Phosphorus Doping of Silicon Using a Solid Planar Diffusion Source at Reduced Pressures

A silicon pyrophosphate-containing (SiP₂O₇) solid planar diffusion source has been evaluated for phosphorus doping of silicon at reduced pressures. A reduction in pressure generally results in improvements in the uniformity of doping. At the edges of 3 in. silicon wafers diffused at 1000°C, maximum deviations in sheet resistance are reduced from approximately 15 to approximately 5%, if total pressure is reduced from 1 atm to 5 torr. This improvement is even more significant for diffusion at 900°C, where the maximum deviation is reduced from approximately 100% to approximately 10% by the same reduction in total pressure. The use of a “closed” wafer boat configuration combined with reduced pressure results in further improvement in doping uniformity, as evidenced by a deviation in sheet resistance of approximately 1% for diffusion at 900°C. The use of low pressure leads to decreased doping performance of the source as reflected in thinner doped oxide formation and higher sheet resistance values for diffused silicon wafers. The rate of evaporation of phosphorus oxide from the source is increased nearly one order of magnitude as pressure is reduced from 1 atm to 0.5 torr, leading to reduced lifetime of the source.