Prolonging optical element lifetime in an EUV lithography system

A method comprising:

• measuring an operating parameter of an EUV light source, the EUV light source comprising a vacuum chamber and a reflective optical element located within the vacuum chamber, wherein the operating parameter is a number of EUV pulses produced by the EUV light source since a predefined starting time;
• and
• controlling, based at least in part on the operating parameter as measured, continuous addition to the vacuum chamber of a mitigation gas that mitigates contamination of the reflective optical element by chemical reaction.

A method comprising:

• measuring an operating parameter of an EUV light source, the EUV light source comprising a vacuum chamber and a reflective optical element located within the vacuum chamber, wherein the operating parameter is a duration of operation of the EUV light source;
• and
• controlling, based at least in part on the operating parameter as measured, continuous addition to the vacuum chamber of a mitigation gas that mitigates contamination of the reflective optical element by chemical reaction.