process of the present invention is carried out such that the formation of the sio 2 film and the formation of the sin film in the step a ii are effected by a heat oxidation method and a cvd method respectively while the heat treatment in the step c is effected after removal of the side wall it is preferred that the thickness of the sio 2 film and sin film preferably ranges between about 50 and 150 u00e5 and that the ion injection is conducted with an acceleration energy of 40 to 90 kev and at a dosage of about 3 to 5 u00d7 10 15 cm 2 while the heat treatment is preferably conducted at a temperature of about 800 u00b0 to 900 u00b0 c according to this method since the ions are injected through the sio 2 film and the sin film implantation of the oxygen atoms from the sio 2 film into the si substrate can be diminished in addition an implantation of nitrogen atom from the sin film into the si substrate is extremely small furthermore the etching during forming the side wall is stopped by the sin film so that the thickness control of the sio 2 film and the sin film is facilitated and qualities.