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.

L. Filipovic: Topography Simulation of Novel Processing Techniques