It is shown that the sheath evolving to a negatively biased wafer during the plasma immersed ion implantation focuses the positive ions to distinct regions on the surface resulting in a poor uniformity of the ion dose. The focusing mechanism is investigated and solutions to improve the ion dose uniformity are proposed based on three-dimensional calculations of the potential distribution and ion trajectories within the sheath structure. Simulations are confirmed at lower scale in a DC Ar plasma which allows an easier visualization of the focusing phenomenon simultaneously with the measurement of the involved currents. (c) 2005 Elsevier B.V. All rights reserved.