KINETIC SIMULATION OF THE TRANSIENT SHEATH IN PLASMA ION-IMPLANTATION

Kinetic particle simulation modeling of a system consisting of plasma, sheath, material, and an external driving circuit is carried out. Plasma ion implantation relies on ion acceleration in the sheath region to a necessary energy level (10-50 keV). The dynamic evolution of the sheath is highly nonlinear and complex, especially when the negative voltage applied to the target material is modulated at a finite frequency to contain the sheath expansion to a manageable size. Our approach uses a kinetic particle-in-cell (PIC) simulation method for a bounded plasma system allowing an external circuit, as in the laboratory experiment. It predicts the kinetic properties which cannot be determined by the fluid approach. When the effect of voltage ramping is considered, the fast dynamic behaviors of the transient sheath such as an initial blow-up of the sheath, can be clearly understood. Except for the very fast ramping, our results are consistent with those of analytic and fluid calculations. The collisional effect is significant, but the temperature effect is negligible.