Abstract: A scan wheel for an ion implanter adapted for carrying a plurality of semiconductor wafers. The scan wheel being rotatable about an axis comprises a central hub, a plurality of separate spoke arms mounted on and extending from the central hub, and a plurality of wafer support elements on the outer ends of respective spoke arms. Each spoke arm has a dimension in the direction of rotation substantially less than the dimension of the corresponding wafer support element, a front face extending generally in the direction of rotation and side faces extending rearwardly from the front face. Each side face does not extend outwards beyond a rearward projection of the corresponding front face which, in a cross section, tapers symmetrically inwards, preferably by 7°, on each side relative to the axis of rotation.

Abstract: An ion implanter for implanting ions into a substrate comprises an ion beam generator for generating a beam of ions, support means for carrying a substrate to be implanted with beam ions, scanning means for scanning at least one of the substrate and the ion beam relative to the other so that the beam traverses the substrate along a predetermined path. Monitoring means are arranged to monitor changes in the proportion of the ion beam cross-sectional area incident on the substrate as the ion beam traverses the edge of the substrate from a position in the scan at which the proportion is finite to a position at which the proportion is zero.