In this work, we studied the influence of combined processing of SiO2/Si(001) substrates using focused ion beams (FIB) and wet chemical etching on the possibility of forming pyramidal cavities and their geometric parameters. It has  been shown that etching FIB-modified samples only in KOH leads to the formation of pyramidal cavities covered with a shell, possibly made of porous silicon layer. We have shown that the use of an isotropic etchant before  anisotropic etching allows to remove a porous silicon layer. An increase in the implantation dose led not only to an increase in the depth of the cavities, but also to an increase in the undercut of the oxide layer.