Effective Work Function Control With Aluminum Postdoping in the Ni Silicide/HfSiON Systems

A simplified method of effective work function (Phi_eff) control to near the Si conduction band edge (Ec) was demonstrated in the Ni fully silicided (Ni-FUSI) gate/HfSiON system. The Phi_eff of NiSi (4.51 eV) decreased and saturated at 4.27 eV, owing to the use of an Al postdoping process, in which the implantation of Al ions into the upper part of the Ni silicide gate electrodes was followed by low-temperature drive-in annealing ( les 500degC) . There is no degradation of the gate leakage characteristics at the Ni-FUSI/HfSiON interface. The metallic state of piled-up Al just at the Ni-FUSI/HfSiON interface seems to be responsible for the Phi_eff near the vacuum work function of Al. The Al postdoping process simplifies a dual metal gate process, owing to single-step Al implantation for nMOS devices without complicated metal etching process for pMOS region. The physical mechanism of bidirectional Phi_eff modulation of Al pileup was also investigated. It was revealed that the opposite Phi_eff modulation, which is the increase in Phi_eff, occurs, owing to the formation of interfacial Al₂O₃ layer at the Ni-FUSI/SiO₂ interface. Although the Al₂O₃ state also formed, it has little influence on the Phi_eff value at the Ni-FUSI/HfSiON interface.