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 the 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 similar to the one in an ordinary television. (Seeing
an image)
When
using a SEM, the column and sample must always be at vacuum. A vacuum
environment means that most of the air molecules have been removed from the
inside of the microscope. There are many reasons for requiring a vacuum in an
SEM. If the filament were surrounded by air, it would quickly burn out, like a
light bulb. If the column were full of air, the electrons would collide with the
gas molecules and never reach the sample. If gas molecules react with the sample
different compounds could form and condense on the sample. This can lower the
quality of the image.
Preparing Samples
Uses