Reference cited in this section

8. S.M. Myers, Ion Implantation, Chapter 10, J.K. Hirvonen, Ed., Academic Press, 1980 Equipment and Processing Times

As of 1994, ion implantation processing technology is still a relatively specialized process compared to many conventional surface treatment techniques. Most implanters in use today for metals applications require highly trained operators because they were originally built for processing silicon wafers, which required very rigorous process control, Such implanters have been subsequently modified for more general use. Typically, the cost of processing components in these types of systems is higher per unit area than for conventional coatings or surface modification techniques.

Figure 4 is a schematic of a typical medium-current semiconductor implanter adapted for metals implantation. This implanter uses a versatile ion source capable of producing heavy metal ion beams of most elements by introducing metal vapor or a volatile compound of the element into the plasma discharge of the ion source. After the positive ions are extracted from the ion source, they are mass analyzed by being passed through a 90° sector magnet. They are subsequently accelerated to their ultimate energy (typically 30 to 200 keV), then directed and scanned over the target.

HIGH-VOLTAGE TERMINAL

ANAL MAC

ANAL MAC

+ 200\

ANALYZER SL(TE

TARGET CHAMBER

TARGET CHAMBER

ANALYZER SL(TE

.JfjUON SOURCE

ENCLOSURE AT GROUND POTENTIAL

Fig. 4 Schematic of Naval Research Laboratory implantation facility for surface alloying. Mass analyzed 52Cr+ beam being transported for the implantation of bearing components to improve their corrosion resistance. The footprint of this machine is about 2.5 x 4 m. Source: Ref 9

An implanter of this design can produce from 1 to 10 mA (6 to 60 x 1015 ions/s) of positive ion beam, depending on the size and design of the ion source. At an energy of 100 keV, this beam current corresponds to energy densities of 100 to 1000 W in the envelope of the beam. It is therefore imperative to either spread this beam energy over a large area or to ensure good thermal conductivity of the substrates to a heat sink in order to limit beam heating. Both of these approaches are used in practice. The actual processing times can be determined by knowing the area to be treated, the ion current intensity, and the geometrical factors involving manipulation of the substrates through the beam to ensure uniform surface coverage. For a stationary substrate, an average beam current of 1 mA will deliver 1017 ions in about 15 s, a typical ion dose required for each square centimeter of the substrate. The actual implantation time will therefore depend on the area over which the beam is scanned to ensure both dose uniformity and adequate cooling. Larger batch sizes help reduce beam heating and improve the economics of processing.

Several approaches have been taken to simplify and reduce the cost of implantation treatments, such as using alternative ion source designs when using mass analysis (Ref 10). Large-scale dedicated systems have also been built without mass analysis to implant nitrogen into fairly massive components. One such unit in the United Kingdom has a processing chamber that is more than 2 m in diameter and 2 m in length and has been used to implant components such as automobile camshafts, dies, and plastic molds weighing more than 500 kg. A similar system has been constructed in the United States (Ref 11). Another large-scale non-mass-analyzed unit, which has a target platen 2 ft in diameter, has been built and installed in a U.S. Army Aviation Depot for high-current (25 mA) nitrogen implantation of metal cutting tools

A second approach, called plasma immersion ion implantation (PIII) or plasma source ion implantation (PSII) (Ref 13, 14), involves immersing the object to be implanted in a plasma and pulsing it to a high negative voltage (50 to 100 kV), thereby extracting the ions from the plasma. Because the plasma completely surrounds the component(s), the line-of-sight restriction of conventional ion implantation is greatly alleviated, with the result that more complex geometries can be implanted and/or less complicated fixturing can be used. A current restriction to this technique, however, is that it is limited to readily ionized plasma species (mainly gaseous elements such as nitrogen) and conducting substrates.

A third approach is to produce heavy ion beams from solids by using a metal vapor vacuum arc (MEVVA) discharge in a pulsed mode (Ref 15), yielding non-mass-analyzed ion beams extracted from a broad beam source (up to 50 cm in diameter). These sources have been demonstrated to produce up to 100 mA currents of metals. The commercial development of this source has been projected to yield low unit costs and high production throughput (Ref 16).

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