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Practical course in scanning electron microscopy
Fortgeschrittenen Praktikum
an der Technischen Universität München
Wintersemester 2017/2018
2
Table of contents
1. Introduction 3
2. Formation of an electron beam 5
3. Lens aberrations 6
4. Interactions of electrons with matter 8
5. Transmission electron microscope (TEM) 10
6. Scanning electron microscope (SEM) 12
6.1 Depth of field 17
7. Detectors 18
7.1 Detection of secondary electrons 18
7.2 Detection of backscattered electrons 19
7.3 Detection of X-rays 20
8. Experiment 21
9. Evaluation of your work 22
10. References 23
3
1. Introduction
Microscopes are used in many fields of science to probe beyond the limits of sight. The
human eye is able to distinguish two points as separate when the distance between them is
at least 0.2 mm. The resolution can be improved using magnifying lenses or whole systems
of these, i.e. optical microscopes. Unfortunately there exists a resolution limit for optical
microscopes based on the wavelength of light. The smallest distance d between two points
which can be resolved is given by the Rayleigh criterion:
𝑑 = 0.61 𝜆
𝑛 sin 𝛼
With:
𝜆: the wavelength of the light,
𝛼: the semi-angle of collection of the lens,
n: the refractive index of the viewing medium
The wavelength of visible light is about 400 nm for blue light and 720 nm for red light. The
resolution which can be reached is therefore about 200 nm equivalent to a magnification of
1000.
To improve resolution further, smaller wavelengths are needed. For doubling the resolution,
ultra violet light can be used. A further increase of the resolution requires electron beams.
In this practical course we are dealing with electron microscopy with a focus on scanning
electron microscopy.
Due to the wave-particle duality the wavelength λ of an electron beam can be calculated
with the well-known de-Broglie equation:
𝜆 = ℎ
𝑝 .
With:
p: the momentum
h: the Planck constant
4
The wavelength depends on the electron momentum after acceleration in an electrostatic
field.
The momentum is:
𝑝 = √2𝑚 ∙ 𝑒 ∙ 𝑈 as 𝐸𝑘𝑖𝑛 = 𝑒𝑈 =𝑝2
2𝑚
With:
e: the elementary charge
U: the accelerating voltage.
This gives for the wavelength:
𝜆 = ℎ
√2𝑚 ∙ 𝑒 ∙ 𝑈 .
At high electron velocity (which is the case at high voltages) a relativistic correction is
needed:
𝜆 = ℎ
√2𝑚 ∙ 𝑒 ∙ 𝑈 (1 +𝑒𝑈
2𝑚0𝑐2)
.
With:
𝑚0: the rest mass of the electron,
c: the light velocity
The relativistic correction is necessary at voltages greater than above 80 kV. Typical voltages
used in scanning electron microscopes are between 1 kV and 30 kV. In this course the
student will operate such a microscope. In a transmission electron microscope higher
voltages of 100 kV to 300 kV are used.
With the above equation we can calculate that for an accelerating voltage of 100, 200,
300 kV we get a wavelength of 3.9, 2.7, 2,2 pm giving a 100 000 smaller wavelength than
light. Inserting into the Rayleigh criterion a resolution of down to 0.2 nm can be theoretically
reached with electron microscopy.
The aim of this course is to acquaint students with electron microscopy with a focus on
scanning electron microscopy. There are mainly two different types of electron microscopes:
the transmission electron microscope and the scanning electron microscope. The way these
two microscopes work is fundamentally different. First a short introduction to transmission
5
electron microscopy is given followed by an introduction to scanning electron microscopy. In
the practical course the students will learn to operate a scanning electron microscope and to
interpret the images.
2. Formation of an electron beam
For creating an electron beam an electron source and an electrical potential for accelerating
the electrons are required. The source is a cathode producing electrons by either thermionic
emission or field emission. The electrons are accelerated by the electric field between the
cathode and the anode. Between the cathode and the anode a cylinder with an aperture,
the so called Wehnelt cylinder, is situated, see Fig.1. This cylinder is set on a negative
electrical potential shaping the electron beam to pass the aperture and being further
accelerated. The electrical field from the Wehnelt cylinder focuses the electrons into a
crossover, which is the virtual electron source for the lenses in the electron microscope
illumination system. The electron beam is scattered on gas molecules when the pressure is
not low enough and becomes wider in diameter. This reduces the imaging quality. The
needed pressure in the gun depends on the type of filament. For stabile operation of the
electron beam the vacuum should be better than 10-4 mbar.
