PURPOSE: To restrain an undercut from being formed to enable an anisotropical etching to be carried out without causing the deformation of a silicon pattern in shape by a method wherein fluorine-based surface active agent is added to a wet type anisotropic etching liquid solution which contains caustic alkali.

CONSTITUTION: Fluorine-based surface active agent is added to a wet type anisotropic etching liquid which is used for processing a silicon wafer and composed of water solution which contains caustic alkali and ethylenediamine- pyrocatechol water solution. Furthermore, an addition amount of fluorine-based surface active agent is made to range from 5ppm to 100ppm. By this setup, undercuts are prevented from being generated, a silicon pattern is protected against deformation caused by anisotropic etching, and furthermore an etched surface as smooth as a mirror surface can be obtained.