Chemically reactive plasma discharges are widely used to process materials. A plasma is a primarily electrically neutral collection of free charged particles moving in random directions. The simplest plasma consists of electrons and one kind of positive ions. This article deals primarily with plasma discharges, which are plasmas having the following features:
(1) They are driven electrically.
(2) Charged-particle collisions with neutral-gas molecules are important.
(3) There are boundaries at which surface losses are important.
(4) Ionization of neutrals sustains the plasma in the steady state.
One simple discharge consists of a voltage source that drives current through a low-pressure gas between two conducting plates or electrodes. The gas “breaks down” to form a plasma, usually weakly ionized—that is, the plasma density is only a small fraction of the neutral-gas density.
The plasmas used in materials processing present an enormous range of charged-particle densities n and of temperatures Te, Ti, and T for electrons, ions, and processing gas, respectively. High-pressure (atmospheric) discharges are in near-thermal equilibrium (Te ~ Ti ~ T ~ 0.1–2 eV). Plasma temperatures are usually given in equivalent electron-volt units: One eV is equivalent to 11600 K through the Boltzmann constant. As discussed in the article by Boulos and Pfender in this issue of MRS Bulletin, these thermal discharges have high densities n ~ 1014-1019 particles/cm3 and are mainly used as heat sources. Low-pressure (1 mTorr–10 Torr) discharges are not in thermal equilibrium (Te ~ 2–5 eV ≫ Ti ~ T) and have low densities n ~ 109–1012 particles/cm3. As discussed in several of the following articles, these discharges are used as miniature chemical factories in which feedstock gases are broken into positive ions and chemically reactive etchants, deposition precursors, etc., which then flow to and physically or chemically react at the surface of a substrate.