In order to be able to image the nanostructure of the sample the electron beam needs to be
further shaped. As the electrons are negatively charged, this is done using electrostatic and
magnetic fields. In a transmission electron microscope higher magnification is reached
(analogous to the optical microscope lenses) lenses which are electromagnets (often
arranged in a quadrupole). In a scanning electron microscope the lenses are used to form a
focused e- beam. The lenses are not used for imaging. The image quality suffers in both
types of microscopes from lens aberration. In the following the three most relevant lens
aberrations are described.
6
Fig.1: Schematic view of a thermionic electron gun. A high voltage is applied between the cathode and the
anode. The electrical potential of the Wehnelt cylinder shapes the electron beam. The electrical field from the
Wehnelt cylinder focuses the electrons into a crossover, which is the virtual electron source for the lenses in
the electron microscope illumination system. [4]
3. Lens aberrations
As in optical microscopes the resolution of the image is limited by aberrations and therefore
the theoretically possible resolution of 0.2 nm is hard to reach in practice. Some
shortcomings of the lenses can be corrected, others can be at least minimized. A summary of
the three most important defects is given in the following:
Chromatic aberration:
The refractive index of a lens always has a small wavelength dependence with different
wavelengths having different focal lengths. This effect is called chromatic aberration (Fig. 2).
The wavelength of the incident electron beam varies due to the thermal Boltzmann energy
distribution. These differences can be neglected. Therefore chromatic aberration does not
play an essential role in a scanning electron microscope. In a transmission microscope,
where the electrons pass through a thick foil (the sample) chromatic aberration can play a
role.
Fig.2: Lens with chromatic aberration: Different colors, i.e. different wavelength have different foci. [5]
Astigmatism:
Astigmatism (Fig. 3) occurs if the focal lengths are different for perpendicular planes of the
incident beam. This defect originates from limitations in manufacturing perfectly cylindrically
symmetric pole pieces or/and due to not perfectly centered beam- limiter- apertures. It can
also originate from external electrical field. Fortunately astigmatism can be easily corrected
using stigmators. These are octupoles which create a compensating electric field. It is
7
necessary to correct the image for astigmatism before taking images with an electron
microscope.
Fig.3: Lens with astigmatism. [6]
Spherical aberration:
Spherical aberration (Fig. 4) is a lens defect focusing off-axis rays differently than on-axis rays.
This lens defect cannot be corrected. It can be only minimized by centering the lenses and by
limiting off axis beam rays by apertures.
Fig.4: Lens with spherical aberration and with focal length F. Off axis beam are focused at different focal
lengths f1, f2, f3. [5]
These limitations of lenses described above are the most relevant ones for electron
microscopy.
4. Interaction of electrons with matter
An electron beam incident on matter interacts in several ways. Electrons can be elastically
and inelastically scattered in forward and backward directions. They can be absorbed or they
can induce X-ray emission. Scattering in the forward direction is used in transmission
electron microscopes and scattering in the backward direction is used in scanning electron
microscopes.
8
Fig 5: showing an overview of the different signals being used in modern electron microscopes. The direction
shown for each signal does not always represent the physical direction. [7]
Fig.6: Above image gives an idea about the signal depth of the different signals. [5]
Electron scattering can be divided into elastic and inelastic scattering. A collision is called
elastic if there is no energy loss or the energy loss is negligible compared to the incident
energy and only the flight direction of the electrons is changed. Inelastic scattering means
the electrons lose energy. The scattering of an electron at an atomic nucleus is elastic if the
energy loss in this scattering event is small (<1 eV) compared to the primary beam energy
which is of the order of 30-100 keV.
Scattering events in which the incident electrons loose energy and change the flight
direction are inelastic. All interactions between the incident electrons and core electrons of
sample atoms are inelastic. Secondary electrons, auger electrons, cathodoluminescence,
characteristic X-rays are a result from inelastic interactions.
