102a and the trench 106, as shown in FIG. 18.
In the inner wall oxidation, the oxidation rate of the polysilicon film 103a is substantially at the same level as the oxidation rate of the silicon substrate 101 (the surface of the trench 106), while the oxidation rate of the pad oxide film 102a is smaller than the oxidation rate of the polysilicon film 103a. For this reason, an oxide film grows faster on the surface of the silicon substrate 101 and on the surface of the polysilicon film 103a than on the surface of the pad oxide film 102a, thereby forming an overhung configuration in which the part of the oxide film that has grown on the side surface of the polysilicon film 103a is hung over theoxide film that has grown on the side surface of the pad oxide film 102a. 
As a result of this, referring to FIG. 19, a recess 120 is formed between the part that has grown on the surface of the silicon substrate 101 and the part that has grown on the surface of the polysilicon film 103a in the oxide film 107. When a buried oxide film 108 is buried in the trench 106 in the step shown in FIG. 20 in a state in which the recess 120 has been formed, the part of the recess 120 is not filled with the buried oxide film 108 and remains as voids 121.
Referring to FIG. 22, these voids 121 form recesses 122 in the trench isolation oxide film 109