PURPOSE: To etch silicon nitride on silicon oxide without causing any residue and particles while a high selective ratio is made compatible with anisotropy by a method wherein the silicon nitride on the silicon oxide is etched by using a gas system which contains carbon, hydrogen and fluorine and a plasma treatment is then executed by using a fluorine-based gas.

CONSTITUTION: An SiO2 film 2 and an Si3N4 film 3 are formed on an Si substrate 1, a photoresist film 4 is formed on them, and the photoresist film 4 is patterned by a G-line stepper by using TSMR-V3. When the Si3N4 film 3 is dry-etched, it is etched by a gas containing C, H and F (CH2F2, CH3F or the like) at a first step. An etching residue which is easily produced at this time is removed by using a gas containing F (SF6, NF3, ClF3 or the like) at a second step. Thereby, the Si3N4 film on the SiO2 film can be etched without any problem of a residue and particles while high selectivity is made compatible with good isotropy.