Considering the electron beam as a wave we can separate the scattered electrons into
coherently and incoherently scattered electrons. Elastically scattered electrons in forward
direction are usually coherent. Inelastically scattered electrons are incoherent. The incident
beam can be assumed to be coherent, that means the incident electron waves are in phase
with each and have a fixed wavelength given by the acceleration voltage. Coherently
scattered electrons are those remaining in phase. The intensity of elastic coherent scattering
is highest at relatively low angles (1 -10°) in the forward direction.
5. Transmission electron microscope (TEM)
The elastic, coherent scattering of electrons in forward direction is used in transmission
electron microscopy. The electron beam passes through a thin sample and the transmitted
electrons are detected. This is analogous to an optical microscope where the light passes
through the sample and produces the image, see Fig. 7. By changing of the current in an
electromagnetic lens its focal length is changed resulting in different magnifications. The
switching of lenses as in an optical microscope to get different magnifications is therefore
9
not required.
As the wavelength of the electron beam is of the order of the distance between two atomic
planes the specimen acts as a diffraction grating. Therefore a diffraction pattern or an image
of the specimen can be recorded.
Depending on the adjustment of the lenses the diffraction pattern or the image of the
sample can be observed. By observing the diffraction pattern the crystallinity and crystal
orientation of the sample can be investigated. By observing the image the microstructure of
the sample can be studied. The dominating contrast in TEM is the diffraction contrast, i.e.
the contrast arising due to different crystallographic orientations.
Fig. 7 shows schematically a light and transmission electron microscope. One can easily see
the similarity between a light and a transmission electron microscope. Fig. 8 shows a
scanning electron microscope. By comparing the set-up of a transmission electron
microscope and a scanning electron microscope it is clear that a scanning electron
microscope works fundamentally different from the way a transmission electron microscope
works.
In a classical transmission electron microscope the electron beam is parallel. When the
electron beam is focused and is scanned point by point through the samples the microscope
is a scanning transmission electron microscope.
Fig.7: Overview of an optical microscope and a transmission electron microscope. [8]
Fig.8: Overview of a scanning electron microscope. [9]
6. Scanning electron microscope (SEM)
In a SEM the incident beam is not transmitted through the sample but the surface is scanned
point by point by a focused electron beam. The focused electron beam is shaped by
electromagnetic lenses. The image observed is not a direct image given by the incident
electrons like in a TEM (by transmitting the sample), it is an image which results from the
interaction between the incident beam and the sample surface. When the incident electron
10
beam hits the sample secondary and backscattered electrons are produced. Backscattered
electrons are electrons which were backscattered at the sample’s atoms and secondary
electrons are electrons which were ejected from the atom shell by the incident electron
beam. The image on the computer screen shows at each point the number of electrons
which are counted in the detector, encoded in brightness, produced at the corresponding
scanned point on the sample by the electron beam. The electron beam is deflected by
electric fields to scan the sample surface point by point. By detecting the signal resulting
from the interaction between the electrons and the sample surface the image is obtained
point by point. Different magnifications are obtained by changing the current in the
deflection coils. By decreasing the scanned area while keeping the number of scanned points
constant a higher magnification is reached. Note that higher magnifications are obtained by
scanning smaller areas of the sample and not by higher magnification through
electromagnetic lenses. This is totally different comparing to the above explained TEM. For
imaging in a SEM the signals from backscattered and secondary electrons can be used.
The probability of a scattering event taking place is given by the cross section σ. The larger
the cross section the more likely scattering occurs. The total cross section σ is the sum of the
cross sections for elastic collision events σel and inelastic events σin.
The three most important signals in a SEM are secondary electrons, backscattered electrons
and X-ray emission. All these three signals will be used in this practical training.
The image can be formed either by detecting the backscattered electrons (BSE) or by
detecting the secondary electrons (SE) or a combination of them.
Backscattered electrons are electrons which were at least once elastically scattered at the
sample’s atoms. The probability of an elastic event is given by the cross section for elastic
scattering σel. σel depends on the incident electron energy σel~1
𝐸02 . The elastic cross section
decreases with increasing energy. Therefore at high acceleration voltages elastic scattering is
infrequent and hence electrons are penetrating deeper into the sample. The elastic cross
section also depends on the atomic number of the analyzed sample and is proportional:
σel~Z2. With increasing Z the incident electrons are more frequently scattered. With
11
increasing Z the scattering angle also increases so at higher Z the incident electrons have a
smaller penetration depth and a larger exit width, compare Fig. 9.
