Enrichments, Chapter 5.2 STEM-in-SEM
Calculating the contrast in STEM-in-SEM operation
The magnitude of the contrast available from small particles imaged with the
STEM-in-SEM mode can be estimated from first principles. The cross section
for elastic scattering greater than or equal to a specified angle is given by:
Q > 0 = 1 .6 2 x 1 0
- 20
Z
2
2
cot
E
2
0
event s > 0
2
e
-
at om
cm
2
(5.2er-1)
where Q(>) is the cross section for an elastic scattering event exceeding a
specified angle , Z is the atomic number of the scattering atom, and E is the
electron energy (keV). Equation (5.2er-1) gives the probability for scattering
through angles greater than a specified value. From Figure 5.2er-1, the angle of
interest is the STEM detector semi-cone angle. Electrons elastically scattered
through an angle greater than the detector semi-cone angle o miss the detector
and contribute to the contrast. From the cross section, the mean free path
(>o) for scattering is given by
(>o) = A/Q(>o)NAcm/event >o
(5.2er-2)
Consider that a condition of single scattering exists, that is, each electron suffers
no more than one scattering event while passing through the specimen. As the
specimen thickness approaches , each electron suffers at least one scattering
event > o. For thinner samples, the contrast is then estimated as the fraction of
the mean free path
C = t/
(5.2er-3)
The thickness for a specified contrast is then:
t = C CA/Q(>o)NA 
(5.2er-4)
Table 5.2er-1 gives the thicknesses of various materials to produce STEM in
SEM contrast levels of 0.10 (10%) and 0.25 (25%) for beam energies of 20 keV
and 30 keV and a STEM detector angle of o= 10o, which is the angle
subtended by a detector 1.8 cm diameter placed 5 cm below the specimen. For
intermediate and high atomic number particles, the contrast sensitivity of the
STEM signal is very high, and very small particles produce high contrast.
Figure 5.2er-2 shows an example of platy clay mineral particles
(aluminosilicates) deposited on a 20 nm (nominal thickness) carbon film and
viewed simultaneously with the conventional positively-biased Everhart-Thornley
detector (secondary and backscattered electrons, collecting above the
specimen) and with a scintillator STEM detector. Note the strong contrast in the
STEM image that permits individual small particles to be recognized.
Table 5.2er-1
Contrast in STEM in SEM Images
Particle Thickness (nm) To Produce Specified Contrast
(STEM Detector Angle, o = 10o)
20 keV
25%
30 keV
Target Z
10%
10%
25%
C
6
46 nm 110 nm
100 nm
260 nm
Al
13
19
46
42
100
Fe
26
3.3
8.2
7.4
18
Au
79
0.5
1.3
1.1
3