Decrease in deposition rate and improvement of step coverage by CF₄ addition to plasma-enhanced chemical vapor deposition of silicon oxide films

The low dielectric constant of F-doped silicon dioxide film makes it suitable for use as an intermetal film to improve the performance of ultra-large scale integrated circuits (ULSIs). One of the properties required by an intermetal film is good gap filling. It is known that fluorine addition to silicon oxide decreases the film deposition rate and improves the step coverage. In order to investigate these phenomena, we study the reaction mechanism of plasma-enhanced chemical vapor deposition (PECVD) silicon oxide film and its change by fluorine addition. Using a two film-forming species model, we explain the dependence of the deposition rate and the step coverage on the residence time for a SiH₄/N₂O-based PECVD system. The precursor produced by the dissociation of SiH₄ has a relatively high sticking probability (≈0.5), while an intermediate species has a low (< 10^-4) sticking probability. The concentration of each species for deposition is changed by the residence time of the gas, thus the deposition rate and the step coverage show dependence on the residence time. The deposition rate of silicon oxide films is decreased and the step coverage is improved by CF₄ addition during SiH₄/N₂O-based PECVD. From the estimation of the sticking probability, we suggest that the reason for the improvement of the step coverage by fluorine addition is not the etching effect by CF₄ addition, but the decrease in sticking probability of the precursor produced by the dissociation of SiH₄.