Introduction to Plasmas

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.

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