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 imposes extensive characterization and understanding of such a novel energy regime, In this paper we investigate the evolution of ultra-low energy B implants (0.25-1 keV) after post-implantation annealing, both in terms of atomic diffusion and electrical activation of the doping atoms. Transient enhanced diffusion (TED) of boron after annealing at 900 degrees C is observed even 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 chosen, the TED can always be observed in samples implanted with a dose at least of 1 x 10(14) cm(2). We suggest a possible microscopic mechanism to justify the dependence of the enhanced diffusion of ultra-low energy implanted boron on the implant dose, energy and annealing temperature. An excess of interstitials occurs giving rise to the formation of interstitials-like complexes containing B, probably due to enhanced annihilation of vacancies at the surface. Such an excess of interstitials is able to promote enhanced diffusion of implanted boron, 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 be strictly connected to its diffusion. With increasing the dose, the motion of B is supported by the increased amount of ion beam generated interstitials. We have also observed, at the same time, that the electrical activation is favored. The electrical activation, which can be achieved by the ultra-low energy implants we have investigated, is between 10 and 40% after annealing 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 after annealing at 1100 degrees C in a range between 250 eV and 1 keV, is below 1000 Omega. Ultra-low energy implantation is therefore extremely appealing for the future generations of semiconductor devices.