Patent Number: 
Section: claims

1. A method to clean one or more optical elements of an apparatus, the apparatus being configured to project a beam of radiation onto a target portion of a substrate and comprising a plurality of optical elements arranged in a sequence in a path of the radiation beam, wherein the method comprises cleaning a second optical element of the sequence, which receives a second radiation dose during operation of the apparatus, utilizing a cumulatively shorter cleaning period than a first optical element of the sequence, which receives a first radiation dose during operation of the apparatus, the second radiation dose being lower than the first radiation dose, wherein the cleaning comprises (i) detecting a radiation loss of radiation passing one or more of the optical elements and automatically starting a cleaning cycle to clean at least one of the optical elements in the case a detected radiation loss reaches a certain amount of radiation loss, or (ii) detecting contamination of at least one of the optical elements and automatically starting a cleaning cycle to clean at least one of the optical elements when the contamination reaches a certain amount of contamination. 2. The method according to claim 1, wherein the second optical element is located downstream with respect to the first optical element, viewed along the path of the radiation beam. 3. The method according to claim 1, further comprising:providing radiation dose information, the information including or relating to the amount of radiation the second optical element receives during operation of the apparatus; andproviding a cleaning period for cleaning the second optical element, such that the length of the cleaning period concerning the second optical element is correlated to the radiation dose information of the second optical element. 4. The method according to claim 1, comprising applying various cleaning cycles between operating periods of the apparatus, wherein during one or more of the cleaning cycles only the first optical element is being cleaned, and not the second optical element. 5. The method according to claim 1, wherein the apparatus comprises a support structure constructed to hold a patterning device at a patterning device position in the path of the radiation beam, wherein the first optical element is located upstream with respect to the patterning device position and the second optical element is located downstream with respect to the patterning device position, viewed with respect to a direction of propagation of the radiation beam. 6. The method according to claim 1, wherein the first radiation dose and the second radiation dose comprise EUV radiation. 7. The method of claim 1, wherein the cleaning comprises the detecting the radiation loss of radiation passing one or more of the optical elements and automatically starting a cleaning cycle to clean at least one of the optical elements in the case a detected radiation loss reaches a certain amount of radiation loss. 8. The method of claim 1, wherein the cleaning comprises the detecting contamination of at least one of the optical elements and automatically starting a cleaning cycle to clean at least one of the optical elements when the contamination reaches a certain amount of contamination. 9. A device manufacturing method, comprising:projecting a beam of radiation onto a target portion of a substrate utilizing a sequence of optical elements, wherein the optical elements receive different radiation doses and the contamination rates of the optical elements are correlated with the radiation doses;executing a number of cleaning cycles to clean at least one of the optical elements, each cleaning cycle involving cleaning an optical element of the sequence based on the radiation dose received by the optical element, so that an optical element that has received a low radiation dose is cleaned during a shorter time period or less often than an optical element that has received a radiation dose that is higher than the low radiation dose, wherein executing the number of cleaning cycles comprises (i) detecting a radiation loss of radiation passing one or more of the optical elements and automatically starting a cleaning cycle to clean at least one of the optical elements in the case a detected radiation loss reaches a certain amount of radiation loss, or (ii) detecting contamination of at least one of the optical elements and automatically starting a cleaning cycle to clean at least one of the optical elements when the contamination reaches a certain amount of contamination. 10. The method according to claim 9, wherein a second optical element of the sequence of optical elements, the second optical element being part of a projection system of a lithographic apparatus used to project the beam of radiation, is being cleaned at least 10 times less often than a first optical element of the sequence of optical elements, the first optical element being part of an illuminator of the lithographic apparatus. 11. The method according to claim 9, wherein the optical element that has received the radiation dose that is higher than the low radiation dose is located upstream with respect to a patterning device in the path of the radiation beam and the optical element that has received the low radiation dose is located downstream with respect to the patterning device, viewed with respect to a direction of propagation of the radiation beam. 12. The method of claim 9, wherein the executing the number of cleaning cycles comprises the detecting the radiation loss of radiation passing one or more of the optical elements and automatically starting a cleaning cycle to clean at least one of the optical elements in the case a detected radiation loss reaches a certain amount of radiation loss. 13. The method of claim 9, wherein the executing the number of cleaning cycles comprises the detecting contamination of at least one of the optical elements and automatically starting a cleaning cycle to clean at least one of the optical elements when the contamination reaches a certain amount of contamination. 