SEMICONDUCTOR DEVICE AND ITS FABRICATION

PURPOSE:To obtain a semiconductor device in which high integration is not impeded by well isolation. CONSTITUTION:An N type shield layer 2 is formed on a P type semiconductor substrate 1 and a P type isolated well 3 is formed on the layer 2. A P type substrate well 5 is formed contiguously to the P type isolated well 3 through an isolation region 10b where a trench deeper than the thickness of the P type well 3 is filled with an insulator. An isolation region 10a, where a trench shallower than the thicknesses of the P type isolated well 3 and the P type substrate well 5 is filled with an insulator, is formed between the elements in the wells 3 and 5. The deep trench type isolation region 10b provides well isolation and the shallow trench type isolation region 10a provided element isolation.