Thermal stability of the HfO2∕SiO2 interface for sub-0.1μm complementary metal-oxide-semiconductor gate oxide stacks: A valence band and quantitative core-level study by soft x-ray photoelectron spectroscopy

Synchrotron-radiation photoelectron spectroscopy is used to study the valence-band structure and the core-level photoemission spectra of HfO2 ultrathin films grown onto SiO2∕Si substrates by atomic layer deposition (ALD). We determine the band offsets (valence and conduction) of HfO2 to Si as a function of postdeposition annealing treatments (under an inert N2 atmosphere or in situ in ultrahigh vacuum) and find a significant evolution, the conduction-band offset remaining larger than 1.5eV. The Si2p and the Hf4f core-level spectra give detailed information on the composition and the spatial extent of the interfacial Hf silicate layer formed between the SiO2 bottom oxide and the HfO2 ALD thin film. By a quantitative treatment of the Si2p core-level intensities, we examine the thermal stability of the interface silicate after postdeposition annealing under N2 and in situ annealing in ultrahigh vacuum (UHV), both at 800°C. The as-deposited layer gives rise to a HfO2∕Hf0.35Si0.65O2∕SiO2 stack with corresponding thicknesses of 0.74∕0.51∕0.73nm. After postdeposition annealing at 800°C in a N2 atmosphere, this becomes a HfO2∕Hf0.31Si0.69O2∕SiO2 stack with corresponding thicknesses of 0.71:0.58:0.91nm. In situ annealing in UHV, on the other hand, gives a HfO2∕Hf0.35Si0.65O2∕SiO2 stack with corresponding thicknesses of 0.65:0.70:0.76nm. The former favors an extension of both the silicate and the SiO2 interface layers, whereas the latter develops only the silicate layer.