Scanning Electron Microscopy and X-Ray Microanalysis

395 23

scattering. As the beam energy is lowered and the electron
range decreases, penetration through the bottom and sides
diminishes, and at E 0 = 5 keV the interaction volume is com-
pletely contained within the 1-μm-diameter aluminum par-
ticle. The beam penetration effect also depends on the
particle size, as shown in. Fig. 23.18 for particles of various
sizes at E 0 = 20 keV, and on composition, as shown for parti-
cles with a range of atomic numbers in. Fig. 23.19. Moreover,
as opposed to the backscattered electrons in the high angle
portion of the cosine distribution which are likely to leave
the vicinity of the particle without further interaction, the
beam electrons that penetrate through the sides and bottom
of the particle are likely to reach the supporting substrate
where they will create X-rays of the substrate material that
contribute to the overall spectrum measured. This effect can
be seen in. Fig. 23.20, which shows the Al and C peaks cal-
culated by the Monte Carlo simulation for 1-μm diameter Al
spherical particles. At a beam energy of E 0 = 5 keV, the elec-
tron trajectories are contained within the particle which
effectively acts like a bulk target. No electrons penetrate the
sides or bottom to reach the substrate so there is no C contri-
bution to the EDS spectrum. With increasing incident

energy, electron penetration through the particle into the

substrate occurs, and the C peak of the substrate increases

relative to the Al-peak from the particle.

23.6.2 Collecting Optimum Spectra

From Particles

Before meaningful qualitative and quantitative analysis of

particles can be attempted, it is important to optimize the

EDS spectrum that is collected. As illustrated in the trajec-

tory plots in the Monte Carlo simulations shown in

. Figs. 23.17, 23.18, and 23.19, and the calculated spectra
shown in 23.20, because of the impact of particle size and
shape (geometry) on electron interactions, the EDS spec-
trum of a particle will always be compromised compared to
the spectrum of a material of identical composition in the
ideal flat, bulk form. The analyst must be aware of the major
factors that modify particle spectra and seek to minimize
these effects. The particle spectrum can be optimized through
careful sample preparation and by understanding of the fac-
tors that affect the strategy for beam placement.

Al spheres on bulk C

E 0 = 20 keV

0.1 μm 0.2 μm

0.5 μm

1 μm

2 μm 1 μm

. Fig. 23.18 DTSA-II Monte Carlo simulation of electron trajectories
in Al spheres of various diameters at E 0 = 20 keV on a bulk C substrate.
Trajectories inside the Al particle are blue. Green shows trajectories that

emerge from the Al particle which change to orange when they enter

the C substrate

23.6 · Particle Analysis