Stereo Scan 440 - DCU

Non-Volatile, Non-Conductors
If it is not possible to obtain suitable resolution by using a low accelerating
voltage and leaving the sample uncoated. then the following applies:-
For most non-conductors which contain no volatile components, eg water, that
would outgas in the vacuum system it is sufficient to coat the sample with a thin
layer of conducting medium such as Au, C, Au/Pd. Al etc. This layer is typically
20-3Onm in thickness. There are several reasons for this coating:-
. Increased conductivity of the sample, thus minimising sample
charge up, which results in deflection of the incident beam and
severe degradation of the fmal image. (see Section 3 .4 Specimen
Charging)
. Increased mechanical stability of the sample due to increased heat
condition.
. Increase in primary and secondary electron emission.
. Decrease in beam penetration, resulting in better spatial resolution.
The two important current techniques of applying a coating are
vacuum evaporation and ion sputtering.
Gold is generally used for the following reasons:-
High secondary emission co-efficient.
High conduction of electrons and heat.
. Does not oxidise.
. Good granularity of evaporated or sputtered particles.
Carbon coating by evaporation is generally used if X-ray microanalysis is to be
undertaken on the sample unless, of course, the element under investigation
happens to be carbon. Aluminium could be used in this case.
More recently Pt/Pd and Au/Pd have been used since their granularity is smaller.
Aluminium can also be used, but it has low mechanical strength and can oxidise
Non-Conductors (Volatile)
Biological and botanical samples, by their nature, require relatively more
complex preparation procedures. The samples fall into two main categories: a)
hard, b) soft.
8.
Hard samples (eg bone, teeth, wood). These, if necessary, can be
washed to remove extraneous fluids such as blood and mucus, dried in
air and coated in the normal way.
Stereoscan 440 Operator Manual