A plasma is a partially ionized gas. Plasmas
actually dominate the visible universe: most of what we (and
telescopes) see in the night sky are various sorts of ionized
gases. Even on our cold, stable, low-energy world we see them
frequently in our daily life: in addition to the fusion plasma
that powers the sun, every fluorescent light and neon sign is
a plasma display device. Plasma-enhanced chemical vapor deposition
(PECVD) is a powerful tool for many film deposition processes
that cannot be achieved with temperature control alone.
Plasmas can be broadly divided into "thermal"
and "cold" varieties. A thermal plasma is just dang
HOT: everything is so toasty that the average energy of particles,
kT, is high enough to separate electrons from their atoms on
a regular basis (typically greater than 5000 K). A cold plasma
is a plasma in which only the electrons are hot, with neutrals
and ions being at modest temperature << ionization energies.
This sort of thing is possible because electrons, being much
lighter than atoms, exchange energy very poorly with them. It
takes thousands of collisions for an electron to exchange energy
with a population of heavy atoms or molecules (that is, to get
"thermalized"). If the ratio of the system size to
the mean
free path is small enough, the electrons don't have enough
time to exchange energy with ions before running into a wall
or being pumped away. You can couple energy into the electrons
without heating the gas. This discrepancy between electron and
gas temperatures makes cold plasmas of great interest for planar
processing, and in particular for chemical vapor deposition.
Hot electrons can ionize, dissociate, and excite, inducing chemistry
that doesn't take place otherwise in a cool gas. Furthermore,
the ions in the plasma can play an important role, especially
at surfaces.
For practical system sizes, cold plasmas
usually involve pressures well below atmosphere, and thus vacuum
technology. The types of reactors are usually classified by the
trick they use to couple energy into the electrons.
