A method includes forming a first layer on a substrate; forming a first plurality of trenches in the first layer by a patterning process; and forming a second plurality of trenches in the first layer by another patterning process, resulting in combined trench patterns in the first layer. A first trench of the second plurality connects two trenches of the first plurality. The method further includes forming dielectric spacer features on sidewalls of the combined trench patterns. A space between two opposing sidewalls of the first trench is completely filled by the dielectric spacer features and another space between two opposing sidewalls of one of the two trenches is partially filled by the dielectric spacer features.