A semiconductor device and a method of fabricating thereof, including an insulator layer having alternately layered insulator films and boundary layers, wherein the boundary layers are more dense than the insulator films to prevent expansion and elongation of string-like defects across the boundary layers. The method includes mixing a nitrogen containing gas and a silane group gas to form an insulator film; temporarily stopping a flow of the silane group gas for approximately one to fifteen seconds to form a boundary layer over the insulator film; restarting the flow of the silane group gas; and repeating the steps of temporarily stopping and restarting for a predetermined number of times to form the plurality of alternately layered insulator films and boundary layers. The plurality of alternately layered insulator films and boundary layers is also etched at an etching rate for the insulator films greater than an etching rate for the boundary layers to form a step-shaped sloped opening.