Fabrication process of a semiconductor memory device having a multiple well structure in a recessed substrate

A method for fabricating a semiconductor memory device, comprising the steps of:

• forming, on a first region of a semiconductor substrate of a first conductivity type, a first well of a second, opposite conductivity type;
• a mask process for forming a mask pattern on a surface of said semiconductor substrate, such that a second region, which is included in said first region, is exposed;
• introducing an impurity element of said first conductivity type into the surface of said semiconductor substrate in correspondence to said second region while using said mask pattern as a mask;
• causing a diffusion of said impurity element in said semiconductor substrate to form a second well of said first conductivity type in said first well in correspondence to said second region;
• causing an oxidation of the surface of said semiconductor substrate while using said mask pattern as a mask, to form an oxide layer on said second well such that said oxide layer covers said second well;
• and
• removing said oxide layer to form a recessed surface on the surface of said semiconductor substrate in correspondence to said second region, such that said recessed surface has a level lower than a level of a surface of said first region.