Scanning electron microscope (SEM) introducton, construction, advantages, disadvantages, application

1. Branch: Mechanical-4C(1)
Collage: S.V.M.I.T.
NAME: (1) KAMAL AKHTAR F.(120450119156)

2. Introduction
 The Scanning Electron Microscope
 Principle
 Construction
 Applications
 Advantages
 Disadvantages

3. The Scanning Electron Microscope
 Principle:
 The basic principle is that a beam of electrons is
generated by a suitable source, typically a tungsten
filament or a field emission gun.
 The electron beam is accelerated through a high
voltage (e.g.: 20 kV) and pass through a system of
apertures and electromagnetic lenses to produce a
thin beam of electrons.
 Then the beam scans the surface of the specimen
Electrons are emitted from the specimen by the action
of the scanning beam and collected by a suitably-
positioned detector.

4. Construction
• Scanning Electron Microscope’s basic
components are as following…
 Electron gun (Filament)
 Condenser lenses
 Objective Aperture
 Scan coils
 Chamber (specimen test)
 Detectors
 Computer hardware and software

5. The Scanning Electron Microscope

6. Electron Guns
 Electron guns are typically one of TWO types.
1) Thermionic guns
2) Field emission guns
 Thermionic guns:
 Which are the most common type, apply thermal
energy to a filament to coax electrons away from
the gun and toward the specimen under
examination.
 Usually made of tungsten, which has a high
melting point

7. Electron Guns
 Field emission guns:
 create a strong electrical field to pull electrons
away from the atoms they‘re associated with.
 Electron guns are located either at the very top
or at the very bottom of an SEM and fire a
beam of electrons at the object under
examination.
 These electrons don't naturally go where they
need to, however, which gets us to the next
component of SEMs.

8. Electron Guns

9. Condenser Lenses
 Just like optical microscopes, SEMs use
Condenser lenses to produce clear and detailed
images.
 The Condenser lenses in these devices, however,
work differently.
 For one thing, they aren't made of glass.
 Instead, the Condenser lenses are made
of magnets capable of bending the path of
electrons.
 By doing so, the Condenser lenses focus and
control the electron beam, ensuring that the
electrons end up precisely where they need to go.

10. Objective Aperture
 The objective aperture arm fits above the objective
lens in the SEM. It is a metal rod that holds a thin
plate of metal containing four holes. Over this fits a
much thinner rectangle of metal with holes (apertures)
of different sizes. By moving the arm in and out
different sized holes can be put into the beam path.
 An aperture holder: this arm holds a thin metal strip
with different sized holes that line up with the larger
holes. The metal strip is called an Aperture strip.
 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.

11. Objective Aperture

12. Scan Coils
 The scanning coils consist of two solenoids
oriented in such a way as to create two
magnetic fields perpendicular to each other.
 Varying the current in one solenoid causes the
electrons to move left to right.
 Varying the current in the other solenoid forces
these electrons to move at right angles to this
direction (left to right) and downwards.

13. Chamber (Specimen Test)
 The sample chamber of an SEM is where researchers
place the specimen that they are examining.
 Because the specimen must be kept extremely still for
the microscope to produce clear images, the sample
chamber must be very sturdy and insulated from
vibration.
 In fact, SEMs are so sensitive to vibrations that they're
often installed on the ground floor of a building.
 The sample chambers of an SEM do more than keep
a specimen still.
 They also manipulate the specimen, placing it at
different angles and moving it so that researchers
don't have to constantly remount the object to take
different images.

14. Chamber (Specimen Test)

15. Detectors
 SEM's various types of detectors as the eyes of
the microscope.
 These devices detect the various ways that the
electron beam interacts with the sample object.
 For instance, Everhart-Thornley detectors register
secondary electrons, which are electrons
dislodged from the outer surface of a specimen.
These detectors are capable of producing the
most detailed images of an object's surface.
 Other detectors, such as backscattered electron
detectors and X-ray detectors, can tell
researchers about the composition of a
substance.

16. Detectors

17. Vacuum Chamber
 SEMs require a vacuum to operate.
 Without a vacuum, the electron beam
generated by the electron gun would
encounter constant interference from air
particles in the atmosphere.
 Not only would these particles block the path
of the electron beam, they would also be
knocked out of the air and onto the specimen,
which would distort the surface of the
specimen.

18. Applications
 SEMs have a variety of applications in a number of scientific
and industry-related fields, especially where characterizations
of solid materials is beneficial.
 In addition to topographical, morphological and compositional
information, a Scanning Electron Microscope can detect and
analyze surface fractures, provide information in
microstructures, examine surface contaminations, reveal
spatial variations in chemical compositions, provide
qualitative chemical analyses and identify crystalline
structures.
 In addition, SEMs have practical industrial and technological
applications such as semiconductor inspection, production
line of miniscule products and assembly of microchips for
computers.
 SEMs can be as essential research tool in fields such as life
science, biology, gemology, medical and forensic science,
metallurgy.

19. Advantages
 Advantages of a Scanning Electron Microscope include its
wide-array of applications, the detailed three-dimensional and
topographical imaging and the versatile information garnered
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.
 This instrument works fast, often completing SEI, BSE and
EDS analyses in less than five minutes. In addition, the
technological advances in modern SEMs allow for the
generation of data in digital form.
 Although all samples must be prepared before placed in the
vacuum chamber, most SEM samples require minimal
preparation actions.

20. 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.
 SEMs are limited to solid, inorganic samples small enough to fit
inside the vacuum chamber that can handle moderate vacuum
pressure.
 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,

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