It was pointed out that appreciable channelling during B+/BF2+ implantation, and enhanced diffusion during subsequent annealing, limited the formation of ultra-shallow p-type junctions. An approach was demonstrated which used sub-amorphous threshold-dose optimum-energy Si+ pre-implantation. This reduced channelling during subsequent dopant implantation, or enhanced B diffusion during annealing. The energy of the pre-implantation ions governed the spatial distribution of the vacancies and interstitials which were formed. The vacancy excess that was created by the pre-implantation produced strains in the lattice by disrupting the periodic potential of the lattice. This then helped to reduce channelling during subsequent dopant implantation. The vacancy distribution which was created by pre-implantation also recombined with the interstitials that were produced by the implanted dopant; thus resulting in fewer interstitials that could contribute to an enhanced B diffusion. This reduction in enhanced B diffusion then resulted in shallower junctions. The energy level of the pre-implantation was chosen so that the excess interstitials which were produced by it lay beyond the depth to which the dopants would diffuse in the absence of pre-implantation. The interstitials which were produced by pre-implantation therefore did not contribute to enhanced B diffusion.

A.Sultan, S.Banerjee, S.List, V.McNeil: Journal of Applied Physics, 1998, 83[12], 8046-50