Patent ID: 7662697

Claim:
A method of forming a semiconductor device, the method comprising: forming a hard mask having a first opening over a semiconductor substrate; etching the semiconductor substrate of a peripheral region to form a first trench in the peripheral region using the hard mask as an etch mask; etching the hard mask to enlarge the first opening and to form a second opening in the hard mask, wherein the second opening is smaller than the first opening, the second opening being formed in a cell region; etching the semiconductor substrate of the peripheral region and the cell region to form simultaneously a second trench in the peripheral region and a third trench in the cell region using the hard mask as an etch mask, wherein the first and second trenches together define a trench having a dual structure in the peripheral region; and forming a first isolation structure within the first and second trenches in the peripheral region and a second isolation structure within the third trench in the cell region, wherein a width of the second trench is larger than a width of the first trench, and a depth of the second trench is smaller than a depth of the first trench.