A device includes a wafer substrate including an isolation feature, at least two fin structures embedded in the isolation feature, and at least two gate stacks disposed around the two fin structures respectively. A first inter-layer dielectric (ILD) layer is disposed between the two gate stacks, with a dish-shaped recess formed therebetween, such that a bottom surface of the recess is below the top surface of the adjacent two gate stacks. A second ILD layer is disposed over the first ILD layer, including in the dish-shaped recess. The second ILD includes nitride material; the first ILD includes oxide material.