SCANNING ELECTRON MICROSCOPY
“Increasing vision is increasingly expensive.”
Electron microscopes are scientific instruments that use
a beam of energetic electrons to examine objects on a very
Electron microscopes were developed due to the
limitations of Light Microscopes which are limited by the
physics of light.
In the early 1930's this theoretical limit had been reached
and there was a scientific desire to see the fine details of
the interior structures of organic cells (nucleus,
This required 10,000x plus magnification which was not
possible using optical microscopes.
The first scanning electron microscope (SEM)
debuted in 1938 ( Von Ardenne) with the first
commercial instruments around 1965. Its late
development was due to the electronics involved in
"scanning" the beam of electrons across the
An electron microscope is a type of microscope that uses a
beam of electrons to illuminate the specimen and produce a
magnified image. Electron microscopes (EM) have a
greater resolving power than a light-powered optical
microscope, because electrons have wavelengths about
and magnifications of up to about
10,000,000x, whereas ordinary, light microscopes are limited
by diffraction to about 200 nm resolution and useful
magnifications below 2000x.
The original form of electron microscope, the transmission
electron microscope (TEM) uses a high voltage electron beam to
create an image. The electrons are emitted by an electron gun
and transmitted through the specimen that is in part transparent
to electrons and in part scatters them out of the beam.
When it emerges from the specimen, the electron beam carries
information about the structure of the specimen that is magnified
by the objective lens system of the microscope.
The spatial variation in this information (the "image") may be
viewed by projecting the magnified electron image onto a
a phosphor or scintillator material such as zinc sulfide.
Image can be photographically recorded by exposing
a photographic film or plate directly to the electron beam, or a
high-resolution phosphor may be coupled by means of a lens
optical system or a fibre optic light-guide to the sensor of a CCD
(charge-coupled device) camera. The image detected by the CCD
may be displayed on a monitor or computer.
The surface features of an object or "how it looks", its texture;
direct relation between these features and materials properties
The shape and size of the particles making up the object; direct
relation between these structures and materials properties
The elements and compounds that the object is composed of
and the relative amounts of them; direct relationship between
composition and materials properties
How the atoms are arranged in the object; direct relation between these
arrangements and material properties.
A scanning electron microscope (SEM) is a
type of electron microscope that images a sample
by scanning it with a high-energy beam
of electrons in araster scan pattern. The electrons
interact with the atoms that make up the sample
producing signals that contain information about
surfacetopography, composition, and other
properties such as electrical conductivity.
Specimen and Electron Detector Geometries:
-position of detectors is a function of relative
energies of the electrons
SEM Sample Chamber [AMRAY 1830] showing positions
of SE and BSE Detectors and the Final Lens
-primary beam electrons
-composition and topography
[specimen atomic number]
SEM Imaging Modes
-sample electrons ejected by the
primary beam [green line]
-surface detail & topography
X ray is produced when outer shell electron
falls in to replace inner shell electron
WORKING OF SPUTTER COATER
Switch power on with main switch.
Flush working chamber several time with argon gas.
Set sputter time with timer digit switch.
Press start button to activate sputter process.
Adjust appropriate gas pressure with argon valve.
Set sputter current with current potentiometer.
Process stops when selected sputter time elapses.
To interrupt running sputter process press stop button.
Switch power off/working chamber will be vented.
What happens when the Electron Beam hits the sample
When the electron is bombarded by the electron beam on the
specimen , electrons are ejected from the atoms of the specimen
Inelastic scattering, place the atom in the excited state. The atom
“wants ” to return to a ground or unexcited state. Hence the atoms
will relax giving off the excess energy.
X-rays, Cathodoluminescence and Auger electrons are the three
ways of relaxation.
A resulting electron vacancy is filled by an electron from a higher
shell, and an X-ray is emitted to balance the energy difference
between the two electrons.
Limitations of Scanning Electron Microscopy (SEM)
Samples must be solid and they must fit into the microscope chamber.
Maximum size in horizontal dimensions is usually on the order of 10
cm, vertical dimensions should not exceed 40 mm.
For most instruments samples must be stable in vacuum . Samples likely
to outgas at low pressures (rocks saturated with hydrocarbons, "wet"
samples such as coal, organic materials or swelling clays, and samples
likely to depreciate at low pressure) are unsuitable for examination in
SEM's cannot detect very light elements (H, He, and Li), and many
instruments cannot detect elements with atomic numbers less than 11.
An electrically conductive coating must be applied to electrically
insulating samples for study in conventional SEM's, unless the
instrument is capable of operation in a low vacuum mode.
Advantages of Using SEM
The SEM has a large depth of field, which allows a large amount of
the sample to be in focus at one time and produces an image that is a
good representation of the three-dimensional sample.
The combination of higher magnification, larger depth of
field, greater resolution, compositional and crystallographic
information makes the SEM one of the most heavily used instruments
in academic/national lab research areas and industry.