This document provides an overview of scanning electron microscopy. It begins with an introduction to microscopy and the need for electron microscopy due to limitations of optical microscopy. It then describes the components and operating principle of a scanning electron microscope. The document explains how electrons interact with matter by producing backscattered electrons and secondary electrons, which are used to form images. It also covers specimen preparation and applications of SEM, concluding with references.

1. SCANNING ELECTRON
MICROSCOPE
Prepared by:Adarsha s
2nd MSc Physics
Reg No:178101
St. Aloysius college
Mangaluru
21 September 2018

2. CONTENTS
 Introduction
 The Need for Electron Microscopy
 Scanning Electron Microscope (SEM)
 Components of SEM
 Operating principle
 Interaction of electrons with matter
 Specimen preparation
Applications
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3. Introduction
Microscopy
It involves the study of objects that are too small to be examined by the unaided eye.
Size of objects
• Micrometer (10-6 m)
• Nanometer (10-9 m)
• Angstrom (10-10 m)
• Picometer (10-12 m)
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4. Important
types of
Microscopes
Optical Electron
Simple Compound ScanningTransmission
Source: Light
Lens: Glass
Source: Electron beam
Lens: Electromagnetic
Electromagnetic Lens
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5. Optical Microscope Electron Microscope
Source: Light Source: Beam of very fast moving electrons
Condenser, Objective and eye piece lenses
made up of glasses
All lenses are electromagnetic
Low resolving power (RP) (~ 0.2 µm) High resolving power (~ 0.0002 µm)
Specimen preparation takes few minutes to
hours
Specimen preparation usually takes few days
The specimen is in µm range The specimen thickness is in nm range
Vacuum is not required Vacuum is essential for operation
There is no cooling system
It has a cooling system to take out heat
generated by high electric current
Filament is not used Tungsten filament is used to produce electrons

6. • Light optical microscopes were developed in early
1600’s.
• Best observations were made by the Dutch
scientist Anton van Leeuwenhoek.
• He used tiny glass lens placed very close to the
object and close to the eye.
• In late 1600’s he observed blood cells, bacteria,
and structure within animal tissue.
• This formed the basis for microstructure analysis.
One of the single-lens microscopes used by
Van Leeuwenhoek.
History of Microscopy
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7. The Need for Electron Microscopy
• Standard light-based microscopes are limited by the inherent limitations of light, and as
such can only magnify to 500 or 1000 times.
• Electron microscopes can exceed this by far, showing details as small as the molecular
level.
• Electron microscopes are very useful as they are able to magnify objects to a much higher
resolution than optical ones.
• Higher resolution can be achieved with electron microscopes because the de Broglie
wavelengths for electrons are so much smaller than that of visible light.
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8. • Light is diffracted by objects which are separated by a distance of about the same size as
the wavelength of the light.
• This diffraction then prevents the transmitted light to be focused into an image.
• Therefore, the sizes at which diffraction occurs for a beam of electrons is much smaller
than those for visible light.
• This is why targets can be magnified to a much higher order of magnification using
electrons rather than visible light.
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9. • de Broglie wavelength is given by : λ = h/p = h/(mv)
where h = 6.626 x10-34 Js (Planck‘s constant), p  momentum, m mass of electron
and v  velocity of electron
• For an electron with K.E. = 1 eV and rest mass energy 0.511 MeV, the associated de Broglie
wavelength is 1.23 nm, about a thousand times smaller than a 1 eV photon. (This is why the
limiting resolution of an electron microscope is much higher than that of an optical
microscope.)
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10. • The wavelength of an electron is inversely related to potential (accelerating voltage) applied
by the anode to the beam of electrons : λ = 1.23/(V)1/2
• The speed of electrons emitted by a cathode (the electron gun in electron microscope) is
directly related to this potential (accelerating voltage).
• Electrons that have been accelerated by V volts have an energy of V electron volts (eV).
• Thus, the higher the accelerating energy applied, the faster the electrons travel and the
smaller the wavelength of the electrons.
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11. Scanning Electron Microscope (SEM)
• To study microscopic structure
• Image is formed by focused electron beam that scans over the surface area of specimen
• Incident beam in SEM  electron probe (diameter ~ 10 nm)
• In general, electrons interact with atoms in the sample producing signals that contain
information about sample surface morphology, surface topography and composition.
• SEM is mainly used to study the surface morphology of the sample.
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12. Figure: SEM images of platinum nanothorns. (a) Large area SEM image; (b) high magnification
SEM image of a platinum nanothorn; (c) side view of a nanothorn; (c) top view of a nanothorn.
The scale bar in (b), (c), and (d) is 100 nm. [Sajanlal et al. 2011, Nano Reviews, 2(1)]
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13. • First SEM image was obtained in 1951.
• First commercial model (built by the AEI Company) was delivered to the Pulp and Paper Research Institute of
Canada in 1958.
• A modern SEM provides an image resolution typically between 1 and 10 nm.
Scanning electron microscope at RCA Laboratories Hitachi-SU5000 SEM resolution 1.2 nm accelerating voltage=30 kV
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14. Components of SEM
• Vacuum System – To avoid deposition of gas molecules on the specimen
• Electron gun - Produces the beam of electrons (Source: Tungsten or LaB6 Accelerating
potential = 30 kV , Size of the probe = 10 nm)
• Condenser lens - Electron beam is condensed by condenser lens (controlled by probe
current) and the amount of current in the beam is limited
• Scan coils - Generate the Magnetic field
• Objective lens - Focuses the scanning beam onto the part of the specimen
• Detector - detects the signal and sends it to the monitor
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15. Operating Principle
• Electron probe of SEM scans the specimen in two
perpendicular directions (x and y)
• x scan is fast, y scan is slow. fx and fy = fx/n are line
frequencies of x and y scan, respectively.
(Generated by two separate saw tooth generator)
• Saw tooth generator supplies current to two scan
coils. These coils generate magnetic field in y
direction, creating a force on an electron that
deflects it in the x-direction.
Saw tooth wave21 September 2018

