Atomic transport properties and electrical activation of ultra-low energy implanted boron in crystalline silicon

The growing importance of ultra-low energy implantation in Si processing im poses extensive characterization and understanding of such a novel energy r egime, In this paper we investigate the evolution of ultra-low energy B imp lants (0.25-1 keV) after post-implantation annealing, both in terms of atom ic diffusion and electrical activation of the doping atoms. Transient enhan ced diffusion (TED) of boron after annealing at 900 degrees C is observed e ven for boron implanted at 250 eV, and also when the implant dose is below the amorphization threshold. At higher temperatures for long anneal times, the TED is overwhelmed by the equilibrium diffusion and it is not visible. However, provided the correct combination of temperatures and times is chos en, the TED can always be observed in samples implanted with a dose at leas t of 1 x 10(14) cm(2). We suggest a possible microscopic mechanism to justify the dependence of th e enhanced diffusion of ultra-low energy implanted boron on the implant dos e, energy and annealing temperature. An excess of interstitials occurs givi ng rise to the formation of interstitials-like complexes containing B, prob ably due to enhanced annihilation of vacancies at the surface. Such an exce ss of interstitials is able to promote enhanced diffusion of implanted boro n, provided the implant dose is high enough to generate a significant total number of point defects. The electrical activation of the ultra-low energy implanted B is shown to b e strictly connected to its diffusion. With increasing the dose, the motion of B is supported by the increased amount of ion beam generated interstiti als. We have also observed, at the same time, that the electrical activatio n is favored. The electrical activation, which can be achieved by the ultra -low energy implants we have investigated, is between 10 and 40% after anne aling at 1100 degrees C, depending, on the implanted dose. For the highest dose we have studied, i.e. 1x10(15) cm(2), the sheet resistance measured af ter annealing at 1100 degrees C in a range between 250 eV and 1 keV, is bel ow 1000 Omega. Ultra-low energy implantation is therefore extremely appeali ng for the future generations of semiconductor devices. (C) 1999 Elsevier S cience Ltd. All rights reserved.