Rise Distance Method
Figure 4: Simulated distribution of 10,000 20-percent
to 80-percent
rise
distances calculated for a FIB system with 1 pA current suffering from shot noise in the beam current proportional to the square root of the current. The average rise distance was 3.7 nm for the conditions chosen, and the standard deviation for the shot noise limited beam was 0.3 nm. Note the vertical scale is logarithmic.
Figure 6b: The rise current I(x) calculated for beam with rFWHM ≈ 4 nm for a knife edge with a radius of curvature rc = 10 nm and for rc = 0 nm. In both cases the results are normalized to the current for the largest value of x. The rise distances are 2.2 and 3.5 nm, respectively. rc = 10 nm = 2.5∗ rFWHM.
Figure 5: A knife edge with a curved edge of radius rc is scanned by an ion beam. The center of the beam has moved a distance δx from the end of the knife edge. e- represents a secondary electron.
Figure 7: A micrograph of a knife edge taken with a Ga+ ion beam. Note the bright line at the right edge of the specimen, corresponding to the effect shown in Figure 6b. The effect is stronger than in the micrograph shown in Figure 3.
Clearly, in the case where there is a curvature larger than
the beam FWHM, the results are distorted, possibly severely. An actual example is shown in Figure 7. Although the dimensions and exact morphology of the non-uniformity of the knife edge are not known, it is clear that there is an edge effect causing a significant increase in secondary electron emission.
Conclusions We believe it is reasonable to conclude that rise-distance
Figure 6a: The rise distance of a beam with FWHM ≈ 4 nm striking a rounded knife edge with radius rc = 1 nm and rc = 0 nm. The 20-percent to 80-percent rise distances are 3.1 and 3.5 nm, respectively, where the maximum value of the current is used in each case. Note there is no statistical noise in these figures.
2011 May •
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measurements offer realistic estimates of the size of the waist of a FIB. However, it is critically important that the measurement be done in a reproducible way so that one can have confidence in its reliability. Tat means making enough measurements
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