Beam stop apparatus for an ion implanter

An ion implanter for implanting ions in a target substrate is arranged to scan the ion beam at the point of extraction of the beam from the ion source. The ion beam extraction assembly includes a tectrode construction in which an extraction electrode adjacent the ion source aperture is split into two halves. A differential voltage is applied across the two halves of the extraction electrode to deflect the ion beam being extracted from the ion source electrostatically. The plane of deflection is arranged to coincide with the plane if dispersion of the ions in a mass analyser magnet downstream of the extraction point and the deflected beam of ions of desired mass/charge ratio is still brought to focus at a common mass selection slit at the exit of the analyser magnet