Advanced vacuum technology is essential for ion implantation, since ions cannot be transported through gaseous media. The quality and reproducibility of the vacuum in the beamline thus becomes an important process parameter. Charge-exchange phenomena are the biggest reasons why vacuum control is important. When an ion collides with a residual gas molecule in the beamline it can either lose or gain an electron from the molecule.
Neutralization occurs when a singly charged ion gains an electron.
Stripping occurs when an ion loses an electron, thereby increasing its charge state. Consequently, ion-molecule collisions do not appreciably change ion trajectories. However, any change in the ion's charge state will greatly affect its response to electric and magnetic fields in the implanter. The dose introduced into a semiconductor is carefully controlled by integrating the current generated when the ion beam strikes a neutral target, assuming a fixed charge on each ion. A net neutralization or stripping of ions in the beamline will cause the actual implanted dose to differ from the calculated dose.