Complex ion-focusing effect by the sheath above the wafer in plasma immersion ion implantation

The ion flux on the wafer surface during plasma immersion ion implantation is investigated by three-dimensional simulations and experiments. Due to the finite size of the wafer and its stage, the evolving sheath acts as a lens that focuses the positive ions to distinct regions on the wafer surface. Depending on the sheath profile, two focusing effects are identified. Discrete focusing involves ions entering the sheath from its frontal side and leads to the formation of a passive surface near the wafer edge, while the modal focusing affects ions entering the sheath from the lateral side of the stage and are eventually directed to the wafer center