US Patent 7736991 Method of forming isolation layer of semiconductor device

A method of forming an isolation layer of a semiconductor device includes forming first trenches in an isolation region of a semiconductor substrate. A spacer is formed on sidewalls of each of the first trenches. Second trenches are formed in the isolation region below the corresponding first trenches. Each second trench is narrower and deeper than the corresponding first trench. A first oxide layer is formed on sidewalls and a bottom surface of each of the second trenches. The first trench is filled with an insulating layer.