MANUFACTURE OF SEMICONDUCTOR DEVICE

PURPOSE: To prevent the deterioration of the dielectric strength of a semiconductor oxide film by forming the oxide film on a semiconductor substrate after the substrate is etched with a mixed gas containing carbon and fluorine and another mixing gas containing fluorine and oxygen at a specific partial pressure ratio or higher. CONSTITUTION: When a silicon substrate 1 in an element forming area 6 is etched with Ar, CF4 , and CHF3 after a field oxide film 2 is covered with a photoresist, a damaged layer, SiC layer, and CFX Si layer are successively formed on the bottom 5 of a trench-like part. When the substrate 1 in the area 6 is etched with a reaction gas containing CF4 and O2 at a specific partial pressure ratio of >=70%, most of the damaged layer, SiC layer, and CFX Si layer is removed. Then a gate oxide film 9 is formed on the surface of the substrate 1 in the area 6. Thus the deterioration of the dielectric strength of the film 9 is prevented.