Doping of the poly-gate by beamline implantation and plasma doping and its influence on the gate oxide reliability

During the ion implantation process, it is well known that the ion beam induces at the wafer surface a potential, which is at the origin of charging effects. We used MOS capacitors with a gate oxide thickness of 4 nm and a polysilicon gate doped by implantation of P for the n+ gate and by B or BF₂ for the p+ gate. The MOS structures were electrically tested by means of charge-to-breakdown (Qbd), capacitance-voltage (C-V) and current voltage (I-V) characteristics. It is shown that the Qbd is significantly sensitive to the charging effects and thus can be used to investigate the oxide degradation during the ion implantation. For the P and B implants, the use of a plasma food gun (PFG) neutralization system reduces drastically the degradation of the gate oxide. However, for BF₂ implants we observed a strong degradation of the gate oxide when using the PFG. We demonstrated, for the first time, that this degradation is mainly due to the charging rather than the fluorine incorporation in the gate oxide, as often suggested in the open literature. In the BF₂ case, we also evidenced a fluorine-induced shift in the C-V plots. This shift corresponds to a high (2×10¹² cm^-2) negative charge density in the gate oxide. The charging effects and thus the oxide degradation are found to depend on the beam current and energy. We suggest a qualitative model, based on the yield of secondary electron emission, to interpret the observed trends