A wafer is provided with a through via extending a portion of a substrate, an interconnect structure electrically connecting the through via, and a polyimide layer formed on the interconnect structure. Surface modification of the polyimide layer is the formation of a thin dielectric film on the polyimide layer by coating, plasma treatment, chemical treatment, or deposition methods. The thin dielectric film is adhered strongly to the polyimide layer, which can reduce the adhesion between the wafer surface and an adhesive layer formed in subsequent carrier attaching process.