PURPOSE: To form a recessed part vertical to the substrate surface with large aspect ratio, by making the thickness of a mask silicon oxide film larger than or equal to a specified value.

CONSTITUTION: A silicon oxide film of 2μm in thickness (larger than or equal to 1.5μm) is formed on one surface of a silicon substrate 1 of 500μm in thickness by thermal oxidation. The oxide film is eliminated by using hydrofluoric acid so as to expose the silicon substrate 1 surface on a part to be worked, and an aperture part 4 is formed. Etching is performed by using a parallel flat plate type dry etching equipment of anode coupling system. As to this equipment, a lower electrode 12 serving as a stage faces an upper electrode 13 so as to keep a distance in the range of 5-100mm, in a reaction chamber 11. Exhaust pipes 15 arranged dispersedly on the peripheral part of a bottom plate 14 of the reaction chamber 11 are opened toward the center of the lower electrode 12 in parallel with the bottem plate 14. A high frequency power supply 17 is connected with the upper electrode 13 via a matching tuner 16. The gap 20 between the top 18 of the reaction chamber 11 and a connection member 19 of the upper electrode 13 acts as the feeding inlet of reaction gas 1.