Scanning Electron Microscopy and X-Ray Microanalysis

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23

Particle Sample Preparation: Bulk Substrate

Because beam electrons can penetrate through the sides and

bottom of a particle and reach the underlying substrate, the

measured spectrum will always be a composite of contribu-

tions from the particle and the substrate, as shown in

. Fig. 23.21 for a K411 glass particle (~1 μm in diameter) on
a bulk carbon substrate. To obtain a spectrum that is repre-
sentative of the particle material alone, it is important when-
ever possible to choose a substrate consisting of an element(s)
not contained in the particle of interest. As part of a quality
measurement system, the EDS spectrum of the blank sub-
strate material should be measured to determine what ele-
ments are present at the major, minor, and trace level.
Additionally, it is desirable that the characteristic peak(s) of
the substrate element(s) should not interfere with character-
istic peaks from the constituents of the particle.
Carbon is an excellent choice for a bulk substrate since it is
available in high purity, is mechanically robust, and supplied
by various vendors as planchets with different surface finishes,
including a highly polished, glassy surface that is nearly fea-
tureless as a background for SEM imaging of small particles.
The low energy of the C K X-ray (0.285  keV) is unlikely to
cause interference with most elements of interest. In addition
to bulk substrates, carbon is often used in the form of carbon-
infused tape with a sticky surface to which particles will read-
ily adhere. If a carbon tape preparation is used, the analyst

must measure a blank spectrum of the tape since the polymer

base of the tape frequently contains elements such as oxygen

in addition to carbon. If carbon is of analytical interest, other

low atomic number substrates are available, including high

purity boron and beryllium (but beware of the health hazard

of handling beryllium, especially in the form of beryllium

oxide which may be released by surface abrasion). Other pure

element substrates such as aluminum and silicon can be used

if that element(s) is not of interest, but because of the high

degree of excitation of the Al K-L2,3 and Si K-L2,3 characteris-

tic X-rays under typical operating conditions, a significant

fraction of the EDS deadtime will be taken up by the substrate

X-rays, diminishing the analytical information collected per

unit time on the particle of interest, as well as contributing

coincidence peaks that might be misinterpreted as minor or

trace constituents, for example, 2 Al K-L2,3 (2.974 keV) is close

to the energy of Ag L 3 -M4,5 (2.981  keV) and 2 Si K-L2,3

(3.480 keV) is close to the energy of Sn L 3 -M4,5 (3.440 keV)

When depositing particles on a substrate, the area density

should be minimized to avoid situations where electrons

scattered off the particle being analyzed can strike nearby

particles and excite X-rays, which will then contribute an

artifact (“cross talk”). While this remote excitation is likely to

be at the equivalent of a minor or trace level constituent, it is

critical to understand such contributions when low level con-

stituents are of interest.

Al Ti Cu

Ag Hf Au

1 μm

. Fig. 23.19 DTSA-II Monte Carlo simulation of electron trajectories
at E 0 = 20 keV in 1 μm-diameter spheres of various elements (Al, Ti, Cu,
Ag, Hf, Au) on a bulk C substrate. Trajectories inside the particles are

blue. Green shows trajectories that emerge from the particle which

change to orange when they enter the C substrate

Chapter 23 · Analysis of Specimens with Special Geometry: Irregular Bulk Objects and Particles