Semiconductor device having dual isolation structure and method of fabricating the same

A method of fabricating a semiconductor device comprising the steps of:

• implanting a second conductivity type impurity into a predetermined region of a semiconductor substrate to form a diffused bottom isolation layer;
• implanting a first conductivity type impurity into the surface of the semiconductor substrate over the diffused bottom isolation layer to form a first conductivity type buried layer;
• forming a second conductivity type epitaxial layer covering the buried layer and the semiconductor substrate;
• patterning the epitaxial layer to form a trench surrounding a predetermined region of the epitaxial layer, the trench exposing a predetermined region of the semiconductor substrate;
• implanting the second conductivity type impurity into the lower side of the trench to form a diffused isolation wall connected to the diffused bottom isolation layer to surround a predetermined region of the semiconductor substrate;
• filing an insulating layer in the trench to form a trench isolation structure in contact with the diffused isolation wall;
• forming a field oxide layer to define a first active area and a second active area on the upper side of the epitaxial layer surrounded by the trench isolation structure, wherein the first active area is surrounded by the field oxide layer, and the second active area is interposed between the field oxide layer and the trench isolation structure;
• forming a gate electrode and a source region on the first active area;
• and
• forming a drain region in the second active area, wherein the trench isolation structure, the diffused isolation wall and the diffused bottom isolation layer define the device region comprising the semiconductor substrate and the epitaxial layer sequentially deposited.