Apparatus and method for neutralizing the beam in an ion implanter

In a particle implantation apparatus of the type for providing a beam of particles for impingement upon a target comprising material of conductive or semiconductive properties on an insulating substrate, the apparatus having a source of ionic particles for providing a beam of ionic particles to the target whereby the target substrate accumulates a charge potential from ions impinging on the material, and means for providing relative movement between the target and the beam at a predetermined rate, wherein the improvement comprises:

• a closed loop feedback circuit including controllable means positioned adjacent the beam and being responsive to a first signal for supplying electrons to the beam of ions to neutralize the net charge of the beam;
• and further including means responsive to the charge potential on the substrate to develop said first signal to control the flow of electrons from the electron supply means to the beam to neutralize the net charge of the beam and thereby reduce the charge potential on the substrate towards zero;
• wherein said electron supply means further includes means to supply electrons to a plate disposed radially outwardly of the supply means, and means responsive to the flow of electrons to said plate for generating a second signal proportional to the ion beam current.