Patent Number: 
Section: claims

1. A system for detecting at least one contamination species in a vacuum, the system including:at least one cathode having a surface comprising a surface material, the at least one cathode configured to emit electrons into the vacuum; andat least one detector configured to detect the electrons emitted by the at least one cathode,wherein the cathode is configured to emit a first current of electrons when a cathode surface is substantially not contaminated by the at least one contamination species, andwherein the cathode is configured to emit a second current of electrons when the cathode surface is contaminated by the at least one contamination species based on an interaction between the at least one contamination species and the surface material. 2. The system according to claim 1, wherein the surface material comprises Lanthanum Hexaboride. 3. The system according to claim 1, further comprising:at least two cathodes having different surface areas. 4. The system according to claim 1, wherein the at least one detector comprises at least one anode configured to receive the electrons that are emitted by the at least one cathode. 5. The system according to claim 1, further comprising:an alarm generator configured to generate an alarm signal if the second current of electrons, detected by the at least one detector, is equal to or less than a threshold value. 6. The system according to claim 5, wherein the threshold value is substantially 0. 7. The system according to claim 1, wherein the second current of electrons is approximately equal to, or less than, half the first current of electrons. 8. The system according to claim 1, wherein the at least one contamination species comprises one or more hydrocarbon chains. 9. The system according to claim 1, wherein the surface material comprises at least one carbon nanotube. 10. A lithographic apparatus comprising at least one system according to claim 1. 11. The lithographic apparatus according to claim 10, further comprising:an illumination system configured to provide a radiation beam;a patterning device configured to impart the radiation beam with a pattern in its cross-section;a substrate holder configured to hold a substrate; anda projection system configured to project the patterned radiation beam onto a target portion of the substrate. 12. The system according to claim 1, wherein the surface material comprises Cesium Hexaboride. 13. A contamination detection system, comprising:a chamber;a cathode situated in the chamber, the cathode comprising a surface coated in a surface material being configured to emit electrons as a first current of electrons, the electrons comprising a first current of electrons when the cathode surface is substantially free of contamination from at least one contamination species, and the electrons comprising a second current of electrons when the cathode surface is contaminated by the at least one contamination species based on an interaction between the at least one contamination species and the surface material;a detector configured to detect the electrons emitted from the cathode; anda controller configured to generate an alarm signal when the detector detects the second current of electrons. 14. The system of claim 13, further comprising:a plurality of cathodes situated in the chamber; anda plurality of detectors corresponding to the plurality of cathodes. 15. The system of claim 14, wherein:a first cathode from among the plurality of cathodes comprises a first surface coated in a first surface material; anda second cathode from among the plurality of cathodes comprises a second surface coated in a second surface material. 16. The system of claim 15, wherein the first surface material comprises a different material than the second surface material. 17. The system of claim 13, wherein the surface material comprises Lanthanum Hexaboride. 18. The system of claim 13, wherein the surface material comprises Cesium Hexaboride. 19. The system of claim 13, wherein the second current is less than the first current. 20. The system of claim 13, further comprising:a current source coupled to the cathode and configured to generate an electric current to heat the cathode to remove the at least one contamination species after the detector has detected the second current of electrons.