Electron Microscopy
Veronika Grünwaldová
Introduction and History
• Electron microscopes are scientific instruments that use a beam of energetic 
electrons to examine objects on a very fine scale.
• Electron microscopes were developed due to the limitations of Light Microscopes 
which are limited by the physics of light.
• In the early 1930's this theoretical limit had been reached and there was a 
scientific desire to see the fine details of the interior structures of organic cells 
(nucleus, mitochondria...etc.).
• This required 10,000x plus magnification which was not possible using current 
optical microscopes.
• The transmission electron microscope (TEM) was the first type of Electron 
Microscope to be developed and is patterned exactly on the light transmission 
microscope except that a focused beam of electrons is used instead of light to 
"see through" the specimen. It was developed by Max Knoll and Ernst Ruska in 
Germany in 1931.
• The first scanning electron microscope (SEM) debuted in 1938 ( Von Ardenne) 
with the first commercial instruments around 1965. Its late development was due 
to the electronics involved in "scanning" the beam of electrons across the
sample.
Comparison of OM,TEM and SEM
The properties of the optical system and the radiation used depends what 
magnification and resolution will be achieved.
Light + glass lens Electron + electomagnetic lens
Principal features of an optical microscope, a transmission electron microscope
and a scanning electron microscope, drawn to emphasize the similarities of
overall design.
Scale and Microscopy Techniques
Transmission electron
microscopy
• In a slide projector light from a light source is made into a 
parallel beam by the condenser lens; this passes through the 
slide (object) and is then focused as an enlarged image onto the 
screen by the objective lens. 
• In the electron microscope, the light source is replaced by an 
electron source, the glass lenses are replaced by magnetic 
lenses, and the projection screen is replaced by a fluorescent 
screen, which emits light when struck by electrons, or, more 
frequently in modern instruments, an electronic imaging device 
such as a CCD (charge‐coupled device) camera. 
The whole trajectory from source to screen is under vacuum and 
the specimen (object) has to be very thin to allow the electrons to 
travel through it ‐ light elements < 1 µm, heavy elements  < 0.1 µm, 
lattice imaginig ~ 10 nm.
Not all specimens can be made thin enough for the TEM. 
Alternatively, if we want to look at the surface of the specimen, 
rather than a projection through it, we use a scanning electron or 
ion microscope.
The transmission electron microscope can be compared with a slide projector. 
Comparison of OM and TEM
TEM
100 000 x kV100
Comparison of TEM and SEM pictures
Scanning electron microscopy
Comparison of OM and SEM
The scanning electron microscope (SEM) provides the competent user with an advantage 
over the light microscope (LM) in three key areas:
• Resolution at high magnification. 
Resolution can be defined as the least distance between two closely opposed points, at which they may 
be recognized as two separate entities. 
The best resolution possible in a LM is about 200 nm whereas a typical SEM has a resolution of better 
than 10 nm (typically 5 nm).
• Depth of field
i.e. the height of a specimen that appears in focus in an image ‐ more than 300 times the depth of field 
compared to the LM. 
This means that great topographical detail can be obtained. For many users, the three dimensional (3D) 
appearance of the specimen image, is the most valuable feature of the SEM. This is because such 
images, even at low magnifications, can provide much more information about a specimen than is 
available using the LM. 
• Microanalysis
i.e. the analysis of sample composition including information about chemical composition, as well as 
crystallographic, magnetic and electrical characteristics.
One drawback to the use of the SEM
is that it operates under vacuum and in many SEMs the samples must be rendered conductive to be 
viewed. This is often achieved by coating with a very thin layer (~10 nm) of metal or carbon. 
This coating is applied for two main reasons:
(1) Non conductive specimens are often coated to reduce surface charging that can block the path of SE
and cause distortion of signal level and image form;
(2) Low atomic number (Z) specimens (e.g. biological samples) are coated to provide a surface layer that
produces a higher SE yield than the specimen material.
It is important to leave the sample uncoated (in its natural state) if compositional information is required because the practice of coating 
samples with metals obscures this. If the sample is non‐conductive then it can be coated with carbon (a low atomic number material) 
which will enhance conductivity without obscuring the compositional detail from below.
