Prolonging optical element lifetime in an EUV lithography system
Degradation of the reflectivity of one or more reflective optical elements in a system for generating EUV radiation is reduced by the controlled introduction of a gas into a vacuum chamber containing the optical element. The gas may be added to the flow of another gas such as hydrogen or alternated with the introduction of hydrogen radicals.
1. Apparatus comprising:
a vacuum chamber;
a reflective optical element arranged in the vacuum chamber;
a gas distribution system for adding gas to the vacuum chamber, the gas distribution system including an interface for interfacing with a source of oxygen-containing gas; and
a gas control system configured to control the gas distribution system to supply the oxygen-containing gas to the vacuum chamber until a partial pressure of the oxygen-containing gas reaches a first value, ceasing supply of the oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a second value less than the first value, and resuming supply of the oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a third value greater than the second value.
2. Apparatus as claimed in claim 1 wherein the reflective optical element comprises a multilayer mirror comprising a capping layer and a plurality of underlying layers, with the capping layer being composed and arranged to protect the underlying layers from damage, the capping layer comprising an oxide.
3. Apparatus as claimed in claim 2 wherein the oxide comprises a metal oxide.
4. Apparatus as claimed in claim 1 wherein the oxygen-containing gas comprises H2O2.
5. Apparatus as claimed in claim 1 wherein the oxygen-containing gas comprises O3.
6. Apparatus as claimed in claim 1 wherein the oxygen-containing gas is mixed with argon or helium.
7. Apparatus as claimed in claim 1 further comprising a gas pressure sensor arranged to sense a partial pressure of the oxygen-containing gas in the vacuum chamber and to generate a first signal indicative of the partial pressure to the gas control system, and wherein the gas control system controls supply of the oxygen-containing gas based at least in part on the first signal.
8. Apparatus as claimed in claim 7 wherein the gas pressure sensor indirectly senses the partial pressure of the oxygen-containing gas by sensing a partial pressure of at least one gas other than the oxygen-containing gas.
9. Apparatus as claimed in claim 7 wherein the gas pressure sensor is arranged to sense the partial pressure of the oxygen-containing gas proximate to the reflective optical element.
10. Apparatus as claimed in claim 1 wherein the gas control system is configured to supply the oxygen-containing gas to the vacuum chamber until a partial pressure of the oxygen-containing gas reaches the first value over a first time interval having a first duration and wherein ceasing supply of the oxygen-containing gas to the vacuum chamber until the partial pressure of the oxygen-containing gas reaches a second value less than the first value occurs over a second time interval having a second duration different in magnitude from the first duration.
11. A method of prolonging an operational lifetime of a reflective surface in a vacuum chamber of an EUV source, the method comprising the steps of:
(a) supplying an oxygen-containing gas to the vacuum chamber;
(b) ceasing supplying the oxygen-containing gas to the vacuum chamber when a partial pressure of the oxygen-containing gas reaches a first value;
(c) supplying more oxygen-containing gas to the vacuum chamber when the partial pressure of the oxygen-containing gas reaches a second value less than the first value;
(d) ceasing supplying the oxygen-containing gas to the vacuum chamber when the partial pressure of the oxygen-containing gas reaches a third value greater than the second value; and
(e) repeating steps (c) and (d) to maintain the partial pressure of the oxygen-containing gas between the first value and the second value.
12. The method as claimed in claim 11 wherein the third value is substantially equal to the first value.
13. The method as claimed in claim 11 wherein the oxygen-containing gas comprises H2O.
14. The method as claimed in claim 11 wherein the oxygen-containing gas comprises H2O2.
15. The method as claimed in claim 11 wherein the oxygen-containing gas comprises O3.
16. The method as claimed in claim 11 wherein the oxygen-containing gas is mixed with an inert gas.
17. The method as claimed in claim 11 wherein each of steps (b), (c), and (d) comprises directly sensing a partial pressure of the oxygen-containing gas in the vacuum chamber.
18. The method as claimed in claim 11 wherein each of steps (b), (c), and (d) comprises sensing a partial pressure of the oxygen-containing gas in the vacuum chamber proximate to the reflective surface.
19. The method as claimed in claim 11 wherein supplying an oxygen-containing gas to the vacuum chamber continues over a first interval having a first duration and wherein ceasing supplying the oxygen-containing gas to the vacuum chamber when a partial pressure of the oxygen-containing gas reaches a first value continues over a second interval having a second duration different in magnitude from the first duration.
20. Apparatus for producing extreme ultraviolet (EUV) radiation, the apparatus comprising:
a vacuum chamber;
a collector mirror arranged in the vacuum chamber to have a primary focus at an irradiation region in the vacuum chamber;
a gas distribution system for adding gas to the vacuum chamber, the gas distribution system having an interface for interfacing with a source of a mitigation gas that mitigates, by chemical reaction, contamination of the collector mirror, the mitigation gas comprising H2O2; and
a gas control system for controlling the gas distribution system, the gas control system having a first state with a first duration in which the mitigation gas from the source of gas is introduced into the interior of the vacuum chamber in a regulated manner by the gas distribution system until a first partial pressure of the mitigation gas is attained and a second state with a second duration in which the mitigation gas from the source of gas is not introduced into the interior of the vacuum chamber by the gas distribution system, the second state continuing a second duration until a second partial pressure of the mitigation gas is attained.
21. Apparatus as claimed in claim 20 wherein the collector mirror comprises a multilayer mirror comprising a capping layer and a plurality of underlying layers, the capping layer being arranged to protect the underlying layers from damage.
22. Apparatus as claimed in claim 21 wherein the capping layer comprises an oxide.
23. Apparatus as claimed in claim 21 wherein the capping layer comprises a nitride or a carbide.
24. Apparatus as claimed in claim 20 wherein the gas distribution system is configured to add mitigation gas from the source of gas into the vacuum chamber by adding a quantity of mitigation gas to a flow of hydrogen-containing gas.
25. Apparatus as claimed in claim 20 wherein the gas distribution system is configured to mix the mitigation gas with an inert gas.