Antimony implantation is a promising technique for fabricating ultra-shallow n+/p junctions for extensions of sub-100-nm n-MOSFETs. By increasing the Sb+ implantation dose to 6×1014 cm−2, sheet resistance (Rs) of an implanted layer was reduced to 260 /sq. for rapid thermal annealing (RTA) at 800°C. The obtained junction depth of 19 nm is suitable for sub-100-nm MOSFETs. However, the reduction in the sheet resistance showed a tendency to saturate. No pileup at the Si-SiO2 interface, which was the major origin of dopant loss in lower dose cases was, observed in Sb depth profiles in this case. However, in the case of 900°C RTA, Sb depth profiles indicated that Sb was nearly immobile in the region where Sb concentration exceeded 1×1020 cm−3. These results imply that the major limiting factor of Rs reduction under the highdose condition is Sb precipitation, which is different from the lower dose cases.