Effect of additive gases on the selective etching of tungsten films using inductively coupled halogen-based plasmas

In this study, the effect of additive gases such as N₂, Ar and O₂ on the selective etching of tungsten (W) films relative to that of poly-Si films was investigated using inductively coupled CF ₄/Cl₂-based plasmas. When CF₄/Cl₂ gas mixtures were used to etch W films and poly-Si, due to the formation of volatile etch products, the etch rates of W and poly-Si were very high. However, because the poly-Si etch product is more volatile than the W etch product, the etch selectivity of W over poly-Si was lower than 0.3. When Ar or N₂ was added to a CF₄/Cl₂ gas mixture, the etch rates of both W and poly-Si were increased, however, the etch selectivity of W over poly-Si remained similar. When O₂ was added to a CF ₄/Cl₂ gas mixture, not only higher W etch rates (approx. 350 nm/min) but also higher etch selectivity of W over poly-Si (approx. 2.4) could be obtained by suppressing the poly-Si etch rate. The increase of W etch rates and etch selectivity by the oxygen addition to the CF₄/Cl ₂ appears from the formation of volatile tungsten halogen oxide on the W surface and involatile silicon oxide on the poly-Si surface.