Scanning Electron Microscopy and X-Ray Microanalysis

400

23

appears to come from the position of the detector immedi-

ately reveals the true EDS position. For this particular parti-

cle, the eclipsing effect of the extended absorption path on

the backside of the particle is readily apparent, even in the

relatively energetic Co K-L2,3 (6.930 keV) intensity map.

Overscanning

“Overscanning” is a strategy by which the beam is continu-

ously scanned over the particle (or rough surface, heteroge-

neous material, etc.) while the EDS spectrum is being

collected. If the magnification control of the SEM is continu-

ously variable, then the size of the scan raster can be adjusted

to bracket the particle, minimizing the fraction of the time

that the beam spends on the surrounding substrate, as shown

in. Fig. 23.28 (yellow box). Alternatively, the scan raster can

be adjusted to fill as much of the particle image as possible

while remaining within the bounds of the particle, thus

avoiding direct beam placement on the substrate, as shown in

. Fig. 23.28 (green box).
While useful for gaining qualitative information on the
constituents of a particle, overscanning only has utility if the
particle (or other target object) is homogeneous. Overscanning
discards valuable information on any possible inhomogeneous

structure within the particle. As described below, overscan-

ning is NOT a means by which an “average” composition can

be found by quantitative microanalysis.

23.6.3 X-ray Spectrum Imaging:

Understanding Heterogeneous

Materials

X-ray spectrum imaging (XSI) involves collecting a com-

plete EDS spectrum at every pixel location visited by the

scanned beam. When applied to particle analysis, the XSI

provides the analyst with an abundance of information

which can be recovered by post-collection processing of the

XSI datacube. Composition heterogeneity down to the sin-

gle pixel level can be detected and interpreted. The particle

X-ray intensity images shown in. Fig. 23.27a, b were

extracted from an XSI, and the localization of Ti and Mo in

inclusions is immediately obvious. Solidification dendrites

are outlined in the Ti K-L2,3 map, a feature that is not obvi-

ous in any of the other elemental intensity maps. Software

tools enable the analyst to extract spectra that are represen-

tative of individual components of the microstructure, such

Photon energy (keV)

Counts

Counts

Photon energy (keV)

K411

15-μm sphere (20 nm C film on Cu grid)

E 0 = 20 keV

Beam position: Top Center

Beam position: R/2, away from EDS

100000

10000

1000

3000

2500

2000

1500

1000

500

0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

100

10

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

K411_sphere_beam-top-center_20kV10nA

K411_sphere_r/2_away_20kV10nA

K411_sphere_beam-top-center_20kV10nA

K411_sphere_r/2_away_20kV10nA

. Fig. 23.24 Comparison of EDS spectra recorded at the top center and on the side away from the EDS

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