scanning electron microscope explained in more details and in flow chart techniques.

1. SCANNING ELECTRON MICROSCOPE (SEM)
CHAITRALI.V.JADHAV
07/M.SC-1
PAPER-4

2. AIM
To study principle and working of scanning
electron microscope.

3. HISTORY
Account of early history - mcmullan.
Photo with 50mm object field-width showing channeling
contrast - Max knoll.
True microscope with magnification - Manfred von Ardenne
1937.
First SEM developed for bulk samples - Zworykin et al. in 1942.
First commercial SEM developed - Cambridge Scientific
Instrument Company as “Stereoscan” in 1965.

4. PRINCIPLE
Basic principle : A beam of eˉ is generated by a suitable
source ( tungsten filament or a field emission gun).
Electron beam is then accelerated through a high
voltage (e.g. 20 kV).
Passed through a system of apertures and
electromagnetic lenses to produce a thin beam of
eˉ.

5. Then the beam scans the surface of the specimen.
Electrons are emitted from the specimen by the
action of scanning beam.
Collected by a suitable positioned detector.

6. CONSTRUCTION
Electron Gun (Filament)
Condenser Lenses
Objective Aperture
Scan Coils
Chamber (specimen test)
Detectors
Computer Hardware & Software

7. INFRASTRUCTURE REQUIREMENTS
 Power supply
Vaccum system
 Cooling system
Vibration-free floor
 Room free of ambient magnetic & electric fields.

8. The scanning electron microscope

9. Electron guns are typically one of two types:
Electron
guns
Thermionic
guns
Field
emission guns

10. Thermionic guns:
Most common type.
Apply thermal energy
to a filament.
Usually made of
tungsten (high M.P).
Field emission guns:
Create strong electric
field.
Located either at very
top or at very bottom
of an SEM.
These eˉ are not
specific in direction…

11. Electron Gun

12. CONDENSER LENSES
 Just like optical microscopes, SEMs use condenser lenses to produce
clear & detailed images.
 The condenser lenses in these devices, however, work differently.
 They aren’t made of glass.
 Instead, made of magnets capable of bending the path of eˉ.
 Hence, focus & control eˉ beam, ensuring that the eˉ end up precisely
where they need to go.

13. 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.

15. 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.

17. 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.

19. 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.

21. VACCUM 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.

22. APPLICATIONS
 Vareity of applications in a # of scientific & industry related fields
(characterisation of solid materials is beneficial).
 In addition to topographical, morphological and compositional &
crystallographic information, SEM 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.
 SEMs have practical, industrial and technological applications such as
semiconductor inspection, production line of miniscale products and
assembly of microchips for computers.
 They can be as essential research tool in fields such as life science, biology,
gemology, medical and forensic science, metallurgy.

23. ADVANTAGES
 Wide array of applications.
 3D & topographical imaging.
 Versatile information gathered
from different detectors.
 Easy to operate (user friendly).
 Works faster ( completing SEI,
BSE & EDS).
 Technological advances allow
generation of data in digital form.
 Most SEM samples require
minimal preparation action.
DISADVANTAGES
 Expensive, large & must be housed
in an area free of electric, magnetic
and 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.

24. POLLEN GRAINS taken on a
SEM show characteristic
depth of field of SEM
micrographs.