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monuclear fusion dominated much of plasma research in the 1960s and 1970s, in the last 20 years it has been the application of plasmas to materials processing that has provided new challenges for many plasma practitioners. It is not surprising that the guest editors and several of the authors for this issue of MRS Bulletin come from a fusion plasma-physics background. Plasmas are used in an incredibly diverse range of materials-processing applications—everything from treating wool in order to make it hydrophilic to the destruction of toxic wastes. This diversity is

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reflected in the articles we have assembled, which concentrate on the application of plasmas to the synthesis or modification of semiconductors, polymers, metals, and ceramics. Some processes, such as the thermochemical heat treatment of metals, the activation of polymers, the thermal spraying of ceramic coatings, and the etching of semiconductors, are well-established in industry while others belong to new and emerging technologies, such as plasma-immersion ion implantation and intense ion-beam processing. Plasma processes for materials synthesis can be grouped into two classes, low and high density, and are often displayed in an electron temperature versus density phase-space plot as in Figure 1. Low-density direct-current and radiofrequency glow discharges are usually nonequilibrium—that is, the electron, ion, and neutral temperatures, Te, T;, and To, respectively, are usually not equal. Moreover the electron-energy distribution often departs from a Maxwellian distribution. Low-density plasmas usually have hot electrons (Te > 10" K) sideby-side with cold ions and neutrals. Energetic electrons collide with and dissociate and ionize low-temperature neutrals, creating highly reactive free radicals and ions. These byproducts enable many traditional "high-temperature" chemical processes to occur with

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Electron density (particles per m3) Figure 1. Classification of plasmas as a function of electron temperature and density. A convenient unit of temperature is the electron volt (eV) where 1 eV equals 11,600 K through the Boltzmann constant.

MRS BULLETIN/AUGUST 1996

Plasma Processing of Advanced Materials

low-temperature feedstock and substrates. The small inventory of particles in low-density plasmas usually results in a small heat capacity. Consequently these plasmas are usually associated with low material-throughput processes such as surface modification. On the other hand, bulk material processing is usually associated with high-density "thermal" plasmas such as atmospheric-pressure arcs and torches. These plasmas are usually highly collisional, with Te » T-, = To, and provide an effective heat source to induce melting and vaporization of a solid feedstock. There are many ways to make a plasma. Plasma sources are chosen to optimize density, uniformity, temperature, composition, an