Nanotechnology-Enabled Sensors

5.7 Electron Microscopy 251
then focused by an objective lens and raster-scanned over the sample by
scanning coils. This is achieved by varying the voltage produced by the
scan generator on the scan coils that are energized, creating a magnetic
field, which deflects the beam back and forth in a controlled pattern.
Electron source
Limiting aperture
Limiting aperture
Objective lens
Secondary electrons
Fig. 5.30 Schematic diagram of an SEM set-up.
e –
Condenser lens
Scanning coils
Everhart-Thornley
detector
Sample
When the primary electrons hit the sample, they give part of their energy
to electrons in the sample, resulting in the emission of secondary electrons.
These secondary electrons have lower energies (around 20 eV).
These secondary electrons are collected by an Everhart-Thornley detector,
converted to a voltage, amplified and build the image. Their intensity is
displayed versus the position of the primary beam on the sample. The
samples placed in the SEM must be either conducting or covered with a
thin metal layer in order to avoid electric charging. Scanning takes place at
low pressures, so that the electrons are not scattered by gas molecules
inside the chamber. 86 Furthermore, with an SEM it is possible to obtain
images from comparatively large area of the sample.