The Different Observation Modes in Electron Microscopy (SEM, TEM, STEM) - Sample Preparation Topic for Transmission Electron Microscopy - page 52

quantitative EDS analysis in TEM are different from those of the SEM because of
sample thickness (5-200 nm). The same ZAF correction programs (atomic number,
absorption, and fluorescence corrections) as those used in SEM and WDS micro-
probe cannot be used, as the sample thickness is not known. In the programs for
TEM analysis, fluorescence effects are ignored because the sample is thin and there
is little fluorescence. The use of quantification in TEM involves the K -factor calibra-
tion for each element, i.e., its emission has a constant value compared to a standard
element, which has an identical chemical environment to that of the phase to be
analyzed. These challenges are especially problematic for light elements. Whereas
quantitative analysis provides correct results for heavy atoms, the quantitative anal-
ysis of light atoms by EDS is not resolved. The error rate on the molar concentration
of chemical elements in EDS is 5% for light elements and 0.1% for heavy elements.
5.6 EELS Spectroscopic Analysis Modes in TEM and TEM/STEM
Energy-loss analysis is used to quantitatively measure the concentration of a chem-
ical element in a material. In the case of EELS quantitative analyses, the problem
of standards is even more pointed since this technique is used to determine the type
of chemical bond as well as the chemical environment of each element. In addition
to this aspect is the fact that this method provides the best results for the analysis
of light elements, making it a complementary technique to EDS. The error rate on
the molar concentration of chemical elements is 0.1% for light elements and 5% for
heavy elements.
6 Conclusion and Information Assessment
A specimen can be analyzed and studied in rather different ways if there are detec-
tors for each signal emitted by the sample during its interaction with an electron
beam. Depending on the signal used, the information is quite different:
- Secondary electrons are used to observe surface topography, which can be
performed using an SEM or a TEM.
- Backscattered electrons are used to produce images of phase distribution and
surface topography for thick samples in SEM. They will also help to highlight
crystallographic orientations using the electron backscattered diffraction mode
(EBSD).
- X-rays are used to perform qualitative or quantitative elemental analysis as well
as chemical mapping. These analyses can be performed in SEM and TEM.
- Auger electrons are used to identify the chemical species of the superficial layers
of the sample (1 nm maximum), which can be done using an SEM dedicated to
spectroscopy.