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Plasma immersion ion implantation is a surface modification technique in which ions to be implanted are drawn directly from surrounding plasma by a pulse biased substrate. Ion acceleration occurs in the electric sheath that forms around the substrate. The technique is most often used to implant ions, such as nitrogen, from plasmas formed by excitation of a gas. More recently it has been applied with cathodic vacuum arc plasmas in order to implant metal ions. These plasmas have a directed ion drift velocity and generally a higher density than gas based plasmas. Both of these differences influence the development of the electric sheath, which determines the implantation profile and the overall stability of the process. If the sheath expands too much during the pulse, the plasma is depleted and implantation ceases. If, however, the sheath is too thin at any point in space breakdown occurs also stopping the implantation process. The major parameters affecting the sheath formation are plasma density, ion drift velocity, and substrate curvature. The influence of each of these factors is discussed and strategies for forming optimum sheath structures are identified. © 2001 American Institute of Physics.