However, there are a number of different types of SEMs which all have specific purposes, often 
associated with additional pieces of equipment like specialised stages or collectors. Some of these do 
not require dry or conductive samples. They include the following:
o Low vacuum scanning electron microscopy (LVSEM)
o Using cryo on a scanning electron microscopy (Cryo‐SEM)
o Environmental scanning electron microscope (ESEM)
o Focused ion beam (FIB) technology
o E‐beam lithography (EBL)
In 1938, von Ardenne added scan coils to a transmission electron microscope (TEM), 
producing a scanning transmission electron microscope (STEM). 
In 1942, Zworykin and his team developed the first scanning electron microscope to 
employ secondary electron detection. 
He and his team recognized that secondary electron emission could be used to generate an image showing the topographic contrast 
of a specimen. The collector was biased to +50 volts to capture the secondary electrons, and the voltage drop across a connected
resistor generated the image. Although the initial spatial resolutions were poor, approximately 200 nm, Zworykin and his colleagues 
reduced the beam spot size and obtained images with a resolution of 50 nm. 
In the late 1940s and early 1950s, researchers Oatley and McMullin introduced several 
notable improvements to the scanning electron microscope, including the 
electromagnetic lens, the stigmator, and signal amplification. By attaching the 
scintillator directly to the face of the photomultiplier, Everhart and Thornley greatly 
improved the signal‐to‐noise ratios.
History
1960s were the development of the LaB6 (Lanthanum‐hexaboride) electron cathode 
and the revival of the field emission tip electron source.
The LaB6 improves resolution via a high‐brightness electron gun. Yielding even higher resolution, the field emission tip electron 
source produces current densities that measure thousands of amps per cm2. Consequently, today's commercial SEMs can obtain 
resolutions of about 10‐20Å.
In 1968, Fitzgerald demonstrated the addition of an energy‐dispersive x‐ray detector 
to an SEM, moving the SEM into the analytical probe arena.
SEM Layout
In simplest terms, an SEM is really nothing more than a television. We use a filament to 
get electrons, magnets to move them around, and a detector acts like a camera to
produce an image.
SEM Layout
Princip: 
The SEM uses a beam of high energy electrons generated by an electron gun, processed 
by magnetic lenses, focused at the specimen surface and systematically scanned 
(rastered) across the surface of a specimen.
Image formation:
Unlike the light in a light microscope (LM), the electrons in a scanning electron microscope (SEM) 
never form a real image of the sample. The SEM image is in the form of a serial data stream i.e. it is 
an electronic image, which is  a reflection of differences (e.g. topographical or compositional) in the 
sample.
The formation of an image requires a scanning system (scan coils). The scanning system 
uses two pairs of electromagnetic deflection coils that scan the beam along a line then 
displace the line position to the next scan so that a rectangular raster is generated both 
on the specimen and on the viewing screen. The first pair of scan coils bends the beam 
off the optical axis of the microscope and the second pair bends the beam back onto the 
axis at the pivot point of the scan. 
Signals generated from the specimen are collected by an electron detector, 
converted to photons via a scintillator, 
amplified in a photomultiplier, 
and converted to electrical signals and used to modulate the intensity of the image on 
the viewing screen.
• An image is obtained by taking the signal from the sample and transferring it to a 
CRT screen. 
• Increased magnification is produced by decreasing the size of the area scanned.
• Magnification is determined by taking the ratio of the lengths of the scans:
Mag. = L/I
Magnification
Resolution
• Resolution is the ability to resolve two closely spaced points.
While you may have to be at a high magnification to see small features, resolution is NOT the 
same as magnification.
• Resolution is dependent on wavelength of the beam we use to see the material.
• Wavelengths of electron beams generated at different accelerating voltages
• One way to improve resolution is by reducing the size of the electron beam that strikes the 
sample.
We can also improve the resolution by:
• Increasing the strength of the condenser lens
• Decreasing the size of the objective aperture
• Decreasing the working distance (WD = the distance the sample is from the objective lens)
Resolution improvements achieved
with aberration correction.
The smallest distance we can see between points in a light microscope (LM) is about 200
nm whereas a typical scanning electron microscope (SEM) can distinguish gaps smaller
than 10 nm.
Low kV
High kV
Resolution x Voltage
Probe diameter becomes larger
as the accelerating voltage is
lowered.
Resolution x Voltage
The height over which a sample can be clearly focused is called the Depth of Field. The SEM has a 
large depth of field which produces the images that appear 3‐dimensional in nature.
