Loss of selectivity during W chemical vapor deposition on Si using the WF₆/SiH₄ process

The undesired deposition of W on SiO₂ during W chemical vapor deposition on Si with the WF₆/SiH₄ process has been studied by in situ time resolved mass spectrometry. This study shows that for [SiH₄]/[WF₆]<0.3 loss of selectivity occurs due to an autocatalytic reaction on the SiO₂ surface. Based on the observations, a model for the process has been proposed. In this model, the rate limiting step is the formation of Si₂H_2n from two SiH₄ molecules. This reaction requires a nucleation site, probably an impurity in the SiO₂ or adjacent metallic areas, and is retarded by WF₆. Subsequently, the Si₂H_2n reacts with WF₆ to a W‐containing species that will then also act as a nucleation site. When the density of W‐containing species approaches monolayer coverage, these species react together with Si₂H_2n, thus forming areas of metallic W.