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SYNTHESIS AND PROCESSING: GENERAL METHODS 29 ION-BEAM PROCESSING Ion implantation is a method of introducing elements into a target material to form a controlled mixture up to 1 µm in depth, with a high degree of precision and without having to heat the material. The ion implantation process begins by forming a plasma of ions containing the elements to be implanted. Ions are then extracted from the plasma, focused, and accelerated to an energy about 10 to 30 keV. The desired ion species is then selected by a magnet and is further accelerated up to many MeV before reaching the surface of the solid. Upon entering the surface of the solid, the incident ion loses its initial kinetic energy by collision with the atoms and excitation of the electrons in the solid. The final distribution of the ions can be accurately predicted by either analytical theories or computer simulations. The penetration 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 implanted. Since the ion depth is typically less than 1 mm, ion implantation is suitable for surface engineering on the submicron scale. In addition to the high degree of precision and control of the distribution of the implanted species, which may be any element in the periodic table, ion implantation is a nonequilibrium process that can be used to make new materials, including metastable crystalline and amorphous materials on surfaces. An increasing effort is being directed toward applications of ion implantation to the fabrication of semiconductor devices, corrosion and wear-resistant surfaces, and high-temperature superconducting thin films. Clearly, the development of micron-or submicron-sized ion beams for patterning will have considerable future impact in these areas. Ion-beam mixing is a mechanism whereby energetic ions bring about the intermixing of layers during ion irradiation. A variety of submicron surface structures, which may be either amorphous or crystalline, can be formed, depending on the chemical composition of the surface layers and the ion-beam irradiation conditions. In many ways ion-beam mixing is analogous to ion implantation. They differ principally in emphasis. In ion implantation, the interest focuses on modifying the solid by the addition of the implanting species. But the maximum concentration of the implanted species in a solid can be severely limited by the process of sputtering that occurs during ion implantation. The highest concentration of the implanted species is typically less than 30 percent. Ion-beam mixing does not suffer from the limitation of sputtering to the extent that ion implantation does. In ion-beam mixing the desired species is first deposited onto the surface of a sample by some conventional means such as evaporation deposition or sputter deposition. An ion beam of sufficient
