Biomedical Engineering Reference
In-Depth Information
a
Parallel
Beam
SAED
c
b
Convergent
Beam
Microdiffraction
d
e
Convergent
Beam
CBED
Large Angle
Convergent Beam
LACBED
Fig. 3.7
Incident beam modes (a) and type of electron diffraction patterns: (b) SAED, (c) microd-
iffraction, (d) CBED and (e) LACBED
using a set of incident beam deflection coils or using an annular aperture located at
the illumination condenser lens C2.
4.3 Analytical TEM/STEM Microscope and “Dedicated STEM”
The most high-performance equipment for microanalysis is the “dedicated STEM,”
the only example of which is the HB5 VG (vacuum generator). The vacuum is very
high (10
-6
Pa in the column and 10
-9
Pa in the gun) in this device in order to help
prevent contamination of the tip and the sample. Its particularity is that it has a FEG
that emits a narrow beam that scans the sample as in an SEM. STEM imaging only
uses transmitted signals. With a narrower size and greater intensity, the emission tips
of these types of guns are the only ones that enable analyses at the atomic column
scale.
In STEMmode (a narrow, convergent beam scanning the sample), various signals
emitted by the sample can be used (e.g., elastic, inelastic, scattered, and non-
scattered transmitted electrons, as in Fig.
3.8a
) and can be detected using an annular
dark-field detector located below the sample.
Depending on both the position of the detector and the detection angle, bright-
field or annular dark-field images (ADF and HAADF) can be formed. STEM mode
allows for nanodiffraction, EDS microanalysis down to the nanometer scale, and
high-resolution
Z
-contrast chemical imaging, which is also called high-angle annu-
lar dark-field imaging, HAADF (Fig.
3.8b
). This mode also allows for the analysis
of the concentration profiles in EELS image-spectrum mode. One of the advantages
of using TEM/STEM instead of a dedicated STEM is the rapid change of the sample
without heating or cleaning it by plasma cleaner. The advantage of STEM mode is
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