Method of fabricating shallow trench isolation

A method of fabricating shallow trench isolation suitable for a semiconductor substrate, comprising:

• forming a pad layer on the semiconductor substrate;
• removing part of the pad layer and the semiconductor substrate to form shallow trenches thereon;
• removing part of the pad layer around the opening of the shallow trenches to expose the surface of the semiconductor substrate thereunder and form a corner between the surface of the semiconductor substrate and the opening of the shallow trenches;
• forming a thermal oxide film on the surface of the semiconductor substrate exposed in the sidewalls of the shallow trenches to constitute a first oxide layer with the pad layer;
• forming a nitride liner film to cover the surface of the first oxide layer on the sidewalls of the shallow trenches and the pad layer on the opening of the shallow trenches;
• forming an insulating layer to fill the shallow trenches;
• forming a first opening by removing part of the insulating layer in the shallow trenches, such that the rest of the insulating layer is divided into top and bottom parts having a top part with greater width than the bottom part does;
• removing the pad silicon nitride layer and the nitride liner film not covered with the top part of the insulating layer to expose the top part of the insulating layer, its sidewalls and part of the first oxide layer, such that the nitride liner film under the top part of the insulating layer is over-etched to form a second opening;
• forming a second oxide layer to cover the upper surface of the top part of the insulating layer and the surface of the first oxide layer exposed at the sidewalls of the top part of the insulating layer, and to fill the second opening;
• removing the second oxide layer and the first oxide layer not covered with the top part of the insulating layer to form a third opening such that the surface of the semiconductor substrate is exposed;
• forming a sacrificial oxide layer on the exposed semiconductor substrate such that part of the third opening is filled to constitute a third oxide layer with the rest of the first oxide layer;
• forming a fourth oxide layer to cover the upper surface of the top part of the insulating layer, its sidewalls and the surface of the third oxide layer, and to fill the third opening;
• and
• removing the fourth oxide layer and part of the third oxide layer such that the upper surface of the top part of the insulating layer, its sidewalls and the surface of the semiconductor substrate are exposed to form a shallow trench isolation comprising the top part and the bottom part of the insulating layer.