The document discusses the components and operation of a scanning electron microscope (SEM). It describes that a SEM has two main components - an electronic console and electron column. The electronic console controls parameters like filament current and magnification. The electron column focuses a beam of electrons onto sample surfaces. As the beam scans the sample, signals like secondary electrons and backscattered electrons are emitted and detected. These signals are amplified and displayed as images on a screen. The document also discusses vacuum requirements, sample preparation, interaction volume, image formation and quality in SEM.

1. Prepared by: Hailay Kidane
hailaykidane@gmail.com
Submitted to:Adane Haile
(Asst.Professor)
1st year Ms.c
semester 1
ID:SGSR/0178/09
13/11/2009 E.c

2. The SEM instrument is made up of two main
components, the electronic console and the
electron column. The electronic console provides
control knobs and switches that allow for
instrument adjustments such as filament
current, accelerating voltage, focus,
magnification, brightness and contrast

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An instrument for observing and
Analyzing the surface micro
structure of a bulk sample using
a finely focused beam of
energetic electrons
What does the SEM do?

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4
The main components of a typical SEM
Electron
column
Scanning system,
Display
Vacuum
system
Detector(s)
Electronics controls,

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Electron column consists of an
electron gun and two or more
electromagnetic lenses operating
in vacuum.
Scanning electron microscope column

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Electron optical column

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generates free
electrons
and
accelerates
these
electrons to
energies in
the range 1-40
keV in the
SEM
:
create small,
focused
electron
probe on the
specimen.
electron
beam is
focused into a
fine probe,
which
is scanned
across the
surface of the
specimen &
emits signal
in the
form of
electromagne
tic radiation

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Collects
Selected
portions of
radiation,
usually
secondary
(SE) and/or
backscattered
electrons
(BSE),
Display: collected
signal is amplified
and displayed on a
TV screen or
computer monitor.
The resulting
image is generally
straightforward to
interpret

9. Vacuum System
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The ability for a SEM to provide a controlled electron
beam requires that the electronic column be under
vacuum at a pressure of at least 5x10-5 Torr. The
reasons are the following.
 A hot tungsten filament will oxidize and burn out in
the presence of air at atmospheric pressure
 Column optics requires clean, dust-free environment.
 air particles and dust inside the column can interfere
and block the electrons before the ever
reach the specimen in the sample chamber

10. Scheme of Scanning Electron Microscope
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11. modes of imaging in the SEM
Beam accelerating voltage
(kV):
Probe convergence angle (αp):
Probe current (ip):
Probe diameter or spot
size (dp):
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11
1
2
3
4
Brightness(B) = 4ip / (π dp αp)2

12. Sample requirements in SEM
Samples must be compatible with high vacuum(~ 10-5 mbar).
Non-conductive materials need coating by thin
conductive film such as Au, Pt, Pd, their alloys, as well as
carbon
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Special
types of
SEM
variable-pressure SEM (VPSEM)
and environmental SEM(ESEM)
Samples containing water and other volatile
components cannot be studied directly. Also fine
powder samples need to be fixed firmly to a specimen
holder substrate so that they will not contaminate the
SEM specimen chamber.

13. Interaction Volume
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 The image details and resolution in the SEM
are determined not by the size of the electron
probe by itself but rather by the size and
characteristics of the interaction volume.
 The resulting region over which the incident
electrons
interact with the sample is known as
interaction volume

14. IMAGE FORMATION IN SEM
 The images in the SEM are formed by
electronic synthesis, no optical
transformation takes place, and no real of
virtual optical images are produced in the
SEM.
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15. Magnification in the SEM
 increase or decrease in
magnification is
achieved by respectively
reducing or increasing
the length of the scan
on the specimen
.
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M = Lmon/Lspec
Length of monitor
magnification

16. Image Quality  Contrast (C) is defined
as:
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contrast is always positive
and varies from 0 to 1
 Measuring the signal (S)
consists of counting the
number of electrons (n), at
the detector. Since the
presence of noise degrades
the signal, the signal
quality is expressed as the
signal-to-noise ratio
(S/N).
10mm
38mm
600 micron

17. Signals and Imaging modes in the SEM
 The signals Are secondary
 (SE) and backscattered
electrons (BSE). SE are
low energy particles& BSE
are high energy particles
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coefficients(C)
On
backscatter
(η1 and η2).
Effect of chemical composition
regions of heavy elements are
brighter than those of light ones

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Image disturbances and their possible causes in SEM

19. (
instrument performance,
nature of the specimen. All three aspects
operate concurrently and neither of them
should or can be ignored or
overemphasized.
,
selection of imaging parameters (e.g.
operator control),
1
2
3

20. reference
1) Bob Hafner ;Scanning Electron Microscopy Primer
2) Brandon Cheney Ant’s ;Introduction to Scanning Electron
Microscopy
3) http://www.jeol.com/sem/docs/sem_guide/tbcontd.html
4) http://en.wikipedia.org/wiki/Turbomolecular_pump
5) D.S. Su: guide of Scanning electron microscopy (SEM)
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THANK
YOU

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