Degradation pattern of thin HfO2 films on Si(100) under ultrahigh-vacuum annealing: An investigation by x-ray photoelectron spectroscopy and low-energy ion scattering

The evolution of microstructure and phase structure of ultrathin HfO2 films on Si(100) under ultrahigh-vacuum annealing is investigated in situ by x-ray photoelectron spectroscopy (XPS) and low-energy ion scattering (LEIS). The onset temperature of degradation is found to depend on film thickness. It is established that, for HfO2 (4 nm)/SiO2 (1 nm)/Si(100) specimens, 5-min annealing at about 900°C causes silicon (LEIS evidence) to appear on the surface, the silicon being uncombined with oxygen or the metal (XPS evidence). A longer annealing at the same temperature produces HfSix; annealing at 950°C converts the entire HfO2 film into polycrystalline silicide whose grains are partly oriented as the Si substrate. With respect to annealing in a low-oxygen environment, the experimental results support a model whereby the degradation of an ultrathin HfO2 film starts with the formation of nanopores by clustering of oxygen vacancies, whose density increases sharply due to partial desorption of oxygen; HfOx with x < 2 then forms in the vicinity of vacancy clusters. It is concluded that the formation of hafnium silicide, the end product of HfO2 degradation, starts in Si surface areas at the bottom of nanopores.