399 23
The Importance of Beam Placement
The placement of the beam on the particle relative to the EDS
detector position can have a strong effect on the measured
spectrum. For particles of intermediate size where the inter-
action volume is contained within the particle, placing the
beam on the side of the particle away from the EDS results in
an extended absorption path. The generated X-rays must
pass through a large mass of the particle to reach the detector.
This effect is illustrated schematically in. Fig. 23.23, where
absorption paths for X-rays generated near the maximum
penetration of beam electrons in the particle are compared
for three beam positions: top center, and at positions directly
facing and away from the EDS. Because absorption follows
an exponential dependence on path length and becomes
increasingly significant for low energy photons below
approximately 4 keV, the low energy portion of the spectrum
will show the strongest absorption effects. Spectra recorded
at the top of a 5-μm-diameter particle and on the side away
from the EDS are compared in. Fig. 23.24 demonstrating a
factor of two difference in the intensity measured at the
energy of Mg K-L2,3 (1.254 keV). For a beam placed on the
curved side of the particle facing the EDS, the absorption
path length is actually reduced relative to the top center, lead-
ing to extra emission from the low energy photons, creating
about 30 % excess for Mg K-L2,3, as shown in the comparison
of spectra in. Fig. 23.25.. Figure 23.26 shows the impact of
beam placement on the accuracy of quantitative analysis, as
discussed below. It is thus critical that the analyst is always
aware of the relative position of the EDS in the SEM image
when choosing locations to analyze. A reliable way to locate
the EDS is to record an X-ray spectrum image and examine
selected X-ray intensity maps, as shown in. Fig. 23.27. The
general rule that the source of the apparent illuminationabc. Fig. 23.22 Particles deposited on a thin (~20-nm) carbon film sup-
ported on a copper grid shown at various magnifications. a Nominal 200
X, field width 351 by 263 micrometers; b nominal 1 kX, field width 70 by
53 micrometers; c nominal 5 kX, field width 14 by 11 micrometers
EDS. Fig. 23.23 Schematic illustration of the effect of beam placement
on a particle on the length of the absorption path to the EDS detector
23.6 · Particle Analysis