Oxidation of Si-rich chemical-vapor-deposited films of tungsten silicide

We have studied dry oxidation characteristics of Si-rich WSi
x
thin films prepared by LPCVD directly on SiO
2
, 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 SiO
2
. Using Auger depth profiling and Rutherford backscattering spectroscopies, we find that silicon in excess of stoichiometric WSi
2
diffuses through the silicide toward the surface to form a SiO
2
passivating overlayer. The extracted activation energy for this oxidation process is E
a
= 1.2 eV, consistent with oxygen diffusion in SiO
2
. A similar value of E
a
is found for WSi
x
deposited on polysilicon. During the anneal, the stoichiometry x of WSi
x
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 WSi
x
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 SiO
2
, 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.