The implanted dose can often be controlled to within a few percent. In addition, doses in the range of 1010 to 1018 cm2 are needed for many applications and are almost impossible to achieve by a thermal diffusion process in many applications. Range and straggle are roughly proportional to ion energy.
In conventional ion implantation devices, an ion beam is extracted from a plasma source, accelerated to the desired energy, and then transported to the target. Typical beam currents are very small (in the microampere range) and the beam “footprint” area is less than 1 cm2. To process large-scale targets, and to avoid shadowing if the target is nonplanar, a combination of beam rastering and target manipulation during the process is required.
In the plasma source ion implantation (PSII) process, the object is immersed in a plasma in which the Debye length is much smaller than the dimensions of the object. The strongest electric field is then in the cathode sheath, which accelerates positive ions to the negatively biased object, which serves as a cathode. If the pressure is kept low enough to prevent an arc discharge, positive ions can be accelerated to energies of 100 kV or more and can be implanted into the cathode surface.
To control the process, the implantation voltage is pulsed. The process begins with the application of a high negative potential to the object relative to the vacuum chamber wall. As the potential of the surface becomes more and more negative, the sheath, or the region surrounding the surface from which the electrons have been expelled, expands into the plasma, reflecting the electrons ahead of it. Being more massive, the ions do not have time to move as the boundary sweeps through them. When the ions find themselves on the other side of the boundary, they are in a region of a strong inward electric field, which accelerates them to the cathode.
Currently, the factors and their interactions that influence this process are poorly understood from the standpoint of the behavior of the material that is implanted. To further advance this field, an understanding of how the bombarding particles interact with the base material and how to apply this knowledge to manufacturing techniques is required for industry to further exploit this technology. Three aspects of this technique, surface hardening and wear resistance, corrosion and oxidation resistance, and semiconductor applications, are ideally suited for the use of statistical methods for experimental design and, in particular, of response-surface methods for exploring interaction effects. These are crucial tools for further developments in this area.
To be applied successfully in industry, many of the applications of ion implantation must be demonstrated to be cost-effective. Here two classes of implantation applications—metals and dielectrics—are discussed. The inherent differences in metallic, as compared with dielectric, implantation need to be considered. In both cases, the substrate requires an applied bias voltage for ion acceleration, but dielectrics will undergo charging and therefore inherently degrade the ion acceleration sheath, thus modifying the implantation. Accordingly, the acceleration voltage will be applied either to a backing conductor or to a conducting metallic, graphitic, or diamond-like carbon film deposited on the substate, which will also act to inhibit its charging.
Recent work using this process has shown remarkable improvement in properties of nitrogen-implanted alloy die steel and of nitrogen-implanted aluminum tools for machining high-temperature alloys. The effects of implanting nitrogen into a surface previously enriched, for instance with carbon and/or boron by vapor deposition, are now being examined.