The present application discloses a method of cleaning a semiconductor wafer by mounting a wafer to a chuck, positioning a gas guard, defining therein a chamber having an open bottom, immediately above the layer of water, spraying de-ionized water onto the wafer while rotating the chuck at a location outside the chamber when the wafer is mounted to the chuck, to thereby form a layer of water on the wafer, and spraying a cleaning gas from a gas spraying unit disposed above said chuck through the chamber and into the layer of water to thereby cause the cleaning gas to dissolve in the layer of water, and at the same time moving the chamber across a surface of the wafer, to thereby clean the wafer, wherein said gas spraying unit includes a gas injection tube oriented to inject the cleaning gas towards the wafer mounted to the chuck, and the gas guard connected to the gas injection tube.