An extreme ultraviolet radiation source is provided, including a vessel and a gas scrubber. The vessel has a gas inlet from which a cleaning gas is supplied into the vessel and a gas outlet from which the cleaning gas exits the vessel. The gas scrubber is disposed within the vessel, arranged such that the cleaning gas leaves the vessel through the gas outlet after flowing through the gas scrubber. The gas scrubber has a number of gas passages to allow the cleaning gas to flow through, and the sizes of the gas passages vary according to the distance between each of the gas passages and the gas outlet.