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 and CF
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
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
are CF
/H
and CHF
. CF
/O
is more selective over SiO
than the previous suggestions,
but it is an isotropic process. A process which can be made selective with respect to SiO
is a combination of chemical etching with a
high ion involvement, such as SF
/O
or SF
/O
/HBr plasmas. Since silicon dioxide serves as the top layer or an effective mask when
silicon is etched, the tapered sidewalls created during SiO
etching must be incorporated into the silicon etching component of the BiCS memory
hole etch process.