gives a brief overview of Scanning electron microscopy technique

1. WORKING
PRINCIPLE OF SEM
SYED JUNAID

2. MICROSCOPY
& types Magnification
Resolution
SEM
Interaction Volume
different kinds of
signals obtained
Parameters-
that alter
results
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3. Microscopy
• Microscopy is the technical field of using microscopes to view objects and
related areas, that cannot be seen with the naked eye .
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4. TYPES
Optical
Microscopy
Electron
Microscopy
scanning
probe
Microscopy
others
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The various types of light
microscopy include
1. bright-field,
2. dark-field,
3. fluorescence, and
4. phase contrast
microscopy
1)SEM
2)TEM
3)STEM
1. X-Ray Microscopy
2. Focused Ion Beam
(FIB)
etc
1. Scanning Tunneling
Microscopy (STM)
2. Atomic Force Microscopy
(AFM)
3. Magnetic Force Microscopy
(MFM)

5. MAGNIFICATION
• Magnification in physical terms is defined as "a measure of the ability of a
lens or other optical instruments to magnify, expressed as the ratio of the
size of the image to that of the object". This means, that an object of any
size is magnified to form an enlarged image.
• In SEM defined as the ratio of the size of the rastered area on the sample to
the size of the rastered area of the output.
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6. RESOLUTION
• Itis the smallest distance at which two neighbouring points can be
distinguished, and is dependent on wavelength.
• The wavelength of accelerated electrons (6 pm) is several orders of
magnitude shorter than that of light (600 nm).
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7. 7
Scanning Electron
Microscope (SEM)
is a type of electron microscope that
produces images of a sample by
scanning the surface with a focused
beam of electrons.
The electrons interact with atoms in
the sample, producing various signals
that contain information about the
sample's surface topography and
composition.

8. PRINCIPLE
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When the accelerated primary electrons strikes the sample , it produces secondary electrons. These
secondary electrons are collected by a positive charged electron detector which in turn gives a 3-dimensional
image of the sample.
OR
The electron detector (Scintillator) is used to collect the secondary electrons and can be
converted into electrical signal. These signals can be fed into CRO through video amplifier
as shown.

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 Stream of electrons are produced by the
electron gun and these primary electrons
are accelerated by the grid and anode.
These accelerated primary electrons are
made to be incident on the sample
through condensing lenses and scanning
coil.
 These high speed primary electrons on
falling over the sample produces low
energy secondary electrons. The collection
of secondary electrons are very difficult
and hence a high voltage is applied to the
collector.
 These collected electrons produce
scintillations on to the photo multiplier
tube are converted into electrical signals.
These signals are amplified by the video
amplifier and is fed to the CRO.
 By similar procedure the electron beam
scans from left to right and the whole
picture of the sample is obtained in the
CRO screen.

10. • The function of the electron gun is to provide a large and stable current in a
small beam.
• There are two classes of emission source: thermionic emitter and field
emitter.
• Thermionic Emitters use electrical current to heat up a filament; the two most
common materials used for filaments are Tungsten (W) and Lanthanum
Hexaboride (LaB6).
• Thermionic sources have relative low brightness , & evaporation of cathode
material and thermal drift during operation occurs.
• Field Emission is one way of generating electrons that avoids these problems.
A Field Emission Gun (FEG); also called a cold cathode field emitter
does not heat the filament.
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IMPORTANCE OF ELECTRON GUN

11. • The emission is reached by placing the filament in a huge electrical potential gradient.
• The FEG is usually a wire of Tungsten (W) fashioned into a sharp point. The significance of
the small tip radius (~ 100 nm) is that an electric field can be concentrated to an extreme
level, becoming so big that the work function of the material is lowered and electrons can
leave the cathode.
• FESEM uses Field Emission Gun producing a cleaner image, less electrostatic distortions
and spatial resolution < 2nm (that means better than SEM with Thermionic gun).
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14. COMPONENTS OF SEM
• 1. Condenser lens :
• The current in the condenser determines the diameter of the beam. a low current results
in a small diameter, a higher current in a larger beam.
• A narrow beam has the advantage that the resolution is better, but the disadvantage that
the signal to noise ratio is worse.
• The situation is reversed when the beam has a large diameter.
• 2. Scan coils :
• The scan coils deflect the electron beam over the object according to a zig-zag pattern.
The formation of the image on the monitor occurs in synchrony with this scan movement.
• The scan velocity determines the refreshing rate on the screen and the amount of noise in
the image (rapid scan = rapid refreshing = low signal = much noise).
• The smaller the scanned region on the object, the larger the magnification becomes at a
constant window size. 14

