Diffusion reaction of oxygen in HfO2/SiO2/Si stacks

We study the oxidation mechanism of silicon in the presence of a thin HfO2 layer. We performed a set of annealing in O-18(2) atmosphere on HfO2/SiO2/Si stacks observing the O-18 distribution in the SiO2 layer with time-of-flight secondary ion mass spectrometry (ToF-SIMS). The O-18 distribution in HfO2/SiO2/Si stacks upon O-18(2) annealing suggests that what is responsible for SiO2 growth is the molecular O-2, whereas no contribution is found of the atomic oxygen to the oxidation. By studying the dependence of the oxidation velocity from oxygen partial pressure and annealing temperature, we demonstrate that the rate-determining step of the oxidation is the oxygen exchange at the HfO2/SiO2 interface. When moisture is chemisorbed in HfO2 films, the oxidation of the underlying silicon substrate becomes extremely fast and its kinetics can be described as a wet silicon oxidation process. The silicon oxidation during O-2 annealing of the atomic layer deposited HfO2/Si is fast in its early stage due to chemisorbed moisture and becomes slow after the first 10 s.