Dose uniformity of sub-keV implants

As device dimensions are reduced below 0.18 μm, low energy ion implantation will be increasingly used for the formation of ultra shallow junctions (<40 nm) required for source/drain extensions. As the ion energy is reduced the projected range decreases and becomes comparable to the native oxide thickness. Due to the proximity of ions to the surface, the sheet resistance uniformity becomes sensitive to the native oxide thickness variation and anneal conditions. In this paper we investigate the dependence of sheet resistance uniformity on ion energy, oxide thickness variation and anneal ambient conditions. Although the sheet resistance nonuniformity increases with decreasing energy we show that highly uniform R₁ maps (<1.5% one sigma) can be obtained with ion energy down to 200 eV by the proper optimization of implant and anneal conditions and wafer surface preparation