Electrical characterization of low-dose ion-implanted silicon annealed with microsecond laser pulses

Abstract

Silicon wafers implanted with low doses (∠1012 cm−2) of either boron or phosphorous ions were annealed with a dye laser and the results compared to those obtained from identically implanted, thermally annealed wafers. The laser was tuned to a wavelength of 600 nm and had a pulse duration of 1 μs with a roughly triangular waveform peaked at approximately 230 ns. Energy densities of the laser pulse were varied from 0.7 J/cm2 to 2.8 J/cm2 in five stages. The thermal anneals were performed at 700 °C or 800 °C for 30 min in dry nitrogen. Examination of electrical activation of the annealed areas using capacitance‐voltage (C‐V) profiling on Schottky barrier structures indicated significant boron activation for the laser‐annealed areas, with peak dopant concentrations 25 to 100% of those obtained from thermal annealing alone. Subjecting thermally annealed, boron‐implanted wafers to laser pulses resulted in profiles identical to those the boron‐implanted wafers that had been only laser annealed. In the phosphorous‐implanted wafers, areas in which laser irradiation was sufficient to cause electrical activation exhibited dopant segregation at the surface similar to zone refining. At the highest power densities examined, surface morphology characteristic of laser melting and regrowth could be detected by Nomarski interference microscopy. Examination of the phosphorous‐implanted wafers using the scanning electron microscope indicated the presence of electrically active defect structures in both the unannealed and thermally annealed areas, but no defect structures could be detected in the areas that had clearly melted. These results imply that temperatures near or above the melting point of silicon must be obtained during laser annealing to activate boron or phosphorous implanted into silicon at low doses and suggest that the melting during laser annealing appears to have removed distinct electrically active defect structures from the implanted regions.