Department of Materials Science and Engineering
MSE 316 – Material Sceince II
SEM – Chemical Analysıs
Nisanur Ayhan
270208035
Assistant : Tuğçe Aybüke Arıca Güvenç
Abstract
The primary purpose of this experiment is to understand how chemical analysis of various
samples is done using Energy Dissipative X-ray Spectrometer (EDS). Chemical analysis can
be performed by detecting the characteristic X-ray emitted from the sample shell as a result of
interactions between the incident electron beam and electrons in the sample shell. Detectors
detect and process these X-rays. The results of chemical analyzes are presented in the form of
a spectrum. Although WDS and EDS perform the same function, their working principles are
slightly different. The wavelength of the emitted and detected X-ray is measured by WDS and
its energy is measured by EDS. As a result, differences between the peaks of the two methods
are observed. Various methods such as point, line, area, mapping can be used while performing
chemical analysis. WDS and EDS are critical and highly sensitive chemical analysis methods.
As a result, many factors affect the accuracy and precision of the analysis, from the electrical
voltage to the polish quality of the material surface.
Introduction
The main aim of this experiment is to understand how chemical analyzes of various samples
are done using Energy Dissipative X-ray Spectrometer (EDS). Apart from atomic contrast and
3D topology analysis, chemical analysis can be performed with SEM by two methods such as
Energy Dissipative X-ray (EDS) and Wavelength Dissipative X-ray Spectroscopy (WDS). The
major difference between EDS and WDS is that EDS measures the energy of emitted X-rays
while WDS measures the wavelength of emitted X-rays [1]. First, how the analyzed X-ray is
emitted is an important issue. When the incoming electron beams hit the target, various
interactions occur between the electron beam and the sample. The incoming electrons interact
with the electrons of the sample in the inner shell and excite them and are struck by an electron
beam. As a result of this excitation, an vacant orbital is formed and this orbital has to be filled
by outer shell electrons at higher energy levels, resulting in emission of X-rays characteristic
of each element. The K, L, M, and N shells have 1, 2, 3, and 4 fundamental quantum numbers,
respectively. Kα, Kβ of La, etc. to the emitted X-rays. is called. Kα stands for X-rays emitted
from the transition of an electron from the L-shell to the K-shell. On the other hand, Kβ means
M shell to K shell and La means M shell to L shell [1,4]. The minimum energy required to
excite an electron from the shell is called the critical excitation energy and increases as the size
of the atom increases. Therefore, the energy of Al Kα is lower than Ni Kα [1]. When the index
electrons interact with the sample, not only the characteristic X-ray is emitted, but also the
continuous X-ray. Incoming electrons are slowed down due to their interaction with atomic
nuclei that have a positive nuclear field, and the energy loss is spread. As a result, continuous
X-rays are produced and they form the background in an x-ray spectrum, which affects the
accuracy of the measurement [1]. This characteristic emitted X-ray is detected by two detectors
called EDS and WDS detectors. An energy dispersive (EDS) detector is used to separate the
unique x-rays of the various elements into an energy spectrum, which is then analyzed by the
EDS system software to calculate the abundance of the individual elements. Lithium-entrained
silicon (Si (Li)-EDS) and silicon-entrained (SDD-EDS) detectors are two commonly used
detector types, and SDD-EDS, which does not require liquid nitrogen cooling, is preferred [6].
The emitted X-rays are absorbed by the semiconductor detector and converted into a charge.
Then, the field-effect transistor converts the charge into a voltage signal, and these signals are
measures to determine the energy of the detected X-ray [7]. WDS works on a different principle
than EDS. In WDS, the analysis of wavelength of the emitted characteristic X-rays is based on
Bragg’s law. There are some important terms in analysis processing. The processing time is
taken while each analysis was performed. Also, dead time is the time between the measurement
of each signal, and the time required for signal collecting is referred to as the live time
Subtracting the dead time from the total measurement time yields the net measurement time
[1]. Note that the more extended time, the higher-energy resolution EDS spectrum. There are
different analysis types of EDS. A single point is one of them and the chemical composition
was determined from a single point. Also, line scanning is another method. The elemental
concentration is achieved along the line. The area scan method gives information about a certain
area. Mapping is the last method and images of qualitative elemental distribution are collected.
