Abstract: In an ion implantation device having a wafer holder assembly with a plurality of wafer holders each with a retractable wafer clamp and a wafer feeder for loading a wafer onto each wafer holder, there is provided a misalignment sensor which includes a light source for directing a collimated beam of light onto each wafer holder once a wafer has been loaded onto a corresponding wafer holder but before being clamped. A light sensor is coupled to the light souce for detecting light reflected at a predetermined angle from a loaded wafer. A display is provided for indicating misalignment of a wafer in response to the absence of reflected light being detected by the sensor at a predetermined time following loading.