Method for forming shallow trench isolation

A method for forming a trench isolation, the method comprising:

• forming a pad oxide layer on a semiconductor substrate;
• forming a silicon nitride layer on the pad oxide layer;
• forming a sacrificial layer on the silicon nitride layer;
• forming a photo-resist layer that defines an active region on the sacrificial layer;
• removing the portions of the sacrificial layer, the silicon nitride layer, the pad oxide layer and the semiconductor substrate to form a trench;
• undercutting portions of the silicon nitride layer;
• forming a dielectric layer over the sacrificial layer, thereby completely filling the trench in the semiconductor substrate, the pad oxide layer, the silicon nitride layer, the sacrificial layer, and completely filling the undercut of the silicon nitride layer;
• planarizing the dielectric layer and the sacrificial layer until the silicon nitride layer is exposed;
• removing the silicon nitride layer;
• and
• removing portions of the pad oxide layer using the remaining dielectric layer as a mask.