Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 95

Incident Ion

High Incidence

Pulverized Ion

Low Incidence

Incident Ion Pulverized Ion

Fig. 5.9 Selective etching as a function of the nature of the material and the angle of incidence of
the ions

The gas used is generally an inert gas, preventing any chemical reaction with the
material. The gas must also have a high atomic weight to be able to pulverize heavy
atoms. Argon (Ar), with an atomic mass of 39.9 amu, is frequently used. It is also an
element that is rarely found in the chemical composition of materials, which helps
to easily identify its implantation in the sample.
In some cases, it is preferable to use a mixture of an inert gas and a reactive
gas in order to accelerate abrasion or reduce artifacts generated by the usual argon
technique. For example, an argon-oxygen mixture can be used to thin diamond,
and iodine can be added to eliminate drops of indium that form on the surface of a
material containing indium during argon thinning.
The ion acceleration voltage is between 100 eV and 10 keV. The use of high volt-
age (and strong current) helps to rapidly abrade the material (up to 100 m/h), but

generates a large number of artifacts. Applying voltage as low as possible (depend-
ing on the type of available equipment) can markedly limit these artifacts. During
treatment, it will be applied only at the end of thinning because of the very low rate
of abrasion resulting from it. The use of a liquid-nitrogen-cooled specimen holder
reduces the temperature rise in the sample.
Two ion guns are generally used. These guns can be situated above, below, or
on both sides of the sample. The incidence angle between the ion beam or beams
and the sample plane can be from 0 ◦ to 90 ◦ , depending on the equipment. The
angles generally used are between 1 ◦ and 15 ◦ . As with acceleration voltage, using
the lowest angle possible will considerably reduce artifacts, but will increase the
time needed to obtain a thin slice.
Some instruments can be used to apply a current (delaying potential) within the
sample via the specimen holder. This arrangement can divert the incident electrons

µ