15. • 3. The objective lens:
• The objective lens is the lowest lens in the column.
• The objective focuses the electron beam on the object. At a short working distance the
objective lens needs to apply a greater force to deflect the electron beam.
• The shortest working distance produces the smallest beam diameter, the best
resolution, but also the poorest depth of field. (The depth of field indicates which range
in vertical direction in the object can still be visualized sharply).
• 4. The stigmator coils:
• The stigmator coils are utilized to correct irregularities in the x and y deflection of the
beam and thus to obtain a perfectly round-shaped beam.
• When the beam is not circular, but ellipsoidal, the image looks blurred and stretched.
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 A light microscope uses light as its illumination source where as an electron microscope uses
electrons.
 Vacuum is needed for the electrons to travel from the electron source to the sample surface
unimpeded.
 Without vacuum the beam of electrons would be scattered (mean free path is low) by air particles.
So, a high level of vacuum is needed for an electron microscope i.e. ( mean free path of the electron
should be larger than the electron column).

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NOVA NANOSEM 450

18. TYPES OF SIGNALS THAT ARE GENERATED
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19. Secondary Electrons
(SE)
• Produced by inelastic interactions of high
energy electrons with valence electrons of
atoms in the specimen which cause the
ejection of the electrons from the atoms.
• After undergoing additional scattering
events while traveling through the
specimen, some of these ejected electrons
emerge from the surface of the specimen.
• Arbitrarily, such emergent electrons with
energies less than 50 eV are called
secondary electrons; 90% of secondary
electrons have energies less than 10 eV;
most, from 2 to 5 eV.
• Being low in energy they can be bent by the
bias from the detector and hence even
those secondary electrons which are not in
the ‘line of sight’ of the detector can be
captured.
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Backscattered Electrons (BSE)
• Backscattered electrons (BSE) are beam electrons whose
trajectories have intercepted a surface usually, but not necessarily,
the entrance surface and which thus escape the specimen.
• Backscattered electrons remove a significant amount of the total
energy of the primary beam.
• Backscattering is quantified by the backscatter coefficient η which is
defined as
• where ηB is the number of beam electrons incident on the specimen
and ηBSE is the number of backscattered electrons (BSE).
• The backscatter coefficient can also be expressed in terms of
currents, where iB refers to the beam current injected into the
specimen and iBSE to the backscattered electron current passing out
of the specimen.

20. • DETECTOR USED FOR SECONDARY ELECTRONS
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Information that can be obtained : Topography and Morphology details

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22. • 4 parameters controls different imaging modes in SEM
• Electron probe size dia. (dp) - Dia. Of electron beam
focused at the specimen
• Electron probe current (ip) – Current impinging upon the
specimen and generating various signals
• Electron probe convergence angle (αp) – Half cone angle
of electrons converging onto the specimen
• Electron beam accelerating voltage (Vo) kV
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23. Mode I -Resolution Mode
• This model is governed by the Electron probe size dia.(dp)
• Resolution refers to the size of finest detail that can be observed.
• To image the finest details dp must be comparable with or smaller than the
feature itself. Resolution mode is only meaningful at high image
magnifications.
• Resolution mode is only meaningful at high image magnifications (>10,000 X),
Beam should contain sufficient current to exceed visibility threshold.
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Mode II –High Current Mode
• This model is governed by the Electron probe current(ip).
• For the best image visibility and quality –Large ip.
• Unless a sufficient amount of current (required to overcome random noise) is
there details, cannot be seen even if the spot size is small enough.

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Mode III –Depth-of-Focus Mode
• This model is governed by the probe convergence angle(αp).
• For the best image quality –αp as small as possible.
• With low beam convergence angle, beam dia. changes only a little over a long
vertical distance, so surface features at different heights will all appear to be
in focus at the same time.

