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CONTENT
• INTRODUCTION
• CONSTRUCTIONAL DETAILS
• SEM OPERATION
• OBTAINING SIGNAL IN SEM
• PERFORMANCE OF SEM
• CHARACTERISTIC INFORMATION: SEM
• SEM IMAGES IN DIFFERENT FIELDS
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INTRODUCTION
• First designed in Britain about 50 years back.
Unlike other optical microscope
• The SEM has a large depth of field, which allows more of aspecimen to be in focus at one time
• The SEM also has much higher resolution similar up to (×2000), so
closely spaced specimens can be magnified at much higher levels
• Can examine object up to 200mm in diameter, weighing up to 3kg
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COMPARISON OF OM,TEM AND SEM
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WHY SEM
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ADVANTAGES OF USING SEM OVER OM
• The SEM has a large depth of field, which allows a large amount ofthe sample to be in focus at one time and produces an image that is
a good representation of the three-dimensional sample.
• The combination of higher magnification, larger depth of field,
greater resolution, compositional and crystallographic informationmakes the SEM one of the most heavily used instruments in
academic/national lab research areas and industry
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CONSTRUCTIONAL DETAILS contd….
CONDENSER LENS
• For example with a thermionic gun, the diameter of the
first cross-over point ~20-50µm.
• If we want to focus the beam to a size < 10 nm on the
specimen surface, the magnification should be ~1/5000,
which is not easily attained with one lens (say, the
objective lens) only.
• Therefore, condenser lenses are added to de magnify
the cross-over points.
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CONSTRUCTIONAL DETAILS contd….
SCANNING COIL
• Fine beam of electron is scanned across the specimen by the scan
coils by changing the magnetic field strength
VACUUM SYSTEMS
• Chamber which "holds" vacuum, pumps are used to
produce vacuum
• Valves to control vacuum, gauges to monitor vacuum
SIGNAL DETECTION
• Detectors which collect the signal
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CONSTRUCTIONAL DETAILS contd….
CATHODE RAY TUBE (CRT)
• Accelerates electrons towards the phosphor coated screen where
they produce flashes of light upon hitting the phosphor.
a)DEFLECTION COIL
• Create a scan pattern forming an image in a point by point manner
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SEM OPERATION
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SEM OPERATION
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SEM OPERATION contd…..
• The SEM is an instrument that produces a largely magnified image
by using electrons instead of light to form an image.
• A beam of electrons is produced at the top of the microscope by an
electron gun.
• The electron beam follows a vertical path through the microscope,which is held within a vacuum.
• The beam travels through electromagnetic fields and lenses, which
focus the beam down toward the sample.
• Once the beam hits the sample, electrons and X-rays are ejected
from the sample.
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SEM OPERATION contd…..
• The region in which the electron penetrates the specimen is known
as interaction volume
• Even though radiation generated within this volume it will not be
detected unless it escapes from the specimen
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OBTAINING SIGNAL IN SEM
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OBTAINING SIGNAL IN SEM contd…
Away from incident light lose moreenergy so less spacial resolution
Closer to incident light havinghighest energy more spacial
resolution contains
crystallographic information
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SECONDARY
ELECTRONS
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OBTAINING SIGNAL IN SEM contd…
• Secondary electrons generated both by primary electron entering
the specimen and by back scattered electrons.
• Hence the diameter of secondary electron originating region is
greater then the diameter of incident beam.
Spacial distribution of secondary electrons
Intensity decreases
with increase in
distance from
incident light
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OBTAINING SIGNAL IN SEM contd…
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DETECTING SECONDARY
ELECTRONS
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OBTAINING SIGNAL IN SEM contd…
Energy of Secondary
electrons are too low(10-
50eV) to excite scintillator for
accelerating it, it is biased.
Purpose
1.Prevents the high voltage of
scintillator affecting incident
electron beam
2.Improves collection efficiency
By attracting the electrons
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DETECTING SECONDARY ELECTRONS
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OBTAINING SIGNAL IN SEM contd…
• At present separate detectors are used for detecting back scattered
electrons
ROBINSON DETECTOR
ADVANTAGE
Rapid response timeDISADVANTAGE
bulky
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DETECTING BACK SCATTERED ELECTRONS
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OBTAINING SIGNAL IN SEM contd…
SOLID STATE SILICON DETECTOR
when electron impregnate on the
semi conductor .It results in
electron-hole formation(voltage)
Which can be further amplifiedDISADVANTAGE
slow response time
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DETECTING BACK SCATTERED ELECTRONS
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ELECTRON DETECTORS
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OBTAINING SIGNAL IN SEM contd….
Information
regarding
shape of
specimen
Chemical
constituents
of the
specimen
Collided
electron, on
detectiongives atomic
no. contrast.
Irregularities
can be
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PERFORMANCE OF SEM
PIXELS
• Minimum spot obtained on the CRT is 0.1mm(100µm)
• The size of the specimen pixel is given by
Where ,
M-magnification
a)If electron probe>specimen pixel
Resolution is degraded
b)If electron probe<specimen
pixel
Signal will be weak
c)If electron probe=specimen pixel
Optimum performance
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PERFORMANCE OF SEM contd……
DEPTH OF FIELD
The depth of field (DOF) is the portion of a
scene that appears acceptably sharp in the image
FOR FINDING DEPTH OF FIELD (H)
Where ,
WD-aperture diameter A -working distance
M-magnification
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PERFORMANCE OF SEM contd……
RESOLUTION
The ultimate resolution of the SEM as being that of the smallest probe
which can provide adequate signal from the specimen
PROBE SIZE
• Decreases with increasing the strength of the condenser lens anddecreasing the working distance
• When probe dia current in the beam
• Relation between these two is given by
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PERFORMANCE OF SEM contd……
MINIMUM USABLE BEAM CURRENT
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PERFORMANCE OF SEM contd……
MINIMUM USABLE BEAM CURRENTRelation between critical current and contrast is given by
Where,
Q is the product of detector efficiency and electron yield
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PERFORMANCE OF SEM contd……
EFFECT OF BEAM TILT
• One of the principal use of SEM is for studying surface features or
topography
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EFFECT OF BEAM
TILT
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PERFORMANCE OF SEM contd……
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Facet fractured surface
viewed in SEM with
secondary electron .Imagetaken at same condition
but exposure at different
angle
TOPOGRAPHIC IMAGES
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CHARACTERISTIC INFORMATION: SEM
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CHARACTERISTIC INFORMATION: SEM
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TOPOGRAPHY
The surface features of an object or "how it looks", its texture;direct
relation between these features and materials properties
MORPHOLOGY
The shape and size of the particles making up the object; direct relation
between these structures and materials properties
COMPOSITION
The elements and compounds that the object is composed of and the
relative amounts of them; direct relationship between composition and
materials propertiesCRYSTALLOGRAPHIC INFORMATION
How the atoms are arranged in the object; direct relation between these
arrangements and material properties
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TOPOGRAPHIC IMAGES
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COMPOSITIONAL IMAGE
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CRYSTALLOGRAPHIC INFORMATION FROM SEM
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SEM IN DIFFERENT FIELDS
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BIOLOGY
SEM (scanning electron microscopy) image of E.coli
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MICRO MACHINING
SEM IMAGES
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REFERENCES
• Electron microscopy and analysis by
• www.purdue.edu
• http://en.wikipedia.org/wiki/Scanning_electron_microscope
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