Abstract: A semiconductor device having a LOCOS region formed on a Si substrate comprises a minute groove filled with non-oxidizable isolation film provided at the boundary between the LOCOS region and an IC area to be formed on the Si substrate. The non-oxidizable isolation film prevents the intrusion of thermal oxidation for the LOCOS region into the IC region, and is of silicon nitride 1000-10000 .ANG. width.A method of manufacturing a semiconductor device having a LOCOS region formed on a Si substrate comprises the steps of depositing a silicon nitride film on the substrate; etching a minute width region at the boundary between an area to be the LOCOS region and an area to be an IC region on the substrate by anisotropic etching with a reactive ion etching mode, thereby to form a groove thereat thermal-oxidizing the Si substrate to form a thin SiO.sub.

Abstract: A semiconductor IC device in which an N-type semiconductor layer is formed in a P-type semiconductor substrate; the N-type layer is divided by a P.sup.+ -type insulation region into plural island regions; and an IIL is formed in a first island region while an NPN transistor is formed in a second island region, wherein an N-type up-diffused layer is formed from the bottom of the first island region up while an N-type well region is formed from the surface of the first island region down, and N.sup.+ -type buried layers are formed near the bottoms of the first and the second island region.