Analysis of interfacial silicates and silicides formed by annealing ultrathin Hf on SiO2:: Effect of Hf/SiO2 thickness ratio

The annealing of two different ultrathin Hf/SiO2 stacks, i.e., Hf rich (1.7 ML/0.3 nm) and SiO2 rich (1 ML/1 nm) is investigated in situ in an ultrahigh vacuum (UHV) by using scanning tunneling microscopy and x-ray photoelectron spectroscopy. To describe the interface structure formed in practical high-k processes, this approach conjectures the effects of underlying SiO2 on the stability of metal-silicon and metal-oxygen bondings, which would subsequently determine the interfacial phases. The annealing of these film stacks causes silicate formation, but the relative thickness ratio between Hf and SiO2 is found to greatly affect a phase stability of interfacial silicates in a high-temperature (greater than or equal to780 degreesC) regime. As the underlying SiO2 thickens, the Hf-Si bondings are expected to be replaced with Hf-O-Si (silicate) bonding units, even at room temperature deposition in an UHV. In the Hf-rich stack (Hf-Si bonding dominant), phase separation into silicides was observed at a relatively low temperature (similar to780 degreesC) compared with a SiO2-rich one (Hf-O-Si bonding dominant) where silicidation occurs above 900 degreesC. Irrespective of the thickness ratio between Hf/SiO2, above 900 degreesC, two types of crystalline silicides, i.e., facetbars (metallic) and platelets (semiconducting), were observed and these silicides were also identified to be oxygen-free phases by analyzing their atomic structures. (C) 2002 American Institute of Physics.