Method of mixing upstream and downstream current measurements for inference of the beam current at the bend of an optical element for realtime dose control

An ion implantation has an ion source and a mass analyzer configured to form and mass analyze an ion beam. A bending element is positioned downstream of the mass analyzer, and respective first and second measurement apparatuses are positioned downstream and upstream of the bending element and configured to determine a respective first and second ion beam current of the ion beam. A workpiece scanning apparatus scans the workpiece through the ion beam. A controller is configured to determine an implant current of the ion beam at the workpiece and to control the workpiece scanning apparatus to control a scan velocity of the workpiece based on the implant current. The determination of the implant current of the ion beam is based, at least in part, on the first ion beam current and second ion beam current.