Boron diffusion in silicon: the anomalies and control by point defect engineering

The fabrication of ultra large-scale integrated (ULSI) circuits with ever shrinking feature size requires a continued reduction of diffusion lengths of dopants in Si. However, boron implants undergo an “anomalous” diffusion upon annealing, which is detrimental to the ultra-shallow junction formation. Over the last decade, boron transient diffusion has been much better understood. This implantation-induced transient-enhanced diffusion is known to be driven by the large supersaturation of self-interstitial silicon atoms emitted from the extended defects. The form and evolution of implantation-induced defects with respect to implant species, energy, dose, and annealing temperature have been systematically studied. The improved understanding has led to the development of new doping techniques, such as carbon co-implant, multiple step annealing and point defect engineering (PDE) using co-implantation of high-energy ions. In the PDE approach, high-energy ion bombardments inject vacancies near the surface region and create excessive interstitials near the end of projected range deep inside the substrate. Such manipulation of point defects can retard boron diffusion and enhance the activation of boron. The control of boron diffusion could be used to form ultra-shallow junctions for next generation Si device. In this article, we review the history and recent progress in PDE.