Kinetic study of chemical vapor deposition of WSi x films from WF 6 and SiH 2Cl 2: Determination of molecular size and reactivity of gas species

Abstract

A kinetic study of chemical vapor deposition of tungsten silicide (WSi x ) films from WF 6 and SiH 2Cl 2 was done, focusing on the deposition rate profile, atomic composition ratio (ratio of Si/W), and the reactivity of film precursors. The deposition mode was changed from blanket WSi x deposition to selective W deposition by decreasing the ratio of SiH 2Cl 2 to WF 6 and also by decreasing the substrate temperature, T sub. The rate-determining step for film deposition was determined to be diffusional mass transport of the film-forming species. The molecular diameter of the film precursor was calculated from measured diffusion coefficients to be 6–7 Å, which is close to the diameter of WF 6 molecules. The temperature at which film formation was extinguished, T ex, was measured as a function of the inner diameter of the reactor. T ex was found to depend on the inner diameter of the reactor, which indicates that the first step of the WF 6/SiH 2Cl 2 process is controlled by radical chain reactions in the gas phase to form active intermediate species. The sticking probability of the film precursor was also calculated by measuring the step coverage in micron-sized trenches. For T sub > 600 °C, the sticking probability was about 0.35 with an activation energy of 19 kJ/mol. However, for T sub = 500 °C, the sticking probability decreased to 0.15, which corresponds to an activation energy of 61 kJ/mol.