Implantation of nitrogen into polysilicon to suppress boron penetration through the gate oxide

Nitrogen implantation into polysilicon gate is a new alternative to retard boron penetration. The effect of nitrogen implantation into polysilicon gates on 4 to 4.5 nm gate oxides was investigated. The implant energies were 60 keV, 120 keV, and 200 keV with doses of 1×10¹³ cm^-2, 1×10¹⁴ cm^-2, 1×10¹⁵ cm^-2, and 5×10 ¹⁵ cm^-2, respectively. The suppression of boron penetration confirmed by SIMS analysis has been attributed to the retardation of diffusion in bulk polysilicon with the presence of nitrogen. No detrimental influence on gate depletion, poly doping, or poly resistivity after nitrogen implantation was seen. Nitrogen implantations with energies of 60 keV and 120 keV showed better barrier properties than implantations at 200 keV. An anneal step after the nitrogen and before the BF₂-implantation of the gate improved barrier properties. An implant dose of 1×10¹⁵ cm^-2 showed the best barrier properties, but the leakage current of the current voltage characteristics was slightly increased. For electrical reliability measurements, i.e. Q_BD-measurements, was found no detrimental effect of nitrogen implantations on electrical characteristics, but the thermal budget of the BF₂-implant anneal step influenced the Q_BD(63%)-value significantly towards better electrical characteristics for lower thermal budgets, especially for negative gate polarity. Under optimum experimental conditions, nitrogen implantation into polysilicon gate is effective in suppressing boron penetration without degrading performance