PURPOSE: To lessen the dust generated by the breakage of an eave and to reduce defective etching caused by the above-mentioned dust by a method wherein etching is conducted using an octagonal etching mask pattern consisting of the side orthogonally intersecting <100>, <110> and <111> directions.

CONSTITUTION: The eave part 3 of a protective film in <100> direction, where an etching side face 5 appears, is almost removed by octagonally forming the mask pattern of anisotropic etching. As a result, the generation of dusts due to the breakage of the eave part can be lessened. Also, the etching bottom face 4 becomes a octagonal shape in the beginning of etching, and the etching operation makes progress. When the etching bottom face becomes a square, the etching is finished. As above-mentioned, by the utilization of configurational change of the <110> etching bottom face 4 in the progress of etching, the finishing point of anisotropic etching can be judged from the thickness of diaphragm and the shape of the <110> etching bottom face.