Investigation on Oxygen Diffusion in a High-k Metal-Gate Stack for Advanced CMOS Technology by XPS

Abstract

X-ray photoelectron spectroscopy was used to study the post deposition annealing of a TiN/HfO2/SiO2/Si stack with a HfO2 thickness varying between 2nm and 5nm. Although the annealing was performed in 5N nitrogen, the growth of a SiO2 layer was observed but only in the presence of the TiN layer. It seems that the TiN layer catalyzes the oxidation with oxide anions crossing the HfO2 layer which acts as a solid electrolyte. Oxidation occurs in two steps, a fast one associated to oxide ions coming from the native TiO2 layer on top of the TiN gate-electrode and a slower one associated to the reduction of residual oxygen in the chamber. The system acts similarly to a solid oxide fuel cell between gaseous oxygen and silicon separated by HfO2. The annealing step leaves residual dipoles at the TiN/HfO2 and SiO2/Si interfaces which are the origins of energy shifts between the XPS peaks.