Patent Number: 
Section: claims

1. An EUV projection optical unit configured to image an object field in an image field, the EUV projection optical unit comprising:a plurality of mirrors configured to guide imaging light from the object field to the image field,wherein:the EUV projection optical unit has an image-side numerical aperture of at least 0.4;an overall reflectivity of the EUV projection optical unit is a product of the reflectivity of each of the plurality of mirrors;the overall reflectivity of the EUV projection optical unit is greater than 7%; andthe EUV projection optical unit has a total of eight mirrors. 2. The EUV projection optical unit of claim 1, wherein:the plurality of mirrors is configured to guide the imaging light from the object field to the image field along a beam path;the plurality of mirrors comprises first and second mirrors;the second mirror is directly behind the first mirror in the beam path; andfor each of the first and second mirrors, an angle of incidence of the imaging light with the mirror is greater than 60°. 3. The EUV projection optical unit of claim 2, wherein, among the plurality of mirrors, only the first and second mirrors have an angle of incidence of the imaging light with the mirror is greater than 60°. 4. The EUV projection optical unit of claim 2, wherein:the plurality of mirrors further comprises third and fourth mirrors; andamong the plurality of mirrors, only the first, second, third and fourth mirrors have an angle of incidence of the imaging light with the mirror is greater than 60°. 5. The EUV projection optical unit of claim 4, wherein:the second mirror is directly behind the first mirror in the beam path; andthe fourth mirror is directly behind the third mirror in the beam path. 6. The EUV projection optical unit of claim 1, wherein:the object field is in an object plane of the EUV projection optical unit;the image field is in an image plane of the EUV projection optical unit; andan angle between the object plane and the image plane is different from 0°. 7. The EUV projection optical unit of claim 1, wherein the EUV projection optical unit comprises at least two mirrors configured to have an angle of incidence with the imaging light that is less than 45°. 8. The EUV projection optical unit of claim 1, wherein the EUV projection optical unit comprises at least four mirrors configured to have an angle of incidence with the imaging light that is less than 45°. 9. The EUV projection optical unit of claim 1, wherein the overall reflectivity of the EUV projection optical unit is greater than 9%. 10. The EUV projection optical unit of claim 1, wherein the EUV projection optical unit has an object-side chief ray angle for a field center point that is less than 7°. 11. The EUV projection optical unit of claim 10, wherein the image field has an extent of more than 13 mm along a field dimension. 12. The EUV projection optical unit of claim 1, wherein the image field has an extent of more than 13 mm along a field dimension. 13. The EUV projection optical unit of claim 12, wherein the extent of the image field is more than 20 mm along the field dimension. 14. The EUV projection optical unit of claim 1, wherein the image-side numerical aperture is at least 0.5. 15. The EUV projection optical unit of claim 1, wherein at least one of the mirrors has a free-form reflection surface. 16. An optical system, comprising:an EUV projection optical unit according to claim 1; andan illumination optical unit configured to illuminate the object field with illumination and imaging light. 17. The optical system of claim 16, further comprising an EUV light source. 18. The optical system of claim 16, wherein the optical system is an EUV projection exposure apparatus. 19. The optical system of claim 18, further comprising a reticle holder configured to hold a reticle, wherein the reticle holder is configured to move the reticle in a scanning direction, and an imaging scale of the EUV projection optical unit in the scanning direction is greater than in a direction perpendicular thereto. 20. A method of using an EUV projection exposure apparatus comprising an EUV projection optical unit and an illumination optical unit, the method comprising:using the illumination optical unit to illuminate an object plane; andusing the EUV projection optical unit to project the illuminated object plane onto a field plane,wherein the EUV projection optical unit comprises EUV projection optical unit according to claim 1.