16. • This procedure is known as raster scanning and it covers
the entire area on the specimen.
• During its x deflection, electron probe moves from A-B.
• It deflects back to C. Then it moves C-D and deflects
back to E.
• This process is repeated until n lines have been scanned
and beam arrives at Z.
• The entire sequence constitutes a single frame of raster
scan.
• From point z it returns to A and it scans the next frame
and process continues for n frames.21 September 2018

17. Interaction of electrons with matter
• SEM produces images by scanning the sample with a
high-energy beam of electrons.
• Electrons interact with the sample and produce
Backscattered electrons. and Secondary electrons
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18. Back scattered Electron Image • Backscattered electron is a primary electron ejected from a
solid when scattering angle θ > 90
∘
(Due to collision between
electrons of incident beam & specimen)
• Backscattered electrons have a probability of leaving the
specimen and re-entering the surrounding vacuum, in which
case they can be collected as a backscattered electron (BSE)
signal.
• Production of backscattered electrons varies directly with
atomic number (Z) of specimen.
• When backscattered electrons are detected, higher Z elements
appear brighter than lower Z elements.
• This principle is used to differentiate parts of specimen that
have different average atomic number.
• Scintillator detector used to detect backscattered electrons.21 September 2018

19. Secondary electron interaction
• Electron beam falls on the sample and interacts with atoms.
As a result, elements will be slowed down due to strong
elastic scattering. Atoms will absorb the energy and get
ionized.
• Some of electrons from the sample atoms will be released as
secondary electrons.
• Secondary electrons leave the atom with very small kinetic
(5 eV) compared to that of primary electrons.
• They escape from a volume near the specimen surface depth
of 5–50 nm. This information is related to the topography.Secondary electron image21 September 2018

20. SEM Specimen preparation
• Specimen does not have to be made thin.
• Specimens of insulating materials do not provide a path to ground for the specimen
current Ix and may undergo electrostatic charging when exposed to the electron probe.
• One solution to the charging problem is to coat the surface of the SEM specimen with a
thin film of metal. This is done in vacuum.
• Films of thickness 10–20 nm conduct sufficiently to prevent charging of most specimens.
• Gold and chromium are common coating materials.
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21. Applications
• Material science (Investigation of nano tubes and fibres, strength of alloys)
• Semiconductor inspection (composition measurement), Microchip assembly
• Forensic investigation (analysis of gunshot residue)
• Medical science (identifying diseases and viruses, testing new medicines)
• Micrographs produced by SEMs have been used to create digital artworks
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22. References
• Physical Principles of Electron Microscopy: An Introduction to TEM, SEM, and AEM-
R.F Egerton Springer International Publications (2016)
• https://www.atascientific.com.au
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23. THANK YOU
21 September 201821 September 2018