Charge control in a ribbon beam high current ion implanter

Wafer charge control has evolved from systems that add gas to the beam or extract electrons from an external source to systems that respond directly to beam potential and current density via an externally coupled plasma. The capability of a plasma to add low energy electrons to an ion beam greatly exceeds the ability of any extracted electron system due to space charge effects. However, even plasma coupled charge neutralization systems still show that effective charge control is more difficult as the beam current density increases. Further improvements have often been obtained by controlling the beam current density by deliberately increasing beam size. This can result in a loss of throughput and can result in possible contamination problems. An alternative approach to further improve charge control is to use a large area ribbon beam, which offers a very low beam current density during high current implantation. This paper presents results for wafer charging on a ribbon beam implanter, the Varian VIISta 80, incorporating a Plasma Flood Gun for charge control. These results are based on sensitive antenna structures using thin oxides with antenna ratios up to 100,000:1 and EEPROM devices