Charge neutralisation during high-current ion implantation

During ion implantation, positive charge accumulates due to excess positive beam ions incident on the wafer as well as through the emission of secondary electrons from the wafer surface. This positive charge build-up is counteracted by the flow of electrons onto the wafer surface. These electrons are drawn from the beam plasma, from supplemental sources of electrons ("flood guns"), and from current flows along the wafer surface and through the bulk [1]. In a typical situation, these charge flows are not exactly balanced, especially in local regions (such as on isolated polysilicon structures), leading to voltage build-up and degradation or destruction of gate oxides and other dielectric films. A successful charge neutralization technology strives to balance the total charged flux into the region near the wafer surface and minimize net charge flow to the wafer. The potential differences between the ion beam plasma and the structures on the wafer surface influence the charge flow to the surface and must be minimized. Since build-up of excess electrons on the wafer surface may also lead to damage in sensitive dielectric films ("overflooding"), the energy of the electrons that are used to balance the positive charge flow must be lower than those potentials that can lead to oxide wearout and breakdown problems. In ion implanters, the beam generates too few electrons by collisional ionization of background gases to balance the positive ion beam current. To neutralize the beam, an auxiliary source of electrons--an electron "flood gun"--must provide additional negative charge