- 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.
- 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.
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