Patent Number: 
Section: claims

1. An optical system for extreme ultraviolet (EUV) lithography, comprisinga reflective optical element, including a substrate with a highly reflective coating emitting secondary electrons when irradiated with EUV radiation, anda source of electrically charged particles, which is arranged such that electrically charged particles are applied to the reflective optical element,wherein the source is a flood gun applying electrons to the reflective optical element as the only charge carrier compensation source. 2. The optical system according to claim 1, wherein the flood gun is arranged to apply the electrons only to a border region of the highly reflective coating of the reflective optical element. 3. The optical system according to claim 1, wherein the flood gun is arranged to apply the electrons to only the border and to cover completely the border of the highly reflective coating of the reflective optical element. 4. The optical system according to claim 1, wherein the flood gun is arranged to apply electrons to a surface of the reflective optical element with an angle larger than 45° to the surface normal. 5. The optical system according to claim 1, wherein the flood gun is a thermionic flood gun with a heating element, the heating element being a planar element. 6. The optical system according to claim 1, wherein the highly reflective coating is a multilayer system. 7. The optical system according to claim 1, further comprising at least one of: at least one further reflective optical element and at least one further electron source. 8. An illumination system for EUV lithography, comprising an optical system according to claim 1. 9. A projection system for EUV lithography, comprising an optical system according to claim 1. 10. An EUV lithography apparatus for EUV lithography, comprising an optical system according to claim 1. 11. An optical system for extreme ultraviolet (EUV) lithography, comprisinga reflective optical element, including a substrate and a reflective coating emitting secondary electrons when irradiated with EUV radiation, anda source of electrically charged particles, configured to apply the electrically charged particles to the reflective optical element,wherein the source consists essentially of a flood gun configured to apply electrons as charge carrier compensation for the emitted secondary electrons.