When two distinct x-rays arrive at the detector at the same time, the detector may treat them
as one and show it at an energy equal to the sum of the two X-rays' energies. This peak is an
artifact known as the sum peak, double peak, or coincidence peak. The energy of characteristic
x-ray peaks from multiple elements might be the same, causing them to overlap in the EDS
spectrum and encountered overlapping peak pairs include S Kα,β-Mo Lα, S Kα,β-Pb Mα, Ti
Kα-Ba Lα, etc. In addition, the electrons of silicon in the detector can be excited by the emitted
X-ray energies from samples and result in a small peak in the EDS spectrum and this is called
Si- escape peak [1].
Procedure
Equipments:
•
Sample holder
•
Cast iron
•
Scanning electron microscopy
•
Secondary electron detector
•
Backscatter electron detector
•
Energy dispersive x-ray detector (EDS)
After the examined material is chosen, the sample chamber is evacuated, the sample is placed
in the chamber and pump the vacuum. When the desired vacuum is achieved, the gun is open.
Then, the accelerating voltage is determined, and filament flow is started. Then, the viewing
mode is chosen and all adjustments like brightness and contrast are made. In the final, chemical
analysis is done with different methods (point, area, line, and mapping) by using an EDS
detector. After the chemical analysis is completed, the filament flow is stopped and the gun is
turned off. Then, the vacuum is evacuated, the sample is removed from the chamber and the
system is vacuumed again.
Results
Figure 1 shows the SEM image of the area where the mapping method is applied on the left
and the results of the mapping analysis method with various element distributions (right). In
Figure 2, mapping analysis and distribution of C, O, Fe, Si elements in the sample are given. In
Figure 3, the result of the line analysis method and the composition and distribution of the
elements along the line are given. Figure 4 shows the location of the point analysis and Figure
5, Figure 6, Figure 7 and Figure 8 show the result of point 1, point 2, point 3 and point 4,
respectively.
Figure 1: SEM surface area and element distributions in that area
Figure 2 : Mapping analysis of C (top, left) , O (top, rigth) , Fe (bottom, left) and Si
(bottom,right) elements.
Figure 3: Line analysis of cast iron (top), and results of line analysis (bottom)
Figure 4 : The point will be analysised
Figure 5: EDS results of spot 1
Figure 6 : EDS results of spot 2
Figure 7: EDS results of spot 3
Figure 8 : EDS results of spot 4
Discussions
Chemical microanalysis of conductive or non-conductive samples can be done with Energy
Dispersion X-ray Spectrometer (EDS) and Wavelength Dispersion X-ray Spectroscopy (WDS)
in SEM. In both methods, it is the method that does not harm the sample for chemical analysis.
These methods are based on the analysis of characteristic X-rays emitted from the sample due
to the interaction of incoming electrons with the electron shells of the sample. However, EDS
measures the energy of this characteristic X-ray, while WDS measures its wavelength. As a
result of this interaction, the electron in the inner shell of the sample is detached and the electron
at higher energy levels fills the emptied electron space. When this phenomenon occurs, the
characteristic X-ray is emitted as the electron descends to a lower energy level. These X-rays
are called characteristic because each element emits a different X-ray and is proportional to its
atomic number. The nuclei hold the electron with some energy, and if the incoming beam of
electrons wants to expel the electron from the shell, it must have an energy of at least twice the
critical excitation energy. This required energy increases as the size of an atom increases as it
requires more energy due to increased excitation energy. Therefore, each element has a different
characteristic X-rays.
