Ion Implantation
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Ion Implantation S.T. Picraux and P.S. Peercy Today's materials scientist is challenged by ever-increasing demands on the surface properties of materials. We live in an age of high technology products where the surface layers of materials must maintain their electrical, optical, mechanical, or chemical characteristics while withstanding such degradation mechanisms as wear and corrosion. Moreover, as electronic and mechanical devices are miniaturized, the decreasing volume-to-surface ratio places even greater demands on the reliability of surface properties. At the same time the optimum surface properties may conflict with other requirements, such as low materials cost, ease of fabrication, and high strength. There is thus a continuing trend in materials science to develop surface modification methods to engineer favorable surface properties independent of the bulk. Because of their high precision and control, new methods such as those discussed in this issue of the MRS BULLETIN are increasingly replacing such traditional methods as the deposition of thin coatings. The high degree of precision and control which ion implantation allows in the tailoring of surfaces is intrinsic to the process. Implantation works by first ionizing the element of interest to create a beam of ions which is accelerated and directed onto the surface of the material. Any element in the periodic table can be selected, a n d the number of ions implanted are counted by measuring the charge carried to the material. Since ions are injected directly into the target material, an intimate mixture is formed without having to heat the material, and adhesion of the modified layer is not an issue. The depth of the ions is controlled by the energy to which they are accelerated, and the final composition is determined by the depth profile and total number of ions added. Historically, ion implantation became possible with the advent of isotope separators during World War II. The realization of the importance of ion implantation for adding dopant species to semiconductors (the most widespread application of ion implantation today) came early and was first reported by Russel Ohl at Bell Labs in 1952. However, the technique was not sufficiently developed for implantation doping of silicon to become a viable alternative to standard furnace diffusion techniques until the late 1960s. A key step in this development was to learn the subsequent annealing steps necessary to remove the damage which was introduced as the ions collided with and displaced the silicon lattice atoms. With the advent of integrated circuit technology in the beginning of the 1970s,
the precision and control of ion implantation brought it into rapid use for microelectronics processing. The use of ion implantation for applications such as adjusting the threshold voltages of fieldeffect transistors allowed the fabrication of low power circuits, making possible such products as electronic watches and hand calculators. Ion implantation processing is now pervasive in microelectronics. Essentiall
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