Numerical studies of collisionless and collisional sheath evolution in plasma source ion implantations

Abstract

In plasma source ion implantations, a target immersed directly in a uniform plasma is biased with a high negative voltage pulse. Then an expanding plasma sheath forms between the plasma and the solid target surface, and implants ions into the material. In this paper, the evolution of collisionless and collisional sheaths in different geometries, and for various gas pressures, has been numerically studied using a one-dimensional fluid model. Collisions are included in the model through a collisional drag term in the equation of ion motion. Besides the sheath evolution, plots for the average kinetic energy as well as the current density of ion implantations at the target surface are presented. For the collisional sheath, it is found that the final sheath thickness in planar and spherical (or cylindrical) geometries differs from each other significantly. Also, computational results of the collisional sheath in different geometries are in good agreement with those obtained by analytic models.