High concentration phosphorus doping of polycrystalline silicon by low temperature direct vapor phase diffusion of phosphine followed by rapid thermal annealing

Phosphorus doping of polycrystalline silicon by direct diffusion of PH3 at 685 and 785 °C followed by rapid thermal annealing (RTA) has been investigated. Unlike at 785 °C, the dopant profile is not uniform at 685 °C but becomes so after RTA at 1050 °C for 10 s, as revealed by secondary ion mass spectroscopy (SIMS). In the absence of any protection layer, RTA in N2 causes an out-diffusion of ∼50% of P from the sample diffused at 785 °C, although it activates ∼100% of the remaining P atoms as indicated by Hall and SIMS measurements. In contrast, RTA in O2 results in 95% activation of the incorporated P from the same sample, accompanying an oxidation of ∼7 nm polycrystalline Si. As regards the sample diffused at 685 °C, RTA in N2 does not cause any out-diffusion but activates less than 80% of the incorporated P atoms. This difference in behavior is attributed to the tendency to attain equilibrium concentrations of P in the crystallites and at the grain boundaries at the RTA temperature. The highest active P concentration achieved is 2.0×1020 cm−3 at 785 °C followed by RTA in O2 yielding a resistivity as low as 9×10−4 Ω cm. The simplicity of this relatively low temperature process is attractive for a single-wafer multisequence fabrication scheme.