Thermal activation of shallow boron-ion implants

Boron implanted into n-type Si at 10¹⁵ cm⁻² dose and energies from 500 eV to 1 keV was activated by annealing in nominally pure N₂ and in N₂ with small admixtures of O₂. Effective process times and temperatures were derived by thermal activation analysis of various heating cycles. The lowest thermal budgets used “spike anneals” with heating rates up to 150°C/sec, cooling rates up to 80°C/sec, and minimal dwell time at the maximum temperature. Dopant activation was determined by sheet electrical transport measurements. Surface oxidation was characterized by film thickness ellipsometry. P-n junction depths were inferred from analysis of sheet electrical transport measurements and secondary ion mass spectroscopy profiles. Boron activation increases with boron diffusion from the implanted region. Electrical activation has a thermal activation energy near 5 eV, while boron diffusion has an activation energy near 4 eV. Surface oxide can retard boron diffusion into the ambient for high-temperature anneals.