Method for manufacturing a semiconductor device and a semiconductor memory device

A method of manufacturing a semiconductor memory device comprising:

• a first step of forming a trench in a surface silicon substrate and forming an insulating film on an inner wall of said trench up to a predetermined height;
• a second step of packing a conductor layer into said trench up to the surface level of said silicon substrate;
• a third step of growing a first silicon oxide film on said silicon substrate and on said conductor layer and depositing a first silicon nitride film thereon;
• a fourth step of etching said first silicon nitride film, said first silicon oxide film, said silicon substrate and said conductor layer at least to the level of said predetermined height by employing a resist pattern as a mask to form a silicon island comprised of a part of said silicon substrate and a part of said conductor layer;
• a fifth step of growing a second silicon oxide film on a surface of said silicon substrate exposed by said second step and on a surface of said silicon island, depositing a second silicon nitride film thereon, and etching said second silicon nitride film so as to leave a portion of said second silicon nitride film deposited on a side wall of said silicon island and deposited on a connection of said silicon island and said conductor layer,
• a sixth step of growing a third silicon oxide film by thermal oxidation of the surface of said silicon substrate to separate electrically said silicon island from said silicon substrate;
• and
• a seventh step of forming a gate electrode on said silicon island and forming a source region and drain region in said silicon island by employing said gate electrode as a mask.