Depth of Field
Depth of field and resolution have a reciprocal relationship:
• Improving resolution in conventional SEM’s
leads to a smaller depth of field
• While increasing depth of field decreases
resolution.
Depth of Field vs. Resolution
Depth of field is improved by:
• Longer working distance
• Smaller objective apertures
• Lower magnifications
Major Components of the Scanning Electron Microscope
Major Components of the Scanning Electron Microscope
All scanning electron microscopes consist of: 
An electron gun (1) which generates a beam of electrons. 
A column (2) which focuses and illuminates the specimen 
A specimen chamber (3) where the electron beam 
interacts with the sample. 
Detectors (4) to monitor the different signals that result 
from the electron beam/sample interaction. 
A viewing system (5) that builds an image from the 
detector signal. 
A water chilling system, which maintains a constant 
temperature of 20°C for the operation of the magnetic 
lenses in the microscope. If the chiller fails and the 
magnetic lenses heat up, the SEM will automatically 
shutdown.
A vacuum system, which consists of an oil rotary (backing) 
pump (for rough evacuation ) and an oil diffusion pump
(higher vacuums). 
Despite the differences between the light and electron microscopes, the components of the SEM have 
an analogous function to the parts of a light microscope.
1
2
3
54
1. Electron gun – generates the electron beam
• Electron guns provide electrons for an electron beam 
• by allowing them to escape from a cathode material. 
• An electron must be supplied sufficient energy to kick it into a high energy state 
within the material and additional energy for it to escape the surface. 
• The electrons are emitted from a small area of the filament (point source). 
A point source is important because it emits monochromatic electrons (with similar 
energy). 
There are two main types of electron sources used in SEMs and microprobes:
Thermionic sources
Field emission source (FEG)
Emission
Thermionic Field Emission
W LaB6 FE
Size (nm) 1 x 105 2 x 104 0.2
Brightness 
(A/cm2.steradian)
104 ‐ 105 105 ‐ 106 107 ‐ 109
Energy Spread (eV) 1 ‐ 5 0.5 – 3.0 0.2 – 0.3
Operating Lifetime 
(hrs)
>20 >100 >300
Vacuum (torr) 10‐4 – 10‐5 10‐6 – 10‐7 10‐9 – 10
1. Electron gun - Thermionic sources
• electrons are produced by heating a conductive material to the point where the
outer orbital electrons gain sufficient energy to escape.
• There are two main types of thermionic sources: tungsten metal filaments and LaB6
crystals.
These two types of sources require vacuums of ~10-5 and ~10-7 torr, respectively.
The tungsten cathode 
 is a fine wire approximately 100mm in diameter that has been bent into 
the shape of a hairpin with a V‐shaped tip.
 The tip is heated by passing current through it; normally, the tip is heated 
to around 2400°C. 
 The tungsten filament lasts approximately 50 hours
Lanthanum hexaboride (LaB6) cathode
The most straightforward method to achieve higher resolution is to find a 
material which prodeces more electrons at a given temperature, hence a 
brighter filament and higher resolution. LaB6, has been the best material 
developed to date for this application. 
 průměr hrotu 20 μm
 The LaB6 filament operates at approximately 2125°C and is five times 
brighter than a tungsten filament under the same conditions. 
However, LaB6 filaments tend to be more expensive than tungsten 
filaments. 
 životnost nad 500 hodin
1. Electron gun - Field emission source (FEG)
• electrons are produced by a large electrical field, 105 to 108 V/cm, which is placed
between cathode and anode.
• the cathode forms a very sharp tip (typically 100 nm or less)
• Although the total current is lower than either the tungsten or the LaB6 emitters, the
current density is between 103 and 106 A/cm. Thus, the field emission gun is hundreds
of times brighter than a thermionic emission source.
Cold source 
works even at room temperature and depends only very slightly 
on temperature, indicating that it is not a temperature activated 
process. Instead, it is a purely quantum mechanical effect called 
"tunneling". 
Field emission sources require vacuums of ~10‐9 torr.
Thermal‐field (TF) source
in which the tungsten point in a field emission source is heated, 
incorporating both thermionic and field emissions; 
this is also referred to as a "'Schottky cathode". 
A TF source requires a vacuum of ~10‐8 torr.
