4.2.3 Etching of Silicon

In order to etch the silicon layer, an etchant with high anisotropy is needed. Since the etching holes need to be vertical, the hole sidewalls should be as vertically flat as possible. The majority of silicon etching processes rely on a halogen gas such as fluorine or bromine to be involved in the reaction [77]. SF$_6$ and CF$_4$ etchant gases are isotropic or near isotropic, but can be made anisotropic with a selective combination of the amounts of gases used in the etch reaction. A gas with some selectivity over SiO$_2$ would be preferred, but a high selectivity is not essential, as silicon dioxide must also be etched anisotropically. Very anisotropic etchants of silicon which are non-selective over SiO$_2$ are CF$_4$/H$_2$ and CHF$_3$. CF$_4$/O$_2$ is more selective over SiO$_2$ than the previous suggestions, but it is an isotropic process. A process which can be made selective with respect to SiO$_2$ is a combination of chemical etching with a high ion involvement, such as SF$_6$/O$_2$ or SF$_6$/O$_2$/HBr plasmas. Since silicon dioxide serves as the top layer or an effective mask when silicon is etched, the tapered sidewalls created during SiO$_2$ etching must be incorporated into the silicon etching component of the BiCS memory hole etch process.