26. Acceleration voltage
• In theory, an increase in accelerating voltage will result in a higher signal (and lower noise)
in the final image (micrograph).
• But the situation is not so simple.
• There are some disadvantages:
• Reduction in structural details of the specimen surface in SE mode.
• Increased electron build up in insulating samples, causing charging effects.
• Increased heating and the possibility of specimen damage.
• With a higher accelerating voltage the electron beam penetration is greater and the
interaction volume is larger. Therefore, the spatial resolution of micrographs created from
those signals will be reduced.
• So there will be a brighter image because the number of backscattered electrons (BSEs)
will increase but the resolution will be worse.
• For secondary electron (SE) imaging at typical voltages (say 15 keV), BSEs can enter the
secondary electron detector and degrade resolution because they come from deeper in
the sample.
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• The solution therefore,
for obtaining fine surface
structure is to exclude
these backscattered
electrons by using lower
kVs such as 3-10kV.
• Hence lower energy
provides better detail of
surface structure.

28. EDGE EFFECTS
• Edge effects are due to the enhanced emission of electrons from edges and peaks
within the specimen.
• They are caused by the effects of topography on the generation of secondary
electrons and are what gives form and outline to the images produced by the
Secondary Electron detector.
• Electrons preferentially flow to and are emitted from edges and peaks. Poor signal
intensity occurs in those regions shielded from the detector, such as depressions.
• Topographic contrast is also enhanced by Back Scattered electrons emitted from
regions of the sample facing towards the detector. Lowering the beam kV can reduce
edge effect.
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29. Charging
• Charging is produced by build-up of electrons in the sample and their uncontrolled
discharge, and can produce unwanted effects, particularly in secondary electron images.
• When the number of incident electrons is greater than the number of electrons escaping
from the specimen, then a negative charge builds up at the point where the beam hits the
sample. This phenomenon is called charging.
• and it causes a range of unusual effects such as abnormal contrast and image deformation
and shift. Sometimes a sudden discharge of electrons from a charged area may cause a
bright flash on the screen. These make it impossible to capture a uniform image of the
specimen and may even be violent enough to cause small specimens to be dislodged from
the mounting stub.
• The level of charge will relate to,
• (1) the energy of the electrons and
• (2) the number of electrons.
• The energy of the electrons is related to the kV ,so reducing kV can reduce charging. The
number of electrons relates to a number of parameters including, beam current, the
emission level of the gun, the spot size, and the apertures between the gun and the
specimen. So reducing the number of electrons by adjusting these parameters can also
reduce charging.
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30. • In a SEM, magnification results
from the ratio of the dimensions of
the raster on the specimen and the
raster on the display device.
• Assuming that the display screen
has a fixed size, higher
magnification results from
reducing the size of the raster on
the specimen, and vice versa.
• Magnification is therefore
controlled by the current supplied
to the x, y scanning coils,, and not
by objective lens power.
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31. • The spatial resolution of the SEM depends on the size of the electron spot,
which in turn depends on both the wavelength of the electrons and the
electron-optical system which produces the scanning beam.
• The resolution is also limited by the size of the interaction volume, or the
extent to which the material interacts with the electron beam.
• The spot size and the interaction volume are both large compared to the
distances between atoms, so the resolution of the SEM is not high enough
to image individual atoms.
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TILTING

32. CONCLUSION
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SEM Advantages
• Advantages of a Scanning Electron Microscope include its wide range of applications, the
detailed three-dimensional and topographical imaging and the versatile information
obtained from different detectors.
• SEMs are also easy to operate with the proper training and advances in computer
technology and associated software make operation user-friendly.
• The instrument works fast, often completing SEI, BSE and EDS analyses in less than five
minutes.
• Although all samples must be prepared before placed in the vacuum chamber, most SEM
samples require minimal preparation actions.

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SEM Disadvantages
• The disadvantages of a Scanning Electron Microscope start with the size and cost.
• SEMs are expensive, large and must be housed in an area free of any possible electric, magnetic or vibration
interference.
• Maintenance involves keeping a steady voltage, currents to electromagnetic coils and circulation of cool water.
• Special training is required to operate an SEM as well as prepare samples.
• In addition, SEMs are limited to solid, inorganic samples small enough to fit inside the vacuum chamber that can
handle moderate vacuum pressure.
• Finally, SEMs carry a small risk of radiation exposure associated with the electrons that scatter from beneath the
sample surface.
• The sample chamber is designed to prevent any electrical and magnetic interference, which should eliminate the
chance of radiation escaping the chamber. Even though the risk is minimal, SEM operators and researchers are advised
to observe safety precautions.

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QUESTIONS?