a semiconductor device , and more particularly a technique for forming the substrate thereof , is provided for the purpose of preventing the occurrence of crystal devects during recrystallization . to accomplish this , while the surface of an insulator on which a material to be recrystallized is flattened , a semiconductor layer which is employed for another use , e . g ., the interconnection between elements or the gate electrode of a mos transistor , is disposed in the insulator . owing to the flattened insulator , the occurrence of the crystal defects in the recrystallization of the material can be prevented .