Flatband voltage shift of ruthenium gated stacks and its link with the formation of a thin ruthenium oxide layer at the ruthenium/dielectric interface

Abstract

A systematic study about the flatband voltage (Vfb) shift of Ru gated metal-oxide-semiconductor stacks after thermal treatment in O2 has been performed. The dependence of the Vfb shift on the anneal time and temperature and the thickness of Ru was studied in detail, and a clear link between the Vfb shift and an oxygen diffusion process in Ru was observed. A high temperature thermal treatment of the devices prior to the O2 anneal has no significant impact on the Vfb shift. The Vfb shift is ascribed to the shift of metal gates' work function, and is not intrinsic to HfO2 gated stacks as similar behavior was also observed on SiO2, from the combination of internal photoemission and conventional capacitance-voltage measurement. No similar Vfb shift was observed for TiN gated stacks and the Vfb shift seems to be more related to the properties of gate electrodes other than those of gate dielectrics. After thermal treatment in O182, from time-of-flight secondary ion mass spectrometry measurement, it was found that O18 penetrated through Ru and was incorporated at around the Ru/dielectric interface, which corresponded to the formation of an interfacial RuOx layer. The thin interfacial RuOx layer was found to have very similar properties as the ones of bulk RuO2 and the mechanism of its formation was discussed from thermodynamics and kinetics points of view. We believe that the formation of a thin RuOx layer at around the Ru/dielectric interface in O2 ambient is responsible for the increase of the Vfb for Ru gated stacks.