Altogether the elastic cross section can be written as σel~𝑍2
𝐸02 ∙
1
sin2 𝜃/2 . In consequence this
allows to gain some information about the composition of the sample. If a sample consists of
two different phases with different Z, then the high Z material will appear brighter in the BSE
image than the low Z material as more electrons are scattered in the high Z material
compared to the low Z material. This contrast is called Z-contrast.
Fig. 9: shows the dependence between the interaction volume of the electron beam (EB) and the acceleration
voltage and the material atomic number Z. [10]
The second signal used for taking images are the secondary electrons. The secondary
electrons are orbit electrons ejected by incident electrons or by already backscattered
electrons. The production of secondary electrons is an inelastic scattering event, i.e. the
incident electrons are losing energy and changing their direction. The cross section for the
production for SE is increasing with decreasing energy of the incident electrons and depends
not on Z.
Secondary electrons have low energies. By definition all electrons with energies below 50 eV
are called secondary electrons. Because of the low energy only secondary electrons
produced close to the surface at depths below 50 nm can exit the sample and can be
detected. As the secondary electron signal originates only from depths up to 50 nm its
intensity mostly depends on the topography of the sample surface giving a high quality
image of the surface structure. When a surface is tilted towards the incident electron beam
the volume from which secondary electrons can exit the sample is larger giving more signal,
i.e. the tilted surface appears brighter, see Fig. 10. This is also the case at edges and needles,
which appear brighter in the image. SE images appear like illuminated from the side because
normally the SE detector is placed at a large angle and acts like a “light source”, giving a
good topography contrast and making the images intuitive. To interpret SE images correctly
it is important to know from which side the SE detector “looks” on the sample.
12
Fig. 10: SE production as a function of the incident angle of the beam electrons (EB). [10]
When analyzing the images, it is important to remember that incident electrons can undergo
several elastic and inelastic collisions before reaching the surface and being detected.
Therefore electrons will not have discrete energies but rather an energy distribution as
shown in Fig. 11. All electrons with an energy >50 eV are called backscattered by definition
because they were at least once elastically backscattered. Electrons with energy E close to
the primary incident energy E0 were backscattered exactly once. Electrons with an energy
between 50 eV and the primary energy (50 eV < E < E0) underwent inelastic scattering events
in addition to backscattering.
Fig. 11: Energy distribution of the BSE and SE leaving the sample surface. [10]
A very important signal used in a SEM is the characteristic X-ray emission which allows
obtaining some information about the chemical composition of the sample. To characterize
the chemical composition of the sample X-ray spectroscopy is used. This method is based on
an interaction between the incident electrons and the sample’s atoms: the characteristic X-
ray emission. An incident electron can eject an electron from an inner atomic shell; say K, L
or M leaving a vacancy in this shell. This vacancy is then filled by an outer shell electron and
X-rays are emitted with an energy equivalent to the energy difference between the higher
and lower energy state, see Fig. 12. The expression “characteristic X-ray emission” originates
from the fact that the energy of the X-ray radiation is characteristic for each chemical
element: This enables the identification of the chemical composition of the sample. The
energy depends on the atomic number Z given by the Moseley law:
𝐸 = [𝐶1(𝑍 − 𝐶2)]2
with E the energy of the X-rays, Z the atomic number and C1, C2 constants.
The emitted X-rays are named after the chemical element and the electron transition
between the two atomic shells. For example the transition from the L to the K shell in Cu is
called Cu-Kα. From M to K it is called Cu-Kβ. The roman capital marks the shell from which
the electron was ejected, the greek character gives the shell distance between the higher
energetic shell and the lower energetic shell (α for a transition from the next higher shell,
β from the second higher shell). In X-ray spectroscopy the spectroscopic lines from the K, L
13
and M shells are most often used. The Kα and Kβ lines are very useful as their intensities are
high. Fig. 12 shows a typical X-ray spectroscopy spectrum.
There are two kinds of the X-ray spectroscopy depending on the spectrometers used. If the
spectrometer measures the wavelength of the X-ray radiation it is called wavelength
dispersive spectrometry WDX. If the energy is measured it is called energy dispersive X-rays
analysis EDX. In this course EDX will be used.
