Intermediate-Pressure Glow Discharge Reactors

With rare exceptions, plasma reactors for the surface treatment of materials that operate below atmospheric pressure are glow discharges in the intermediate- pressure regime between 6.7 Pa (50 mTorr) and 1.3 kPa (10 Torr). The intermediate-pressure glow discharge sources used for surface treatment include DC and RF parallel-plate, as well as magnetron plasma sources. These sources have been discussed in section 16.1 of this volume.

Figure 17.17. A planar surface layer plasma reactor on a 5 cm diameter cylinder. A pair

of wires is wrapped helically on the surface of the cylinder, providing adjacent electrodes of opposite polarity.

Figure 17.18. The cylinder shown in figure 17.17, operating in helium gas, at a frequency

(a)

(b)

Figure 17.19. A single insulated rod and insulated planar electrode configuration for the

generation of a sheet of OAUGDP. (a) Schematic diagram of configuration. (b) Photograph of a rod-and-plane OAUGDP configuration operating in helium gas. The rod length is 15 cm.

(a)

(b)

Figure 17.20. A double insulated rod configuration for the generation of an OAUGDP.

(a) Schematic diagram of configuration. (b) Photograph of a double-rod OAUGDP configuration operating in helium gas. The rod length is 15 cm.

Figure 17.21. A parallel-plate plasma reactor.

17.1.4.1 Parallel-Plate Reactors

Parallel-plate glow discharge reactors intended for surface treatment can be generated by DC or RF capacitive excitation, using the physics and technology discussed in chapters 9 and 12 of Volume 1, and section 16.1.1 of this volume. Additional information on parallel-plate intermediate-pressure plasma reactors may be found in Batenin et al (1992) and Raizer et al (1995). A characteristic parallel-plate reactor is illustrated in figure 17.21. The workpiece to be exposed to active species from the plasma is usually on or near an electrode. In industrial parallel-plate glow discharge reactors, the ratio L/d of plate width, L, to plate separation, d, may range over values from three to more than 100.

The location of glow discharges on the voltage–current characteristic of the DC low-pressure electrical discharge is discussed in section 4.9 of Volume 1, and is illustrated in figure 17.22. DC glow discharges used in surface treatment characteristically operate between 100 V and 1 kV, and at currents from a few milliamperes to approximately one ampere. The workpieces either are mounted on or comprise the cathode. Most applications require that the plasma be uniform over the surface of the workpiece. To ensure this uniformity, DC glow discharges are normally operated as an abnormal glow discharge, as discussed in section 9.5.2 of Volume 1. Such a discharge is established between the points G and H on the voltage–current curve of figure 17.22, where the plasma completely

Figure 17.22. The voltage–current characteristic of the classical DC low-pressure electrical discharge tube.

covers the cathode uniformly, and higher voltages are required to further increase the current.

Electron number densities, kinetic temperatures, and active-species production rates and fluxes available from the positive column are less than that from the negative glow. The positive column therefore is eliminated by operating the discharge as an obstructed abnormal glow discharge, leaving only the higher density and more energetic negative glow plasma to produce active species. Obstructed abnormal glow discharges are those for which the width, d, of the cathode region, shown on figure 17.23, is less than the Paschen minimum distance appropriate for the gas and pressure, as discussed in section 9.2 of Volume 1. In such obstructed abnormal glow discharges, a positive column and other structures are absent between the negative glow and the anode, and the cathode fall voltage is higher than the Paschen minimum breakdown voltage. This high sheath voltage is useful when energetic ions are required to etch the cathode surface, to sputter atoms off the cathode surface, or to treat workpieces mounted on the cathode.

17.1.4.2 Magnetron Discharges

Magnetron discharges, discussed in section 9.5 of Volume 1, are characterized by crossed electric and magnetic fields in the negative glow plasma, with the sheath (or cathode fall) electric field also at right angles to the magnetic field. When used

Figure 17.23. The obstructed abnormal glow discharge plasma reactor, with a schematic

potential distribution.

for treatment of individual workpieces or electrically conducting surfaces, the

DC parallel-plate magnetron, illustrated in figure 17.24, is normally preferred. In

some applications, supplemental RF excitation is used because it makes possible a higher electron number density in the negative glow, a higher sheath voltage drop, and greater discharge stability. The DC magnetron discharge has been discussed in section 9.5.6 and the RF magnetron in section 12.4.1 of Volume 1.

