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 Al
₂
O
₃
layer at the Ni-FUSI/SiO
₂
interface. Although the Al
₂
O
₃
state also formed, it has little influence on the Phi
_eff
value at the Ni-FUSI/HfSiON interface.