A method of forming an integrated circuit includes forming a plurality of gate structures longitudinally arranged along a first direction over a substrate. A plurality of angle ion implantations are performed to the substrate. Each of the angle ion implantations has a respective implantation angle with respect to a second direction. The second direction is substantially parallel with a surface of the substrate and substantially orthogonal to the first direction. Each of the implantation angles is substantially larger than 0°.