Influence of halogen plasma atmosphere on SiO2 etching characteristics

The influence of halogen plasma atmosphere on SiO2 dry etching characteristics has been investigated using various halogen gases (SF6, Cl2, HBr and HI). It was found that in Cl2 and HBr plasma atmospheres, when Si and SiO2 are etched simultaneously, the SiO2 etch rate increases to more than 4 times larger than the SiO2, etch rate obtained when only SiO2 is etched. It was also found that the SiO2 etch rate increases linearly in proportion to the total amount of silicon halide produced by etching Si. Low-order silicon halides such as SiX and SiX2 (X = Cl or Br) connect with oxygen atoms in solid SiO2 by Coulomb force since both silicon halide and SiO2 are electrically polarized. Silicon oxyhalide (e.g., SiOX) as an etching product of SiO2 is produced and desorbed by ion sputtering or thermal evaporation. This is because that the Si-O bond strength in solid SiO2 (108 kcal/mol) is weaker than that in a diatomic molecule (191 kcal/mol) composed of a Si atom from silicon halide and an O atom from SiO2. Consequently, SiO2 etching progresses when silicon chloride or silicon bromide is contained in the plasma, which drastically decreases the etching selectivity of n+ poly-Si to SiO2.