Patent Number: 
Section: claims

1. A method of measuring contamination of a lithographical element comprising:providing a process chamber with a first lithographical element, a second lithographical element, and a contaminant therein;covering part of the first lithographical element, thus providing a reference region on the first lithographical element being the covered part of the first lithographical element and a test region on the first lithographical element being the uncovered part of the first lithographical element;directing an exposure beam via the test region of the first lithographical element towards the second lithographical element such that at least one of the lithographical elements gets contaminated by the contaminant;uncovering the covered part of the first lithographical element, thus providing an uncovered reference region; andoptically measuring the level of contamination of the at least one contaminated lithographical element in the process chamber. 2. The method according to claim 1, wherein the covering of part of the first lithographical element comprises, either providing a shield in direct contact with the first lithographical element to cover at least part of the first lithographical element, or providing a shield for shadowing at least part of the first lithographical element. 3. The method according to claim 1, wherein the optically measuring of the contamination level comprises measuring an optical parameter on the test region and the reference region. 4. The method according to claim 1, wherein the first lithographical element is a lithographical optical element. 5. The method according to claim 1, further comprising removing the contaminant from the process chamber before measuring the level of contamination of the at least one contaminated lithographical element in the process chamber. 6. The method according to claim 1, wherein the optically measuring of the contamination level comprises measuring an optical parameter on the test region and the uncovered reference region. 7. The method according to claim 1, wherein the measuring of the level of contamination of the first lithographical element in the process chamber comprises:measuring a value for a parameter at the uncovered reference region of the first lithographical element, the measured value being the reference value of the parameter,measuring a value for the parameter at the test region of the first lithographical element, the measure value being the test value of the parameter,determining the difference between the reference value and the test value. 8. The method according to claim 7, wherein the measuring of the reference value of the parameter comprises redirecting the exposure beam via the reference region of the first lithographical element towards a sensor. 9. The method according to claim 7, wherein the measuring of the test value of the parameter comprises redirecting an exposure beam via the test region of the first lithographical element towards a sensor. 10. The method according to claim 1, wherein the second lithographical element is a mask. 11. The method according to claim 1, wherein the second lithographical element comprises the contaminant. 12. The method according to claim 11, wherein the second lithographical element comprises resist, the resist comprising the contaminant, the contaminant being provided in the process chamber by outgassing of the resist. 13. The method according to claim 1, wherein the optically measuring of the level of contamination of the at least one contaminated lithographical element in the process chamber comprises measuring the level of contamination of the second lithographical element in the process chamber. 14. The method according to claim 13, wherein the optically measuring of the level of contamination of the second lithographical element further comprises:measuring a second element test value for the parameter at the test region of the second lithographical element by redirecting the exposure beam via the reference region of the first lithographical element towards the second lithographical element and further towards a sensor. 15. The method according to claim 14, wherein the measuring of the second element test value further comprises redirecting the exposure beam via the test region of the first lithographical element towards the second lithographical element and further towards a sensor. 16. The method according to claim 1, wherein the first lithographical element is a mirror or a lens. 17. The method according to claim 16, wherein the parameter is the reflectivity of the mirror or the transmissivity of the lens. 18. The method according to claim 17, wherein the sensor is either a reflectivity sensor or a transmissivity sensor, for the mirror or the lens, respectively. 19. The method according to claim 1, wherein the test region comprises a plurality of test zones. 20. The method according to claim 1, wherein the reference region comprises a plurality of reference zones. 21. A system for measuring contamination of a lithographical element comprisinga process chamber;a first lithographical element and a second lithographical element;a contamination source for providing a contaminant,a first energy source for providing a first energy beam in the process chamber, the energy beam at least adapted to cause contamination of at least one of the first and second lithographical elements by directly or indirectly exposing the at least one of the first and second lithographical elements in the presence of the contaminant;a sensor for measuring contamination; anda cover element configured to cover at least part of the first lithographical element such that the covered part of the first lithographical element is not contaminated when the first lithographical element is exposed by the first energy beam. 22. The system according to claim 21, wherein the first energy source emits electromagnetic radiation in the extreme ultra-violet range. 23. The system according claim 22, wherein the sensor is an optical sensor. 24. A system for measuring contamination of a lithographical element comprising:means for accommodating a first lithographical element, a second lithographical element, and a contaminant;means for covering part of the first lithographical element, thus providing a reference region on the first lithographical element being the covered part of the first lithographical element and a test region on the first lithographical element being the uncovered part of the first lithographical element;means for directing an exposure beam via the test region of the first lithographical element towards the second lithographical element such that at least one of the lithographical elements gets contaminated by the contaminant;means for uncovering the covered part of the first lithographical element, thus providing an uncovered reference region; andmeans for optically measuring the level of contamination of the at least one contaminated lithographical element in the process chamber.