PURPOSE: To obtain a U-shaped groove isolation formation method, by which a crystal defect is never generated by a method wherein a groove is formed between an element region of the surface of a semiconductor substrate and other element region of the surface, the interior of the groove is filled with a polycrystalline or amorphous silicon layer insulated electrically from the substrate and the surface of the groove is coated with an insulating material film adhered by deposition.

CONSTITUTION: A mask is formed on a P-type single crystal Si substrate 1 by a photolithography, an isolation groove is made by a selective ion etching(RIE) method and the surface of this groove is thermally oxidized to form an SiO2 film 2. When a polycrystalline silicon layer 3 is grown by deposition by a reduced CVD method, the deposition is completely proceeded isotropically. Therefore, the groove not only is completely filled but also the final surface of the layer 3 also becomes almost flat. When this layer 3 is etched back by an RIE method, a state that the layer 3 is left only in the groove is obtained. Moreover, an SiO2 layer 4 is applied on the whole surface of the substrate by a CVD method, subsequently, a selective etching is performed by a normal photolithography and an RIE method and an SiO2 layer 4' is left only on the upper part of the groove. Thereby, a U-shaped groove isolation structure is realized.