A method includes forming a low-k dielectric layer over a substrate of a wafer, forming a first dielectric layer over the low-k dielectric layer, forming a second dielectric layer over the first dielectric layer, forming a stress tuning dielectric layer over the second dielectric layer, forming an opening in the stress tuning dielectric layer to expose a top surface of the second dielectric layer, and etching the stress tuning dielectric layer and the second dielectric layer to form a trench. The formation of the opening and the etching of the stress tuning dielectric layer are performed in separate etching steps. The method further includes etching the first dielectric layer to form a via opening connected to the trench, and filling the trench and the via opening to form a metal line and a via, respectively.