no more visible, hence the need for details on the surface used as landmarks.


leaving two traces (one on each side of the wafer) that looked vertically aligned. Inside this selected area, a FIB lamella was then extracted in a way to induce a low angle between the twin boundary and the surface of the lamella. This angle could simply be controlled by rotating the milling frame relative to the boundary on the surface of the wafer in the FIB, as seen in Figure 2. In the meantime, the twin boundary is assumed to be roughly perpendicular to the observation plane in Figure 2 (bear in mind that Fig. 2 presents a side view of the lamella being excavated). During the excavation, the angle of the twin boundarywith the surface of the lamella was thus perfectly known, and was purposely set to 8°. The next steps consisted in the typical lift-out, transfer to a grid and thinning of the lamella. During these final preparation steps, the exact positioning of the twin boundary in the lamella was lost, especially during the thinning step. How- ever, the angle of the twin boundary with the surface of the lamella remained largely unchanged. Besides, as it is detailed in the Methodology for the Determination of the Depth Resolution section, the position of the twin boundary could be determined by diffraction later on. The last step consisted in applying a low energy cleaning at 5 keV, 240pA for 60 s on each side in order to remove as much as possible the amor- phous layer induced by the milling at 30 keV. With such final polishing, we estimate that there is about 5 nmof amorphous Si on each side of the lamella according to Tee (Tee et al., 2009). Because this layer is likely thin, but also because we have no way to measure its thickness, it is neglected for the remainder of the study. (Note that the thicker the amorphous layer, the more underestimated, i.e. shorter, the depth resolution would be with the present methodology.) At this point, the thickness of the lamella was largely unknown, but seemed to range from under 100nm up to


Figure 3. Scheme of the Si lamella produced by FIB and used for the determination of the depth resolution in on-axis transmission Kikuchi diffraction (TKD). SEM, scanning electron microscope.


220nm according to side views, with a thickness gradient


across the lamella (Fig. 3). This thickness variation, although first unintended, was then purposely not corrected by addi- tional milling. This way, it became possible to study the dependence of the depth resolution with the sample thick- ness. For this, the local thickness of the lamella at the point of measurement of the depth resolution is determined accord- ing to the methodology described in the next section.


Methodology for the Determination of the Depth Resolution Every time the text refers to the depth resolution, we are using the following definition: we define the depth resolution in TKD as the minimal thickness of material at the bottom of the sample such that the material above does not produce a contribution to the Kikuchi diffraction pattern that can be detected, in the general case by analysis software, but in the


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