Mechanism of suppressed change in effective work functions for impurity-doped fully silicided NiSi electrodes on Hf-based gate dielectrics

We investigated the dependence of the effective work function (Phi(eff)) on interfacial Hf density ([Hf]) for fully silicided (FUSI) NiSi/Hf-capped SiON and FUSI-NiSi/Si-capped HfSiON stacks to study the mechanism that suppresses the threshold voltage (V-th) change induced by impurity segregation on Hf-based gate dielectrics. We discuss possible mechanisms for the suppressed V-th change: i) Fermi-level pinning, ii) increased dielectric constant (epsilon(inter)) at the electrode/dielectric interface, and iii) suppressed formation of the interfacial dipole causing the Vth change. We found that the Vth change was suppressed with increasing [Hf] in the low-[Hf] region (0.1-0.5 monolayers), where epsilon(inter) is similar to that of SiON. We also found that an increase in equivalent oxide thickness (EOT) of about 0.1 nut was induced by impurity segregation. We attribute this EOT increase to substitution of the segregated impurity atoms for electrode atoms bonded to the dielectric, which causes the formation of an interfacial dipole. Our results indicate that Fermi-level pinning is a predominant factor in suppressing the V-th change in the low-[Hf] region. In the high-[Hf] region, the EOT of an impurity-doped NiSi electrode was almost the same as that of an undoped NiSi electrode. We consider that the suppressed V-th change in the high-[Hf] region is due to the diffusion of segregated impurity atoms into the HfSiON dielectric.