Comparison of tungsten  cathode and FEG
Magnetic lens system
The magnetic lens system consists of a:
• Condenser lens which is composed of one or more lenses, controls the intensity of the 
electron beam reaching the specimen
• The probe‐forming lens, often called the objective lens brings the electron beam into focus 
(de‐magnifies) on the specimen.
• Scanning coils deflect the electron beam horizontally and vertically over the specimen 
surface. This is also called rastering.
Objective lens (OL) aperture
This aperture is used to reduce or exclude extraneous (scattered) electrons. An optimal aperture 
diameter should be selected for obtaining high resolution secondary electron images.
2. Electron column - The electron beam is focused using electromagnetic lenses.
Lenses
The purpose of a lens is to change the path of the rays in a desired direction. 
Glass or transparent plastic may bend light and so are used in optical lenses. However, glass or 
plastic lens will stop electrons. Therefore, it is not appropriate to use glass or plastic as lenses in an 
electron microscope. 
Since electrons are charged particles and they can be bent in a magnetic field. Lenses for 
electrons are constructed with ferromagnetic materials and windings of copper wire. These 
produce a focal length which can be changed by varying the current through the coil. They are 
called electromagnetic lenses. The magnetic field bends electron paths in a similar way that solid 
glass lenses bend light rays. Under the influence of a magnetic field, electrons assume a helical 
path, spiralling down the column.
An electromagnetic lens is a coil of wire through which current flows. Because the current flow 
produces a magnetic field at right angles, the field pushes inwards into the hole in the centre. This 
acts to shape a beam of electrons travelling in their natural spiral path down the central hole.
Abberations
• Spherical aberration results from nonuniformity of the lenses. Electrons which pass through the lens
further off the optical axis are pulled more strongly than those that pass through near the center of the
lens. The outer zones of a lens focus more strongly than inner zones. To reduce this effect, the final
aperture can be reduced, but this reduction results in a lower beam current.
• Chromatic aberration results from differences in electron velocity through the lenses. The magnetic
lenses will bend electrons with higher velocity or energy more strongly, resulting in a dull or blurred image.
Electrons of slightly different energies are focused differently. No method exists to correct this problem
other than using a more expensive LaB6 or field emission instrument. Tungsten filament systems typically
have a 2 eV spread when leaving the cathode; LaB6 systems have a 1 eV spread, and field emission
systems have a 0.2 to 0.5 eV spread.
Astigmatism results from the fact that magnetic lenses do not have perfect symmetry- the electron
beam may be elliptical. The stigmator can correct astigmatism by a correcting magnetic field to produce
symmetrical electron beam at the sample. The operator can change both the strength and orientation (angle)
of the magnetic field produced by stigmators to control the final beam shape.
Diffraction occurs because of the wave nature of electrons and the aperture size of the final lens. The only
way to reduce diffraction problems is to increase the final aperture size.
Astigmatic el. beam
Stigmatic el. beam
Coils
The condenser lens convergences the cone of the electron beam to a spot below it, 
before the cone flares out again and is converged back again by the objective lens and 
down onto the sample. 
This initial convergence can be at different heights, that is, close to the lens, or further 
away. The closer it is to the lens, the smaller the spot diameter at the point of 
convergence. The further away, the larger the diameter of this point. 
So the condenser lens current controls this initial spot size and is referred to as the spot 
size control. The diameter of this initial convergence (also called a cross‐over point) 
affects the final diameter of the spot the beam makes on the sample.
Condenser lens
• By changing the current in the objective lens, the magnetic field strength changes and 
therefore the focal length of the objective lens is changed.
• Its main role is in focusing the beam onto the sample. 
• The objective lens also has some influence over the diameter of the spot size of the 
electron beam on the specimen surface. A focused beam produces a smaller spot on 
the surface than an under or over‐focused beam.
SEM works on a voltage between 2 to 50kV and its beam diameter that scans the
specimen is 5nm‐2μm.
Objective lens
Aperture
• The objective aperture fits above the objective lens in the SEM.
• It is a thin metal strip with different sized holes that line up with the larger holes. 
• The aperture stops electrons that are off‐axis or off‐energy from progressing down 
the column. It can also narrow the beam below the aperture, depending on the 
size of the hole selected.
A large aperture is chosen for low magnification imaging to increase signal and for BSE and 
microanalysis work.
