A semiconductor device includes a first layer, first and second active areas disposed on the first layer; a trench disposed between the first and second active areas, an insulating oxide that fills the trench to a level below a surface of the first and second active layers, and a nitride cap disposed on top of the insulating oxide so that the first and second active areas can be cleaned without damaging the insulating oxide. A top surface of the nitride cap in regions adjacent to the first and second active areas in aligned with a top surface of the first and second active areas, a top surface of the nitride cap in a center region of the nitride cap is stepped below the top surface of the adjacent regions, and a void is formed between the top surface regions adjacent to the first and second active areas.