Mechanism of Hf-silicide formation at interface between poly-Si electrode and HfO₂/Si gate stacks studied by photoemission and x-ray absorption spectroscopy

We have investigated the mechanism for silicidation by chemical reactions at polycrystalline-Si (poly-Si) Hf O2 Si gate stacks by annealing in ultrahigh vacuum using photoemission spectroscopy and x-ray absorption spectroscopy. Si 2p, Hf 4f, and O 1s high-resolution photoemission spectra have revealed that a Hf-silicide formation starts at as low temperature as 700 °C and that a Hf silicate is also formed at the interface between poly-Si electrodes and Hf O2. The metallic Hf silicide is formed at the interface between Hf O2 and Si substrates, which changes the band offsets on Si substrates. We have found that poly-Si electrodes promote the interfacial reaction between Hf O2 and Si substrates, while the crystallization in a Hf O2 layer is independent of the silicide formation. The silicidation mechanism based on photoemission spectra is also confirmed from the thermodynamical analysis considering the Gibbs' free energy.