Model of Collisional Sheath in Spherical and Cylindrical Geometries for Plasma Source Ion Implantation

A model is developed that describes the temporal evolution of the sheath during a pulse of high negative voltage applied to a spherical and cylindrical target in a plasma such as that present in plasma source ion implantation for the case in which the pressure of the neutraJ gas is large enough that the ion motion in the sheath can be assumed to be highly collisional.The analytic expressions of the sheath expanding velocity are obtained.The positions of the shea th edge as a function of time predicted by the model are in agreement with those obtained by Huid equations.