field stop ion implantation may be performed using a dopant such as boron or the like thereafter photoresist 71 is removed then as shown in fig4 f fourth oxide layer 93 and first oxide layer 91 and second oxide layer 92 which are exposed to the outside are removed by carrying out an oxide layer etching process and fifth oxide layer 102 is formed on the inside of the trench and around the polysilicon plug by carrying out a thermal oxidation process this process is carried out in such a manner that thermal oxidation layer 102 is formed in a thickness of 300 15 000 u00e5 on the inside of the trench and around polysilicon plug 111 by carrying out a heat treatment at a temperature of over 600 u00b0 c in an atmosphere including oxygen the growth of thermal oxidation layer 102 is inhibited on the portion of the wafer covered by the nitride layer under this condition polysilicon plug 111 which is surrounded by an oxide layer is connected along the edges of active region 1 and trench 2 and therefore a metal plug contact may be formed to connect polysilicon plug 111 to vcc or ground during a back end metallization process thereby utilizing it as an element insulating means then as shown in fig4 g silicon nitride layer 62.