Patent Number: 
Section: claims

1. An optical sensor apparatus for use in an extreme ultraviolet (EUV) lithographic system comprising:an optical sensor comprising a sensor surface, wherein the optical sensor comprises an EUV sensitive scintillation material comprising YAG:Ce and a detector configured to detect photonic energy emitted from the scintillation material in response to scintillation caused by incident EUV radiation; anda removal mechanism configured to continuously remove debris from the sensor surface, the removal mechanism having a hydrogen radical supply system. 2. The optical sensor apparatus of claim 1, wherein the hydrogen radical supply system comprises a hydrogen gas supply system and a radical generator. 3. The optical sensor apparatus of claim 2, wherein the radical generator comprises at least one of a filament and microwave. 4. The optical sensor apparatus of claim 1, wherein the sensor surface comprises a cap layer having a hydrogen radical recombination constant less than 0.2. 5. The optical sensor apparatus of claim 4, wherein the cap layer Comprises Si3N4. 6. The optical sensor apparatus of claim 1, wherein:the optical sensor apparatus is configured to be used with an EUV radiation system; andthe EUV radiation system includes an EUV source having a laser induced plasma source or a discharge produced plasma source. 7. The optical sensor apparatus of claim 6, wherein the plasma source comprises Sn or Xe. 8. An optical sensor apparatus for use in an extreme ultraviolet (EUV) lithographic system comprising:an optical sensor comprising a sensor surface; anda removal mechanism configured to continuously remove debris from the sensor surface, the removal mechanism having a hydrogen radical supply system,wherein the sensor surface comprises a surface part of a rotatable plate and wherein the removal mechanism is configured to remove debris from another part of the rotatable plate. 9. The optical sensor apparatus of claim 8, wherein the rotatable plate is an EUV translucent plate or an EUV sensitive scintillation material. 10. The optical sensor apparatus of claim 9, wherein the EUV translucent plate is configured to selectively transmit a spectral range of a beam of EUV radiation from the EUV source. 11. The optical sensor apparatus of claim 9, wherein the EUV translucent plate selectively passes radiation with wavelengths in a range of 10-20 nm. 12. The optical sensor apparatus of claim 9, wherein the EUV translucent plate comprises a Nb filter, a Zr/Si multilayer filter, or a Zr/Nb multilayer filter.