The present disclosure provides an embodiment of a method. The method includes patterning a substrate to form trenches; etching the substrate, thereby modifying the trenches with round tips; forming a stack including conductive layers and dielectric layers in the trenches, wherein the conductive layers and the dielectric layers alternate with one another within the stack; forming an insulating compressive film in the trenches, thereby sealing voids in the trenches; and forming conductive plugs connected to the conductive layers, respectively.