Projection objective for microlithography
View Patent ↗A projection objective for microlithography is used for imaging an object field in an object plane into an image field in an image plane. The projection objective comprises at least six mirrors of which at least one mirror has a freeform reflecting surface. The ratio between an overall length (T) of the projection objective and an object image shift (d OIS ) can be smaller than 12. The image plane is the first field plane of the projection objective downstream of the object plane. The projection objective can have a plurality of mirrors, wherein the ratio between an overall length (T) and an object image shift (d OIS ) is smaller than 2.
1. A projection objective for imaging an object field in an object plane into an image field in an image plane, the projection objective comprising:
at least six mirrors of which at least one mirror has a freeform reflecting surface, wherein:
the projection objective is a microlithography projection objective;
the image plane is the first field plane of the projection objective downstream of the object plane;
the projection objective has an overall length, T, which is a distance between the object plane and the image plane in a direction perpendicular to the object plane;
a first point in the image plane is a projection in a direction perpendicular to the object plane of a central field point of the object field;
during operation of the projection objective, a ray of light from the central field point of the object field intersects the image plane at a second point in the image plane;
an object image shift, d OIS , is a distance between the first and second points in the image plane in a direction parallel to the image plane; and
a ratio of T to d OIS is smaller than 5.
2. The projection objective of claim 1 , wherein the ratio of T to d OIS is smaller than 1.5.
3. The projection objective according to claim 1 , wherein the ratio of T to d OIS is smaller than 1.1.
4. The projection objective according to one of claims 2 , wherein the freeform reflecting surface is a biconical surface.
5. The projection objective according to claim 2 , wherein d OIS is greater than 200 mm.
6. The projection objective according to claim 2 , wherein during operation the projection objective directs light along a path from the object field to the image field and a ratio of a difference between a largest and a smallest incidence angle of the light impinging upon one of the mirrors to a numerical aperture on the image side of the projection objective is 60° or less.
7. The projection objective according to claim 1 , wherein a numerical aperture on the image side of the projection objective is at least 0.25.
8. The projection objective according to claim 1 , wherein the object field and/or image field have dimensions of at least 2 mm×26 mm.
9. The projection objective according to claim 1 , wherein during operation the projection objective directs light along a path from the object field to the image field and an incidence angle of the light at the central field point of the object field is in a range between 5° and 9°.
10. The projection objective according to claim 1 , wherein the ratio of T to d OIS is smaller than 2.
11. The projection objective according to claim 10 , wherein the freeform reflecting surface is a biconical surface.
12. The projection objective according to claim 10 , wherein d OIS is greater than 200 mm.
13. The projection objective according to claim 10 , wherein during operation the projection objective directs light along a path from the object field to the image field and a ratio of a difference between a largest and a smallest incidence angle of the light impinging upon one of the mirrors to a numerical aperture on the image side of the projection objective is 60° or less.
14. The projection objective according to claim 10 , wherein a numerical aperture on the image side of the projection objective is at least 0.25.
15. The projection objective according to claim 10 , wherein the object field and/or image field have dimensions of at least 2 mm×26 mm.
16. The projection objective according to claim 10 , wherein during operation the projection objective directs light along a path from the object field to the image field and an incidence angle of the light at the central field point of the object field is in a range between 5° and 9°.
17. The projection objective according to claim 1 , wherein the freeform reflecting surface is a biconical surface.
18. The projection objective according to claim 1 , wherein d OIS is greater than 200 mm.
19. The projection objective according to claim 1 , wherein during operation the projection objective directs light along a path from the object field to the image field and a ratio of a difference between a largest and a smallest incidence angle of the light impinging upon one of the mirrors to a numerical aperture on the image side of the projection objective is 60° or less.
20. The projection objective according to claim 1 , wherein a numerical aperture on the image side of the projection objective is at least 0.25.
21. The projection objective according to claim 1 , wherein the object field and/or image field have dimensions of at least 2 mm×26 mm.
22. The projection objective according to claim 1 , wherein during operation the projection objective directs light along a path from the object field to the image field and an incidence angle of the light at the central field point of the object field is in a range between 5° and 9°.
23. An apparatus, comprising:
a light source;
an illumination system comprising one or more optical elements; and
a projection objective according to claim 1 , wherein the apparatus is a projection exposure apparatus.
24. The apparatus according to claim 23 , wherein the freeform reflecting surface is a biconical surface.
25. The apparatus according to claim 23 , wherein d OIS is greater than 200 mm.
26. The apparatus according to claim 23 , wherein during operation the projection objective directs light along a path from the object field to the image field and a ratio of a difference between a largest and a smallest incidence angle of the light impinging upon one of the mirrors to a numerical aperture on the image side of the projection objective is 60° or less.
27. The apparatus according to claim 23 , wherein a numerical aperture on the image side of the projection objective is at least 0.25.
28. The apparatus according to claim 23 , wherein the object field and/or image field have dimensions of at least 2 mm×26 mm.
29. The apparatus according to claim 23 , wherein during operation the projection objective directs light along a path from the object field to the image field and an incidence angle of the light at the central field point of the object field is in a range between 5° and 9°.
30. A method comprising:
providing a reticle and a wafer; and
projecting a structure on the reticle onto a light-sensitive layer of the wafer using a projection exposure apparatus,
wherein the projection exposure apparatus comprises:
a light source;
an illumination system comprising one or more optical elements; and
a projection objective according to claim 1 .
31. The method according to claim 30 , wherein the freeform reflecting surface is a biconical surface.
32. The method according to claim 30 , wherein d OIS is greater than 200 mm.
33. The method according to claim 30 , wherein during the method the projection objective directs light along a path from the object field to the image field and a ratio of a difference between a largest and a smallest incidence angle of the light impinging upon one of the mirrors to a numerical aperture on the image side of the projection objective is 60° or less.
34. The method according to claim 30 , wherein a numerical aperture on the image side of the projection objective is at least 0.25.
35. The method according to claim 30 , wherein the object field and/or image field have dimensions of at least 2 mm×26 mm.
36. The method according to claim 30 , wherein during the method the projection objective directs light along a path from the object field to the image field and an incidence angle of the light at the central field point of the object field is in a range between 5° and 9°.
37. The projection objective according to claim 1 , wherein:
the projection objective has an axis perpendicular to the object plane;
the axis intersects the central field point of the object field; and
during operation of the projection objective, light follows a path from the central field point of the object field to the image field without intersecting the axis in a region between the object plane and the image plane.