A method for forming a semiconductor device. One embodiment provides a semiconductor substrate having a trench with a sidewall isolation. The sidewall isolation is removed in a portion of the trench. A gate dielectric is formed on the laid open sidewall. A gate electrode is formed adjacent to the date dielectric. The upper surface of the gate electrode is located at a depth d1 below the surface of the semiconductor substrate. The gate oxide is removed above the gate electrode. An isolation is formed simultaneously on the gate electrode and the semiconductor substrate such that the absolute value of height difference d2 between the isolation over the gate electrode and the isolation over the semiconductor substrate is smaller than the depth d1.