Physical Principles of Electron Microscopy: An Introduction to TEM ...

The Scanning Electron Microscope 135
emitting visible-light photons when bombarded by charged particles such as
electrons. The number of photons generated by each electron depends on its
kinetic energy Ek (= eVs) and is of the order 100 for Vs � 10 kV. The
scintillator material has a high refractive index, so advantage can be taken of
total internal reflection to guide the photons through a light pipe (a solid
plastic or glass rod passing through a sealed port in the specimen chamber)
to a photomultiplier tube (PMT) located outside the vacuum.
The PMT is a highly sensitive detector of visible (or ultraviolet) photons
and consists of a sealed glass tube containing a hard (good-quality) vacuum.
The light-entrance surface is coated internally with a thin layer of a material
with low work function, which acts as a photocathode. When photons are
absorbed within the photocathode, they supply sufficient energy to liberate
conduction or valence electrons, which may escape into the PMT vacuum as
photoelectrons. These low-energy electrons are accelerated toward the first
of a series of dynode electrodes, each biased positive with respect to the
photocathode.
At the first dynode, biased at 100 � 200 V, the accelerated photoelectrons
generate secondary electrons within the escape depth, just like primary
electrons striking an SEM specimen. The dynodes are coated with a material
with high SE yield (�) so that at least two (sometimes as many as ten)
secondary electrons are emitted for each photoelectron. The secondaries are
accelerated toward a second dynode, biased at least 100 V positive with
respect to the first, where each secondary produces at least two new
secondaries. This process is repeated at each of the n (typically eight)
dynodes, resulting in a current amplification factor of (�) n , typically � 10 6 for
n = 8 and ��4.
Figure 5-8. A typical scintillator/PMT (Everhart-Thornley) detector, employed for secondaryelectron
imaging in the SEM. From Reimer (1998), courtesy of Springer-Verlag.