The magnetic field employed in magnetron discharges is usually generated by inexpensive permanent magnets, strong enough to magnetize and trap the negative glow electrons, but not the ions, in a local magnetic mirror above the cathode surface. This magnetic mirror trapping increases the electron number density and the active species production rate. The E/B ‘magnetron’ drift is into or out of the plane of the diagram in figure 17.24, thus making the plasma itself and its effects on a workpiece more uniform in the E× B/B2 drift direction. A porous fabric, electrically conducting film, or other electrically conducting material may be moved past the magnetron discharge at the electrode surface position A if ion bombardment is desired, or at the anode surface position Aif a

Figure 17.24. A low-pressure parallel-plate magnetron reactor. Fabrics or films may be

exposed either at the position A or A.

Figure 17.25. A schematic diagram of the low-pressure co-planar magnetron reactor. A

fabric or film (usually electrically insulating) passes by the plasma at the position Afor sputtering or other plasma surface treatment.

flux of sputtered atoms or other active species is required.

If the workpiece material is an insulator, or if charge build-up on the workpiece is undesirable, the co-planar magnetron geometry shown in figure 17.25 can be used with either RF or DC excitation. Placement of the workpiece material at the position Aallows it to receive a flux of active species, without having to interrupt real or displacement currents, or be immersed in a sheath.

17.1.4.3 Microwave Energized Reactors

Microwave plasma reactors may be used to generate active species needed for the plasma treatment of surfaces. They have the advantage of being ‘electrodeless’, and thus produce a plasma with fewer sputtered impurities than result from ion bombardment of the cathode of DC glow discharges or the powered electrode of RF glow discharges. A survey of microwave plasmas in surface treatment technologies has been published by Dusek and Musil (1990). Other more specialized references include Asmussen (1989) and Batenin et al (1992). Microwave glow discharge reactors used for surface treatment characteristically operate at the standard frequency of 2.45 GHz, and at power levels of a few hundred watts. The free-space wavelength at this frequency is 12.24 cm, a circumstance that may lead to the formation of undesirable modal patterns in the power density of such reactors.

The waveguide-coupled reactor has been discussed in section 13.5.1 of Volume 1, and is illustrated in figure 17.26. In this configuration, a microwave- generated plasma is created inside a quartz or dielectric tube inserted through a tapered waveguide. A feed gas flows through the plasma, and the resulting active species are convected to the workpiece by the gas flow. This arrangement is usually used at operating pressures below 13.3 kPa (100 Torr). At atmospheric pressure, such high power densities (0.1–10 kW/cm3_{) are required to maintain a}

microwave plasma that the power flux of active species is likely to damage the workpiece.

A second microwave reactor is the microwave cavity reactor, illustrated in figure 17.27, and discussed in section 13.5 of Volume 1 and in section 16.2.2. This reactor resembles a domestic microwave oven, except that it characteristically operates below 1.33 kPa (10 Torr), rather than at 1 atm. The microwave radiation arrives through a waveguide maintained at 1 atm, and passes into the evacuated reactor cavity through a ceramic window. A good impedance match—which yields a minimum of reflected power—is obtained by adjustment of tuning stubs in the input waveguide.

In the resonant cavity microwave reactor, also illustrated by figure 17.27, the free-space wavelength may be comparable to the radius or the axial dimension of the cavity, or a small integer multiple of these. Such a resonant cavity is undesirable for many applications, for reasons similar to those that apply to domestic microwave ovens. The modal concentrations of microwave power density produce a non-uniform plasma, which yields a non-uniform flux of active species and effect on a workpiece.

More satisfactory for applications requiring uniform surface treatment is the

non-resonant multimode cavity reactor, an example of which was discussed in

section 13.5.3 of Volume 1. In these reactors, the wavelength is smaller than any dimension of the cavity, thus resulting in a multimode, uniform plasma and a uniform flux of active species on workpieces.

Figure 17.26. A microwave waveguide coupled reactor, in which the active species of a

microwave-generated plasma are convected to a workpiece by the neutral gas flow.

17.1.5 Operational Issues

The intermediate-pressure glow discharge reactors discussed here operate below 1.33 kPa (10 Torr), and require vacuum systems and batch processing of treated materials. The requirement of operation in a vacuum system reduces the attractiveness of these plasmas for applications requiring continuous processing. If vacuum exposure and batch processing of the material are acceptable, however, these reactors offer a relatively simple way to generate the active species required with off-the-shelf hardware.

17.2

PLASMA REACTORS FOR ION IMPLANTATION

DC abnormal glow discharges have been used for low-energy plasma thermal

diffusion treatment (nitriding, carbonizing, etc) for nearly a century. The use of

glow discharges for plasma ion implantation as a competing technology to ion-

beam implantation, however, is as recent as the past two decades (Conrad 1988).

Each of these ion-implantation technologies requires its own characteristic plasma source, the subject matter of this section.

Figure 17.27. A microwave cavity reactor, in which active species from a microwave-generated plasma impinge on a workpiece located at the wall.