37 3
other surface for specimens with more complex topography
than a simple flat bulk target), the SE that they continue to gen-
erate as they approach the surface region will escape and add to
the total secondary electron production, as shown in. Fig. 3.9.
This class of SE is designated SE 2 and they are indistinguishable
from the SE 1 class based on their energy and angular distribu-
tions. However, because of their origin from the backscattered
electrons, the SE 2 class actually carries the degraded lateral spa-
tial distribution of the BSE: because the relative number of SE 2
rises and falls with backscattering, the SE 2 signal actually car-
ries the same information as BSE. That is, the relative number
of the SE 2 scales with whatever specimen property affects elec-
tron backscattering. Finally, the BSE that leave the specimen
are energetic, and after traveling millimeters to centimeters in
the specimen chamber, these BSE are likely to hit other metal
surfaces (objective lens polepiece, chamber walls, stage compo-
nents, etc.), generating a third set of secondary electrons desig-
nated SE 3. The SE 3 class again represents BSE information,
including the degraded spatial resolution, not true SE 1 infor-
mation and resolution. The SE 1 and SE 2 classes represent an
inherent property of a material, while the SE 3 class depends on
the details of the SEM specimen chamber. Peters ( 1984 ) mea-
sured the three secondary electron classes for thin and thick
gold targets to estimate the relative populations of each class:
Incident beam footprint, high resolution, SE 1 (9 %)
BSE generated at specimen, low resolution, SE 2 (28 %)
BSE generated remotely on lens, chamber walls, SE 3 (61 %)
A small SE contribution designated the SE 4 class arises from
pre-specimen instrumental sources such as the final aperture
(2 %) that depends in detail on the instrument construction
(apertures, magnetic fields, etc.). These measurements show
that for gold the sum of the SE 2 and SE 3 classes which actually
carry BSE is nearly ten times larger than the high resolution,
high surface sensitivity SE 1 component. These three classes of
secondary electrons influence SEM images of compositional
structures and topographic structures in complex ways. The
appearance of the SE image of a structure depends on the
details of the secondary electron emission and the properties
of the secondary electron detector used to capture the signal,
as discussed in detail in the image formation module.References
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M (1993) Electron-specimen interactions in low voltage scanning
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measurements for polycrystalline copper with a retarding-field
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enriched signal suitable for high resolution SEM on bulk specimens.
In: Kyser DF, Niedrig H, Newbury DE, Shimizu R (eds) Electron beam
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scanning electron microscope. Scanning 1:195
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scope. Scanning 3:35SE (^1) SE 2
SE 3
. Fig. 3.9 Schematic diagram
showing the origins of the SE 1 , SE 2 , and
SE 3 classes of secondary electrons. The
SE 1 class carries the lateral and
near-surface spatial information defined
by the incident beam, while the SE 2 and
SE 3 classes actually carry backscattered
electron information. The blue rectangle
represents the escape depth for SE, and
the cylinder represents the volume from
which the SE 1 escape
References