this presentation includes the principle,construction of scanning electron microscope and the problems-solutions it faces when dielectric surfaces are imaged along with normal imaging

1. Vinaykrishnan k
11-521

2.  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.

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

5.  Electron guns are typically one ofTWO types.
1) Thermionic guns
2) Field emission guns
1) 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

6. 2) 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.  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.

9.  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.  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.

12. The specimen(dielectric material) is placed
on a teflon holder.
This isolates the dielectric material from
typically mounted aluminium stub on which
a sample is usually placed for imaging in
SEM.

13.  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.  Cleaning of dielectric surface
 Stabilization of dielectric material
 Rinsing of material
 Dehydration of surface
 Drying of surface

17.  Charging effect takes place on the surface of
dielectric material due to electron irradiation.
 As a result, electrostatic charge formation
occurs due to charge trapping by the
dielectric material.
 Moreover various surface defects
characterized by surface roughness,
microstructural lattice damages also lead to
electrostatic charge formation.

19.  Coating the dielectric surface with a thin layer
of approx. 20-30 nm conductive metal(gold,
gold-palladium, platinum) which is grounded.

20.  Applying a defocused flux of soft landing
positive ions.
 Applying high energy electrons on dielectric
surface followed by bombardment of low
energy electrons.