Oxygen migration in TiO2-based higher-k gate stacks

We report on the stability of high-permittivity (high-k) TiO2 films incorporated in metal-oxide-silicon capacitor structures with a TiN metal gate electrode, focusing on oxygen migration. Titanium oxide films are deposited by either Ti sputtering [physical vapor deposition (PVD)] followed by radical shower oxidation or by plasma-enhanced atomic layer deposition (PEALD) from titanium isopropoxide (Ti{OCH(CH3)(2)}(4)) and O-2 plasma. Both PVD and PEALD films result in near-stoichiometric TiO2 prior to high-temperature annealing. We find that dopant activation anneals of TiO2-containing gate stacks at 1000 degrees C cause 5 angstrom or more of additional SiO2 to be formed at the gate-dielectric/Si-channel interface. Furthermore, we demonstrate for the first time that oxygen released from TiO2 diffuses through the TiN gate electrode and oxidizes the poly-Si contact. The thickness of this upper SiO2 layer continues to increase with increasing TiO2 thickness, while the thickness of the regrown SiO2 at the gate-dielectric/Si interface saturates. The upper SiO2 layer degrades gate stack capacitance, and simultaneously the oxygen-deficient TiOx becomes a poor insulator. In an attempt to mitigate O loss from the TiO2, top and bottom Al2O3 layers are added to the TiO2 gate dielectric as oxygen barriers. However, they are found to be ineffective, due to Al2O3-TiO2 interdiffusion during activation annealing. Bottom HfO2/Si3N4 interlayers are found to serve as more effective oxygen barriers, reducing, though not preventing, oxygen downdiffusion. (C) 2010 American Institute of Physics. [doi:10.1063/1.3298454]