1. Principle
1.1.What does the word FESEM mean?
FESEM is the abbreviation of Field Emission Scanning Electron Microscope. A
FESEM is microscope that works with electrons (particles with a negative charge)
instead of light. These electrons are liberated by a field emission source. The object is
scanned by electrons according to a zig-zag pattern.
1.2. What can be done with a FESEM?
A FESEM is used to visualize very small topographic details on the surface or entire
or fractioned objects. Researchers in biology, chemistry and physics apply this
technique to observe structures that may be as small as 1 nanometer (= billion of a
millimeter). The FESEM may be employed for example to study organelles and DNA
material in cells, synthetical polymeres, and coatings on microchips. The microscope
that has served as an example for the virtual FESEM is a Jeol 6330 that is coupled to a
special freeze-fracturing device (Oxford Ato).
1.3. How does a FESEM function?
Electrons are liberated from a field emission source and accelerated in a high
electrical field gradient. Within the high vacuum column these so-called primary
electrons are focussed and deflected by electronic lenses to produce a narrow scan
beam that bombards the object. As a result secondary electrons are emitted from ech
spot on the object. The angle and velocity of these secondary electrons relates to the
surface structure of the object. A detector catches the secondary electrons and
produces an electronic signal. This signal is amplified and transformed to a video
scan-image that can be seen on a monitor or to a digital image that can be saved and
processed further.
1.4.How does een cryo-FESEM look like?
A cryo-FESEM looks like a cylindrical column (1) that is mounted on a desk. The
column hosts the electron beam. Knobs for the regulation of the electron beam can be
found at various levels on the column. There are also tubes present to mainatin the
cacuum and the temperature in the instrument and the cryo-unit.The microscope is
operated from the steering panel (2; on the desk). A close copy of this panel has been
used for the simulations. The cryo-unit with a binocular (3) is located left of the
column. When conventional (not cryo) microscopy is applied the exchange chamber
in front, below the columns (4) is used to introduce the object into the high vacuum
area. The object can be observed on the large screen (5) while it is scanned. The small
screen (6) serves to watch the object chamber. The computer for image archiving and
processing is located right (7). The cupboards below the desk contain (LOT OF)
electronics (8). On the background the sound of the pumps that maintain the vacuum
in the caolumsn canbe heared as well as the sissing of the boiling nitrogen for the
freeze-unit and the cooling of the column.
2. Preparation
In order to be observed with a SEM objects are first made conductive for current. This
is done by coating them with an extremely thin layer (1.5 - 3.0 nm) of gold or goldpalladium.
Furtheron, objects must be able to sustain the high vacuum and should not
alter the vacuum, for example by losing water molecules or gasses. Metals, polymers
and crystals are usually little problematic and keep their structure in the SEM.
Biological material, however, requires a prefixation, e.g. with cold slush nitrogen
(cryo-fixation) or with chemical compounds. This particular microscope is forseen of
a special cryo-unit where frozen objects can be fractured and coated for direct
observation in the FESEM. Chemically fixed material needs first to be washed and
dried below the critical point to avoid damage of the fine structures due to surface
tension. Coating is then performed in a separate device.
3. Source of electrons
In standard electron microscopes electrons are mostly generated by heating a tungsten
filament by means of a current to a temperature of about 2800°C. Sometimes
electrons are produced by a crystal of lantanumhexaboride (LaB6) that is mounted on
a tungsten filament. This modification results in a higher electron density in the beam
and a better resolution than with the conventional device. In a field emission (FE)
scanning electron microscope no heating but a so-called "cold" source is employed.
An extremely thin and sharp
tungsten needle (tip diameter
10–7 –10-8 m) functions as a
cathode in front of a primary
and secondary anode. The
voltage between cathode and
anode is in the order of
magnitude of 0.5 to 30 KV.
Because the electron beam
produced by the FE source is
about 1000 times smaller than
in a standard microscope, the
image quality is markedly
better. As field emission
necessitates an extreme
vacuum (10-8 Torr) in the
column of the microscope, a device is present that regularly decontaminates the
electron source by a current flash. In contrast to a conventional tungsten filament, a
FE tip last theoretically for a lifetime, provided the vacuum is maintained s