4.2.2 Etching of Silicon Dioxide

Similar to silicon etching, in order to etch Silicon dioxide, an etchant with high anisotropy is required. A very good wet chemical etchant for silicon dioxide with a high selectivity over silicon is HF (49% in water) [175], however, because the etching is due to a wet chemical process, the memory hole sidewalls would be lost due to the etchant penetrating laterally into the SiO$_2$ layer. Some plasma etching processes for SiO$_2$ involve at least one of fluorine or carbon atom. SF$_6$, NF$_3$, CF$_4$/O$_2$, and CF$_4$ are generally isotropic, but can be made more anisotropic with a higher involvement of ion species. Plasma etching of SiO$_2$ is most commonly performed with the injection of a fluorocarbon gas into the processing chamber, where it is ionized and accelerated to the surface for etching [77]. There are many etchants which are selective over Si and are anisotropic, such as CHF$_3$/O$_2$, C$_2$F$_6$, C$_3$F$_8$, and C$_5$F$_8$/CO/O$_2$/Ar. High ratios of fluorine atoms versus carbon atoms are important in achieving a high selectivity of SiO$_2$ over Si. An etchant such as CF$_4$ gas is a viable option, but the main issue regarding SiO$_2$ etching with fluorocarbons is the deposition of precursors during the etching process. With CF$_4$ gas, the main deposition precursor is suggested in [86] to be CF$_2$ gas and its deposition onto the SiO$_2$ surface must be considered when modeling the etching process. This deposition effect affects the sidewall angles of the silicon dioxide.