149 10
energy and composition dependent.. Table 10.2 gives the
diameter of the footprint of the area that contains 90 % of the
BSE, SE 2 , and SE 3 emission, which is compositionally depen-
dent, as calculated from the cumulative radial spreading plot-
ted in. Fig. 2.14. The radial spreading is surprisingly large
when compared to the distance between pixels in. Table 10.1.
For a beam energy of 10 keV, the BSE (SE 3 ) and SE 2 signals
will delocalize out of a single pixel at very low magnifications,
approximately 40× for C, 200× for Cu, and 1000× for Au. Even
allowing for the fact that the average observer viewing an SEM
image prepared with a high pixel density scan will only per-
ceive blurring when several pixels effectively overlap, these are
surprisingly modest magnification values. Considering that
high resolution SEM performance is routinely expected and is
apparently delivered, this begs the question: Is such poor reso-
lution actually encountered in practice and why does it not
prevent useful high resolution applications of the SEM?. Figure 10.1a shows an example of degraded resolution
observed in BSE imaging at E 0 = 20 keV of what should be
nearly atomically sharp interfaces in directionally solidified
Al-Cu eutectic. This material contains repeated interfaces
(which were carefully aligned to be parallel to the incident
beam) between the two phases of the eutectic, CuAl 2 interme-
tallic, and Al(Cu) solid solution. A similar image is shown in
. Fig. 2.14 with a plot of the BSE signal (recorded with a large
solid angle semiconductor detector) across the interface. The
BSE signal changes over approximately 300 nm rather than
being limited by the beam size, which is approximately 5 nm
for this image. The same area is imaged with the Everhart–
Thornley detector(positive bias) in. Fig. 10.1b and shows
finer-scale details, that is, “better resolution.” The positively
. Table 10.1 Relationship between nominal magnification
and pixel dimension
Nominal magnification
(10 × 10-cm display)Edge of
scanned area
(μm)Pixel pitch (1000 x
1000-pixel scan)40× 2500 2.5 μm
100× 1000 1 μm
200× 500 500 nm
400× 250 250 nm
1000× 100 100 nm
2000× 50 50 nm
4000× 25 25 nm
10,000× 10 10 nm
20,000× 5 5 nm
40,000× 2.5 2.5 nm
100,000× 1 1 nm
200,000× 0.5 500 pm
400,000× 0.25 250 pm
1,000,000× 0.1 100 pm. Table 10.2 Diameter of the area at the surface from which
90 % of BSE (SE 3 ) and SE 2 emerge
E 0 C Cu Au30 keV 11.8 μm 2.6 μm 1.2 μm
20 keV 6.0 μm 1.4 μm 590 nm
10 keV 1.9 μm 410 nm 180 nm
5 keV 590 nm 130 nm 58 nm
2 keV 128 nm 28 nm 12 nm
1 keV 41 nm 8.8 nm 3.9 nm
0.5 keV 12.7 nm 2.8 nm 1.2 nm
0.25 keV 4.0 nm 0.9 nm 0.39 nm
0.1 keV 0.86 nm 0.19 nm 0.08 nmab. Fig. 10.1 Aluminum-copper eutectic alloy, directionally solidified.
The phases are CuAl 2 and an Al(Cu) solid solution. Beam energy = 20 keV.
a Two-segment semiconductor BSE detector, sum mode (A + B). b Ever-
hart–Thornley detector(positive bias)
10.3 · Pixel Size, Beam Footprint, and Delocalized Signals