14. An apparatus, comprising:an illumination system configured to condition a radiation beam;a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;a substrate table constructed to hold a substrate;a projection system configured to project the patterned radiation beam onto a target portion of the substrate; anda cleaning system configured to clean one or more optical elements of the apparatus, the cleaning system being configured to clean an optical element of the projection system during a cumulatively shorter cleaning period than an optical element of the illumination system, the cleaning system comprising or being, in use, coupled to (i) a radiation loss detector configured to detect a radiation loss of radiation passing one or more of the optical elements and wherein the cleaning system is configured to automatically start a cleaning cycle to clean the optical element of the projection system, the optical element of the illumination system, or both, in the case a detected radiation loss reaches a certain amount of radiation loss, or (ii) a contamination detector configured to detect contamination of the optical element of the projection system, the optical element of the illumination system, or both, and wherein the cleaning system is configured to automatically start a cleaning cycle to clean the optical element of the projection system, the optical element of the illumination system, or both, when the contamination reaches a certain amount of contamination. 15. The apparatus according to claim 14, wherein the cleaning system is configured to clean only one or some of the optical elements that are arranged in the path of the radiation beam, depending on the amount of radiation received by each of such elements during the projection of the patterned beam. 16. The apparatus according to claim 14, further comprising a memory holding (i) radiation information including or relating to the amount of radiation one or more of the optical elements receives during operation of the apparatus, or (ii) a cleaning period for cleaning one or more of the optical elements, the length of the cleaning period concerning an optical element being correlated to the respective radiation dose information of that optical element, or (iii) a cleaning rate of a cleaning process for cleaning one or more of the optical elements, or (iv) any combination of (i)-(iii), wherein the cleaning system is configured to use the radiation information, or the cleaning period, or the cleaning rate, or any combination thereof, to clean the optical element of the projection system, the optical element of the illumination system, or both. 17. The apparatus according to claim 14, comprising or being coupled to the radiation loss detector and wherein the cleaning system is configured to automatically start a cleaning cycle to clean the optical element of the projection system, the optical element of the illumination system, or both, in the case a detected radiation loss reaches a certain amount of radiation loss. 18. The apparatus according to claim 14, comprising or being coupled to the contamination detector and wherein the cleaning system is configured to automatically start a cleaning cycle to clean the optical element of the projection system, the optical element of the illumination system, or both, when the contamination reaches a certain amount of contamination. 19. The apparatus according to claim 14, configured to clean the optical element of the projection system at least 10 times less often than the optical element of illumination system. 20. A cleaning system adapted to clean one or more optical elements of an apparatus, the optical elements being arranged in a path of a radiation beam used during a process carried out by the apparatus, wherein the cleaning system is configured to clean only one or some of the optical elements that are arranged in the path of the radiation beam, depending on the amount of radiation received by each of such elements during the process, the cleaning system comprising or being, in use, coupled to (i) a radiation loss detector configured to detect a radiation loss of radiation passing one or more of the optical elements and wherein the cleaning system is configured to automatically start a cleaning cycle to clean at least one of the optical elements in the case a detected radiation loss reaches a certain amount of radiation loss, or (ii) a contamination detector configured to detect contamination of at least one of the optical elements and wherein the cleaning system is configured to automatically start a cleaning cycle to clean at least one of the optical elements when the contamination reaches a certain amount of contamination. 21. The cleaning system according to claim 20, further comprising a memory holding (i) radiation information including or relating to the amount of radiation one or more of the optical elements receives during operation of the apparatus, or (ii) a cleaning period for cleaning one or more of the optical elements, the length of the cleaning period concerning an optical element being correlated to the respective radiation dose information of that optical element, or (iii) a cleaning rate of a cleaning process for cleaning one or more of the optical elements, or (iv) any combination of (i)-(iii), wherein the cleaning system is configured to use the radiation information, or the cleaning period, or the cleaning rate, or. any combination thereof, to clean the one or some optical elements. 22. The cleaning system according to claim 20, comprising or being coupled to the radiation loss detector and wherein the cleaning system is configured to automatically start a cleaning cycle to clean at least one of the optical elements in the case a detected radiation loss reaches a certain amount of radiation loss. 23. The cleaning system according to claim 20, comprising or being coupled to the contamination detector and wherein the cleaning system is configured to automatically start a cleaning cycle to clean at least one of the optical elements when the contamination reaches a certain amount of contamination. 24. The cleaning system according to claim 20, configured to clean one or more of the optical elements during a cumulatively 100 times shorter cleaning period than one or more other of the optical elements. 25. The cleaning system according to claim 20, configured to clean only one or some of the optical elements that are arranged in the path of the radiation beam, depending on the amount of EUV radiation received by each of such elements during the process.