In a method for fabricating a semiconductor device, a trench is etched in a semiconductor substrate having a top surface, and a lining oxide layer is formed conformal to the trench. A negatively-charged liner covering the lining oxide layer and conformal to the trench is formed. The trench is partially filled with a flowable oxide to a level below the top surface of the semiconductor substrate, and the flowable oxide in the trench is cured. The negatively-charged liner above the cured flowable oxide is optionally removed. A silicon oxide is deposited in the remaining portion of the trench, and a planarization process is performed to remove excess portions of the silicon oxide over the top surface of the semiconductor substrate.