After the characteristic x-ray emitted from the detector is collected and analyzed, its chemical
analysis is given in EDS or WDS spectrum graphs. In the EDS and WDS spectrum, the x-axis
is given as energy in keV and the y-axis is the count number. Various kinds of signals are
emitted from various depths of the interaction volume. The maximum width of the interaction
volume created by electrons or X-rays projected onto the sample surface is defined as the Xray spatial resolution [1]. In Figure 9 a diagram of the different X-ray ranges can be seen.
Figure 9 : Schematic showing x-ray range [1].
As the depth of penetration increases, lowering the X-ray spatial resolution. Because X-rays are
generated from a larger depth and width of the sample, materials with low density (low Z) will
yield X-ray signals with limited spatial resolution, as can be seen in Figure 10. The X-ray spatial
resolution attained in a specimen is reduced as the accelerating voltage is increased. Spectral or
energy resolution in EDS is called as minimum energy of a peak in the spectrum that can be
resolved, and it directly depends on the detectors. Note that WDS has better resolution than
EDS.
Figure 10 : Electron range and x-ray spatial resolution for Al-Cu alloy of
different compositions at a beam energy of 20 keV. a) low density, b) high density [1].
Even Though both EDS and WDS methods are used for chemical analysis, they use different
parameters. EDS analysis the energy of the X-ray. On the other hand, WDS analyzes according
to the wavelength of the X-ray, and results in a different spectrum. The difference between EDS
and WDS spectra can be seen in Figure 11.
Figure 11: BaTiO3 EDS and WDS spectra. The red portion of the spectrum depicts a lowresolution EDS spectrum. The WDS spectrum of the same spectral region is shown in the blue
part of the spectrum. EDS does not resolve Ba L1 and Ti K1,2 but partially resolves them in
WDS [8].
The EDS spectrum has more background and peaks are not as sharp as in the WDS spectrum.
Ba Lα1 and Ti Kα1,2 are essentially entirely resolved from one another in the WDS spectrum,
however, they are not resolved in the EDS. The main benefit of WDS is its higher spectral
resolution, which allows it to solve complicated elemental determinations that are more difficult
or impossible to perform with EDS. WDS has better energy resolution and lower background
compared to EDS. So, WDS can give more accurate and precision results than EDS. There are
four analysis types, point, line, area, and mapping. The chemical analysis is done from the point,
along the line, and from the determined area, respectively. The mapping mode is applied to all
the images, and it shows all element distribution at one time or shows each element's
distribution alone. Figures 1 and 2 show the mapping analysis of the cast iron sample. The
distribution of the elements can be seen. Especially, iron and oxygen atoms can be detected
clearly as red and yellow, respectively. The dominance of the red areas is expected because of
the cast iron sample, and evident yellow areas mean that the oxide-forming in cast iron. In
addition, according to Figure 2, the homogenous distribution of Fe (red) and Si (turquoise)
elements can be seen. However, the C elements (purple) can be seen as uncertain and clustered
at the centre of the image. Also, the distribution of oxide can be analysed from the mapping
images. In Figure 3, a line analysis of the same sample is given. The image is in BSE mode,
and it is an advantage to see the difference between darker and brighter areas, and it helps in
determining where the line will pass. The darker area has oxygen content and the brighter has
Fe. The line analysis result tells us the same. When the line passes on the brighter area, Fe
elements become more. However, the amount of O is increased and Fe is decreased when the
line passes on the darker area. At last, the point analysis gives us information about the chemical
composition of the chosen point on the image, and also it gives the atomic and weight per cent
of the composition at the spot. In Figure 5, the weight per cent of Fe is almost 96.53 % and O
is 1.01% at spot 1. On the other hand, in Figure 7, the weight per cent of O is 12.01 % and Fe
is 35.34 % at spot 3. The appropriate accelerating voltage is a crucial parameter for EDS
because of the critical excitation energy. Generally, the energy of the incident electron beam
should be a minimum of two times of excitation energy. The EDS detector has no built-in
focusing capabilities. There is, however, an ideal operating distance that permits the most Xrays to enter the EDS detector. So, too high or too low WD affects the number of X-rays that
enter the detector. To eliminate the topography that causes the geometric effects, the sample
must be extensively polished. In addition, Chemically etching the highly polished surfaces can
affect the composition of the near-surface area that is probed in the electron-excited X-ray
analysis. A semiconductor detector monitors the energy of incoming photons in an energydispersive X-ray spectrometer, and the detector heats up throughout the measurement. To
maintain its integrity and resolution, the detector must be cooled using liquid nitrogen or Peltier
cooling. Also, the sample should be prepared properly before the analysis, and vacuum, voltage,
and WD are some of the parameters that should be careful with appropriate chemical analysis.