An EDX spectrum (Fig. 12) contains not only characteristic X-ray radiation but also X-ray
radiation resulting from the deceleration of the electrons in the electrostatic potential of the
atoms. This is called Bremsstrahlung and is visible as background in the X-ray spectrum.
Fig.12: schematic view of the origin of characteristic X-ray radiation [11] (right). Example spectrum (left)[12].
6.1 Depth of field in SEM images
Besides the excellent resolution of SEM images the high depth of field of SEM images is a
further advantage.
The depth of field describes the depth ranges where the image is in focus. The depth of
focus D depends on the magnification M, d the resolution limit of the eye and α the angular
aperture of the objective lens for the ideal case of no lens defects and an electron beam with
no diameter.
𝐷 = 𝑑/𝑀𝛼
When taking into account the diameter of the beam db the depth of field is:
𝐷 =
𝑑𝑀 − 𝑑𝑏
𝛼
The high depth of field of SEM images results from the very small angle α as the electron
beam is focused. Because of the high depth of field of SEM images rough surfaces can be
imaged well giving a 3D impression of the topography.
14
7. Detectors
7.1. Detection of secondary electrons
For the detection of secondary electrons an Everhart-Thornley detector (scintilator-
photomultiplier detector) is used. Usually it is mounted at an angle of about 45° from the
incident beam direction and has a collector mesh. Between the sample and the detector
collector mesh a potential (of about 50 V) is applied which sucks the low energetic secondary
electrons towards the detector. Due to this attractive potential the effective solid angle for
detection of secondary electrons is large as electrons originating from the far side of the
sample also reach the detector. After reaching the detector mesh the electrons are further
accelerated by an inner potential towards the scintillator where they generate light signals.
These light signals are then conducted through an optical fiber to the photomultiplier. In the
photomultiplier the light signal is converted in electrons and these electrons are multiplied.
This signal is used by the electronics to generate the image.
The Everhart-Thornley detector also detects backscattered electrons. As these electrons
have high energies the additional potential between the sample and the mesh hardly
influences them. Therefore only backscattered electrons exiting the sample towards the
detector can reach the detector. This results in a very small solid angle. Therefore when the
attractive potential is turned on the number of the detected BSE is negligible in comparison
to the number of detected SE.
Of course the Everhart-Thornley detector can be also used to record a BSE image. To prevent
secondary electrons reaching the detector an inverse potential of -50 V between the sample
and the mesh can be applied. In that case the signal originates only from backscattered
electrons, the secondary electrons are suppressed. As the solid angle for the detection of
backscattered electrons is small the BSE image is noisy.
15
7.2. Detection of backscattered electrons
To get high quality BSE images usually different types of detectors are used. Typically a
semiconductor detector is used. A semiconductor detector is a diode biased reversed. The
basic module of a semiconductor diode is the junction between a n-doped semiconductor
and a p-doped semiconductor or a junction between a metal and a semiconductor. When
biasing in inverse direction such a diode a depletion layer is formed. When the electrons
reach the depletion layer they generate a number of electron-hole pairs which create a
current pulse. This current pulse is then amplified and measured.
BSE detectors are often located above the sample having a large opening to ensure a large
solid angle for the BSE detection. To enable small working distances usually the BSE detector
is mounted close below the end of the SEM column (pole piece). It is circular with a hole for
the incident electron beam. This position is called high take-off angle. At this position the Z-
contrast is excellent and the topography contrast is suppressed. Sometimes there is an
additional BSE detector at a smaller angle or the BSE detectors can be tilted to that angle in
order to get a better BSE topography contrast. This position is called low take -off angle. A
BSE detector usually is either a semiconductor detector or scintillator-photomultiplier
detector. A semiconductor BSE detector located at the high take-off position is often divided
into several segments. The signals from each segment can be add or subtracted. With this
the contrast can be influenced giving a better topography image or Z-contrast.
