The recently observed phenomenon of boron uphill diffusion during low-temperature annealing of ultrashallow ion-implanted junctions in silicon has been investigated. It is shown that the effect is enhanced by preamorphization, and that an increase in the depth of the preamorphized layer reduces uphill diffusion in the high-concentration portion of boron profile, while increasing transient enhanced diffusion in the tail. The data demonstrate that the magnitude of the uphill diffusion effect is determined by the proximity of boron and implant damage to the silicon surface.

We demonstrate that boron diffuses at high concentrations during low-temperature thermal annealing in amorphous silicon pre-amorphized by germanium ion implantation. For a typical boron ultrashallow junction doping profile, concentrations as high as 2ϫ10 20 cm Ϫ3 appear to be highly mobile at 500 and 600°C in the amorphous silicon region before recrystallization. In crystalline silicon at the same temperatures the mobile boron concentration is at least two orders of magnitude lower. We also show that boron diffusivity in the amorphous region is similar with and without fluorine. The role of fluorine is not to enhance boron diffusivity, but to dramatically slow down the recrystallization rate, allowing the boron profile to be mobile up to the concentration of 2 ϫ10 20 cm Ϫ3 for a longer time.

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