Scanning Electron Microscopy and X-Ray Microanalysis

385 23

etching is usually required to produce contrast from grains at
different crystallographic orientations and from composi-
tionally distinct phases by creating surface relief through dif-
ferential chemical attack and dissolution or by staining
through chemical reactions. Such microscopic physical relief
creates unwanted topography similar to mechanically pro-
duced scratches that can affect SEM/EDS analysis.
Additionally, in some cases chemical etching can actually
modify the chemical composition of the surface region, so
that it is no longer representative of the bulk of the material.

23.3 Consequences of Attempting Analysis

of Bulk Materials With Rough Surfaces

To illustrate the impact of surface topography on microanal-
ysis results, a microscopically homogenous glass (NIST SRM
470  K411) containing several elements (O, Mg, Si, Ca, and
Fe) that provide a wide range of range of photon energies, as
listed in. Table 23.2, was analyzed with a range of surface
topography (Newbury and Ritchie 2013b). Analysis was per-
formed with NIST DTSA-II at E 0 = 20  keV using elemental
(Mg, Si, Fe) and multi-element (SRM 470 glass K412 for Ca)
standards, with oxygen calculated by assumed stoichiometry,
followed by normalization of the raw result. When analyzed
in the ideal, highly polished (100-nm alumina final polish)
flat form, the analyzed concentrations for the Mg and Fe con-
stituents, selected because of their wide difference in photon
energies, measured at 20 randomly selected locations show
the distribution of results plotted in. Fig. 23.6. The mean of
the 20 analyses falls within +1.8 % relative for Fe and −1.0 %
relative for Mg (SRM certificate values). Of the 20 analyzed
locations, 19 fall within a symmetric cluster that spans

approximately 1% relative along the Mg and Fe concentra-

tion axes, while the results for one location fall significantly

outside this cluster. This anomalous value was found to be

associated with a shallow scratch that remained on the pol-

ished surface (location noted on the inset SEM image).

When this highly polished surface was degraded by direc-

tional grinding with 1-μm diamond grit, 20 analyses at ran-

domly selected locations produce a much wider scatter in the

normalized Mg and Fe concentrations, as shown in. Fig. 23.7,

a direct consequence of the effect of surface geometry.

Creating an even more severe topographic feature by

gouging the polished surface of K411 with a diamond scribe

created the crater seen in the SEM(ET+) SE + BSE image

shown in. Fig. 23.8a. Many locations in this gouge crater

were analyzed, and the results are plotted in. Fig. 23.8b,

showing a very wide range of Mg-Fe results. For comparison,

note that the 20 Mg-Fe results from the highly polished sur-

face, including the outlier seen In. Fig. 23.6, are contained

within the small red box noted on the plot in. Fig. 23.8b.

. Table 23.2 NIST SRM 470 (Glass K411)

Element Mass concentration Characteristic X-ray

energy (keV)

O 0.4236 0.523

Mg 0.0885 1.254

Si 0.2538 1.740

Ca 0.1106 3.690

Fe 0.1121 6.400

11.3

11.4

11.5

8.6 8.65 8.7 8.75 8.88.858.9

BULK

Mg (normalized weight percent)

Fe (normalized weight percent)

*Av

1% relative

1% relative

Analysis of K411: Bulk polished

1 s

Mean compared to

SRM values

Fe 11.41 (11.21%) +1.8% rel

Mg 8.76 (8.85 %) -1.0% rel

Outlier possibly a

surface finish artifact

. Fig. 23.6 Analysis of polished
(0.1-μm alumina final polish)
NIST SRM 470 (K411 glass) at
E 0 = 20 keV with NIST DTSA-II
and standards: elemental (Mg,
Si, Fe) and multi-element (SRM
470 K412 glass for Ca), with oxy-
gen calculated by assumed stoi-
chiometry. Normalized results.
Note cluster of results and one
outlier

23.3 · Consequences of Attempting Analysis of Bulk Materials With Rough Surfaces