Scanning Electron Microscopy and X-Ray Microanalysis

153 10

structure, but these energetic BSEs will continue to travel,
backscattering off other nearby specimen surfaces and the
SEM lens and chamber walls, producing additional genera-
tions of SEs at each surface they strike. These additional SEs
will be collected with significant efficiency by the E–T (pos-
itive bias) detector and assigned to each pixel as the beam
approaches the edge, further increasing the signal at a thin
edge relative to the interior and thus increasing the contrast
of edges.

10.4.3 Too Much of a Good Thing: The Bright

Edge Effect Can Hinder

Distinguishing Shape

As the dimensions of a free-standing object such as a particle

or the diameter of a fiber approach the secondary electron

escape length, the bright edge effects from two or more edges

will converge, as shown schematically in. Fig. 10.7 and in

the image of TiO2 particles (e.g., objects in magenta circles)

SE 1 SE 1

SE 1

SE 1

SE 1

SE 1

SE 1

Scan position

SE signal

SE 1

SE 1

SE 2 SE (^2) SE 2
SE (^2) SE 2
SE 2
SE 2
SE 2
SE 1
SE 1
SE 1
SE 1
SE 2
SE 2
SE 1 SE 1
SE 1
SE 1
SE 1
SE 1
SE 1
Scan position
SE signal
SE (^2) SE 2
SE 2
SE 2
SE 2
SE 1
SESE 21
SE 1
SE (^2) SE 2
SE 1
SE 1
SE 1
SE 2
SE 2
a
b

. Fig. 10.7 Convergence of
bright edges as feature dimen-
sions approach the SE escape
distance. a Object edges sepa-
rated by several multiples of the
SE escape distance so that edge
effects are distinct; b object
edges sufficiently close for edge
effects to begin to merge

10.4 · Secondary Electron Contrast at High Spatial Resolution