A method includes forming fin semiconductor features on a substrate. A dopant-containing dielectric material layer is formed on sidewalls of the fin semiconductor features and the substrate. A precise material modification (PMM) process is performed to the dopant-containing dielectric material layer. The PMM process includes forming a first dielectric material layer over the dopant-containing dielectric material layer; performing a tilted ion implantation to the first dielectric material layer so that a top portion of the first dielectric material layer is doped to have a modified etch characteristic different from an etch characteristic of a bottom portion of the first dielectric material layer; and performing an etch process to selectively remove the top portion of the first dielectric material layer and the top portion of the dopant-containing dielectric material layer.