The disclosure provides methods and structures for preventing exposing polysilicon layer and silicon substrate on the substrate backside to polysilicon etching chemistry during removal of the dummy polysilicon layer in replacement gate structures. A thermal deposition process or processes are used to deposit a dielectric layer for offset spacers and/or a contact etch stop layer (CESL) to cover the polysilicon layer on the substrate backside. Such mechanisms reduce or eliminate particles originated at bevel of substrate backside, due to complete removal of the polysilicon layer at the backside bevel and the resultant etching of silicon substrate.