Conclusion
To summarize, scanning electron microscope images are very important in chemical analysis
as well as the study of various surface properties or atomic contrast and provide a lot of critical
information about the sample; EDS and WDS are two methods used for this purpose. Its purpose
is to determine the chemical composition of the sample. WDS measures the wavelength, while
EDS measures the energy of the characteristic X-ray. The energy dispersive spectroscopy
(EDS) method is mostly used for qualitative material inspection. Different inspection methods
such as point, line, area and mapping are used during the analysis. The weight and atomic
percentage of each element at a point is determined by point analysis. In addition, mapping is
an important method that provides information about the distribution of each element. In
addition, mapping is an important method that gives information about the distribution of each
element in the entire image and can determine whether the elements are uniformly distributed.
WDS has some advantages over EDS, such as higher energy resolution, whereas EDS can miss
overlapping peaks. In addition, the entire energy of the characteristic X-rays emitted by each
element is determined and recorded in the database. Consequently, the voltage must be chosen
with this in mind. Other factors such as WD or surface quality also have an impact on the quality
and accuracy of chemical analysis. Since EDS or WDS is a quantitative method, it is possible
to obtain an accurate and sensitive chemical analysis by adjusting all the parameters following
the sample. EDS or WDS is a very sensitive method and depends on many parameters, so
sensitive and precise chemical analysis can be.
References
1- Ul-Hamid, Anwar. A beginners' guide to scanning electron microscopy. Vol. 1. Cham,
Switzerland: Springer International Publishing, 2018.
2- Nanakoudis, A. (2022, February 18). EDX analysis with sem: How does it work?
Accelerating Microscopy. Retrieved March 20, 2022, from
https://www.thermofisher.com/blog/microscopy/edx-analysis-with-sem-how-does-itwork/
3- Shindo, D., & Oikawa, T. (2013). Analytical electron microscopy for materials science.
Springer Science & Business Media. doi:10.1007/978-4-431-66988-3_4
4- https://www.matsusada.com/column/sem-tech3.html
5- https://quizlet.com/224914640/radiation-physics-flash-cards/
6- Goodge, J. (2017, April 26). Energy-dispersive detector (eds). Geochemical
Instrumentation and Analysis. Retrieved March 20, 2022, from
https://serc.carleton.edu/research_education/geochemsheets/eds.html#:~:text=An%20e
nergy%2Ddispersive%20(EDS),the%20abundance%20of%20specific%20elements.
7- Goldstein, J. I., Newbury, D. E., Echlin, P., Joy, D. C., Lyman, C. E., Lifshin, E.,
Sawyer, L., & Michael, J. R. (2003). X-ray spectral measurement: EDS and WDS.
Scanning Electron Microscopy and X-Ray Microanalysis, 297–353.
https://doi.org/10.1007/978-1-4615-0215-9_7
8- https://assets.thermofisher.com/TFS-Assets/MSD/Application-Notes/WP52608principles-applications-parallel-beam-wds-xray-spectroscopy.pdf