Process that follows immediately after photolithography step is the removal of material from areas unprotected by photoresist. This process must be selective; that is SiO2 is removed while leaving photoresist and silicon intact. It must also be anisotropic; that is etching should be in one direction only.
Two types of etching processes are used in practice; namely, chemical and physical etching. In purely chemical etching material is removed by dissolution which is highly selective but not anisotropic. In purely physical method material is removed by bombardment of high energy ions which is inherently anisotropic but unselective.
As an example, SiO2 which is used as a mask for drive in diffusion is removed by exposure to hydrogen fluoride. Hydrogen fluoride reacts with SiO2 to form volatile SiF4 which is swept away by inert argon gas.