An apparatus and a method of ion implantation using a rotary scan assembly having an axis of rotation and a periphery. A plurality of substrate holders is distributed about the periphery, and the substrate holders are arranged to hold respective planar substrates. Each planar substrate has a respective geometric center on the periphery. A beam line assembly provides a beam of ions for implantation in the planar substrates on the holders. The beam line assembly is arranged to direct said beam along a final beam path.