1. An extreme ultraviolet radiation source, comprising:a vessel configured to produce extreme ultraviolet light therein, wherein the vessel has a gas supply from which a cleaning gas is supplied into the vessel and a gas outlet from which the cleaning gas exits the vessel;
a vacuum pump coupled to the gas outlet; and
a gas scrubber disposed within the vessel, arranged such that the cleaning gas leaves the vessel through the gas outlet after flowing through the gas scrubber;
wherein the gas scrubber has a plurality of ribs and a plurality of gas passages to allow the cleaning gas to flow through, wherein the ribs form the gas passages therebetween, and sizes of the gas passages vary according to a distance between each of the gas passages and the gas outlet.