A smaller aperture is chosen for high resolution work and better depth of focus but has 
the disadvantage of fewer electrons and therefore a less bright image.
Some examples of aperture size and purposes
Scale
Aperture
diameter
(microns)
Probe
current
Purpose
4 30 Smallest
Ultrahigh resolution; Low
probe current; Large depth
of field
3 50 Usual observation
2 70
High resolution at high
probe current; Reduced
depth of field
1 110 Largest
Observation at high probe
currents; Shallow depth of
field
0 1,000 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ Axis alignment
Aperture
4. Specimen chamber
The specimen chamber is maintained at high vacuum that 
minimises scattering of the electron beam before 
reaching the specimen. This is important as scattering or 
attenuation of the electron beam will increase the probe
size and reduce the resolution, especially in the SE mode. 
A high vacuum condition also optimises collection 
efficiency, especially of the secondary electrons.
Specimen stage
The specimen holder is fixed to the specimen stage. 
The stage can be moved along the X, Y (in the 
specimen plane), and Z directions (at right angles to 
the specimen plane). The Z adjustment is also known 
as the specimen height. The specimen stage can also 
rotate continuously or to be tilt. 
Electron-matter interactions
When the electron beam hits a sample it interacts with the atoms in that sample.
Electron‐matter interactions can be divided into two classes:
1. Elastic scattering – the electron trajectory within the specimen changes, but its kinetic energy 
and velocity remains essentially constant. The result is generation of backscattered electrons 
(BSE).
2. Inelastic scattering– the incident electron trajectory is only slightly perturbed, but energy is 
lost through the transfer of energy to the specimen. 
Inelastic scattering, places the atom in an excited (unstable) 
state. The atom “wants” to return to a ground or unexcited 
state. Therefore, at a later time the atoms will relax giving off 
the excess energy. XRays, cathodoluminescence and Auger 
electrons are three ways of relaxation. The relaxation energy 
is the fingerprint of each element.
Electron-matter interactions
The volumes involved in the production of secondary electron (SE), backscattered electron (BSE) 
and X‐rays, form into a shape that ranges from a tear‐drop to a semi circle within the specimen. 
This shape is called an interaction volume and its depth and diameter depends on the kV as 
well as the density of the specimen. 
Approximately the top 15nm of the volume comprises the zone from which SE can be collected, 
the top 40% is the region from which BSE can be collected and X rays can be collected from the 
entire region.
An example of a typical Interaction volume for:
• Specimen with atomic number 28, 20 kV
• 0° degrees tilt, incident beam is normal to 
specimen surface
Secondary electrons
• Secondary electrons are low energy electrons ( E ˂ 50 eV) formed by inelastic scattering. 
• The low energy of these electrons allows them to be collected easily. This is achieved by placing a 
positively biased grill on the front of the SE detector, which is positioned off to one side of the 
specimen. The positive grill attracts the negative electrons and they go through it into the 
detector. This is the case for the Everhart‐Thornley detector which is most commonly used but 
there is another kind of In‐lens SE detector in some machines. 
Since this lets us collect a large number of the secondaries (50 ‐ 100%), we produce a “3D” type of 
image of the sample with a large depth of field. 
Electrons emitted from a surface that faces away from the detector or which is blocked by the 
topography of the specimen, will appear darker than surfaces that face towards the detector. 
• The major influence on SE signal‐generation is the shape (topography) of the specimen surface. 
In the image of the beetle, the electron detector is in the 
top left corner, hence that region looks brightest. It is, 
however, not the only factor that contributes to the contrast 
and brightness in an SEM.
Secondary electrons
• The major influence on SE signal‐generation is the shape (topography) of the specimen surface. 
Vespula vulgaris - antennaPollen, http://www.mansic.eu/documents/PAM1/Giannakopoulos1.pdf
• Secondary electrons provide particularly good edge detail. Edges (and often pointy parts) look 
brighter than the rest of the image because they produce more electrons. 
