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 suitab le for use as an intermetal film to improve the performance of ultra-large scale integrated circuits (ULSIs). One of the properties required by an int ermetal film is good gap filling. It is known that fluorine addition to sil icon oxide decreases the film deposition rate and improves the step coverag e. In order to investigate these phenomena, we study the reaction mechanism of plasma-enhanced chemical vapor deposition (PECVD) silicon oxide film an d 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 t he residence time for a SiH4/N2O-based PECVD system. The precursor produced by the dissociation of SiH4 has a relatively high sticking probability (ap proximate to 0.5), while an intermediate species has a low (<10(-4)) sticki ng probability. The concentration of each species for deposition is changed by the residence time of the gas, thus the deposition rate and the step co verage show dependence on the residence time. The deposition rate of silico n oxide films is decreased and the step coverage is improved by CF4 additio n during SiH4/N2O-based PECVD. From the estimation of the sticking probabil ity, 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 decre ase in sticking probability of the precursor produced by the dissociation o f SII-IJ.