Phosphorus Diffusion in Silicon from Enclosed Solid Planar Sources

Phosphorus doping of silicon using solid planar sources has been observed to result in anomalously higher values of sheet resistance when diffusion occurs in ambients that are stagnant or partially contained. Partial enclosure of the diffusion boat results in an increase in sheet resistance of approximately 30% for phosphorus diffusion at 1000°C and 1 atmosphere total pressure; however, at a reduced pressure of 5 torr, this enclosure results in a 30% decrease in sheet resistance. SIMS analysis of the phosphosilicate glass layers formed on silicon during diffusion at 1 atmosphere reveal that the level of hydrogen is approximately doubled if the boat is enclosed. We conclude that moisture, which is adsorbed on the source during wafer loading operations, is desorbed during diffusion. If contained, the desorbed moisture competes with phosphorus oxide in the oxidation of silicon, resulting in a degradation in doping performance.