Method of forming shallow trench isolation regions with improved corner rounding

A method of forming a shallow trench isolation (STI), region in a semiconductor substrate, comprising the steps of:

• forming a silicon nitride shape on said semiconductor substrate, featuring an opening which exposes a first region of said semiconductor substrate;
• performing an ion implantation procedure to a top portion of said first region of said semiconductor substrate;
• performing a first thermal oxidation procedure to grow a first silicon dioxide layer in said first region of said semiconductor substrate, consuming implanted region, with said first thermal oxidation procedure allowing the sides of second regions of said semiconductor substrate, located underlying said silicon nitride shape and adjacent to said first silicon dioxide layer, to be defined with rounded corners;
• performing a first wet etch procedure to selectively remove said first silicon dioxide layer resulting in a recessed first region of said semiconductor substrate;
• depositing a first silicon oxide layer;
• performing a first anisotropic reactive ion etching (RIE) procedure to form silicon oxide spacers on the sides of said silicon nitride shape, with said silicon oxide spacers overlying said rounded corners of said second regions of said semiconductor substrate;
• performing a second anisotropic RIE procedure to form a shallow trench shape in a portion of said recessed first region of said semiconductor substrate not covered by said silicon oxide spacers;
• performing a second thermal oxidation procedure to grow a second silicon dioxide layer on the sides of said shallow trench shape;
• depositing a second silicon oxide layer completely filling said shallow trench shape;
• performing a chemical mechanical polishing procedure to remove portions of said second silicon oxide layer from the top surface of said silicon nitride shape resulting in said STI region comprised of a silicon oxide plug obtained from said second silicon oxide layer located in center of said STI shape, of said second silicon dioxide layer located on the sides of said shallow trench shape, and of said silicon oxide spacers located on said rounded corners;
• and
• performing a second wet etch procedure to remove said silicon nitride shape.