Ion purification for plasma ion implantation

Abstract

Plasma source ion implantation is carried out within an evacuated chamber having a target, such as a semiconductor wafer, supported on a target stage, with pulses of high voltage applied to the target periodically to implant ions from a plasma into the target. The ions in the plasma are purified after formation of the plasma by passing the plasma through an ion cyclotron resonance system composed of excitation electrodes and ion collector electrodes surrounding an ion purification region, with a magnet providing a unidirectional magnetic field through the ion purification region. A radio frequency time varying electric field from the excitation electrodes drives unwanted ions having charge-to-mass ratios greater than or less than that of the desired ion species to resonance with the electric field. During resonance, the undesired ions are driven outwardly in expanding spirals until reaching the ion collector plates, where the unwanted ions are removed. The purified plasma is passed to a plasma implantation region within the chamber from which the desired ions are implanted into the target as pulses of voltage are applied to the target. The ion purification may be carried out in a periodic manner between the repetitive pulses of voltage that are applied to the target.