Rhyzopertha dominica egg
http://old.padil.gov.au/pbt/index.php?q=node/15&pbtID=187
http://www.google.cz/imgres?q=SE+SEM+images&hl=cs&sa=X&biw=1229&bih=786&tbm=isch&tbnid=sUAEy6GXHncLaM:&imgrefurl=htt
p://nanolab.me.cmu.edu/projects/geckohair/hierarchy.shtml&docid=3vJxPIzIFkA0RM&imgurl=http://nanolab.me.cmu.edu/projects/geckoh
air/images/CMU_NanoRobotics_Lab_Hierarchy_SEM.jpg&w=800&h=600&ei=SVNdUdbbHYq20QW33IBY&zoom=1&iact=hc&vpx=2&vpy
=420&dur=2340&hovh=194&hovw=259&tx=82&ty=133&page=3&tbnh=138&tbnw=172&start=60&ndsp=34&ved=1t:429,r:87,s:0,i:349
http://crysa.fzu.cz/MMaterialu2012/lectures.htm
http://www.google.cz/imgres?q=SE+SEM+images&hl=cs&sa=X
&biw=1229&bih=786&tbm=isch&tbnid=QOSlKElBi5AmHM:&img
refurl=http://www.edlin.cz/fei/24magellan.htm&docid=JeYYDar7
CCaeNM&imgurl=http://www.edlin.cz/fei/sem_magel2.jpg&w=26
8&h=247&ei=oVVdUbHNHNKk0AXS24CwDw&zoom=1&iact=rc
&page=2&tbnh=138&tbnw=141&start=27&ndsp=33&ved=1t:429,
r:46,s:0,i:226&tx=57&ty=78
http://www.google.cz/imgres?q=SE+SEM+images&hl=cs
&sa=X&biw=1229&bih=786&tbm=isch&tbnid=cEAJq7xY
b3GrwM:&imgrefurl=http://www.xray.cz/xray/csca/kol201
0/abst/slouf.htm&docid=F-D4JCHey-
gE8M&imgurl=http://www.xray.cz/xray/csca/kol2010/abst/
slouf_files/image002.jpg&w=292&h=252&ei=oVVdUbHN
HNKk0AXS24CwDw&zoom=1&iact=hc&vpx=145&vpy=2
84&dur=3931&hovh=201&hovw=233&tx=161&ty=121&p
age=3&tbnh=133&tbnw=147&start=60&ndsp=34&ved=1t:
429,r:74,s:0,i:310
Backscattered electrons
• Backscattered (BS) electrons are high‐energy electrons (>50 eV), which  arise due to elastic 
collisions between the incoming electron and the nucleus of the target atom (i.e. Rutherford 
scattering). 
• These BSE are used to produce a different kind of image. Such an image uses contrast to tell 
us about the average atomic number of the sample. The higher the average atomic number, 
the more primary electrons are scattered (bounced) back out of the sample. This leads to a 
brighter image for such materials.
• The angle of scattering can range from 0 to 180°. 
• Since BSE have high energies, they can’t be pulled in like secondaries. If you placed a 
potential on a grid to attract them, you would also attract the ncident beam!!
• The most common detector used is called a surface barrier detector. It sits above the 
sample, below the objective lens. BSE which strike it are detected.
Microanalysis - The relaxation energy is the fingerprint of each element.
A technique to analyse composition in regions of a sample in the micron and nano
ranges.
Inelastic scattering
Techniques include spectroscopy
 Energy Dispersive or Wavelength dispersive X-ray
 Cathodoluminescence
• Electron Backscatter Diffraction
Energy Dispersive or Wavelength dispersive microanalysis
When the sample is bombarded by the electron beam of the SEM, electrons are ejected 
from the atoms on the specimens surface. A resulting electron vacancy is filled by an 
electron from a higher shell, and an X‐ray is emitted to balance the energy difference 
between the two electrons. The EDS X‐ray detector (also called EDS or EDX) measures 
the number of emitted x‐rays versus their energy. The energy of the x‐ray is 
characteristic of the element from which the x‐ray was emitted.
• the elemental analysis or chemical characterization of a sample
• each element has a unique atomic structure allowing unique set of peaks on its X‐ray 
spectrum 
Cathodoluminescence (CL)
• Cathodoluminescence (CL) is the emission of photons of characteristic wavelengths
from a material that is under high-energy electron bombardment.
• The CL response of the sample is recorded with digital images from the CL detector.
• The CL images can be obtained over a range of magnifications (10-10,000x), but the
lowest magnification is constrained by the specific configuration of the CL detector
system.
