Plasma
A plasma is a mixture of positive
ions, negative ions, and electrons produced by an electrostatic or electromagnetic
field. The ions or charged molecules (excited radicals) remove contaminants
by physically sputtering them from the surface or reacting with them chemically.
In chemical removal, the plasma breaks the contaminant molecules into water
vapour, carbon dioxide gas, and small, volatile organic molecules. These are
then exhausted from the area. Most plasma cleaning processes are done in vacuum
chambers with argon or oxygen plasmas.
Plasmas are effective for removal of very thin organic layers. Plasma cleaning
is common in the semiconductor industry. It often improves wire bonding on hybrid
microcircuits. Plasma cleaning can strip wire insulation from coils and magnetic
assemblies. It is also used to prepare surfaces before plating on plastics.
Plasmas are best suited for line-of-sight cleaning. Some work has been done
to develop atmospheric pressure plasmas, which may expand the applications for
this technique.
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