A first dielectric layer made of a first dielectric material is deposited over a semiconductor substrate. A buffer layer is then deposited on an upper surface of the first dielectric layer. A trench is opened to extend through the buffer layer and the first dielectric layer. A second dielectric layer made of a second dielectric material is the deposited in a conformal manner on the buffer layer and filling the trench. Chemical mechanical polishing of the second dielectric layer is performed to remove overlying portions of the second dielectric layer with the buffer layer being used as a polish stop. After removing the buffer layer, the first dielectric layer and the second dielectric material filling the trench form a pre-metallization dielectric layer having a substantially planar upper surface.