A method includes depositing a first dielectric layer covering an electrical connector, depositing a second dielectric layer over the first dielectric layer, and performing a first etching process to etch-through the second dielectric layer and the first dielectric layer. An opening is formed in the first dielectric layer and the second dielectric layer to reveal the electrical connector. A second etching process is performed to laterally etch the first dielectric layer and the second dielectric layer. An isolation layer is deposited to extend into the opening. The isolation layer has a vertical portion and a first horizontal portion in the opening, and a second horizontal portion overlapping the second dielectric layer. An anisotropic etching process is performed on the isolation layer, with the vertical portion of the isolation layer being left in the opening.