Boron pile-up phenomena during ultra shallow junction formation

The redistribution during annealing of low-energy B implants in SOI structures and in bulk Si have been investigated by comparing Secondary Ion Mass Spectrometry (SIMS) and simulated profiles. Samples preamorphised with Ge at different implantation energies have been prepared in order to investigate the effects of the damage position on B diffusion. The specimens have been subsequently B implanted at 500 eV with doses 2×10 and 2×10 cm and annealed between 700 and 1100°C. SIMS profiles show a B pile-up in the first few nanometres of the Si matrix on the Si surface. Simulations of diffused profiles indicate that the B redistribution upon annealing can be explained by assuming that the mobility of the dopant which arrives in proximity of the surface is practically annulled. The amount of B trapped at the surface is maximum at the temperatures around 800°C, when more than 80% of the implanted dopant is made immobile and electrically inactive. The trapped B increases with reducing the depth of the amorphous layer and it is higher in the bulk Si than in SOI. By comparing Hall measurements and the amount of B not trapped at the surface, we also estimate the amount of B that aggregates inside the Si lattice in form of clusters (BICs). For the B dose of 2×10 cm, after isochronal annealing of 60 s, the amount of BICs is about 3-4×10 cmat the lowest temperatures and tends to vanish at high temperatures. © 2007 IEEE