A method is given to form a thin film on a surface of a semiconductor wafer. The surface has at least a first region, containing an inner portion of the wafer, and a second region, containing an outer portion of the wafer, and slopes outward from the first region to the second region. The method starts with performing an in-situ inert gas plasma treatment on the surface of the semiconductor wafer to generate different temperatures from the first region to the second region. Different deposition rates of a precursor A from the first region to the second region are thus generated so as to form a flat surface. Then a precursor A-chemical vapor deposition (CVD) process is performed to form the thin film with the flat surface immediately after performing the inert gas plasma treatment.