What is an Electron Microscope?
The development of the Scanning
Electron Microscope in the early 1950's
brought with it new areas of study in the
medical and physical sciences because it
allowed examination of a great variety of
specimens.
As in any microscope the main objective
is for magnification and focus for clarity.
An optical microscope uses lenses to
bend the light waves and the lenses are
adjusted for focus. In the SEM,
electromagnets are used to bend an
electron beam which is used to produce
the image on a screen. By using
electromagnets an observer can have
more control in how much magnification
he/she obtains. The electron beam also
provides greater clarity in the image
produced.
A modern version of the SEM.
The first modern scanning electron microscope,
constructed by D. McMullan in the Cambridge
University Engineering Laboratory in 1951.
Source: Electron Optics and Electron Microscopy,
P.W. Hawkes.
The SEM is designed for direct
studying of the surfaces of solid
objects. By scanning with an
electron beam that has been
generated and focused by the
operation of the microscope, an
image is formed in much the
same way as a TV. The SEM
allows a greater depth of focus
than the optical microscope. For
this reason the SEM can produce
an image that is a good
representation of the threedimensional sample.
How the SEM Works
The SEM uses electrons instead of light to form an image. A beam of electrons is produced at the
top of the microscope by heating of a metallic filament. The electron beam follows a vertical path
through the column of the microscope. It makes its way through electromagnetic lenses which focus
and direct the beam down towards the sample. Once it hits the sample, other electrons (
backscattered or secondary ) are ejected from the sample. Detectors collect the secondary or
backscattered electrons, and convert them to a signal that is sent to a viewing screen similiar to the
one in an ordinary television, producing an image.
How an Image is Produced
To produce an image on the screen, the electron beam scans over the area to be magnified and
transfers this image to the TV screen. The electron beam stops at 1,000 points as it scans
horizontally across the sample and down 1,000 lines vertically. This gives 1,000,000 points of
information. The signal read from the electrons coming off each point is transfered to a
corresponding point on the TV screen. Since the TV screen also has 1,000 points horizonally and
1,000 lines vertically, there is a 1:1 correspondance between the scan on the specimen and the TV
screen. Since the length of the electron beam scan on the specimen is smaller than the length of the
TV screen, a magnification is produced equal to the following equation:
Length of TV scan
Magnification =
Length of Electron Beam Scan
By changing the size of the scan on the sample, the magnification can be changed. The smaller the
area of the electron beam scan, the higher the magnification. Obtaining different degrees of
magnifications are important in any practical uses of the SEM.
Electron Beam/Specimen Interactions
While all these signals are present in the SEM, not all of them are detected and used for
information. The signals most commonly used are the Secondary Electrons, the Backscattered
Electrons and X-rays.
Backscattered Electrons
When the electron beam strikes the sample
some of the electrons will interact with the
nucleus of the atom in much the same way a
space craft will interact with the gravity of a
planet. The negatively-charged electron will be
attracted to the positive nucleus but if the angle
is just right instead of being captured by the
"gravitational pull" of the nucleus it will circle
the nucleus and come back out of the sample
without slowing down. These electrons are
called backscattered electrons because they
come back out of the sample. Because they are
moving so fast, they travel in straight lines. In
order to form an image with BSE
(backscattered electrons), a detector is placed in
their path. When they hit the detector a signal is
produced which is used to form the TV image.
All the elements have different sized nuclei. As
the size of the atom nucleus increases, the
number of BSE increases. Thus, BSE can be
used to get an image that showed the different
elements present in a sample.
Secondary Electrons and Detection
Sometimes beam electrons interact with the
electrons present in the atom rather than the
nucleus. Since all electrons are negatively
charged, the beam electrons will repel the
electrons present in the sample. This interaction
causes the beam electrons to slow down as it
repels the specimen electrons, The repulsion
may be so great that the specimen electrons are
pushed out of the atom, and exit the surface of
the sample, these are called secondary
electrons. Unlike the BSE, the secondary
electrons are moving very slowly when they
leave the sample.
Since they are moving so slowly, and are
negatively charged, they can be attracted to a
detector which has a positive charge on it. This
attraction force allows you to pull in electrons
from a wide area and from around corners in
much the same way that a vacuum pulls in dust
particles. The ability to pull in electrons from
around corners is what gives secondary
electron images a 3-dimensional look.
Source: Iowa State MSE Department, http://mse.iastate.edu/microscopy/ For a more advanced description see the
same source.
Energy Dispersive X-ray analysis
EDS Spectrum for contamination on stainless steel mesh.
DESCRIPTION OF TECHNIQUE
Energy dispersive x-ray spectroscopy (EDS) is a chemical microanalysis technique performed in
conjunction with a scanning electron microscope (SEM) . The technique utilizes x-rays that are
emitted from the sample during bombardment by the electron beam to characterize the elemental
composition of the analyzed volume. Features or phases as small as about 1µm can be analyzed.
When the sample is bombarded by the electron beam of the SEM, electrons are ejected from the
atoms comprising the sample's surface. 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.
The EDS x-ray detector measures the number of emitted x-rays versus their energy. The energy of
the x-ray is characteristic of the element from which the x-ray was emitted. A spectrum of the
energy versus relative counts of the detected x-rays is obtained and evaluated for qualitative and
quantitative determinations of the elements present in the sampled volume.
TYPICAL APPLICATIONS
 Surface contamination analysis
 Corrosion evaluations
 Coating composition analysis
 Rapid material alloy identification
 Small component material analysis
 Phase identification and distribution
Source: http://www.mee-inc.com/eds.html
Examples:
Jämförelse av spår på kula hittad på brottplatsen med
kula avfyrad från ett misstänkt vapen.
Tjocklekmätning av tunna skikt
Grundämnes analys längs en linje. Denna metod
används också för tjockleksmätning av tunna skikt..
Brott av lödbump i BGA
ESD-tyg med metalledare
Tvärsnittsbild på billack
komponent brott orsakad av icke tillräcklig mängd
lodpasta .
Slitage på guldbelagt kontakterings pinne (SEM bild)
(topp vy)
Kontakterings pinne (optisk mikroskop bild)
(sid vy)