A method includes forming a first layer on a substrate; forming a first plurality of trenches in the first layer by a first patterning process; and forming a second plurality of trenches in the first layer by second patterning process, wherein a first trench of the second plurality merges with two trenches of the first plurality to form a continuous trench. The method further includes forming spacer features on sidewalls of the first and second pluralities of trenches. The spacer features have a thickness. A width of the first trench is equal to or less than twice the thickness of the spacer features thereby the spacer features merge inside the first trench.