The image produced by cathodoluminescence showing otherwise invisible
microstructural defects and impurities. It is used to examine internal structures of
semiconductors, rocks, ceramics, glass, etc. in order to get information on the composition,
growth and quality of the material.
The distribution of the CL in a material gives fundamental insights into such processes as
crystal growth, replacement, deformation and provenance.
Zircon, showing crystal growth patterns.
Microanalysis can provide details on
composition, defects, and impurities.
This photo is a CL image from
granite and shows two minerals
intergrown with each other. The
bluish cross-hatched area is
occupied by a grain of potassium
feldspar (microcline), and the purple
red-rimmed mineral in the upper half
of the image is a grain of sodium
feldspar (albite)
http://serc.carleton.edu/research_education/geochemsheets/semcl.html
Electron backscatter diffraction (EBSD)
• EBSD is used to examine the crystallographic orientation of many materials, which
can be used to elucidate texture or preferred orientation of any crystalline or
polycrystalline material.
• EBSD can be used to index and identify the seven crystal systems, and as such it is
applied to:
• For an EBSD measurement a flat/polished crystalline specimen is placed in the SEM
chamber at a highly tilted angle (~70° from horizontal) towards the diffraction camera, to
increase the contrast in the resultant electron backscatter diffraction pattern.
• An electron backscatter diffraction pattern is formed when many different planes diffract
different electrons to form Kikuchi bands which correspond to each of the lattice diffracting
planes.
• Each band can be indexed individually by the Miller indices of the diffracting plane
which formed it.
• In most materials, only three bands/planes which intercept are required to describe a
unique solution to the crystal orientation (based upon their interplanar angles) and most
commercial systems use look up tables with international crystal data bases to perform
indexing.
 crystal orientation mapping,
 defect studies,
 phase identification,
 grain boundary and morphology studies,
 egional heterogeneity investigations,
...
Electron backscatter diffraction (EBSD)
An orientation map of a quartz clast in a pseudotachylite
(earthquake rock) showing recrystallisation around the
outer edge. Microanalysis can provide details on
crystallographic information.
EBSD characterization of grain boundary network: (a) SE image from grain boundary, (b) EBSD pattern of grain boundary and (c)
overlaying of ferrite pattern on the measured EBSD pattern.
http://www.sciencedirect.com/science/article/pii/S092150930602
5172
Types of SEM
Conventional (high vacuum) scanning electron microscopy (SEM)
• This is the most common type of machine.
• It requires a dry, conductive sample (often achieved by applying a thin layer
of metal to the surface with a technique called sputtering).
• The sample must be able to withstand a high vacuum.
• This type of machine is used for routine imaging, using either secondary
electrons (SE) or backscattered electrons (BSE).
Variable Pressure or Low Vacuum scanning electron microscopy (LVSEM)
• This type of machine is basically like a conventional SEM but has the advantage in low
vacuum (LV) mode that the pressure can be adjusted in the sample chamber until the
artefact of "electron charging" is removed from images.
• This means LVSEM can be used to image the surface of non-conductive samples (no
metal needs to be added to the surface of such samples). It is particularly useful for
viewing polymers, biological samples, and museum samples that cannot be changed in
any way, particulate samples, and geological materials. Imaging uses backscattered
electrons (BSE).
• Backscattered electron imaging (BSE) of nonconductive, uncoated samples can provide
information about composition via the contrast of the image: whiter regions have a
higher average atomic number than darker regions.
• The LV mode can also be used to freeze-dry samples for viewing.
Using cryo on a scanning electron microscope (Cryo-SEM)
Cryo stands for frozen.
• A cryo-scanning electron microscope is a conventional SEM that has been fitted with
specific equipment that allows samples to be viewed in the frozen state. This is
particularly useful for directly viewing hydrated (wet) samples, delicate biological
samples, hydrogels, food, biofilms, foams, fats, and waxes, suspensions,
pharmaceuticals and nanoparticles.
• The sample can be snap frozen outside the machine and then inserted in its frozen state,
or placed into the machine in an unfrozen state and frozen more slowly in the machine.
• Frozen samples can also be fractured or cut during preparation to reveal internal
structures.
• It is imaged using either secondary electrons (SE) or backscattered electrons
(BSE).
Environmental scanning electron microscope (ESEM)
• This machine is designed to view a sample in its natural state, without the need for
desiccation.
