We have studied dry oxidation characteristics of Si-rich WSix thin films prepared by LPCVD directly on SiO2, with x = 2.7 for as-deposited films. It has been reported previously that thin (less than 100 nm) CVD tungsten silicide adheres well to SiO2. Using Auger depth profiling and Rutherford backscattering spectroscopies, we find that silicon in excess of stoichiometric WSi2 diffuses through the silicide toward the surface to form a SiO2 passivating overlayer. The extracted activation energy for this oxidation process is Ea = 1.2 eV, consistent with oxygen diffusion in SiO2. A similar value of Ea is found for WSix deposited on polysilicon. During the anneal, the stoichiometry x of WSix decreases monotonically with the annealing temperature, reaching x = 2 after 30 min at 900°C or 20 min at 950°C. Longer times or higher temperatures result in silicon depletion, with x = 1.7 after 30 min at 1000°C. At the same time, the resistivity of WSix also decreases from ≃90 Ω/□ for 1500 Å as-deposited film to 5 Ω/□ for the films annealed at 1000°C, the value obtained in a standard homogenization anneal. A scanning electron micrograph (SEM) of 0.5-µm fine lines patterned using e-beam lithography reveals that the integrity of fine line structures, their adhesion to SiO2, and their vertical profiles remain unchanged after the oxidation process. We suggest that such Si-rich tungsten silicide can be useful as a gate electrode without the polysilicon underlayer, since no extra passivation is necessary and reoxidation and homogenization steps in the FET processing sequence can be accomplished simultaneously.
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IBM Journal of Research and Development
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06 April 2010
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