the steps shown in fig1 a fig1 b and fig1 c first referring to fig1 a a gate insulating film 20 is provided on a substrate top surface 12 of a semiconductor substrate 10 a gate electrode 30 of the amorphous silicon is provided on the gate insulating film 20 a silicon nitride film 40 is then deposited on the gate electrode 30 further a silicon oxide film 50 is deposited on the silicon nitride film 40 the silicon oxide film 50 the silicon nitride film 40 and the gate electrode 30 are selectively etched into a predetermined pattern by using the photolithography technique referring to fig1 b then using the silicon oxide film 50 as a mask the gate insulating film 20 and semiconductor substrate 10 are selectively removed by an etching in this etching a trench 60 is formed to penetrate the gate insulating film 20 and reach the semiconductor substrate 10 when the semiconductor substrate 10 is etched to form the trench 60 an rie process under a high pressure atmosphere in which an etching gas including cl 2 and hbr is used is added to the ordinary rie process these rie processes using cl 2 and hbr and the ordinary rie process are implemented consecutively in the same chamber subsequently the side and bottom surface portions of the trench 60 are oxidized by rto in oxygen o 2 atmosphere held at 1000 u00b0 c in fig1 b the trench 60 and the surrounding structure of the trench 60 after rto treatment are shown in an enlarged scale this oxidation process may be implemented in a hydrogen h 2 and oxygen o 2 atmosphere or in ozone o 3 atmosphere in lieu of oxygen o 2 atmosphere the curvature radius of the boundary portions 80 may be.