Method of manufacturing a semiconductor device including forming well comprising epi in trench

A method for fabricating a semiconductor device comprising:

• providing a first conductive silicon substrate having an active region and field regions thereon and having a formed pad oxide layer on the surface;
• forming a trench having a width and including an active region and field regions at both sides of the active region by etching the pad oxide layer and the silicon layer;
• forming a first sidewall layer on the inner wall of the trench and on the pad oxide layer by an oxidation process;
• depositing a first oxide layer on the first sidewall layer to have a thickness similar to the width of the field region;
• forming a spacer exposing the active region of the silicon substrate at both sidewalls of the trench by etching the first oxide layer;
• forming a second conductive well by growing an in-situ doped silicon epi layer having a height similar to a surface of the silicon substrate on the exposed active region of the silicon substrate;
• forming a second sidewall layer on the surface of the well by an oxidation process;
• depositing a second oxide layer having a predetermined thickness on the resultant structure to fill a gap between the spacer and the well;
• performing a first planarization to the second oxide layer to expose the first sidewall layer;
• and
• performing a second planarization to the resultant structure to expose the surface of the well and to form an isolation layer.