Environmental SEM System
Introduction
The typical scanning electron microscope is comprised of several components.
First, an electron gun supplies a beam of electron ranging in energy from
1 to 40 keV. Next, the electron beam passes through a vacuum electron column
which contains the elect ron lens. In turn, these lens reduce the diameter
of the primary electron beam to create a focused high intensity electron
beam. With the aid of scanning coils the focused electron beam is scanned
across the surface of the specimen to obtain a raster im age. The scanning
coils deflect the beam to first strike the surface in a straight line.
Then, a scanning circuit steps the beam incrementally until a rectangular
raster image of the surface is obtained. Several phenomena such as absorption,
radiation, and transmission are observed after the primary incident beam
strikes the specimen. However, backscattered electrons and secondary electrons
are primarily used to obtain an image of the surface. Back scattered electrons
are electrons from the impinging electron beam which scatter laterally
after striking the surface. Since they are deflected with only a slight
loss of energy, these electrons can escape be detected if near the surface.
However, some of the high energy electrons from the impinging beam penetrate
the surface of sample. Once inside the sample, the high energy electrons
excite electrons within the sample. The excited electrons which have sufficient
energy to escape the surface are known as secondary electrons. The signals
from both of these electrons is detected and intensified. Next, the created
image is displayed on a cathode ray tube. In terms of operation, the envirnomental
scanning microscope (ESEM) is fairly similar to the conventional scanning
electron microscope (SEM). As before, the incident electron beam travels
through a vacuum column. However, in the ESEM, the envirnomental chamber
is maintained at pressures ranging from 0.1 to 1 kPa. Furthermore, depending
on the type of material to be examined, different gases are introduced
into the envirnomental chamber. The interactions between the gas molecules
and the incident electron beam are neuralize surface charging on samples
which act as insulators and amplify the output signal.
Figure 1 Environmental SEM Surface Assessment System