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What is an Electron Microscope

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					                               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           The first modern scanning electron microscope,
  more control in how much magnification        constructed by D. McMullan in the Cambridge
  he/she obtains. The electron beam also        University Engineering Laboratory in 1951.
  provides greater clarity in the image         Source: Electron Optics and Electron Microscopy,
  produced.                                     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 three-
                                                                dimensional sample.
    A modern version of the SEM.


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          All the elements have different sized nuclei. As
come back out of the sample. Because they are        the size of the atom nucleus increases, the
moving so fast, they travel in straight lines. In    number of BSE increases. Thus, BSE can be
order to form an image with BSE                      used to get an image that showed the different
(backscattered electrons), a detector is placed in   elements present in a sample.
their path. When they hit the detector a signal is
produced which is used to form the TV image.
      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            Since they are moving so slowly, and are
repels the specimen electrons, The repulsion            negatively charged, they can be attracted to a
may be so great that the specimen electrons are         detector which has a positive charge on it. This
pushed out of the atom, and exit the surface of         attraction force allows you to pull in electrons
the sample, these are called secondary                  from a wide area and from around corners in
electrons. Unlike the BSE, the secondary                much the same way that a vacuum pulls in dust
electrons are moving very slowly when they              particles. The ability to pull in electrons from
leave the sample.                                       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)   Kontakterings pinne (optisk mikroskop bild)
(topp vy)                                              (sid vy)

				
Jun Wang Jun Wang Dr
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