Method and apparatus for purifying target material for euv light source

A method, comprising:

• loading a target material in a crucible, the target material to be used in a droplet generator of an extreme ultraviolet (EUV) light source;
• inserting the loaded crucible into a vessel and sealing the vessel;
• melting the target material in the crucible;
• flowing a gas containing hydrogen over a free surface of the molten target material;
• measuring a concentration of water vapor in gas exiting the vessel;
• and
• after the measured concentration of water vapor in the gas exiting the vessel reaches a target condition, allowing the molten target material to cool.

An apparatus, comprising:

• a metal vessel having an open end and a closed end, the metal vessel having a cylindrical shape;
• a crucible disposed within the metal vessel, the crucible having an open end and a closed end, the crucible disposed within the metal vessel such that the open end of the crucible faces the open end of the metal vessel;
• a closure device covering the open end of the metal vessel, the closure device configured to form a seal having vacuum and pressure capability;
• an input tube having a first end located outside the vessel and a second end located inside the vessel, the second end of the input tube being positioned to direct an input gas flowing into the vessel through the input tube toward the crucible;
• and
• an exhaust tube having a first end located outside the metal vessel and a second end in flow communication with the inside of the metal vessel.