• Sample temperature and specimen chamber vapor pressure can both be
controlled, allowing samples to be heated, cooled, wetted or dried.
Relative humidity (RH) can be controlled within the chamber by adjusting the temperature
of the conventional stage (±20° C) along with the pressure.
• Dynamic experiments can also be carried out on wet samples in real time, involving
heating on a specialized hot-stage, anywhere up to 1500° C, cooling, wetting and
drying. The samples can be imaged while these dynamic processes are occurring.
Focused ion beam (FIB) technology
• This technology involves using an ion beam (typically gallium ions) directed onto a hard
sample. The beam is focused to an extremely fine probe size (<10 nm) onto the surface of
a specimen. The sample can be sectioned or shaped with the ion beam while it is
being monitored by scanning electron microscopy (SEM).
FIB can cut 10-nm-thick sections from very hard materials. These sections can be taken off as sequential
sections, each viewed in turn with the SEM mode, and this imaging information used to construct a 3D
image.
FIB can also be used to shape needles that can then be viewed by other techniques such as transmission
electron microscopy or atom probe tomography.
• It can also be used for deposition of materials in a small area (approx 100nm) from
chemical vapor from specific gasses.
Electron-beam lithography (E-beam lithography or EBL)
• EBL is a maskless lithography technique used for patterning of computer generated layout
structures on photoresists on Si wafers.
• Upon irradiation of focused electron beam, electron-sensitive resists undergo chainscission
or crosslinking, resulting in solubility switch of materials during the subsequent
development process (remove/retain exposed material in development depending on the
tone of the resist).
• To date, EBL remains the highest resolution patterning tool in lithography, it is widely used
in photomask fabrication and low volume production of semiconductor components.
Summary - Applications and practical uses - what the SEM can do
Scanning electron microscopy is a remarkably versatile technique. There are many different
types of SEMs available, tailored to specific needs. With SEM one can:
• Image morphology of samples (e.g. view bulk material, coatings, sectioned material,
foils, even grids prepared for transmission electron microscopy).
• Image compositional and some bonding differences (through contrast and using
backscattered electrons).
• Undertake micro and nano lithography: remove material from samples; cut pieces out
or remove progressive slices from samples (e.g. using a focussed ion beam).
• Heat or cool samples while viewing them (while possible in many instruments it is
generally done only in ESEM or during Cryo-scanning electron microscopy).
• Wet and dry samples while viewing them (only in an ESEM)
• View frozen material (in an SEM with a cryostage)
• Generate X-rays from samples for microanalysis (EDS; WDS)
• Study optoelectronic behaviour of semiconductors using cathodoluminescence
• View/map grain orientation/crystallographic orientation and study related information
like heterogeneity and microstrain in flat samples (Electron backscattered diffraction).
• Electron diffraction using electron backscattered diffraction. The geometry may be
different to a transmission electron microscope but the physics of Bragg Diffraction is the
same
Summary - What the SEM can't do
There are some things SEM can't do:
• SEM cannot take colour images. The colour is often added artificially in coloured SEM images.
However, some SEMs can collect true colour images via a wavelength selective cathodolumenence
(CL) detector.
• SEM cannot image through water. An ESEM using a wet Scanning Transmission Electron
Microscope (STEM) detector can be used to image through thin water films.
• Generally, SEMs are not used for experiments involving liquids, chemical reactions, and airgas
systems although some specialised machines and sample chambers do allow for these
experiments.
• The resolution of the SEM is not high enough to image individual atoms (use a transmission
electron microscope).
• The SEM cannot reliably image charged molecules that are mobile in a matrix. For example,
some species (e.g. Na+) are volatile under the electron beam because the negative electron beam
exerts a force on charged material.
• SEM is not good for quantifying surface roughness at small scale. Atomic Force Microscope
(Scanning Probe Microscopy) is more useful for this task.
• Elemental analysis below micron scale. Analysis in the < 7kV range can provide elemental
information on the sub-micron scale but is often problematical.
Recrystalization of API
Drying under different conditions
Crystalization under different conditions
Pelets after dissolution tests
Tablets
folie
blistr
ACLAR
PVC
ACLAR
PVC
Al
Microanalysis – identification of package
Microanalysis – identification of package
Blistr: ACLAR, PVC a Al.
F
Cl
Cl
Al
Folie: ACLAR a PVC
F
Cl
Cl