Effects of the phosphorus doping level and excess silicon on the oxidation of tungsten polycide

A tungsten silicide film was deposited by low pressure chemical-vapor deposition (LPCVD) onto phosphorus (P)-doped polysilicon/SiO₂/Si substrates. The tungsten polycide films were annealed in a nitrogen ambient at 850°C for 20 min prior to oxidation. The kinetics of the thermal oxide growth of the annealed tungsten polycide (WSi_2.5/P-doped polysilicon) films were studied as a function of the phosphorus doping level in the polysilicon and as a function of the excess silicon in the tungsten silicide. The activation energy for a linear rate constant (B/A) for tungsten polycide oxidation increased with increasing phosphorus doping in the polysilicon layer, while it was almost independent of the doping level for a parabolic rate constant (B). The excess Si from within the tungsten silicide (WSi_2.5) film was consumed during the oxidation at first; then, the underlying polysilicon provided the necessary Si for further oxidation. The excess Si in the tungsten silicide enhanced the oxidation.