1. PRESENTED BY,
MUGILAN N
M.TECH NAST 1st year
Reg no:16305012
NAST-614: Synthesis and Characterization of Nanostructured
Materials
TITLE: SCANNING ELECTRON MICROSCOPE
COURSE INSTRUCTOR;
A.VADIVEL MURUGAN
Centre for Nano Sciences & Technology
Madanjeet School of Green Energy
Technologies
3. ELECTRON MICROSCOPE
Electron microscopes are scientific instruments that use
beams of energetic electrons to examine objects on a
very fine scale.
Electron microscopes magnifications over 1,000,000×
Electrons cannot travel freely in air, electron
microscopes are built into airtight metal tubes or
“columns” and use vacuum pumps to remove all the air
from within the microscope.
4. Scanning electron microscope (SEM).
SEM
Electron beam scans over surface of sample.
Image shown on TV monitor.
Image is of the surface of the sample.
SEM FEATURES
Is a microscope that uses electrons rather than light to
form an image and to examine objects on a very fine
scale.
This examination can yield the following information:
Topography – surface features.
Morphology – shape and size.
Composition – elements and compounds/ relative
amount of them.
Crystallographic information - how the atoms are
arranged in the sample.
5. SETUP OF A SCANNING ELECTRON MICROSCOPE EQUIPMENT
6.
7. DEFINITION
Scanning electron microscope is a type of electron microscope
that images a sample by scanning it with a beam of electrons in a
raster scan pattern.
WORKING PRINCIPLE
In SEM, an electron beam is emitted from an electron gun fitted
with a tungsten filament and passed towards the anode.
This makes the electron to travel in vertical path through the
column of the microscope in a concentrated manner
The electrons to pass through electromagnetic lenses which focus
and direct the beam down towards the specimen
The detectors detect the back scattered electrons and secondary
electrons and convert them to a signal that is sent to a viewing
screen.
The signals from the detectors are received and converted into
images and displayed on the screen.
8.
9. Secondary electrons (SE)
Generated when a primary electron dislodges a specimen electron from
the specimen surface.
Produced by inelastic interactions.
SE energy level is only 3 ~ 5 eV.
Can be easily collected
The maximum escape depth is about 5 nm in metal and 50 nm in
insulators.
BACKSCATTERED ELECTRONS(BSE)
Produced by elastic interactions.
BSE energy level is about 60 ~80 % of incident one.
Special detector is required to collect BSE.
The maximum escape depth varies inversely with the average atomic
number,
The range from a fraction of micrometer to several micrometers.
(Larger atomic number, larger escape depth).
High energy electrons that are reflected or back scattered out of
the specimen.
BSE images provide information about distribution of different elements
in the sample
10. X-rays
Generated when electron from incident beam knocks off electron from inner shell
leaving hole.
Electron from higher energy level jumps to inner shell emitting the excess energy as
X rays.
Energy and wavelength of x-ray can be used for chemical analysis.
Electron Detectors
11. Detecting electrons
Detectors used to amplify the signal rather than detected
secondary andbackscattered electrons directly.
i.e. Electrons Photons Electrons.
Backscattered electrons are high energy (i.e. 1 - 40 keV)
Secondary electrons are low energy (~0 - 50 eV)
Must use different detectors to collect the backscattered and
secondary electron signals.
Detecting backscattered electrons
Two specialized detectors are used to detect BSE.
Detector sits directly beneath the objective lens.
Large surface area to maximize collection efficiency.
Scintillator detectors
These comprise a scintillator (phosphor) with a light pipe and a
photomultiplier
Fast response time and high gain, which
makes them suitable for use at TV rates.
Bulky and may restrict the working distance of the microscope.
12.
13.
14. Interaction between electron beam and sample;
The interactions may be elastic or inelastic.
The elastic interactions:
Only trajectory change
KE and velocity remains constant
little energy loss
The inelastic interactions:
Incident electrons will collide and displace electrons from their orbit
heavy energy loss
15. The photo shows a lung cancer cell during cell division –
taken with a scanning electron microscope.
16. Advantages of SEM
The SEM is routinely used to generate high-
resolution images of shapes of objects.
SEMs can magnify objects at upward of
300,000 times the size of the object .
fast speed.
Higher magnification.
Depth of field will be more.
3D image of the object can be viewed.
Image can be directly viewed on the screen.
17. Disadvantages
Samples must be solid and they must fit into
the microscope chamber.
Very costly and large size.
Scanning electron microscopes are radiation-
generating devices and should be well
protected.
Some may loss their structural property due
to the interaction of the electron.
18. Wide range of applications in scientific and industry
related field.
Forensic science.
Structural analysis
Small feature measurements
Fracture mode preliminary identification
Grain size
Corrosion failure inspection
Surface contamination evaluation
Chemical Composition
Crystallographic information
Applications
19. Conclusion
Although SEMs are large, expensive pieces of
equipment, they remain popular among
researchers due to their wide range of
applications and capabilities, including the
high-resolution, three-dimensional, detailed
images they produce.
