Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 116

Generally, an oscillating quartz microbalance is placed near the object, providing
real-time measurement of layer thickness. Another way is to put a filter paper close
to the sample. Part of the filter paper is protected from evaporation to allow seeing
the coloring changes, by comparing the grey color of the two filter paper areas.
The gray coloring changes depend on the coating thickness. This system is used to
determine approximate layer thickness.
The metal evaporation source, generally a wire with a diameter of approximately
1 mm, is located in a tungsten basket placed between the two electrodes. When the
chosen vacuum is reached, the setup is heated until evaporation begins.
The evaporated metallic particles are projected into all directions, particularly
downward, where the stage holds the object. Evaporation is very rapid. Once the
desired thickness is reached, monitored by the in situ thickness control, the heat
is suddenly shut off and the object can be removed from the chamber once air is
allowed inside.
The evaporation temperatures of different metals used are found in special-
ized publications and are essentially dependent upon the pressures in the vacuum
chambers.
There are many other source-support devices. In particular, cradles that require
inverting the positioning of the substrate, since evaporation can only occur upward.
Also in the specialized publications (L. Holland) are tables with different setups to
be used and their conflicts with different metals.
For the evaporation of carbon by the Joule effect, the evaporation device is
replaced by two carbon rods. Figure 5.22 shows two carbon-rod device setups. The
first setup shows that the carbon rod is thinner in the center so as to prevent less
resistance during heating. It is this part that will evaporate first when a very intense
current goes through it. In the second setup, one of the rods is sharpened to a fine
point and placed into contact with the unsharpened rod. The system contains a spring
that ensures contact between both during evaporation. Evaporation is obtained by an
electrical arc generated by the passage of a very intense current.
Fig. 5.22 Device for carbon
evaporation by the Joule
effect
The carbon rods can be replaced by a carbon braid. This system is very useful
for obtaining good reproducibility.
Evaporation Using an Electron Gun
Evaporation by means of electron bombardment is used to obtain metallic coatings
with very fine grain sizes.