SEMICONDUCTOR DEVICE

PURPOSE:To obtain an insulative separation structure easy to prepare, highly reliable and easy to make minute by a method wherein a cavity having a slender section, which is left in polycrystalline silicon filled in a groove formed for insulative separation, is formed to have an approximately round section by heat treatment. CONSTITUTION:An SiO2 film 2 is formed on one main surface of a semiconductor single-crystal substrate 1, and then a groove 3 having a substantially vertical wall is formed. After an SiO2 film 2' is formed on the exposed surface of the substrate inside the groove, polycrystalline silicon 4 is applied for coating. When heat treatment is applied for 15min at 1,100 deg.C in dry N2 gas subsequently, the shape of a cavity is changed to be elliptic approximately. After photoresist is applied to make the surface flat, sputter etching is conducted to remove a polycrystalline silicon layer on the surface, and an SiO2 film 2'' is formed by a thermal oxidation method. By this constitution, an insulative separation structure having a narrow separating area can be formed easily.