16
7.3. Detection of X-rays
A classical semiconductor detector for EDX is a silicon semiconductor crystal. As in the X-ray
spectroscopy the energy of photons is to be measured and these energies are in the order of
a few keV it is important to minimize the background signal. Therefore only crystals with a
high chemical purity and a low number of intrinsic crystal defects are used. To minimize
further the conductivity the EDX detector is cooled with liquid nitrogen. When a photon
reaches the depletion area it generates a number of electron-hole pairs which create a
current pulse. As the energy needed to create an electron-hole pair in Si is 3.8 eV the
number of created electron hole pairs by a photon is large. The energy of the electrical pulse
is equal to the energy of the incoming X-ray. The electrical pulses are sorted by height in a
multichannel analyzer which allows determining the X-ray energy giving the spectrum shown
in Fig. 10.
17
8. Experiment
First get acquainted with the microscope: look into the vacuum chamber, try to identify the
different detectors, think about the geometry. You should use gloves.
Sample 1: Observation of a screw and a coil filament
Exercise 1: Try to take images in the SE mode and BSE mode
To take an image:
Check the cross over: if it is not in the middle of the axis move it there. Adjust brightness/contrast. Focus roughly your image. Recheck the cross over in the image Focus your image in a high magnification. When the sample is in focus click
Z <-> FWD to tell the microscope that the sample is focused Check whether there is astigmatism; if so try to correct the astigmatism by adjusting
the stigmator settings. Take your images in the SE mode. Switch to the BSE mode. Adjust contrast/brightness. Take some images.
Exercise 2: Record an EDX spectrum and identify the elements with the EDAX Genesis software
Start the program EDAX Genesis and collect the spectrum by clicking button “Collect”. Identify the peaks with Peak ID and consider whether the peak identification by the
program is correct. Identify all peaks.
Sample 2: “pyramid” W on Fe sample
Exercise 1: Take SE images of the sample and BSE images. How to make topography contrast with BSE? Make a topography contrast BSE image.
Exercise 2: Rotate with scan rotation and consider how to identify “Berge und Täler”
Sample 3: polished sample
Exercise 1: Take SE and BSE images.
What do you learn from this? What do you see?
Exercise 2: Take an EDX spectrum.
18
Sample 4: unknown sample
Exercise 1: Take SE image and BSE (A+B), (A-B) and record an EDX spectrum.
Why are the images so different?
9. Evaluation
Sample 1:
Show your images.
Sample 2:
Explain the differences between a SE and BSE image and a topography BSE image. Explain how to see topography with the BSE detector.
Discuss how to identify rough surface. How do you see which part of the sample is higher.
Discuss the scan rotation and why you should be very careful when interpreting the images.
Sample 3:
Explain how to distinguish in a BSE image whether you see material contrast or something else. Evaluate the taken EDX spectra and identify the element/elements on the sample. Describe your procedure in identifying the peaks.
Sample 4:
Evaluate the taken EDX spectra and identify the element/elements on the sample.
Please add the taken images and EDX spectra to your evaluation.
19
10. References:
[1] Peter Fritz Schmidt et al.: „Praxis der Rasterelektronenmikroskopie und
Mikrobereichsanalyse“, 1994, expert verlag, Renningen-Malmsheim
[2] Andrzej Barbacki: „Mikroskopia elektronowa“, 2007, Wydawnictwo Politechniki
Poznanskiej, Poznan
[3] David B. Williams, C. Barry Carter “Transmission Electron Microscopy part 1 basics”,
1996, Springer Science+Business Media, New York
Images references:
[4] https://lp.uni-goettingen.de/get/text/353
[5] wikipedia.de; wikipedia.org, wikipedia.pl
[6] https://www.onlinesehtests.de/astigmatismus/
[7] David B. Williams, C. Barry Carter “Transmission Electron Microscopy part 1 basics”, 1996,
Springer Science+Business Media, New York
[8] http://wiki.polymerservice-erseburg.de/index.php/Transmissionselektronenmikroskopie
[9] http://www.technoorg.hu/news-and-events/articles/high-resolution-scanning-electron-
microscopy-1/
[10] Peter Fritz Schmidt et al.: „Praxis der Rasterelektronenmikroskopie und
Mikrobereichsanalyse“, 1994, expert verlag, Renningen-Malmsheim
[11] http://geo-consulting.warnsloh.de/rfaxrf-allgemein/
[12] Andrzej Barbacki: „Mikroskopia elektronowa“, 2007, Wydawnictwo Politechniki
Poznanskiej, Poznan