Si (001) surface defects after extended high temperature annealing

Deep diffusion of dopants in Si 001 requires temperature annealing as high as 1250°C and more for several days, in a quasi-neutral atmosphere. We have shown that such thermal treatment strongly affects the surface morphology, creating several-micrometers long, 10–50 nm deep square-shaped pits faceted along the 110 directions, with a density of ∼10 defects/cm2 for a 50-h anneal. These kind of defects, although reported in the literature for different experimental conditions, have not been studied in details. These defects grow as the annealing time increases. A second annealing in similar conditions shows a striking difference of behavior between CZ- and FZ-grown wafers, the latter having a defect density rising up to 104/cm2. It is believed that the metallic contaminants accumulated during the first annealing precipitate during cooling and eventually dissolve during further thermal treatment, leaving small marks on the surface which grow to micron-sized squares via a surface reconstruction. Comparisons have been made between the evolution of these defects and patterns with various depths followed by a high-temperature annealing.