Critical issues in ion implantation of silicon below 5 keV: Defects and diffusion

Continued use of ion implantation for doping of silicon integrated circuits will soon require implantation energies below 5 keV in order to form electrical junctions <50 nm deep. At such low energies, dopant diffusion and formation of extended defects may be modified by both the proximity of the surface and by the large volume concentrations of point defects and dopant atoms that will arise from reduced range straggling. This brief review summarizes our recent experiments measuring defect formation and evolution, as well as transient enhanced diffusion (TED), in silicon implanted with Si+ and B+ ions <10 nm deep. The results have demonstrated that {311}-type extended defects are generated from Si+ implants even within 3 nm of the surface. However, when these defects eventually dissolve, the surface acts as a perfect sink to efficiently annihilate the released interstitials. As a result, the amount of TED measured at epitaxially-grown B markers decreases approximately linearly with decreasing ion energy (at least, for Si+ implantation). For low energy B+ implants typical doses required for source-drain doping lead to the formation of a fine-grain polycrystalline silicon boride phase during activation annealing.