Optimized Conditions for PVC Observations
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In many cases, it is hard to obtain distinguishable PVC. The main factors determining the degree of PVC are the generation of charge and the reduction of charge due to leakage current from the observing structure. Therefore, the condition should be optimized in order to maximize the PVC signal:

       i) Increase the current of the primary beam. The generation of charge increases with the beam current. However, the structure can be destroyed at a high beam current if an ion beam is used in FIB observation. The optimized Ga ion current density for PVC observation is in the range of 1.75 x 10-3 to 2.5 x 10-2 pA/µm2. Figure 3844a shows contrast comparison between low and high beam currents in FIB observation.

       ii) Increase the scanning area on the structure. This can be achieved by tilting the sample. In this case, the rate of charge generation can be higher.

       iii) Lower the speed of beam scan. This can also improve the charge generation.

       iv) Avoid deposition-induced leakage current. For instance, metal deposition on the adjacent structures can cause leakage current.

       v). Optimize the beam-scanning direction relative to the shape of the observing structure. The simultaneous scaning on the floating structure can more efficiently generate charges, for instance, scanning in y-direction provides higher PVC signal than that in x-direction in Figure 3844b.

Contrast comparison between high and low currents in FIB observation

Figure 3844a. Contrast comparison between low (a) and high (b) currents in FIB observation. Adapted from [1].

Optimized Conditions for PVC Observations

Figure 3844b. Schematic illustration of optimized scanning direction (y-direction)
relative to the shape of the observing structure.

 

[1] Ruediger Rosenkranz, Failure localization with active and passive voltage contrast in FIB and SEM, J Mater Sci: Mater Electron (2011) 22:1523–1535.

 

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