edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 2 14 on a larger scale in the µSHD curve of Fig. 3h signals only an information loss as the current drops to the lowest level. From Figs. 3i and j, it is clearly observed that the range within which the µSHD vary is between 0.03 and 1.54 for the effect of separation distance, while that is between 0.05 and 1.40 for the effect of current. As Orozco qualitatively noted in the original reference, the impact of reducing the current on the quality of the images is evident, but clearly less than the effect of distance.[18] Similar approaches and procedures as discussed in the SAM section can be followed to obtain data insight on the interrelationships between the measurement distance, current, and µSHD curves of the MFI images. SCANNING ELECTRON MICROSCOPY Due to some technique differences between SEM and the focused ion beam (FIB), Vanderlinde[16] pointed out that the adjustment of the focus and astigmatism corrections is the most difficult for a novice electron microscopist. Fortunately, such experience and skills can now be systematically quantified and demystified with the tool of µSHD curves. Figures 4a to d are the examples provided by Vanderlinde representing the same image under focus, over focus, true focus, and true focus after correction for astigmatism, respectively. The µSHD curves of the under- and overfocus images, i.e., curves a and b in Fig. 4e, are found to oscillate with increasing amplitude Fig. 3 MFI current density image from backside scanning under 1 mA with the separation distance decreasing from 200 µm (a) to 75 µm (b), 40 µm (c), 5 µm (d), and with a fixed separation distance of 5 µm under 1 mA (e), 500 µA (f), 100 µA (g), 25 µA (h). Images courtesy of M. Hechtl.[18] µSHD curves of (a) to (d) plotted in (i) and (e) to (h) in (j). (a) (b) (i) (j) (c) (d) (e) (f) (g) (h)
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