A method for forming a shallow trench isolation (STI) structure with improved electrical isolation performance including providing a semiconductor substrate including an overlying silicon oxide layer on the semiconductor substrate and a hardmask layer on the silicon oxide layer; dry etching in a first etching process to form a patterned hardmask opening for etching an STI opening; dry etching in a second etching process the semiconductor substrate to form an upper portion of an STI opening to form a polymer layer along sidewall portions of the STI opening; and, dry etching in a third etching process the STI opening to form rounded bottom corners and rounded top corners.