Method of fabricating STI

A fabrication method for a STI, wherein a substrate is provided with a patterned mask layer and pad oxide layer formed thereon, with a first opening which exposes a part of the substrate, the method comprising steps of:

• forming a shallow trench in the substrate, with the mask layer and the pad oxide layer serving as a mask;
• forming a silicon oxide layer on the substrate to fill the shallow trench;
• performing CMP, so that a part of the silicon oxide layer is removed until the surface of the mask layer is exposed;
• removing another part of the silicon oxide layer, so that a surface of the remaining silicon oxide layer is lower than a surface of the substrate, and a part of the substrate forming a sidewall of the shallow trench is exposed;
• removing a part of the mask layer and the pad oxide layer, so that the first opening is enlarged to form a second opening which exposes a part of the substrate;
• performing a local ion implantation, so that a doped region is formed on an exposed part of the substrate;
• forming an insulating layer which fills the shallow trench and the second opening;
• performing a densification step;
• and
• removing the mask layer and the pad oxide layer in sequence so as to form a STI in the shallow trench.