US Patent 7989912 Semiconductor device having a compressed device isolation structure

The semiconductor device includes a lower device isolation structure formed in a semiconductor substrate to define an active region. The lower device isolation structure has a first compressive stress. An upper device isolation structure is disposed over the lower device isolation structure. The upper device isolation structure has a second compressive stress greater than the first compressive stress. A gate structure is disposed over the active region between the neighboring upper device isolation structures.