PURPOSE: To obtain a narrow and deep separation-groove by a method wherein, when an insulating film is to be buried in the separation groove, the central part of the groove with a high etching-speed is removed and the insulating film or a semiconductor film with a slow etching-speed is buried in a recessed part produced by the removal.

CONSTITUTION: After a polysilicon film 3 and a CVD SiO2 film 4 have been deposited on an SiO2 film 2 on a p-type Si substrate, this assembly is etched selectively by reactive ion etching so that a narrow separation-groove 6 and a wide separation-groove 7 can be made. After the film 4 has been removed, a CVD SiO2 film 9 is formed on the wall of the groove via a thermal-oxidization film 8. Then, after this assembly has been etched by an HF solution, the central part of the narrow groove 6 is etched quickly so that a recessed part 10 can be made. This part is covered with polysilicon 11 and the polysilicon on the flat part is etched by reactive ion etching so that the polysilicon 11 can be buried in the recessed part 10. After the wide groove 7 has been buried by a CVD SiO2 film 12 and photoresist has been spin-coated, the films 12 and 9 are etched simultaneously so that the CVD SiO2 films 9, 12 can be buried in the grooves 6, 7. Then, the polysilicon film 3 is etched by reactive ion etching and the SiO2 film 2 is removed by the HF solution so that a gate oxide-film 15 and a polysilicon gate-electrode 16 can be formed. Through this constitution, it is possible to obtain a high integration-density LSI with a small area for the separation layer at a good yield rate.