A study of the role of HBr and oxygen on the etch selectivity and the post-etch profile in a polysilicon/oxide etch using HBr/O₂ based high density plasma for advanced DRAMs

The role of HBr and oxygen on the etch selectivity and the post-etch profile in a polysilicon/oxide etch using HBr/O₂ based high density plasma was studied. HBr/O₂-based polysilicon etch process used in this study seems to be highly selective to the underlying oxide and produce a dielectric fill-friendly post-etch profile depending on the flow rates of HBr and oxygen. When appropriate amounts of HBr and oxygen (∼30 sccm of HBr and ∼3 sccm of oxygen) are present in the etch plasma, brominated silicon oxide seems to be deposited on the original gate oxide and the gate stack sidewall from the reaction of SiBr_x (reaction product during polysilicon etch step) and oxygen during the HBr/O₂-based oxide etch process. The deposited brominated oxide on the thin gate oxide seems to make the HBr/O₂-based plasma etch process extremely selective to the thin gate oxide by protecting the underlying gate oxide. The deposited brominated oxide on the gate stack sidewall seems to prevent the notching by protecting the sidewall during gate stack etching. The etch rate of the brominated oxide seems to be much faster than that of the thermal oxide during the 200:1 diluted HF cleaning. However, the deposited brominated oxide on the thin gate oxide and the gate stack sidewall during the plasma etching survived the following 1 min 200:1 diluted HF cleaning, as was observed in a TEM micrograph (Fig. 2(a)).