Conferències, actes

Plasma sources have found an enormous range of industrial applications, including many niche applications when physical or chemical-processing methods proved inadequate. New applications are being reported at plasma- related meetings on an almost monthly basis. In terms of sheer numbers, plasma sources are most often found in the well-established areas of surface treatment of materials, microelectronic deposition and etching, ion-beam sources, and the thermal plasma processing of bulk materials.

15.1.6.1 Surface Treatment

Plasma surface treatment involves using direct plasma exposure to change the

surface energy, wettability, wickability, and bonding of fabrics, films, and solids, and the sterilization, cleaning, and decontamination of surfaces. To achieve industrially important effects, plasmas used for surface treatment must have a sufficiently high electron number density to provide useful fluxes of active species, but not so high or energetic as to damage the material treated. These constraints rule out dark discharges other than coronas for most applications, because of their low production rate of active species, and arc or torch plasmas, which have power densities and active-species flux intensities high enough to damage exposed material. Glow discharge plasmas, whether operated at 1 atm or under vacuum, possess the appropriate density and active-species flux for nearly all plasma surface treatment applications.

RF glow discharge plasmas can be generated by inductive, capacitive, or microwave excitation, using the physics and technology discussed in chapters 11, 12, and 13 of Volume 1, respectively, while DC corona and glow discharge plasmas may be generated with the respective physics and technology discussed in chapters 8 and 9 of Volume 1. The location of dark and glow discharges on the voltage–current characteristic of the DC low-pressure electrical discharge was discussed in connection with figure 15.1.

above 13.3 Pa (100 mTorr) is adequate. More advanced etching technologies below a design rule of 0.5µm, however, require plasma sources that operate in the low-pressure regime below 6.7 Pa (50 mTorr). Several low-pressure plasma sources are under development for microelectronic processing, and are discussed for completeness in chapter 16.

15.1.6.3 Ion-Beam Sources

A selection of ion-beam sources used in aerospace, industrial, and research applications was discussed in chapter 6 of Volume 1. Many of these sources were originally developed for high-energy and solid-state physics research, or as ‘ion engines’ for electrostatic space propulsion. This technology matured over the period from 1930 to 1970 and was adapted to industrial uses. These uses include ion-beam implantation to improve the hardness and wear (tribological) characteristics of medical prostheses and other high-value items; ion-beam doping of semiconductors and optical components; thin-film deposition by ion- beam sputtering; and ion-beam-enhanced plasma chemical vapor deposition and epitaxy.

These applications of ion beams require long mean free paths, and hence operation in the low-pressure or high-vacuum regimes. At these very low pressures, ionizing collisions are less frequent, and efficient generation of plasmas dense enough to provide the required ion fluxes is difficult. Of the sources discussed in chapter 6 of Volume 1, the Kaufman (or electron bombardment)

ion source is preferred when electrical or gas utilization efficiency is important;

the Penning ion source when it is required to produce ions of a wide range of elements or compounds; and the Freeman ion source when physical robustness and reliability are the dominant considerations.

15.1.6.4 Processing Bulk Materials

The oldest industrial applications of plasmas involve the processing of bulk materials using electrical arcs or plasma torches (see Boulos et al 1994, Cobine

1958, Hoyaux 1968, Gross et al 1969). Plasma torches and arcjets convert electrical power into thermal enthalpy and operate at or near thermodynamic

equilibrium, in which the ion, electron, and neutral working gas equilibrate to

temperatures ranging from 10 000 to 20 000◦C. These factors ensure a high power density and heat flux to a workpiece in direct contact with the plasma.

Bulk materials can be melted at 1 atm of pressure, where plasma torches and arcjets normally operate. These sources make it possible to weld and refine metals, melt scrap metals in arc furnaces, plasma spray thick films of ordinary and refractory materials, and maintain the temperature of ladles and tundishes in metalworking foundries (see Barber 1983, Cary 1979, Paschkis and Persson 1960, Smith and Novak 1991). Plasma torches and arcjets are also used to heat gas flows for energy addition in aerospace wind tunnel applications, and to increase the efficiency of blast furnaces (less coke and oxygen are needed to melt and refine the ore with plasma heat addition).

15.2

ATMOSPHERIC PRESSURE CORONA SOURCES

In section 15.1.5 it was pointed out that a corona should not be confused with the dielectric barrier discharge, an entirely different type of plasma. There exist at least two broad motivations to study corona in industrial applications. First, a corona can be a regulatory nuisance and safety hazard in the operation of high voltage equipment, and it must be well enough understood to engineer these problems to acceptable levels. Second, a corona can generate relatively low positive and negative ion fluxes that, however, are sufficient for some industrial plasma-processing applications, including plasma surface treatment. Important contributions to the literature of corona discharges have been made by Cobine (1958) and Loeb (1965).