A method for fabricating a semiconductor device includes the steps of: forming a fin-shaped structure on a substrate, forming a gate material layer on the fin-shaped structure, performing an etching process to pattern the gate material layer for forming a gate structure and a silicon residue, performing an ashing process on the silicon residue, and then performing a cleaning process to transform the silicon residue into a polymer stop layer on a top surface and sidewalls of the fin-shaped structure.