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. In this article, we used MOS capacitors with a gate oxide thickness of 4nm and ii 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 plasma food gun (PFG) neutralization system reduces drastically the degradation of the gate oxide. However, for BF2 implants, although the use of PFG, we observed a strong degradation of the gate oxide. 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 (2x10(12) 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.