Novel rare-earth dielectric interlayers for wide NMOS work-function tunability in Ni-FUSI gates

Abstract

A novel method of forming rare-Earth (RE)-based interlayers to engineer the work function (φm) 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 φm. Band edge φm was retained even after a high-temperature annealing was conducted before FUSI. Ni-FUSI gate φm was tunable to ∼4.11-4.39 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 φm 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 φm could be engineered by intentionally inserting the LaIL at the HfO2/SiO2 interface. © 2008 IEEE.