Novel Rare-Earth Dielectric Interlayers for Wide NMOS Work-Function Tunability in Ni-FUSI Gates

A novel method of forming rare-earth (RE)-based interlayers to engineer the work function (Phi _m) of nickel fully silicided (Ni-FUSI) gates was investigated. An extensive range of RE metals comprising yttrium (Y), erbium (Er), dysprosium (Dy), terbium (Tb), gadolinium (Gd), ytterbium (Yb), or lanthanum (La) were sputtered to form RE-based interlayers (REil's) on SiO₂ dielectric. The interposed REIL enabled Si conduction band-edge (Ec) modulation (~3.8-4.0 eV) of midgap NiSi Phi _m . Band edge Phi_m was retained even after a high-temperature annealing was conducted before FUSI. Ni-FUSI gate Phi_m was tunable to ~4.11-.39 and ~4.25- 4.48 eV by reducing the interlayer thickness and varying the Ni silicide phase, respectively. Improved gate leakage and breakdown voltage were observed for the REiL-incorporated gate stacks. RE-O-Si bonding confirmed that the REIL 's that were formed on SiO₂ were thin RE silicates. The modulation of Ni-FUSI gate Phi_m was attributed to the presence of interfacial RE-oxygen (RE-O) dipoles and correlated well with the calculated RE-O dipole magnitude. The application of La-based interlayer (LaIL) in a HfO₂ dielectric stack was also investigated, and band-edge NiSi Phi_m could be engineered by intentionally inserting the La_IL at the HfO₂/SiO₂ interface.