A method includes transferring a wafer to a position over a wafer chuck; ejecting a first gas from a purging device above the wafer to clean a top surface of the wafer; after ejecting the first gas, lifting a lifting pin through the wafer chuck to receive the wafer; and after the wafer is received by the lifting pin, ejecting a second gas from first openings in a sidewall of the lifting pin to a region between a bottom surface of the wafer and a top surface of the wafer chuck.