Ion density variation effects in plasma source ion implantation

Plasma-source ion implantation (PSII) is a well-developed technique for introducing ions into a material specimen for enhancement of the surface properties. The technique is moving from the research to the industrial level, where specimens of large surface area need to be treated simultaneously. However, such large areas demand an enormous amount of ion current from the plasma during application of the negative bias. As in most cases the rate and mechanism of plasma production is independent of the bias conditions of the specimen, there can be a drop in the plasma density during the application of the bias for large areas of the specimen. This paper analytically investigates the consequences of this drop in density on sheath expansion and ion flux using quasi-static expansion of Childʹs law. The results indicate a drastic drop in ion flux to the specimen and the production of large ion sheaths if the plasma production mechanism is not strong enough to cope with the ion loss.