A solid planar diffusion source which contained pyrophosphate (SiP2O7) was evaluated for P doping at reduced pressures. A reduction in pressure generally resulted in improvements in doping uniformity. At the edges of 3in wafers which were diffused at 1000C, the maximum deviations in sheet resistance were reduced from about 15% to about 5% when the total pressure was reduced from 1atm to 5torr. This improvement was even more significant for diffusion at 900C, where the maximum deviation was reduced from about 100% to about 10% by the same reduction in total pressure. The use of a closed wafer boat configuration, combined with a reduced pressure, resulted in further improvements in doping uniformity; as reflected by a deviation in sheet resistance of about 1% for diffusion at 900C. The use of low pressures led to a decreased doping performance of the source, as reflected by thinner doped oxide formation and higher sheet resistance values for diffused Si wafers. The rate of evaporation of P2O5 from the source increased by almost an order of magnitude when the pressure was reduced from 1atm to 0.5torr, thus leading to a reduced lifetime of the source.

J.R.Flemish, R.E.Tressler, J.Ruzyllo: Journal of the Electrochemical Society, 1991, 138[1], 233-8