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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. Plasmas, which are quasineutral (wi ~ n e ), are joined to wall surfaces across thin (0.1-5 mm) positively charged layers called sheaths. The net positive charge (Ms > > ne) within the sheath leads to an electric-potential profile that is positive within the plasma and falls sharply to zero near the wall. This profile acts as a confining potential "valley" for the fastmoving electrons and a "hill" for ions, thus equalizing their loss rates to the wall. The plasma potential with respect to the wall is a few Te in order to confine most of the electrons. The energy of ions bombarding the wall is then a few Te. The separation of discharges into bulk plasma and thin sheath regions is an important paradigm that applies to all discharges. The bulk region is quasineutral, and both instantaneous and time-averaged fields are low. The bulk-plasma dynamics are described by diffusive ion loss at high pressures and by free-fall ion loss at low pressures. In most cases, the flow is ambipolar—that is, the ion and electron fluxes are equal everywhere. In the positive space charge sheaths, large electric fields exist, leading to dynamics that are described by various ion spacecharge-sheath laws. The plasma and sheath dynamics are usually joined at their interface by requiring that the mean ion velocity at the plasma-sheath edge be equal to the ion sound (or Bohm) velocity: «B = (eTe/M)V2 where M is the ion mass.

Fundamentals of Low-Pressure Discharges The electrical power is coupled most efficiently to plasma electrons in lowpressure discharges. In the bulk plasma, energy is transferred inefficiently from electrons to ions and neutrals by weak collisional processes. The electronenergy distribution function does not necessarily have to be Maxwellian but insightful estimates of source operation can be obtained by approximating it as a Maxwellian, with Te and the various electron collisional rates assumed to be uniform within the bulk plasma. Electron-neutral collisional processes are important not only for particle production (ionization, dissociation) but also for other collisional-energy losses (excitation, elastic scattering). Ion-neutral collisions (charge transfer, elastic scattering) are also important in determining particle production, plasma transport, and ion-energy distributions at a substrate surface. The myriad of collisional processes that can occur in molecular feedstock gas mixtures 1 can obscure the fundamental principles of particle and energy balance. Consequently a noble gas, such as argon, is often used as a reference for describing discharge operation. However most process gases are molecular and electronegative (the discharge contains some negative ions), leading to significant d