Method for forming floating gate in flash memory device

Abstract

A method for fabricating a nonvolatile memory device including successively forming a first oxide layer, an electrically conductive layer, a second oxide layer, a nitride layer and a third oxide layer on a semiconductor substrate. The method also includes patterning the third oxide layer, forming spacers at sidewalls of the third oxide layer, forming a trench in the substrate by selectively etching the substrate with the third oxide layer as a mask, filling the trench with fourth oxide layer, and removing the third oxide layer, the nitride layer and the second oxide layer. Before filling the trench with the fourth oxide layer, a liner oxide layer is formed on inner walls of the trench. The fourth oxide layer is high density plasma (HDP) oxide and tetrafluoroethane (Si(OCH)). During the filling the trench, lower corners of the conductive layer are made have rounded structure or bird's beak structure.