Implant energy determination from time-of-flight measurement

High-energy ion implanters using RF acceleration have advantages over electrostatic implementations. However, beam energy from such systems is not simply related to DC voltages as it is in electrostatic systems. Traditionally, energy control in these machines relies on beam collimation from a final energy magnet. Axcelis has developed a novel approach that calculates the energy from the time delay of pulses generated by two spatially separated beam current probes. The compact and cost-effective design is based on a precision, programmable delay line and a unique pulse correlation technique. The prototype hardware has yielded a delay resolution and repeatability of better than &plusmn;100 picoseconds. Absolute energy calibration of the system is established through a nuclear interaction measurement. This patented Time-of-Flight (TOF) methodology has demonstrated the potential for <1.0 % measurement accuracy.