Diffusion of ion-implanted boron impurities into pre-amorphized silicon

Boron ion implantation into pre-amorphized silicon is studied. Pre-amorphization is performed either by F⁺ or Si⁺ implantation prior to B⁺ implantation at 10 keV with 3×10¹⁵ ions/cm². Broadening of the boron profile can be suppressed markedly in the pre-amorphized layers. For instance, the as-implanted depth at a B concentration of 1×10¹⁸ atoms/cm³ decreases from 0.19 to 0.1 μm for implantation into a pre-amorphized layer compared to B implantation into crystalline silicon. After annealing at 950°C, B atoms diffuse much more rapidly in the pre-amorphized layers than in the crystalline silicon case. Nevertheless, shallower junctions are obtained with the use of pre-amorphization. For dual F⁺ and B⁺ implantation at F⁺ doses above 1×10¹⁵ F⁺/cm², fluorine is found to segregate to the peak of the boron profile during annealing. Fluorine is also trapped at the peak of the as-implanted fluorine profile peak and near the amorphous–crystalline interface. The effects of fluorine dose and anneal temperature on the F precipitation are described and compared to results for BF⁺₂ implants.